Power transformer Y-D Y grounded 2

[jschwei314]

Are there any benefits from grounding the neutral on a Y-D transformer? This is a small 750 KVA, 13.8KV to 2.3 KV. The secoundary, 2300 volt side is ungrounded. The 13.8KV that supplies this transformer is grounded.

 

[busbar]

There may be potential problems, per IEEE C62.92.4-1991 …Application of Neutral Grounding… §4.3 If the wye-delta neutral of the transformer bank is connected to the primary neutral, the transformer bank may:
1) Circulate current in an attempt to balance any unbalanced primary phase-to-neutral loads
2) Act as a grounding bank and supplying fault current to any fault on the circuit
3) Circulate triplen harmonic currents in the delta.

An ungrounded secondary winding is subject to damaging transient ground overvoltage.

 

[jbartos]

Suggestion to schweitzereaton (Electrical) Dec 17, 2003 marked ///\\Are there any benefits from grounding the neutral on a Y-D transformer? This is a small 750 KVA, 13.8KV to 2.3 KV. The secondary, 2300 volt side is ungrounded. The 13.8KV that supplies this transformer is grounded.
///It depends. How is system grounding upstream?
What is the transformer load in terms of the transformer availability and cost of the process or load downtime?
In some applications, the ungrounded power distribution systems are used since they can survive one fault to ground that is alarm only without any tripping. Also, any overvoltages caused by arcing may be suppressed and alarmed. This way the availability of the ungrounded power supply is high. This is usually referred to as a power supply for a dedicated load, or dedicated power supply.
If the above is not the case, then the grounding will limit overvoltage transients caused by arcing, cause positive trips of protective relays on fault conditions, break or modify the zero sequence current path, etc.\\

 

[RajT]

There is no merit in grounding the N of the prim Y winding as the 13.8kV supply is already grounded. On the other hand if you did ground it then there would be circulating currents flowing between the 13.8kV neutral and the transformer neutral which would be problematic.

 

[jbartos]

Comment: It is not stated anywhere in the original posting that the 13.8kV upstream has a neutral, and that this neutral is grounded. What if the upstream winding is delta? Certainly, there would still be the ground zero sequence currents flowing between the grounded points, if the transformer 13.8kV neutral is grounded. However, this is not normally implemented.

 

[gwgroup]

In a Y-delta bank, whether the Y is primary, the grounding of the neutral of the Y permits grading of the transformer insulation and also tends to stabilize the 60-cycle potencials of its terminals.
When the source (grounding upstream) is efectively grounded, a line fault eliminates one phase voltage regardless of the connections of the step down bank.
When the source is isolated, and the source impedance is negligible , in case of a line-to-ground fault, the primary voltages are maintained. This points up the fact that an isolated system can not be effectively grounded by grounding the neutral of a small Y-delta bank capable of drawing only a small short-circuit current in case of a line-to-ground fault.

 

[jbartos]

Suggestion to the previous posting: The small y-delta grounding transformer may be used for an alarm or tripping in case of the line to ground fault.