132/33kV Star Delta Tx configuration 1

[bazzj]

Hi All,

Can someone explain to me why we have a star/delta Tx on the 132/33kV network and then a delta/star on the 33/11kV network. I understand the reason on the 33/11kV, I just don’t understand why it is a Star/Delta on the 132/33kV side. This makes the network a Delta Delta from the 132/33kV to the 33/11kV, so we have two Delta tx’s together. Am I correct in thinking that this is due to harmonics and Voltage imbalances?

 

[Mbrooke]

I could be wrong, but played right the 11kv system is in phase with the 132kv system. Only the 33kv system is out of phase by 30*. I don't know the specifics behind it, but that is standard practice for some POCOs.


Another reason is that a wye primary is cheaper at 132kv than delta, the insulation can be progressively reduced as the primary coils head toward the neutral point. Theoretically the voltage is near zero at the start of the grounded neutral point while in delta the lowest voltage is at the center of each winding at 57% relative to earth. The delta secondary comes in because its usually a good idea to have a delta winding on any transformer to allow for zero sequence currents a place to circulate.


Delta-wye on the 33/11kv units because the 33kv system is in most cases only 3 wire.


Earth resistance itself varies, and as such faults occurring on the 11kv side will pass to the 33kv. Any primary neutral impedance will cause L-N voltage distortion on the 11kv system- especially true on the the 33kv system is being operated unearthed or impedance earthed at the substation- a good possibility with a 132/33kv Ynd11. Unearthed or impedance earthed means a service continuity scheme, so an ungrounded primary 33/11kv transformer is the only thing that will work. Even for a solid earthed 33kv system and guaranteed low earth resistance, relaying becomes more difficult since fault current on the 11kv passes to the 33kv. Relaying settings will have to take this into account, and may result in neutral 50/51 elements being set such that a high impedance ground fault (like wire down on a side walk or resting on a tree limb) on the 33kv system may persist. On the other hand high impedance fault at 132kv are far more likely to clear even with basic 21 elements.



But to be honest I am only guessing on the 132/33kv trafo, I have no idea- delta-wye would work well also.

 

[prc]

The reason for the sub transmission transformer (132/33 kV having delta secondary - same as of the function of a stabilizing delta tertiary in star-star transformers. It will take care of unbalanced loading+ neutral stabilization..In some parts of the world, the delta connection is on primary side ie 132 kv side, esp feeding large factory loads. When the 33 kV is completely isolated one(ie both sides delta connection) usually grounding transformers are provided at both ends-sending and receiving ends of line.

 

[RRaghunath]

There is one more reason why Star is provided in 132kV system.
The star winding provides additional earth in 132kV system (132kV star point), which helps in reducing the overall zero-sequence impedance of 132kV system and is especially beneficial in weak systems.
In case of Transformers with OLTC, the OLTC for star winding will be cheaper than that for delta winding.

Rompicherla Raghunath