Power Transformer Delta Tertiary Insulation 1

[CKent]

Good day!

For a 115kV/34.5k/13.8 kV 3-wdg power XFs, what should be the minimum insulation level of the 13.8 kV wdg if:
a. Unloaded tertiary, with no leads taken out
b. Tertiary shall be loaded, either via station service or capacitor for system improvement.

Note that tertiary is not corner grounded.

I am assuming that the 15 kV insulation level (110kV BIL) will suffice. However, some transformer manufacturers suggest to have 25kV insulation (which is about 150kV or 170kV BIL insulation). Of course this results to higher transformer costs.

Additionally, if we intend to ground one corner of the delta tertiary in cases we need to load it, what would be the voltage across the different phases?

 

[prc]

Your proposed insulation level(110 kV BIL ) is sufficient.Some times an higher BIL for tertiary will be required to take care of the transferred surges from the winding nearest to it.In case it is a 220/132/13.8 Kv auto-transformer,BIL of tertiary must be atleast 170 BIL to take care of transferred surges.If it is a 400/220/13.8 kV unit,BIL of tertiary shall be 250KV.Otherwise you should provide suitable surge capacitor/arrester to take care of transferred surges.

When one corner of delta is grounded,voltage to ground at other terminals will be 13.8 kV ie line-line voltage.So protective arresters also should of 13.8KV class.

 

[RRaghunath]

prc,

Good explanation. Star for you.

Could you also please inform in case of unloaded tertiary with corner grounding, do we still need higher insulation / surge arresters!

Thanks.

 

[prc]

With one corner tertiary grounding,transferred surges will not come down,and the same surge protection will be required if BIL level is not enhanced. In India, some 30 years back,many large 220 kV autos failed in service due to these transferred surges to tertiary. With one corner of tertiary in grounded condition, trasferred surges caused flash over at tertiary bushings resulting in transformer LL faults.In Europe and US(where tertiary is used for caterring local loads or capacitors)nominal BIl is used for tertiary,but with surge protection provided.In India,where most of the time, tertiary is not loaded,higher BIL is adopted.Many times,only the tertiary bushing rating need to be enhanced,as the tertiary winding has to meet the transferred surges anyway during tests.

 

[CKent]

Thanks prc...did you mean that the 15kV insulation level is enough to withstand the transferred surges in a 115kV/34.5kV/13.8 kV YG-YG-D powerXF?

from your last post, I understand that this transferred surge occurs only when the XF is being tested, is that correct? what test produce this transferred surge?

 

[RRaghunath]

Thanks once again for detailed explanation, prc. It is enlightening, Appreciate.

 

[prc]

CKent.Your first question.Yes 15 Kv insulationlevel is sufficient.More BIL required only when LV voltage is 132 KV or 220 KV.
Trasferred surges to tertiary occur when LV or Hv is tested for impulse or switching surges.In service due to travelling surges from lightning or from switching surges.So transferred surges are ocurring not only during testing.In fact all tests are done to simulate max service severe conditions!