THREE WINDING TRANSFORMERS TERTIARY WINDING GROUNDING PURPOSE

[arjun_athi]

Dear Members,

In some cases for the three winding transformers , the tertiary winding is shorted and only two bushings are coming out from the tertiary side.
What is the reason for this?

Arjun Athiyaman.
KSA.

 

[templdl]

Do you have a picture of the nameplate of the transformer which should include the connection and winding diagram that you could post?

 

[AdrianWint]

If the two main windings of the transfomer are WYE/WYE winding then it is fairly common to provide a DELTA connected tertiary winding. This tertiary winding my not be brought out of the transformer can.

The purpose of the tertiary winding is to provide a path for the ampere-turns balancing current which will flow if the secondary must supply an unbalanced load & the star point of the primary winding cannot be connected to the system neutral. If the tertiary where not provided, then attempting to supply an unabalnced load would result in the neutral point on the secondary side shifing & the voltages become unsymetrical.

 

[rcwilson]

Many utilities use the tertiary winding in substations as a convenient source for station service power for the lights, heat, battery chargers and controls. This is secondary to its main purpose already mentioned.

The winding voltage is usually a medium voltage "standard" for the utility like 13.8 kV. The 480V or 240/120V station service power comes from a "standard" pole top or padmount distribution transformer connected to the delta tertiary winding.

Without the diagram we can't tell how your tertiary is connected. A couple of possibilities:

1. Phase B grounded internally, only A&C brought out through bushings- provides full 3 phase service to the station service transformers, using the ground lug for the grounded third leg. Two single phase transformers connected open delta is the most common (cheapest) method.

2. Single phase power only with one phase terminal not brought out. If only single phase power is needed, there is no need for the third bushing. I have not seen this approach, but it is possible.

 

[templdl]

If the two main windings of the transfomer are WYE/WYE winding then it is fairly common to provide a DELTA connected tertiary winding. This tertiary winding my not be brought out of the transformer can.

The purpose of the tertiary winding is to provide a path for the ampere-turns balancing current which will flow if the secondary must supply an unbalanced load & the star point of the primary winding cannot be connected to the system neutral. If the tertiary where not provided, then attempting to supply an unabalnced load would result in the neutral point on the secondary side shifing & the voltages become unsymetrical.

this sounds like a design that would be used with a 3 legged core transformer design to deal with the unbalance magnetic flux with a Y-Y connected transfomer.

 

[gpedens]

Three winding Transformer with grounded tertiary

Three winding Transformer with grounded tertiary

As others have stated the tertiary is basically for the zero sequence currents during unbalanced voltage conditions for 3 phase 3 leg core wye wye transformers. With out the tertiary the flux from the zero sequence would be forced into the air and couple to the tank. It can cause tank heating. It is called a stablizing winding on 3 of the 4 winding transformers (3 wye plus delta tert) in our switchyard. There are no external connections or bushings and the "A" phase is shown grounded internally on the nameplate.

As others have stated the phases are sometimes brought out for auxiliary loads and we have one of those on a 230000 volt to 525000 volt 1000000 KVA auto transformer that supplies 22000 volt 16000 KVA auxiliary load.

Four or five leg wye wye or 3 leg delta wye dont need these internal delta windings as the zero sequence flux gets routed through the extra leg.

 

[Phil Corso]

resuming the tertiasry (delta) winding is not conneced to an external load, then, the two terminals are used to close the tertiary Delta-Winding. They permit the addition of one CT in the "closing conductor" to close the delta, and/or a second CT in the GroundING conductor

The CT in the "close conductor" will detect asymmetrical-faults such as turn-to-turn, or winding-to-winding!

The CT in the GroundING conductor will detect ground-faults in the delta winding and in the 4 associated bushings.

Regards, Phil Corso,

 

[rcwilson]

Phil, Great explanation. I was trying to remember how the delta teriary winding CT's were installed. What you described is a cost effective way.