Generator Unit Transformer Protection

[Cerkit]

Hi,

I am trying to understand why for large power stations the Unit Transformer normally would have it's own separate overall protection?

Thanks

 

[RRaghunath]

The generator step-up transformer is unit transformer and is without redundancy.
Hence, it is important to ensure in the eventuality of any fault in the transformer, the damage is minimised so as to limit the downtime for repair / avoid the need for total replacement.
Hope this answers your question!

 

[ScottyUK]

Are we talking about the generator step-up transformer or the unit auxiliary transformer?

 

[j_p_c]

This is a broad question. In general separate protection is provided for generators vs GSU Transformers in power plants as they are logically distinct electrical entities. Though in some (let's say often small and or remote) applications it's fine to protect the GSU and Generator at a single point.

Depending on configuration you have enough signals for each piece of equipment to warrant it's own protective device any way. The generator will often have one or more neutral references (current and voltage), plus a number of voltage references, not to mention tie-ins with excitation, governing (at times), these are all logical connections related to the generator. The GSU is it's own logical device, with again a number of it's own reference signals. If the GSU is on the other side of a generator breaker, then you especially have a good case for separate protection, as you don't want to take the GSU offline for a pure generator electrical fault.

The IEEE tutorial on generator protection is a useful learning resource:

http://www.pes-psrc.org/kb/published/reports/IEEEGenProtTutorial_20110506.pdf

 

[Cerkit]

Thanks very helpful.

Does anyone have any guidance info/docs on how to select suitable CT ratios for biased differential protection for a transformer. So utilising Star/Delta CT configuration on one side and a Star/Delta/Star (interposing CT configuration) on the other end.

Thanks

 

[davidbeach]

Any decent relay these days can accept CTs connected in wye, no need to throw away perfectly good data by connecting in delta any more. CT primary rating on each side should be close to full load rating, but use the full winding of any multi-tap CTs to minimize risk of saturation.

 

[Cerkit]

Hi,

I forgot to mention it is for combined bias differential and restricted earth fault protection hence the interposing CT.

Thanks

 

[davidbeach]

Relays are available that would not require interposing CTs.

 

[crshears]

I have seen thermal generating units with dedicated GSUs, station service auxiliary trafos, generators and turbines, each with their own discrete protection and trip annunciation windows but all tripping to the same zone [ "diameter" ] isolating devices. This makes sense when all said elements are matched to each other and function as a unit.

A different case is where multiple generators supply a common LV bus with, say, two or more main plant output trafos, each one of which can be tripped o/s individually; depending on the capacities of the remaining trafos, generator runback schemes may or may not be necessary to protect them form overloads.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Cerkit]

Hi,

I am looking at a legacy system which does not have modern relays that account for the ratio compensation hence why I was asking about literature to support how to determine CT ratios for the attached configuration.

The line CTs need to account for the ratio of the transformation and the neutral CTs need to be 3 times the line side CT ratio for the HV side, and for the LV side because of the neutral earthing resistor will need to be defined by the fault current for an earth fault is this correct?

Thanks