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Anonymous Poster #1

# MV Circuit Breaker Device Evaluation

08/12/2017 1:42 AM

We evaluate MV breakers by the short circuit current at 3-5 cycles. The generator fault contribution is computed using xd''. Thought after 3-5 cycles, we can use xd'. Can you please direct me to references? or advise why xd'' is used not xd'?

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#1

### Re: MV Circuit Breaker Device Evaluation

08/12/2017 2:37 AM
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Guru

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#2

### Re: MV Circuit Breaker Device Evaluation

08/12/2017 2:57 AM

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Power-User

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#5

### Re: MV Circuit Breaker Device Evaluation

08/14/2017 12:25 PM

Subtransient values can be used to calculate upper limit of mechanical bracing of components in the fault path, and also to predict the worst case current that the protective relaying will see, for selective coordination & protection.

The current at 3-5Hz is what you might expect your protective device to interrupt, after responding to the protective relay

Guru

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#3

### Re: MV Circuit Breaker Device Evaluation

08/12/2017 10:31 AM

Here is a good explanation of subtransient and transient reactance:

"An explanation of the origin of a generator’s subtransient reactance

Posted on December 11, 2014

The term “subtransient reactance” is denoted by the symbol X’’d and is used to calculate available short-circuit generator fault currents. But where does it come from? What’s its origin?

A generator’s short-circuit current is calculated from its subtransient reactance. The subtransient reactance is an impedance value that entirely neglects the resistance component.

This is justifiable based on the physical characteristics and construction of electric generators. The subtransient reactance is a transient effect that’s directly related to the electromagnetic relationships between the various physical components of the generator. The resistance of the windings of a synchronous generator are generally negligible compared to their reactance, but they do play a role in the decay rates of the transient currents in the form of L/R time constants." more...

http://www.nuclearelectricalengineer.com/explanation-origin-generators-subtransient-reactance/

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#4

### Re: MV Circuit Breaker Device Evaluation

08/12/2017 11:30 AM

I am not a "protection" engineer only an engineer_ designer of electrical part of power stations, so I am not familiarized with "settings".

However, in my opinion, one could follow the recommendation of IEEE 242 IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems Chptr.2.4 Protective device currents.

According to Table 2-1—Short-circuit impedances for protective device application and evaluation, the protection of a generator presents 3 points of settings following type of protection device employed.

For instantaneous currents the considered impedance it is R+jX"d , for time-delay currents R+jX'd and for long-time currents R+jXd.

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