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14
LINE PROTECTION WITH DISTANCE RELAYS
295
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 mho type is best suited for phase-fault relaying for longer lines, and particularly where
severe synchronizing-power surges may occur. It is the least likely to require additional
equipment to prevent tripping on synchronizing-power surges.
1
When mho relaying is
adjusted to protect any given line section, its operating characteristic encloses the least
space on the R-X diagram, which means that it will be least affected by abnormal system
14
LINE PROTECTION WITH DISTANCE RELAYS 296
LINE PROTECTION WITH DISTANCE RELAYS
conditions other than line faults; in other words, it is the most selective of all distance
relays. Because the mho relay is affected by arc resistance more than any other type, it is
applied to longer lines. The fact that it combines both the directional and the distance-
measuring functions in one unit with one contact makes it very reliable.
The impedance relay is better suited for phase-fault relaying for lines of moderate length
than for either very short or very long lines. Arcs affect an impedance relay more than a
reactance relay but less than a mho relay. Synchronizing-power surges affect an impedance
relay less than a reactance relay but more than a mho relay. If an impedance-relay
characteristic is offset, so as to make it a modified relay, it can be made to resemble either
a reactance relay or a mho relay but it will always require a separate directional unit.
There is no sharp dividing line between areas of application where one or another type of
distance relay is best suited. Actually, there is much overlapping of these areas. Also,
changes that are made in systems, such as the addition of terminals to a line, can change
the type of relay best suited to a particular location. Consequently, to realize the fullest
capabilities of distance relaying, one should use the type best suited for each application.
In some cases much better selectivity can be obtained between relays of the same type, but,
if relays are used that are best suited to each line, different types on adjacent lines have no
appreciable adverse effect on selectivity.
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
To convert primary impedance to a secondary value for use in adjusting a phase or ground
distance relay, the following formula is used:
CT ratio
Z
sec
= Z
pri
×

VT ratio
where the CT ratio is the ratio of the high-voltage phase current to the relay phase current,
and the VT ratio is the ratio of the high-voltage phase-to-phase voltage to the relay phase-
to-phase voltageall under balanced three-phase conditions. Thus, for a 50-mile, 138-kv
line with 600/5 wye-connected CTs, the secondary positive-phase-sequence reactance is
600 115
about 50
×
0.8
×

× = 4.00 ohms.
5 138,000
It is the practice to adjust the first, or high-speed, zone of distance relays to reach to 80%
to 90% of the length of a two-ended line or to 80% to 90% of the distance to the nearest
terminal of a multiterminal line. There is no time-delay adjustment for this unit. LINE PROTECTION WITH DISTANCE RELAYS
297
The principal purpose of the second-zone unit of a distance relay is to provide protection
for the rest of the line beyond the reach of the first-zone unit. It should be adjusted so that
it will be able to operate even for arcing faults at the end of the line. To do this, the unit
must reach beyond the end of the line. Even if arcing faults did not have to be considered,
one would have to take into account an underreaching tendency because of the effect of
intermediate current sources, and of errors in: (1) the data on which adjustments are
based, (2) the current and voltage transformers, and (3) the relays. It is customary to try
to have the second-zone unit reach to at least 20% of an adjoining line section; the farther
this can be extended into the adjoining line section, the more leeway is allowed in the
reach of the third-zone unit of the next line-section back that must be selective with this
second-zone unit.
The maximum value of the second-zone reach also has a limit. Under conditions of
maximum overreach, the second-zone reach should be short enough to be selective with
the second-zone units of distance relays on the shortest adjoining line sections, as
illustrated in Fig. 1. Transient overreach need not be considered with relays having a high
ratio of reset to pickup because the transient that causes overreach will have expired before
the second-zone tripping time. However, if the ratio of reset to pickup is low, the second-
zone unit must be set either (1) with a reach short enough so that its overreach will not
extend beyond the reach of the first-zone unit of the adjoining line section under the same
conditions, or (2) with a time delay long enough to be selective with the second-zone time
of the adjoining section, as shown in Fig. 2. In this connection, any underreaching
tendencies of the relays on the adjoining line sections must be taken into account. When
an adjoining line is so short that it is impossible to get the required selectivity on the basis
of react, it becomes necessary to increase the time delay, as illustrated in Fig. 2. Otherwise,
the time delay of the second-zone unit should be long enough to provide selectivity with
the slowest of (1) bus-differential relays of the bus at the other end of the line,
(2) transformer-differential relays of transformers on the bus at the other end of the line,
Fig. 1. Normal selectivity adjustment of second-zone unit.
Fig. 2. Second-zone adjustment with additional time for selectivity
with relay of a very short adjoining line section. 298
LINE PROTECTION WITH DISTANCE RELAYS
or (3) line relays of adjoining line sections. The interrupting time of the circuit breakers
of these various elements will also affect the second-zone time. This second-zone time is
normally about 0.2 second to 0.5 second.
The third-zone unit provides back-up protection for faults in adjoining line sections. So far
as possible, its reach should extend beyond the end of the longest adjoining line section
under the conditions that cause the maximum amount of underreach, namely, arcs and
intermediate current sources. Figure 3 shows a normal back-up characteristic. The third-
zone time delay is usually about 0.4 second to 1.0 second. To reach beyond the end of a
long adjoining line andstill be selective with the relays of a short line, it may be necessary
to get this selectivity wi