Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROTECTIVE CIRCUIT FOR
ZINC OXIDE VARISTORS
Background of the Invention
Series capacitor protective equipment can employ a
non-linear ~inc oxide varistor to limit the magnitude of
the voltage across the protected series capacitor. Under
normal operating conditions load currents ~low through ~he
series capacitor such that the voltage across the capacitor
is the produc~ of the load current and the capacitive
reactance. The voltage withstand of the capacitor is
selected such that the capacitor voltage caused by the
flow o load current is well within the Yoltage with-
stand capability o~ the capacitor. Tha varistor
_ characteristic is selected such that under normal load
current conditions the varistor current is limited to
a few milliamperes. ~en a fault condition, for example
a line to ground fault, occurs on ~he transmission line
in which the series capacitor is connectèd the current
through ~he capacitor increases. The current increase
causes the capacitor voltage to increase and if the
capacitor voltage is sufficientiy high its voltage with-
stand capability is exceeded. To prevent the occurrence
of excess voltage across the capacitor the zinc oxide
varistor provides an alternative path for the fault
current causing the excess capacitor voltage. However
the current flow through ~he zinc oxide varistor during
line fault conditions may cause damage to the varistor
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if allo~ed to continue for prolonged periods o~ time.
Because excessive energy is dissipa~ed in the varistors
some means must be provided therefore for limiting
the total energy dissipation wi~hin the varistor itsel~.
One means commonly employed to protect equipment
from excess energy dissipation is the employment of a
pa~allel air gap to bypass at least a part o~ the energy
developed during a ~ault situation. One of the problems
involved with the employment of triggered air gap devices
is that a means must be provided to determine when the
energy dissipated by the equipment becomes excessive.
Another problem involved is to determine when the rate
at which the energy is dissipated within the equipment
becomes excessive. When the rate at which energy is
lS dissipated in the equipment is too high the gap will not
have sufficient time to operate before the eqtlipment fails.
One of the purposes of this invention is to deter-
mine when the magnitude of rate o~ rise of energy dissi-
pation is excessive and to provide low voltage pulses to
initiate operation of a high voltage pulse generator for
triggering an air gap when eith,er of these conditions
exist. A second purpose of the invention is to provide
the low voltage initiating pulses at times when the
voltage across the air gap is at or near its maximum value.
Summary o~ the Invention
A current sensing device coupled with a combined
~hermal analog and low voltage pulse generator circuit
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generates low Yoltage pulses or initiating the opera-
tion o a high voltage pulse generator to trigger an
air gap device.
The combined thermal analog and low voltage pulse
generator circuit comprises a combination of current
sensors- and resisti~e elements coupled with a switching
deYice driven by a voltage comparator. A voltage
rec~ifier is used to charge a sensing capacitor for
- providing input to the voltage comparator.
Brief D-es'cri'pti'o'n of the Dra'wings
.
FIGURE l is a block diagram representation o one
type o~ a series capacitor protective circui~;
FIGURE 2 is a detailed circuit representation o~
the series capacitor protective circuit o FIGURB 1;
. FIGURE 3 is a circuit representa~ion o the thermal
analog and low voltage pulse generator according to the
invention; and - :
FIGURE 4 is a circuit diagram of one type o~ a
voltage comparator circuit for ùse within the circuit
of FIGURE ~ '
General Description o the Invention '
FIGURE 1 shows a seriès capacitor protective circuit
which is used for example for protecting the series
capacitor of a power transmission line. A metal oxide
varistor lO'is electrically connected in parallel with the
capacitor 11 in order to bypass current through capacitor
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11 when the voltage across the capacitor is excessive.
~cessive voltages develop, for example, when a line
to ground fault occurs on the transmission line. A
triggered air gap device 14 is electrically coupled in
parallel with both the metal oxide varistor and the
capacitor to bypass both the varistor and the capaci-
tor when the magnitude or rate of energy dissipation with-
in the varistor becomes excessive. An inductive element
17 is electrically connected in series with the alr gap
in order to limit the current through both the air gap
and the capacitor when the air gap becomes conductivè.
A sensor device 12 is used to monitor the current through
the varistor for providing input to a low voltage pulse
generator, and thermal analog circuit 13. The combined
1~ low vol~age pulse generator and thermal analog circuit
is connected to a high voltage pulse generator 15 which
in turn provides high voltage~ pulses to the triggered
air gap 14. The series capacitor protectivè circuit is
coupled to the transmission line at terminal L and also
at common terminal G.
Descri~tion o the Preferred Embodiment
. . _ , .
FIGURE 2 is a detailed illustration of the series
capacitor protective circuit of the invention wherein
the sensor circuit 12 includes a first current trans-
former CTl and a second current transformer CT2 for
monitoring the current through varistor -10 and for
providing input ~o the thermal analog and lot~ voltage
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pulse generator (TA) 13. A second pair of current
sensors CT3, CT4, are provided for redundant operation
of the sensor circuit and are connected with a second
TA 13 t . The first pair of current transormers CTl,
CT2, in coopera~ion with TA 13 provide low voltage
pulses on the positive portion Qf the varistor current
wave and the second pair of current transformers CT3,
CT4, in cooperation with the second TA 13' pro~ide
low voltage pulses on the negative portion of the
varistor current wa~e. The output from both TA 13~ 13'
are coupled to the input of high voltage pulse generator
15. The high voltage pulse generator can consist ~or
example, of two pulse forming ne~works which are dis- .
charged through two separate switching. devices into one
lS ~ommon pulse transformer 19. The output of the high
voltage pulse generator is connected ~o the input of
pulse transormer 19, and the output from the high
voltage pulse transformer is connected to the trigger
electrode 9 o~ triggered air gap 14.to cause the air ~ap
to become conductive. The output of the pulse trans~ormer
provides a se~uence of high Yol,tage pulses in correspondence
with the low voltage pulses.~Purther current transformer
.
CT5 is also coupled to the transmission line and provides
input pow~r to battery charger 17 which supplies power
to platform battery 16. The platform battery is used to
provide power to operate elements 13, 13l9 and 15. The
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elements of the series capacitor bypass circuit depicted
within FIGURE 2 are located within separate and complete
enclosures which in turn are supported upon a raised
platform 20. The raised platform is electrically isola~ed
from groundby means of a plurality of insulating columns
~1 .
FIGURE 3 shows the TA circuit of FIGURE 1 and 2
in greater detail. CTl is connected by line 22 to the
anode of a first diode Dl for rectifying one-half of the
output from current transformer CTl. Line 24 connects
between a second diode D2 and current ~ransormer CTl
for rectifying the other half of the current wave of CTl.
Line 23 connects the center point o CTl to a common terminal
G. the cathodes of diodes Dl, D2, are coupled together and
lS are connected with capacitor Cl, resistor Rl, and to ~he
input of voltage comparator 16. The other lead of Cl
connects to common terminal G. The current flow~ through
diodes Dl, D2 and charges capacitor Cl. The voltage
across capacitor Cl is proportional to the energy dissi-
pated within varistor 10 because the varistor voltage is
nearly constant and the current-time integral of the
varistor current is proportiona~ to the voltage existing
across the capacitor. The proportionally constant is
determined by the values selected for components CTl, C
and varistor 10. This is an important feature of the
thermal analog and low pulse generator circui~ of ~he
invention.
The thermal recovery of varistor 10 after experienc;na
a fault condition ("thermal duty") is approximated tnrough
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the sPlection of the discharge time constant (RlCl).
The residual voltage existing across capacitor Cl a
short time after a fault condition accounts ~or the fact
that the thermal capability of the varistor is reduced
when the time between successive fault occurrences is
sufficiently short. Resistors R2, P~3 electrically
- coupling between the anodes of diodes Dl, D2 and lines
22, 23~ 24, provide an electrical path for the ou~put
current from CTl under normal operating conditions when
~he varis~or current is in the order of a few milli-
amperes. This prevents capacitor Cl from becoming
charged under normal operating conditions.
The function of voltage comparator 16 is to compare
the voltàge existing across capacitor Cl to a predeter-
lS minea voltage representing the maximum thermal capability
of varistor 10. The input impedance of the vol~age
comparator is selected at a high enough value to prevent
Cl from becoming discharged through the voltage compara-
tor circuit. In the event that the voltage existing
across capacitor Cl exceeds a standard reference voltage
the output from comparator 16 r,ises from a low voltage
to a higher voltage.
The output from voltage comparator 16 is connected
to the base of a transistor Ql The tr~nsistor is biased
into a low current state when the output voltage of the
comparator is low, and is forced into saturation ~hen
the voltage comparator output is high. The emitter of
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transistor Ql is connected to one lead of a resistor
R4 and the other lead of resistor R4 is connected to
common terminal G.
The gate of an SCR is connected to the emitter of
Ql and to R4. When Q is off the voltage across R~ is
low so that the gate of the SCR is of. ~en Ql satu-
rates the ~oltage across R4 rises to a high enough
value to cause the gate of the SCR to operate.
Transistor Ql can be eliminated when the power
output from voltage comparator is sufficient to drivs
the gate of SCR directly. CT2 is connected by means of
lead 25 to common terminal G and by means of lead 26 ~o
one side of a burden resistor R5, one side of non-linear
resistive element Zl' and one side of resistor R6. The
other side of Zl and R5 are connected to common terminal
G. The other side of resistor R6 is coupled with a
second non-linear resistive element Z2~ resistor R7 and
one side of ~he low voltage winding of a transormer T~
The other ends o non-linear resistor Z2' resistor R7
and the low voltage winding of transformer T are connected
together and to the anode of the SCR and one end of non-
linear resistor Z3. The cathode of the SCR and the other
side of non-linear resistor Z3 are connected to common
terminal G.
The mechanism by which the above described circuit
detects high rates of rise of energy within varistor 10
and generates low vol~age pulses is described as follows.
Because the rate at which energy is abosorbed by varistor
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10 is proportional to the current through the varis~or
the rate at which energy is absorbed within the varistor
can be determined from the crest magnitude of the
varistor curren~. The varistor current is represented
by a voltage which is developed across resistor R5;
therefore, the rate at which energy is dissipated in the
varistor is represented by the crest voltage magnitude
across resistor R5. The magnitude of this voltage is
sensed by the resistor combina~ion R6, R7, and non-linear
resistor Z3. Non-linear resistor Zl protects CT
against excessively high voltage values. ~en the
voltage across resistor R5 is less than the turn-on
voltage of non-linear resistor Z3, very little current
10ws through resistors R6, R7, and non-linear resistor Z3,
lS so that substantially all the voltage across resistor R5
appears across Z3. When the voltage across resistor R5
is greater than the tuTn-on voltage of non-linear resistor
Z3 current flows ~hrough resistors R6, R7, and non-linear
resistor Z3. The voltage in excess of the turn-on voltage
of non-linear Z3 appears across resistors R6, R7, and the
relative values of R6 and R7 are adjusted such tha~ the
majority of the excess voltage appears across resistor R70
The voltage across resistor R7 however is made small
relative to the total voltage across resistor R5 so that
small voltage values in excess of the required ~urn-on
voltage of non-linear resistor Z3 will be sufficient to
generate the required voltage pulses for transformer T.
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This increases the sensitivity o the circuit to small
fault current increases over a predetermined value. The
voltage across resistor R7 is increased by means o
transformer T to a value high enough to initiate the
operation of the high voltage pulse generat.or 15. Since
the voltage across resistor R5 varies over a wide range,
non-linear resistor Z2 is included in order to limit
- the maximum voltage which may appear across resistor R7
and thereby prevents excessive voltage pulse magnitudes
from damaging the high voltage generator circuits. ~en
non-linear resistor Z2 conducts, all the remaining excess
voltage appears across resistor R6.
The voltage pulses which appear across the high
voltage side of transformer T are in nearly exact electri-
1~ cal phase with the voltage developed across varistor 10.
This means that the high voltage pulses developed by the
high voltage pulse generator 15 are in electrical phase
with the voltage maxima which appear across the triggered
air gap. This electrical phase rela~ionship is another
important feature of the invention.
One lead of the high voltage winding of transformer T
is connected to common terminal G, and to one side of
resistor R8. The other side of resistor R8 is connected
to one lead of capacitor C2. The other lead of capacitor
C2 is connected to the other terminal of the high voltage
side of transformer T. Capacitor C2 and resistor R8 form
a high-pass filter which shapes the voltage wave which
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appears across the high voltage winding o Transformer T.
The voltage which appears across resistor R8 is the signal
tha~ initiates the operation of high-voltage pulse
generator 15.
When the SCR is caused to conduct by means of the
output from voltage comparator 1$ the voltage across non-
linear resistor Z3 drops to near zero. This causes current
to flow through resistors R6 and R7 when any voltage
appears across resistor R5. Voltage pulses therefore
appear across the high voltage side of transformer T
whenever the SCR is caused to conduct.
FIGURE 4 is one type of a voltage comparator circuit
16 for use within the circuit of FIGURE 3. The voltage
comparator 16 of FIGURE 4 contains a plurality of transis-
tors Q2' Q3. Q4 interconnected by means of a plurality of
9, ~ Rl~ ~2~ and R13 and at least ane non-
linear resistor Z4 ~i.e. Zener-diode) for the purpose of
providing an output voltage on line 27 when the prede~er-
mined threshold voltage is exceeded. The output voltage
~ comparator 16 is connected to the base of transistor Ql
(FIG. 3) by means of lead 27 and causes transistor Ql to
become operational as described earlier. Although the
configuration of transistors, resistors and non-linear
resistive element is used for the voltage comparator 16
of FIGURES 3 and 4 it is to be clearly understood that
other types of voltage comparator circuits may also be
employed .
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Although the zinc oxide varistor protective circuit
of the invention is disclosed or the purpose of pro-
tecting varistors in series capacitor applications on
high voltage transmission lines this is by way of
example only. The zinc oxide protective circuit of the
invention finds application wherever zinc oxide varistors
are to be protected.
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