Parallel Wye to Double-Delta voltage problem - Electrician Talk - Professional Electrical Contractors Forum
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Old 04-18-2020, 05:15 PM   #1
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Default Parallel Wye to Double-Delta voltage problem

I have a Bell System - AC Delco KS 20523 Gen Set.
It is rated 120/208v or 139/240v 3 phase 4 wire 30kw and was factory wired in a parallel Wye configuration. It has a 12 lead generator and all the leads have the original factory tags from T1 to T12.
The lead identification matched the wiring diagram for Parallel Wye exactly.
for 20+ years I have been using this setup as a whole house standby generator using it to back up my single phase 120/240 residential service.
It has never had a problem, but I have always been concerned that I was unevenly loading the generator with no load on L3 and I also didn't like the idea of running my AC compressors and well pump at 208v.
So, with Covid 19 time on my hands, I decided to bite the bullet and change the generator connections to Double-Delta in order to load the generator more evenly and to derive 120/240v single phase 3 wire.
I have double checked my connections. But my voltage is now 155/311v.
Operating in Parallel Wye, the normal excitation current of 1.0 dc amps resulted in 120/208v.
The excitation current is now 2.65a and the alternator voltage rheostat will not adjust any lower. It doesn't seem to adjust higher either.
I cleaned the rheostat and tested its function with an ohm meter.
It tests steady and linear.
I'm thinking that this may have to do with three CTs that had generator leads double wrapped around their exteriors.
Coil 1 had T6 and T12
Coil 2 had T5 and T11
Coil 3 had T4 and T12
These were the six leads going to T0 or N.
With the Double Delta,
I double wrapped coil 1 with T2 and T4 and coil 2 with T7 and T12 with both sets going to N.
Any ideas where I went wrong?
Thanks,
Chris
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Old 04-18-2020, 10:02 PM   #2
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After reviewing everything I discovered that:

Coil 1 was wrapped with T6, T12 / L3

Coil 2 was wrapped with T5, T11 / L2

Coil 3 was wrapped with T4, T10 / L1

Since I am now using L1 and L2, I swapped the control wires between Coil 1 and Coil 3 so that they are now sensing the lines in use.

No difference.

I'm pretty sure that I need to reduce the field current, but ...

If the voltage regulator is monitoring all three phases, and I now have no voltage on L3, it might be overcompensating. If it is measuring phase to neutral, I could try a jumper from L2 to L3.

If it is looking for a phase relationship, maybe I could extend a signal wire from either the T3,T5 splice or the T9,T11 splice. I'm not sure what the voltage would be.

If it is also looking for each phase to N, I'n not sure what that voltage would be from the T3,T5 splice to N.

I guess I need to break out the books (after blowing off the dust).


Thanks for any help. I don't really know who else I can contact for help with this.
Attached Files
File Type: pdf Wiring Diagram.PDF (529.1 KB, 34 views)
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Old 04-19-2020, 01:29 PM   #3
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The 3 CTs are looking at current, not voltage.

Where is the voltage regulator connected? Is it single or 3 phase?

Depending on the load, you might be better off connecting it series ∆ and not using the high leg. The disadvantage is the you could use only about half its capacity this way.
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Old 04-19-2020, 03:38 PM   #4
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Micromind,
I considered a center-tapped Delta and a Zig-Zag or Dog Leg before deciding on the Double-Delta.
What I did not consider was the voltage regulator and its limitations.
I realize that the CTs are measuring current, but since this unit can be paralleled with other units and has synchronized controls, I don't know if it is using the CTs in the voltage regulator circuit to maybe measuring for droop? The truth is, I don't know anything about the voltage regulator and the way the wiring is managed, I can't trace the wires. I assume that it is measuring all three phases to each other and to neutral.
My very limited understanding is that the voltage regulator receives AC voltage from the generator windings and rectifies it into DC to excite the field windings. The field voltage is probably around 60vdc and the field current proportionally controls the generator output voltage. The higher the field current, the higher the generator AC voltage.
Even though it is not a simple operation, the end result is that the voltage regulator monitors the AC output and controls it by varying the field current.
I'm debating whether to just restore the connections to original or investigate the option of installing a modern automatic voltage regulator.
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Old 04-19-2020, 04:11 PM   #5
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from a power tronic manual it looks like you use a single phase input (even on 3 phase) to the control board which is shown as hooked between a and c (1&3) as you are now using 1 and 2 you probably do not have one side of the regulator input hooked up so its sensing low voltage and maxing out the dc trying to get the voltage to come up.

The ct's are most probably for a analog meter (display use only) and have no control on the generator (unless there's one for nesting with another generator or the grid which is a option that you are probably not using)

download the voltage regulator manual and you will probably be able to spot the problem
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Old 04-19-2020, 04:20 PM   #6
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Quote:
Originally Posted by chrisrappl View Post
Micromind,
I considered a center-tapped Delta and a Zig-Zag or Dog Leg before deciding on the Double-Delta.
What I did not consider was the voltage regulator and its limitations.
I realize that the CTs are measuring current, but since this unit can be paralleled with other units and has synchronized controls, I don't know if it is using the CTs in the voltage regulator circuit to maybe measuring for droop? The truth is, I don't know anything about the voltage regulator and the way the wiring is managed, I can't trace the wires. I assume that it is measuring all three phases to each other and to neutral.
My very limited understanding is that the voltage regulator receives AC voltage from the generator windings and rectifies it into DC to excite the field windings. The field voltage is probably around 60vdc and the field current proportionally controls the generator output voltage. The higher the field current, the higher the generator AC voltage.
Even though it is not a simple operation, the end result is that the voltage regulator monitors the AC output and controls it by varying the field current.
I'm debating whether to just restore the connections to original or investigate the option of installing a modern automatic voltage regulator.
Don't over think it. The controller need to know what the output ac voltage is and it only needs single phase. It can not adjust independent legs so it doesn't need to know them.

If AC voltage is low it will increase DC which increase the mag field and causes AC to raise. On more modern ones it monitors the hertz and stuff for alarms.

If its being nested then it has a ct to work out its load but yours should be jumpered on the board as it not being used that way.

the extra ct's are for alarms or a simple analog meter on the generator to show you the amps.
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Old 04-19-2020, 05:43 PM   #7
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Thanks gpop,
More to think about before I throw in the towel.
I don't think I can find a manual for the voltage regulator.
It was probably built to Western Electric specs. I believe it has markings for L1, L2 and L3, but I'm interested in finding out because if it only needs two phase conductors, I can give it that.
Update:
I just tried powering A & C with no joy.
The regulator board has T1, T2 and T3 identified, so I'm quite sure that it is looking for all three phases.
Sorry for any confusion regarding Power-Tronics. The wiring diagram is just a generic drawing, not specifically for my generator.
I think I have three choices:
1) Rewire back to 120/208
2) Center tap Delta (don't know if the 120v capacity will be an issue)
3) Replace the voltage regulator

Last edited by chrisrappl; 04-19-2020 at 06:19 PM.
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Old 04-21-2020, 08:32 AM   #8
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Thanks to everyone for their help.

Yesterday afternoon I went out to the shop, undid all my beautiful wiring and restored the connections to the original parallel Wye configuration.

I started the generator, adjusted the voltage and then ran it with a 12 KW load for an hour. I'm running at 211v phase to phase which gives me 120v A to N and 124v B to N once loaded. I'm sure having no load on C is affecting the difference in voltage. Testing with no load I can adjust to 208 phase to phase and read exactly 120v from each phase to N.

I know a lot more about this machine now, and I think I may go ahead and place an order with McPherson Controls for a replacement regulator, just to have everything on the shelf in case someday the original regulator fails (which it probably never will).

Now it's time to go out and clean up my mess in the shop.
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