Sequence for substation energization 1

[Power0020]

a 230/115 kV interconnection station ,needs to be energized from 230 kV side,

After clearing commissioning steps and tests, I presume the following sequence:

1- the 230 kV bus will be livened first by one incoming feeder with all other breakers open.
2?- the power transformer 230 kV CB is closed.

3- *overriding the interlock* after about 24 hours, 115 kV CB is closed and other CBs accordingly.

This looks very systematic. However, I am worried about energizating a 230 kV transformer from a long OHTL and leaving it unloaded. I had similar experience when investigation system transients with ferroresonace arising from no load current, line capacitance and core non-linearity.

Any clue?

 

[davidbeach]

What interlocks? We always let a transformer like that soak for a day or more energized from the high side. I don't recall hearing about ferroresonance at the voltage level.

 

[wbd]

I agree with davidbeach as all the large substation commissioning has always let the transformer remain energized with no load (soak) for a day. Usually will grab a DGA oil sample at end of soak also.

Usually ferroresonance would be associated with cables attached to the secondary side on a delta delta configuration.

 

[Power0020]

There is an interlock between both 230 kV and 115 kV CBs to avoid energizing the transformer with no load! I think this was a result of the ferroresonance study carried out.

The 230 kV line is about 150 km and terminated into the transformer. It is an autotransformer with neutral earthed.

 

[davidbeach]

You're going to have to do something to control the 230kV voltage. Without shunt reactors it will be considerably higher than 230kV. You may need the load simply to keep the voltage under control.

 

[stevenal]

Can you soak it from the 115?

 

[bacon4life]

Why does it have to be energized from the 230 kV side for soaking? If you do stick with the 230 kV side, moving the LTC to the position that minimizes core flux may give you some additional margin for tolerating higher than typical voltage and would minimize core nonlinearity.

This paper mentions a couple of different ferroresonance issues at EHV levels, including both transformer terminated lines and CCVTs.

http://www.ece.mtu.edu/faculty/bamork/FR_WG/Panel/paper03gm0984.pdf

 

[Power0020]

I think ferroresonace is much related with system abnormalities. However, the consequence of having a ferroresoance during energization may be really confusing as the station TFR and relays oscillograms will need to be analyzed.

The case is quoted in the paper shared by bacon4life : *Transformer supplied through a long transmission line or cable with low short-circuit power* and I personally experienced that on a 380 kV network.

LTC seems a good option to drive it to high position (i.e. high # of turns) to get the soak done. How can I energize the 115 kV bus then? the LTC have to be adjusted to match the bus voltage before synchronization.

 

[davidbeach]

If the 115kV is energized you'd be much better off in this case soaking from the low side. You then just have to close in to the high 230 side. As soon as there is a networked connection the voltage will sort itself out.

 

[prc]

When energizing a Power Transformer with out load connected, I prefer as per below sequence:
1) Select side where the line length is minimum
2) Select HV side (to minimize the inrush current)
3) Select side with OLTC. Put the LTC at highest voltage tap(ie highest turns) - it will reduce inrush + avoid the floating regulating winding going in to "part winding resonance"
4) After the soaking period, check air or gas in gas operated relay, take oil sample for DGA
5) Before synchronization on load side operate LTC and select appropriate tap.

 

[crshears]

There is an interlock between both 230 kV and 115 kV CBs to avoid energizing the transformer with no load!

Maybe I'm being linear, but isn't a transformer always placed on potential first, then on load? or in this instance is it the case that the interlocks are present to allow for energization only in one specific sequence?

Within my utility, transformers are often energized from either the high or low side, depending on circumstances, generally without adverse consequence...although autotransformers, HV in-line regulators and phase shifters are almost universally energized with ULTCs on the neutral tap. Our 230 kV and below transformers are routinely placed on potential by means of their high-side disconnect switch [the same cannot be said of our 500 kV equipment, since the individual phases of those switches are normally separately motorized and blade timing issues would cause undesirable equipment trips].

Hope this helps.

CR

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

 

[bacon4life]

Does having slightly staggered switch blade closing change the magnitude of inrush? On our 115kV and 230 kV air break switches on transmission lines, we have seen enough difference in blade timing to cause fast ground elements to trip even though the switches are gang operated.

I think there is probably a large difference in risk between leaving the transformer open ended for 24 hours to soak and the more routine case of having a momentary delay between the 230 kV and 115 kV circuit breakers closing.