Setting Guidelines (Compensated Networks) - GE MiCOM P40 Agile Technical Manual

Chapter 6 - Current Protection Functions P14D
Directionality is usually implemented using a Wattmetric function, or a transient earth fault detection function
(TEFD), rather than a simple directional function, since they are more sensitive.
7.5.3

SETTING GUIDELINES (COMPENSATED NETWORKS)

The directional setting should be such that the forward direction is looking down into the protected feeder (away
from the busbar), with a 0° RCA setting.
For a fully compensated system, the residual current detected by the relay on the faulted feeder is equal to the coil
current minus the sum of the charging currents flowing from the rest of the system. Further, the addition of the two
healthy phase charging currents on each feeder gives a total charging current which has a magnitude of three
times the steady state per phase value. Therefore, for a fully compensated system, the detected unbalanced
current is equal to three times the per phase charging current of the faulted circuit. A typical setting may therefore
be in the order of 30% of this value, i.e. equal to the per phase charging current of the faulted circuit. In practise,
the exact settings may well be determined on site, where system faults can be applied and suitable settings can
be adopted based on practically obtained results.
In most situations, the system will not be fully compensated and consequently a small level of steady state fault
current will be allowed to flow. The residual current seen by the protection on the faulted feeder may therefore be
a larger value, which further emphasises the fact that the protection settings should be based upon practical
current levels, wherever possible.
The above also holds true for the RCA setting. As has been shown, a nominal RCA setting of 0º is required.
However, fine-tuning of this setting on-site may be necessary in order to obtain the optimum setting in accordance
with the levels of coil and feeder resistances present. The loading and performance of the CT will also have an
effect in this regard. The effect of CT magnetising current will be to create phase lead of current. Whilst this would
assist with operation of faulted feeder IEDs, it would reduce the stability margin of healthy feeder IEDs. A
compromise can therefore be reached through fine adjustment of the RCA. This is adjustable in 1° steps.
106 P14D-TM-EN-8