VFD Operation on "V-Phase" distribution circuit

[dpc]

We have some rural utilities with miles of "V-phase" distribution - basically two out three phases on a grounded-wye system.

For small VFD installations - say up to 50 hp at 480 V, can VFDs be obtained that will operate satisfactorily with only two of three phases? I have been assuming that, as long as the VFD supplier is aware of the incoming power, the VFD can be oversized or specially configured to operate like this.

What would be an upper size limit for this type of installation, assuming a 480 V drive?

Thanks,

Dave

 

[waross]

High Dick;
To use V phase power you have to have two transformers in open delta. In a perfect world it will work perfectly.
In the real world, a couple of factors enter.
1> Transformer voltage drop. Open delta bank transformers may not be the same size and may not have the same actual loading or percentage loading. Your voltages may not be exactly equal.
2> Primary neutral currents. The neutral currents in a rural V system cause voltage drops on the neutral. This implies that the neutral position is shifted from the electrical neutral position. This results in phase angle errors. While in line single phase voltage regulators improve the voltages, they make the phase angle errors worse.
The bottom line/
Don't be surprised by badly unbalanced phase currents at light loads that will improve as the load increases.
Phase angle errors and unbalanced voltages both lead to unbalanced phase currents to a rectifier bank.
I would be looking for a VFD with a dumb front end and oversizing it a little.
Hope this helps

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

 

[ScottyUK]

Dave's description sounds more like two lines of a three phase group, essentially a single-phase supply at line voltage.

Who is Dick?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[dpc]

Just imagine a standard 4 wire grounded system with a neutral. Now take away one of the phase wires. V-phase. Typical single-phase services are connected line-to-neutral.

For a "three-phase" service, two transformers are connected.



David Castor

http://www.cvoes.com

 

[ScottyUK]

I can imagine what you're describing, but VFDs don't normally use a neutral, so you only have the two phase lines, i.e. a single phase supply.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

If you have a single phase service from this, you can use a VFD to produce 3-phase for the motor, so long as the VFD doesn't have phase loss protection. If you have a three phase service (open wye-open delta) you can still use the single phase VFD or you can put in a delta-wye transformer to give you a balanced (at least in terms of voltage to ground) three phase system. While the three phase VFD won't need a neutral connection they can be really picky about phase to ground voltage balance.

 

[stevenal]

dpc,

I don't understand your question. The distribution is V phase, but when you install two transformers on it in open wye - open delta, your secondary will have full three phase three wire to supply to your VFDs. The only question left is the one waross answered regarding voltage balance.

For those who think Dave is describing single phase, see faq238-777.

 

[dpc]

stevenal,

Yes, that has always been my assumption, but I guess I just wanted to make sure the drive was not going to have any issues with the open delta "three-phase" service, and if the VFD manufacturer take this into account in any way. We have situations where a customer wants to install a 150 hp motor and only V-phase is available.

Dave

 

[waross]

Hi Scotty:
You may enjoy a description of common distribution circuits in rural North America.
In wheat and cattle farming areas the load is mostly residential lighting. Distances are relatively large. For instance, regulations limit grazing densities to about 250 animals per square miles. One of my neighbors is running several thousand head. Another neighbor seeded 3.75 square miles to canola. He also has several square miles of hay fields and grazing fields.
Distribution is typically single phase. Four wire three phase primary distribution circuits are typically carried along main roads with single phase radiating out on the side roads. Our area uses about 25,000V and a ground return. There are multiple ground rods at each transformer installation.
Installing three phase requires miles of primary conductors to be installed. Typically a farm may install a pump or grain crusher or other machine with a motor too large for reasonable single phase operation. In these instances a second primary phase is run and two transformers are used in open wye/open delta configuration.
The electrical characteristics are not ideal but the savings may be tens of thousands of dollars for an installation that may be used only seasonally or for a few hours a day. Another factor is that even if three phase conductors were used, the nature of the majority of the loads, widely spaced single phase loads, is such that neutral voltage drops and the accompanying phase angle errors may be present anyway. Just oversize the motor somewhat so that it can reject the extra heat resulting from the unbalanced voltages and currents.
Some utilities that usethese solutions in rural area may notallow them in built up areas where three primary phases are available. There the North American standard of a wye connection for three phase power may be required.



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

 

[stevenal]

If phase balance were an issue, I suppose the most severe situation would be a loss of phase. I attended a demonstration where a fuse was yanked on a running VFD and motor. Motor kept running without a hiccup. I'm sure the VFD was specified for this type of duty.

 

[DickDV]

Maybe I'm missing something here but, the OP clearly stated that the supply was V phase, GROUNDED WYE.

To me, that would be single phase from ground out to one phase, and a second single phase at 120 degrees from the first from ground out to the second phase.

If those single phase voltages were each 277V, wouldn't you have single phase 480V from phase to phase?

I understand about the economics of running open side delta and getting three phase from that but, I don't see any economical benefit to doing this with a grounded wye source.

Someone want to help me out here?

 

[stevenal]

You need to look beyond the initial post.
The secondary supply to the VFD is three phase three wire 480 from an open delta. No neutral.

The distribution primary is V phase. Note that V looks like Y missing one phase. He didn't state the voltage, but 12.47/7.2 kV is a common one. 7.2 from each of the two phases to ground, 12.47 between. The economics state that three phase loads can be served with the existing V phase line, and there is no reason economically or otherwise to reframe miles of distribution to bring in a fourth conductor.

 

[dpc]

Dick,

My first post probably wasn't too clear. I was referring to the primary system (12.47/7.2 kV). At the service, there would be a two-transformer bank connected open wye on the primary and open delta on the secondary (480 V) as stevenal describes. The V-phase distribution saves one phase conductor.

The open delta secondary provides a poor man's three-phase supply. It is often used for small three phase motors out in the boondocks. We generally don't use it over 20 hp or so for constant speed motors.



David Castor

http://www.cvoes.com

 

[waross]

We had a discussion recently concerning rectifier loads on unbalanced three phase voltages. The rectifier clips the top of the voltage wave form and with light loads the higher voltage phase tends to supply more than it's share of the current.
I worked at a sawmill in the third world that was at the end of the three phase section a wye distribution line. Only two phases and a neutral were carried on for over twenty miles. There were several villages, a small town, a small dairy and a couple of small saw-mills. All the phase current returned on the neutral which was a couple of gage sizes smaller than the phase conductors. The voltage drop on the neutral moved the neutral position out of the electrical neutral quite a bit. This causes phase angle errors. We had soft starts shutting down several times a day on excess phase angle errors. We were on full three phase but the relatively heavy neutral current extended past us towards the source for about twenty miles before there was enough load to balance the phase currents and cancel the neutral current.
An open delta on a solid system gives good three phase. Do some vector sketches of the voltage drops on an open delta transformer bank and you will see that they act in such a way as to produce an equal voltage drop on the third, open, phase. But the common application of open delta connections often implies a long, soft circuit. It often implies a long primary neutral conductor which causes the phase shifts that are the source of most of the issues.

My solution is to estimate the amount of neutral shift to anticipate. This often calls for ingenious improvisations in the field rather than office calculations.
From that I estimate the effect on the load.
The last step is to oversize the motors or drives to accommodate the extra heat caused by the unbalances.
Note: If you can get the utility to float a wye delta bank with the primary neutral connected at the end of the full three phase line, it will do a good job of balancing the voltages at that point and prevent neutral issues from passing back onto the full three phase section.

And another point of interest, several years ago an engineer from New England shared a connection to develop full three phase wye power (120:208V) from two primary phases and a primary neutral. I later heard this connection described as a Winipeg connection.

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

 

[ScottyUK]

stevenal, dpc, Bill,

Thanks for the explanations. I had arrived at the same [ignore](mis-)understanding [/ignore]as Dick a few posts earlier, and although I could see that there were some possibilties to get a third phase using transformers I hadn't realised that the 'V' connection was on the HV system.

You guys do some weird things to your electrons!


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[DickDV]

Aha! That I understand! And I am familiar with open delta as well.

It was the two-legged grounded wye that stopped me dead in my tracks. I see it now.

Thanks!

 

[Electic]

I think the open wye primary would be used to supply two transformer windings creating an open delta 480V supply. I don't believe the UPS would care much whether the supply is delta or wye connected. However, typically the center of one transformer secondary winding is grounded (240/240 winding) or a corner point of the transformer bank is grounded.

I am guessing you would have to derate the transformers for harmonic content plus account for the load being supplied by two windings, but before I did anything I would check with the UPS mfg to see what they think about corner grounded delta supply.

 

[Electic]

Edit previous post: change UPS to VFD. Same idea wrong loads

 

[waross]

dpc; If it hasn't been mentioned, a VFD for 480V wye connection is expecting to see 277 Volts from each phase to neutral/ground.
With a four wire delta (center tap grounded at 240:480V) One phase will be 416 Volts to ground rather than 277V. With a floating delta the VFD must be capable of withstanding 480V to ground on any phase. An economy model VFD designed for 277:480V only may fail on delta service due to possible overvoltages to ground. But you knew that once you thought about it Dave.
Yours
Bill

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