Out-Of-Step Tripping; Description; Ost Characteristic Shape - GE LPS-O Instruction Manual

1.9 OUT-OF-STEP TRIPPING

1 PRODUCT DESCRIPTION
1.9 OUT-OF-STEP TRIPPING

1.9.1 DESCRIPTION

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The Out-of-Step Tripping (OST) feature is a positive-sequence static distance relay designed to provide
out-of-step tripping protection. It uses three modified Mho characteristics, all with the same reach along a com-
mon maximum reach angle, but each having a different shape (such as lens, circle or tomato). It includes logic
and time delay circuits that work with the modified Mho characteristics to detect an out-of-step condition.
The OST function does not have a dedicated output relay, nor does it automatically initiate tripping via the LPS-
O trip output relays. Rather, the output of the OST logic must be assigned by the user to an output relay, or to
the configurable trip bus input (CNFTRP, Index #122) if it is desired to trip the same circuit breaker as for a line
fault.
Loss of synchronism or a power swing between two areas of a power system is detected by measuring the
positive sequence impedance seen by the relay as the power swing develops over a period of time. Since the
OST option measures positive-sequence impedance as seen at the relay location, there is less chance of set-
ting up blocking for non-swing conditions (such as close in single phase to ground faults on series compen-
sated lines where gap flashing and capacitor reinsertion can cause the fault impedance to vary greatly) than
there is for a relay that measures phase impedance.
The OST option includes three positive-sequence offset Mho distance relay characteristics. These characteris-
tics have the same reach along a common angle of maximum reach, but have different reaches along the R-
axis. An out-of-step condition is detected by measuring the time for the apparent impedance seen by the OST
to travel from a load impedance value to a point along the protected line. A fault on the line will cause this
impedance change to occur immediately and will not set up out-of-step tripping. For a swing condition the
impedance will change slowly. If the impedance passes through the three characteristics in the proper
sequence, an out-of-step trip output is produced.
The basic mode of operation is with all three characteristics (OUTER, MIDDLE, INNER). However, with Protec-
tion Setting 1402: NUMBRCHAR, the MIDDLE characteristic can be bypassed, thereby using only the OUTER
and INNER characteristics for out-of-step detection. The decision whether to use two or three characteristics
depends largely on the maximum slip frequency expected during a swing condition. Three characteristics are
preferred if the slip frequency is low enough to permit making the minimum recommended settings on TLOS1,
TLOS2 and TLOS3. In some applications only two characteristics may be necessary, because the combination
of a fast moving swing locus (high slip frequency) and a restricted OUTER characteristic (due to heavy load
transfer) prevents setting up the proper logic sequence.
There is another setting option that provides a choice between tripping as soon as the out-of-step condition is
confirmed: on entering the INNER characteristic (Protection Setting 1403: TRIPIN_OUT = IN) or delaying trip-
ping until the two system voltages are more nearly in phase; that is, on leaving the OUTER characteristic (Pro-
tection Setting 1403: TRIPIN_OUT = OUT). Some users prefer this delayed tripping to reduce the risk of
damage to the circuit breaker doing the tripping.

1.9.2 OST CHARACTERISTIC SHAPE

The shape of the offset Mho characteristic may be expanded or restricted by changing the characteristic timer
setting. Figure 1-16 shows three Mho characteristics used in the OST function. All three have the same for-
ward and reverse reach along the maximum reach angle, but the OUTER characteristic is an expanded Mho
(tomato-shaped) while the INNER characteristic is a restricted Mho (lens-shaped). The angle of maximum
reach of the three characteristics is determined by the positive-sequence impedance angle of the protected
line (Protection Setting 1201: POSSEQANG).
In Figure 1–9: OST MHO CHARACTERISTIC on page 1–23, the dotted line, SWING LOCUS, represents one
possible path that would be followed by the apparent impedance resulting from a swing It may travel either left
to right or right to left. Also, the SWING LOCUS may be above the location shown, depending on the relation-
ship between the relay location and the electrical center of the power system.
1- LPS-O Line Protection System GE Power Management
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