Generator Protection

[jbond]

Hi,

we are putting a generator specific protection relay to protect a gen-set, and I have a few questions:

1) the gen-set is star-pointed to earth, and we have a toroid on the MEN there. Our line CT's (3) are located just above the star-point and before the gen-set. What are the advantages to having the CT's located there as opposed to after the gen-set?

this may shed some light -> a SLD would have these items in this order: earthing point, MEN toroid, neutral star point, line CT's, gen-set, incomer cable, switchboard CB, switchboard

2) The protection relay has quite a few functions (obviously) but the ones that I thought were most important were and most critical to utilise are: Undervoltage (27), Overvoltage (59), Overfrequency (81), Overcurrent (51) and Earth Fault (51G).

The other functions such as: Reverse Active Power (32P), Reverse Reactive Power (32Q), Negative Sequence (46), Thermal Image (49) and Stator Temperature (49T) - Would you consider these functions as something that I should be utilising? How important do you regard them?

Thanks,
John.

 

[davidbeach]

What is a MEN? CTs on the neutral end of the generator allow internal ground faults to be detected with the breaker open. If the CTs are at the terminal end of the generator, current will only flow at the CT location if the breaker is closed.

The 32P gives you motoring protection; the 32Q may be used as loss of field; the 46 can protect against extreme heating due to negative sequence currents, damage that happens at currents well less than load; 49 and 49T are machine temperature, the 49T is likely to be RTD based and the 49 done in the relay software.

 

[ScottyUK]

Adding slightly to David's post:

What is your prime mover? If it is reciprocating or a steam turbine, reverse power becomes significant to prevent machine damage. Less critical on a GT, but still important.

The 46 NPS relay is designed to protect the rotor of the machine. Without it, unbalanced loads could cause severe thermal damage to the rotor winding and in extreme cases to the forging itself.

You should consider some form of sensitive zone protection, either a 64 REF relay which would be economical for a small set, or a proper 87G differential relay protecting the windings. There are other more specialised generator relays such as 51V voltage restrained overcurrent, 40 loss of field, etc. The free download Network Protection & Automation Guide (NPAG) by Alstom / Areva is referenced elsewhere in Eng-Tips and has a good chapter on generator protection.


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