I have to confess that I don't know much of anything about what comes from the POCO. I have a potential customer who has had 3PH machines burn up. The POCO came and metered 265 volts where there should have been 240 volts. They told him that an open delta can vary by 10%. Their suggested solution for him is to bring in another type of transformer if someone will do the load calculations. Meanwhile he has a lot of buck/boost transformers on the machines to bring the voltage to 240.

Help!

Dave

I would wonder if this high voltage is constant. I would start with a recording voltmeter left in place for a week to determine how much voltage variation this customer has. (if any)
Also, measure the voltage at each machine before it starts, then with no load, then again under full load.
Load calculations will be necessary for every piece of equipment, and for the shop distribution layout.
Possible solutions require a lot more information than you have provided. Maybe an electrical engineer should have been required to lay out the entire building.
Is 480 volt service available?

...I'm gonna show my ignorance here,gang,..."Open Delta" means no neutral,..or am I mistaken??,..Please advise..Thanx,
Russ

Is the POCO calling an ungrounded delta system "open delta"? Or are they pointing out the customer may have an "open delta load" (broken load phase) that is causing the higher voltage?

Is the building serviced with 240V 3-phase or is it transformed in-house from a higher voltage?

Open Delta is a delta hook up with one side open. With an open Delta each secondary winding must suppy 100% of the current.Lots of times an open delta well be used where they feel larger furture loads are possible , They would then add the 3th transformer. Ifo out od Howard W. Sams & Company (Transformers & Motors)dated 1989

We one time closed up an open delta on a project - in order to increase overall system capacity (as the previous poster indicated). As I understand it, an open delta behaves exactly the same as a normal delta (secondary connection) except at only about 67% of nameplate KVA capacity - which makes sense because one of the three transformers is missing. External wiring is exactly the same.

Also, if I remember correctly, the POCO primary was not a delta, but a wye, which I could not ever seem to explain to the POCO personnel we encountered.

Radar

Could the setup be an Open Wye?


Russ,
The Open Delta could be a 3 Phase 3 Wire setup (Grounded or Ungrounded), or a 3 Phase 4 Wire Grounded setup, which would include a Grounded Neutral Conductor.

Not a bad question! Most Open Delta "Vee" Setups discussed are targeted towards Motor Loads, but there are a lot of them in areas with Residential/Commercial customers fed from the same bank of Transformers.
Those would be 4 Wire Delta setups.

All the Open Delta connections I see, and have seen for years, are of the "Vee" configuration. Have yet to see any 3 Phase 3 Wire Open Delta "Tee" configurations on Utility Poles (excluding the Ancient Port areas of Long Beach, San Pedro and such, where 2 Phase systems were still in use in the 1980s. These would be "Tee" configs.).

Scott35

These are great for apartment houses with a 3 phase elevator motor. The local POCO requires all single phase loads be connected to the 120/240 volt transformer. Only the elevator is connected to the high leg (every third circuit breaker position in a 3 phase panel is the high leg).

[This message has been edited by elecbob (edited 10-28-2004).]

[This message has been edited by elecbob (edited 10-28-2004).]

Sorry but the voltage from the grounded point on the lighting (larger, center tapped) transformer to the B phase is 208 volts nominal, not 265 volts. This configuration is normally used as an open wye/open delta and each transformer serves 57.7% of the three-phase load. The lighter serves the entire single-phase load.

The voltages from A to ground to C are usually very stable and the voltage from the common end of the open delta to the high phase are also usually very stable. In both cases, there is a real transformer in the circuit. From the other end of the lighter to the high phase, the voltage is usually very stable as long as the motor load is not too great.

In this case, I am betting that either the bank is very close to a substation or a capacitor bank is on top of the installation (the utility's capacitor bank). Of course it could be both.

Most of the state public service commissions require the investor owned electric utilities to maintain their voltages to about 5% of nominal. It may be time to call them if you haven't been able to get through to the correct people to get their problem taken care of. I highly recommend that you get through to their power quality engineer or at least a distribution engineer first.

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Charlie Eldridge, Indianapolis, Utility Power Guy

I thought the 10% variation sounded too high also. I have doubts about the POCO here going to any great lengths for customer service, though.

Is there any kind of UPS or other equiptment for 3PH 240 volt maybe around 20 amp where no mater what you put into it (within obvious limits), that it would stabalize voltage and frequency variations?

I have a cut sheet for the new machine and it's outlining dire consequences for problems with the power including electrical noise.

Dave

 
Open-delta transformer banks always consist of two 1ø transformers, where delta banks {sometimes called ‘closed-delta’} use three 1ø transformers. See the two compared in Scott35’s Technical Reference Area under https://www.electrical-contractor.net/ubb/Forum15/HTML/000054.html

It’s important to understand that [in elecbob’s sketch] that there should almost always be very close to 240V across the two shown transformer-secondary windings. But also, there should be 240V across the open-delta ‘ends’ at 12 o’clock and 4 o’clock in elecbob’s sketch.

For most utilities, open-delta configurations are reserved for smaller motor loads, on the order of 7½-20hp. Voltage imbalance is the biggest problem—and 3ø induction motors are sensitive to stator heating—with reducing the motor’s mechanical load to prevent overheating.

Note that one way to illustrate open-delta imbalance is using two 10kVA transformers and a 10kW 240V 1ø 2-wire resistive load. If the 10kW load is connected across the lower or left winding in elecbob’s sketch, voltage drop will be about this same. But if the same load is connected across the open ends of the two windings [still at 240V] somewhat greater voltage drop will be produced.

I would take exception to that statement. The only way that would be true is if the load were to be large enough to collapse the delta a bit and thus lowering the voltage. That doesn't happen in a normally loaded transformer bank. If we have permitted the bank to have too much load that does indeed happen. What happens is that the voltage is lower to start with if the bank is overloaded.

Don't lose sight of the fact that the rated voltage output of a transformer will only happen when no load is on the transformer and the rated voltage is on the primary side. As soon as load is applied, the impedance will cause voltage drop through the transformer.

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Charlie Eldridge, Indianapolis, Utility Power Guy

 
A high-leg-to-ground reading of 265V almost sounds like there may be a misplaced 277V-secondary “power” transformer, with a correct 240V unit for the “lighting” pot.

CharlieE, pardon me, but that’s the idea of having other than a theoretical zero-impedance transformer. Very roughly, the change in voltage on secondary terminals decreases by the per-unit impedance when nameplate loaded versus zero load—e.g., a 3% Z, nominal 240V [or ~247V] transformer terminal voltage drops to about 97% ~233V [or ~240V] when fully loaded.

Equal loading across the ‘open’ terminals [versus across one of the two individual windings] on the open-delta bank approximately doubles. While a 3ø induction motor can succesfully operate with a lower overall [balanced] voltage like a 5% reduction, it will typically experience much greater stator heating if terminal-voltage imbalance is 5% per ANSI/NEMA standard C84.1-1995 Annex D. Discussed at www.joliet-equipment.com/voltage_unbalance.htm

I have a feeling we are saying the same thing. I normally think of voltage drop as just on the conductors and I was pointing out that more voltage drop will be experienced through transformation. I thought you were talking about the drop on just the conductors and you were intending to use the transformation in the calculations. With that, I agree.

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Charlie Eldridge, Indianapolis, Utility Power Guy

Although I haven't measured it myself, Bjarney, my understanding is that the 265 volt measurement was phase-to-phase, not phase to ground. After spending a few hours online I'm wondering if this isn't a power quality problem. It's a tool & die shop with CNC machines and possibly a fair computer load with CAD for blueprints. I'm wondering if there are grounding problems, neutral problems, harmonics, and service load imbalance. I also read about installing a zig-zag transformer close to the load. Any thoughts?

By the way, if you're not an electrical engineer and want to feel like an idiot, check out www.eng-tips.com


Dave

 
Dave55 — Zigzag transformers from drytype transformer producers are intended for triplen-harmonic mitigation and neutral ‘stiffening’ in {symmetrical} four-wire wye circuits. They would be a misapplication on the described 4-wire delta system, even it they were available for 240V. One type is the Acme ‘I-Trap’ with a nominal 208V rating— www.acmepowerdist.com/pdf/Page%2041.pdf See also www.electrical-contractor.net/ubb/Forum15/HTML/000072.html




[This message has been edited by Bjarney (edited 10-30-2004).]

Thanks for the info, Bjarney. When they talk about close to the load, I'm assuming this means next to the macnine? These sizes could cover the whole shop...(2) 200 amp panels, but there would be some 150' runs to the machines.

Dave

 
Dave55, these particular zigzag transformers are intended for use only with 208Y/120V (remote) panelboards, and not 240/120V ∆ service.

Are the ø-ø voltages all high [A-B, B-C and C-A]?




[This message has been edited by Bjarney (edited 10-31-2004).]

Dave55, check out American Power Conversion:
http://www.apc.com

They make systems up to several kva for mainframes, etc. and I'm sure they have someting to help you with the machine you're working on...

[This message has been edited by mxslick (edited 10-31-2004).]

Thanks, guys. Sorry about not understanding the first time about the zig-zag transformer, Bjarney. I thought the link was to something other than a zig-zag transformer.

Am I understanding correctly that elecbobs diagram is correct, except the 12 o'clock position to ground is 208 volt? I believe this is a typical configuration for our POCO with two pole mounted transformers.

I'll check out that APC site some more, but I belive it's for 480 volt. The more I read on this, the more I think this should be going to an electrical engineer. This is over my head and I like working from experience. It has, however been very educational.

Dave