Choice of Vector Group for a 6.6/0.415kV Auxiliary Transformer 1

[JSLeith]

I have two 6.6kV supplies from different sources. The 6.6kV supplies will be used to provide a low voltage supply to auxiliary switchboards via step-down transformers, 2MVA in size. One supply will be the normal supply the other standby. Unfortunately there is a 30 deg phase shift between sources. For operational purposes we will occasionally need to parallel the two supplies. Going through all the different transformer vector groupings commonly available and I believe the solution is to use a Dyn11 on one source and either a Dy0 or a YNyn0 for the other.

The question I have relates to out of the Dz0 and YNyn0 which would be the preferred option. With respect to the YNyn0 option I was intending to earth the star point on the LV side only.

 

[RalphChristie]

With regards to the Dz0 transformer - I am not really familiar with it, but I think it can be difficult finding one or someone who manufacture it. There will be manufacturers who will build it, but most probably it will be an one of a kind order.

With regards to the Ynyn0 transformer: I would recomend using a stabilizing winding (tertiary delta winding - thus a three-winding transformer) to provide a low-impedance path for the third harmonic currents. Even if the neutral of the star-connected winding is unearthed, the neutral point can oscillate above and below earth at a voltage equal in magnitude to the third-order harmonic component. The use of a delta tertiary will prevent this.

Regards
Ralph

 

[RalphChristie]

JSLeith:

How long do you intend to parallel this two supplies?
Will it only be for a short period when you switch over from one supply to the other?

 

[skm]

I have used Dzn0 transformer of 2MVA & 1 MVA as this gives the most optimal solution for application where one requires zero shift between the primary and secondary and also provides the possibility of neutral earthing on lV side. These transformer are used for supplying station aux loads and are suitable for parallel operation with emergency DG sets.

These transformers are genrally available with leading manufacturers. We sourced it from India.

 

[JSLeith]

RalphChristie
The transformers would be paralleled for only short periods of time to transfer load without interruption of supply, ie to take one transformer circuit out of service for maintenance etc.