Loading the tertiary winding of autotransformer 2

[rsherry]

Does anyone have any comments on the idea of putting load on the tertiary winding of an autotransformer - in this case a 400/132kV earthed Star autotransformer with a 33kV delta tertiary winding?
It is not uncommon in the UK to have reactive compensation connected to such tertiaries but this idea is to use it to connect consumer load.

 

[Mazoon]

In our transmission system, we already plan to use auto transformers 2x315 MVA 220/132 kV with a 33 kV, 40 MVA Delta tertiary winding; this tertiary winding will supply the nearest consumers to substation on distribution system.

 

[Alex68]

Usually the tertiary winding is close with a resistor (or short-circuited) to damp the fault currents in case of 1ph s.c.
What could be the consequence of a 1ph fault on the HV side for the loads connected to the tertiary winding?

 

[dpc]

It is OK to put load on a tertiary winding, provided it is rated for it.

 

[RRaghunath]

33kV system may require earthed neutral as per the national electrical safety rules (it is so in India).

If so, you may need to create an artificial neutral using a zigzag transformer or otherwise.

raghunath_n00@rediffmail.com

 

[Mazoon]

On 33kV side, you can use a Delta/star earthing transformer as well, and use it as an auxiliary transformer supply to the substation auxiliary loads.

 

[cuky2000]

Yes, you can load the tertiary winding up to the kVA rating. This is commonly used for distribution application and other local service requirement instead to use an additional transformer.

For internal tertiary winding, without external connection this can not be possible.

CAUTION: There are record of severe ferroresonance in some cases loading the tertiary of the power transformer. Also for grounding application this could require additional investment as suggested in other posts.

 

[rsherry]

Thanks for the posts, yes there would be a separate 33kV earthing transformer.
Most standard units have equal primary and secondary winding ratings thus reducing loading capacity for the secondary if the tertiary is loaded.
May go for a higher rated primary on any new units to get round this but otherwise I couldnt think of any reasons not to do this.

 

[mc5w]

The Ohio Power division fo American Electric Power does this with transmission transformers that step down from 80Y138 KV down to 40Y69 KV. They use the 13.2 KV delta teritary to supply local 7,620Y13,200 volt distribution using a grounding transformer of the wye-delta type.

One word of caution is that a lot of delta tertiaries are only 1/3 of the transformer rating. You also need to have an overload relay in series with each of the 3 windings of the delta tertiary rather than the bushings for the delta tertiary. You also need to have temperature monitoring of each of the 3 windings of the delta tertiary as well as the 6 main windings. Another word of caution is that differential protection that is intended to detect a turn to turn short or excessive core loss or an internal ground fault could be a bit more complicated because a short on your local distribution could create a nuisance trip of the differential protection.

Your distance relays on the primary side of the transformer could also accidentally trip in response to a fault in the local distribution. I have seen an instance on Ohio Power where the engineers are depending on the 138 KV distance relays to protect the tertiary winding and the local distribution - a bit too cheap for my comfort but I do not work for them or live in their area.

 

[prc]

Large auto-transformers are provided with a tertiary winding .Unless specifically called for ,such stabilising windings will have a rating much less than 331/3% ,percentage that is normally provided for two winding transformers.

Normally such tertiaries are loaed with reactors or capacitors and customer normally specify the load and power factor for such loads .Then manufacturer can design the unit for ,say 100/100/30 MVA load ie you can load both secondary and tertiary for full rating with out primary getting overloaded.Then temperature rise test also will be done for 100/100/30 MVA loading.

But if the tertiary is used for a consumer load of say 30 MVA ,then the rating of the secondary shall be adjusted such that the primary is not overloaded ie secondary load will be reduced to that extent.Normally tertiary impedance will be low and any fault on terminals will cause heavy currents in auto-transformer .So normally in such cases users specify specially high impedances for tertiary or put a reactor in series.In USA one utility which was using tertiary for station loading was putting the trf on the main auto itself,ie "piggy riding"on main unit so that chances of a 3L fault on tertiary terminals are avoided.

Greatest disadvatantage as mentioned by previous contributors is the disturbance to your main power flow due to line faults on your tertiary distribution circuit.Many times this will be counterproductive.I remember a case when a utility was planning to use the tertiary of a 1500 MVA auto bank for loading station load of 5-10 MVA. I could convince the utility the futility of such a provision as any fault in the tertiary circuit will affect his 1500 MVA power flow.It may be better to put a separate MV line for such loadings rather than piggy ride on your critical large capacity auto transformer.