|
If fault current can flow in both directions through a relay location, it is necessary to add directionality to the overcurrent relays in order to obtain correct co-ordination. Typical systems which require such protection are parallel feeders (both plain and transformer) and ring main systems, each of which are relatively common in distribution networks. In order to give directionality to an overcurrent relay, it is necessary to provide it with a suitable reference, or polarizing, signal.
The basic principle used in determining directionality is that, in a power system operating
at unity power factor, phase current and phase-to-neutral voltage are in phase, and that phase current leads the
phase-to-phase voltage between the other two conductors by 90° as can be seen below. Fault current in that conductor lags its
phase-to-neutral voltage by the angle of system impedance.
Therefore, the phase angle of fault current in Phase C will
always lead the angle of the A-B voltage by an angle which
can never exceed 90°. However, if the direction of the current
in Phase C reverses, then its' angle will lag the A-B voltage
by up to 90°.
It is therefore important to ensure the correct phasing of all current and voltage inputs to the relay, in line with the supplied application diagram. Under system fault conditions, the fault current vector will lag its nominal phase voltage by an angle dependent upon the system X/R ratio. It is therefore a requirement that the relay operates with maximum sensitivity for currents lying in this region. This is achieved by means of the relay characteristic angle (RCA) setting; this defines the angle by which the current applied to the relay must be displaced from the voltage applied to the relay to obtain maximum relay sensitivity. |