U.S. patent application number 09/742606 was filed with the patent office on 2002-06-27 for electrical system with capacitance tap and sensor for on-line monitoring the state of high-voltage insulation and remote monitoring device.
Invention is credited to Golubev, Alexander, Rashkes, Viktor S..
Application Number | 20020079906 09/742606 |
Document ID | / |
Family ID | 24985507 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020079906 |
Kind Code |
A1 |
Rashkes, Viktor S. ; et
al. |
June 27, 2002 |
ELECTRICAL SYSTEM WITH CAPACITANCE TAP AND SENSOR FOR ON-LINE
MONITORING THE STATE OF HIGH-VOLTAGE INSULATION AND REMOTE
MONITORING DEVICE
Abstract
A sensor and associated circuits are provided for on-line
monitoring of the state of the high-voltage insulation in
electrical equipment with capacitance tap. In this arrangement,
both the power frequency signal and the radio frequency signals
associated with partial discharge activity are sensed. These
signals are transmitted from the sensor to remote monitoring
instrumentation via one connecting cable. The sensor contains a
surge arrester in parallel with a capacitor shunt, a radio
frequency current transformer and a connecting circuit. The
polarity terminal of the primary winding of the radio frequency
current transformer is connected to the tap output. The
non-polarity terminal is connected to the common connection point
of the surge arrester and the capacitor shunt while the second
terminals of these components are connected to the local ground.
The non-polarity terminal of the secondary winding of the radio
frequency current transformer is connected to the non-polarity
terminal of its primary winding, and the polarity terminal-to the
signal conductor of the connecting circuit. The second conductor of
the connecting circuit is connected to the local ground. Both the
power frequency signal and the radio frequency signal are
transmitted on the same cable from the sensor to the remote
monitoring instrumentation.
Inventors: |
Rashkes, Viktor S.;
(Plymouth, MN) ; Golubev, Alexander; (Maple Grove,
MN) |
Correspondence
Address: |
Martin J. Moran
Eaton Corp. Cutler-Hammer Technology Center
170 Industry Drive, RIDC Park West
Pittsburgh
PA
15275
US
|
Family ID: |
24985507 |
Appl. No.: |
09/742606 |
Filed: |
December 21, 2000 |
Current U.S.
Class: |
324/544 |
Current CPC
Class: |
H02B 11/26 20130101;
G01R 15/16 20130101; H02B 11/04 20130101; G01R 15/14 20130101; G01R
31/1227 20130101 |
Class at
Publication: |
324/544 |
International
Class: |
H04B 003/46; H01H
031/02 |
Claims
1. A partial discharge determination system for an electrical
system which includes a conductor at a given voltage potential,
electrical insulation disposed proximate said conductor, insulator
capacitance in said insulation which conducts a partial discharge
radio frequency electrical current and a power frequency current
component: a sensor current transformer, said sensor current
transformer having a sensor current transformer primary winding and
a sensor current transformer secondary winding, said sensor current
transformer primary winding having a sensor current transformer
primary winding first end and a spaced sensor current transformer
primary winding second end, said sensor current transformer
secondary winding having a sensor current transformer secondary
winding first end and spaced current transformer sensor secondary
winding second end; a sensor capacitor shunt, said sensor capacitor
shunt having a sensor capacitor shunt first end and a spaced sensor
capacitor shunt second end; said sensor current transformer primary
winding first end being connected electrically to said insulator
capacitance in said insulator to conduct said partial discharge
electrical current through said sensor current transformer primary
winding; said sensor current transformer primary winding second
end, said sensor current transformer secondary winding second end,
and said sensor capacitor shunt first end being connected together
electrically; a coaxial cable, said coaxial cable having an inner
conductor, said inner conductor having an inner conductor first end
and a spaced inner conductor second ends, said coaxial cable having
an outer conductor, said outer conductor having an outer conductor
first end and a spaced an outer conductor second end; said coaxial
cable inner conductor first end being connected electrically to
said sensor current transformer primary winding first end; a
monitoring power frequency capacitor having a monitoring power
frequency capacitor first end and a spaced monitoring power
frequency capacitor second end; a monitoring radio frequency
isolation transformer having a monitoring radio frequency isolation
transformer primary winding and a monitoring radio frequency
isolation transformer secondary winding, said monitoring radio
frequency isolation transformer primary winding having a monitoring
radio frequency isolation transformer primary winding first end and
a spaced monitoring radio frequency isolation transformer primary
winding second end, said monitoring radio frequency isolation
transformer secondary winding having a monitoring radio frequency
isolation transformer secondary winding first end and a spaced
monitoring radio frequency isolation transformer secondary winding
second end; a monitoring choke coil, said monitoring coil having a
monitoring coil first end and a spaced monitoring choke coil second
end; said monitoring choke coil first end, and said monitoring
power frequency capacitor first end being interconnected
electrically with said coaxial conductor inner conductor second
end; said monitoring radio frequency isolation transformer primary
winding first end being interconnected electrically with said
monitoring power frequency capacitor second end; a monitoring surge
arrester, said monitoring surge arrester having a monitoring surge
arrester first end and a spaced monitoring surge arrester second
end; said monitoring choke coil first end, said monitoring surge
arrester first end, and said monitoring power frequency capacitor
first end being interconnected electrically with said coaxial
conductor inner conductor second end; said monitoring radio
frequency isolation transformer primary winding second end being
interconnected electrically with said coaxial conductor outer
conductor second end; a first signal representative of said partial
discharge electrical current power frequency current component
existing between said monitoring choke coil second end and said
monitoring surge arrester second end; and a second signal
representative of said radio frequency current component existing
between said monitoring radio frequency isolation transformer
secondary winding first end and said monitoring radio frequency
isolation transformer secondary winding second end.
2. The partial discharge determination system of claim 1, wherein
said sensor current primary winding first end comprises a polarity
terminal of said sensor current primary winding, and said sensor
current secondary winding first end comprises a polarity terminal
of said sensor current secondary winding.
3. The partial discharge determination system of claim 1,
comprising a sensing surge arrester, said sensing surge arrester
having a sensing surge arrester first end and a spaced sensing
surge arrester second end; and said sensor current transformer
primary winding second end, said sensor current transformer
secondary winding second end, said sensing surge arrester first end
and said sensor capacitor first end being connected together
electrically.
4. The partial discharge determination system of claim 3, said
sensing surge arrester second end being connected to ground.
5. The partial discharge determination system of claim 1, said
sensor capacitor shunt second end being connected to ground.
6. The partial discharge determination system of claim 1, said
coaxial cable outer cable first end being connected to ground.
7. The partial discharge determination system of claim 1, said
monitoring surge arrester second end being interconnected
electrically with ground.
8. A partial discharge determination system for an electrical
system which includes a conductor at a given voltage potential,
electrical insulation disposed proximate said conductor, insulator
capacitance in said insulation which conducts electrical current
therethrough, said electrical current having a power frequency
current component and a radio frequency current component,
comprising; a current sensor; a first signal representative of said
power frequency current component produced in said current sensor;
a second signal representative of said radio frequency current
component produced in said current sensor; two conductor cable with
a first end and a second end, said two conductor cable being
connected electrically to said current sensor at said first end and
conducting said first and second signal concurrently therethrough;
signal splitting device, said signal splitting device being
connected to said two conductor cable at said second end and
splitting said first signal from said second signal; a monitoring
power frequency device; a monitoring radio frequency device; and
said monitoring power frequency device and said monitoring radio
frequency device each being connected to said signal splitting
device, said monitoring power frequency device monitoring said
first signal and providing information about said power frequency
current component, said monitoring radio frequency device
monitoring said second signal and providing information about said
radio frequency current component.
9. The combination as claimed in claim 8, wherein said current
sensor comprises a sensor current transformer, said sensor current
transformer having a sensor current transformer primary winding and
a sensor current transformer secondary winding; said sensor current
transformer primary winding being connected serially electrically
to said insulator capacitance in said insulator to conduct said
sensor electrical current through said sensor current transformer
primary winding; a sensor capacitor, said sensor capacitor being
interconnected serially electrically with said sensor current
transformer primary winding; said first signal being produced in
association with said sensor current transformer secondary winding;
and a second signal being produced in association with said sensor
capacitor shunt.
10. The combination as claimed in claim 9, comprising a sensing
surge arrester said a sensing surge arrester being interconneced
electrically with said sensor current transformer primary winding
and said sensor capacitor.
11. The combination as claimed in claim 8, wherein said signal
splitting device comprises a monitoring power frequency capacitor
and a serially connected radio frequency isolation transformer
having a monitoring radio frequency isolation transformer primary
winding and a monitoring radio frequency isolation transformer
secondary winding; said first signal being represented electrically
in association with said monitoring radio frequency isolation
transformer secondary winding; and said second signal being
represented electrically in association with sensor monitoring
power frequency capacitor.
12. The combination as claimed in claim 11, comprising a monitoring
choke coil, said monitoring choke coil being interconnected
electrically with said sensor monitoring power frequency capacitor
and said monitoring power frequency device.
13. The combination as claimed in claim 11, comprising a monitoring
surge arrester said monitoring surge arrester being interconnected
electrically with said sensor monitoring power frequency capacitor
and said monitoring power frequency device.
14. The combination as claimed in claim 9, wherein said signal
splitting device comprises a monitoring power frequency capacitor
and a serially connected radio frequency isolation transformer
having a monitoring radio frequency isolation transformer primary
winding and a monitoring radio frequency isolation transformer
secondary winding; and said first signal being represented
electrically in association with said monitoring radio frequency
isolation transformer secondary winding; and said second signal
being represented electrically in association with said sensor
monitoring power frequency capacitor.
15. The combination as claimed in claim 14, comprising a monitoring
choke coil, said monitoring choke coil being interconnected
electrically with said sensor monitoring power frequency capacitor
and said monitoring power frequency device.
16. The combination as claimed in claim 14, comprising a monitoring
surge arrester said monitoring surge arrester being interconnected
electrically with said sensor monitoring power frequency capacitor
and said monitoring power frequency device.
17. An electrical system, comprising: a conductor at a given
voltage potential; electrical insulation disposed proximate said
conductor; insulator capacitance in said insulation which conducts
electrical said current having a power frequency current component
and a partial discharge radio frequency current component; a
partial discharge determination system comprising: a sensor current
transformer, said sensor current transformer having a sensor
current transformer primary winding and a sensor current
transformer secondary winding, said sensor current transformer
primary winding having a sensor current transformer primary winding
first end and a spaced sensor current transformer primary winding
second end, said sensor current transformer secondary winding
having a sensor current transformer secondary winding first end and
spaced current transformer sensor secondary winding second end; a
sensor capacitor shunt, said sensor capacitor shunt having a sensor
capacitor shunt first end and a spaced sensor capacitor shunt
second end; said sensor current transformer primary winding first
end being connected electrically to said insulator capacitance in
said insulator to conduct said partial discharge electrical current
through said sensor current transformer primary winding; said
sensor current transformer primary winding second end, said sensor
current transformer secondary winding second end, and said sensor
capacitor shunt first end being connected together electrically; a
coaxial cable, said coaxial cable having an inner conductor, said
inner conductor having an inner conductor first end and a spaced
inner conductor second ends, said coaxial cable having an outer
conductor, said outer conductor having an outer conductor first end
and a spaced outer conductor second end; said coaxial cable inner
conductor first end being connected electrically to said sensor
current transformer primary winding first end; a monitoring power
frequency capacitor having a monitoring power frequency capacitor
first end and a spaced monitoring power frequency capacitor second
end; a monitoring radio frequency isolation transformer having a
monitoring radio frequency isolation transformer primary winding
and a monitoring radio frequency isolation transformer secondary
winding, said monitoring radio frequency isolation transformer
primary winding having a monitoring radio frequency isolation
transformer primary winding first end and a spaced monitoring radio
frequency isolation transformer primary winding second end, said
monitoring radio frequency isolation transformer secondary winding
having a monitoring radio frequency isolation transformer secondary
winding first end and a spaced monitoring radio frequency isolation
transformer secondary winding second end; a monitoring choke coil,
said monitoring choke coil having a monitoring choke coil first end
and a spaced monitoring choke coil second end; said monitoring
choke coil first end, and said monitoring power frequency capacitor
first end being interconnected electrically with said coaxial
conductor inner conductor second end; said monitoring radio
frequency isolation transformer primary winding first end being
interconnected electrically with said monitoring power frequency
capacitor second end; a monitoring surge arrester, said monitoring
surge arrester having a monitoring surge arrester first end and a
spaced monitoring surge arrester second end; said monitoring choke
coil first end, said monitoring surge arrester first end, and said
monitoring power frequency capacitor first end being interconnected
electrically with said coaxial conductor inner conductor second
end; said monitoring radio frequency isolation transformer primary
winding second end being interconnected electrically with said
coaxial conductor outer conductor second end; a first signal
representative of said partial discharge electrical current power
frequency current component existing between said monitoring choke
coil second end and said monitoring surge arrester second end; and
a second signal representative of said radio frequency current
component existing between said monitoring radio frequency
isolation transformer secondary winding first end and said
monitoring radio frequency isolation transformer secondary winding
second end.
18. The electrical system of claim 17, wherein said sensor current
primary winding first end comprises a polarity terminal of said
sensor current primary winding, and said sensor current secondary
winding first end comprises a polarity terminal of said sensor
current secondary winding.
19. The electrical system of claim 17, comprising a sensing surge
arrester, said sensing surge arrester having a sensing surge
arrester first end and a spaced sensing surge arrester second end;
and said sensor current transformer primary winding second end,
said sensor current transformer secondary winding second end, said
sensing surge arrester first end and said sensor capacitor shunt
first end being connected together electrically.
20. The electrical system of claim 19, said sensing surge arrester
second end being connected to ground.
21. The electrical system of claim 17, said sensor capacitor shunt
second end being connected to ground.
22. The electrical system of claim 17, said coaxial cable outer
cable first end being connected to ground.
23. The electrical system of claim 17, said monitoring surge
arrester second end being interconnected electrically with
ground.
24. An electrical system comprising: a conductor at a given voltage
potential; electrical insulation disposed proximate said conductor;
insulator capacitance in said insulation which conducts electrical
current therethrough, said electrical current having a power
frequency current component and a radio frequency current
component; a partial discharge determination system comprising: a
current sensor; a first signal representative of said power
frequency current component produced in said current sensor; a
second signal representative of said radio frequency current
component produced in said current sensor; a two conductor cable
with a first end and a second end, said two conductor cable being
connected electrically to said current sensor at said first end and
conducting said first and second signals concurrently therethrough;
a signal splitting device, said signal splitting device being
connected to said two conductor cable at said second end and
splitting said first signal from said second signal; a monitoring
power frequency device; a monitoring radio frequency device; and
said monitoring power frequency device and said monitoring radio
frequency device each being connected to said signal splitting
device, said monitoring power frequency device monitoring said
first signal and providing information about said power frequency
current component, said monitoring radio frequency device
monitoring said second signal and providing information about said
radio frequency current component.
25. The combination as claimed in claim 24, wherein said current
sensor comprises a sensor current transformer, said sensor current
transformer having a sensor current transformer primary winding and
a sensor current transformer secondary winding; said sensor current
transformer primary winding being connected serially electrically
to said insulator capacitance in said insulator to conduct said
sensor electrical current through said sensor current transformer
primary winding; a sensor capacitor, said sensing capacitor being
interconnected serially electrically with said sensor current
transformer primary winding; said first signal being produced in
association with said sensor current transformer secondary winding;
and a second signal being produced in association with said sensor
current transformer secondary winding; and a second signal being
produced in association with said sensor capacitor shunt.
26. The combination as claimed in claim 25, comprising a sensing
surge arrester, said sensing surge arrester being interconnected
electrically with said sensor current transformer primary winding
and said sensor capacitor.
27. The combination as claimed in claim 24, wherein said signal
splitting device comprises a monitoring power frequency capacitor
and a serially connected radio frequency isolation transformer
having a monitoring radio frequency isolation transformer primary
winding and a monitoring radio frequency isolation transformer
secondary winding; said first signal being represented electrically
in association with said monitoring radio frequency isolation
transformer seconding winding; and said second signal being
represented electrically in association with said sensor monitoring
power frequency capacitor.
28. The combination as claimed in claim 27, comprising a monitoring
choke coil, said monitoring choke coil being interconnected
electrically with said sensor monitoring power frequency capacitor
and said monitoring power frequency device.
29. The combination as claimed in claim 27, comprising a monitoring
surge arrester said monitoring surge arrester being interconnected
electrically with said sensor monitoring power frequency capacitor
and said monitoring power frequency device.
30. The combination as claimed in claim 25, wherein said signal
splitting device comprises a monitoring power frequency capacitor
and a serially connected radio frequency isolation transformer
having a monitoring radio frequency isolation transformer primary
winding and a monitoring radio frequency isolation transformer
secondary winding; said first signal being represented electrically
in association with said monitoring radio frequency isolation
transformer secondary winding; and said second signal being
represented electrically in association with said sensor monitoring
power frequency capacitor.
31. The combination as claimed in claim 30, comprising a monitoring
choke coil, said monitoring choke coil being interconnected
electrically with said sensor monitoring power frequency capacitor
and said monitoring power frequency device.
32. The combination as claimed in claim 30, comprising a monitoring
surge arrester said monitoring surge arrester being interconnected
electrically with said sensor monitoring power frequency capacitor
and said monitoring power frequency device.
33. A sensor for the on-line monitoring of the state of the
high-voltage insulation in electrical apparatus equipped with a
capacitance tap, said electrical apparatus experiencing partial
discharge activity, said partial discharge activity causing the
existence of a power frequency component and a radio frequency
component, said sensor being connected between said capacitance tap
and local ground, comprising: a surge arrester with first and
second ends said surge arrester second end being connected to said
local ground; a capacitor shunt with first and second ends said
capacitor shunt second end being connected to said local ground; a
conducting cable having a signal conductor and a second conductor,
each said signal conductor and second conductor having a local end
and a remote end, said local end of said second conductor of said
conducting cable being connected to said local ground; a radio
frequency current transformer with a primary winding having a
polarity terminal and a non-polarity terminal and a secondary
winding having a polarity terminal and a non-polarity terminal;
said polarity terminal of said primary winding being connected to
said capacitance tap; said non-polarity terminal of said primary
winding being connected to said capacitor shunt first end; said
polarity winding of said secondary winding being connected to said
signal conductor; said first end of said surge arrester being
connected to said first end of said capacitor shunt; and said
non-polarity terminal of said primary winding and said non-polarity
terminal of said secondary winding being connected together and to
said first terminal of said capacitor shunt and said first terminal
of said surge arrester.
34. A remote input circuit for a radio frequency measuring device
with input terminal and second terminal and a power frequency
signal measuring device with input terminal and ground terminal for
the on-line monitoring of the state of the high-voltage insulation
in electrical apparatus experiencing partial discharge activity,
said partial discharge activity causing the existence of a radio
frequency signal, there also being a separate power frequency
signal, said radio frequency signal and said power frequency signal
being concurrently present, comprising: a conducting cable having a
signal conductor and a second conductor, each signal conductor and
second conductor having a local end and a remote end, said local
end of said second conductor of said conducting cable being
connected to said local ground and having a signal representative
of said power frequency signal and a signal representative of said
radio frequency signal concurrently disposed thereupon; said remote
end of said second conductor being ungrounded; a remote surge
arrester having a firs end and a second end, said first end of said
remote surge arrester being connected to said remote end of said
signal conductor, said second end of said remote surge arrester
being connected to said remote ground; a remote inductance having a
first end and a second end, said first end of said remote
inductance being connected to said remote end of said signal
conductor, said second end of said remote inductance being
connected to said input terminal of said power frequency signal
measuring device, said ground terminal of said power frequency
signal measuring device being connected to remote ground; a remote
capacitance having a first end and a second end; a remote radio
frequency isolating transformer with a primary winding having a
first terminal and a second terminal and a secondary winding having
a first terminal and a second terminal; said second end of said
remote capacitance being connected to said first end of said
primary winding of said remote radio frequency isolating
transformer; said first end of said remote capacitance being
connected to said remote end of said signal conductor of said
conducting cable and said second end of said primary winding of
said remote radio frequency isolating transformer being connected
to said second conductor of said conducting cable; said first
terminal of said secondary winding of said remote radio frequency
isolating transformer being connected to said input terminal of
said radio frequency signal measuring device, said second terminal
of said radio frequency signal measuring device being connected to
said second terminal of said secondary winding of said remote radio
frequency isolating transformer.
35. Electrical apparatus of the kind which experiences partial
discharge in the high-voltage insulation thereof, said partial
discharge activity causing the existence of a radio frequency
component, there being a separate power frequency component, said
electrical apparatus including a capacitance tap associated with
the capacitance in said insulation, comprising: a sensor for the
on-line monitoring of the state of the high-voltage insulation in
said electrical apparatus, comprising: a surge arrester with first
and second ends said surge arrester second end being connected to
said local ground; a capacitor shunt with first and second ends
said capacitor shunt second end being connected to said local
ground; a conducting cable having a signal conductor and a second
conductor, each said signal conductor and second conductor having a
local end and a remote end, said local end of said second conductor
of said conducting cable being connected to said local ground; a
radio frequency current transformer with a primary windinging
having a polarity terminal and a non-polarity terminal and a
secondary winding having a polarity terminal and a non-polarity
terminal; said polarity terminal of said primary winding being
connected to said capacitance tap; said non-polarity terminal of
said primary winding being connected to said capacitor shunt first
end; said polarity winding of said secondary winding being
connected to said signal conductor; said first end of said surge
arrester being connected to said first end of said capacitor shunt;
and said non-polarity terminal of said primary winding and said
non-polarity terminal of said secondary winding being connected
together and to said first terminal of said capacitor shunt and
said first terminal of said surge arrester.
36. Electrical apparatus of the kind which experiences partial
discharge in the high-voltage insulation thereof, said partial
discharge activity causing the existence of a radio frequency
component, there being a separate power frequency component, said
radio frequency component and said power frequency component
generating a local radio frequency signal and power frequency
signal, respectively, in a local sensor for transmittal to a remote
input circuit for a radio frequency measuring device with input
terminal and second terminal and a power frequency signal measuring
device with input terminal and ground terminal for the on-line
monitoring of the state of the high-voltage insulation in said
electrical apparatus experiencing said partial discharge activity,
comprising: a conducting cable having a signal conductor and a
second conductor, each said signal conductor and second conductor
having a local end and a remote end, said local end of said second
conductor of said conducting cable being connected to said local
ground and having a signal representative of said power frequency
component and a signal representative of said radio frequency
component concurrently disposed thereupon; said remote end of said
second conductor being ungrounded; a remote surge arrester having a
first end and a second end, said first en of said remote surge
arrester being connected to said remote end of said signal
conductor, said second end of said remote surge arrester being
connected to said remote ground; a remote inductance having a first
end and a second end, said first end of said remote inductance
being connected to said remote end of said signal conductor, said
second end of said remote inductance being connected to said input
terminal of said power frequency signal measuring device, said
ground terminal of said power frequency signal measuring device
being connected to remote ground; a remote capacitance having a
first end and a second end; a remote radio frequency isolating
transformer with a primary winding having a first terminal and a
second terminal and a secondary winding having a first terminal and
a second terminal; said second end of said remote capacitance being
connected to said first end of said primary winding of said remote
radio frequency isolating transformer; said first end of said
remote capacitance being connected to said remote end of said
signal conductor of said conducting cable and said second end of
said primary winding of said remote radio frequency isolating
transformer being connected to said second conductor of said
conducting cable; and said first terminal of said secondary winding
of said remote radio frequency isolating transformer being
connected to said input terminal of said radio frequency signal
measuring device, said second terminal of said radio frequency
signal measuring device being connected to said second terminal of
said secondary winding of said remote radio frequency isolating
transformer.
37. A method for detecting and monitoring partial discharge in
electrical apparatus of the kind which experiences partial
discharge in the high-voltage insulation thereof, said partial
discharge activity causing the existence of a radio frequency
component, there being a separate power frequency component also,
comprising the steps of: locally sensing said radio frequency
component; locally sensing said power frequency component;
providing both said radio frequency component and said power
frequency component to a single conductor for simultaneous
transmittal to a remote location.
38. The method of claim 37 comprising the steps of: transmitting
both said radio frequency component and said power frequency
component on said single conductor to said remote location;
extracting said power frequency signal from said cable at the
remote end thereof; providing said extracted power frequency signal
to a power frequency system for monitoring said power frequency
signal; extracting said radio frequency signal from said cable at
the remote end thereof; and providing said extracted radio
frequency signal to a radio frequency system for monitoring said
radio frequency signal.
39. A sensor for an on-line monitoring of the state of the
high-voltage insulation in electrical equipment/apparatus equipped
with a capacitance tap, whereas the sensor is connected between the
output of the said tap and the local ground, whereas the
measurements are performed simultaneously at the power frequency
and the radio frequencies associated with partial discharge
activity, and whereas the measured signals are transmitted to a
remote location via connecting circuits (cables), containing: a
surge arrester (varistor) and a capacitor shunt, both being
connected to the local ground with their second terminals; a radio
frequency current transformer, the polarity terminal of its primary
winding being connected to the output of the capacitance tap, the
non-polarity terminal of its primary winding being connected to the
first terminal of the said capacitor shunt, and the polarity
terminal of its secondary winding being connected to the signal
conductor of the connecting circuit (cable); a connecting circuit
(cable), with its second conductor (shield) connected to the same
local ground; wherein the improvement comprises of: the first
terminal of the surge arrester (varistor) is joined together with
the first terminal of the capacitor shunt; the non-polarity
terminals of both the primary and the secondary windings of the
said radio frequency current transformer are joined together and
connected to the first terminals of the surge arrester (varistor)
and the capacitor shunt.
40. The input circuit for the connection of measuring devices to
the remote end of the connecting cirlcuit (cable) originated at the
sensor as claim 39, comprising of two separate measuring circuits,
one for the power frequency signal and the other for the radio
frequency signal, both connected to the signal conductor of the
connecting circuit (cable); wherein the improvement comprises of:
the second conductor (shield) of the connecting circuit (cable) is
ungrounded at its remote end; an additional surge arrester
(varistor) is connected between the signal conductor of the said
connecting circuit (cable) and the local ground; an additional
small inductance (choke) is connected between the signal conductor
of the connecting circuit (cable) and the output to a power
frequency measuring device, whereas the second terminal of the
output eing connected to the local ground; an additional small
capacitor in series with the primary winding of an additional radio
frequency isolating transformer are connected between the signal
conductor and the second conductor (shield) of the said connecting
circuit (cable), whereas the secondary winding of the said
isolating transformer serves as an output to a radio frequency
measuring device.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] This invention relates to sensors used for on-line
monitoring of the state (condition) of high voltage insulation in
electrical equipment with capacitance (potential) taps and the
interconnection therewith to remote measuring devices. The
electrical equipment may include bushings of power transformers,
shunt reactors or circuit breakers and current transformers.
[0003] 2. Description of Prior Art
[0004] The On-line monitoring of high-voltage insulation of
electrical equipment is performed on the equipment under operation,
i.e. in the actual operating condition. Equipment de-energization
is required only for the initial sensor installation. As
increasingly reliable and cost-and labor-effective, this technology
is now widespread in numerous applications. Particularly, such
monitoring provided concurrently on power frequency and radio
frequencies may be an effective tool in prediction and prevention
of in-service failures for high-voltage bushings and other
equipment with capacitance (potential) taps.
[0005] Attention is called to the following Publications:
[0006] "Methods and Tools for High-Voltage Equipment Diagnostics",
Energoatomizdat Publishing House, Moscow, by P. Svy 1992.
[0007] "Experience in the Application of the On-Line Monitoring
System Using Power Frequency and Partial Discharges to High Voltage
Transformer and Bushing Insulation", by Z. Berler, L. Letitskaya
and P. Svy, EPRI Substation Equipment Diagnostic Conference VI,
Feb. 16-18, 1998, New Orleans, La.
[0008] Bushings of power transformers, shunt reactors or circuit
breakers and current transformers, with their internal insulation
of oil-impregnated paper similar to that used in cables or
capacitors, are equipped with so called capacitance or potential
taps. A capacitance tap is connected to a metal foil shield
inserted inside the insulation. The insulation has certain
capacitance and conductance between the high voltage
current-carrying conductor and the foil shield. Both the
capacitance value and the power factor of the insulation depend
upon the insulation condition and could be quantified at the tap
output with the equipment on-line. Furthermore, the electrical
impulses that accompany partial discharges inside the insulation
are also coupled to the output of the capacitance tap and can be
detected using circuits of a suitable design.
[0009] The capacitance taps were originally designed only for
relatively rare off-line insulation tests using a suitable test
source at power frequency. During equipment operation they remained
grounded. It was recognized that these taps lend themselves as
excellent means of on-line monitoring of the insulation. The use of
the capacitance tap for an on-line monitor requires a sensing
device to be inserted permanently between the live tap contact and
the ground. The aforementioned publications teach such an
arrangement.
[0010] The sensor designed for the power frequency measurement
produces a signal proportional to the capacitive current through
the bushing insulation. The sensor designed for partial discharges
senses the radio frequency impulses and produces a signal of
magnitude proportional to the dissipated electrical charges. The
repetition rate of such discharges can be determined by a measuring
device.
[0011] Sensors based on application of current transformers are
described in U.S. Pat. No. 5,471,144 "System for Monitoring the
Insulation Quality of Step Graded Insulated High Voltage Apparatus"
issued Nov. 29, 1995; U.S. Pat. No. 5,574,378 "Insulation
Monitoring System for Insulated High Voltage Apparatus" issued Nov.
12, 1996; U.S. Pat. No. 5,640,154 "Insulation Monitoring System for
Insulated High Voltage Apparatus" issued Jun. 17, 1997; and U.S.
Pat. No. 5,652,521 "Insulation Monitoring System for Insulated High
Voltage Apparatus" issued Jul. 29, 1997 and in the Svy reference,
P. 107. They consist of a current transformer with a primary
winding created by the capacitance tap grounding conductor, and a
secondary toroidal winding consisting of several or many turns.
This current transformer can be coreless (so-called Rogovsky coil),
as suggested in the above mentioned patents for power frequency
measurements, or with a ferrite core, as recommended in the Svy
Reference for the radio frequency impulse measurements. The
advantage of the current transformer-based sensor is its
simplicity. A current transformer with its secondary winding loaded
with a small resistance has small input impedance, so there is
usually no need for a special tap overvoltage protection.
[0012] Monitoring of radio frequency (partial discharge) impulses
imposes different requirements on sensor design, as opposed to
monitoring of signals at power frequency. For partial discharge
monitoring it is desirable to detect a frequency band generally
between 0.5 and 20 MHz with high sensitivity. Ferrite radio
frequency transformers with a small number of turns in the
secondary winding are appropriate for this task as they are capable
of accurately transmitting short and steep pulses, but they block
power frequency signal. A coreless current transformer with a large
number of turns in the secondary winding can be employed for power
frequency measurement, but it is practically insensitive to weak
partial discharge pulses. To meet both requirements, two separate
transformers, one of each type, are necessary.
[0013] A coreless Rogovsky coil has a low sensitivity even at the
power frequency signals. For this reason it was replaced with a
resistor shunt connected between the output of the tap and local
ground (Russian Patent 292,062, published Feb. 12, 1971). The
measured quantity, a power frequency voltage drop across the
resistor shunt, is directly proportional to the capacitive current
through the bushing insulation. The magnitude of the power
frequency signal can be conveniently controlled by the resistance
chosen for the shunt. The disadvantage of such an arrangement is
that the tap capacitance, between the high voltage line and the
output of the capacitance tap, in series with the resistance of the
sensor shunt represents a frequency dependent voltage divider. As a
result, switching and lightning transients can cause severe
overvoltages at the output of the tap due to their very high
frequencies. These transients have the potential of destroying not
only the measuring circuit, but also the insulation of the tap
output or even the bushing. To limit the transients, a surge
arrestor is added in parallel to the resistor shunt, as shown in
the Svy Reference, on its FIG. 8.2.
[0014] A further improvement of the sensor consisted of replacing
the resistor shunt with another capacitor, see U.S. Pat. No.
4,757,263 "Insulation Power Factor Alarm Monitor" issued Jul. 12,
1988; U.S. Pat. No. 5,903,158 "Monitoring of Internal Partial
Discharges in a Power Transformer" issued May 11, 1999; and U.S.
Pat. No. 6,028,430 "Method for Monitoring a Capacitor Bushing, and
Monitoring System" issued Feb. 22, 2000. This arrangement features
a capacitor divider ratio that is essentially independent of
frequency, thus minimizing the exposure of the tap and the low
voltage circuits to destructive switching and lightning impulses. A
surge arrester is kept in place as a "second line of defense" for
rare cases of extremely severe overvoltages.
[0015] All of the sensor designs described above are mutually
exclusive in that they can satisfy only one application at a time;
a power frequency signal detection or a partial discharge
detection, but not both. With only one capacitance tap available
per bushing, this represented a serious disadvantage as the
replacement of a bushing sensor requires outage.
[0016] A Publication entitled "On-Line Monitoring of Power
Transformer-Trends, New Developments and First Experiences" by T.
Leibfried, W. Knorr, K. Viereck, CIGRE, 1998, #12-211, teaches a
sensor that can contain both circuits. The sensor relies on the
capacitor shunt connected to the tap output and the radio frequency
current transformer the primary winding of which is connected in
series with the capacitor shunt, either on its grounded side or on
its "live" side. Two separate coaxial cables carry power frequency
and radio frequency signal signals respectively. Similar sensors
were used by Cutler-Hammer starting in 1996.
[0017] These sensors have disadvantages. It is the necessity to use
two cables to carry the information extracted from a sensor.
Another disadvantage is that, compared with the sensor that
utilizes a current transformer only, this sensor has lower
sensitivity to partial discharge impulses: on high frequencies the
stray capacitance of the surge arrester shunts the circuit of
series connected radio frequency current transformer and capacitor
shunt, thus diverting part of high frequency current from entering
into current transformer.
SUMMARY OF THE INVENTION
[0018] In accordance with the invention, a partial discharge
determination system for an electrical system which includes: a
conductor at a given voltage potential, electrical insulation
disposed proximate the conductor and an insulator capacitance
disposed in the insulation which conducts partial discharge
electrical current is taught. The partial discharge electrical
component may be random and occasional. It has a radio frequency
impulse associated therewith. A steady state generally continuous
power frequency current component may flow in parallel at the same
time. There is a sensor capacitor shunt, the primary winding of the
sensor current transformer is connected electrically to the
insulator capacitance to conduct partial discharge electrical
current there through. There is also a coaxial cable, the coaxial
cable is connected to the sensor current transformer primary
winding. There is also present a monitoring power frequency
capacitor. A monitoring radio frequency isolation transformer is
also present. A monitoring choke coil is present. The monitoring
choke coil and the monitoring power frequency capacitor are
interconnected electrically with the coaxial conductor. There is a
monitoring surge arrester. The monitoring choke coil, the
monitoring surge arrester and the monitoring power frequency
capacitor are interconnected electrically with the coaxial
conductor. A first signal representative of an electrical power
frequency current component exists between the monitoring choke
coil and the monitoring surge arrester. A second signal
representative of the radio frequency current associated with
partial discharge component exists between the monitoring radio
frequency isolation transformer and the monitoring radio frequency
isolation transformer secondary winding second end.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] For a better understanding of the invention reference may be
had to the preferred embodiment thereof shown in the accompanying
drawings in which:
[0020] FIG. 1 depicts side elevation, partially in section of a
switchgear circuit breaker cell utilizing concepts of the present
invention;
[0021] FIG. 2 depicts a side elevation, partially broken away, of a
circuit breaker connection spout of a kind shown in FIG. 1;
[0022] FIG. 3 depicts a front view of the spout of FIG. 2;
[0023] FIG. 4 depicts a rear view of three of the spouts shown in
FIGS. 1 and 2 disposed in a structure for monitoring partial
discharge;
[0024] FIG. 5 depicts a side view of a spout similar to that shown
in FIG. 2 but with shield access conductors present;
[0025] FIG. 6 depicts a front view of a spout of FIG. 5 similar to
that shown in FIG. 3;
[0026] FIG. 7 depicts a rear view similar to that shown in FIG. 4
of three of the spouts of FIG. 5 disposed in an arrangement for
partial discharge monitoring;
[0027] FIG. 8 is a side elevation, in section showing a line
conductor insulator for the switchgear of FIG. 1;
[0028] FIG. 9 shows a side elevation, partially cut away, of a line
conductor insulator similar to that shown in FIG. 8 but with an
offset;
[0029] FIG. 10 shows an arrangement for the line conductors of FIG.
8 and FIG. 9 including partial discharge monitoring structure;
[0030] FIG. 11 is a schematic, mechanical diagram of a line
conductor wiring arrangement similar to that shown in FIG. 1;
[0031] FIG. 12 shows an alternate arrangement for the line
conductor terminals depicted in FIG. 11;
[0032] FIG. 13 shows an elevation, cut away and in section of an
alternative switchgear utilizing current transformer bottles;
[0033] FIG. 14 shows a side elevation of a current transformer
bottle depicted in FIG. 14;
[0034] FIG. 15 shows a front view of the current transformer bottle
of FIG. 14;
[0035] FIG. 16 shows a rear view of the current transformer bottle
of FIG. 14;
[0036] FIG. 17 shows an arrangement of current transformer bottles
of the type shown in FIG. 14 disposed in a structure for partial
discharge sensing;
[0037] FIG. 18 shows a stand-off insulator similar to that depicted
in FIG. 1 in elevation and in section depicts internal partial
discharge sensor elements; FIG. 19 shows an orthogonal view,
partially broken away of a horizontal circuit breaker utilizing the
teachings of the current invention;
[0038] FIG. 20 shows an orthogonal view of a transformer utilizing
a bushing, which maybe utilized for partial discharged sensing;
[0039] FIG. 21 shows a prior art schematic diagram of a partial
discharge sensor and monitor;
[0040] FIG. 22 shows another embodiment of a prior art partial
discharge sensor and monitor;
[0041] FIG. 23 shows still another embodiment of a prior art
partial discharge sensor and monitor;
[0042] FIG. 24 shows still another embodiment of a prior art
partial discharge sensor and monitor;
[0043] FIG. 25 shows still another embodiment of a prior art
partial discharge sensor and monitor;
[0044] FIG. 26 shows a partial discharge sensor, monitor and
measuring currents embodying the teachings of the present
invention; and
[0045] FIG. 27 depicts an orthogonal view, partially broken away of
a gas insulator conductor system;
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0046] Referring now to FIG. 1 there is depicted a switchgear
system or apparatus cabinet or side entry switchgear 10 with side
entry line buses. There is disposed within the side entry
switchgear 10 a front compartment or circuit breaker compartment 12
shown on the left, and a central lower or load compartment 14 shown
in the lower center. Shown to the right is a rear compartment or
cable compartment 16. Movably disposed within the circuit breaker
compartment 12 is a medium voltage circuit breaker 23. Circuit
breaker 23 includes a line side circuit breaker terminal bushing 24
and a load side circuit breaker terminal bushing 26. The latter two
bushings, in this embodiment of the invention, are disposed one
above the other and protrude into the load compartment 14. Load
side terminal bushing 26 is interconnected with a load terminal
current transformer spout bushing 32. There is provided, a load
terminal current transformer 36 (CT) which is disposed annularly
around the load spout bushing 32. The load spout bushing 32
insulatingly surrounds a current transformer spout bushing
conductor 60. Bushing conductor 60 is interconnected electrically
with a load bus 38. Load bus 38 may be supported within the cable
compartment 16 by way of an inventive insulator with partial
discharge sensor 40. The function and operation of stand-off
insulator 40 will be described in greater detail hereinafter. There
is also provided a central upper or line compartment 15. In line
compartment 15 may be disposed a line current transformer spout
right off-set vertical bushing 42A and a line current transformer
spout left off-set vertical bushing 42B. There may be also provided
a line current transformer spout non-offset vertical bushing 44.
The interconnections of the immediately aforementioned bushings
with the various line side terminal bushings 24 of the circuit
breaker 23 will be described hereinafter in greater detail with
respect to FIG. 11. Each of the vertical bushings 42A, 42B and 42C
may have identical line current transformer spout upper vertical
bushing portions 45 as is also shown in FIGS. 8 and 9, for example.
Conductors in each of the upper vertical bushing portions 45 may be
interconnected at right angles with line bus conductors 46 as is
best shown schematically in FIG. 11. A line spout insulating
support plate 48 may be utilized to space and support the line bus
conductors 46. There may be provided line spout current
transformers 50 annularly disposed around each of the line current
transformer spout upper vertical bushing portions 45.
[0047] Referring now to FIGS. 2-4 as well as FIG. 1, further
description of the load spout bushing 32 is set forth. Load spout
bushing 32 includes an axially aligned load terminal bushing wider
cylinder 54 axially interconnected with a hollow load terminal
bushing narrower cylinder 56. These may be alternatively referred
to as the wider cylinder 54 and narrower cylinder 56 respectively.
The latter two cylinders insulatingly encircle a bushing conductor
60 which terminates at the external end thereof with a load
terminal current transformer spout bushing conductor to load bus
connector 58. Conductor 60 terminates at the internal end thereof
with a current transformer spout-connecting stub 62. There is
provided radially internally of the wider cylinder 54, a load
terminal current transformer bushing spout voltage stress shield 64
which circumferentially surrounds the bushing conductor 60 in
common axial alignment between the outer cylindrical surface of the
conductor 60 and the cylindrical surface of the wider cylinder 54.
There is provided axially adjacent the rear of wider cylinder 54 a
load terminal current transformer bushing spout connecting stub
support 65. It is through the central axis of the stub support 65
that the connecting stub 62 protrudes for interconnection with the
load side terminal bushing 26. There are disposed at the ends of
the line side terminal bushing 24 and load side terminal bushing
26, circuit breaker load terminal connector arrays 66. It is these
arrays which interconnect with the connecting stub 62 in the load
spout bushing 32. There is provided a load terminal current
transformer spout voltage stress shield lead wire 67 which
interconnects the stress shield 64 with a stress shield-to-aluminum
support piece connecting bolt 75. The latter connecting bolt is
disposed in the load terminal current transformer spout support
piece flange 77. Part of the load spout bushing 32 includes a
hollow cylindrical load terminal current transformer spout shell
wall 68. It is axially into this hollow region formed by the shell
wall 68 that the aforementioned connecting stub 62 protrudes for
interconnection with the breaker terminal connector array 66. The
shell wall 68 terminates on one end at the stub support 65 and at
the other end in a load terminal current transformer spout mounting
flange 69.
[0048] Referring now more particularly to FIG. 4 as well as FIGS.
1-3, an arrangement for disposing load spout bushings 32 in a
three-phase electrical disposition within the side entry switchgear
10 is depicted. In particular right load terminal current
transformer spout bushing 32A, central load terminal current
transformer spout bushing 32B, and left load terminal current
transformer load spout bushing 32C are arranged side by side in an
aluminum support piece 74. In particular right load terminal spout
support piece flange 77A, central load terminal spout support piece
flange 77B, and left load terminal spout support piece flange 77C
are secured against the aluminum support piece 74 by way of a
stress shield-to-aluminum support piece connecting bolt 75.
Consequently, it can be seen that there is electrical continuity
for each spout bushing 32A, 32B, and 32C through its spout stress
shield lead wire 67 from the stress shield 64 to the aluminum piece
74 by way of the connecting bolts 75. Aluminum support piece 74 is
affixed to right support piece-to-side wall spacer 76R on one side
and left support piece-to-side wall spacer 76L on the other side by
way of support piece-to-side wall spacer fasteners 80. In turn the
right side wall spacer 76R and left side wall spacer 76L are
affixed, respectively, to switchgear cabinet right side vertical
wall 70R and left side vertical wall 70L by way of appropriate side
wall spacer-to-switchgear cabinet side wall vertical wall fasteners
82. In the depiction of FIG. 4 the three thusly aligned and
arranged spout bushings 32A, 32B and 32C are firmly disposed in
load compartment 14 between the aforementioned left vertical wall
70L and right vertical wall 70R beneath the horizontal separating
shelf 72 between the central lower compartment and central upper
compartment. In this embodiment of the invention or arrangement, it
is to be noted that an intrinsic conductor-to-ground capacitance C1
exists between the bushing conductor 60 and the stress shield 64.
This capacitance is coupled by way of an electrical connection from
stress shield 64 through spout stress shield lead wire 67,
connecting bolt 75, aluminum piece 74, partial discharge sensor
lead wire and current transformer primary winding 90 to ground G.
The partial discharge sensor transformer secondary winding 92 is
shown disposed in electromagnetic relationship with the primary
winding 90. The function of this arrangement will be described
hereinafter.
[0049] Referring now to FIGS. 5-7 as well as FIG. 1 another
embodiment of the invention or arrangement is shown. In this
embodiment of the invention or arrangement a load terminal CT spout
bushing with stress shield lead wire 32' which is very similar to
load spout bushing 32 is depicted. The embodiment or arrangement of
FIGS. 5-7 is similar to the embodiment or arrangement of FIGS. 2-4,
respectively, except for the interconnection between the shield 64
and the region external to the spouts 32 or 32' as the case may be.
With regard to FIGS. 5-7 those features of the embodiment which are
the same as the features depicted in the embodiment represented in
FIGS. 2-4 are identified by similar reference characters. With
respect to the embodiment or arrangement of FIGS. 5-7 there is
provided a bushing hole 71 for load terminal CT spout bushing
voltage stress shield lead wire radially disposed in the wider
cylinder 74 to provide a clear path between the shield 64 and the
external surface of the wider cylinder 54. An electrical conductor
84 or a stress shield lead wire for load terminal CT spout bushing
is electrically affixed to the shield 64 and fed through the
opening 71 and interconnected externally of the right spout bushing
with lead wire 32A', central spout bushing with lead wire 32B', and
left spout bushing with lead wire 32C' and then transformer primary
winding 90 and ground G. The electrical current provided therein is
monitored by the secondary winding 92. Consequently, it can be seen
that the three aligned spout bushings 32A', 32B', and 32C' have
right bushing hole 71A, central bushing hole 71B and left bushing
hole 71C, respectively, through which the various interconnecting
wires 84 feed to the various shields 64.
[0050] Referring now to FIGS. 8-10 as well as FIG. 1 an arrangement
for the line terminals for a switchgear cabinet apparatus or system
10 with top entry line buses is depicted. In this embodiment of the
invention or arrangement the switchgear arrangement maybe similar
to that shown in FIG. 1, except that the electric line terminals
resident in line compartment exit the switchgear from above rather
than from the side as depicted in FIG. 1. In this embodiment of the
invention or arrangement like reference characters represent like
elements of the embodiments or arrangement. In this embodiment of
the invention or arrangement there are provided two kinds of line
vertical bushing breaker terminals. There is provided the
non-offset vertical bushing 44 shown in FIG. 8 and the left offset
vertical bushing 42B shown in FIG. 9. Left offset vertical bushing
42B shown in FIG. 9 may be reversed to form a right offset vertical
bushing 42A as will be described with respect to FIG. 10.
Non-offset vertical bushing 44 includes an upper vertical bushing
portion 45 and a line current transformer spout non-offset vertical
bushing main conductor 52 traverses through the axial center of the
non-offset vertical bushing 44. The main conductor 52 protrudes
outwardly from the bottom of the bushing casing to form a line
current transformer spout non-offset vertical bushing breaker
terminal 86 and protrudes outwardly upwardly from the top of the
bushing casing to form a line current transformer spout non-offset
vertical bushing line terminal 88. The upper vertical bushing
portion 45 has disposed therein, circumferentially surrounding the
axial line CT spout non-offset vertical bushing main conductor 52,
a line transformer spout stress shield 95 which is interconnected
electrically to a line contact spout stress shield external
terminal 93. Intrinsic capacitance C1 exists between the shield 95
and the line current transformer spout non-offset vertical bushing
main conductor 52.
[0051] Referring to FIG. 9, a left offset vertical bushing 42B
similar to non-offset vertical bushing 44 is depicted. In this
embodiment of the invention or arrangement, similar reference
characters represent identical or similar portions of the two
bushings 42B and 44. The upper vertical bushing portion 45 of the
left off-set vertical bushing 42B is identical to the upper
vertical bushing portion 45 of the non-offset vertical bushing 44
of FIG. 8. The similarity is existent from the CT line spout
non-offset line terminal 88 in FIG. 8 and the CT line spout offset
line terminal 88A in FIG. 9 downwardly to the stress shield
external terminals 93. From there downward the arrangements vary.
In the embodiment of FIG. 9 the left offset line main conductor 87B
has two angled bends therein thus offsetting the line CT spout
offset line terminal 88A from the left offset line breaker terminal
86B.
[0052] Referring, now to FIG. 10, the line compartment 15 of the
top entry switchgear 10' with the various line terminals in place
is depicted. In the center is disposed the non-offset vertical
bushing 44. Disposed above and the left thereof as viewed in FIG.
10 is the left offset vertical bushing 42B and disposed to the
right and lower thereof as viewed in FIG. 10 is the right offset
vertical bushing 42A. Disposed on the left as shown in FIG. 10 is
the vertical separating panel 73 between the circuit breaker
compartment and the central lower and upper compartments. Panel 73
is attached at one end to the central upper compartment left side
wall 100 and at the other end to the central upper compartment
right side wall 102, these side walls 100 and 102, extend at right
angles from the vertical separating panels 73. Each of the vertical
bushings 42A, 42B and 44 are interconnected by way of line current
transformer spout stress shield external terminal connecting bars
96 which are interconnected electrically with the stress shield
electrical terminals 93 in each case. The right offset vertical
bushing 42A is interconnected with right side wall 102 by way of a
right side wall to flange insulating spacer 94R. The left offset
vertical bushing 42B is interconnected with left side wall 100 by
way of a left side wall-to-flange insulating spacer 94L. Therefore,
it can be seen that there is electrical continuity between all of
the line spout stress shields 95 by way of the terminals 93 and the
external terminal connecting bars 96. The transformer primary
winding 90 is interconnected with one of the common terminals 93
and ground G. The secondary winding 92 is disposed in proper
electromagnetic relationship with the primary winding 90 as was
discussed previously.
[0053] Referring now to FIGS. 11-12 as well as FIG. 1, the
mechanical schematic arrangement of two possible line terminal
layouts are depicted. FIG. 11 schematically represents the
embodiment set forth in more detail in FIG. 1 shown in the side
entry switchgear 10. FIG. 12 schematically reflects the top entry
switchgear embodiment 10'. In both FIGS. 11-12, as well as FIG. 1,
like reference characters represent like features. In each case, on
the left the vertical-separating panel 73 is shown. Beneath that in
each case is the horizontal-separating shelf 72. The closest side
represents the right side wall 102 and the furthest side represents
the left side wall 110. Together these planes form part of a rough
cube corresponding generally schematically to the line compartment
15. In each case, schematically represented right side terminal
bushing 24A, central terminal bushing 24B and left side terminal
bushing 24C are shown exiting the vertical separating panel 73 into
the volume of the line compartment 15. In the embodiment or
arrangement of FIG. 11, the non-offset vertical bushing 44 is shown
rising vertically from the end of central terminal bushing 24B,
right offset terminal bushing 42A is shown rising vertically, and
offset somewhat to the front from left terminal bushing 24C, and
right offset vertical bushing 42A is shown rising vertically, and
somewhat to the rear from the right terminal bushing 24A.
Interconnected with left offset vertical bushing and extending
outwardly therefrom is left line bus 46C. Interconnected with
non-offset vertical bushing 44 and extending outward therefrom and
in the same direction is central line bus 46B. Extending from right
offset vertical bushing 42A is right line bus 46A. Line buses 46A,
46B and 46C extend outwardly through right side wall 102 as
depicted at 88 for example in FIG. 1. With respect to the
embodiment or arrangement of FIG. 12, three identical non-offset
vertical bushings 44 may extend upwardly at right angles from the
right side terminal bushing 24A, central terminal bushing 24B and
left side terminal bushing 24C, respectively, to the line at
terminals across the diagonal of the top part of the cube
representing line compartment 15. Either one embodiment or
arrangement or the other may be favored depending upon the needs of
the user of the switchgear equipment. Both embodiments or
arrangements are amenable to being utilized in the partial
discharge diagnostic techniques to be described hereinafter.
[0054] Referring now to FIG. 13, still another embodiment of the
invention is depicted. FIG. 13 shows a side elevation, partially
broken away, of short switchgear cabinet apparatus or system with
rear entry line buses 10". Switchgear 10" includes a short
switchgear cabinet front compartment 12' in which is disposed the
circuit breaker 23 as was described previously. As was described
previously, circuit breaker 23 has a line side terminal bushing 24
and a load side terminal bushing 26 located one above the other.
These bushings protrude through a vertical separating panel 73'
between the circuit breaker compartment 12' and the line and load
terminal regions 15' and 14' respectively of the short switchgear
cabinet. Mounted on the short switchgear vertical separating panel
73' are current transformer terminal bottles 112. The upper one
represents the line terminal bottle and the lower one represents
the load terminal bottle. Terminal bottles 112 are described
hereinafter in greater detail with respect to FIGS. 15-16. Sufficed
to say at this time that circuit breaker line or load terminal
arrays 66 interconnect in a convenient manner with current
transformer terminal bottle load terminal 114 and current
transformer terminal bottle line terminal 116 in a manner to be
described hereinafter. The region to the right of the short
switchgear vertical separating panel 73' within the switchgear
cabinet 10" is the line terminal region 15' at the top and the load
terminal region 14' at the bottom. There may be disposed in the
terminal region 14' the transformer primary winding 90 as
interconnected with Ground G and as electromagnetically interacted
with secondary winding 92 to assist in a partial discharge
diagnosis in a manner which will be described hereinafter.
[0055] Referring now to FIGS.14-17 as well as FIG. 13 the latter
embodiment of the invention or arrangement will be described in
greater detail. In particular, the terminal bottle 112 has a bottle
load or line terminal 114 or 116 as the case may be. It is to be
understood that an identical bottle may be used for either load
terminal or line terminal operation. There is a central conductor
120 which has on the right the load or line terminal 114 or 116 and
on the left the current transformer terminal bottle circuit breaker
connecting stub 122. Disposed in the face of the bottle load
terminal 114 or bottle line terminal 116 are convenient current
transformer bottle bus connector threaded holes 121. There is
provided at the end right of the bottle, a bottle bus connector lip
123. There is also provided intermediate the main body of the
terminal bottle 112 a terminal bottle flange 124. Forward of that
and circumfrentially disposed around the conductor 120 are terminal
bottle insulator rings 125 which provide the normal high voltage
insulating function. There is provided on the main body, a body
outer voltage stress shield 126 on the outer surface thereof and a
body intervoltage stress shield 127 on the inner surface of the
shell-inside wall 128. The breaker terminal connector arrays 66
circumfrentially attaches itself to the bottle connecting stub 122
when the circuit breaker 23 is completely operationally inserted
into short switchgear cabinet 10". This provides a circuit breaker
connection between the line terminal 112 and load terminal 114.
There are provided in the bottle flange 124, bottle outer voltage
stress shield flange terminals 130 which are electrically
interconnected with the bottle outer stress shield 126. In this
embodiment of the invention or arrangement the intrinsic
capacitance C1 exists between the bottle inner stress shield 127
(and the bottle central conductor 120 which is electrically
connected thereto) and the outer bottle stress shield 126. In the
embodiment or arrangement shown in FIG. 17, the tiered arrangement
of three sets of bottle load terminals and bottle line terminals
are shown. There is depicted on the left, the short switchgear
right wall 140 and on the right the short switchgear left wall 138.
On the bottom aligned in a tier are the right bottle load terminal
114A, the central bottle load terminal 114B and the left bottle
load terminal 114C. Aligned on the top in a similar manner are the
right bottle line terminal 116A, the central line bottle terminal
bottle 116B and the left line bottle terminal 116C. These are all
disposed in a short switchgear insulating support 136. On the top,
the terminals 130 are electrically joined together by a connector
strap 142 for the flange terminals. The flange connector strap 142
is interconnected with the transformer primary 90 and thence to
Ground G as was the case previously. The transformer secondary
winding 92 is electromagnetically interconnected with transformer
primary winding 90.
[0056] Referring now to FIG. 18 a standoff insulator 40 such as the
one shown in FIG. 1 is depicted again. In particular, standoff
insulator 40 may comprise molded epoxy insulating material 159 in
which are moldingly disposed concentric shells. Resins or other
suitable molding materials may be used for the insulating material
159. There may be a larger high voltage outer shell 174
concentrically disposed about a higher voltage inner shell 176 both
of which are axially aligned with and concentrically attached to a
conductor to bus support member 168. The concentric arrangement is
not limiting. Conductor to bus support member 168 may have axially
disposed therein a threaded opening 169. Threaded opening 169
communicates or connects with an external portion of the insulating
material 159 at the bottom of standoff insulator 40 as viewed in
FIG. 18. Axially aligned with the latter arrangement and coaxially
interleaved therewith may be a smaller grounded inner shell 178 and
a larger grounded outer shell 180 both of which are joined at the
root to an axial conductor to frame support member 164. Conductor
to frame support member 164 or grounded support member as the case
may be may include therein a threaded opening 166. Conductor to
frame support member 164 communicates within an external portion of
the insulating material 159 at the top of the standoff insulator 40
as shown in FIG. 18. As shown in FIG. 1 and depicted again in
broken off section in FIG. 18 there may be provided a conductor bus
standoff insulator support member 162 which supports the standoff
insulator 40 by way of a support base to insulator support member
fastener 172 which captures support member 162 and impresses it
against the top of the insulator 40 as member 172 is threaded into
the threaded opening 166. In a like manner, on the bottom of
insulator 40, load bus 38 is captured between the head of a
conductor bus to conductor bus support fastener 170 and the bottom
surface of the standoff insulator 40 as the threaded member 170 is
threaded into threaded openings 169. In this embodiment of the
invention therefore the standoff insulator 40 spaces the insulator
supports base 162 from the load bus 38 and supports the load bus
38. In addition, in the present embodiment of the invention the
insulator member 40 may perform another function and that is to act
as a partial discharge sensor member. In particular, conductor to
frame support member 164 acts as the transformer primary winding 90
in a manner which was described previously. This member is
electrically interconnected with the support member 162 which in
turn is Grounded at G. Surrounding the transformer primary winding
90 is the transformer secondary winding 92. Electromagnetic
interaction between the primary 90 and the secondary 92 is enhanced
by the presence of a ferrite current transformer core 182. The
secondary winding wires 92A and 92B, also depicted in FIG. 26, as
will be described hereinafter exit the epoxy insulating material
159 through a grommet 190 which may feed through the support member
162. By observation of the arrangement of the interleaved
concentric plates in the insulator 40, it can be seen that at least
three regions of intrinsic capacitance exists. One region of
intrinsic capacitance C1 exists between plate 178 and plate 174
another region of intrinsic conductor-to-ground capacitance C1A
exists between the two outer shells 174 and 180 and still another
intrinsic capacitor-to-ground capacitance C1B exists between the
inner shells 178 and 176. All of this capacitance combines
electrically to form the total capacitance that is necessary and
required to perform the partial discharge sensing and monitoring
function. It is to be understood that the foregoing arrangement is
not limited to one with a bus, a wire or cable may be used
instead.
[0057] Referring now to FIG. 19, there is shown another
arrangement. In particular, a horizontal, large air circuit breaker
system or apparatus 191 is shown which is spaced apart from a large
air circuit breaker air standoff insulator base 195 by a circuit
breaker standoff insulator with partial discharge sensor 40A.
Horizontal breaker 191 includes a circuit breaker casing 192 and a
main conductor 193 which is axially intermediate the casing 192.
There may be provided in a hollow recess in the casing 192 a set of
separable contacts 194. The intrinsic capacitance C1 in the
standoff insulator 40A between the main conductor 193 and Ground G
may feed through the transformer primary winding 90 for magnetic
interaction with the transformer secondary winding 92.
[0058] Referring now to FIG. 20 a high voltage transformer 200 is
shown which provides power to a high voltage power line 202 by way
of a high voltage transformer terminal bushing 212 and through a
high voltage current transformer 214. High voltage transformer
current transformer secondary winding leads 92A and 92B are shown
feeding schematically outwardly from the high voltage transformer
current transformer 214 to a high voltage transformer partial
discharge sensor SEN which may provide dual signals on cable 244 to
remote monitor REM in a manner to be discussed hereinafter. Lead 90
is also shown.
[0059] Referring now to FIG. 27 still another arrangement is shown
in which a gas insulator or insulated conduction section system or
apparatus 267 is shown. Section 267 comprises a generally circular
cylindrical casing 268 in which is axially disposed and aligned a
main conductor 269. Electrical insulating gas 270 insulates main
conductor 269 from the gas insulated casing 268, which may, in one
embodiment of the invention, be grounded. The insulating gas 270
may comprise sulfur hexalfluoride or a similar insulating gas. A
standoff insulator with partial discharge sensor 40B may be
provided between the main conductor 269 and the casing 268 to
support the main conductor 269 within the 268 casing. Within the
body of gas insulated standoff insulator 40B resides the intrinsic
capacitance C1 which is interconnected with the gas insulated
casing 268 from whence the primary transformer winding 90 is
interconnected with Ground G. The primary transformer winding 90 is
electromagnetically interconnected with the secondary winding 92 in
a manner described previously.
[0060] Referring now to FIG. 21, a prior art, circuit of a sensor
employing a current transformer is shown. The bushing tap 234 is
permanently grounded at the vicinity of the tap location, and the
primary winding 90 of the current transformer 91 is formed by the
grounding conductor (single-turn winding). The power frequency
signal IC or radio frequency signal RFI arrives to the tap 234
through the capacitance C1 formed by the current carrying parts of
the bushing conductor 230, the bushing shield BS and the bushing
insulation INS connected with the tap. The signal then travels to
the ground, through the primary winding 90 of the current
transformer 91. Conductor 230 represents a high voltage conductor
of the type which may be found in FIGS. 1, 13, 19, 20 or 27. The
current transformer secondary winding 92 consisting of several
turns, depending on the required sensitivity, is connected with the
connecting circuit 244 (usually a coaxial cable or twisted pair
cable) that transmits the signal to remote measuring equipment.
[0061] Referring now to FIG. 22, a prior art sensor for measuring
power frequency capacitive current through a bushing, employing a
resistor shunt 241 is shown. A capacitive tap 234 is connected
through the resistor shunt 241 and a surge arrester or varistor 240
in parallel, to the Ground G in the vicinity of the tap location.
The connecting circuit (usually a control or coaxial cable, or a
twisted pair cable) 244 carries the signal to remote measuring
equipment (not shown). The resistance R of the shunt resistor 241
is chosen to limit the power frequency voltage at the output of the
tap to a safe value in the event of accidental circuit opening,
such as a disconnecting of the measuring equipment at the remote
end of the circuit or an open-circuit fault. The surge arrester 240
limits the tap-to-ground voltage to a safe level with respect to
the tap and the measuring circuit insulation, in the event of
occurrence of switching and lightning overvoltages originated in
the primary circuits.
[0062] A prior art sensor designed to measure capacitive current
through the bushing insulation at power frequency employing a
capacitor shunt 250 is shown in FIG. 23. This circuit is identical
to the circuit of FIG. 22, except that the resistor shunt 241 is
substituted with a capacitor shunt 250 of capacitance value C2.
Thus the voltage divider C1-C2 has a ratio practically independent
of frequency. The value C2 of the capacitor 250 is chosen to limit
both the power frequency voltage and the switching and lightning
impulses to a safe level with respect to the tap and the measuring
circuit insulation. The surge arrester 240 is installed as a second
line of defense.
[0063] A prior art sensor to measure both the radio frequency
impulses and the power frequency capacitive current of the bushing
is shown in FIG. 24. Two separate circuits or cables carry the
measured signals to the remote recording instrumentation (not
shown). The surge arrester 240 and the capacitor shunt 250 are
connected in parallel between the capacitance tap 234 and the
Ground G, as in the circuit of FIG. 23. The primary winding of the
radio frequency current transformer 91, in the form of a single
turn, is inserted between the capacitor shunt 250 and the Ground G.
The secondary winding 92 of the current transformer 91 is connected
to the connecting circuit 244. The shield of the circuit and the
respective end of the secondary winding 92 are grounded at the tap
location. The second connecting circuit 244A carries the power
frequency signal in a manner similar to the circuit described in
FIG. 23. The sizing requirements for the capacitor shunt 250 and
the surge arrester 240 are identical to that of the circuit of FIG.
23.
[0064] FIG. 25 represents a prior art sensor similar to the sensor
of FIG. 24, except that the primary winding 90 of the radio
frequency current transformer 91 is located on the "live" side of
the capacitor shunt 250 instead of its grounded side.
[0065] An embodiment of the present invention which may be an
integral part of any of the electrical systems or apparatus of
FIGS. 1, 13, 19, 20 or 27 for example, is depicted in FIG. 26 and
described hereinafter includes a sensor SEN that permits
transmitting both the radio frequency and power frequency signals
simultaneously using one connecting circuit as shown in FIG. 26.
The connecting circuit 244 may be, for example, a coaxial or
twisted pair cable. The capacitor shunt 250 and the parallel surge
arrester 240 are connected to the non-polarity terminal 90B of the
primary winding 90 of the radio frequency current transformer 91,
while its polarity terminal 90A (*) is connected to the tap 234.
The opposite terminals of the capacitor shunt 250 and the surge
arrester 240 are grounded at G. The polarity terminal 92A of the
secondary winding 92 of the current transformer 91 is connected to
the signal conductor 246 of connecting circuit 244, while the
second conductor 248 (for example, the shield of a coaxial cable),
is grounded at G at the tap or local location. The opposite
(non-polarity) terminal 92B of the secondary winding 92 of the
current transformer 91 is joined with the non-polarity terminal 90B
of the primary winding 91.
[0066] An input circuit for the remote measuring device REM for the
sensor signal is also shown in FIG. 26. To prevent circulation of
induced currents in the second conductor 248 of the connecting
circuit 244 this conductor 248 is ungrounded at the instrumentation
REM end. An additional surge arrester 254, preferable of identical
rating to the one, 240, in the sensor SEN, is placed between the
signal conductor 246 of the connecting circuit 244 and the remote
ground GM for added protection and safety. The power frequency
measuring/record device PMD is connected across the additional
surge arrester 254 via a small inductance 260. The size of the
inductance or choke 260 is chosen such that the power frequency
signal is let through, while the radio frequency signal is blocked.
The induced current in the loop created by the signal conductor of
the connecting circuit 244, the capacitor shunt 250 in the sensor
SEN and the input impedance ZI of the measuring/recording device
PMD, as well as the noise level on the input, are reduced by the
choice of the capacitance C2 for the capacitor shunt 250. Radio
frequency measuring/recording equipment RFMD is connected across
the secondary winding 264A of radio frequency isolating transformer
264. The primary winding 256 thereof is connected in series with
capacitor 258 and inserted between the signal conductor 246 and the
second conductor or shield 248 of the connecting circuit 244. The
capacitor 258 cuts off the power frequency signal and narrows the
band of the radio frequency signal let through.
[0067] Referring to the prior art circuit of FIG. 21, the power
frequency capacitive current IC flows through the capacitance C1 of
the bushing insulation BS to the Ground G, with a relatively small
voltage drop across the primary winding 90 of the current
transformer 91. The radio frequency electrical impulses RFI that
accompany the partial discharges inside the bushing insulation BS
travel the same path. The voltage drop from the power frequency
signal IC as well as switching and lightning impulses is usually
small in this type of a sensor, thus there is no need for any
additional overvoltage protection of the tap.
[0068] In the sensor designed to detect only a power frequency
signal, the current transformer 91 is typically of an air core type
(Rogovsky coil) with a linear response characteristic (output
signal vs. input current magnitude). The capacitive current through
the insulation is typically in the order of 5-100 aM rms, depending
upon the capacitance of the insulation C1 and the rated voltage. If
the Rogovsky coil design is employed, its low sensitivity requires
a measuring instrumentation of higher sensitivity. In the sensor
designed to detect only the radio frequency impulses, the current
transformer 91 is typically of a ferrite core type. A radio
frequency signal RFI associated with partial discharges in the
insulation yields very weak output signals (from microvolts of
tenths of volt), also requiring more sensitive measuring
instrumentation. In any of these two cases the connecting circuit
244 transfers the measured signal to the remote monitoring
instrument (not shown). Although no additional overvoltage
protection is necessary at the sensor location, it may be required
at the measuring equipment.
[0069] In the prior art design of FIG. 22, a significant voltage
drop is generated across the resistor shunt 241 by the power
frequency capacitive current IC that flows through the bushing
insulation. This voltage drop is transferred to the monitoring
equipment (not shown) through the connecting circuit 244. If the
input impedance of the remote measuring device is significantly
lower than that of the shunt 240, the voltage drop is controlled by
the input impedance of the instrumentation. The value of the
resistance of shunt 240 is chosen such as to limit the power
frequency voltage at the tap 234 to a safe value in the event of
the monitoring device disconnection or accidental open circuit
fault in the connecting circuit 244. When the input impedance of
the remote instrumentation is comparable with the resistance of
shunt 241, a precision resistor must be employed as it directly
controls the accuracy of the measurement. Switching or lightning
overvoltages that occur on the high voltage primary circuit are
transferred to the output of the capacitor tap 234, their magnitude
being controlled by the capacitive-resistive voltage divider. This
divider consists of the bushing insulation capacitance C1 and the
parallel combination of the resistance R of the shunt 241 and the
surge resistance (not shown) of the connecting circuit 244. The
divider ratio is frequency dependent; hence the high frequency
transients from the high voltage primary circuits lead to very high
transient voltages at the tap 234. To keep the voltages at the safe
level, the surge arrester 240 is required. As switching and
lightning overvoltages in a high voltage switchyard are relatively
common, the arrester 240 duty is high. The thermal stability of the
resistor shunt 241 has to be sufficiently high to survive the
prolonged power frequency overvoltages resulting from open circuit
faults in the connecting circuit 244. The dielectric strength of
the resistive shunt 241 has to be coordinated with the residual
voltage of the arrester 240.
[0070] In the prior art design of FIG. 23, the voltage divider
consists of the bushing insulation capacitance C1 and the capacitor
shunt 250 of capacitance C2. The voltage divider ratio is
essentially independent of frequency; thus both the power frequency
voltage drop and the voltage drop from switching and lightning
transients can be reduced in the same proportion. As a result,
transients are reduced to a much lower level than in the circuit of
FIG. 22. The surge arrester 240 merely serves as a second line of
defense, in the event of extremely severe overvoltages. In
comparison with the circuit of FIG. 22, much less frequent
operation of the surge arrester 240 is expected. To insure adequate
accuracy and frequency response, a special impulse capacitor 250
should be used, of essentially constant capacitance over a wide
range of frequencies, typically from 50 Hz to 1-10 MHz. The
capacitor 250 should also feature high temperature stability. The
capacitor shunt insulation has to withstand the maximum possible
residual voltage of the surge arrester 240, a requirement similar
to one for the resistive shunt 241 in the circuit of FIG. 22.
[0071] In the prior art designs of FIGS. 24 and 25, the principle
of detection of the power frequency signal IC and the requirements
for overvoltage protection of the tap insulation and the associated
circuits are similar to the sensor of FIG. 23. Therefore, the same
requirements apply for the selection of the capacitor shunt 250 and
the surge arrester 240. Radio frequency impulses RFI associated
with the partial discharges in the bushing insulation generate the
radio frequency current impulses traveling through the bushing
insulation capacitance C1 and the capacitor shunt 250. The higher
the frequency of the current the lower the impedance of this
circuit, hence steeper impulses of the same magnitude result in
higher current magnitudes. Due to this phenomenon, even very weak
high frequency signals can be successfully captured by the radio
frequency current transformer 91 inserted in the circuit of the
capacitor shunt 250. The impulses are conducted from the secondary
winding 92 of the current transformer 91 to the connecting circuit
244A. In order to keep the power frequency voltages, as well as the
switching and lightning overvoltages, within a safe limit and to
ensure an optimal sensitivity of the radio frequency impulse
detection, the capacitance C2 of the capacitor shunt 250 should be
typically within 0.1-1 uF. Although functionally identical to the
circuit of FIG. 24, locating the radio frequency current
transformer 91 at the grounded end of the capacitor shunt 250, as
in FIG. 25, is preferred for the safety reasons. The circuits for
transmitting of the power frequency and the radio frequency signals
are formed by two separate circuits (cables) 244 and 244A. This
arrangement allows the use of a radio frequency monitoring
equipment of high sensitivity while eliminating the potential of
its damage by the power frequency signal.
[0072] The circuits of FIGS. 24 and 25 have two disadvantages.
First, two circuits (cables) are required, complicating the sensor
design. Second, the surge arrester (varistor) stray capacitance,
being in parallel with the capacitance C2, in series with the
impedance of the current transformer 91 (consisting of the current
transformer inductance and its resistive load), causes diversion of
a significant part of the high frequency current impulses from the
current transformer 91 into the varistor 240. This phenomenon
reduces the sensitivity of the sensor to current impulses,
especially to the steep and short impulses associated with partial
discharges, but can be saturated on long pulses.
[0073] With regard to what is shown in FIG. 26, the requirements
for selection of the capacitor shunt 250 and the surge arrester
(varistor) 240 are similar to those for the sensors of FIGS. 24 and
25. The power frequency current IC travels from the tap 234 into
the primary winding 90 of the current transformer 91 and then into
the capacitor shunt 250. The radio frequency current impulses RFI
travel the same way. As the primary 90 and the secondary windings
92 of the radio frequency current transformer 91 have a common
point at the capacitor shunt 250 "live" terminal 90B, the radio
frequency signal induced in the secondary winding 92 becomes
superimposed on the power frequency signal in the connecting
circuit (cable) 244. These two signals have to be separated at the
remote end REM of the connecting circuit.
[0074] The placing of the surge arrestor 240 in the circuit of FIG.
26 requires further clarification. The two methods of connecting
the surge arrestor 240 represented by FIGS. 22-25 and FIG. 26,
respectively, offer virtually an identical protection, as the
impedance of the primary winding 90 of the current transformer 91
is effective only during a fraction of the impulse duration
(usually no more than 50-100 ns), until the ferrite core (see 182
in FIG. 18 for example) of the current transformer 91 saturates.
After the core has saturated, the transformer 91 input impedance
drops to a negligible value, and the voltage at both ends of the
primary winding 90 practically coincide electrically. The surge
arrester 240 which is designed to absorb impulse currents in the
order of hundreds to thousand amperes cannot provide an adequate
protection during such short time intervals, i.e. it will be
ineffective during first 50-100 ns, regardless of its connection to
the tap output 234 or to the capacitor shunt 250. Consequently, in
both discussed configurations, the sensor circuit SEN should be
designed to withstand initial overvoltages as if no surge arrester
240 were present. It has been proven by test that with a proper
design, these initial overvoltages can be confined to a level
acceptable for both the capacitance tap insulation INS and the
sensor components. After the initial time interval the overvoltage
protection provided by the arrester 240 together with the capacitor
shunt 250 is identical in both designs. But due to the placement of
the surge arrester 240 after the primary winding 90 of the radio
frequency current transformer, all impulse current flows through
it, thus providing the maximum sensitivity of the sensor to the
impulses created by partial discharges.
[0075] Locating the remote instrumentation REM far from the device
being monitored may pose concerns. First, the difference in ground
impulse potentials between the sensor SEN and measuring equipment
REM locations during switching and lightning transients originated
on the high voltage side of the equipment being monitored can
distort the measurements or even damage the measuring devices.
Second, power frequency currents induced in the loops including the
connecting cable 244 can create an essential error in the measured
values. For these reasons the input circuit of the remote measuring
or recording equipment REM has to be coordinated with the sensor
circuit SEN. FIG. 26 shows an embodiment of an input circuit for
the remote monitoring devices PMD and RFMD to be used with the
sensor circuit SEN.
[0076] The second conductor (or shield) 248 of the connecting
circuit 244 is left ungrounded at the remote end of the circuit,
thus preventing the formation of a ground loop through this
conductor (shield) 248. To provide safety and overvoltage
protection, an additional surge arrester 254 is used as the part of
the input circuit at the remote end REM of the connecting circuit
244 between the signal conductor 246 and the remote Ground GM. To
reduce the possible induced current in the loop, created by the
capacitor shunt 250 in the sensor SEN, the signal conductor 246 in
the connecting circuit 244 and the input impedance ZI of the power
frequency measuring/recording device PMP at the remote end REM, and
especially the interference at the input of device PMD, some
limitations have to be imposed on the choice of the capacitor shunt
250 in the sensor SEN. The impedance of the chosen capacitor shunt
250 at the power frequency has to be much higher than the input
impedance ZI of the device PMD. High impedance in this loop reduces
the magnitude of the current; and the voltage induced in the loop
will be divided between this capacitor shunt 250 impedance and the
input impedance of the instrumentation while most of the voltage
will appear across the capacitor shunt 250 instead of the measuring
device.
[0077] Power frequency and impulse signals transmitted via the
single connecting circuit 244 from the sensor SEN have to be
separated at the remote end REM to be fed into the proper
measuring/recording instruments. The power frequency signal,
related to IC, is obtained across the surge arrester 254 via a
small inductance (choke) 260 that blocks the radio frequency
signals from penetrating into the power frequency
measuring/recording device PMD, but does not interfere with the
power frequency signal. To satisfy the two requirements the
inductance 260 is typically of the order 0.1-1 mH. To detect the
radio frequency signals, related to RFI, a small capacitor 258 and
the primary winding 256 of the radio frequency isolating
transformer 265 connected in series are connected between the
signal conductor 246 and the second conductor or sheath 248 of the
connecting circuit 244. The high impedance of the small capacitor
258 at power frequency blocks the power frequency current from
traveling to the remote Ground GM through the primary winding 256
of the radio frequency isolating transformer 265, thus only the
radio frequency signals are detected at the transformer secondary
winding 264 that the radio frequency measuring device RFMD is
connected to. The impedance of the capacitor 258 has to be low at
radio frequencies. To satisfy these two conditions the required
capacitance is typically in the range of 1-10 nF. The capacitor 258
also limits the frequency band to assist in rejecting unwanted
noise. The small capacitor 258 and the isolating radio frequency
transformer 265 provide isolation between the radio frequency
measuring/recording equipment RFMD and the remote end of the
connecting circuit 244.
[0078] The sensor circuit SEN of FIG. 26 is capable of a
simultaneous online detection of two separate signals reflecting
the condition of the monitored high voltage apparatus insulation,
namely, the power frequency capacitive current IC through the
bushing insulation BS and the radio frequency current impulses RFI
associated with partial discharges occurring inside the bushing
insulation BS. Both signals are transmitted using a single
connecting circuit (cable) 244, common to both signals, to the
remote instrumentation REM where the signals have to be separated.
The power frequency signal IC is detected in the sensor SEN using a
capacitor shunt 250 allowing good sensitivity and accuracy of
detection by conventional measuring devices and also providing a
good suppression of overvoltages. A surge arrester 240 connected in
parallel to the capacitor shunt 250 serves as a second line of
defense. The radio frequency signal RFI is detected using a radio
frequency current transformer 91, the primary winding 90 of which
is in the circuit of the capacitor shunt 250. The polarity terminal
(*) for the winding 91 is connected to the tap 234 output. The
superposition of both signals in one circuit is accomplished by the
connection of the non-polarity terminals of the secondary 92 and
the primary winding 90. The polarity terminal (*) of the secondary
winding 92 of the radio frequency current transformer 91 is
connected to the signal conductor 246 of the connecting circuit
244. High sensitivity of the sensor SEN to the radio frequency
signals RFI is accomplished by placing the surge arrester 240 in
parallel with the capacitor shunt 250, instead of its usual
connection directly to the capacitor tap output 234.
[0079] The input circuit for the remote measuring devices REM of
FIG. 26 is to separate the power signal related to signal IC and
the radio frequency signals related to signal RFI, to reduce
interference and the susceptibility of the system to the hazardous
differences in transient ground potentials at the opposite ends of
the connecting circuit, and to eliminate the formation of power
frequency current loops through the connecting circuit 244. This is
accomplished by the combination of several things: the grounding of
the second conductor (shield) 248 in the connecting circuit 244 is
made at the sending end SEN only; an additional surge arrester 254
is installed between the signal conductor 246 of the connecting
circuit 244 and the remote Ground GM at the remote end REM; and the
impedance of the capacitor shunt 258 at the power frequency is
chosen much higher than that of the measuring equipment. The power
frequency signal is detected across the additional surge arrester
254 through a small inductance (choke) coil 260, thus preventing
the radio frequency signal from penetration into the power
frequency measuring equipment PMD. The radio frequency signals are
detected between the signal conductor 246 of the connection circuit
244 and its second conductor 248, and the galvanic isolation of the
radio frequency measuring circuit RFMD is provided with a radio
frequency isolating transformer 265 of which primary winding 256 is
combined in series with a small capacitor 258 to block the
penetration of power frequency current into this circuit.
[0080] It is understood that the transformer 91, primary current
transformer winding 90, the secondary winding 92, the conductor 230
and the capacitance C1 depicted in FIG. 26 are depicted elsewhere
in this Specification. For instance, elements 90, 91 and 92 may be
found in FIGS. 1, 4, 7, 10, 13, 17, 18, 19 and 27. Capacitance C1
of FIG. 26 is also depicted in FIGS. 2, 5, 8, 9, 14, 18 (including
C1A and C1B), 19 and 20 Conductor 230 of FIG. 26 may also be 60 in
FIG. 2 and FIG. 3, 86 and 88 in FIG. 8 and FIG. 10, 120 in FIG. 14,
38 and 168 in FIG. 18, 193 in FIG. 19, 200 in FIG. 20, and 269 in
FIG. 27. Points 92A and 92B of FIG. 26 are also depicted in FIG. 1
and FIG. 18. It is also to be understood that the embodiment of
FIGS. 1, 13, 18, 19 and 20 may sense and monitor only partial
discharge impulses to be fed to prior art monitors such as shown in
FIGS. 21-25 or may sense both power frequency current and/partial
discharge impulses to be treated in the manner set forth in and
described with respect to FIG. 26. Of course, none of these
arrangements are limited t the illustrative embodiment shown
herein.
* * * * *