Toshiba GR200 Series - (iii) Detection of power swing and out-of-step

6F2S1914 (0.49)
GRL200 (Soft 033 & 037)
- 8 -
fault arc is considered to be almost equivalent to pure resistance. But if the phase of a current
that flows into a fault point from the remote end is different from the phase of the local current,
the voltage at the fault will have a phase angle difference with respect to the local current,
producing a measuring error in the distance relay with the principle of measuring the
reactance component. The existence of a zero-sequence current on the protected line and
adjacent line can also cause errors in the earth fault relay. The zero-sequence current normally
acts in the direction of relay underreaching due to the effect of the induced voltage. The
compensation method will be described in detail in the next section. The earth fault relay
contains more errors than the phase fault relays even with these compensation methods.
Therefore, the earth fault relays are usually set with a greater margin than the phase fault
relays.
Regarding measuring errors in the relay, it is also necessary to consider hardware errors
in the relay itself, errors introduced by coupling capacitor voltage transformers (CCVT), and
transient overreach errors caused by the DC component of the fault current. For the GR200
series relays, the errors are defined to be less than 5%.
(iii) Detection of power swing and out-of-step
Power swings occur when the output voltages of generators at different points in the power
system slip relative to each other, which result in system instabilities that may be caused by
sudden changes in load magnitude or direction, or by power system faults and their subsequent
clearance. During the course of such a power swing, the impedance seen by a distance relay
may move (relatively slowly) from the load area into the operating characteristic of the distance
protection. In fact, this phenomenon appears to the distance protection measuring elements
like a three phase fault condition and may result in tripping if no countermeasure is applied.
Most power swings are transient conditions from which the power system can recover after a
short interval, and distance protection tripping is therefore highly undesirable in such cases.
GR200 series relay provides the block function for power swing (PSB) so that preventing
unwanted tripping is possible during the power swing. Figure 1.1-5 illustrates the typical
impedance locus as seen by the distance relay during the transient power swing.