U.S. patent application number 10/672770 was filed with the patent office on 2005-03-31 for method and apparatus detecting shorted turns in an electric generator.
This patent application is currently assigned to Siemens Westinghouse Power Corporation. Invention is credited to Cates, Stephen W., Nelson, Robert John, Nieves, Abraham, Prole, Aleksandar.
Application Number | 20050068058 10/672770 |
Document ID | / |
Family ID | 34376461 |
Filed Date | 2005-03-31 |
United States Patent
Application |
20050068058 |
Kind Code |
A1 |
Nelson, Robert John ; et
al. |
March 31, 2005 |
METHOD AND APPARATUS DETECTING SHORTED TURNS IN AN ELECTRIC
GENERATOR
Abstract
Methods and systems consistent with the present invention
provide improved online detection of one or more shorts in rotor
turns (18) of a field winding (22) of an electric generator. An
initial reference inductance L.sub.REF is determined by an
impedance-measuring circuit (50). A subsequent inductance L is
determined by the impedance measuring circuit (50). A data
processing system (54) compares L.sub.REF to L to determine whether
they differ by a predetermined amount. If L.sub.REF and L differ by
the predetermined amount, an alarm is provided to operators to
indicate the presence of one or more shorted rotor turns.
Inventors: |
Nelson, Robert John;
(Orlando, FL) ; Prole, Aleksandar; (Winter
Springs, FL) ; Cates, Stephen W.; (Oviedo, FL)
; Nieves, Abraham; (Orlando, FL) |
Correspondence
Address: |
Siemens Corporation
Intellectual Property Department
170 Wood Avenue South
Iselin
NJ
08830
US
|
Assignee: |
Siemens Westinghouse Power
Corporation
|
Family ID: |
34376461 |
Appl. No.: |
10/672770 |
Filed: |
September 26, 2003 |
Current U.S.
Class: |
324/765.01 |
Current CPC
Class: |
G01R 31/346
20130101 |
Class at
Publication: |
324/772 |
International
Class: |
G01R 031/06; G01R
031/34 |
Claims
What is claimed is:
1. A system for detecting one or more shorted rotor turns in a
field winding of an electric generator, comprising: an electrical
tap for measuring a first voltage drop V.sub.f1 and a subsequent
voltage drop V.sub.f2 across said field winding; an electrical
shunt for measuring a first current I.sub.f1 and a subsequent
current I.sub.f2 entering said field winding; an
impedance-measuring circuit for determining a reference inductance
L.sub.REF based on V.sub.f1 and I.sub.f1 and for determining a
subsequent inductance L based on V.sub.f2 and I.sub.f2; a memory
circuit for storing L.sub.REF; and a data processing system for
comparing L.sub.REF to said subsequent inductance L to determine
whether L.sub.REF and said subsequent inductance L differ by more
than a predetermined amount and for providing an alarm to indicate
the presence of said one or more shorted rotor turns when L.sub.REF
and said subsequent inductance L differ by more than said
predetermined amount.
2. The system of claim 1 wherein said impedance-measuring circuit
comprises circuitry for isolating harmonic components V.sub.0 and
I.sub.0 and an associated harmonic frequency .omega..sub.0 using
Fourier analysis and for determining L.sub.REF and L via a formula
L=V.sub.o/(.omega..sub.- 0*I.sub.o).
3. The system of claim 1 wherein said impedance-measuring circuit
comprises an inductance-measuring bridge.
4. The system of claim 1 wherein said impedance-measuring circuit
is located onboard a spinning rotor of said electrical
generator.
5. The system of claim 4 further including a telemetry circuit for
transmitting data from said impedance-measuring circuit to a remote
location.
6. A method for detecting one or more rotor turn shorts in a field
winding of an electric generator, comprising the steps of: taking a
first measurement of a voltage V.sub.f1 across said field winding;
taking a first measurement of a current I.sub.f1 entering said
field winding; analyzing said first measurement V.sub.f1 and
I.sub.f1 to isolate a harmonic component V.sub.o1 of V.sub.f1 and a
harmonic component I.sub.o1 of I.sub.f1 and an associated harmonic
frequency .omega..sub.01; calculating a reference inductance
L.sub.REF based on said first measurements; taking a subsequent
measurement of a voltage V.sub.f2 across said field winding; taking
a subsequent measurement of a current I.sub.f2 entering said field
winding; analyzing said subsequent measurement V.sub.f2 and
I.sub.f2 to isolate a harmonic component V.sub.o2 of V.sub.f2 and a
harmonic component I.sub.o2 of I.sub.f2 and an associated harmonic
frequency .omega..sub.02; calculating a subsequent inductance L
based on said subsequent measurements; comparing said reference
inductance L.sub.REF to said subsequent inductance L to determine
whether said reference inductance L.sub.REF and said subsequent
inductance L differ by more than a predetermined amount; and
providing an alarm indication if said reference inductance
L.sub.REF and said subsequent inductance L differ by more than said
predetermined amount.
7. The method of claim 6 wherein said predetermined amount is a
difference between L.sub.REF and L of about 5%.
8. The method of claim 6 wherein said harmonic component is a
fundamental harmonic component.
9. The method of claim 6 further comprising the step of
transmitting said alarm via telemetry to a remote location.
10 The method of claim 6 wherein said step of calculating a
reference inductance L.sub.REF and said step of calculating a
subsequent inductance L comprises using a formula
L.sub.REF=V.sub.o1/(.omega..sub.01*I.sub.o1) and
L=V.sub.o2/(.omega..sub.o2*I.sub.o2), respectively.
11. A method of detecting a shorted rotor turn in a field winding
of an electric generator, comprising the steps of: determining a
reference inductance L.sub.REF for said field winding at an initial
time; determining a second inductance L for said field winding at a
time subsequent to said initial time; comparing said reference
inductance L.sub.REF to said second inductance L to determine
whether said reference inductance L.sub.REF and said second
inductance L differ by more than a predetermined amount; and
providing an alarm if said reference inductance L.sub.REF and said
second inductance L differ by more than said predetermined
amount.
12. The method of claim 11 wherein said predetermined amount is a
difference between L.sub.REF and L of about 5%.
13. The method of claim 11 further comprising the step of
transmitting said alarm via telemetry to a remote location.
14. The method of claim 11 wherein said step of calculating a
reference inductance L.sub.REF and said step of calculating a
second inductance L includes isolating harmonic components V.sub.0
and I.sub.0 with a harmonic frequency .omega..sub.0 and using the
formula L=V.sub.o/(.omega..sub.0*I.sub.o).
15. The method of claim 14 wherein said harmonic component is a
fundamental harmonic component.
16. The method of claim 11 wherein said step of calculating a
reference inductance L.sub.REF and said step of calculating a
second inductance L includes obtaining an indication of L.sub.REF
and L from an inductance measurement bridge.
17. The method of claim 11 wherein said step of determining a
reference inductance L.sub.REF and said step of determining a
second inductance L comprises taking measurements from said field
winding while said electrical generator is in operation.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to the monitoring of
electric machinery, and more particularly to methods and
apparatuses for detecting shorted turns in an electric
generator.
BACKGROUND
[0002] Electric generators, such as those used in the power
generation industry, essentially comprise a rotor and a stator. The
rotor is wound with conductors to form a field winding. The stator
is wound with conductors to form a stator winding. The field
winding is supplied with an excitation current in order to create a
magnetic field on the rotor. When the rotor spins inside the
stator, electric power is induced in the stator winding.
[0003] The rotor of an electric generator is generally machined
from a solid steel forging. Slots are provided along the length of
the rotor for inserting the conductors that make up the field
winding. FIG. 1 illustrates a slot 10 of a typical synchronous
generator rotor 12. The slot 10 is filled with multiple copper
conductors 14. Each copper conductor is separated from adjacent
copper conductors by an insulation layer 16. The conductors 14
extend down the axial length of the rotor 12. A pair of
electrically connected conductors 14 is referred to as a rotor turn
18.
[0004] When an electric generator is first manufactured, each rotor
turn 18 is electrically insulated along the axial length of the
rotor from adjacent rotor turns by the insulation layer 16. Over
time, various factors related to the normal operation of the
generator may cause damage to the insulation layer 16. Damage to
the insulation layer 16 may permit adjacent rotor turns to come
into electrical contact. This situation is referred to as a rotor
turn short. Rotor turn shorts significantly reduce the overall
inductance of the field winding of a rotor and impair the
efficiency and output capability of a generator. When sufficient
rotor turn shorts occur, a rotor must generally be rewound to
repair the shorts.
[0005] The conventional approach to detecting shorted rotor turns
in an operating generator involves the use of a flux probe. A flux
probe is basically a coil in which a voltage is induced by a
varying magnetic flux. The flux probe is used to measure the
magnetic field associated with each rotor pole by placing the probe
inside the generator air gap and then observing and comparing the
flux associated with each rotor pole. The main problems with flux
probes are that the data acquired by flux probes are sensitive to
generator load and the interpretation of the data is relatively
subjective. Flux probe systems are also quite expensive, generally
costing in excess of $25,000 per unit.
SUMMARY OF THE INVENTION
[0006] With the foregoing in mind, methods and systems consistent
with the present invention enable improved online detection of
rotor turn shorts in an operating generator. An exemplary
embodiment of the present invention utilizes an AC component of the
excitation power provided to a field winding to calculate a
reference inductance L.sub.REF. Over time, new inductance values
are calculated. The new inductance values are compared to
L.sub.REF. If a new inductance value differs from LREF by more than
a predetermined value, one or more rotor turn shorts have occurred.
Upon detection of the shorts, an alarm may be provided to operators
to indicate that the generator should be rewound.
[0007] These and other objects, features, and advantages in
accordance with the present invention are provided in one aspect by
a system that comprises (a) an electrical tap for measuring a first
voltage drop V.sub.f1 and a subsequent voltage drop V.sub.f2 across
the field winding, (b) an electrical shunt for measuring a first
current I.sub.f1 and a subsequent current I.sub.f2 entering the
field winding; (c) an impedance-measuring circuit for determining a
reference inductance L.sub.REF based on V.sub.f1 and I.sub.f1 and
for determining a subsequent inductance L based on V.sub.f2 and
I.sub.f2, (d) a memory circuit for storing L.sub.REF, and (e) a
data processing system for comparing L.sub.REF to the subsequent
inductance L to determine whether L.sub.REF and the subsequent
inductance L differ by more than a predetermined amount and for
providing an alarm to indicate the presence of a shorted rotor turn
when L.sub.REF and the subsequent inductance L differ by more than
the predetermined amount.
[0008] In another aspect, the present invention is provided by a
method comprising the steps of (a) determining a reference
inductance L.sub.REF for the field winding at an initial time, (b)
determining a second inductance L for the field winding at a time
subsequent to the initial time, (c) comparing the reference
inductance L.sub.REF to the second inductance L to determine
whether the reference inductance L.sub.REF and the second
inductance L differ by more than a predetermined amount, and (d)
providing an alarm if the reference inductance L.sub.REF and the
second inductance L differ by more than the predetermined
amount.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention is explained in more detail by way of example
with reference to the following drawings:
[0010] FIG. 1 illustrates a typical synchronous generator rotor
slot.
[0011] FIG. 2 illustrates a schematic of a static excitation system
consistent with the present invention.
[0012] FIG. 3 illustrates a typical voltage waveform associated
with the static excitation system of FIG. 2.
[0013] FIG. 4 illustrates a schematic of a brushless excitation
system consistent with the present invention.
[0014] FIG. 5 illustrates a typical voltage waveform associated
with the brushless excitation system of FIG. 4.
[0015] FIG. 6 illustrates an impedance-measurement circuit
consistent with an exemplary embodiment of the present
invention.
[0016] FIG. 7 illustrates steps associated with an exemplary method
consistent with the present invention.
[0017] FIG. 8 illustrates steps associated with an exemplary method
consistent with the present invention.
DETAILED DESCRIPTION
[0018] An electrical generator requires a source of excitation
current to energize its field winding. Excitation current is
generally provided by an excitation system. Examples of excitations
systems include static excitation systems and brushless excitation
systems. Static and brushless excitation systems are described in
detail below to provide an exemplary context for the invention.
However, any other excitation system that provides an excitation
power with an AC component is also suitable for use with the
present invention.
[0019] FIG. 2 illustrates a static excitation system 20 suitable
for use with the present invention. The static excitation system 20
is provided with three-phase power (represented by V.sub.AB,
V.sub.CA and V.sub.BC) from the main leads of a generator or from
the station power. The three-phase power is fed to a voltage
regulator 24, where it is rectified and regulated to produce an
excitation current I.sub.f. The excitation current I.sub.f is
supplied to the field winding 22 of the generator and produces a
voltage drop across the field winding 22 of V.sub.f.
[0020] FIG. 3 illustrates the waveform representation of a typical
voltage drop V.sub.f versus time for a field winding 22 in a
generator with a static excitation system. V.sub.f includes a DC
component, V.sub.AV, and an AC (or periodic) component. The AC
component of V.sub.f results from the rectification performed by
the static excitation system. V.sub.f includes a number of
harmonics. The first harmonic of V.sub.f is V.sub.0. V.sub.0 has a
period T.sub.0, which is related to an angular frequency
.omega..sub.0 by the formula .omega..sub.0=2.pi./T.sub.0. V.sub.f
includes other harmonic voltages with periods T.sub.n such that
.omega..sub.n=2.pi./T.sub.n.
[0021] FIG. 4 illustrates a brushless excitation system 40 suitable
for use with the present invention. The brushless excitation system
40 may include a permanent magnet generator (PMG) 44, which may be
located on the outboard end of the excitation system 40. The
permanent magnet generator 44 or other AC source provides AC power
to a voltage regulator 42, which rectifies and regulates the power
to produce an exciter current I.sub.EX, which is fed to a
stationary exciter field winding 45 to produce a voltage drop of
V.sub.EX. The power provided to the stationary field winding 45
induces a three-phase voltage V.sub.AC in a rotating armature 46
located on the shaft of the excitation system 40. The three-phase
voltage V.sub.AC is fed to a rotating rectifier 47, where it is
rectified and regulated to produce an excitation current I.sub.f.
The excitation current I.sub.f is supplied to the field winding 22
of the generator and produces a voltage drop across the field
winding 22 of V.sub.f.
[0022] FIG. 5 illustrates the waveform representation of a typical
voltage drop V.sub.f versus time for a field winding 22 in a
generator with a brushless excitation system. V.sub.f includes a DC
component, V.sub.AV, and an AC component. The AC component of
V.sub.f, arises from the rectification of the rotating armature
output. V.sub.f, includes a number of harmonics. The first harmonic
of V.sub.f is V.sub.0. V.sub.0 has a period T.sub.0, which is
related to an angular frequency .omega..sub.0 of V.sub.f by the
formula .omega..sub.0=2.pi./T.sub.0. V.sub.f may include other
harmonics with periods T.sub.n such that
.omega..sub.n=2.pi./T.sub.- n.
[0023] Methods and systems consistent with the present invention
take advantage of the AC component (or periodic) of the excitation
current I.sub.f and the field winding voltage V.sub.f supplied by
an excitation system in order to determine the inductance of a
generator's field winding. The inductance L of a field winding is
related to a generator's rotor turns by the formula L=N.sup.2P,
where N is the number of turns in the filed winding and P is the
permeance of the rotor flux path, which is generally a constant for
a given set of operating conditions. Therefore, changes in the
inductance L of the field winding are indicative of shorts between
rotor turns. When one or more rotor turns short together, as occurs
when an electrical generator ages and its insulation layers
deteriorate, the number of turns N decreases. For example, if a
rotor has 112 rotor turns, as with a 32-slot rotor with 7
conductors per slot, a single short would cause a decrease in the
rotor inductance of about 2%. Two shorts, on the other hand, would
decrease the inductance by 4%. After sufficient rotor turn shorts
occur, a generator must generally be rewound to maintain a suitable
efficiency and output capability.
[0024] FIG. 6 illustrates an online impedance-measurement circuit
50 consistent with an exemplary embodiment of the present
invention. Measurements of I.sub.f and V.sub.f are fed into the
impedance-measurement circuit 50, for example, by communications
cables or telemetry or any other available means of signal
transfer. The measurement V.sub.f may be achieved, for example via
one or more electrical taps across the field winding 22. The
measurement of I.sub.f may be achieved, for example, via a current
shunt that is inline with the field winding 22.
[0025] The impedance-measurement circuit 50 may be implemented as
an analog or digital circuit. If digital circuitry is utilized, the
impedance-measurement circuit 50 may include, for example, a
digital-to-analog converter (A/D) 52 for converting the I.sub.f and
V.sub.f into a digital format to facilitate analysis and
calculations. In the exemplary embodiment illustrated in FIG. 6,
the digital representation of I.sub.f and V.sub.f is fed to a
digital signal processor (DSP) 56. The DSP 56 analyses the AC
component of I.sub.f and V.sub.f to determine their fundamental
frequency .omega..sub.0 and the associated components V.sub.0 and
I.sub.0 using well-known Fourier analysis techniques. Once the
fundamental frequency .omega..sub.0 has been determined, it is used
by a calculation circuit 54, which may comprise for example a
microprocessor or central processing unit, to calculate the
inductance L of the field winding using the formula
L=V.sub.o/(.omega..sub.0*I.sub.o). A memory circuit 58 is provided
to store at least one reference inductance L.sub.REF. The reference
inductance should be value representing the inductance of the field
winding when field winding is known to be functioning properly, for
example, when the generator is new or has recently been rewound. If
the impedance-measurement circuit 50 is located on the spinning
rotor of the generator, the results of the calculations performed
by the impedance-measurement circuit 50 may be transmitted to a
remote location for further processing via a wireless
communications link.
[0026] Referring now to FIGS. 7 and 8, a initial reference
calculation phase of an exemplary method consistent with the
present invention will now be described. As illustrated in FIG. 7,
the exemplary method begins with the measurement of the field
winding voltage drop V.sub.f and the field current I.sub.f (step
62). In a preferred embodiment, V.sub.f and I.sub.f are then
sampled and converted to a digital format by an analog-to-digital
converter (A/D) 52. The A/D 52 feeds a digital representation of
V.sub.f and I.sub.f to a DSP circuit 56, which analyses V.sub.f and
I.sub.f to determine a harmonic frequency .omega..sub.0 and
associated harmonic components V.sub.0 and I.sub.0 using Fourier
analysis (step 64). These values are then used to calculate the
inductance L of the field winding using the formula L=V.sub.o/
(.omega..sub.0*I.sub.o). An initial reference value of the
inductance L.sub.REF should be calculated at a time when the field
winding is known to be functioning properly, for example, when the
generator is new or has recently been rewound. This reference
inductance LREF may then be stored in memory for later calculations
(step 68).
[0027] FIG. 8 illustrates the subsequent monitoring phase of the
exemplary method. Over time, new measurements of V.sub.f and
I.sub.f are taken at some predetermined interval of time (step 70).
In the exemplary digital embodiment, the measurements are converted
to a digital format by the AND 52. The A/D 52 feeds the digital
representation of V.sub.f and I.sub.f to the DSP 56, which analyses
the signals to isolate a harmonic frequency .omega..sub.0 and
associated harmonic components V.sub.0 and I.sub.0 (step 72). These
values are then used to calculate a subsequent value L of the
inductance of the field winding (step 74). This subsequent
inductance value L is then compared to the previously established
L.sub.REF (step 76). If L is different from L.sub.REF by more than
about 5%, for example, the change in L is indicative of one or more
shorted rotor turns and an alarm is provided to operators (step
80). The alarm may signal to operators, for example, that the
generator should be rewound. If L is within the acceptable range,
steps 70 through 76 are repeated until a sufficient change in L is
detected.
[0028] The present invention has been described with reference to
the accompanying drawings that illustrate preferred embodiments of
the invention. The invention may, however, be embodied in many
different forms and should not be construed as limited to the
embodiments set forth herein. For example, it will be understood
that harmonics other than the fundamental may be used with methods
and systems consistent with the present invention to determine
inductance. In addition, inductance may be measured in other ways
than described in the exemplary embodiment, such as by the use of
bridges or by digital calculation of voltage and current phase
relationships. The embodiments described above are provided so that
this disclosure will be thorough and complete, and will fully
convey the scope of the invention to those skilled in the art. The
scope of the invention should be determined based upon the appended
claims and their legal equivalents, rather than the specific
embodiments described above.
* * * * *