Autotransformer Tertiary Winding Configuration

[111R]

What is the typical configuration for the tertiary winding on a Y-Y neutral grounded autotransformer with a delta tertiary?

I've seen cases where:

[ol 1]
[li]The delta is intact internally with two corners tapped and these leads are brought out through bushings. This is used as a power source for station service, etc.[/li]
[li]The delta is open at one corner and these two leads are brought out through bushings. These two bushings are tied together on top and grounded.[/li]
[li]The delta has the configuration of #2, but with another corner also brought out for station service.[/li]
[li]The delta is closed internally and all phases are brought out through bushings for connecting to a reactor or other large load.[/li]
[li]Completely buried[/li]
[/ol]

Does the tertiary need to be grounded on one corner or is it allowed to float with respect to ground?

If station service is not desired, what benefits are gained from being able to break the delta above the tank? Is it mainly for testing or for protection purposes?

Thanks

 

[odlanor]

- Modern Power Transformer Practice – R. Feinberg – John Wiley & Sons
suggest grounding delta by zigzag transformer, in case of delta can operate unloading a long time

 

[prc]

The reason for (2) is you can check the tertiary winding health by testing resistance,continuity etc. When one corner is brought out, it is better to earth this terminal as it will (a) drain out charges from tertiary on switching off the transformer (safety issue -after energisation in case some one climb up and touch the tertiary terminal, he will get a rude electric shock).During assymetry on HV or LV side, power frequency overvoltages can be capacitively transferred to tertiary. Hence IEC 60076-3 ed3.0-2013 for Power Transformers recommends earthing one corner of tertiary .(see appendix B)

In case of (5) usually one corner will be earthed inside to tank surface.

 

[Mbrooke]

It may be floating or corner grounded, however 3 common applications with it:

1. left floating. It does its job with circulating currents.

2. Used for station service via connected step down transformer. Oftn the most reliable way to obtain station service.

3. Used for distribution. Often a zig zag transformer provides a neutral where needed.

 

[111R]

Thank you.

What is recommended if one wants the ability to test the winding as well as station service? Would it be like option #3 where there are three total bushings coming out of the case? Two are tied together on top and the third one is connected to the L-G station service transformer?

If so, and the tertiary is a typical distribution voltage (12.5kV, 13.8kV, etc.), what is recommended for the bushing ratings? The corner brought out for station service will obviously need to be rated for the L-L voltage of the tertiary. Does the other corner that is tied to ground on top of the transformer need to have bushings rated for the same voltage or can these be much smaller due to it being tied to ground potential?

 

[prc]

Since the tertiary is delta the voltage across the grounded corner and the line connected to service tarnsformer will be L-L voltage. A good practice followed by many utilities is to use bushings of one step higher voltage rating ie use 33 kV bushings for 11kV winding and 52 kV for 33 kV windings. This is done to take care of the transferred voltages from HV and MV windings.

In case you want to to do all LV tests on the tertiary, the third corner of delta also is to be brought out. Then take care to avoid L-L faults across the bushings(say insulating bushing live terminals, using metallic separators between bushings etc,etc)