VT Protection

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Thanks, actually I read a lot, I would like to know a methodology to calculate the short circuit impedance of the VT and its corresponding short circuit current with the nameplate information given, because is the information that you have in real life.

Best regard

Fault calculatio'ns of industrial! commercial ,power systems - IEEE Xplore

Chap 5 Sol

Calculating Short-Circuit Current | IAEI Magazine

TESTING POWER TRANSFORMERS

Could you help me calculating the short circuit impedance of the VT with this information:

Thermal Burden: 1000VA

Nominal Prim. Volt = 69/square(3)

1a-1n 2a-2n 1a-1n

10 VA 10 VA 10 VA

CL 0.2 CL 3P CL 3P

600:1 600:1 600:1

Best Regard
Thanks in advance

No. I'm just the reference librarian here.

Did you read anything in #3?

Can you show the work that you have completed already?

We don't answer homework questions as a matter of policy....You have been given the methodology, the 'how to do it' if you will, now you must attempt to answer these questions.....we will check your work if you will take time to show it to us....

Thanks SolarEagle

I'm not asking for my homework this is something that I have to do at my work, I've been reading on differents standards trying to find a methodology to sizing the MCB on the secondary of the PT with the information given for the manufacter.

I really appreciate the information given to me but it is not according what I need.

I'm reading rigth now this book "AC Circuits and POWER SYSTEMS in PRACTICE - Graeme Vertigan" if I find a proper way to solve this, I'll share the methodology here, if there somebody who know how to do it this I'll appreciate the help.

Best regards

Thanks in advance

This might help....

https://electricalnotes.wordpress.com/2017/05/09/setting-of-overload-short-circuit-ground-fault-protection-of-mccb-part-1/

http://www.studiecd.dk/cahiers_techniques/Calculation_of_short_circuit_currents.pdf

<...have to do at...work...>

There should be a standard procedure available at the workplace that details how to carry this out, so that others can follow, or at least a Mentor at the workplace that can guide others through this work. Anything other than that would be grossly unfair by the employer, and absence of both presents a significant business risk.

You don't give information on where you are located and what jurisdictional codes and standards apply in your location. Protection of PTs is not usually based on short-circuit current, but on maximum full-load current (FLA), which can be calculated from the VA rating and the winding voltage.

In the US per the NEC, for a transformer with secondary voltage less than 600V, it should have secondary protection set at 125% of FLA in "any location" or 250% of FLA in a "supervised location."

Also, I am assuming that your PT is 69kV on the primary? Otherwise the 600:1 ratio seems kind of unusual. That would make your secondary voltage = 115V, and your FLA = 8.69A based on the 1000VA thermal rating. Typical secondary fuse or breaker, depending on situation, would be 10A to 25A.

Thanks currently I'm working in Guatemala, here we use US standards.

Your asumming is correct, 69kV on the primary side, according to what I read, your don't sizing the MCB on the secundary of the PT on based of the thermal burden, because this is even greater than the nominal burden. On the PT Test you guarantee do not overload the connected burden above the nominal burden of the PT that is why the thermal burden is not a criterian. The protection on the secondary of the PT find to protect the secondary from downstream faults and to provide a convenient method of isolating the VT. That is why I'm trying to calculate the short impedance circuit according the information given by the manufacter of the PT. To calculate the short circuit current on the secundary of the PT.

If PT secondary voltage is 115V nominal and burden is 10VA , that makes load current 10/115 = 0.09 amp which is the maximum load you would connect. So any current exceeding this could be set for an overload device

But thermal rating is given as 1000VA - not clear if this is per winding or total - take as total - one winding is 333 VA, so current rating is 333/115 = 2.9 amps. It is not likely all 3 windings would get overload at same time, so 3 amp is a safe setting.

So you could fit a 3 amp fuse or breaker. Or less, because no-one should connect more than 10VA load.

But the fuse/breaker must be able to break the secondary short circuit current and non of the information given defines that. If the test document or rating plate gave an impedance or a short circuit test result that would be helpfull.

You could do a short circuit test by shorting secondary and applying 230V to primary. Supposing S/C is 300 amp @115V, you would get 300 x 230/69000 = 1 Amp applying 230V to primary (but note secondary OC volts are just 0.38 so you should/connect direct to PT terminals/ not use long thin wire or an ammeter with more than mV drop at 1 amp).. Or you could measure DC resistance of HV & LV windings, transfer HV resistance Rp to LV by adding Rp/600² to LV resistance to get Rs'.

Then SC current is 115/Rs'. Any leakage reactance will reduce the actual current from that estimate.

Any 230V MCB is likely to have a 6kA breaking capacity and a type B breaker would trip instantly [magnetic] at <= 5amp for a 1 amp rating. You would have to check that the most distant S/C fault on PT 115V circuit will exceed 5 amp.

For the protection windings, you have to be carefull that reliability of protection is not affected, so you would go for a 3 amp bolt-on fuse - less easy to flip off & forget than a breaker. Or you might decide a 10 amp fuse giving S/C protection alone is enough - even 1 sq.mm site wire is rated >10 amp.