Toshiba GRZ100 B Series, Instruction Manual, Page: 16 / 447

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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 protection relay measuring errors, 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 GRZ100, the total of these
errors is specified to be less than 5%.
2.1.5 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, as a result of system instabilities which 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 distance protection operating characteristic. 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 period of time, and
distance protection tripping is therefore highly undesirable in such cases. GRZ100 provides a
power swing blocking function (PSB) to prevent unwanted tripping during a power swing. Figure
2.1.5.1 illustrates the typical impedance locus as seen by a distance relay during a transient power
swing.
Figure 2 1.5.1 Impedance Locus during Transient Power Swing
A special case of the power swing condition occurs when the power system disturbance is so
severe that generators lose synchronism with each other and are said to be out-of-step. During an
out-of-step condition the phase angle between generators continues to increase and pass through
180
°
, at which point a distance relay measures an impedance equal to that for a three phase fault at
the centre of the power system. The impedance locus typically describes an arc passing through
the electrical centre, as shown in Figure 2.1.5.2.
X
R
Load Area
Distance protection
characteristic (Mho)
Impedance locus during
transient power swing