CA1123895A - Protective circuit for zinc oxide varistors - Google Patents
Protective circuit for zinc oxide varistorsInfo
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- CA1123895A CA1123895A CA329,404A CA329404A CA1123895A CA 1123895 A CA1123895 A CA 1123895A CA 329404 A CA329404 A CA 329404A CA 1123895 A CA1123895 A CA 1123895A
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Abstract
PROTECTIVE CIRCUIT FOR
ZINC OXIDE VARISTORS
Abstract of Invention 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 pro-tective air gap for bypassing the metal oxide varistor device. One application for the invention is for use within a series capacitor protective circuit.
ZINC OXIDE VARISTORS
Abstract of Invention 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 pro-tective air gap for bypassing the metal oxide varistor device. One application for the invention is for use within a series capacitor protective circuit.
Description
3~395 5~5595 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 ~23i~
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 1123~5 5D5595 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 .
-~ 1 Z3~ 9 5 5DS595 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 . ~23~5 5D5595 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 ~i2389$ SD5595 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 ~.2~
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 38~i 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 -11238~5 s DS595 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.
`` 1 1 23~ ~ 5 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 i 1~ 38 ~ 5 5D5595 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 .
~3 8 ~ ~
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.
. -12-
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 ~23i~
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 1123~5 5D5595 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 .
-~ 1 Z3~ 9 5 5DS595 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 . ~23~5 5D5595 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 ~i2389$ SD5595 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 ~.2~
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 38~i 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 -11238~5 s DS595 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.
`` 1 1 23~ ~ 5 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 i 1~ 38 ~ 5 5D5595 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 .
~3 8 ~ ~
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.
. -12-
Claims (21)
1. A protective circuit for zinc oxide varistors comprising in combination:
at least one zinc oxide varistor connected within an electric circuit;
a triggered switching device electrically coupled in shunt relationship with the varistor for bypassing current from the varistor under a condition of varistor overload;
at least one current sensor electrically connected within the circuit for sensing current flow through said varistor and for generating an electric signal in response to the varistor overload condition, said current sensor being directly connected to an input of a rectifier circuit having an output directly connected to a first capacitor and to a first resistor for generating a voltage across the capacitor which is directly proportional to the energy dissipated within the varistor during said overload condition; and pulse generator circuit means for providing a high voltage pulse to the triggered device for causing the triggered device to become conductive when said overload condition occurs.
at least one zinc oxide varistor connected within an electric circuit;
a triggered switching device electrically coupled in shunt relationship with the varistor for bypassing current from the varistor under a condition of varistor overload;
at least one current sensor electrically connected within the circuit for sensing current flow through said varistor and for generating an electric signal in response to the varistor overload condition, said current sensor being directly connected to an input of a rectifier circuit having an output directly connected to a first capacitor and to a first resistor for generating a voltage across the capacitor which is directly proportional to the energy dissipated within the varistor during said overload condition; and pulse generator circuit means for providing a high voltage pulse to the triggered device for causing the triggered device to become conductive when said overload condition occurs.
2. The circuit of claim 1 wherein the rectifier comprises a pair of first and second diodes.
3. The circuit of claim 2 wherein the current sensor comprises at least one current transformer coupled with said first and second diodes and a common terminal within the circuit to provide charging current to said first capacitor.
4. The circuit of claim 1 further including a voltage comparator circuit for comparing the voltage developed across the first capacitor to a reference voltage and for providing an output signal when the voltage across the first capacitor exceeds the reference voltage.
5. The circuit of claim 4 including a gated switch controlled by the output signal of the voltage comparator whereby said switch becomes conductive when the output signal reaches a predetermined value.
6. The circuit of claim 5, including an amplifier for increasing the output signal power to a level sufficient to operate said switch.
7. The circuit of claim 6 wherein the switch comprises a silicon controlled rectifier.
8. The circuit of claim 5 further including a first transistor amplifier electrically connected with an output from the voltage comparator and with a gate of the silicon controlled rectifier for providing gating voltage to the silicon controlled rectifier when the output signal from the voltage comparator causes the transistor to become conductive.
9. The circuit of claim 8 further including a second resistor electrically connected between the gate of the silicon controlled rectifier, the cathode of the silicon controlled rectifier, and the common terminal within the circuit for generating a voltage of sufficient value to operate the gate.
10. The circuit of claim 9 further including a third resistor connected between the anode of the first diode and the common terminal and a fourth resistor connected between the anode of the second diode and said common terminal for providing a current path for said first current transformer under normal varistor operation.
11. The circuit of claim 10 further including a transformer having a primary and secondary winding wherein the anode of the silicon controlled rectifier is connected to one lead of the primary for providing a voltage across the primary when the silicon controlled rectifier becomes conductive.
12. A protective circuit for zinc oxide varistors comprising in combination:
12. A protective circuit for zinc oxide varistors comprising in combination:
Claim 12 continued:
at least one zinc oxide varistor connected within an electric circuit;
a triggered switching device electrically coupled in shunt relationship with the varistor for bypassing current from the varistor under a condition of varistor overload;
at least one current sensing means electrically connected within the circuit for generating an electric signal in response to the varistor overload condition;
pulse generator circuit means for providing a high voltage pulse to the triggered device for causing the triggered device to become conductive;
an electrical analog circuit having at least one rectifier electrically coupled with a first capacitor for developing voltage across said first capacitor in direct proportion to energy dissipated in the varistor for determining when the varistor overload occurs and for causing the pulse generator circuit to provide the high coltage pulse when the overload occurs, and consisting of a first resistor electrically connected in parallel with the first capacitor for providing an RC circuit to generate an electrical analog of the thermal recovery of the varistor, a second resistor electrically connected between the gate of the silicon controlled rectifier, the cathode of the silicon controlled rectifier, and the common terminal within the circuit for generating a voltage of sufficient value to operate the gate, a third resistor connected between the anode of the first diode and the common terminal and a fourth resistor connected between the anode of the second diode and said common terminal for providing a current path for said first current transformer under normal varistor operation; and a fifth resistor connected in parallel with a second current transformer for generating a voltage across the fifth resistor in proportion to current through the second current transformer, said second current transformer being connected at one end to said common terminal.
at least one zinc oxide varistor connected within an electric circuit;
a triggered switching device electrically coupled in shunt relationship with the varistor for bypassing current from the varistor under a condition of varistor overload;
at least one current sensing means electrically connected within the circuit for generating an electric signal in response to the varistor overload condition;
pulse generator circuit means for providing a high voltage pulse to the triggered device for causing the triggered device to become conductive;
an electrical analog circuit having at least one rectifier electrically coupled with a first capacitor for developing voltage across said first capacitor in direct proportion to energy dissipated in the varistor for determining when the varistor overload occurs and for causing the pulse generator circuit to provide the high coltage pulse when the overload occurs, and consisting of a first resistor electrically connected in parallel with the first capacitor for providing an RC circuit to generate an electrical analog of the thermal recovery of the varistor, a second resistor electrically connected between the gate of the silicon controlled rectifier, the cathode of the silicon controlled rectifier, and the common terminal within the circuit for generating a voltage of sufficient value to operate the gate, a third resistor connected between the anode of the first diode and the common terminal and a fourth resistor connected between the anode of the second diode and said common terminal for providing a current path for said first current transformer under normal varistor operation; and a fifth resistor connected in parallel with a second current transformer for generating a voltage across the fifth resistor in proportion to current through the second current transformer, said second current transformer being connected at one end to said common terminal.
13. The circuit of claim 12 further including a sixth resistor, a seventh resistor and a first non-linear resistor connected in series and across the fifth resistor for generating a voltage across the seventh resistor when the voltage across the fifth resistor is higher than a predetermined turn-on voltage for the first non-linear resistor
14. The circuit of claim 13 further including a second non-linear resistor electrically connected in parallel with the seventh resistor for limiting the magnitude of the voltage across the seventh resistor to a predetermined value.
15. The circuit of claim 14 wherein the seventh resistor is electrically coupled with the primary of the transformer for providing a voltage across the primary.
16. The circuit of claim 13 wherein the resistance value of the sixth resistor is selected to limit the total current flow through the series parallel combination of the second non-linear resistor, the seventh resistor, the silicon controlled rectifier and the first non-linear resistor to a predetermined value.
17. The circuit of claim 16 further including a third non-linear resistor, in parallel with the fifth resistor to limit the total voltage developed across the fifth resistor to a predetermined value.
18. The circuit of claim 17 wherein the secondary winding of the transformer is connected to the high voltage pulse generator input for causing the high voltage pulse generator to deliver a high voltage pulse to the trigger electrode when a voltage appears across the seventh resistor.
19. The circuit of claim 18 wherein the trigger gap device further includes a pair of main gap electrodes and wherein high voltage pulse at the trigger electrode is caused to be in electrical phase with the crest voltages appearing across the main gap electrodes.
20. The circuit of claim 18 further including a second capacitor and an eighth resistor connected in series and across the transformer secondary for decreasing the pulse width of the signal applied to the high voltage pulse generator input.
21. A control circuit for eliminating varistor over-load comprising:
at least one current transformer for sensing the varistor current and for charging a capacitor through at least one diode and having a voltage value in proportion to energy dissipated within the varistor;
a voltage comparator for comparing the capacitor voltage to a reference voltage and for causing a silicon controlled rectifier to conduct whereby voltage signals are transmitted to a high voltage pulse generator to operate a triggered air gap connected across the varistor.
at least one current transformer for sensing the varistor current and for charging a capacitor through at least one diode and having a voltage value in proportion to energy dissipated within the varistor;
a voltage comparator for comparing the capacitor voltage to a reference voltage and for causing a silicon controlled rectifier to conduct whereby voltage signals are transmitted to a high voltage pulse generator to operate a triggered air gap connected across the varistor.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA329,404A CA1123895A (en) | 1979-06-08 | 1979-06-08 | Protective circuit for zinc oxide varistors |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA329,404A CA1123895A (en) | 1979-06-08 | 1979-06-08 | Protective circuit for zinc oxide varistors |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1123895A true CA1123895A (en) | 1982-05-18 |
Family
ID=4114396
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA329,404A Expired CA1123895A (en) | 1979-06-08 | 1979-06-08 | Protective circuit for zinc oxide varistors |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1123895A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4636910A (en) * | 1985-04-11 | 1987-01-13 | Canadian General Electric Company Limited | Varistor overvoltage protection system with temperature systems |
-
1979
- 1979-06-08 CA CA329,404A patent/CA1123895A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4636910A (en) * | 1985-04-11 | 1987-01-13 | Canadian General Electric Company Limited | Varistor overvoltage protection system with temperature systems |
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