Toshiba GR200 Series Instruction Manual page 32

Figure 1.1-5
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 center of the power system. The impedance locus typically describes
an arc passing through the electrical center, as shown in Figure 1.1-6.
Figure 1.1-6
In the case of a full out-of-step condition (as opposed to a transient power swing), it is
desirable to separate the system in the vicinity of the center of the out-of-step condition. GR200
series can provide an out-of-step detection element (OST) which can provide tripping in these
circumstances.
Although the power swing and out-of-step conditions are very closely related (in fact one
is simply the most severe form of the other), completely different actions are required from the
protection relay. The PSB function must ensure stability of the distance protection during
X
Distance protection
characteristic
(Mho)
Impedance locus during transient power swing
X
Distance protection
characteristic
(Mho)
Impedance locus during out-of-step condition
- 9 -
Impedance locus during
transient power swing
R
Load Area
Impedance locus during
out-of-step condition
R
Load Area
6F2S1914 (0.49)
GRL200 (Soft 033 & 037)