Page 275 - Offshore Electrical Engineering Manual

262 CHAPTER 7 Protection and Discrimination

WORKED EXAMPLE: SETTING VOLTAGE CONTROLLED
OVERCURRENT RELAYS

Some overcurrent relays provide dual inverse characteristics. The less sensitive
characteristic gives a longer time interval for clearance of downstream faults, and
the more sensitive characteristic removes close-up faults more quickly. Faults
close to the generator have less impedance and hence produce greater voltage
dips. At a set level of voltage dip, the relay will switch from one characteristic
to the other.
In this case we will set the voltage to discriminate between a fault upstream
(giving rise to a large voltage dip) and a fault downstream of a transformer (pro-
ducing a smaller voltage dip). The inset on Fig. 4.7.6 shows the equivalent dia-
gram. The maximum voltage dip occurs when the transformer reactance X is a
x

FIGURE 4.7.6
Graph of voltage downstream of transformers during system fault: Example for voltage
controlled relays.