3 phase neutral & grounding problems?

[comaqc]

While browsing the web for information about a problem our company is trying to solve at a customer site, I found enlightening information on your web site. Here is the problem we are troubleshooting right now. It is not be relevant whether you can provide assistance or not at this point. The main purpose of this message is to get my thoughts on paper.


Our company manufactures induction heating power supplies which we install on industrial equipment. Our power supplies are rated at 6 kwatts each and run at about 30 kHz. They run on 208 Volts 3 phase power. The AC line sees a three phase rectifier and a 30 ufd filter capacitor. No power factor correction circuits are installed within our power supplies. Total current harmonic distortion is around 30 %. Demand for each power supply is modulated from 0 to maximum power by a process control system.


A typical installation will have anywhere from 50 to 100 of our induction power supply connected to the secondary of a Wye connected main transformer. Standard electrical distribution panels with circuit breakers feed individual power supplies. Our power supplies are floating from ground as we use fully encapsulated workcoils. All power supplies are housed in a steel cabinet which is grounded to the equipment. Of course, the chassis of our power supplies are grounded to the cabinet.


We have more than two dozens induction heating system installed since 2001, totalling close to 1900 induction power supplies. All starting up successfully except one. system started up in 2003 which showed a weird problem. This system had 72 power supplies installed in one cabinet. Some power supplies started failing randomly right after the installation. Being unable to find anything wrong with our equipment or with the installation, our service engineer decided out of despair to disconnect individual ground wires running from the neutral of the transformer secondary to each power supplies. The power supplies failures stopped immediately. We scratched our heads while unsuccessfully trying to understand why running wires directly from each one of our power supplies chassis back to the main transformer neutral would cause our power supplies to fail.


Two week ago, we installed a 61 power supplies system which is behaving strangely like the one described above. We lost 8 power supplies in one day, then 10, then everything ran well for a week. Now we are loosing a couple of power supplies a day. Our service engineer is not back from the site, but he says that the power supplies are not individually wired back to the transformer neutral.


Could a large quantities of small individual power supplies, running at varying power demands, thus at varying current harmonics distortion, cause a weird phenomenon on either the voltage or the current fed from the transformer to each power supply?

 

[petersonra]

Re: 3 phase neutral & grounding problems?

I would be inclined to get some more data before you blow up anymore power supplies. You seem to have some kind of design flaw in your system that needs a serious look-see.

If the design flaw is so serious that it damages power supplies at this rate, I have to wonder if there is not also a hazard to human life that it might also be introducing into the equation.

You might want to give serious consideration to shutting these installations down until you uncover the problem. I know that is not a pleasant thought, but it might be something you decide needs to be done after you give it some thought.

 

[comaqc]

Re: 3 phase neutral & grounding problems?

Thanks for the suggestion, Bob. Our power suppies are UL and CSA International certified.They are also CE marked. I invented this industrial control system back in 1983. There are over 50, 000 power supplies using almost the same design that are still running successfully around the world. The 1900 units functionning well taht I alluded to are of the exact same generation as the 61 showing problems

The problem I mentionned became apparent two weeks ago at the startup of one system. The only otther place where we saw a similar phenemenom was in 2003 at at installation in Nothern Europe.

While things are now running OK at the recent troublesome installation, I want to get to the bottom of this issue.

I had a wild scheme in my head that some other electrical equipment installed in the same facility, such as 100 HP motor for instance, which would be connected on a totally separate distribution transformer, would sometimes prefer a return path through our distribution transformer, thereby causing a large neutral current to flow in our transformer, thus creating large voltage differences between phases.

 

[GENEM]

Re: 3 phase neutral & grounding problems?

I think I would be chasing your wild scheme but looking for a loose ground/nuetral connection somewhere in the parent plant that your system is installed in. This could be allowing a surge to be hitting your power system rather than feeding out on ground. I ran into a similar problem and found a loose ground. It was in a plant with a lot of motors turning on and off resulting in surges which was knocking out PLC's. Just a wild suggestion.

 

[comaqc]

Re: 3 phase neutral & grounding problems?

Thanks Genem.
The plant in question is a large paper mill. Some of the big 300 to 500 hp motors on the paper machine are running constantly. But,there are similar size motors starting and stopping on winders located in the same room. There are also lots of 50 to 100 hp pumps starting and stopping.

Is there a convenient way, except through visual inspection to locate poorly grounded electrical equipment? Is there equipment we can purchase to assist us in troubleshooting poor grounds on equipment?
I guess the paper mill electrical maintenance department must have that type of equipment.

 

[hurk27]

Re: 3 phase neutral & grounding problems?

Does your power supply use a grounding conductor for a load in any way? Is your power supply's using a neutral? If not then the only thing that I could come up with is if this plant is feed by an open delta bank transformer and it is a 120/240 4-wire delta that has to much neutral current. Have your guys done any power quality monitoring? Does the utility's responsibility stop at the primary drop to plants substation?

I they do have an open delta 4-wire service closing the delta should fix the problem with one more transformer.

 

[GENEM]

Re: 3 phase neutral & grounding problems?

I don't know of a convienant way to check grounds other than a megger or hi-pot tester but that is not usually practical in an operating plant. Normal metering does not always show up the problem since the ground may not actually fail until hit by a current surge. You might try monitoring at your system to see if that is the problem but to check the ground the best I can suggest is physical inspection.

 

[comaqc]

Re: 3 phase neutral & grounding problems?

Hurk27
We specify a Wye secondary power transformers for each of our induction heating systems. The primary is invariably delta connected to the high voltage mil supply. A typical induction system requires anywhere from a 350 kVA to 1 MkVA dedicated transformer. No other equipment except auxiliary pumps and air blowers are allowed on this transformer. The transformer output is routed to circuit breakers panels for distribution to individulal power supplies Our power supplies require only the 208 -240 Volts phases. The neutral is not utilized. The transformer output should always see a balanced load in the form of a 3 phase rectifier bridge. A computer control system varies our individual power supplies output from minimum to 100 % in response to process requirement.

No power quality monitoring has been done at this customer site yet. Being an OEM for a large multi national organisation (name witheld on their request) which resells our products under their own brand name, we have to work through them to have access to the end user. We will be going to site next August 17.

 

[hurk27]

Re: 3 phase neutral & grounding problems?

With that knowledge I can't see a grounding or neutral problem as there should be no loads on it, If these power supply's try to increase power in the event of a failure of a component then you could have a runaway cascading effect. Or like in a case of a voltage drop problem the power supply increases output to try to maintain a set level of power the same thing could happen. But I'm shooting in the wind without knowing the circuits involved. It could even go all the way back to the grid feeding the transformer.

 

[comaqc]

Re: 3 phase neutral & grounding problems?

Thanks to all for the comments. It looks like I will have to look elsewhere than grounding problems for explanations as to what caused the high failure rate of power supplies at that particular mill site.

The situation at the mill is OK right now; everything is running fine. I am still uneasy about not having identifed the root cause of the original problem.

We will be at the site in two weeks to inspect things. I will post any findings on this thread.

 

[benaround]

Re: 3 phase neutral & grounding problems?

comaqc,

I read in your other post that you have the ground wires from the power supplys going to thje neutral of the transformer. Is this correct? If so you are grounding the transformer neutral to your equipment! By the sizes of motors that you mention,I'm thinking High impedance ground might be in use,therefore,a fault might find less resistance thru your grounded equipment. Just a guess.

frank

 

[comaqc]

Re: 3 phase neutral & grounding problems?

beneround,

Grounding of the power supplies by running wires back to the neutral occured only once on a seventy power supplies system (420 kW) back in 2003. As you suggested, we had unexplained and random failures of our power supplies until the ground wires going back to the transformer neutral were disconnected.
Our specifications is to have our the metal enclosure housing the power supplies grounded to the paper machine frames. We do not specify what to do with the neutral connection of the dedicated power transformer. Some mills ground the neutral connection at the transformer. Others run the neutral to the distribution circuit breaker panel, where it ends without being connected anywhere else. I assumed those different neutral point connections to be local elctrical code related.


I have forgotten to mention that the power transformer is invariably a good distance (200 feet on average)away from the power supplies. There could be a significant difference between the ground in the electrical distribution room where the power transformer is normally located and the paper machine frame ground where the power supplies cabinet is installed.

 

[sofroniosoriano]

Unbalanced Voltage reading between phase to ground

Unbalanced Voltage reading between phase to ground

We are currently having a problem regarding the unbalanced voltage reading of one of the five feeders in our 6.6 Kv cubicle. Our secondary side is connected in delta and it doesn't have any grounding terminal that's why they got one terminal to be grounded.

Rescently, machine installers from production find it out that there is inbalance in the system. what could be the possible causes. hope for your help. thanks

 

[brian john]

What I would do:

1.Verify with your suppliers that their components meet your criteria (no faulty parts).
2.Verify the control software is operating properly.
3.Install one or more line disturbance analyzers on the distribution equipment monitoring continuously for all phase, neutral (where a neutral is installed) and ground current.
4.Perform a visual inspection of all connections ground and phase conductors also check for wiring errors and code violations.
5.Megger the feeders phase to phase to phase and phase to ground.
6.Verify the equipment as installed meets spec.s, for correct installation and the equipment itself, has no issues )possibly damaged in shipping or during installation).
7. Look at operational sites and sites with issues what's different from site to site?

While factory maintenance personnel are often very good electricians you might be better served by employing someone with power quality expertise in resolving this issue. Many of the items listed above could be performed and documented prior to hiring a PQ expert.

Leave no stone unturned, something is causing this problem.

 

[Ben There]

Wonder if original poster found their problem. The floating neutrals or ungrounded wye with delta secondary xfrmer's sound like a classic case of ferroresonance.