Star/star or star/delta 1

[reactive]

Haven't been on the forum for a long time. Hoping I can get some advice.

A client is designing a network for a mine. The supply transformers are 2 x 80MVA 132/33kV Yd earthed via a NECR.
The 33kV is required due to the heavy load and can be considered as an intermediate voltage.

The mine and plant are fed via overhead lines at 33kV and then stepped down to 11kV via 40MVA transformers (3 off at the mine and 2 off at the plant). The LV transformers are 11kV/525V Dyn.

The question - What vector group is recommended for the 40MVA 33/11kV transformers ? Our client has proposed Yd earthed via NECR. I would imagine that Yy could be considered. If so how would they be earthed ? What is the best solution, technically and financially ?

Thankyou.

 

[Marmite]

I would suggest Yy or Dy for the 33/11kV transformers. The advantage of the 11kV secondary being Y is that you can earth the star point either directly or through an impedance. If you go for delta secondary you will need to buy an earthing transformer, which will cost you more money for the transformer and possibly for a circuit breaker to control it. If there are no single phase 11kV loads connected ph-n then either Yy or Dy will do. The costs will be very similar. If you go for Yy the 33kV star point can be connected to earth via a VT to provide neutral displacement protection, which is used to prevent backfeeding grounded overhead line conductors.
Regards
Marmite

 

[davidbeach]

You will have much better control over the ground fault current with a [Δ]-Y transformer than you will with a Y-Y. The [Δ]-Y completely separates the zero-sequence network on the secondary from the zero-sequence network on the primary. The Y-Y does not provide that separation.

 

[rbulsara]

This has been discussed here before. The bottom line is unless you have a very good reason (like to match an existing vector group etc.) for any other configuration, use delta-wye, which is superior as davidbeach said and most common set up.

 

[vennivivi]

To complement davidbeach`s post,zero sequence currents cannot flow in the lines connected to a delta connected winding because no return path is available for these zero sequence currents.However, the zero sequence currents, caused by the presence of zero sequence voltages,can circulate through delta connected windings themselves. Delta conncetion winding is also desirable in many power transformers for reasons of harmonic reduction.

Regards.

Grundig

 

[waross]

I agree completely with davidbeach on this.

You will have much better control over the ground fault current with a ?-Y transformer than you will with a Y-Y. The ?-Y completely separates the zero-sequence network on the secondary from the zero-sequence network on the primary. The Y-Y does not provide that separation.

However, what may not be clear from this statement is that instead of flowing as a zero sequence current on one phase and the neutral of the primary as in the case of a YY transformer, the secondary zero sequence is transposed into phase currents on the primary circuit. There will be no neutral or ground currents reflected onto the primary, but there will be a corresponding phase current on the primary side.
I see circulating currents in a wye/delta secondary when, contrary to engineering recommendations, the primary neutral is connected, but I see supporting currents in a delta primary when there is a zero sequence fault on the wye secondary.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Bronzeado]

As Marmite said "if you go for delta secondary you will need to buy an earthing transformer...".

Also, for ungrounded systems, beware on overvoltages on the sound phases in the case of one of them touch the ground.

Best regards,

Herivelto Bronzeado

 

[sberbece]

Because of the risk of overvoltage in the healthy phases of the 33 kV network, during a single phase fault to earth, preferable is to use 132/33 kV Yd and 33/11 kV Yd, both transformers earthed through a NER. This configuration will allow you to pick up any earth fault and maintain the voltage stable in the other two phases during an earth fault.

Since the 33 kV system total impedance is symmetrical (i.e. the impedance of the windings of the two power transformers and the transport line), there shouldn’t be high zero sequence current circulation through the 33 kV winding neutral connection. However, this is not the case during a single phase to earth fault, case when the NEC became very handy in limiting the current circulation through the neutral connection.

I’d go with Client’s suggestion, it is easier to control and protect.

Regards,
Stefan