Insulation Co-ordination of Power Transformer

The reason you need a neutral-to-ground connection is that the transformer secondary may be in parallel with other sources of power, and a single connection prevents currents circulating in the neutral otherwise. The circuit protective devices will still work just fine.

This worries me in multiple ways on the same topic. People who do not know or understand the power distribution rules for a region should not be allowed to touch any of it, let alone get a contract to do this kind of work.

Don't worry. The poster is Anonymous. :-$

Clearly for a good reason.

<...We are doing a project...>

Consult an Electrical Engineer within the organisation, to organise mentoring.

I agree with PWSlack: Consult an Electrical Engineer specialist in this field.

I am not a specialist in this field. In my opinion you have to consult the manufacturer.

However, I know two standards recommending the impulse test requirement and procedure:

IEEE 32/1972/1990 ch.5.2.1 Impulse Tests (see definition in Section 13).

and IEC 60076-3/2000 ch.7.4.3 Neutral terminal not directly earthed.

If there are no codes, specifications, regulations, inspecting agencies, safety requirements, insurance companies underwriting the loss of life and property associated with this project in "Eastern Europe", then don't worry, be happy.

Forge ahead with blind abandon!

I'd leave the country PRIOR to commissioning of the project, if at all possible.

For the record, I don't have a clue.

1. Can Non-uniform insulated transformers be used without Neutral Grounding? (Ungrounded). Yes if the OEM design allows it. Contact the OEM.

2. Assuming the system is ungrounded, what will happen if one phase is faulted (Single line to ground fault)? Does the Power Frequency Voltage of HV (Neutral) is enough? Nothing. This method is used regularly in order to adapt system protection relay schemes for ungrounded 3 phase systems. However, if a second phase should go to ground there can be catastrophic damage experienced by the transformer.

You have star connected 132kV winding with non-uniform insulation. Neutral may be isolated from ground using disconnecting switch but still the surge arrester will earth the neutral in case of over voltage appearing at the neutral terminal. This is my understanding and hope it is correct.

With the system as above, the 132kV winding is not designed for earth isolated operation.

In case the disconnecting switch is kept open, the surge arrester will have to protect the winding insulation closer to neutral by conducting the overvoltage to earth, when there is an single phase to earth fault elsewhere in 132kV system. This is the case if the transformer has source on the LV side.

If the subject transformer is at the load end, stepping down the grid voltage to supply loads downstream, there shouldn't be a problem of overvoltage and no issue in operating the transformer 132kV winding neutral unearthed.

Dear Sir,

Thank your for your reply.

In case the disconnecting switch is kept open, the surge arrester will have to protect the winding insulation closer to neutral by conducting the overvoltage to earth, when there is an single phase to earth fault elsewhere in 132kV system. This is the case if the transformer has source on the LV side.

What happens if Line to ground fault occurs at the 132kV in actual?

If the subject transformer is at the load end, stepping down the grid voltage to supply loads downstream, there shouldn't be a problem of overvoltage and no issue in operating the transformer 132kV winding neutral unearthed.

Why there will not be a problem, for the transformer at the load end?

When there is line-to-ground fault in 132kV system eslewhere (such as OHL or Cable feeder upstream), the current tries to circulate through the transformer neutral, provided the transformer has source(s) connected on its secondary. If the DSis open and thus current circulation is blocked, the neutral assumes voltage with reference to ground that is close to phase-to-ground voltage of the system. Since the transformer is non-uniformly insulated, the transformer neutral is unlikely to withstand this voltage stress and transformer may fail.

If the subject transformer is at the load end, the transformer neutral doesn't form part of earth fault loop for circulation of currents (instead the transformer at the source end will take that role) and hence would be safe.