14-08-2013, 02:28 PM
LINE PROTECTION WITH DISTANCE RELAYS
INTRODUCTION
Distance relaying should be considered when overcurrent relaying is too slow or is not
selective. Distance relays are generally used for phase-fault primary and back-up protection
on subtransmission lines, and on transmission lines where high-speed automatic reclosing
is not necessary to maintain stability and where the short time delay for end-zone faults can
be tolerated. Overcurrent relays have been used generally for ground-fault primary and
back-up protection, but there is a growing trend toward distance relays for ground faults
also.
Single-step distance relays are used for phase-fault back-up protection at the terminals of
generators, as described in Chapter 10. Also, single-step distance relays might be used with
advantage for back-up protection at power-transformer banks, but at the present such
protection is generally provided by inverse-time overcurrent relays.
Distance relays are preferred to overcurrent reIays because they are not nearly so much
affected by changes in short-circuit-current magnitude as overcurrent relays are, and,
hence, are much less affected by changes in generating capacity and in system
configuration. This is because, as described in Chapter 9, distance relays achieve selectivity
on the basis of impedance rather than current.
THE CHOICE BETWEEN IMPEDANCE, REACTANCE, OR MHO
Because ground resistance can be so variable, a ground distance relay must be practically
unaffected by large variations in fault resistance. Consequently, reactance relays are
generally preferred for ground relaying.
For phase-fault relaying, each type has certain advantages and disadvantages. For very
short line sections, the reactance type is preferred for the reason that more of the line can
be protected at high speed. This is because the reactance relay is practically unaffected by
arc resistance which may be large compared with the line impedance, as described
elsewhere in this chapter. On the other hand, reactance-type distance relays at certain
locations in a system are the most likely to operate undesirably on severe synchronizing
power surges unless additional relay equipment is provided to prevent such operation.
THE ADJUSTMENT OF DISTANCE RELAYS
Chapter 9 shows that phase distance relays are adjusted on the basis of the positive-phase-
sequence impedance between the relay location and the fault location beyond which
operation of a given relay unit should stop. Ground distance relays are adjusted in the
same way, although some types may respond to the zero-phase-sequence impedance. This
impedance, or the corresponding distance, is called the "reach" of the relay or unit. For
purposes of rough approximation, it is customary to assume an average positive-phase-
sequence-reactance value of about 0.8 ohm per mile for open transmission-line
construction, and to neglect resistance. Accurate data are available in textbooks devoted to
power-system analysis.2
THE EFFECT OF ARCS ON DISTANCE-RELAY OPERATION
Chapter 9 shows the effect of fault or arc resistance on the appearance of different kinds of
short circuits when plotted on an R-X diagram in terms of the voltages and currents used by
distance relays. Chapter 13 gives data from which arc resistance can be estimated for
plotting such fault characteristics on the R-X diagram. It is only necessary, then, to
superimpose the characteristic of any distance relay in order to see what its response will be.
The critical arc location is just short of the point on a line at which a distance relay's
operation changes from high-speed to intermediate time or from intermediate time to
back-up time. We are concerned with the possibility that an arc within the high-speed zone
will make the relay operate in intermediate time, that an arc within the intermediate zone
will make the relay operate in back-up time, or that an arc within the back-up zone will
prevent relay operation completely. In other words, the effect of an arc may be to cause a
distance relay to underreach.
OVERREACH OF GROUND DISTANCE RELAYS FOR PHASE FAULTS
Reference 8 shows that if phase-to-neutral voltage and compensated phase current are used,
one of the three ground distance relays may overreach for phase-to-phase or two-phase-to-
ground faults. This is not a transient effect like overreach because of offset waves; it is a
consequence of the fact that such ground relays do not measure distance correctly for
interphase faults. (Phase distance relays do not measure distance correctly for single-phase-
to-ground faults, but, fortunately, they tend to underreach.) For this reason, it is necessary
to use supplementary relaying equipment that will either permit tripping only when a
single-phase-to-ground fault occurs or block tripping by the relay that tends to overreach.
THE CONNECTIONS OF GROUND DISTANCE RELAYS
Reference 11 shows that for accurate distance measurement, a ground relay may be
supplied with a phase-to-neutral voltage and the sum of the corresponding phase current
and an amount proportional to the zero-phase-sequence current. If there is another line
nearby that can induce voltage in the line under consideration when a ground fault occurs
anywhere, there must also be added to the phase current an amount proportional to the
zero-phase-sequence current of the other line. The addition of these zero-phase-sequence
quantities is called "current compensation." Reference 11 also describes an alternative to
current compensation called "voltage compensation," whereby the voltage is compensated
by zero-phase quantities. Compensation is necessary because variations in the distribution
of zero-phase-sequence current relative to the distribution of positive- and negative-phase-
sequence current would otherwise cause objectionable errors in distance measurement.
PURPOSEFUL TRIPPING ON LOSS OF SYNCHRONISM
When generators have gone out of synchronism, all ties between them should be opened
to maintain service and to permit the generators to be resynchronized. The separation
should be made only at such locations that the generating capacity and the loads on either
side of the point of separation will be evenly matched so that there will be no interruption
to the service.14 Distance relays at those locations are sometimes suitable for tripping their
breakers on loss of synchronism, and in some aystems they are used for this purpose in
addition to their usual protective functions. However, as mentioned in Chapter 9,
additions or removals of generators or lines during normal operation will often change the
response of certain distance relays to loss of synchronism. Therefore, each application
should be examined to see if certain distance relays can always be relied on to trip.
AUTOMATIC RECLOSING
Chapter 13 introduces the subject of automatic reclosing and describes the practices with
overcurrent relaying. Here, we are concerned with the practices with distance relaying.
Lines protected by distance relays usually interconnect generating sources. Consequently,
the problem arises of being sure that both ends are in synchronism before reclosing. "High-
speed reclosing," defined here as reclosing the breaker contacts in about 20 cycles after the
trip coil was energized to trip the breaker, cannot be used because of the inherent time
delay of distance relaying for faults near the ends of a line