U.S. patent number 3,614,355 [Application Number 04/815,674] was granted by the patent office on 1971-10-19 for sf system with constant density and constant pressure differential maintaining means for a high-voltage switchgear.
This patent grant is currently assigned to N.V. COQ. Invention is credited to Rintje Boersma.
United States Patent |
3,614,355 |
Boersma |
October 19, 1971 |
SF SYSTEM WITH CONSTANT DENSITY AND CONSTANT PRESSURE DIFFERENTIAL
MAINTAINING MEANS FOR A HIGH-VOLTAGE SWITCHGEAR
Abstract
Metal clad switch gear for high voltages comprising switches and
accessories, low-overpressure and high-overpressure compartments
filled with SF.sub.6 -gas and a gas-controlsystem controlling the
flow of gas to and from said compartments and keeping both the
pressure difference between said compartments and the density of
the SF.sub.6 -gas in the low-overpressure compartment(s)
substantially constant.
Inventors: |
Boersma; Rintje (Harmelen,
NL) |
Assignee: |
N.V. COQ (Utrecht,
NL)
|
Family
ID: |
19803398 |
Appl.
No.: |
04/815,674 |
Filed: |
April 14, 1969 |
Foreign Application Priority Data
|
|
|
|
|
Apr 22, 1968 [NL] |
|
|
68.05638 |
|
Current U.S.
Class: |
218/83 |
Current CPC
Class: |
H01H
33/56 (20130101); B01J 23/885 (20130101); H01H
33/562 (20130101); H02B 13/055 (20130101) |
Current International
Class: |
B01J
23/885 (20060101); B01J 23/76 (20060101); H01H
33/02 (20060101); H01H 33/56 (20060101); H02B
13/035 (20060101); H02B 13/055 (20060101); H01h
033/56 () |
Field of
Search: |
;200/148R,148E,148B,84
;174/14 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Schaefer; Robert K.
Assistant Examiner: Vanderhye; Robert A.
Claims
What we claim is:
1. A switchgear system for high voltage electrical circuits,
comprising in combination;
an electrically conductive envelope housing at least a gas blast
circuit breaker and related portions of a high voltage electrical
circuit, said envelope being adapted for connection to earth and
defining low-pressure and high-pressure compartments of fixed
volumes, said low pressure compartment being filled with an amount
of gaseous sulfur hexafluoride sufficient to establish a relatively
low pressure in such compartment which will insulate live parts of
the switchgear from earth, and said high-pressure compartment being
filled with an amount of gaseous sulfur hexafluoride sufficient to
establish a relatively high pressure in such compartment which is
capable of extinguishing a switching arc caused by operation of
said circuit breaker;
means for maintaining a substantially constant density of gas in
said low-pressure compartment whereby the low-pressure compartment
is subjected to limited gas pressure variation in response to
temperature variations; and
means for varying the density of the gas in said high-pressure
compartment to maintain a substantially constant gas pressure
differential between said high-pressure and low-pressure
compartments.
2. Metal clad switchgear as claimed in claim 1 wherein said means
for maintaining a substantially constant density of gas in said
low-pressure compartment comprises a discharge conduit (32) and a
supply conduit (29), said conduits (32, 29) being connected to the
low-pressure compartment (14) and cooperating with flow regulating
means (45, 31, 28) acting on the density of the gas in said
compartment (14), said flow regulating means ensuring that gas
flows from and to the low-pressure compartment (14) when the
density of the gas in said compartment (14) becomes higher and
lower than the required density, respectively, said means for
varying the density of the gas in said high-pressure compartment
comprising a conduit (33) provided with a valve (34) and a conduit
(35) provided with a compressor (36), said conduits being connected
between the low-pressure compartment (14) and the high-pressure
compartment (2), and a differential-pressure relay (48, 49, 50)
connected to both compartments (2, 14) and either opening said
valve (34) or starting said compressor (36), when the pressure
difference between both compartments (2, 14) becomes higher or
lower than the required pressure difference, respectively (FIGS. 2,
3, 4).
3. Metal clad switchgear as claimed in claim 2, comprising a check
valve (55) closing towards the low-pressure compartment (14) and
being provided in the discharge conduit (32) which is connected to
a low-pressure storage tank (54), the supply conduit (29) being
connected through a valve (28) to a high-pressure storage tank
(60), a conduit (58) provided with a second compressor (59) being
connected between both storage tanks (54, 60), a relay (45) acting
on the density of the gas in the low-pressure compartment (14) and
adapted to open and to close said valve (28) when the density of
the gas in the low-pressure compartment 14 becomes a given value
lower than or becomes equal to the required density, respectively,
and means (63, 65) for starting the second compressor (59), when
the low-pressure storage tank (54) contains too much gas at the
temperature obtaining in said tank (FIG. 3, 4).
4. Metal clad switchgear as claimed in claim 3, in which the relay
(45) acting on the density of the gas in the low-pressure
compartment (14) starts and stops the second compressor (59) when
the density of the gas in said compartment (14) becomes higher or a
given value lower than the required density, respectively, a
differential-pressure relay (63) being provided which is connected
both to the low-pressure compartment (14) and the low-pressure
storage tank (54) and is adapted to start the second compressor
(59), when the pressure in the low-pressure compartment (14) (FIG.
3).
5. Metal clad switchgear as claimed in claim 3, comprising also a
relay (65) acting on the density of the gas in the low-pressure
storage tank (54), said relay starting and stopping the second
compressor (59) at density values which are so much higher than the
density values, at which the relay (45) acting on the density of
the gas in the lower-pressure compartment (14) closes and opens the
valve (28), that at the maximum temperature difference to be
expected between the gas in the lower-pressure compartment (14) and
the gas in the lower-pressure storage tank (54) the second
compressor (59) is started at a higher gas pressure than that, at
which the valve (28) is closed, and is stopped at a higher gas
pressure than that, at which the valve (28) is opened (FIG. 4).
6. Metal clad switchgear as claimed in claim 3, comprising a gas
blast circuit breaker provided with an expansion tank (15) to
collect the used extinguishing gas, said expansion tank (15)
communicating with the low-pressure storage tank (54) through a
conduit (37) provided with a check valve (56) closing towards said
tank and a filter (38), said expansion tank (15) also communicating
with the low-pressure compartment (14) through a conduit (39)
provided with a check valve (40) closing towards said compartment
(14).
7. Metal clad switchgear as claimed in claim 6, comprising a buffer
tank (57) provided between the check valve (56) and the filter (38)
in the conduit (37) extending between the expansion-tank (15) and
the low-pressure storage tank (54).
Description
The invention relates to metal clad switch gear for high voltages
comprising a metal envelope adapted to be connected to earth and
containing at least a gas blast circuit breaker, a low-pressure
compartment filled with SF.sub.6 -gas of lower overpressure for the
insulation of the live parts of the switch gear from earth and a
high-pressure compartment filled with SF.sub.6 -gas of higher
overpressure for the extinction of the switching arc and a
composite gas-control system to control the flow of gas to and from
said compartments, said gas-control system being arranged to keep
the pressure difference between both compartments substantially
constant by controlling the density of the SF.sub.6 -gas in each
one of the two compartments.
In switch gear filled with SF.sub.6 -gas changes of the state of
the gas economy occur repeatedly as a result of the properties of
said gas and the operation of the switch gear. In switch gear local
temperature variations can occur which are produced by the current
flowing through the conductors and the continuously varying
temperature of the outer air. These temperature variations can
have, during operation, a range of -25.degree. C. to +85.degree. C.
For the extinction of the switching arc and, if necessary, for
driving the switches gas from the high-pressure compartment is used
and either directly or by a roundabout way returned to the
low-pressure compartment, Moreover, the change of pressure of a
SF.sub.6 -gas mass of a given volume due to a change of temperature
is greater as the density of said mass is higher, which means that
the isochores in the pressure-versus-temperature diagram are
steeper at higher density than at lower density. Within the range
of operation of the switch gear the pressure and the temperature of
the SF.sub.6 -gas can have values, at which said gas condenses. The
gas-control system must reckon with these properties and also
ensure that the density of the SF.sub.6 -gas in no one of the
compartments containing live parts falls down below a given value,
since otherwise the insulation from earth is endangered, and that
the pressure difference between the high-pressure compartment and
the low-pressure compartment remains above the minimum value
required for a sufficient extinguishing action and, if necessary,
for a satisfactory driving of the switches.
Switch gear of the described kind, in which the various above
mentioned facts are reckoned with, has been disclosed in U.S. Pat.
3,390,241. In said switch gear the gas-control system tries to keep
both the total quantity of SF.sub.6 -gas contained in the switch
gear and the pressure difference between the high-pressure gas mass
and the low-pressure gas mass constant as much as possible. This
means, that not only during the switching process, but also at a
rise in temperature in the stationary state of the switch gear
SF.sub.6 -gas must be transported from the high-pressure
compartment towards the low-pressure compartment, so that the
pressure in the low-pressure compartment then will increase more
than would have been the case, when in the low-pressure compartment
only a rise in temperature occurs. This additional increase of
pressure in the low-pressure compartment has the disadvantage that
the envelope of said compartment, which is considerably larger than
that of the high-pressure compartment, must be made stronger and
that also the sealing and the packing between the separable parts
of the envelope of the low-pressure compartment must be made proof
against higher pressures. This sealing may give difficulties in
view of the large dimensions of the joints to be sealed between
said parts of the envelope. The stronger envelope and the heavier
sealing make the switch gear more expensive. The attention is drawn
to the fact that an increase of the density of the SF.sub.6 -gas in
the low-pressure compartment has no real advantage regarding to the
safe state of insulation in the switch gear, since the minimum
density of the gas in the low-pressure compartment, said density
obtaining in the known switchgear at the lowest temperature to be
expected, say a temperature of -25.degree. C., guarantees already a
sufficiently safe state of insulation.
The invention has for its object to provide switchgear of the kind
referred to which is simpler, has a cheaper construction, can be
easily made gastight and for which a relatively simple gas-control
system is required. It consists in that the gas-control system is
so arranged and so operates as to keep also, under all
circumstances, the density of the SF.sub.6 -gas in the low-pressure
compartment substantially constant. This measure has the advantage
that at a rise in temperature the pressures in the low-pressure
compartment and in the high-pressure compartment will increase
relatively slightly, so that lighter structures can be used and a
satisfactory seal can easily be realized. Moreover, a relatively
simple gas-control system can be used, since the control has only
to do with two quantities, viz the density in the low-pressure
compartment and the said pressure difference.
It is observed that switchgear filled and operating with SF.sub.6
-gas and comprising high- and low-pressure compartments, in which
the density of the SF.sub.6 -gas in the low-pressure compartment is
kept constant, has been disclosed by the U.S. Pat. specification
3,129,309. Also in this switchgear the density of the SF.sub.6 -gas
in the high-pressure compartment is kept constant. This results in
that the pressure difference between both compartments and in
consequence thereof the switching process strongly depend on the
temperature and in that at great rises in temperature high
pressures will obtain in the high-pressure system.
Advantageously, the switchgear can be so constructed that the
low-pressure compartment is connected to a discharge conduit and a
supply conduit, said conduits cooperating with means acting on the
density of the gas in said compartment, said means ensuring that
gas flows from and to the low-pressure compartment, when the
density of the gas in said compartment becomes higher and lower
than the required density, respectively, whereas a conduit provided
with a valve and a conduit with a compressor are connected between
the low-pressure compartment and the high-pressure compartment,
both compartments being connected to a differential-pressure relay
which opens said valve or starts said compressor when the pressure
difference between both compartments becomes higher or lower than
the required pressure difference, respectively. In this switchgear
only a relay acting on the density of the SF.sub.6 -gas contained
in the low-pressure compartment and a differential-pressure relay
are necessary. The high-pressure compartment can only receive gas
from or supply gas to the low-pressure compartment, so that the gas
supply of the switchgear takes place only through the low pressure
compartment.
As SF.sub.6 -gas is too expensive to be discarded, switchgear is
recommended in which gas discharged from the switchgear is
collected in a storage tank. The switchgear and its gas-control
system may then be constructed in such a manner that the discharge
conduit is connected through a check valve closing towards the
low-pressure compartment with a low-pressure storage tank and the
supply conduit is connected through a valve to a high-pressure
storage tank, whereas the two storage tanks are interconnected by a
conduit provided with a second compressor and a relay acting on the
density of the gas in the low-pressure compartment is used said
relay opening and closing said valve, when the density of the gas
in the low-pressure compartment becomes a given value lower than or
becomes equal to the required density, respectively. In addition,
means are provided which start the second compressor, when the
low-pressure storage tank contains too much gas for temperature in
said tank. Owing to the use of storage tanks in the gas-control
system no SF.sub.6 -gas will be lost except by leakage. By using a
high-pressure storage tank in the gas circuit a relatively large
stock of SF.sub.6 -gas can be stored in a relatively small volume
and it becomes possible to supply the insulating compartments of
the switchgear very quickly with gas, when therein the density of
the gas, for whatever reason, falls down below a value which is
dangerous to the state of insulation.
If each switching unit of the switchgear has a gas-control system
provided with its own gas storage tanks, this system may
advantageously be made in such a manner, that the relay acting on
the density of the gas in the low-pressure compartment starts and
stops the second compressor, when the density of the gas in said
compartment becomes higher or a given value lower than the required
density, respectively the low-pressure compartment and the
low-pressure storage tank being connected to a
differential-pressure relay which starts the second compressor,
when the pressure in the low-pressure storage tank becomes higher
than the pressure in the low-pressure compartment. The
differential-pressure relay provided between the low-pressure
compartment and the low-pressure storage tank ensures that the
pressures in said compartment and said tank remain substantially
equal, notwithstanding the fact that the temperature of the gas in
the storage tank, said gas having the temperature of the outer air,
can differ considerably from that of the gas in the low-pressure
compartment, through which the main current heating the gas passes.
At a change of this temperature difference, said change resulting
in a change of the pressure difference between the low-pressure
compartment and the low-pressure storage tank, the gas from the
low-pressure compartment could circulate continuously through the
two storage tanks. The differential-pressure relay prevents the
unnecessary circulation of the gas in the low-pressure system.
The switchgear can be made considerably simpler, when the
gas-control systems of a plurality of switching units cooperate
with a common low-pressure storage tank, a common high-pressure
storage tank and a conduit connected between these storage tanks
which is provided with a second compressor. In that case the
differential-pressure relay can be omitted. However, also a relay
acting on the density of the gas in the low-pressure storage tank
is then provided, said latter relay starting and stopping the
second compressor at density values which are so much higher than
the density values, at which the relay acting on the density of the
gas in the low-pressure compartment closes and opens the valve, as
to guarantee that at the maximum temperature difference to be
expected between the gas in the low-pressure compartment and the
gas in the low-pressure storage tank the second compressor is
started at a higher gas pressure than that, at which the valve is
closed, and is stopped at a higher gas pressure than that, at which
the valve is opened. If the density relays in the low-pressure
compartments of the switching units are adjusted in this manner in
respect of the density relay provided in the common low-pressure
storage tank, the gas systems of said switching units cannot
influence one another.
If the gas blast circuit breaker is provided with an expansion tank
to collect the used extinguishing gas, it is advantageous to
connect the expansion tank through a conduit having a check valve
closing towards said tank and a filter to the low-pressure storage
tank and through a conduit having a check valve closing towards the
low-pressure compartment to said compartment. During the switching
process the extinguishing gas, which is deionized in the expansion
tank and cleansed by the filter, will then reach the low-pressure
compartment not directly but by a roundabout way, so that in the
low-pressure compartment a sudden rise of the density will not be
produced, since said rise of density is neutralized relatively
slowly by the control system. Moreover, the used highly heated
extinguishing gas will get sufficient time to cool down, before it
is returned into the switchgear. Preferably, a buffer tank is
inserted in the conduit extending between the expansion tank and
the low-pressure storage tank, said buffer tank being provided
between the check valve and the filter. This buffer tank damps the
velocity of the extinguishing gas which escapes from the expansion
tank during the switching-off process, so that it flows slowly
through the filter and can be well cleansed. The conduit having the
check valve and extending between the low-pressure compartment and
the expansion tank ensures that in the stationary state of the
switchgear the pressure in the expansion tank is about equal to
that in the low-pressure compartment.
If at a predetermined temperature and pressure of the SF.sub.6 -gas
condensation should occur in the high-pressure compartment, the
pressure in said compartment would remain constant, notwithstanding
the supply of gas from or the discharge thereof to the low-pressure
compartment, as long as condensate is to be found in the
high-pressure compartment. In that case it is not possible to keep
the pressure difference between the high-pressure compartment and
the low-pressure compartment constant, when the circumstances
change. In order to avoid this difficulty the high-pressure
compartment can be connected by a condensate conduit and a vapor
conduit to an evaporator adapted to be heated by a heating element
and provided with a relay acting on the liquid level in the
evaporator, said relay putting the heating element into operation,
when said level exceeds a predetermined value. By this measure the
SF.sub.6 -gas contained in the high-pressure compartment is heated
above its dew point.
The invention will be further elucidated with the aid of the
accompanying drawing. Therein is:
FIG. 1 a temperature-versus-pressure diagram of SF.sub.6 -gas at
constant volume for the range which is interesting to switch gear
constructed according to the invention, said FIG. 1 showing lines
of constant density (isochores),
FIG. 2 a schematic view of the part of the switchgear which is
necessary to understand the invention, said switchgear being
provided with a gas-control system carried out in accordance with
the invention,
FIG. 3 a similar schematic view of an other embodiment of the
gas-control system of said switchgear,
FIG. 4 a schematic view of a variant of the gas-control system
shown in FIG. 3 and
FIG. 5 a pressure-versus-temperature diagram to determine the
adjustment of density relays used in the gas-control system shown
in FIG. 4.
From the diagram shown in FIG. 1, in which the pressure of the
SF.sub.6 -gas has been plotted vertically and the temperature
thereof horizontally, it appears that at constant volume the
pressure of the gas rises less at lower density than at higher
density. The lines 1 of constant density (isochores) thus show a
greater steepness as the density of the SF.sub.6 -gas is
higher.
The metal clad switchgear illustrated in FIGS. 2 and 3 comprise a
gas blast circuit breaker 3 accommodated in a high-pressure
compartment 2 filled with SF.sub.6 -gas, said circuit breaker
having four places of interruption to interrupt the main current
circuit, an isolator switch 4 connected in series with said circuit
breaker and adapted to keep the main current circuit interrupted
and to close said circuit, two bus bar isolators 5, 6 connected in
series with the gas blast circuit breaker 3, two rails 7, 8 forming
part of different bus bar systems, a cabel terminal box 9, an
earthing switch 10, an overvoltage protective device 11 and current
transformers 12, 13 for measuring and protection.
The container enclosing the high-pressure compartment 2 and
accommodating the gas blast circuit breaker 3 is positioned in a
low-pressure compartment 14 also filled with SF.sub.6 -gas and
surrounded by a metal envelope adapted to be connected to earth.
Mounted in said compartment 14 are also the isolator switch 4, the
movable contacts of the bus bar isolator switches 5, 6, the current
transformers 12, 13 and an expansion tank 15, in which the
extinguishing gas from the high-pressure compartment 2 and used to
extinguish the switching arcs during the switching-off process is
collected and deionized. The fixed contacts of the bus bar
isolators 5, 6, the rails 7, 8, the cabel terminal box 9, the
earthing switch 10, the overvoltage protective device 11 and the
connecting conductors extending between the isolator switch 4 and
the cabel terminal box 9 and between said box and the overvoltage
protective device 11 are accommodated in individual low-pressure
compartments 16, 17, 18, 19, 20, 21, 22, 23, 24 filled with
SF.sub.6 -gas, said latter compartments communicating in a not
shown manner with the low-pressure compartment 14, so that therein
the same pressure obtains and all these compartments may be
considered as one single low-pressure compartment. The SF.sub.6
-gas contained in the low-pressure compartment serves mainly to
insulate the live parts from earth. The SF.sub.6 -gas contained in
the high-pressure compartment 2 is in the first place used to
extinguish the switching arcs but it may also be used for the
pneumatical driving of the gas blast circuit breaker and the
isolator switches 4, 5, 6.
According to the invention the density of the SF.sub.6 -gas in the
low-pressure compartment 14 and, consequently, also in the other
low-pressure compartments is kept constant as much as possible,
e.g. at a value of 40 gram per liter. This value has been indicated
in the diagram shown in FIG. 1 by the thick isochore 25. In
addition, the density in the high-pressure compartment 2 is so
controlled, as to keep also the pressure difference between the
high-pressure compartment 2 and the low-pressure compartment 14
substantially constant, e.g. at a value of 10 kg./cm..sup.2 In
order to maintain this pressure difference under all circumstances
the density of the gas contained in the high-pressure compartment
must be changed in accordance with the thick line 26 shown in the
diagram given in FIG. 1 at a change of temperature.
For the desired gas economy in the switchgear a gas-control system
has been provided which in the embodiment shown in FIG. 2 is
carried out and operates as follows:
The low-pressure compartment 14 is connected both to a supply
conduit 29 provided with an electromagnetically controlled valve 28
and connected with a gas bottle 30 and to a discharge conduit 32
provided with an electromagnetically controlled valve 31. Connected
between the low-pressure compartment 14 and the high-pressure
compartment 2 are a conduit 33 provided with an electromagnetically
controlled valve 34 and a conduit 35 provided with a compressor 36.
The expansion tank 15 is connected to the low-pressure compartment
14 both by means of a conduit 37 provided with two filters 38,
which can be cleaned alternately, and by means of a conduit 39
provided with a check valve 40 closing towards the low-pressure
compartment 14. The high-pressure compartment 2 is connected by
means of a condensate conduit 41 and a vapor conduit 42 with an
evaporator 44 adapted to be heated by an electric heating element
43.
For controlling the gas-control system a relay 45 acting on the
density of the SF.sub.6 -gas, contained in the low-pressure
compartment 14, and having contacts 46, 47 for the electric control
of the valves 28, 31 is provided in the low-pressure compartment 14
and a differential-pressure relay 48 having contacts 49, 50 for the
electric control of the valve 34 and the compressor 36 is connected
to the low-pressure compartment 14 and the high-pressure
compartment 2. Moreover, mounted in the evaporator 44 is a relay 51
acting on the level of the condensate, collected therein, and
having a contact 52 for the control of the heating element. The
control is effected as follows:
In the stationary state of the switchgear the gas in the
low-pressure compartment 14 has the required density and the gas in
the high-pressure compartment 2 has such a density as to guarantee
that the pressure difference between the compartments 2 and 14 has
the required value. The pressure in the expansion tank 15 is equal
to that in the low-pressure compartment 14.
During operation of the switchgear different changes of the
conditions may occur. If the temperaturs in the switchgear rises,
the pressure in the high-pressure compartment 2 will increase more
than that in the low-pressure compartment 14 (see the diagram shown
in FIG. 1). In that case the pressure difference between both
compartments will become greater than desired. This has the effect
that the differential-pressure relay 48 comes into action and
closes the contact 49, so that the valve 34 is opened. Gas from the
high-pressure compartment 2 then flows through the conduit 33 to
the low-pressure compartment 14 till the required pressure
difference has been restored. The gas fed to the low-pressure
compartment 14 increases the density of the gas in said
compartment, which has the result that the density relay 45 comes
into action and closes the contact 47. Due thereto the valve 31 is
opened, so that the excess gas can flow away from the low-pressure
compartment 14 through the conduits 27 and 32. Should the
temperature in the switchgear fall down, the pressure in the
high-pressure compartment 2 will fall further down than that in the
low-pressure compartment 14. Also in that case the relay 48 comes
into action, so that the contact 50 is closed, the compressor 36 is
put into operation and gas is pumped from the low-pressure
compartment 14 to the high-pressure compartment 2. Owing thereto
the density in the low-pressure compartment 14 decreases, the relay
45 comes into action, the contact 46 is closed and the valve 28 is
opened, so that gas from the gas bottle 30 is supplied to the
low-pressure compartment 14 to supply the gas deficiency in said
compartment.
Not only a change of temperature but also the switching-off process
upsets the stationary state. During the switching-off process gas
flows from the high-pressure compartment 2 through the switching
contacts of the gas blast circuit breaker 3 to the expansion tank
15 and thereafter from this tank through the conduit 37 and a
filter 38 to the low-pressure compartment 14. This results in that
the pressure difference between the compartments 2 and 14 decreases
and the density in the low-pressure compartment 14 increases. The
differential-pressure relay 48 then closes the contact 50, whereby
the compressor 36 is started. Also the density relay 45 comes into
action and closes the contact 47. However, the closing of this
contact 47 does not result in opening the valve 31 of the discharge
conduit 32, since this contact is connected in series with an
auxiliary contact 53 of the differential-pressure relay 48, said
auxiliary contact 53 being closed in the state of equilibrium of
the relay 48 and in the position thereof, in which the contact 49
is closed, but being opened in the position of the relay 48, in
which the contact 50 is closed and the compressor 36 operates. In
that case no gas is lost through the discharge conduit 32
notwithstanding the density in the compartment 14 is too high.
At a predetermined pressure and a predetermined temperature within
the pressure-temperature range, in which the switchgear operates,
the SF.sub.6 -gas can condense. This danger only occurs in the
high-pressure compartment 2. As the condensation of the gas and the
evaporation of the condensate have a disturbing effect on
maintaining the pressure difference between the high- and
low-pressure compartment the heating element 43 is set into
operation as soon as condensate flows into the evaporator 44. Due
thereto the SF.sub.6 -gas in the high-pressure compartment 2 is
heated to get a temperature above the dew point.
As SF.sub.6 -gas is too expensive to be discharged to the open air
the gas-control system shown in FIG. 3 is provided with a
low-pressure storage tank 54, the discharge conduit 32 being
connected to said storage tank 54 through a check valve 55 closing
towards the low-pressure compartments 14 of the switchgear. The
expansion tank 15 is connected to the low-pressure storage tank 54
through the conduit 37, a check valve 56 closing towards the
low-pressure compartment 14, a buffer tank 57 and the filters 38.
The low-pressure storage tank 54 is connected to a high-pressure
storage tank 60 by a conduit 58 provided with a second compressor
59, said high-pressure storage tank 60 being connected to the
supply conduit 29 provided with the electromagnetically operating
valve 28. The gas bottle 30 is connected with the high-pressure
storage tank 60 by a conduit 61 provided with a valve 62 and it has
the task to cover losses of SF.sub.6 -gas by leakage. In this case
the differential-pressure relay 48 is connected to the low-pressure
compartment 14 through the expansion tank 15 and the conduit 39
provided with the check valve 40. This has the advantage that the
differential-pressure relay 48 becomes earlier active and also
starts the compressor 36, during the switching-off process, earlier
than it does in the system shown in FIG. 2, since, during a
switching-off operation, the pressure in the expansion tank 15
rises much quicker than the pressure in the low-pressure
compartment 14.
The relay 45 acting on the density of the gas in the low- pressure
compartment 14 controls by its contact 46 the valve 28 provided in
the supply conduit 29 and by its contact 47 the second compressor
59. Finally a second differential-pressure relay 63 is provided
between the low-pressure compartment 14 and the low-pressure
storage tank 54, the contact 64 of said relay controlling the
compressor 59. This gas-control system operates as follows:
If, due to a rise in temperature in the switchgear, the pressure in
the high-pressure compartment 2 becomes too high in respect of that
in the low-pressure compartment 14 the valve 34 is opened by the
relay 48. This has the effect that gas flows from the compartment 2
to the compartment 14 and from the latter through the conduit 32
and the check valve 55 to the low-pressure storage tank 54. In that
case the density in the low-pressure compartment 14 increases, so
that the relay 45 closes its contact 47 and starts the compressor
59. This compressor sucks the gas excess from the low-pressure
compartment 14 and the low-pressure storage tank 54 and pumps it
into the high-pressure storage tank 60.
If, due to a drop of temperature in the switchgear, the pressure in
the high-pressure compartment 2 becomes too low in respect of that
in the low-pressure compartment 14 the relay 48 starts the
compressor 36. Owing thereto the compartment 2 receives gas from
the compartment 14 and the density of the gas in the low-pressure
compartment 14 decreases and becomes too low, so that the relay 45
closes its contact 46 resulting in that the valve 28 is opened and
gas flows from the high-pressure storage tank 60 to the
low-pressure compartment 14 to supply the gas deficiency. Since no
gas can flow from the low-pressure storage tank 54 to the
low-pressure compartment 14 the pressure in the tank 54 will
temporarily become higher than that in the compartment 14, so that,
depending on the sensibility of the relay 63, the contact 64 is
closed and thereby the compressor 59 is started. The latter
continues to operate till the moment, at which the pressures in the
compartment 14 and the tank 54 will be equal again. From that
moment on the low-pressure compartment 14 and, through this
compartment, the conduit 27, 32, the check valve 55 and the
low-pressure storage tank 54 are fed with gas from the
high-pressure storage tank 60 till the gas in the low-pressure
compartment 14 has reached the right density.
In the stationary state of the switchgear the same pressure obtains
in the expansion tank 15 as in the low-pressure compartment 14
owing to the connection through the conduit 39 provided with the
check valve 40. During the switching-off process the pressure in
the expansion tank rises considerably and the pressure in the
high-pressure compartment 2 falls down. The result thereof is that
the relay 48 comes immediately into action and it starts the
compressor 36 over the contact 50. The density of the gas in the
low-pressure compartment 14 then decreases, so that also the relay
45 is put in action and it opens the valve 28 by means of its
contact 46. Due thereto gas flows from the high-pressure storage
tank 60 to the low-pressure compartment 14. Owing to the fact that
at first the density and the pressure in the low-pressure
compartment 14 decrease, as well as due to the fact that the
extinguishing gas flows from the expansion tank 15 through the
conduit 37, the check valve 56, the buffer tank 57 and a filter 38
to the low-pressure storage tank 54, so that the pressure in the
tank 54 rises, the differential-pressure relay 63 comes into action
and the contact 64 is closed, whereby the compressor 59 is started
and the gas excess is pumped away from the low-pressure storage
tank 54 to the high-pressure storage tank. As soon as the right
pressure difference between the high-pressure compartment 2 and the
low-pressure compartment 14 has been restored and the density in
this compartment 14 has again reached the right value, the relays
45, 48 and 63 return to their rest positions, the compressors 36
and 59 are stopped and the valve 28 is closed.
The storage tanks 54 and 60 are situated outside the switchgear and
no electric current flows through them, so that great temperature
differences can be set up between the gas in these tanks and the
gas in the switchgear. Said temperature differences could disturb
the stationary gas economy in the system. If the switchgear has
been dead for a long time the temperature in the switchgear and
that in the low-pressure storage tank 54 will be equal, so that the
gas densities in the compartment 14 and the tank 54 will also be
equal, since the pressures are equal too. However, if current flows
through the switchgear the temperature in the low-pressure
compartment 14 will become higher than that in the low-pressure
storage tank 54. This results in that, if the pressure does not
change, the density in the storage tank 54 will become higher and
the density in the low-pressure compartment 14 will become lower.
Consequently, there will be no gas transport from the compartment
14 to the storage tank 54. If the density in the compartment 14
becomes too low the relay 45 will come into action, the contact 46
will be closed and the valve 28 will be opened, so that the gas
deficiency in the low-pressure compartment 14 will be supplied from
the high-pressure storage tank 60. It is also possible that in the
considered case, due to the decrease of the density in the
compartment 14, the pressure difference between the high-pressure
compartment 2 and the low-pressure compartment 14 makes the relay
48 operative, so that the valve 34 is opened, which will last till
the moment, at which the required pressure difference has been
restored. From that moment on the pressure in the low-pressure
compartment 14 continues to rise due to the supply of gas from the
storage tank 60, so that the relay 48 comes into action and starts
the compressor 36 again till, at the right density in the
low-pressure compartment 14, the right pressure difference between
the compartments 2 and 14 has again been reached. If after the
termination of this control action the temperature difference
between the gas in the switchgear and the gas in the storage tank
54 decreases again, either owing to an unintended heating of the
tank 54 or, which is more probable, owing to the fact that the
switchgear becomes currentless, the density in the low-pressure
compartment 14 will not change but there will be set up a pressure
difference between the storage tank 54 and said compartment 14,
said pressure difference putting the relay 63 into operation so
that the compressor 59 is started and the pressure in the tank 54
is made equal again to that in the low-pressure compartment 14. The
same control occurs when the temperature in the storage tank 54
would, for whatever reason, become higher than that in the
low-pressure compartment 14.
By the right adjustment of the sensibility of the relays 45, 48 and
63 it is avoided that the control becomes instable. The sensibility
of these relays defines, within which range the density in the
low-pressure compartment 14 and the pressure difference between the
compartments 2 and 14 can be kept constant. It will be obvious,
that it is not possible to keep these quantities exactly
constant.
The buffer tank 57 has the object to level the sudden pressure rise
in the expansion tank 15 due to the switching-off of the gas blast
circuit breaker 3, so that the extinguishing gas which is
contaminated by the switching process flows with low speed through
the filter 38 to the low-pressure storage tank 54. The effect of
the filter is thereby considerably increased.
In the switchgear shown in FIG. 3 each switching unit is provided
with its own set of storage tanks 54, 60 and accessories. In many
cases it is more advantageous to use one single low-pressure
storage tank and one single high-pressure storage tank for a
plurality of switching units. The gas-control system then must be
so arranged, that change of the conditions in a given switching
unit does not affect the conditions in other switching units of the
switchgear. An example of switchgear consisting of a plurality of
switching units which are connected in parallel to a set of storage
tanks 54, 60 is illustrated in FIG. 4.
The gas-control system in FIG. 4 differs from that shown in FIG. 3
in that the differential-pressure relay 63 has been omitted, the
density relay 45 in the low-pressure compartment 14 has the contact
46 only and in the low-pressure storage tank 54 also a relay 65
acting on the density of the gas in said tank has been provided.
The density relay 65 opens and closes the contact 66 inserted in
the circuit of the second compressor 59. This system operates as
follows:
From FIG. 5 it appears that the density relay 65 tries to keep the
density of the gas in the low-pressure compartment 14 at a value
lying on the isochore 25, that means in the present case at a value
of 40 g./liter, by opening and closing the valve 28. Has this value
been reached the relay 45 opens the contact 46 and the valve 28
admitting gas from the high-pressure storage tank 60 into the
compartment 14 is closed. At the temperature T.sub.1 this closing
occurs at the pressure indicated by point 67 and at the higher
temperature T.sub.2 this closing takes place at the higher pressure
indicated by point 68. The density relay 45 closes the contact 46,
when the pressure in the low-pressure compartment 14 becomes a
value equal to one-half kg./cm..sup.2 lower than that defined by
the isochore 25. Consequently, at the temperature T.sub.1 the valve
28 is opened, when the gas pressure in the compartment 14 has
reached the value indicated by point 69 but at the temperature
T.sub.2 the valve 28 will open at the pressure indicated by the
point 70. The density relay 65 in the low-pressure storage tank 54
has been adjusted at higher density values. At the temperature
T.sub.1 the contact 66 of the relay 65 is closed and the second
compressor 59 is started, when the density in said storage tank has
reached a value, at which the pressure in said tank has the value
indicated by the point 71. The contact 66 is opened and the
compressor 59 is stopped again, when the pressure in the storage
tank 54 is returned to the pressure indicated by the point 72, said
pressure being, in the present case, one-half kg./cm..sup.2 lower
than the pressure at the point 71 which starts the compressor. Now,
the adjustment of the density relay 65 is so chosen as to render
the density values indicated by the parallel lines 73 and 74, at
which the second compressor 59 is started and stopped,
respectively, so much higher than the density values given by the
isochore 25 and the line 75 parallel thereto, at which the valve 28
is closed and opened, respectively, that even at the maximum
difference to be expected of say 15.degree. C. between the lower
temperature of the gas in the low-pressure storage tank 54 and the
higher temperature of the gas in the low-pressure compartment 14
the relay 65 puts the compressor 59 in operation at a pressure
which is higher than the pressure, at which the relay 45 closes the
valve 28, and stops the compressor 59 at a pressure, which is
higher than the pressure at which the relay 45 opens the valve 28.
This means, that in the given example, in which the temperatures
T.sub.1 and T.sub.2 differ the expected maximum value of 15.degree.
C. under normal operational circumstances, the point 71 comes to
lie higher than the point 68 and the point 72 comes to lie higher
than the point 70. Only, when the relays 45 and 65 are thus
adjusted, the control of the gas-control system of each switching
unit is stable, which will appear from the following:
If one starts from the state, in which a switching unit is dead,
the gas in the low-pressure compartment 14 of said unit has the
right density defined by the isochore 25 (FIG. 5) and the
temperature of the gas in the low-pressure compartment 14 is equal
to the temperature of the gas in the low-pressure storage tank 54,
that means is equal to T.sub.1, the gas densities and the gas
pressures in said compartment 14 and said storage tank 54 will be
equal too. The gas pressure then is given by the point 67. If
thereafter the switching unit is switched in, so that current flows
through it, the temperature in the low-pressure compartment 14
could, in the most unfavorable situation, rise 15.degree. C. with
respect to the value T.sub.2, so that the pressure in said
compartment 14 will rise to the point 68 and gas will flow from the
compartment 14 through the conduit 32 and the check valve 55 to the
storage tank 54 till the pressure in the tank 54 has become equal
to that in the compartment 14 again. The pressure in the
compartment 14 then is somewhat lower than that indicated by the
point 68 and the pressure in the tank 54 then is somewhat higher
than that indicated by the point 67. Should, for whatever reason, a
further loss of gas occur from the compartment 14, so that the
pressure indicated by the point 70 is reached, the valve 28 will be
opened by the relay 45 till the pressures in the compartment 14 and
the storage tank 54 have again reached the value indicated by the
point 68. Since the point 71, at which the compressor 59 is put in
action, lies higher than the point 68 said compressor will remain
inoperative.
If, owing to the switching-off of the gas blast circuit breaker 3,
gas flows from the high-pressure compartment 2 to the low-pressure
storage tank 54 the pressure therein can exceed the value indicated
by the point 71, so that then the compressor 59 is started and gas
is pumped from the low-pressure storage tank 54 to the
high-pressure storage tank 60 till the pressure in the low-pressure
storage tank 54 (that means also the pressure in the low-pressure
compartment 14) has reached the value indicated by the point 72. As
this value lies above the value indicated by the point 70, at which
the valve 28 is opened by the relay 45, said valve remains closed.
It follows therefrom, that the compressor 59 will not be put into
operation when the valve 28 is in its open position and also that
the valve 28 will not be opened, when the compressor 59 is in
operation, so that the instable situation, in which the valve 28 is
open and the compressor 59 is in operation, whereby gas would be
continuously pumped from the low-pressure storage tank 54 to the
high-pressure storage tank 60 and hence to the low-pressure
compartment 14 and through the conduit 32 back to the low-pressure
storage tank 54, can not occur.
If one has to reckon with a higher maximum temperature difference
than say 15.degree. C. the adjustment of the density relay 65 and
that of the density relay 45 must differ more, that means the lines
73 and 74 will have to lie higher in respect of the lines 25 and 75
than has been illustrated in FIG. 5.
* * * * *