MV neutral grounding

[WECSman]

We have a system needing neutral grounding, and lead times are way out. Can an existing pad mount transformer be reconfigured for Zig-Zag or grounding neutral configuration?

35 kv collection system on a wind farm.

 

[davidbeach]

Nope. The 3-phase zig-zag has six windings, two half windings per core and the standard pad mount transformer won't let you get at the mid-point of each winding to make the zig-zag connection.

What you could do instead is use a grounded-wye/delta transformer with the wye connected to the system to be grounded. The delta can be left floating. A grounding resistor can be added if desired.

 

[WECSman]

A pad mount has 6 internal windings, 2-per core, separated internally and brought out. Could we not reconfigure the internals to create a grounding unit? Existing windings are 34.5 delta, and 575 wye.

 

[davidbeach]

Nope. If the two sets of windings each had the same number of turns it might be possible, but you would be far from a zig-zag with that difference in the number of turns per winding.

 

[dpc]

A standard delta-wye, or wye-delta transformer can be used instead of a zig-zag transformer (for grounding the system connected to the wye winding, obviously). This is done all the time and is generally more cost-effective these days than trying to get a zig-zag grounding transformer.

Since your question is "MV neutral grounding", I assume you're trying to establish a neutral for the 34.5 kV system? Even if you re-configured the delta connection to a wye, you still have to get the neutral through the tank, and deal with the secondary voltage issues.

It would be simpler to buy a new wye delta transformer. You don't need any load on the delta side.

 

[waross]

I understand that all the winding ends are brought out.
If so you can reconnect the primary in star and the secondarys in delta.
The voltage on each primary winding will be 19,900V. Not a problem on a 34.5 winding.
Likewise the delta secondary winding will have a voltage of 575V/1.73 = 332V when the primary windings are connected in wye and 332V/1.73 = 192V.
If one primary phase is grounded the corresponding secondary voltage will be zero. the voltage of the other two secondary windings will rise to 332 volts. The resulting circulating current in the delta will be determined by the transformer impedance. Adequate primary current will flow to ground to alow the operation of protective devices. If high resistance grounding is desired, this can be accomplished by inserting resistance in series with the secondary delta connection.
Primary current to ground may be measured directly or infered by monitoring the secondary current in the delta.
respectfully

 

[davidbeach]

waross, I've never seen a pad-mount transformer where both ends of each winding was accessible, but if they were your suggestion should work. If a grounding resistance is desired, it can be inserted between the neutral of the wye and ground.

 

[waross]

Agreed davidbeach.
I have never seen one either.
However I understood from the original poster's 27 May 06 16:28 post that this transformer was the exception.
Agreed also, the resistance may be inserted as you suggest.
Inserting the resistance in the 575 delta secondary is analogous to the secondary resistance in a single phase grounding transformer. The voltage across the resistance is lower during faults, but it's the designer's choice.
respectfully