Figure 66: Phasor diagrams for insulated system with C phase fault
The current imbalance detected by a core balanced current transformer on the healthy feeders is the vector
addition of Ia1 and Ib1. This gives a residual current which lags the polariing voltage (–3Vo) by 90°. As the healthy
phase voltages have risen by a factor of Ö3, the charging currents on these phases are also Ö3 times larger than
their steady state values. Therefore, the magnitude of the residual current IR1, is equal to 3 times the steady state
per phase charging current.
The phasor diagram indicates that the residual currents on the healthy and faulted feeders (IR1 and IR3
respectively) are in anti-phase. A directional element (if available) could therefore be used to provide discriminative
earth fault protection.
If the polarising is shifted through +90°, the residual current seen by the relay on the faulted feeder will lie within
the operate region of the directional characteristic and the current on the healthy feeders will fall within the
The required characteristic angle setting for the SEF element when applied to insulated systems, is +90°. This is for
the case when the protection is connected such that its direction of current flow for operation is from the source
busbar towards the feeder. If the forward direction for operation were set such that it is from the feeder into the
busbar, then a –90° RCA would be required.
Discrimination can be provided without the need for directional control. This can only be achieved, however, if it is possible to
set the IED in excess of the charging current of the protected feeder and below the charging current for the rest of the system.
SETTING GUIDELINES (INSULATED SYSTEMS)
The residual current on the faulted feeder is equal to 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 per phase value. Therefore, the total imbalance current is equal
to three times the per phase charging current of the rest of the system. 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 remaining system. Practically though,
the required setting may well be determined on site, where suitable settings can be adopted based on practically
When using a core-balanced transformer, care must be taken in the positioning of the CT with respect to the
earthing of the cable sheath:
An RCA setting of ±90º shifts the
"centre of the characteristic" to here
Chapter 6 - Current Protection Functions
IR3 = (IH1 + IH2)