Page 276 - Offshore Electrical Engineering Manual

Worked Example: Setting Voltage Controlled Overcurrent Relays 263

minimum compared with the generator reactance X . In this case we will assume
g
this is when one generator is operating and two transformers are paralleled. The
voltage at the generator (high-voltage) terminals is given by:
X 1
V = − E (where E is the generated voltage) .
(X t + X t )
The generator reactances from say 0–2 s can be calculated from the generator
decrement curves or from values given by the manufacturer. In this case the genera-
tors are identical, each with reactances of 15.1% and 33% , and each transformer
t=0
t=1
has a reactance of 20.12% at the same MVA base.
Therefore with one transformer:
V t = 0 = 20.12/(20.12 + 15.1) = 57 %
V t = 1 = 20.12/(20.12 + 33) = 38 %
With two transformers in parallel:
V t = 0 = 10.06/(10.06 + 15.1) = 40 %
V t = 1 = 10.06/(10.06 + 33) = 23.4 %

Therefore a setting of 20% should prevent the relay from switching to the more
sensitive characteristic on a low-voltage fault.
Two further points may be made:
First, this type of relay is known as a voltage controlled overcurrent relay, and
should not be confused with the voltage restrained type where the sensitivity is
continuously variable over a voltage range. The voltage restrained type is not rec-
ommended since sensitivity increases too quickly with dipping voltage so that dis-
crimination with downstream fuses is lost.
Second, some American relays are also known as voltage controlled, but with
these the inverse characteristic is inhibited at low levels of voltage dip, leaving only
a definite time delay element to clear the fault (Fig. 4.7.7).