High Leg on Delta-Delta Transformer causing problems
High Leg on Delta-Delta Transformer causing problems
(OP)
I was called in to review a problem with a delta-delta transformer that is connected to 240V 3-phase in and running 480V 3-phase out. This transformer is only feeding one machine that requires 480V 3-phase.
The Buss feeding the transformer apparently has a grounded delta, the voltage between each leg is 240V, phase to phase. Voltage to neutral is 120V at legs A & C, but it is 240V between leg B and neutral.
The transfomer only has the three phase wires feeding both in and out of the transformer. No neutral is connected to the load, or the transformer.
The problem is the machine will trip the main breaker to the packaging area. This breaker is two breakers upstream from the breaker feeding the transformer. The transformer breaker doesn't trip. When the breaker trips, the middle leg of the breaker sparks excessivly.
While I understand the issues with a grounded delta, what could be causing the tripping? Is it possible, something at the machine is grounded improperly, or simply an overload?
What is interesting, is the machine runs most of the time, it just trips off occasionally.
Thanks for the help!
The Buss feeding the transformer apparently has a grounded delta, the voltage between each leg is 240V, phase to phase. Voltage to neutral is 120V at legs A & C, but it is 240V between leg B and neutral.
The transfomer only has the three phase wires feeding both in and out of the transformer. No neutral is connected to the load, or the transformer.
The problem is the machine will trip the main breaker to the packaging area. This breaker is two breakers upstream from the breaker feeding the transformer. The transformer breaker doesn't trip. When the breaker trips, the middle leg of the breaker sparks excessivly.
While I understand the issues with a grounded delta, what could be causing the tripping? Is it possible, something at the machine is grounded improperly, or simply an overload?
What is interesting, is the machine runs most of the time, it just trips off occasionally.
Thanks for the help!





RE: High Leg on Delta-Delta Transformer causing problems
RE: High Leg on Delta-Delta Transformer causing problems
Start with insulation-resistance and current checks.
That it is a 3ø 3w transfomer fed from 4-wire-delta servce likely does not matter.
RE: High Leg on Delta-Delta Transformer causing problems
RE: High Leg on Delta-Delta Transformer causing problems
RE: High Leg on Delta-Delta Transformer causing problems
RE: High Leg on Delta-Delta Transformer causing problems
RE: High Leg on Delta-Delta Transformer causing problems
1- For balanced three phase voltages, you should have 208 V = 240*.8666 between B and neutral.
2- For a motor faulty condition, the first protection trips off. (It is supposed to be coordinated).
3- 240 Volts seems too low for industrial application. This will result in operation with high currents.
4- The upstream breaker seems to have damaged contact surfaces, most probably due to high current operation.
RE: High Leg on Delta-Delta Transformer causing problems
RE: High Leg on Delta-Delta Transformer causing problems
1. If the machine's transfomer breaker is not tripping, what makes you beleive that the machine is causing the main breaker to trip?
2.According to you description, even a ground fault on the secondary side will only be seen as over curret (not a ground fault) on the primary side as the primary and secondary sides are electrically isolated. So the GF issue is ruled out.
3. The voltage from N to phase B should read 208V as indicated by other posts.
4. Its sounds more like a case of overload or a bad breaker. Post the amps readings.
5. Post the loads on the breaker that trips when the 'machine' is running and not running.
6. Sizes of the equipemnt involved, amps and hp'kvA etc.
RE: High Leg on Delta-Delta Transformer causing problems
I was called in to review a problem with a delta-delta transformer that is connected to 240V 3-phase in and running 480V 3-phase out. This transformer is only feeding one machine that requires 480V 3-phase.
The Buss feeding the transformer apparently has a grounded delta, the voltage between each leg is 240V, phase to phase. Voltage to neutral is 120V at legs A & C, but it is 240V between leg B and neutral.
///Please, would you clarify how the delta-delta transformer has its neutral implemented? Do you call the grounded corner of the upstream transformer the neutral?\\\
The transformer only has the three phase wires feeding both in and out of the transformer. No neutral is connected to the load, or the transformer.
///Then, how could have been voltage measurement performed to neutral if there is no neutral connected to the load or the transformer? Is there a neutral terminal on the delta delta transformer?\\\
The problem is the machine will trip the main breaker to the packaging area. This breaker is two breakers upstream from the breaker feeding the transformer. The transformer breaker doesn't trip. When the breaker trips, the middle leg of the breaker sparks excessivly.
///Yes, this can happen when the corner grounded upstream delta is connected to an ungrounded downstream delta. The downstream delta may have unbalanced voltages due to the secondary load asymmetry.\\\
While I understand the issues with a grounded delta, what could be causing the tripping? Is it possible, something at the machine is grounded improperly, or simply an overload?
What is interesting is the machine runs most of the time, it just trips off occasionally.
///If the machine trips occasionally only, there is probably an overload that trips the circuit upstream; especially, if the upstream tripping circuit breaker has a ground fault protection.\\\
RE: High Leg on Delta-Delta Transformer causing problems
The power system in this building is a 4-wire 3-phase 240V system. The 4-wires connect to a circuit breaker panel which feeds the transformer, but the neutral stops at the circuit breaker panel. The voltage readings I have made are at the circuit breaker panel, not the transformer. The electrician who called me in, was concerned because of the high leg on this delta system. The power systems he worked on for twenty years were all delta-wye systems where all legs were of equal voltage
This transformer is used as a step up transformer (240 to 480). This is the only 480 volt machine in the building, everything else is 240V 3-Phase.
There are only three wires connected to the high side, and three wires connected to the low side fo the transformer. The neutral is not connected anywhere on this transformer.
There is only one packaging machine connected to this transformer and when the upstream breaker trips it is always when this machine is running.
I have not seen any ground fault type breakers anywhere in this circuit. They are standard Cutler Hammer 3-phase breakers.
The problem may lie in the machine itself, I will be taking measurements from several locations tomorrow and that should help clarify the problem.
Thanks again for the responses!
RE: High Leg on Delta-Delta Transformer causing problems
I wouldn't concern myself too much with the source. Distribution equipment is pretty rugged (transformers, etc.) and as long as the equipment has worked in this arrangement with NO problems in the past than I can't see your transformer as the problem. Check the insulation of your transformer and inspect the wiring for wear.Check for proper voltage out of the transformer. If you have proper voltage out than your primary is connected properly to the source. Who cares what arrangement the source has. It doesn't matter.
The problem is likely that packaging machine of yours. It may have an intermittent gnd fault. A tough thing to find.
I'll lay odds however that the problem is simply a tired overworked undersized overheated breaker that has seen better days.
RE: High Leg on Delta-Delta Transformer causing problems
There are only three wires connected to the high side, and three wires connected to the low side fo the transformer. The neutral is not connected anywhere on this transformer.
///If the neutral terminal exists on the transformer secondary, then the transformer is probably Delta,primary - Ungrounded Wye,secondary, not a delta-delta as stated in the original posting. The ungrounded wye transformer secondary is not considered good power supply source since there may be high transients and the neutral may circulate.\\\
RE: High Leg on Delta-Delta Transformer causing problems
The building source is 240V three phase with a grounded center tap on one phase to handle 120V loads.
The transformer being used as a step up has the same connection and the center tap is grounded as well. If you isolate the center tap on this unit it should work fine.
The breaker that is tripping is probably the first upstream device that has ground fault protection. By grounding the center tap on the step up unit you are creating a ground fault as "seen" from the source end.
RE: High Leg on Delta-Delta Transformer causing problems
"""Voltage to neutral is 120V at legs A & C, but it is 240V between leg B and neutral."""
Apparently, this statement needs clarification.
RE: High Leg on Delta-Delta Transformer causing problems
This is a 3-phase delta-delta transformer.
There is NO neutral on either side of the transformer. The voltage measurements taken from phase to neutral is measured in the power panel feeding the transformer. There are three wires feeding the feed side of the transformer, and three wires connected to the load side of the transformer. 3 wires in, 3 wires out. No ground, no neutral.
The power system in the building is 3-phase 240 volt delta, with the so called "wild leg, that gives the 120/240/208 situation.
After doing some testing today, we were concerned the transformer had some phase wiring wrong. But after moving the wires around, the breaker still tripped. Apparently none of the electricians in this building have paid attention to keeping all phases 1-2-3 inline throughout the building.
What has me baffled, is the breaker that always trips is two breakers upstream from the transformer. The breaker feeding the transformer and the breaker feeding that power panel never trip. All of the three of these breakers are rated at 100A. Apparently no coordination has been applied here.
After some more investigation, I learned the breaker that trips gave them problems before, but was replaced recently. The other strange thing is, the power panel that contains the breaker that trips has several other three phase breakers, but they are not installed inline. In other words, there is a three phase breaker, as single breaker, then another three phase breaker. No spaces between the three phase breaker, the single, then the next three phase breaker.
It looks something like this:
3-phase breaker 1
3-phase breaker 1
3-phase breaker 1
1-phase breaker 2
3-phase breaker 3
3-phase breaker 3
3-phase breaker 3
It appears the 3-phase breakers are not all lined up in three's. We ran out of time for more testing, but I feel the problem is more with this panel than the transformer itself. Since this system has the delta high leg, and this is the only transformer I know in the system, I am wondering if the transformer is creating some sort of phase-phase short with the rest of the power system, because these breakers are not line up in sequence.
More testing tomorrow.
Thanks for all the posts!!!
RE: High Leg on Delta-Delta Transformer causing problems
From everything that has been said so far; to me it sounds like if the the breaker feeding the distribution panel is tripping and not just the breaker to the transformer: it sounds like the panel is over loaded. or out of balance(more amperage on A phase or B phase or C phase. To me the the issue of the Wild leg (B phase) is a mute point. the machine is hooked up right and runs fine. It just really sounds like the panel is overloaded.
When this started to happen, did they add any new equipment in recent times? have they started to run more equipment at the same time? have ther been shift changes?
MOST NEEDED INFO. Current draw on all three phases when all equipment is running. size of beakers that tripps.
RE: High Leg on Delta-Delta Transformer causing problems
Then it is no mystery that it is the breaker upstream that trips, it is more loaded than the breaker in front of the transformer.
You must measure ALL currents in ALL phases everywhere in the distrubition panel during normal production (clamp ammeter) !
The "arching" phase on the tripping breaker is particularly interesting.
It could be as simple as the breaker (and the rest of the distibution) is underdimensioned for the load.
Seems like the electrical supply in this plant is a mess, and you have simply found the first place that is starting to give problems.
There may be a lot of other surprises waiting for you !
RE: High Leg on Delta-Delta Transformer causing problems
It could be because of sloppiness, but it could also be a way to compensate for the high inrush current of the transformer. Getting a breaker made for "transformer" loads will allow you to dimension it properly.
(here in europe its called breakers with "D" characteristic).
RE: High Leg on Delta-Delta Transformer causing problems
Try answering rock1's questions.
RE: High Leg on Delta-Delta Transformer causing problems
Here is the one line diagram
200A Main 240V three phase delta system with "high leg"
This breaker feeds
|
Distribution panel 1 with multiple breakers
100A breaker (A) in this panels feeds
|
Distribution panel 2 with multiple breakers
100A breaker (B) in this panel feeds
|
Distribution panel 3 with multiple breakers
100A breaker (C) in this panel feeds
|
30kVA Delta-Delta Tranformer 240 to 480 volt step up
The breaker that always trips is 100A breaker (A) in distribution panel 1.
We shut everything off in all these panels, except the circuit feeding the transformer. Turned the tranformer breaker on and it stayed on. Nothing trip. At least during this test.
The Breaker (C) was on and the system was working properly.
Amperage readings on the 240V side of the transformer
A - 11.1
B - 10.3
C - 3.8
Amperage readings on the 480V side of the transformer
A - 4.1
B - 4.1
C - 0
Voltage measurements on 480 Volts side
AB - 491
BC - 492
AC - 496
Voltage measurements on 480 volt side to ground
Remember this transformer is not grounded nor does it have a neutral connected anywhere in this circuit either. This measurement was taken at the incoming power disconnect on the 480v machine. The ground wire runs all the way back to the distribution panel 3. The ground wire does not go through the transformer.
A-G 278
B-G 245
C-G 327
We shut off breaker (c) waited a minute then turned it back on. It tripped. There was nothing else on in distribution panels 1, 2 or 3. We reset the breakers then tried again. It tripped. But sometimes it will hold.
Breaker (A) trips about 80% of the time when you turn Breaker (C) on to the transformer. Breaker (B) NEVER trips.
The machine is the only device connected to the step transformer. The voltage and amperage readings are what the systems sees when the breaker is turned on and the 480 volt circuit is seeing with the machine powered up, but not running. There are three motors, which are all controled by drives. The only 480 direct connecte motor is a 2HP vacuum pump motor, but it is not on until started by the control system. Normally it is off. There are three small drives on this machine, all controlling 1HP motors. But the drives are not "on" during power up.
This has been one weird problem, and I am wondering if there is more than one problem involved here.
Again, thanks to all, so far.
RE: High Leg on Delta-Delta Transformer causing problems
From what I read in your last post, that the breaker tripping while 'energizing' the transfomer is possibly due to high inrush current.
Does the breaker trip, when the tranformer is already energized and motors are running?
or does it trip only when switching the transformer on?
RE: High Leg on Delta-Delta Transformer causing problems
I just spoke with the machine manufacturer and they say there is a control tranformer on this machine that has a high inrush current and the circuit breaker that it tripping needs to have a higher delay setting.
We shall see.
RE: High Leg on Delta-Delta Transformer causing problems
But it is the inrush that is causing the breaker trip. Untill you upgrade the breaker (and possibly the wiring), leave the transformer engergized all the time.
RE: High Leg on Delta-Delta Transformer causing problems
First, rbulsara has done an outstanding job of pro bone remote-control troubleshooting and advice. Not sure I’d have the patience to type out all the piecemeal questions and information he has given. I hope it is very sincerely recognized by the OP and duly appreciated.
It’s time to consider reviewing the overcurrent-devoice provisions of NEC Article 450. If this reference is not clearly understood, pay for some help. Enough manhours of handholding…
RE: High Leg on Delta-Delta Transformer causing problems
But you are right that this forum is no substitute for hiring right professionals.
RE: High Leg on Delta-Delta Transformer causing problems
What has me baffled, is the breaker that always trips is two breakers upstream from the transformer. The breaker feeding the transformer and the breaker feeding that power panel never trip. All of the three of these breakers are rated at 100A. Apparently no coordination has been applied here.
///In some cases, the upstream breaker trips, if a loop in the downstream installation materializes and some irregularity in the loop exists, e.g. load is connected to the upstream CB as well as downstream CB via voltage displacement angle of the transformer, "wild leg" of the upstream transformer is connected to 120V load, etc. Therefore, the power distribution circuit tracing and load examination are needed as well as a correct single line diagram of the as-built power distribution.\\\