Open wye - Open delta problem

[rstumph]

I have a distribution engineering question. I work for an electric cooperative. Our standard voltage is 7200/12470. On our system we have an open wye open delta xfmr bank served at the intersection of two single phase lines. Each line is fused. During a recent storm, one fuse blew due to lightning. A customer served by this supposed "dead" line called in to complain regarding having "part" lights which caused his refrigerator to stop working.

I am interested as to whether a backfeed could exist through the secondary of the system when one phase of the primary is opened. I am looking for reference material regarding troubleshooting of transformer problems as well.

If a backfeed can exist, what voltage would be present on the 7200 volt single phase line.

 

[al hildenbrand]

Welcome to the Forum, rstumph.

Interesting question. I'll quess that your PoCo primary is connected to the open wye. Also, that the customer's refrigeration is 3 ?.

Loosing one of the primary lines would result in the refrigeration being "single phased".

 

[jtester]

I know of no reason why a loss of one phase on your bank would burn up single phase loads. There is no way to backfeed an open wye-open delta bank into the high side with only a blown high side fuse. It sounds like you could have lost the high side neutral, at which point you easily could have wierd voltages.

I suggest you try to find a couple of reference manuals that I have had for 30 years, a GE Distribution Transformer Manual, and another distribution manual by Westinghouse, but I can't remember its name. These are small booklets that offer a wealth of information on transformers, particularly 3 phase connections, loading, grounding, etc.

I got mine long ago, and really don't know if they are available any more. I suspect that most co-ops have one on somebody's bookshelf.

Jim T

 

[al hildenbrand]

A customer served by this supposed "dead" line called in to complain regarding having "part" lights which caused his refrigerator to stop working.

After rereading this, assuming this is a single phase customer, I wonder. . .back at the wye-delta 3 ? bank, the customer there, are there large 3 ? motors there?

Jim, couldn't the motors, still moving, generate the third leg and phase relationship and back feed the PoCo primary?

 

[jtester]

After rereading this, assuming this is a single phase customer, I wonder. . .back at the wye-delta 3 ? bank, the customer there, are there large 3 ? motors there?

Jim, couldn't the motors, still moving, generate the third leg and phase relationship and back feed the PoCo primary?

Al

Probably yes. I was reading a steady state backfeed into the post, not a momentary one. With three phase motors there could be some, albeit brief, backfeed.

Jim T

 

[rstumph]

Jim & Al,

Thanks for the input. There are 3 ph motor loads on the load side of a primary meter on this same line. The largest motor is 10 hp. The oil well company's electrician told me that when he arrived at the site, the motor was running backward and he measured p-p voltages of 440, 440 and 220 volts.

The customer on the "dead" line is a single ph residential customer. He was away when the event happened and returned the next day to see the dim lights and the problem with the refrigerator. The system neutral was intact. The primary wye connection is grounded to the system neutral as required.

rstumph

 

[al hildenbrand]

I have heard of situations where a 3 ? load was supplied off a single phase source, by taking a 3 ? motor, connecting A & B of the motor to the single phase source as well as A & B of the load. Then C of the motor is connected to C of the load.

The 3 ? motor has to be hand started, but, at a threshold rpm, will run by itself off the single phase source and will generate a voltage and current on C that resembles 3 ?. This is not a transient state, and I am sure is not a pretty "state".

The situation I'm thinking of was in a rural setting with little money, lots of salvaged equipment, and the driving will to power several 3 ? woodworking tools by this "method".

If the load out in the area of the outage caused by the single distribution fuse opening was relatively small, it seems conceivable that the 3 ? motors could move enough power to back feed the area of the outage.

 

[LarryFine]

I have heard of situations where a 3 ? load was supplied off a single phase source, by taking a 3 ? motor, connecting A & B of the motor to the single phase source as well as A & B of the load. Then C of the motor is connected to C of the load.

It's called a rotary converter.

 

[al hildenbrand]

It's called a rotary converter.

I'll be darned! I've never worked with one. Cool.

I've got some reading to do.

 

[jtester]

Jim & Al,

Thanks for the input. There are 3 ph motor loads on the load side of a primary meter on this same line. The largest motor is 10 hp. The oil well company's electrician told me that when he arrived at the site, the motor was running backward and he measured p-p voltages of 440, 440 and 220 volts.

The customer on the "dead" line is a single ph residential customer. He was away when the event happened and returned the next day to see the dim lights and the problem with the refrigerator. The system neutral was intact. The primary wye connection is grounded to the system neutral as required.

rstumph

Are there more transformer banks than just the one open wye-open delta on the line that single phased? If you have another bank, a 480 bank for example, you might well generate less than nominal voltage on an isolated phase. Also as Al mentioned, if you have a motor spinning on the load side of the open fuse, you could likely generate a voltage.

Jim T

 

[iwire]

It's called a rotary converter.

You can certainly buy rotary converters, they have no output shaft and are design built for changing single phase to three phase and are 'self starting'

IMO what Al was describing was the home made version where you literally use a standard three phase motor to do the job.

As Al mentioned it seems to be a common sawmill in the boonies remedy.

On one of the forums a sawmill operator explained his system it was backwoods genius at work.

 

[iwire]

I found the thread, interesting set up.

HillBillySawMiller

 

[al hildenbrand]

Thanks Bob,

Great link.

Using a plain old 3 ? motor is exactly what I was trying to describe.

As I read the opening post at your link, I recalled reading it before. I'd forgotten the sawmill. Man, what a setup that must be!

The woodworking tools I recall come from a rural setting about 35 miles from me. I can still see, in my minds eye, the open wiring and the motor sitting on the floor at one end of the woodshop, covered in sawdust. . . .

 

[Wes G]

I have heard of situations where a 3 ? load was supplied off a single phase source, by taking a 3 ? motor, connecting A & B of the motor to the single phase source as well as A & B of the load. Then C of the motor is connected to C of the load.

The 3 ? motor has to be hand started, but, at a threshold rpm, will run by itself off the single phase source and will generate a voltage and current on C that resembles 3 ?. This is not a transient state, and I am sure is not a pretty "state".

I am aware of a fairly good sized machine shop in rural Mich. about 80 miles from chicago that ran on this principle. They used a small single phase motor to get an aprox. 25HP motor mooving than pulled their 3rd phase from it.

The question that I have is this, If the largest motor is only 10Hp, and it is pulling a load, how much power could it produce? How could it reverse when you lost one side of an open phase Delta which would in effect drop out 2 phases?