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Utility Unbalance loads

Utility Unbalance loads

Utility Unbalance loads

(OP)
What is the maximum allowable utility unbalance loads at distribution level(25KV)? There are single phase loads and three phase loads.  

RE: Utility Unbalance loads

Load balance is dictated by voltage regulation and economics. So the limits a utility applies depends on how unbalanced loads affect the system. Are they are line to line or line to neutral and what are the related system impedances. Or how close a particular circuit is loaded to its capacity. The motivation would be higher to balance loads as one approaches capacity limits than on a lightly loaded circuit.

RE: Utility Unbalance loads

The maximum depends on who is asking.

Traditionally, many utilities set an arbitrary goal of reducing unbalance to less than 10% as measured at the substation. Sometimes regulators will have specific requirements.

As noted by PHovnanian, a practical constraint is economic (higher losses for heavily loaded unbalances).

From a customer perspective, current unbalance is not important, but voltage unbalance for 3-phase customers is very important (don't want more than about 2% voltage unbalance).

So, who is asking about the 'maximum'?
- your regulator?
- your shareholders?
- your customers?

RE: Utility Unbalance loads

(OP)
10% for voltage regualtion? Sounds too high. At motor terminals, currnet unbalance is 6 times voltage unbalance. This doesn't apply Utility level.

RE: Utility Unbalance loads

If you are connecting a wye:delta transformer bank to a wye distribution system you should float the wye tie point. If the wye point is connected to the system neutral any primary voltage unbalance will cause circulating currents in the delta. The circulating current is related to the transformer PU impedance. A few percent primary unbalance may cause a circulating current greater than the transformer capacity.
When you float the wye point, you my be subject to switching transients. I would not choose a wye primary/delta secondary transformer arrangement for power applications.
Wye primary/delta secondary may be used for protection and ground detection, and even for system grounding but these applications often use a broken delta secondary and the open corner on the delta avoids the circulating currents.
Note to our IEC friends; Nema- broken delta = IEC open delta.
NEMA open delta = IEC "V" connection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Utility Unbalance loads

(OP)
Warass, Our generator is connected to the Utility through Wye-Delta system. We are adding a zig-zag transformer to create a neutral in case the power supply on the utility side is disconnected.

RE: Utility Unbalance loads

Thanks for that update, Hamid.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Utility Unbalance loads

Hamid. It sounds like you are worried about the effects of utility unbalance on your zig zag transformer. You'll need to contact the w.r.t. their specifications for maximum voltage and phase errors. They'll worry about balancing their loads to stay within these limits.

Quote:

10% for voltage regualtion? Sounds too high.
That 10% figure is a rule of thumb for load balance, not voltage. For a 10% load imbalance on a system with 5% Z, the voltage unbalance will only be 0.5% (an overly simplified example).  

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