No load transformer tertiary configuration

[Bronzeado]

Dear "threaders" (?),

To avoid short-circuits on three-phase transformer tertiary terminals (13.8kV), it has been decided to leave them without feeding loads, with surge aresters phase-to ground being connected to each phase.

In the case of Y-connected tertiary, there are the options:

a) Leaving the neutral point floating, with a surge arrester being conneceted neutral-to-ground or

b) Grounding the neutral point (solidly or throught an impedance).

I will appreciate to receive your thoughts on these configurations.

Thank you in advance.

Regards,

H. Bronzeado

 

[sberbece]

Hi,

The surge arresters (SA) connected on each phase-ground will stabilize the voltage on each phase and protect the windings against overvoltage.
However, in a Y configuration, the windings usually have gradient insulation, lower close to the neutral point where the voltage to ground (transformer’s body) supposes to be low.

If you let the neutral point floating, there is a good chance to stress the insulation around the neutral point and have a fault due to an elevated neutral voltage.
Same effect is you connect the neutral point to ground through a MV SA.

Personally, I would connect the neutral point directly to earth, saving the neutral impedance (NER) for other applications.
The NER is used to limit the return current in case of a fault to earth. Because you do not have a load connected to the transformer terminals it seems that there will be no return current through the point of connecting the neutral terminal to earth (pig tail). Though, there will be the SA-s leakage current circulation between the SA-s and the windings neutral point. I assume that the SA-s are installed on the transformer and their earthing terminals are connected to the transformer’s earth terminal, not directly to the earth grid.

Obvious, apart of transformer’s mechanical protections (i.e. temperature, Buchholz and oil pressure, if any) the transformer differential protection relay shall supervise the tertiary winding as well, in order to protect the transformer against internal faults.

Regards,
Stefan

 

[Bronzeado]

Hi Stefan,

Thank you very much for your reply and advices. I agree with you.

However, there is a opinion that in the case of grounding solidly the neutral point, there is a risk to have a short-circuit if just one SA fails.

If the neutral is floating, the short-circuit will occur only if the SA of neutral point fails together with a phase SA. Also, some manufacturers say that the isolation of the neutral point is similar to the phase isolation.

That's is why I ask for the global experience on this.

Regards,

Herivelto

 

[REM76]

Hello

Leaving the Y point ungrounded could result in tremendous voltage escalation during arcing ground faults. This will damage transformer insulation and blow up surge arresters.

Solidly grounding the Y point will result in an excessive ground fault current during bolted fault currents.

With keeping the SA on the phase bushings, I would also recommend high resistance grounding the Xo point utilizing a step down single phase transformer with a resistor and a 59G relay connected across its secondary terminals. The value of the resistor should be selected to limit the ground fault current to a value higher than the charging current of the system and below 10 Amps.

Thanks,

Rakan

 

[Bronzeado]

Thanks Rakan.

Why using a step down transformer insteady connecting a resistance straight to the neutral?


Regards,

Herivelto

 

[REM76]

I believe an 8 KV resistor rated would be more expensive verses a 8000 V /240-120 V step-down transformer; 8000= 13.8/1.73. In additon, you need the transformer anyway for the 59G relay.

Thanks,

Rakan

 

[mgtrp]

Although I would certainly not profess to have much expertise in this area, two questions/points:

1/ What am I protecting against by installing a surge arrestor on the tertiary? There are no incoming lightning strikes or switching surges from the tertiary winding, so any issues would be those transmitted through the primary or secondary windings, which should be fitted with their own surge arrestors in addition to the impedance presented by the transformer itself/

2/ By doing anything other than grounding the neutral point of the tertiary winding, what am I protecting against? A ground fault on the winding itself is going to result on replacement of the winding as a minimum, if not the transformer. Or would you add resistance grounding, hoping to minimize the effect of a fault on the tertiary winding (which I would see as fairly unlikely in an unloaded winding) on the primary and secondary windings?

As an aside, I come from a background of 11 kV nominal voltages and am still adjusting to 13.8 kV nominal voltages in my adopted country, but would expect an 11 kV winding to be fully insulated (as opposed to graded insulation), and therefore would probably also expect a 13.8 kV winding to be fully insulated.

 

[jghrist]

I agree with mgtrp. Why use arresters? For that matter, why would you have a wye connected tertiary if you are not going to load it for fear of having faults? I can see having an unloaded delta tertiary, but of what use is an unloaded wye tertiary?

 

[slavag]

Hi.
Are possible see this case, as AT with unloaded delta tertiary?
Regards.
Slava

 

[Bronzeado]

Thank you all for the repplies.

"Why use arresters?
- The use of SA is due to overvortages that can be generated by switching, system faults and lightning. We have already faced short-circuit in tertiary during transformer energizing. It seemed to happen due to overvoltages caused by transformer internal ressonances.

"... why would you have a wye connected tertiary if you are not going to load it for fear of having faults?"
- Some of the old transformers were designed with tertiary windings to supply loads, utility services and/or to connect reactive power equipment for voltage control. Nowadays, transformer tertiary is avoided when possible.

Regards,

Herivelto Bronzeado