Protective circuit for zinc oxide varistors

The invention provides a circuit for determining excessive energy magnitudes or rates of rise of energy in metal oxide varistor devices. The circuit further provides a series of low voltage control pulses for energizing a high voltage pulse generator. The output of the high voltage pulse generator triggers a protective air gap for bypassing the metal oxide varistor device. One application for the invention is for use within a series capacitor protective circuit.

BACKGROUND OF THE INVENTION 
Series capacitor protective equipment can employ a non-linear zinc oxide 
varistor to limit the magnitude of the voltage across the protected series 
capacitor. Under normal operating conditions load currents flow through 
the series capacitor such that the voltage across the capacitor is the 
product 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 of load current is well within the voltage withstand 
capability of the capacitor. The varistor characteristic is selected such 
that under normal load current conditions the varistor current is limited 
to a few milliamperes. When a fault condition, for example a line to 
ground fault, occurs on the transmission line in which the series 
capacitor is connected to current through the capacitor increases. The 
current increase causes the capacitor voltage to increase and if the 
capacitor voltage is sufficiently high its voltage withstand 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 the 
zinc oxide varistor during line fault conditions may cause damage to the 
varistor if allowed to continue for prolonged periods of time. Because 
excessive energy is dissipated in the varistors some means must be 
provided therefore for limiting the total energy dissipation within the 
varistor itself. 
One means commonly employed to protect equipment from excess energy 
dissipation is the employment of a parallel air gap to bypass at least a 
part of the energy developed during a fault 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 dissipated in the equipment is too high 
the gap will not have sufficient time to operate before the equipment 
fails. 
One of the purposes of this invention is to determine when the magnitude of 
rate of rise of energy dissipation is excessive and to provide low voltage 
pulses to initiate operation of a high voltage pulse generator for 
triggering an air gap when either 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 OF THE INVENTION 
A current sensing device coupled with a combined thermal analog and low 
voltage pulse generator circuit generates low voltage pulses for 
initiating the operation of 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 resistive elements coupled 
with a switching device driven by a voltage comparator. A voltage 
rectifier is used to charge a sensing capacitor for providing input to the 
voltage comparator.

GENERAL DESCRIPTION OF THE INVENTION 
FIG. 1 shows a series capacitor protective circuit which is used for 
example for protecting the series capacitor of a power trasmission line. A 
metal oxide varistor 10 is electrically connected in parallel with the 
capacitor 11 in order to bypass current through capacitor 11 when the 
voltage across the capacitor is excessive. Excessive 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 capacitor when the magnitude or rate of energy dissipation within the 
varistor becomes excessive. An inductive element 17 is electrically 
connected in series with the air gap in order to limit the current through 
both the air gap and the capacitor when the air gap becomes conductive. 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 low voltage 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 protective circuit is coupled to the transmission line at 
terminal L and also at common terminal G. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 is a detailed illustration of the series capacitor protective 
circuit of the invention wherein the sensor circuit 12 includes a first 
current transformer CT.sub.1 and a second current transformer CT.sub.2 for 
monitoring the current through varistor 10 and for providing input to the 
thermal analog and low voltage pulse generator (TA) 13. A second pair of 
current sensors CT.sub.3, CT.sub.4, are provided for redundant operation 
of the sensor circuit and are connected with a second TA 13'. The first 
pair of current transformers CT.sub.1, CT.sub.2, in cooperation with TA 13 
provide low voltage pulses on the positive portion of the varistor current 
wave and the second pair of current transformers CT.sub.3, CT.sub.4, in 
cooperation with the second TA 13' provide low voltage pulses on the 
negative portion of the varistor current wave. 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 for example, of two pulse forming 
networks which are discharged through two separate switching devices into 
one common pulse transformer 19. The output of the high voltage pulse 
generator is connected to the input of pulse transformer 19, and the 
output from the high voltage pulse transformer is connected to the trigger 
electrode 9 of triggered air gap 14 to cause the air gap to become 
conductive. The output of the pulse transformer provides a sequence of 
high voltage pulses in correspondence with the low voltage pulses. Further 
current transformer CT.sub.5 is also coupled to the transmission line and 
provides input power to battery charger 17 which supplies power to 
platform battery 16. The platform battery is used to provide power to 
operate elements 13, 13', and 15. The elements of the series capacitor 
bypass circuit depicted within FIG. 2 are located within separate and 
complete enclosures which in turn are supported upon a raised platform 20. 
The raised platform is electrically isolated from ground by means of a 
plurality of insulating columns 21. 
FIG. 3 shows the TA circuit of FIGS. 1 and 2 in greater detail. CT.sub.1 is 
connected by line 22 to the anode of a first diode D.sub.1 for rectifying 
one-half of the output from current transformer CT.sub.1. Line 24 connects 
between a second diode D.sub.2 and current transformer CT.sub.1 for 
rectifying the other half of the current wave of CT.sub.1. Line 23 
connects the center point of CT.sub.1 to a common terminal G. The cathodes 
of diodes D.sub.1, D.sub.2, are coupled together and are connected with 
capacitor C.sub.1, resistor R.sub.1, and to the input of voltage 
comparator 16. The other lead of C.sub.1 connects to common terminal G. 
The current flows through diodes D.sub.1, D.sub.2 and charges capacitor 
C.sub.1. The voltage across capacitor C.sub.1 is proportional to the 
energy dissipated within varistor 10 because the varistor voltage is 
nearly constant and the current-time integral of the varistor current is 
proportional to the voltage existing across the capacitor. The 
proportionally constant is determined by the values selected for 
components CT.sub.1, C.sub.1 and varistor 10. This is an important feature 
of the thermal analog and low pulse generator circuit of the invention. 
The thermal recovery of varistor 10 after experiencing a fault condition 
("thermal duty") is approximated through the selection of the discharge 
time constant (R.sub.1 C.sub.1). The residual voltage existing across 
capacitor C.sub.1 a short time after a fault condition accounts for the 
fact that the thermal capability of the varistor is reduced when the time 
between successive fault occurrences is sufficiently short. Resistors 
R.sub.2, R.sub.3 electrically coupling between the anodes of diodes 
D.sub.1, D.sub.2 and lines 22, 23, 24, provide an electrical path for the 
output current from CT.sub.1 under normal operating conditions when the 
varistor current is in the order of a few milliamperes. This prevents 
capacitor C.sub.1 from becoming charged under normal operating conditions. 
The function of voltage comparator 16 is to compare the voltage existing 
across capacitor C.sub.1 to a predetermined voltage representing the 
maximum thermal capability of varistor 10. The input impedance of the 
voltage comparaor is selected at a high enough value to prevent C.sub.1 
from becoming discharged through the voltage comparator circuit. In the 
event that the voltage existing across capacitor C.sub.1 exceeds a 
standard reference voltage the output from comparator 16 rises from a low 
voltage to a higher voltage. 
The output from voltage comparator 16 is connected to the base of a 
transistor Q.sub.1. The transistor is biased into a low current state when 
the output voltage of the comparator is low, and is forced into saturation 
when the voltage comparator output is high. The emitter of transistor 
Q.sub.1 is connected to one lead of a resistor R.sub.4 and the other lead 
of resistor R.sub.4 is connected to common terminal G. 
The gate of an SCR is connected to the emitter of Q.sub.1 and to R.sub.4. 
When Q.sub.1 is off the voltage across R.sub.4 is low so that the gate of 
the SCR is off. When Q.sub.1 saturates the voltage across R.sub.4 rises to 
a high enough value to cause the gate of the SCR to operate. 
Transistor Q.sub.1 can be eliminated when the power output from voltage 
comparator is sufficient to drive the gate of SCR directly. CT.sub.2 is 
connected by means of lead 25 to common terminal G and by means of lead 26 
to one side of a burden resistor R.sub.5, one side of non-linear resistive 
element Z.sub.1, and one side of resistor R.sub.6. The other side of 
Z.sub.1 and R.sub.5 are connected to common terminal G. The other side of 
resistor R.sub.6 is coupled with a second non-linear resistive element 
Z.sub.2, resistor R.sub.7 and one side of the low voltage winding of a 
transformer T. The other ends of non-linear resistor Z.sub.2, resistor 
R.sub.7 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 
Z.sub.3. The cathode of the SCR and the other side of non-linear resistor 
Z.sub.3 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 voltage pulses is 
described as follows. Because the rate at which energy is abosorbed by 
varistor 10 is proportional to the current through the varistor the rate 
at which energy is absorbed within the varistor can be determined from the 
crest magnitude of the varistor current. The varistor current is 
represented by a voltage which is developed across resistor R.sub.5 ; 
therefore, the rate at which energy is dissipated in the varistor is 
represented by the crest voltage magnitude across resistor R.sub.5. The 
magnitude of this voltage is sensed by the resistor combination R.sub.6, 
R.sub.7, and non-linear resistor Z.sub.3. Non-linear resistor Z.sub.1 
protects CT.sub.2 against excessively high voltage values. When the 
voltage across resistor R.sub.5 is less than the turn-on voltage of 
non-linear resistor Z.sub.3, very little current flows through resistors 
R.sub.6, R.sub.7, and non-linear resistor Z.sub.3, so that substantially 
all the voltage across resistor R.sub.5 appears across Z.sub.3. When the 
voltage across resistor R.sub.5 is greater than the turn-on voltage of 
non-linear resistor Z.sub.3 current flows through resistors R.sub.6, 
R.sub.7, and non-linear resistor Z.sub.3. The voltage in excess of the 
turn-on voltage of non-linear Z.sub.3 appears across resistors R.sub.6, 
R.sub.7, and the relative values of R.sub.6 and R.sub.7 are adjusted such 
that the majority of the excess voltage appears across resistor R.sub.7. 
The voltage across resistor R.sub.7 however is made small relative to the 
total voltage across resistor R.sub.5 so that small voltage values in 
excess of the required turn-on voltage of non-linear resistor Z.sub.3 will 
be sufficient to generate the required voltage pulses for transformer T. 
This increases the sensitivity of the circuit to small fault current 
increases over a predetermined value. The voltage across resistor R.sub.7 
is increased by means of transformer T to a value high enough to initiate 
the operation of the high voltage pulse generator 15. Since the voltage 
across resistor R.sub.5 varies over a wide range, non-linear resistor 
Z.sub.2 is included in order to limit the maximum voltage which may appear 
across resistor R.sub.7 and thereby prevents excessive voltage pulse 
magnitudes from damaging the high voltage generator circuits. When 
non-linear resistor Z.sub.2 conducts, all the remaining excess voltage 
appears across resistor R.sub.6. 
The voltage pulses which appear across the high voltage side of transformer 
T are in nearly exact electrical 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 
relationship 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 R.sub.8. The other side of 
resistor R.sub.8 is connected to one lead of capacitor C.sub.2. The other 
lead of capacitor C.sub.2 is connected to the other terminal of the high 
voltage side of transformer T. Capacitor C.sub.2 and resistor R.sub.8 form 
a high-pass filter which shapes the voltage wave which appears across the 
high voltage winding of Transformer T. The voltage which appears across 
resistor R.sub.8 is the signal that initiates the operation of 
high-voltage pulse generator 15. 
When the SCR is caused to conduct by means of the output from voltage 
comparator 16 the voltage across non-linear resistor Z.sub.3 drops to near 
zero. This causes current to flow through resistors R.sub.6 and R.sub.7 
when any voltage appears across resistor R.sub.5. Voltage pulses therefore 
appear across the high voltage side of transformer T whenever the SCR is 
caused to conduct. 
FIG. 4 is one type of a voltage comparator circuit 16 for use within the 
circuit of FIG. 3. The voltage comparator 16 of FIG. 4 contains a 
plurality of transistors Q.sub.2, Q.sub.3, Q.sub.4 interconnected by means 
of a plurality of resistors R.sub.9, R.sub.10, R.sub.11, R.sub.12, and 
R.sub.13 and at least one non-linear resistor Z.sub.4 (i.e. Zener diode) 
for the purpose of providing an output voltage on line 27 when the 
predetermined threshold voltage is exceeded. The output voltage comparator 
16 is connected to the base of transistor Q.sub.1 (FIG. 3) by means of 
lead 27 and causes transistor Q.sub.1 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 
FIGS. 3 and 4 it is to be clearly understood that other types of voltage 
comparator circuits may also be employed. 
Although the zinc oxide varistor protective circuit of the invention is 
disclosed for the purpose of protecting 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.