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Join Date: Nov 2016
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# Differential Protection

08/08/2017 7:02 AM

Dear all,

Good day. Can anybody explain what the difference is between high impedance and low

impedance differential relay, please? Also, where is each used? Is it possible to replace

one another?

Thanks

Pathfinder Tags: Differential Protection
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#1

### Re: Differential Protection

08/08/2017 8:16 AM

<...anybody...> Yes. Google can: 1,510,000 hits.

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#2

### Re: Differential Protection

08/08/2017 1:54 PM

..."Differential protection is usually applied on bus protection because of its high selectivity since it does not need to coordinate with other relays. Bus differential protection in power system networks operates on a principle defined by Kirchoff's current law. The law states that the vector sum of all currents entering and leaving a node or bus is equal to zero. It is this principle that is used in all bus differential protection regardless of the relay type used. This paper discusses the fundamentals of bus protection with a focus on the two common methods typically used: high-impedance and low-impedance bus differential relaying. Included are the basic theories of high impedance and low-impedance differential relaying and their operational concepts. A comparison between the two methods which points out the benefits, drawbacks, concerns and considerations is also given. The importance of current transformer selection and performance are considered in this paper as part of the consideration in bus differential protection scheme design."...

http://ieeexplore.ieee.org/document/6799038/

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#3

### Re: Differential Protection

08/11/2017 4:49 AM

High impedance and low impedance differential relays are typically use for the protection of buses. High impedance relays requires dedicated cts, for each of the feeders connected to the bus, aside from this, class,ratio , accuracy and length of wires from the ct to the relay terminals should be also identical, compare to low impedance relays that accepts unidentical cts. Also low impedance relays allows sharing or paralleling of cts for other protection purposes such as back up protection (50/51) or other protection function wherein a high impedance relay will require dedicated CTs.

Consideration for using wether high or low depends on the cts available, say one have different rated cts then a low impedance relay is recommended. If there are identical cts available upon application then go for High impedance.

Though they have differences in requirement, back up protection such as open ct alarm, short ct alarm and MOV degradation protection are available for both high and low.

In terms of selectivity low impedance offers much that high impedance relays

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