Autotransformer Tertiary Winding Configuration
Autotransformer Tertiary Winding Configuration
(OP)
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:
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
I've seen cases where:
- 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.
- 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.
- The delta has the configuration of #2, but with another corner also brought out for station service.
- The delta is closed internally and all phases are brought out through bushings for connecting to a reactor or other large load.
- Completely buried
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






RE: Autotransformer Tertiary Winding Configuration
suggest grounding delta by zigzag transformer, in case of delta can operate unloading a long time
RE: Autotransformer Tertiary Winding Configuration
In case of (5) usually one corner will be earthed inside to tank surface.
RE: Autotransformer Tertiary Winding Configuration
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.
RE: Autotransformer Tertiary Winding Configuration
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?
RE: Autotransformer Tertiary Winding Configuration
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)