Note: Descriptions are shown in the official language in which they were submitted.
P55-8488
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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
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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
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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
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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
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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
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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
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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
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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.
09
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
21
22
23
24
26
27
28
29
31
32
33
34
36
37
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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
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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
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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
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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.
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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
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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.
