Page 215 - Practical Power System and Protective Relays Commissioning

Protection Relays Chapter | 18 215

FIGURE 18.6.8 Load encroachment in distance protection.

When we set this zone we should take care that it does not encroach the
load area, which causes unnecessary tripping, as shown in Fig. 18.6.8.
To overcome this problem we use a characteristic with a lenticular shape
or eight and quadrilateral characteristics, which avoid the load area
encroachment, as shown in Fig. 18.6.9.
As can be seen also from Fig. 18.6.5, the relay setting reach is set at
about 80% only of the line length to prevent the relay from over-reaching or
underreaching for the following reasons:

1. Due to the error in CT ratio and VT ratio, which can cause the relay to
work in the wrong way in close-up faults near the remote end of the line.
2. Subtransient and transient short circuit (SC) currents have values more
that the steady-state value of the SC current, which can cause the relay to
under-reach. This means the measured impedance is less than the actual
impedance of the fault, meaning that if the fault is outside the
transmission line the relay will see it within the transmission line and trip
in the wrong way and not follow the protection selectivity feature of the
distance relay.
3. Due to high-resistance earth faults, which include a tower foot
resistance in the path of the fault which can cause the relay to measure
an impedance more than the actual impedance of the fault, this means
for a boundary fault near the remote end of the transmission line if
the fault is on the line the relay will see the fault as outside the