automaticsetdistrelayonsriescompensatedlines
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*Rua Delmiro Gouveia, 333, anexo II, sl. B-216, Bongi, 50761-901, Recife-PE, tel:81-3229.4434
VII Seminrio Tcnico de Proteo e Controle22 a 27 de Junho de 2003Rio de Janeiro - RJ
Artigo: 37558002
AUTOMATICALLY SETTING DISTANCE RELAYS FOR USE ON
SERIES COMPENSATED LINES
Gustavo Arruda1*
Patrcia Ramos Morais1
Ashok Gopalakrishnan 2
(1) CHESF, Brasil.
(2) Electrocon International Inc., USA.
1 ABSTRACT
This paper presents a novel procedure for modeling the
Metal Oxide Varistor (MOV) for use in steady-statephasor based short-circuit programs. The MOV ismodeled iteratively in phase coordinates, rather than in
the sequence domain. A method for automaticallycomputing the protection settings of distance relays, inthe context of series compensated lines, is also
described.
Keywords: series compensation, MOV, distance
relays, relay setting.
2 INTRODUCTION
Power system engineers are familiar with the use ofseries capacitors for voltage compensation on long
transmission lines. To protect the capacitor fromovervoltages that develop in the presence of short-circuit faults, metal oxide varistors (MOVs) are used in
parallel with the capacitor. The MOV is basically anon-linear resistor that limits the voltage across thecapacitor during a fault condition. When the fault
condition is cleared, the MOV allows almost
instantaneous re-insertion of the capacitor into thetransmission line.
Protective relaying of series compensated lines iscomplicated by a number of factors voltage
inversion, current inversion and sub-synchronousoscillations for faults near the capacitor. Thesephenomena cause the zone 1 element of the distance
relays to overreach, and the relay will have to performsome special functions to avoid overreaching. It isimportant to note that even relays not directly on the
compensated line can also be affected by the seriescapacitor.
Further complications are introduced by the action ofthe MOV that protects the series capacitor from over-
voltages. The MOV is typically studied using transient(time-domain) programs like the ATP. Modeling theMOV for use in steady-state phasor domain programs
is a challenging task because of the non-zero inter-sequence coupling terms that appear in the systemimpedance matrices for unbalanced faults. The separate
phase MOVs operate independently and may conductdifferently.
When setting a relay, the relay engineer typically usesa phasor based short-circuit program to perform anumber of fault studies. The results of these studies are
used to determine the relay settings. If a seriescapacitor/MOV is present near the relay, it must beaccurately modeled to get realistic short-circuit results.
Several methods of modeling the MOV for use inshort-circuit programs have been proposed:
Goldsworthy [1], Coursol et al [2] and Mahseredjian etal [3]. In [1], a linearized model for MOV-protectedseries capacitors was first introduced. For balanced
faults, the capacitor and MOV are replaced by anequivalent linear impedance in the system impedancematrix. This equivalent impedance is a function of the
total capacitor/MOV current and is denoted by ZEQ.
The parameters of the function are determined bycomputer simulations. An iterative procedure is used toarrive at the final value of ZEQ.
When the fault is unbalanced, the unequal conduction
in the phases of the MOV will produce a sequence-domain matrix that is no longer diagonal. In [2] and[3], the authors compensate for the non-zero inter-
sequence coupling terms by different methods. Thesolution however takes place in the sequence domainand the sequence networks have to be correctly put
together. This means that the algorithmic equationsdepend on the type of fault.
The technique presented in this paper uses phasecoordinates rather than sequence coordinates. This
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2
method also uses the ZEQ of [1]. However, since eachphase is iterated independently of the other phases, theinter-sequence coupling does not affect the
computations. That is, both balanced and unbalancedfaults can be easily handled. Further, the MOVcomputation does not need any knowledge of the fault
dependent sequence circuits. This means that any typeof system unbalance can be treated correctly.
The MOV conduction is represented by a series faultacross the capacitor. Since the capacitors are retainedin the computation, recalculation of the system
impedance matrices is not required.
The computation algorithm is described in the next
section of this paper. The method was tested byincorporating it into an existing short-circuit program[4].
The paper also describes a protection simulation
environment that helps the relay engineer determinethe settings of protective relays. Many different short-circuit computations are automatically performed,based on rules that are specified by the engineer and
the appropriate relay settings are determined, [5].Finally, the relay settings are validated by a stepped-event simulation of the tripping sequence [6], using
detailed relay comparator equations. Miscoordinationsbetween primary and backup protection can be foundin this way.
3 MOV COMPUTATION ALGORITHM
3.1 Representing the MOV Device
Figure 1 shows a single MOV device in parallel with a
capacitor. The total current through the capacitor/MOVcomponent is IC. If the triggered gap and the bypassswitch are open, IC will flow through the capacitor
(ICAP) and the MOV (IMOV). The capacitor and MOVcurrents by themselves are not sinusoidal, but IC, thesum of these two currents, is approximately sinusoidal
[1].
CONTROL
SERIES CAPACITOR
ZMOV
TRIGGERED GAP
BYPASS SWITCH
ZCAPIC ICAP
IMOVBUS1 BUS2
Figure 1: Series Capacitor and MOV Arrangement.
First, the current IPU is defined as
II
IPU
C
PR
= (1)
where IPR is the capacitor protective level current and
is usually 2 to 2.5 times the rated current of thecapacitor bank.
The equivalent phase impedance ZEQ of the capacitorand MOV, as a function of IPU can be written as (1):
Z (I ) R (I ) jX (I )EQ PU C'
PU C'
PU= (2)
By means of extensive computer simulations,Goldsworthy [1] showed that the MOV begins to
conduct for values of IPU 0.98, and equation 2 can beused to compute the equivalent MOV/Capacitorimpedance. Please see (1) for the actual equations that
are used to compute RC'and XC
'. For values of IPU