REF - In Zone Faults - Voltage across relay terminals with Stabilizing resistor 1

[NickParker]

Typically, a Stabilizing Resistor is incorporated into a High Impedance Type REF Protection Scheme to reduce the sensitivity of the relay and prevent it from triggering during through fault conditions, particularly if there is saturation in one of the Current Transformer (CT). The resistor is carefully calculated to ensure that under external fault conditions (at max fault level), the voltage generated by an unsaturated CT does not cause a current flow exceeding the relay's preset threshold. This setup aims to avoid unnecessary relay operation during external faults.

However, a concern arises regarding whether the relay might remain desensitized even during internal faults (In Zone faults), where the fault current magnitude is generally lower than that of a full external fault. The question arises because, by introducing the stabilizing resistor, the relay has already been desensitized against responding to a full fault current. In this scenario, how does the relay still receive sufficient current during an internal fault?

 

[9wearSuave]

Why would the measured fault current be lower than a through fault? In any case, it doesn’t matter so much the magnitude of the fault, as much as the comparison between the residual current and measured In (3I0). If it’s an external fault, Ig and In would be in phase and wouldn’t operate the relay, no matter how large the current was. Stabilizing resisters help take care of angle errors due to CT saturation.

 

[RRaghunath]

During internal faults and when there is no CT saturation (due to low fault currents as mentioned by you) all the differential current will flow in to the relay coil.
No current can be diverted, despite having a stabilising resistor in series with relay coil as the unsaturated CT presents a very very high impedance.
Sensitivity check is part of the REF protection calculations to confirm not less than 90% of the windings are covered under the protection.

R Raghunath