transformer protection: which relay trips which circuit breakers?

[leoliu]

A distribution transformer with two circuit breakers (CB) and severl different protection relays including overload, overcurrent, time delay overcurrent, instantaneous overcurrent, differential relay, ground differential relay. Anybody can tell me whether the relays trip the HV and LV CBs at the same time or just trip one of them specifically? Many thanks to you.

 

[ScottyUK]

It's impossible to say, but normally I would expect protection specific to the LV side to open the LV breaker and leave the HV breaker closed. Conversely for protection on the HV side it is common to inter-trip the LV breaker from the HV 86 relay.

The HV and LV thermal overcurrent relays normally grade with each other so that LV clears first.

Differential relays will normally trip the HV and intertrip the LV.

Transformer tank instruments such as buchholz, WTI and OTI will normally trip the HV and intertrip the LV.

Some protection types are specific to the winding type: e.g. you would not expect to see REF protection on a delta winding for example. Also ground faults on the LV side of an earthed star winding appear as overcurrents in two phases of a delta wound primary, so the HV E/F relay will not respond to the same fault as the LV E/F relay.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[leoliu]

ScottyUK:

Thank you very much for the reply. Would you like tell me what book I can read to understand more clearly about this matter?

 

[tommom]

Try these "standard" references:

Applied Protective Relaying (Westinghouse)
Protective Relaying, Blackburn (Dekker)
IEEE Buff Book
The Art of Protective Relaying (viewable on-line

http://www.geindustrial.com/industrialsystems/pm/notes/artsci/artsci.pdf

)

There are others, but these are pretty good. Hope this helps.

 

[RalphChristie]

It depends a lot on your operating philosophy, but if you clearly think about it, it is fairly easy to figure out:

Any fault inside the transformer (between HV- and LV-breakers) should be cleared by the HV-breaker (and intertrip the LV-breaker) and faults outside the transformer (behind the LV-breaker) should be cleared by the LV-breaker alone.

· Any kind of zone protection (differential, REF, etc.) indicate faults inside the transformer and should be cleared by the HV-breaker. (HV should intertrip the LV)
· LV overcurrent, LV instantaneous overcurrent, LV earthfault protection etc. indicate faults outside the zone and should just hit the LV-breaker.
· HV overcurrent, HV instantaneous overcurrent, HV earthfault protection etc. should co-ordinate with downstream (LV) devices and should trip the HV-breaker. (HV should intertrip the LV) It is kind of a back-up for the LV-protection, but can also be an indication of faults inside the transformer zone.
· Buchholz relays, pressure relays, etc indicate faults inside the transformer zone and should trip the HV-breaker (and intertrip LV-breaker)
· Oil temperature devices (top-oil, winding temp, etc) are normally an indication of transformer overloading (note: an overload condition and not an overcurrent condition) and should trip the LV-breaker.

Hope it makes sense,

Regards
Ralph

 

[dpc]

Instantaneous relay response time does vary, but there is data available. For electro-mechanical relays, it will be in the 1-2 cycle range. The greater the fault magnitude, the fast the response. Also, keep in mind that these old IT units do respond to dc offset current as well as ac, in general.

For digital relays, the response may not be as fast as you think. The relay must sample enough points to be able to predict the magnitude of the current and to apply harmonic filtering and dc offset filtering done in software. The processor then has to issue an command to energize the auxiliary relay that is the output contact (in most cases).

So, again, response times of 1 to 2 cycles seem to be fairly typical.

It would be nice to eliminate the entire "instantaneous" terminology, but that isn't going to happen in my lifetime, at least in the US.

Hope that helps.

 

[wbd]

The newer microprocessor relays can also be set to mimic an electromechanical relay delay. This allows the original coordination, if relay delays were taken into account, to remain as originally designed.

 

[Chad12]

A good reference would be one of the most common electromechanical relays around. It is the IAC relay from GE. Figure 7A on page 7 of this PDF shows an operating curve for the instantaneous function. Keep in mind that total operating time for the circuit includes mechanical operating time of the affected circuit breaker.

http://www.geindustrial.com/products/manuals/iac/geh1788.pdf

Chad Snyder
Director of Operations
American Power Technology

http://www.ampowertech.com