Overvoltage Relay in Transformer Protection

In the normal way of life, transformers are fed from sources of controlled voltage - like the public mains supply.

The source can go over-voltage but the transformer cannot - unless it is a voltage regulating transformer.

Even if it was a voltage regulating transfo, the overvoltage protection would be to protect the load, not the transfo.

So the overvoltage protection is part of the SOURCE and you do not need input overvoltage protection on individual transfos.

I acknowledge that, for a transfo, low input voltage is harmless, while input overvoltage and consequent overfluxing could cause damage.

I suggest the undervoltage protection associated with the transfo you write about is for the sake of the load.

Overvoltage protection in main power systems is usually on the generators and the output of any regulators down-stream of them. On sensitive loads, the consumer might add his own overvoltage protection - but his first option would to be to add an automatic voltage regulator feeding the sensitive load.

tnks frnd.... but... pls explain me clearly tis...

Even if it was a voltage regulating transfo, the overvoltage protection would be to protect the load, not the transfo...???

for example iam using 11/0.663kv transfo... in my primary side if i receive 12kv... now wat will happen...??? we are not using overvoltage protection relay...??

Several points :-

One :- 12 kV is 9% high on 11 kV. Why worry? Mains supplies vary +/- 10% by statute and the design limits of equipment recognize this, usually accommodating the worst of world wide statutory voltage ranges. See ----

http://en.wikipedia.org/wiki/Mains_voltage

-- and because a significant part of this voltage range is due to the (approximately 4%) variation of secondary voltage between no-load and full load, the voltage variation at 11 kV level is less than permitted at [secondary winding] consumer supply voltage.

Two :- If you exceed the maximum voltage on a transformer, the flux density will increase and the magnetizing current will increase. Resistive losses will increase in the primary winding and iron core losses will increase. But, unless these cause major increases in temperature, the transformer is not going to fail quickly. Even then, it matters a lot if the actual average loading is enough to make transfo average temperature reach its continuous rated limits.

Three :- Following on from Two, are the life-reducing periods at over-normal voltage compensated by life-increasing periods at below normal voltage? Unless there is an actual overload, transformer life depends on average temperature.

Four :- Even if you have an overvoltage relay, this cannot stop the overvoltage itself, it can only give a warning. To stop the overvoltage, there must be an electrically- tripped circuit breaker which can disconnect the input voltage to the transformer - to this the overvoltage relay could be connected.

Suddenly disconnecting all consumers from the power source is usually found to cause more trouble than the possibility the transformer might not last so long! And 11 kV circuit breakers add considerably to cost - most distribution transfos only have fuse protection.

Five :- If it is really high load voltage you worry about, as I already wrote, a wide input range voltage regulator feeding sensitive equipment is a solution.

tnks frnd for u valuable suggestion....

i agree wid all ur points...

but... as u said in ur first reply... u told me tat over voltage is related to source and we are using over voltage protection in source side but not in transfo side.... as similar under voltage voltage is also related to source side... ???? why we are using under voltage relay in transfo side....???

WHY undervoltage?? What would you want to happen if there were an undervoltage?

1. Ask everyone who might know. Electric supply distributor?
2. Find out voltage setting.
3. Does undervolt relay have output contacts or lamps?
4. If yes to 3. , follow the circuit or wiring diagrams to destination. Or follow the wires.

i) The probability of overvoltage is lesser than the probability undervolatge.

ii) To save cost (Overvolatge relays are costly)

iii) For High Voltage & High Capacity transformers, we do provide Over voltage Relay.