Star/star or star/delta
Star/star or star/delta
(OP)
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.
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.






RE: Star/star or star/delta
Regards
Marmite
RE: Star/star or star/delta
RE: Star/star or star/delta
RE: Star/star or star/delta
Regards.
Grundig
RE: Star/star or star/delta
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
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"Why not the best?"
Jimmy Carter
RE: Star/star or star/delta
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
RE: Star/star or star/delta
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