High Impedance REF - Stabilizing resistor

[charz]

The stabilizing resistor in high Impedance REF assists in such a way that the derived current in the relay circuit (Rst + Rp) cannot reach the relay setting threshold when the maximum through current saturates a CT with its DC component.

Generally, the faults closer to neutral have less magnitude of fault current while the through current fault would be higher.

So, will the stabilizing resistor allow the magnitude of current for the faults closer to neutral to cause the operation of the relay?

 

[RRaghunath]

When one of the CTs in the HiZ REF protection loop is saturated, that provides a low impedance path for the fault currents (when compared to the path through relay/stab res).
In case of internal fault, that too close to the neutral with low current magnitude, there is no saturation of CT and all the CTs present high impedance paths forcing the fault current to flow throgh the relay/stab res.
Moreover, during internal faults, the relay is making a decision in < 10ms that is required for CT saturation.

 

[charz]

Thanks RRaghunath,

Can I understand that for external faults, if one of the phase CT gets saturated, the secondary currents of the unfaulted phase CTs and the secondary current from the neutral CT would circulate through the faulted CT secondary.

Is the High impedance REF relay current operated or Voltage operated?

 

[RRaghunath]

That's right.
With regard to voltage or current operated, it is essentially current operated being connected in the CT secondary circuit.
However, some relays have settings in terms of voltage like MFAC of Alstom and 7VH6000 of Siemens.
These are also current operated but calibrated to set in terms of stability voltage and the stabilising resistor is made integral to the relay.
The voltage setting is like 'operating current multiplied by the stabilising resistor value' when one wants to understand the relay working with the understanding of the relay that is set in terms of current (and has stabilising resistor external to the relay).

 

[dfdt]

By the way fault closer to neutral point is infact quite high as the impedance of shorted winding is very low. This is inspite of the fact that voltage developed for closein fault is also low