High Leg on Delta-Delta Transformer causing problems

[jjdad]

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!

 

[sreid]

Perhaps the upstream breaker that sparks when it trips is the clue. If there is an intemittant wire connection between the power panel and the transformer, the motor might get only two phases of power from the panel and the third phase of power from three phase motors in the packaging area with the current running backwards through the packaging area circuit breaker. Circuit breakers don't know whick way the current is traveling. This would also explain the excessive sparking on only one CB phase.

 

[busbar]

Start with insulation-resistance and current checks.

That it is a 3ø 3w transfomer fed from 4-wire-delta servce likely does not matter.

 

[jbartos]

Question: Does the delta-delta transformer consist of three legs or two legs (two single phase transformers connected into open delta = V connection)?

 

[jjdad]

This is one 3-phase delta-delta transformer using three legs.

 

[PWR]

A grounded center tap on one leg of the transformer should give you 120/120/208 to ground on the three phases. You say you ran three wires but mention line to neutral voltages. Where is this neutral? What is the winding connection on the source transformer for the 240V side of the problem unit? I suspect there is an incompatibility between how that transformer is connected and the problem unit.

 

[tarzan56007]

I think some more meter readings are in order here, are you saying that the voltage from phase B to nuetral is only 240 when that one machine is running, or is it all the time, if you can go into this facility when machines are offline and check voltages, and if you have normal voltage readings with no machines running then monitor it as you bring machinery online one at a time.

 

[aolalde]

Comments to jjdad post:

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.

 

[MakeItSpin]

Does the main breaker for the area that is tripping have a Ground Fault trip unit? If so it is likely to trip at lower fault levels to ground than the breakers downstream of it.

 

[rbulsara]

Some clarifications and additional data needed.

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.

 

[jbartos]

Comment on jjdad (Electrical) May 24, 2004 marked ///\\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.\\

 

[jjdad]

Thanks to all who have responded so far. This is a consulting job and I was not able to visit the plant today to get the readings, but I will be there tomorrow. Here are more details about the problem.

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!

 

[skiier]

Please tell us the breaker size and measure the peak steady amp draw. If the breaker in question is running near full capacity for an extended period of time and is "crowded" in a distribution panel with other heat generating breakers it will tend to trip at a lower current value than it is rated for.
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.

 

[jbartos]

Comment on jjdad (Electrical) May 25, 2004 marked ///\\\

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.\\

 

[PWR]

While some of the stated facts don't exactly line up for this scenario, I postulate the following;

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.

 

[jbartos]

Comment on the previous posting: The original posting states:
"""Voltage to neutral is 120V at legs A & C, but it is 240V between leg B and neutral."""
Apparently, this statement needs clarification.

 

[jjdad]

Update on this problem and to clarify some questions.

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!!!

 

[rock1]

When you talk about this breaker that trips is 2 breakers up stream from the transformer, is that feeding a disribution panel that feeds the transformer? does the breaker trip when you start the machine or just while it's running(under load). What is the amperage draw of the machine. What is the amperage of the distibution panel ( total Load connected. What is the size of the br4eaker that tripps? what size is the breaker that feeds the transformer? How many KVA is the transformer?

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.

 

[JesperMP]

All these post seems to tell me that you have a breaker rated 100A supplying several branches, one of the branches is the breaker rated 100A that supplies the 240V/480V transformer.
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 !

 

[JesperMP]

By the way, the 100A breaker in front of the transformer is probably overdimensioned. The question is why ?
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).