"False 3-Phase"

[JDBrown]

I was talking the other day with a County employee about an issue they've been having with some 3-phase lift pumps up in the mountains. Please keep in mind that this guy doesn't have an electrical background, and was only repeating to me what he remembers being told by other Engineers and Electricians.

Apparently, any time there's bad weather up there and a power line comes down, their lift pump motors burn up. Of course, my immediate reaction was to say that they need to get single phasing protection for their motors, but he claims they installed "top of the line" (whatever that means) single phasing protection after the first time the motors burned up.

Whatever this "top of the line" single phasing protection is, it hasn't helped. The County employee reported that "somebody" told him there must have been a "false 3-phase" which caused the single phasing protection to be ineffective.

Have any of you guys ever run into a similar situation, or heard the term "false 3-phase" before? I feel like I'm way outside my area of expertise here.

 

[meternerd]

Most utilities, when supplying underground services, use pad-mount transformers that have a "common core" winding. That means that the three phases are not wound on individual cores like overhead would be. Instead, the core uses three vertical and two top and bottom horizontal members. The magnetic flux during a loss of one primary phase can allow the "dead" phase to still supply voltage, usually less than nominal, but still enough to "fool" a phase monitor. Motors will usually run OK during this condition, but during starting, when current is high, voltages drop enough to damage the motor. It can also happen on overhead if the secondary line is long, due to mutual induction. Most phase monitors have adjustments for phase imbalance. Set it to its lowest setting and it may help. Also set the trip delay to a very short duration.

 

[Jraef]

Most utilities, when supplying underground services, use pad-mount transformers that have a "common core" winding. That means that the three phases are not wound on individual cores like overhead would be. Instead, the core uses three vertical and two top and bottom horizontal members. The magnetic flux during a loss of one primary phase can allow the "dead" phase to still supply voltage, usually less than nominal, but still enough to "fool" a phase monitor. Motors will usually run OK during this condition, but during starting, when current is high, voltages drop enough to damage the motor. It can also happen on overhead if the secondary line is long, due to mutual induction. Most phase monitors have adjustments for phase imbalance. Set it to its lowest setting and it may help. Also set the trip delay to a very short duration.

In addition to that, and for similar reasons, if the motors happened to be RUNNING at the time that a phase was lost, the running motors act like rotary phase converters to create a voltage on the missing phase, which has been known to fool voltage based phase monitors. For that reason, I always recommend using CURRENT based phase monitors for motor loads. And making that much much simpler now is the fact that most Solid State Overload Relays come with that capability, so all you really need to do is use that for both functions. Many come with Equipment GF protection as well, even better.

 

[Sierrasparky]

This is very good information , I am glad someone brought that up.

 

[ActionDave]

Have any of you guys ever run into a similar situation, or heard the term "false 3-phase" before? I feel like I'm way outside my area of expertise here.

I wonder if someone is talking about an Open Delt, three phase from two transformers. That could look like a "false three phase" if you weren't familiar with that power system.

 

[kwired]

I wonder if someone is talking about an Open Delt, three phase from two transformers. That could look like a "false three phase" if you weren't familiar with that power system.

Yes but to a phase monitor on the secondary an open delta derived system would appear no different then a full delta system.

But OP did say "up in the mountains" indicating to me that this is a remote location and is likely either a open delta or even single phase with a phase converter somewhere.

Motor already running when phase is lost is sort of a rotary phase converter and may continue to run as long as torque demand is not too high, motor trying to start when phase is lost will have no torque and will draw high enough current to trip overload protection.

 

[meternerd]

As I've said in other posts, we had problems with phase monitors on delta systems, whether open or closed delta Mostly 3W. It helps if it's corner grounded. Problem is that a delta with one missing primary phase will still put out voltage on all three secondary phases, but the voltages may be weird. Can often confuse a phase monitor. Ungrounded delta is a bigger issue, because when you lose a primary phase on ungrounded, the phase to phase voltages on the secondary can go VERY high...high enough to blow phase monitors (usually the varistors) and Kwh meters. This I know from personal experience. Most Kwh meter manufacturers (Itron & GE were what we used) no longer list their meters for 480V 3W Delta (Form 12S or 5S). They say it costs too much to build them capable of withstanding the high voltages. You can ground the primary neutral of a 12,470/7200 distribution system at the transformers to prevent the high voltages, but you risk more blown primary fuses or transformer damage during close in ground faults. Best to just go 277/480, even with the added cost of the neutral wire.

 

[JDBrown]

Thanks for all the great information, guys. I'm in the process of trying to run some of this down (transformer OH or padmount, winding configuration, secondary conductor length, etc.), but it's likely to take a while.

Meternerd, when you say, "Best to just go 277/480, even with the added cost of the neutral wire," do you mean 480Y/277V grounded secondary to prevent the strange voltage issues you get with a delta secondary?

 

[meternerd]

Thanks for all the great information, guys. I'm in the process of trying to run some of this down (transformer OH or padmount, winding configuration, secondary conductor length, etc.), but it's likely to take a while.

Meternerd, when you say, "Best to just go 277/480, even with the added cost of the neutral wire," do you mean 480Y/277V grounded secondary to prevent the strange voltage issues you get with a delta secondary?

Yes...that's what I meant. Since the transformer secondary will have a grounded neutral, per Code, you have to run a neutral to the service disconnect panel even if you don't use it. It will solve some of the issues with the phase monitor, but not all, depending on the transformer type. I personally think it's a much safer system though. Won't go into the reasons.

 

[kwired]

Delta transformer with missing input phase is same thing as an intentional open delta system.

I can see voltages being effected if the load is high enough all three transformers are needed though.

 

[GoldDigger]

Delta transformer with missing input phase is same thing as an intentional open delta system.

I can see voltages being effected if the load is high enough all three transformers are needed though.

But a delta primary with a missing (open) phase *line* is just single phase. That single line is part of two phases.

 

[meternerd]

Delta transformer with missing input phase is same thing as an intentional open delta system.

I can see voltages being effected if the load is high enough all three transformers are needed though.

Not the way I see it....an open delta still has all three primary phases, but it lacks one phase to phase winding. A missing primary on a full delta puts one winding phase to phase, but the other two are essentially a series set of windings across those same two phases. For that set of windings, load voltages can be pretty much anywhere, depending on load.

 

[kwired]

But a delta primary with a missing (open) phase *line* is just single phase. That single line is part of two phases.

Not the way I see it....an open delta still has all three primary phases, but it lacks one phase to phase winding. A missing primary on a full delta puts one winding phase to phase, but the other two are essentially a series set of windings across those same two phases. For that set of windings, load voltages can be pretty much anywhere, depending on load.

Ok if you have a delta primary then you do have a problem if you lose an input phase.

Around here all you ever see for pole top configured delta systems are wye primary, and a two transformer open delta secondary is two phases and the neutral conductor on the primary - a three phase delta secondary/wye primary essentially becomes same thing as the two pot open delta if you lose an input phase, and can run a long time without being noticed if not loaded very heavily.

Padmount transformers may have delta primary I really don't know for certain what is most common, but pole top three phase banks are all configured with same pots with single primary bushing that they use for single phase services - less inventory that way.

 

[Electric-Light]

Ok if you have a delta primary then you do have a problem if you lose an input phase.

Around here all you ever see for pole top configured delta systems are wye primary, and a two transformer open delta secondary is two phases and the neutral conductor on the primary - a three phase delta secondary/wye primary essentially becomes same thing as the two pot open delta if you lose an input phase, and can run a long time without being noticed if not loaded very heavily.

Padmount transformers may have delta primary I really don't know for certain what is most common, but pole top three phase banks are all configured with same pots with single primary bushing that they use for single phase services - less inventory that way.

If the primary is wye, you can have two phases N-A, N-B by three wires.
If the primary is delta, you will single phase with a loss of pole.