Overvoltages in Consumer Premises

What are the primary and secondary voltages (both intended and actual)? Single or three-phase? Star or delta? If one phase to neutral is 280V, what about the other phase(s)?

its phase to neutral voltage..three phase transformer..dy11 phase group..one phase vrn 256;vyn 268;vbn 280 volts...

It can happen due to light load condition.

It can happen if the load has excessive capacitive element.

It can happen if the supply voltage itself is high. Obviously the problem is not with the transformer in this case.

I guess this is a typical Indian rural situation.

Regards.

It is impossible to be a capacitive-load in electrical distributions.

Always it is inductive and Power factor correction is implemented by Power suppliers on line or the industrial consumers are obligated to use PFC to bring the power factor as near to 0.9 [not 1.0 which is never acheived].

My comments only point out causes for over voltages. It does not say what is the specific cause for the over voltage experienced as indicated by OP. Please read my comments carefully.

Is this trafo located near the utility co. voltage regulator/OLTC? In a bid to compensate voltage drop for the far end customer, may also result in nearby customer seeing higher voltages. By the way, can you tell tap positions of the subject trafo.

have you checked open circuit voltages on the secondary? L-L? and L-N?

have you checked primary voltages?

while checking primary voltages, have you also checked if the same are balanced?

if primary voltages are balanced and secondary open circuit voltages are still unbalanced then please check the transformer.

in a village i presume there will be many single phase consumers so unbalance under load does happen.

has the unbalance surfaced recently? was there "good" voltage earlier? if yes then check history of new connections added after which the problem is first observed.

generally 100 kVA transformers in rural India service anything between 25 to 100 small LV consumers. it is very unlikely that any consumer may have connected large capacitors to the system. in such rural scenarios cable capacitance is also rarely a predominant condition. however voltages going as high as 300 Volts at night are often seen. poor villagers have just reconciled to living with failed appliances at their homes and farms. changing taps creates problems in day time when voltages fall to levels around 170 Volts.

i suggest you first address the unbalance and resolve as best as possible and then look for over voltage solutions. this is only to improve your chances of success.

This is a common phenomenon in rural electrical distribution system in India. Following steps should be taken to mitigate the problem :

The transformer neutral earthing should be thoroughly checked. If spike earthing is used, it should be relpaced by double pipe earth stations and should be connected with G.I. flats and earth resistance should be measured.

The loads, specially single phase loads should be made balance as far as possible.

Checks all jumper connections in overhead conductors to reduce the junction resistances and also connections in consumer premises.

Check whether there are unauthorised tappings in the overhead lines vis-a-vis distribution system. Also check the condition of transformer tap switch contacts.

Regards,

Manindra

Hi Nhadkine, the supply transformer is Delta (primary) Star (secondary).

I would suspect that one of the consumers on the transformer neutral connection, no that can't be right because it would only affect that customers load.

I think you have to go back and take a good look at the neutral connection, testing the voltage to the neutral is not good enough, you have to check the connections on your neutral, this is a supply problem, not a consumer problem, check for possible loose neutral connections and also possible poor crimping on the neutral connection.

Best Regards

Joe

refer to raghunath7 comments + what is tap changer position