Substation Commissioning - short circuit

[Paschoal]

Hello,

A few days ago I was on a commissioning and see something new.

They short circuit the PTs and apply 380V on the power transformer, with this they basically "see everything" on the substation, but I don't see how a short circuit current can show this much.
Image "ENTRADA LINHA".
Any of you can help me to undestand the teory and the math on this situation ?

[duak]

I'm not a power systems engineer so I'm going from my schooling and experience with small transformers and power systems.

Most of the basic characteristics of transformers is determined by both open circuit and short circuit testing.  An open circuit test should be obvious.  For an introduction to short circuit testing see https://en.wikipedia.org/wiki/Short-circuit_test.  By using a much lower excitation voltage, the substation components can be tested at high current without the hazards of high voltage.  The high currents will quickly show any high resistance connections or loose or incorrectly mounted parts.  They will also confirm the connections and phasing of the Current Transformers.

Hope this helps,

Cheers,

[rstofer]

Probably unrelated but transformers have "percent impedance" rather than some Ohmic value.  The percent impedance is defined as the percent of full primary voltage required to produce full rated secondary current with the secondary shorted.  It's usually a pretty small number - like 6% or so.  So a 12 kV transformer with 6% impedance will deliver full rated current into a short circuit with only 720 volts on the primary.

This percent impedance is then inverted to predict the maximum short circuit current of the transformer with full primary voltage.  In the 6% impedance case, the maximum secondary fault current is 16.7 times nominal full load current.

A 1000 kVA 480V transformer can deliver, nominally, 1200 Amps.  Under fault conditions, the transformer can deliver 16.7 times 1200A or about 20,000 Amps.  This is assuming infinite bus for the primary.  That's a pretty good assumption when the primary is 12 kV and the 12 kV is derived from 115 kV a few hundred yards away.

In any event, it gives a worst case fault contribution.

I have no idea why they would short a potential transformer.

[Paschoal]

Thank you guys, this help me a lot !!

Do you have recommendation of book about this theme ?

[floobydust]

Look for "substation commissioning" to find articles and books.
The job involves long hours working on-site, away from home, lots of travel. Very high stress. I am glad I quit that but the pay was good.

It takes many years to learn the craft because each substation (project) is unique, any mistakes cause a power outage (at best) or blow up expensive equipment if wired wrong. Safety is extremely important and you work with many different crews/departments such as SCADA, Telecom, Distribution, Lines, Protection, Civil Engineering for structures, towers and buildings.

I'm not sure what you saw during commissioning.
CT's are always shorted when disconnected from the burden resistor (in the protective relay), to ensure no arcing from HV generated which can happen when a CT is operated with no load.

Rarely, PT's can be connected backwards i.e. apply mains to their secondary to generate (low current) HV on the bus. This is for transformer-type PT's only. Capacitive-type PT's do not step-up. Sometimes to verify correct phasing of the CT/PT and their wiring.
If you switch in full power to the "big" transformer or the bus, and there is a problem, you want to ensure the protection relay is going to work.