embeded vs normal vs no tertiary on 500 MVA 230/115 kV transformer

[bacon4life]

I am going to be purchasing a new 230/115 kV auto transformer and am considering what kind of tertiary, if any to specify.

The bank this is replacing originally had a 13.8 kV tertiary connected synchronous condenser, however that was removed a decade ago. There are no plans to use the tertiary for power factor correction or customer load, although it might be a backup source for station service. Any thoughts if a tertiary is even required, and if so, whether it should be brought out for possible use as station service or just embedded to protect against faults?

One of the possible reasons for a tertiary is for 3rd harmonics or voltage unbalance. All of our 115/15 kV transformers are delta-wye, so I don't think there should be much 3rd harmonic. However I have been unable to check what the actual harmonic levels are.

For reference, thread238-125717 was somewhat similar.

 

[prc]

You can safely eliminate the tertiary if you are purchasing a 3 phase 3 limbed core transformer.For this rating it may be a bit difficult and normally 5 limbed core is the practice.I have seen 3 phase 5 limbed autos, without tertiary,in operation with out problem in Itallian grid.In US, banks of transformers of this size are in operation with out tertiary.Considering that you have delta connected transformers on secondary side,even 5 limbed 3 phase trfs and single phase banks may operate without problem evenwithout tertiary.

In case, you are going for a stabilizing tertiary,never opt for embedded one.Better to bring one corner of delta outside (broken delta) and take out through two bushings and earth it after shorting the bushings.Advantages - you can avoid tertiary by opening the bushing shorting( no current flows through tertiary during a LG fault), in service you can check the healthiness of tertiary through these bushings.

In the earlier thread, I have mentioned the risks of station loading from tertiary.In such a case, tertiary has to be specified for 3L fault withstand and ensure adequate impedance between primary to tertiary and secondary to tertiary.