Ungrounded Wye Power Transformer Neutral Point Voltage Rise

[Power0020]

While reviewing a 220/66 kV substation design in IEC world, I noticed that the power transformer has two neutral points connected to ground through disconnectors, I learnt that these DS are normally closed during operation and opened only for maintenance.

The 220 kV neutral side is equipped with 123 kV surge arrester, disconnector and current transformer. 36 kV rated similar equipment is used for 66 kV neutral side.

Some points I noticed here need to understand:

1- Why, ever, should the neural point be opened? what maintenance requires that?.

2- If, once the DS is open, a fault occurs somewhere on 220 kV network, the faulted phase to ground will cause the "float" neutral voltage to rise up, while the two other phases are still connected to some other transformers having their neutrals solidly grounded, heavy circulating current will appear on these healthy phases!, again, the neutral point voltage with respect to ground will be almost the phase voltage, i.e. 220 kV rated equipment should be placed, not 123 kV "L-L". The same for 66 kV side.

In other words, the neutral to ground voltage will be 220/1.732 = 127 kV, the proposed DS has a phase-to-phase insulation"though it is made of single of 123 kV i.e.71 kV phase-to-ground insulation!.

I tried to calculate the surge arrestor TOV capability, it is still lower that the tripping time of 1 S in that case and an explosion may well occur!, if no flashing takes place.

3- Why the CT isn't placed after the disconnector to reduce the required insulation voltage? the CT will never read a surge current "lightning, maybe" going through the surge arrestor when the DS is left open "the healthy case with no faults".

4- The Transformer neutral bushing is clearly rated at 123 kV "graded insulation" as well for 220 kV side and 36 kV for 66 kV side, I wonder if this permits running it open, ever! any IEC or IEEE recommendations for that?

 

[waross]

I would consider replacing the disconnect switch with a bolted jumper.
This would preclude the possibility that the switch may be opened when it should not be.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

I think it has to do with ease of testing. The windings are floated when leads are taking measurements, and before being put back into service the wye points are grounded back down.


Someone would know more then I do, but because you have a tertiary I think you could actually float the primary.


In typical 220 volt networks you have zero sequence sources else where, and power is flowing 220 to 66kv (generation >>> load)

 

[prc]

I have seen similar arrangement in some specific markets eg Syria. Actually the transformer will be switched on with DS open and after a few seconds it will be closed. When Ds is closed LA has no effect. Why it is done, I forgot and I could not locate the page where I had jotted down the reason.

 

[Power0020]

Well, seems to be something related with switching surge maybe? but, if so, why not to divert it to earth initially?

 

[waross]

There is a case for disconnecting the primary wye point from ground/neutral on a wye delta transformer bank.
A four wire wye:delta transformer is subject to severe circulating currents.
The circulating currents may be avoided by floating the wye point, however floating the wye point may lead to damaging transient over-voltages when the bank is energized.
Back when wye:delta banks were more common and were also commonly protected by individual fused disconnects field service could cause issues.
It was common for these banks to have four fused disconnects, the fourth disconnect in the neutral connection.
Service crews always carried spare fuse holders on the service truck.
When such a bank was to be re-energized, a spare holder/fuseholder would be installed in the neutral connection.
The bank could then be energized phase by phase with the individual cutouts without fear of transient over-voltages.
When the transformer bank had been safely energized the neutral fuse/fuseholder would be removed and taken away in the service truck.
About ten years ago I saw a pole mounted transformer bank with a wye:delta connection complete with the four fused cutouts. I have since searched in vain on Google Street View, and driven the area when I was in the locality hoping to get a picture.
It must have been replaced or removed. I can no longer find it.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

This odd-looking configuration isn't uncommon in Kuwait where rapid growth of loads and increased generation to support those loads mean that system fault conditions now exceed the capability of switchgear close to the generating stations. By selectively opening the disconnectors on certain transformers the earth fault levels can be managed to a tolerable level.

 

[prc]

A correction on my posting-during switching on,the DS is closed to avoid ferro-resonant over voltages and soon it is opened(automatically) to limit the LG fault as mentioned by Scotty.

 

[Power0020]

Makes sense, so it is to limit the LG fault current into the substation.

The overvoltage that may appear on the neutral in the case of running it open can be destructive, the surge arrestor will be loaded beyond its TOV capability, the other issue is that the fault current is very small, means that the overvoltage duration will be prolonged due to the slow remote end protection operation.