short circuit calculations 9

[stevemechanical]

how are these calculations performed?

 

[waross]

Hello cuky2000
I love your graphics. Thanks for ilustrating what I was trying to explain.
davidbeach
Have you chosen the wrong word or have I misunderstood you? Adding impedance will cause a voltage drop that is proportional to the current. If a reactance is added in series with a 2% impedance transformer of a value that results in a combined transformer plus reactor impedance of 3% the symetrical short circuit current and the voltage regulation will be the same as it would be with a 3% impedance transformer without reactors.
The phrase "standing voltage drop" may be misleading.
Respectfully

 

[davidbeach]

waross, if the voltage is set at the secondary of the 2% impedance transformer and there is then an additional 1% impedance in series with the secondary side of the transformer, the voltage drop across the impedance will be subtracted from the voltage set at the secondary of the transformer. If a 3% impedance transformer were used instead, the voltage would have been set at the secondary of the transformer and there would not be the additional voltage drop between the transformer and the load. The utility is unlikely to adjust their transformer taps to account for an additional chunk of impedance added by the customer.

I had not addressed voltage regulation, and you are correct that voltage regulation to the load would be the same across 3% worth of impedance whether it is one 3% impedance transformer or a 2% transformer plus a 1% impedance.

My phrase "standing voltage drop" referred to the situation where there is a reasonably steady load and the utility transformer is set for a secondary voltage. If the separate impedance were in the circuit, there would be a voltage drop (probably more of a phase angle shift than drop depending on the X/R of the impedance) across the impedance that wouldn't be there if the impedance wasn't there and that is the standing voltage drop.

 

[waross]

Hi davidbeach
Thanks for the reply. I think we are in agreement.
As to voltage adjustment, In my experience this is usually done on the basis of open circuit voltage to compensate for cronically high or low primary voltages. If there is a further problem, usually with fluctuating primary voltages, the taps may be adjusted to a different value. This is invariably a compromise.
In regards to added impedance, a 2% transformer and a 3% transformer will usually be set on the same taps and deliver the same open circuit voltage. If a 2% transformer is used and the impedance seen from the main breaker is increased to 3% either by the feeder impedance, an added reactor or a combination of the two, the effect will be the same as if a 3% transformer was used and there was no feeder impedance.
Respectfully