High leg electrical system

[LaRoacha]

I'm in the HVAC field as a service, startup, and controls tech and I recently ran into a beast I've never seen before. We did a renovation on a old (1890's) courthouse in a small town in Texas and when I showed up to do startup I found that the power being supplied to all my equipment had a strangely high phase. By this I mean on sigle phase equipment I would get one leg 120v to ground and the other leg 212v to ground. Simalarly on 3 phase it would be 120/212/120 to ground. When you go phase to phase you still only read 240 whether your going 120/212 or 120/120. This concerns me because I have several motors that are running above nameplate current draws whether they are under a load or not and I can find no miswiring or loose conections. I asked the electrical contractor about the voltages and he just matter of factly told me it was a high leg system, which I've never heard of. Most equipment I've worked on specificly warns against running with a phase inbalance much smaller than what I have here. Can anyone please explain to me what exactly I have here?
Thanks

 

[DickDV]

These high-leg systems are actually quite common and are a neat way of coming up with 120V single phase and 230V three phase in the same distribution system. It is not necessarily damaging to your electrical loads if the single phase loads are actually seeing 120V to neutral and your three phase loads are seeing 230V between the three phases.

The problem that often arises is that, with heavy single phase loads, the three phase balance starts to go off. Then you have problems with your three phase loads. So, the three phase source has to be rather stiff and the single phase loads need to moderate.

There is no particular hazard in a system like this but do not be tempted to try a similar arrangement on 460V three phase. That would be almost as bad as running with a floating delta distribution system or worse, a floating delta system with one leg grounded. (You can tell I'm no fan of floating systems having almost lost my life to one once feeding an old analog DC drive)

 

[itsmoked]

LaRoacha: What You have is a common situation.

Essentially you have three phases with one caveat.

You may run single phase loads between only two of your three phases and the neutral.

Single phase 240V loads may only be connected between the two low phases.

Three phase loads will happily run between all three of these phases.

The high phase is often called, in the field, the "stinger leg" for obvious reasons. Often it is over 200V to the neutral.

The stinger can seriously louse up your loads or you if misapplied.

I believe it is generally created by utilities to make three phase power for a customer by using only two step down transformers instead of three. It is called a "Scott Tee" and makes a sort of distorted three phase. Hence the high center phase. But the high leg is not high to a three phase load. It all works fine if you follow the rules above.

 

[LaRoacha]

So what your telling me is that if my single phase 230 loads have a 120 and 212 leg, that's a problem, right?

By the way, thanks for the awesome info.

 

[itsmoked]

Well you can run single phase 230V loads from the stinger to another phase. I wouldn't. But it will only be 230/240V from the stinger to either low voltage leg.

The problems occur because a lot of industrial equipment only breaks one leg for control. If only the low leg is broken by the controls the stinger can cause other controls to still operate that in normal hook ups would stop as expected. This means if you really do want to run, using that stinger on a single phase 240V load, YOU MUST USE TWO POLE contactors.

Personally I wouldn't bother I'd just use the low legs.

You may need to though, for service load balancing. Yuck.

My pleasure.

 

[ccjersey]

Whoa! high leg delta or 120/240 3phase 4 wire or center tapped delta or whatever you want to call it has nothing to do with a Scott T connection. The Scott T connection is to convert between true 2 phase and 3 phase.

I have never seen any problem with using ANY two legs for a single phase 240V load. If the service is supplied with only two transformers, "open delta", the transformer bank is derated to something like 57% of the total kVA of the two transformers. The voltage between the high leg and ONE of the low voltage legs, (but not the other) is succeptable to browning out under heavy loads in an open delta service. Obviously, you can't use the high leg for 120V loads, but I have used it L-N for 208V contactor coils and other tiny loads upon occasion.

(Almost)no way to get the usual building/farm load to within 10-15% imbalance. The problem with the imbalance will come if you want to supply this service with a transformer or a generator. Having said that I have a couple of old 50kW standby generators which supply two services we have here which have had no problems with the unbalanced load.

If you are using the high leg as a feed for control power or as one of the legs for a single phase load, and you break either the high leg or the low leg to the load, it WILL stop, it can't do anything else, there is no longer a circuit for current to flow. This is an uncommon service, but it does obey the laws of physics.

You might want to roll the phases ( A to motor terminal B, B to motor terminal C, and C to motor terminal A) on the motors with the unbalanced amperages and see if you can find a combination with a better balance. Make sure not to get any 120V control feed on the high leg. Best to roll them at the output of the control where the motor leads hook up or in the motor terminal box. If you still cannot get a good balance and it seems to be having negative consequences, the power co may need to stiffen up the service or convert it to 120/208. Power quality monitoring could support the request for bigger service equipment. Converting to 120/208 might require the replacement of a lot of older equipment not able to run on the lower voltage.

Jim

 

[itsmoked]

ccjersey, Yes, that is one use of a Scott TEE what I mentioned is the OTHER.

Of course the motor will stop when you break the connection what may not stop are control transformers run off the other side of the motor that now still sees 80-90Vac. This is a common problem in refrigeration. The only solution is to cut off both sides of the source. Like I said.

 

[ccjersey]

Here is a link showing the delta transformer schematics

http://www.electrical-contractor.net/ubb/Forum15/HTML/000054.html

You have to remember the moderator's name is Scott!

You can see from the first two diagrams of 3 phase 4 wire transformers that if you only load the two legs adjacent to the neutral (the low voltage ones), you ignore the other two transformers on a closed delta bank and one other on an open delta bank. Not great for balancing the loads. The 2 or 3 transformers may be sized differently for the different loads expected to be placed on them. I see there is a tee connection to make an open delta which I was not familiar with. Not sure how it relates to 3 phase 4 wire though. Our services are supplied open wye/centertapped open delta and the V configuration is used for the delta as shown in the second graphic.

And here is a link to the Scott tee connection

http://www.electrical-contractor.net/ubb/Forum15/HTML/000055.html

Jim

 

[ccjersey]

Couldn't get the links to display correctly. You have to copy the http etc. part to your browser

 

[itsmoked]

Yeah YEAH! That looks like it! The second one on the first site. 3 phase to 3 phase using two transformers!

Scott TEE stuff is hard to find on the web! Nice find.

 

[weh3]

La Roacha,

Dominion Virginia Power (DVP) uses open deltas all over the place, and they are readily identifiable by their two differently sized transformers, one with two secondary connections, and the other with three. DVP even uses an open wye primary in some places, and they call it an open wye-delta.

Only the center-tapped phase should be used for single-phase loads, and you are obligated to switch all ungrounded conductors. There is no need to use 208V or 240V control.

Your 212V leg is theoretically 208V, but I would not consider it unusual on a utility secondary to have that kind of variation.

Regards,
William