Protective Circuit for Zinc Oxide Varistors

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.

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.

FIG. 1 is a block diagram representation of one type of a series capacitor protective circuit;

FIG. 2 is a detailed circuit representation of the series capacitor protective circuit of FIG. 1;

FIG. 3 is a circuit representation of the thermal analog and low voltage pulse generator according to the invention; and

FIG. 4 is a circuit diagram of one type of a voltage comparator circuit for use within the circuit of FIG. 3.

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.

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.