High Z vs Low Z diff protection?

[veritas]

Hi

I am wondering if someone could tell me exactly just what is meant by high Z differential protection (e.g. restricted earthfault) and what is meant by low impedance diff protection, e.g. biased differential. What does the high and low refer to?

Thanks.

 

[Bung]

The high and low refers to the burden of the relay. High impedance protection is voltae based - it uses the voltage output of the CT, so usually requires CTs with a known knee-point voltage that is high enough to meet certain criteria. Low impedance lust uses the current output directly, so the burden of the relays is as low as possible. Saturation effects are dealt with differently in each scheme, and are really what separate the philosophies.

Most of the major relay manufacturers have good descriptions in the tech literature on their websites.

Bung
Life is non-linear...

 

[RalphChristie]

Agree with Bung.

High impedance systems - voltage operated
Low impedance systems (bias or unbias) - current operated

Main disadvantage of circulating current protection using low impedance relays is through fault instability due to CT saturation.
Main disadvantage of high-impedance protection is the need for dedicated CTs.

The trend in modern relays today is to go with low impedance schemes.



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[dpc]

The high impedance relay has been used very successfully for decades. You can read about it in any of the old relaying application guides. The CTs are connected in parallel, usually in a terminal junction box centrally located, and only two leads need to be brought to the relay.

With the advent of microprocessor relays, there is a renewed interest in using low impedance differential protection, similar to generator differentials and transformer differentials. This requires a separate CT input to the relay for each CT.