Voltage Transformers

what i can understand,that the voltage transformere (240/120) is used to transform the voltage to suitable value for the relay .

I would advise checking the relay manual as the relay may not be able to operate directly off 240V ac (hence the step-down transformer).

in addition the fuse canot be used as a protection this because that we need continous grounding for the protection of the generartor.

I am not familiar with your local electrical regulations and standards regarding the fusing of voltage transformer outputs. Only the primary side of the transformer is directly connected to the earth so secondary winding fusing may be allowed, operation of the secondary fuse will only effect the relay it is connected to.

I do not fully understand your application.I do know that we can have protection fuses on the primary ( to protect against short turns in VT) and secondary sides of the voltage transformers ( to protect over loading & shorting in meter or control ckts connected to VT).VT burden or loading output is in va and is used to operate voltage coils of varous meters such as voltmeter,kw,kwh,kvar,pf,phase indicator,under & over voltage relays,control voltage for electronic devices and even printer etc.

In your case, if you use VT for under voltage relay operation besides many other connected devices,you may have a secondary fuse blown due to the other devices and causing the undervoltage relay to trip the main supply which is undesirable.

Regards,

Khor

You never, never, never put a fuse in a grounding transformer circuit. If it blows, your neutral is then ungrounded. If you don't have someone standing there watching it 24 hours/day seven days/week (kind of a boring job), the ungrounded condition can go un-detected.

In the design described, the neutral will NOT become ungrounded. The neutral is grounded through the primary winding of the transformer with no fusing. The fuse in question would be on the secondary side, completely separate from the neutral circuit.

However, Post #4 by raghun is correct - putting a fuse in the secondary will protect a $500 relay, at the possible expense of failing to protect a $5M generator. Not the best cost-benefit ratio.

Now I understand. It is NOT voltage transformer that we are talking about.It is neutral grounding transformer with secondary output voltage.Sorry for wasting time.

Regards,

Khor

AbdELaziz Taman, Yes, VT across Resistor in case of distribution trafo type Generator grounding system is different from a conventional VT. The conventional VT has voltage at its secondary terminals all the time when the generator is working and hence, the fuse will save the VT in case of wiring fault on VT secondary. The VT across grounding resistor will have no voltage except when there is a ground fault in generator system. Hence, the fuse if provided will not serve the same purpose as that in case of conventional VT. When a fault occurs in the generator system and there is voltage at the VT, you do not want protection to fail to operate because of a (stupid) fuse, I mean protection operation at that point of time is critical. Hope it is helpful.

dear sir,

I would like to thank you.very thanks for your support.

in addition i would like to inform you that i believe this answer

BEST REGARDS

Eng.AbdElaziz Taman

Egypt.Nubaria Power Station