CA1202669A - Series capacitor protective circuit - Google Patents

Abstract

ABSTRACT

A series capacitor protective circuit is provided in which a linear resistor in series with a non-linear resistor are connected in parallel with the series capacitor. A spark gap is also connected in parallel with the series capacitor and the series connected resistors.
The spark gap conducts current, to protect the series capacitor, when triggered into conduction or when the volt-age across it exceeds a preselected value.

Description

lZ~Z~9 SERIES CAPACITOR PROTECTIVE CIRCUIT

BACKGROUND OF THE INVENI'ION
The present invention relates to circuits employed to protect capacitors which are placed in series with power lines. More particularly, this invention relates to a protective circuit which employs both linear and non-linear resistors in a series branch circuit which is connected in parallel with the series capacitor.
Networks of power lines are employed to transmit electrical power from generating sites to places o consump-tion. Each power line has multiple conductors which have distributed self and mutual inductances. In medium and long distance power lines, it is desirable to compensate for the effective inductance of the power line by inserting series capacitors in each current carrying conductor and thereby promoting more efficient power flow and network stability.
It is desirable to maintain, to the maximum extent possible, the contribution to system stability provided by series capacitors. Should network stability be lost~ generators must be removed from the network to permit resynchronization of the network. Duxing the resynchronization period, service is typically lost over wide geographic areas.
As with other elements, capaci~ors have finite voltage, current, and power ratings, which cannot be greatly exceeded without threat of serious injury to, or destruction of, the capacitors. It is not economically feasable to use greatly overrated capacitors for a given application due to their increased cost At present, suggested industry standards specify a minimum lifetime of 30 minutes for a capacitor ~ Z~z ~ ~ P55-~88 subject to a voltage of 1.35 per unit: i.e. a voltage 35%
over the nominal voltage appearing across the capacitor terminals in normal operation. Unfortunately, fault condi-tions frequently occur in a power network which, absent some means of protection, would stress the capacitor to failure.
If the fault is on the same line as the capacitor, failure would be rapid because of the magnitude of the resulting fault current. When the ~ailure is on another line in the network; a lesser off line fault current results and failure would be less rapid.
Various protective circuits have been employed to protect series capacitors under fault conditions. I~he simpler series capacitor protective networks employ a spark gap in shunt, or connected in parallel, with the series capacitor. The current through a capacitor is proportional to the rate at which voltage changes across it. when a spark gap begins to conduct, the voltage changes across it in a nearly discontinuous manner. Therefore a damping reactor is usually inserted in series with the spark gap to protect the series capacitor. After the spark gap fires and begins to conduct during fault conditions, essentially all current flows around the capacitor, thereby preventing excess capacitor voltage. During this period of gap conduc-tion, the series capacitor's contribution to stability is almost totally lost~ This loss occurs simultaneously with the additional threat to stability posed by excessive fault currents. If spark gaps which are not self-extin~uishing are employed~ delays associated with circuit breaker opera-tion are likely, thereby lengthening the duration of gap conduction. Further, circuit breakers have a relatively -- 3 ~

~L2~Zf~ P55-8488 high statistical probability of fallure, and are often regarded as the weakest link in a power system. But when spark gaps of the self-extinguishing type are used, they may cause the introduction of high frequency power transients, resulting in high capacitor voltages.
With either type of spark gap, once the fault is cleared, reinsertion of ~he series capacitor is li~.ely to occur near a current zero or reversal. Unless reinsertion resistors are employed to limit current into the series capacitor, voltages high enough to cause the spark gap to conduct again are possible. As line current is reintroduced into the series capacitor near a current ~ero, the absence of charge on the capacitor may result in a direct current offset. The discontinuous change in circuit impedance, associated with a direct current offset, can produce sub-synchronous voltage oscillations which may cause high capacitor voltage, possibly exceeding twice rated, and which may persist for several cycles. Another adverse effect of protective networks for series capacitors primarily relying on spark gaps is high frequency standing wave phenomena, which produce excessive voltages on unfaulted lines.
The above disadvantages associated with spark gaps can be avoided by restricting the use of spark gaps to situations where other protective components would not be able to dissipate the energy associated with bypassing high fault currents. Non-linear resistors, ~or example those manufactured principally from silicon carbide ox metal oxides, are such other protective components. Their higher resistance at lower voltage, which decreases to a much lower resistance at higher voltage, results in a voltage-amperage ~ Z()~ P55-84 characteristic which is analogous to that associated with spark gaps. When the vol~age is low, only relatively small amounts of current wlll flow through the non-linear resistor; above the knee of the characteristic in the transistion between high and low resistance values, roughly corresponding to the arcing voltage of a comparable spark gap, relatively high current flow occurs without a great incremental increase in voltage. This voltage-amperage characteristic is superior to that of a spark gap in khat abrupt changes, amounting to discontinuities, do not exist.
However, since the non-linear resistor is conducting the major portion of the fault current under fault conditions~
it must be able to dissipate the resulting energy, unless other protective means are also employed. It is not presently economical to provide non-linear resistors capable of dissipating all of the energy associated with all possible fault currents foreseeable in typical instal-lations. Customarily back up protective means of the con-ventional spark gap type are employed to conduct fault current which would otherwise cause excessive power dissipation in the non-linear resistor. When the back up protective means is appropriately coordinated with the non-linear resistor ~o avoid excessive dissipation, the non-linear resistor will conduct at lower fault current levels allowing the series capacitor to continue making its con-tribution to stability. These lower fault currents are often encountered when the actual fault occurs on another power line than when the fault occurs on con~uctors in the power line.

U. S. Patent No. 4,028,S92 issued June 7, 1977 to ~Z~2669 P55-8~88 Fahlen shows in Fig. 1 the application of a non-linear resistor shunting a series capacitor. Other figures show a non-linear resistor in series with a ~park gap shuntiny a series capacitor. These later arranyements are particularly advantageous if the non-linear resistor is manufactured principally from silicon carbide. The typical voltage-amperage characteristic for a silicon carbide non-linear resistor, exhibits a lower resistance below the knee than many other non-linear resistors, for example most of those manufactured principally from metal oxides.
Relatively high levels of current would be conducted by a silicon carbide non-linear resistor during normal opera-tion; but ~or the spark gap~
The comparatively high resistance of zinc oxide varistors operating below the knee oE a typical voltage-amperage curve, renders the use of a spark gap, in series with it, unnecessary and avoids the drawbacks which accompany the use of series spark gaps. U. S. Patent No.
4,174,529 issued November 13, 1979 to Hamann shows the use of a zinc oxide non-linear resistor (varistor) in a series capacitor protective circuit. The varistor is connected in parallel with the series capacitor as the sole element of its branch circuit. The series capacitor and the varistor are both protected by a triggerable spark gap in series with a damping reactor connected in parallel across them. The triggerable spark gap is to be triggered into conduction when current sensors in the varistor branch or associated circuitry indicate or anticipate excessive current in the varistor branch Present technology will not economically allow 12~2~ P55-8488 the manufacture of metal oxide varistors, with the required ability to withstand the high voltages and dissipate the high power associated with fault currents in power lines, in a single monolithic block. Individual blocks are assembled in series, forming stacks to withstand high voltages.
Series stacks are assembled in parallel to attain the required power dissipation levels. The individual series stacks to be used within a single assembly must be carefully tested, at some expense, to insure that each series stack has nearly identical voltage-amperage characteristics. If one of the series stacks should have a signiEicantly lower resistance than the others, it may fail from excessive dissipation well before the others and lead to a progressive total failure of the assembly. Since metal oxides resistors are relatively expensive, compared to linear resistors, and their failure may jeopardize either the series capacitors or system stability, or both; such progressive failures should be guarded against.

SUMMARY OF THE INVENTION
With the present invention there is provided a series capacitor protective circuit employing non-linear resistor elements to obtain maximized utilization of the series capacitor during low current fault conditions~ while reducing power dissipation in the non-linear resistor elements and enhancing the reliability of the protective circuit.
The series capacitor protective circuit of the present invention includes a first branch circuit connected in parallel relationship with the series capacitor to be ~ 7 --~2~2~

01 protected. The Eirst branch circuit includes ~
02 non-linear resistor connected in series with a linear 03 resistor. Physically, the non linear and lin~ar 04 resistors may be interconnected in series and parallel 05 relation. The relationship of the ohmic values 06 between the linear and non-linear resistors is 07 selected to dissipate maximum po~er in the linear 08 resistor when it is conduc-ting low fault currents 09 without permitting voltages harmful to the series capacitor. A second branch circuit contains a 11 triggerable spark gap connected in parallel 12 relationship with the first branch circuit to conduct 13 high levels of fault current and pro-tect both the 14 series capacitor and the resistors in the first branch circuit.
16 More generally, the preferred embodiment 17 of the invention is a series capacitor protective 18 circuit comprising of ~irst branch circui-t connected 19 in parallel with the series capacitor, ~he first branch circui~ including non-linear and linear 21 resistive elements, which are interconnected so tha-t 22 the electrical characteristics of the first branch 23 circuit can be represented by a non-linear resistor 24 connected in series with a linear resistor. The resistive value of the non-linear and linear resistive 26 elements is selected to maximize power dissipation in 27 the linear resistive element and minimizes power 28 dissipation in the non-linear resistive element when 29 low level fault currents arise, without permitting excessive series capacitor voltages. A second branch 31 circuit is connec-ted in parallel with both the first 32 branch circuit and the series capacitor, the second 33 branch circuit including a spark gap. The ~irst 34 branch circuit conducts low levels of fault current and the second branch circuit conducts high levels of 36 fault current.
37 In this manner an economic and reliable ~2~3~;69 01 series capacitor protective circui-t, which maximizes 02 utilization of the series capacitor during low fault 03 current conditions, is provided.
04 Other advantages, and eatures of this 05 invention will hereinafter appear, and for the 06 purposes of illus-tration; but not of limitation an 07 exemplary embodiment of the subject invention is shown 08 in the appended drawing.

BRIEF DESCRIPTION OF THE DRAWINGS
ll Figure l is a schematic diagram of the 12 series capacitor protective cixcuit of the presen-t 13 invention.
14 Figure 2 illustrates an improvement in current sharing between parallel non-linear resis-tor 16 stacks achieved by the presen-t invention.
17 Figure 3 illlustrates one embodiment of 18 the physical relationship between the resistive 19 elements of the present 38 - 8a -~ P55-8~88 invention.

DETAILED DESCRIPTION OF THE PREF~RRED EMBODIMENT
A schematic diagram of the electrical relation-ship between components in the present invention is illustrated in Fig. 1. A series capacitor 1 to be protected is shown connected in series with one conductor of a multi-phase power line. It is to be understood that other series capacitors are connected in series with the other current carrying conductors of -the mul~iphase line. In the interest of brevity, conventional elements which may be employed in conjunction with this invention are ommitted. Conservative engineering practice would probably result in the use of auxillary protective, bypass, and transient suppression means.
A irst branch clrcuit containing a non-linear resistor 2 in series with a linear resistor 3 is shown con-nected in shunt relationship with the terminals of the series capacitor 1~ A second branch circuit containing a triggerable spark gap 4 is connected in shunt relationship with both the first branch and the terminals of series capacitor 1. In normal operation, neither the first branch circuit nor the second branch circuit conduct any signiEi-cant current. A small current flows in the first branch circuit limited by the relatively high resis-tance value of the non-linear resistor. When low leakage non-linear resistors are used, as is preferred, the need for a series spark gap and any associated circuit breakers in the first circuit branch is avoided. Typical voltage amperage 9 _ ~2~
P55-~488 characteristics 5a, 5b oE metal oxide non-linear resistors are shown in Fig. 2. Characteristics 5a and 5b, each, represent a voltage-amperage characteristic of a series stack of metal oxide non-linear resistors o~ slightly dif-fering resistances which are increasingly apparent above knee point 6. Characteristics 7a and 7b represent a voltage-amperage characteristic for similar stacks of metal oxide non-linear resistors of slightly differing resist-ances which are connected ln series with a linear resistor.
When the resistive elements are connecte~ in parallel the voltage across them is the same. However, the effect of the slightly differing resistances of the individual series stacks is considerably reduced when a linear resistance is employed in series with the series stacks. Ak an arbikrary voltage Vl, above knee point 6, of the voltage-amperage characteristics in Fig. 2, the difference in current between the individual series stacks for the non-linear resistors alone in characteristics 5a, 5b is denoted DELTA I; however, the di~ference in current between individual series skacks which are in series with a linear resistor, charackeriskics 7a, 7b is not discernable in Fig. 20 Knee point 6 of the non-linear resistor to be employed in the present invention is selected to be safely above normal capacitor voltage to avoid significant leakage current Elow throu~h the non-linear resistor. The leakage currenk flowing through the first branch circuit during normal operation is insuf ficient to cause an appreciable volkage drop across the linear resistor, as may be seen in Fig. 2 by comparing khe charackeristics for metallic oxide non-linear resistors 5al 5b, with the combined voltage-amperage characteristic 7a, 7b ~2~ 9 P55-~88 of the non-linear resistors and linear resistors in the region below knee point 6, where they essentially coincide.
In this normal region of operation the ohmic value of the linear resistors is very much less than that of the non-linear resistors. Should the voltage across the first branch circuit rise above knee point 6, the ohmic value of the non-linear resistor rapidly decreases to a value much less than that of the linear resistor, such that an appreci-able voltage appears across the linear resistor. The higher the ohmic value of the linear resis~or, the less power the non-linear resistor will dissipate. But the higher its ohmic value, the greater the capacitor voltage will be during a fault condition. The resistance of the linear resistor may in many cases be determined on a basis of economics.
A method of economically selecting resistor values resulting in a reliable protective circuit is as follows. A maximum capacitor voltage is selected, and the maximum available fault current from an off line fault is determined. A knee point voltage sufficiently above normal voltage to avoid signiicant leakage current is selected.
A linear resistance ohmic value is determined, such that at maximum off line ault current a capacitor voltage, equal to maximum capacitor voltage, exists. This calculated resist~
ance value will normally result in an economical and reli-able arrangement. Conservative engineering practice would suggest tha~ some safety allowance for tolerances be provided, and several selected values be examined to deter-mine the most economical arrangement for a particular installation. Because of the interdependency of the various ~ 2r)2 ~ 6~ P55-8~88 parameters there is no single order in which they must be selected.
A second branch circuit containing a triggerable spark gap 4 is shown in Fig. 1. ~Iowever, since -the trigger circuitry may fail, it is desirable ko avoid exclusive reliance on the trigger means to initiate conduction through the gap. If the spark gap is designed to initiate conduc-tion at approximately maximum capacitor voltage without reliance on the trigger means, the series capacitor and the first branch circuit are rapidly and reliably bypassed upon the occurence of excessively high fault currents, which often result from faults on the same line. When basic protective circuitry ~unctions as designed, faults in the networks are of relatively short duration. The power dissipation capabilities of components of the protective network including the non-linear and linear resistors can be designed, with appropriate safety factors, for short fault duration. Should the basic network protective circuitry, such as line circuit breakers, fail to function, it is desirable to trigger the spark gap into conduction. Conven-tional current sensors, or temperature sensors, or both, in conjunction with timing, or rate of current rise circuitry, may be employed to detect imminently hazardous conditions and initiate spark gap conduction.
The prior discussion of the first branch circuit of the present invention has largely been in terms of a single non-linear resistive element in series with a single linear resistive element. The limitations of present tech-nology being what they are, it is believed to be impossible to commercially obtain a single non-linear resistive element ~)2669 P55-~488 which has either the required high knee point 6 voltage, or suficient power dissipation capabilities. These limita-tions are overcome by connecting non-linear resistors in series, forming stacks, to obtain sufficient knee point 6 voltage; and connecting these stacks in parallel, to achieve sufficient power dissipa~ing capabilities. It appears to be more advantageous to assemble linear resistors in series with each non-linear resistor stack; rather than connecting parallel stacks in series with a linear resistor of greater power dissipating abili~y to minimize unequal current sharing. This circuit arrangement is shown in Fig. 3.
Linear resistors 8 are connected in series with series stacks of non-linear resistors 9. The values of resistance for elements 8 and 9 are chosen such that the equivalent circuit represented by Fig. l of the circuit shown in Fig. 2 has ohmic values determined in the above-mentioned manner.
More than two series stacks will usually be employed.
In light of presen~ technical and economic factors, the knee point 6 for the non-linear resistor 2 is typically set in the region of twice normal capacitor voltage (2 p.u.), and the level at which the triggerable spark gap 4 will fire without triggering, is set at three times normal capacitor voltage (3 p.u). The triggerable spark gap 4 in the second branch circuit protects the series capacitor 1 and the linear and non-linear resistors 3 and 2 against high fault currents. An effect of the linear resistor 3 is to cause the voltage across the first branch circuit to rise with increasing current. When that voltage rises to the level at which triggerable spark gap 4 will fire, typically 3 p.u., the fault current is shunted around the series capacitor 1 and the first branch circuit.

3L~ 0 2~;6g P5~-8488 Typically power lines with associated trans-mission equipment are protected by basic protective cir-cuitry which employs line circuit breakers, whether or not series capacitors are employed. The energy dissipation requirements of the elements in the first branch circuit and the series capacitor 1 may be further reduced by recognizing that most faults are cleared by line circuit breakers within 13 cycles of their initiation. These components need only be sized to dissipate the power resulting from 13 cycles of the maximum calculated off line fault current, if a circuit in shunt with them is provided to conduct low level fault`
current when the fault is not cleared within 13 cycles. The triggerable spark gap 4 is provided for that purpose and when used with conventional current or temperature sensors, or both, will protect both the series capacitor and the elements of the first branch circuit against long duration low current faults which would cause dissipation in excess of their design level.
The energy dissipation requirements for non-linear resistive elements employed in series capacitor protective circuits are greatly reduced when the present invention is employed. In one study, of a 180 mile double line network with capacitors located near the 60 mile point, employing conventional transient network analysis means 7 it was determined that, in the worst case, the energy require-ments for the non-linear resistors employed in the present invention were a third of that required in a conventional protective circuit. Further, the energy requirements for the series capacitors and its protective circuitry remain essentially unchanged if the series capacitors are moved to P55-8~8 ~2~266~
the ends of the line when the present invention employing linear and non-linear resistors in ser.ies, connected in parallel with a triggerable and voltage activated spark gap is empLoyed; but rise greatly when it is not. In addition to reducing expense by reducing the power dissipation requirements, the present invention allows locating series capacitors at the line ends, saving the expense of instal-ling and mainkaining a mid line substation.
It should be understood that various modifica-tions changes and variations may be made in the arrangement, operation, and details of construction of the series capacitor protective circuit disclosed herein, without departing from the spirit and scope of this invention.

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A series capacitor protective circuit comprising:
a first branch circuit connected in parallel with a series capacitor, said first branch circuit including non-linear and linear resistive elements, which are interconnected so that the electrical characteristics of said first branch circuit can be represented by a non-linear resistor connected in series with a linear resistor, the resistive value of said non-linear and linear resistive elements being selected to maximize power dissipation in the linear resistive element and minimizes power dissipation in the non-linear resistive element when low level fault currents arise, without permitting excessive series capacitor voltages; and a second branch circuit connected in parallel with both said first branch circuit and the series capacitor, said second branch circuit including a spark gap;
whereby said first branch circuit conducts low levels of fault current and said second branch circuit conducts high levels of fault current.

2. A series capacitor protective circuit as claimed in claim 1 wherein said spark gap is triggerable into conduction when the power dissipation in said first branch circuit caused by low level fault currents approaches a predetermined magnitude.

3. A series capacitor protective circuit as claimed in claim 2, wherein said spark gap may be triggered into conduction, but will conduct without being triggered when high fault currents cause the series voltage to rise significantly above the series capacitor voltage produced by low level fault current.