Distance relay (ANSI 21) in Generators

[guiyermo]

Hi everyone,

I have been doing some research on generator protection and I have found that in some protection schemes the distance function (ANSI 21) is used as a backup protection.

I am having trouble to understand its philosophy (e.g. where should CT's be placed: before or after the generator, etc), how to set this device and wondering if it should be coordinated with the distance relays of transmission lines (my guess is they don't have to since generator's distance relay has a delay because it is for backup purposes)

I would appreciate if you have some information regarding this area.

Thank you very much,

 

[slavag]

Hi.
Usually, 21 function ( BTW, we use underimedance term instead distance) is back-up of generator-trafo block.
It will be protect 75% of tafo winding for the selectivity with HV protections ( distance, directional ground fault, etc.). But this function is curcule form with "0" in middle. Becouse it isn't important whre is CT.
For my pinion ( we haven't excperiece yet) classical 21 function request connection to CT installed in the neutral point of generator., of course on separate core of CT.
First zone with 0 delay same is underimpedance, is protected 75% of trafo.Second zone-- will be coordinate with HV protections. Don't forgot, as back-up protection also used voltage restraint ( or controlled) overcurrent protection
Best Regards.
Slava

 

[lz5pl]

One additional point to slavaq explanation:
I would recommend to add enabling of 21-function via binary input of the relay from auxiliary contact of circuit breaker, i.e. 21 to be activated after closing the breaker. Otherwise it is possible to have a trip during the excitation process of the generator (when voltage is still under the nominal).

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It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant

 

[davidbeach]

Distance, using CTs on the neutral end of the generator will look at toward the line through the generator. Using CTs at the terminal end of the generator you can look outward at the system as back up protection to detect uncleared system faults or you can look inward toward the generator and provide loss of excitation protection.

 

[rcw retired EE]

CT's on neutral allow the 21 function to protect the generator prior to breaker closure if the settings can be configured to allow normal energization current without tripping. CT's on the generator line terminals do not see current during an off-line generator winding fault.

I have problems visualizing whether the generator is in front or behind the 21 relay. Remember, it is the VT location and not the CT that determines the origin of the relay's R-X diagram. With VT's on the generator terminals and CT's at the neutral, a generator fault appears in the relay’s forward direction even though the generator impedance is drawn on the R-X diagram as being behind the relay. Using a circular (constant impedance) characteristic provides protection for the generator winding and transformer without the forward/reverse issue.

To answer the original question: as David says, use fast zone 1 for 75% of the transformer and delayed zone 2 for the rest. (Often the generator step up transformer impedance is so large compared to the T-Line and system impedance that a setting of 20% beyond the transformer terminals covers the rest of the world. Suitable time delays are needed.)

 

[guiyermo]

Sorry for the delay on responding.

I would like to thank everyone for your support. I really appreciate that.

Thanks,
Guillermo

 

[guiyermo]

Regarding Slava answer I have a question:

If the ANSI 21 function is used as a backup protection, why the first zone should be set with 0 second delay? Shouldn't it be set with enough delay so that other primary protection, such as differential, trips in case of a short circuit?

Thanks,
Guillermo

 

[slavag]

Hi Guiyermo.
Word "backup" is only old definition of this type of protection from era of EM relays.
Today are prefered use Main1 and Main2.
We try use today fully duplicate protection systems with new numerical relays. That means: twice 87 diff protection and twice 21 function in the separated relays ( for big generators, separate cubicles too).
Operation time isn't issue, if 21 will faster than 87, will only better for generator or block generator - trafo.
Is my opinion only.
Best Regards.
Slava

 

[kssschsekhar]

Mr Slavag

For your information. Don't use 21 and 87 relays as primary relay.

As 87 will be for internal fault of the generator/Generator Transformer. Whereas 21 will be back up. Incase of external faults like remote phase fault/ground fault 21 relay may pick up. Incase of no time delay then nauisence tripping may occur.

Hence 21 should have time delay to avoid nauisence trip. However one can use 51v/51R (Voltage controlled/Restrained over current relay) as back upto 50 MW generators.

greater than 50 MW generators use 21 is must for back up protection.

 

[slavag]

Hi Kssschsekhar.
Is OK, we told about different things.
You are right, for generator back-up protection used 51V/51R and of course with "long" delay and "low" level current, for provide selectivity with other protections.
for generator directly connection to the grid 21 function with delays exactly as you wrote. With block.. we used circuit type 21 function w/o any direction

 

[slavag]

BTW, according to the classical books:
51V/51R is back-up of 87G ( generator diff)
21 is back-up of 87U ( overall diff or block gen-trafo).
Lot of application for the medium size genrators up to 75MW
aren't include 21 functionality ( I told about EM relays).
21 as distance protection in this application was started with start of numerical/digital relays and in case of memory problems or type problems provided this combination:
21+87G in one relay, 51V/51R with 87U in second.
Actually in lot of case puted time delay for the 21 ( underimpedance, not directional distance) was about 0.5sec.
But all case by case.
Best Regards.
Slava