Transformer Impedance Question

[cslater]

I used to work with an EE who claimed that the impedance losses from the transformer are overlooked and shouldn't be. According to him, at full load, the voltage drop is about 2/3 of the impedance percentage.

Because of this, he would always specify very low impedance transformers. I argued that the trade-off in Short Circuit current isn't worth the voltage stability.

What do you all think?

 

[tlrols]

Low impedance transformers are typically preferred if your short circuit protection can interrupt short circuit currents. The utility I work for almost always specifies low impedance transformers and swaps out circuit breakers that aren't up to snuff for fault current. Keep in mind this is at 100 kV and up basically. You may have some issues with lower voltage systems close to strong sources...still, I would use a current limiting reactor at the location where fault duty was an issue instead of a tolerating a high impedance transformer. Regulators, capacitors, and tap changers might not be needed if a low impedance transformer is used. Like most things it depends, do a study and see what shakes out...

 

[prc]

Even if the breaker clears the fault, the transformer windings will see the first peak of fault when the forces will be maximum.The short circuit with stand strength will be an issue with transformers of low impedance.

 

[ScottyUK]

Depending on network conditions and operational switching requirements it may not be possible, or not economically viable, to buy switchgear capable of withstanding the fault infeed from two sources operating in parallel. I know some operators allow equipment ratings to be exceeded during short-term paralleling; personally I don't agree with this practice because it is usually avoidable through good engineering, and it creates the conditions for a very serious incident followed by an almost indefensible legal position.

 

[cslater]

The other concern is Arc Fault. High Short Circuit ratings mean more damaging faults, even if you have equipment with enough withstand.

In our "discussions" about this with my EE friend (and don't get me wrong - I have a lot of respect for this guy) I argued that the tradeoff in safety and cost didn't seem worth it when you could adjust using the taps. His argument is that if you have a variable load, the taps will cause you to drift too high during low load cycles.

I get the argument - I guess I just don't agree. If your load is that sensitive to voltage fluctuations, you can put it behind a UPS or some other power conditioner, right?

 

[Zogzog]

Higher fault current does not always mean higher arc flash hazards, clearing time play a big role.