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Old August 22nd, 2019, 01:59 PM   #1
lesmar96
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VFD install on open delta system

My customer, from Northern Cailifornia, has an open delta or some people call it high leg delta utility. I am not sure what the correct name is for it.

As I understand it, it is regular delta with one winding of the transformer center-tapped to connect the neutral. So if A-B, B-C and C-A are all 240 volts, then A-N and C-N will both be 120 volts, but B-N will be 208 volts.

He also says they struggle with load imbalance on the motor phases. The current on each leg is not equal, up to 10% imbalance.

Anyway, we were checking into adding a vfd to an existing pump installation. What all should we be thinking about installing a vfd? Line reactor? Derate the drive?

Look forward to your thoughts. Thanks!
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Old August 22nd, 2019, 02:27 PM   #2
kamenges
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Check the drive manufacturer's installation instructions. Different drives have different tolerance for such things.

Worst case you are looking at a 3-phase wye secondary isolation transformer. I don't think a line reactor will do you any good relative to the grounded delta thing.

Keith
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Old August 22nd, 2019, 02:42 PM   #3
GaryS
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if its a 3 ph motor the single phase components on the incoming line should not have any effect on motor current imbalance.
I think you should be looking at the incoming line voltage an imbalance in the incoming line voltage will cause an imbalance in the motor leads.
if its and older system also meg the motor and motor wiring you may have a bad spot in insulation
a good VFD would help with that
you said that this is in Northers California their power grid is not as good as it is in our part of the country
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Old August 22nd, 2019, 03:55 PM   #4
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Do you expect the imbalance is due to a weak grid or is that typical of the open delta? In other words, is this two different things to deal with or are they related?
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Old August 22nd, 2019, 04:53 PM   #5
Gene Bond
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That sounds close to our 'normal' 240v system around here.

The current imbalance may well be due to a voltage imbalance from the utility. I forget the numbers, but it's something like 3x the voltage imbalance % for the current imbalance, and anything over 8% IIRC, is unacceptable due to the heat associated.

On the VFD, you'll want to disconnect any filters and MOV's which are connected to ground. They won't be rated for the excessive voltage caused by the Delta configuration. On some (Non-CE) drives, the MOV's are just connected in Delta, so they are OK.

If the incoming voltage is over 1-2% imbalance, get the utility to fix it!
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Old August 22nd, 2019, 10:26 PM   #6
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“Open delta” is something different, it’s when you use only two transformers to put together a 3 phase system. What you are describing has a lot of different colloquially derived names; “high leg delta”, “red leg delta”, “stinger leg delta” are a few of the most common. Officially though, it is referred to as “240/120V 3 phase 4 wire delta”.

As a general rule, delta power sources of any sort are problematic for VFDs because the drive mfr will put protective devices like MOVs as well as Common Mode Capacitors in the drive that are all referenced to a ground point and in a delta system the reference voltage to ground is at BEST uneven between the phases, or at worst is floating (in an ungrounded delta system). The “best practice” for connecting a VFD to a delta system is to install a drive isolation transformer with a solidly grounded wye secondary. In your case the secondary would be 240Y138, but the 138V is irrelevant, the issue is that it establishes a consistent L-G reference for the drive internals. Hardly anyone does this however and just take their chances. Some of the bigger drive mfrs decrease the likelihood of component failure by allowing you to disconnect their internal ground reference points. But doing so also removes some of the protection that provided.

That will not fix your voltage imbalance however, which is all but guaranteed on a high leg delta system. There really isn’t a lot you can do about that.
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Old August 23rd, 2019, 06:23 AM   #7
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We use an ungrounded delta system here. It is a problem for drives. Remove the MOV protection - easy with an AB drive would have to check on how to remove from the others. We use Yaskawa, AB, MItsubishi drives without issues. MUST INSTALL A LINE REACTOR ON THE LINE SIDE or you will be replacing drives.

We use a 3% line reactor and size the drive to the motor requirements. No need to derate.
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Old August 26th, 2019, 02:56 PM   #8
lesmar96
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Quote:
Originally Posted by jraef View Post
“Open delta” is something different, it’s when you use only two transformers to put together a 3 phase system.
Hello Jraef,

This comment has caught my attention. Can you explain a little how this type of utility works?
He had mentioned along the way that the utility only has 2 transformers to produce 3 phase system. I was working at understanding that, but maybe I threw everybody off by the voltage to neutral measurements I posted?
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Old August 26th, 2019, 03:25 PM   #9
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Open Delta transformer configuration:







Basic Closed Delta configuration for comparison:







Closed Delta 4 wire (240/120V 3 phase 4 wire) configuration:








Technically, you CAN have an Open Delta 3 phase 4 wire configuration. I have never seen one other than in textbooks.

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Old August 26th, 2019, 03:51 PM   #10
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Old August 26th, 2019, 03:56 PM   #11
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Quote:
Originally Posted by jraef View Post
<snip>
Technically, you CAN have an Open Delta 3 phase 4 wire configuration. I have never seen one other than in textbooks.

I believe this is the configuration we see on farms. I darn near ran out of hair scratching my head about where the weird voltages came from!
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Old August 26th, 2019, 04:01 PM   #12
lesmar96
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Quote:
Originally Posted by Gene Bond View Post
I believe this is the configuration we see on farms. I near ran out of hair scratching my head about where the weird voltages came from!
How is this different from the 3PH 4wire Hi-Leg Grounded Open Delta on the diagram that you posted?
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Old August 26th, 2019, 04:16 PM   #13
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It is the same, but I had never run across it before
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Old August 26th, 2019, 04:17 PM   #14
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Quote:
Originally Posted by lesmar96 View Post
How is this different from the 3PH 4wire Hi-Leg Grounded Open Delta on the diagram that you posted?
It's not different.


You said that the customer is in Northern California, as far as I know PG&E does not provide an open delta 4 wire connection, they use closed delta. But they are not the only utility here, some small irrigation districts in the central valley have their own distribution systems.
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Old August 26th, 2019, 04:32 PM   #15
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If this is an industrial site the open delta portion might be a separate feed off the plant 480VAC 3-phase supply. The 480 might be closed delta and the 240 derived from that might be open.

Keith
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