mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
It depends on a number of things:
- system configuration (transformers in parallel, radial feed, or secondary selective system)
- primary side system stiffness - if we already have fairly low fault current, we probably don’t want to knock it down further with a high impedance transformer
- what kind of load you’re feeding (steady, cyclical, any big motors to start)
- downstream distribution system characteristics - what additional voltage levels are involved, how large is the distribution system, what losses are expected downstream
- power price per kWh to pay for transformer losses
- voltage regulation on the primary
- what voltage regulation is required on the secondary.
Without assessing all of those details, the short answer is 8% sounds high. It might allow you to use 31 kA MV gear, but the voltage regulation and efficiency benefits may justify 6-7% Z even though the switchgear may need to be rated for more fault current.
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Primary 115kv is allowed to vary + or - 5%.
Voltage on the secondary is regulated at +5% during peak periods, sometimes a tad higher under extreme contingencies.
I'm curious how radial overhead vs network is effected by a low Z bank vs a high Z bank.
FWIW, normally open secondary 20/33MVA, 30/40/50MVA and 40/50/60MVA is serving loads in POCO right now satisfactorily. This is the base concpet I am comparing everything to.