We are having some issues with volt

[deetz]

We are having some issues with voltage at our facility. We had the power company check the incoming and this is what they came up with. Does any of this make sense?

Sorry to send this to you on a Friday. Do you have an electrical engineer on staff? If not, you will probably need to contact a power quality consultant. You can give them the attached charts for reference.

The voltage and KVA are okay at the transformer; well within the voltage limits. You have the 5Th harmonic above 3% and the THD slightly above 5%. High 5th harmonics are usually from 6 pulse rectifiers. You also have a load that has sub cycle events above 1000 amps. You should hire a power quality electrical contractor Hunt / Olympia Tech. > Parsons > Etc.

 

[Overvoltage]

Well, you really do need to probably hire a consultant to come look at your system, first of all. Saying you have "voltage issues" is pretty vague.

With that said, based on what the utility said, do you have DC loads that you are supplying through rectifiers? Has anything changed with those loads or the configuration of your facility?

I'd first try to figure out when the problems started happening and if anything else happened at around the same time, whether unexpected or planned.

 

[deetz]

Yeah I know it was a little vague. I just didn't want to maybe sway ones thinking in a certain direction right off the bat. We installed a new Heading machine that required us to get a bigger transfomer to supply the facility from our Utlity company. It also included an addon to our switchgear with a breaker and line just for the Induction Heater. I know this is probably the cause of most of the problems, but what would be a cure for the issues? I would assume that everything is sized right for this machine, and the new transformer from the Utility company must be okay too right? We have a 91% power factor, so we really don't have an imbalance. Would a line filter be an answer? I just want to be have a little understanding of what is going on and such so that when we do get a Company in here to do the assessment that we aren't getting snowballed. Thanks for the quick response by the way.

 

[pwrengrds]

If you won't tell us what the problem is besides a voltage issue we can't tell you what might cause it or how to fix it. If you tell me you have a car issue that could mean that it doesn't work, you don't have one, you have speeding tickets, or you don't know how to drive, it's hard to advise without more information.

To provide answers we need some information on what the voltage problem might be. Power factor does not have anything to do with voltage imbalance. Power factor is the relation between voltage and current, Voltage imbalance is the relation between the 3 phases of voltage.

 

[deetz]

Okay. Our 480v line is sometimes down to 467. Go farther down the line to our machines that have transformers for lower voltage and now we are at 98v on our 115v controls. Also the dc side is alot lower. We are having power supplies going out in our CNC machines. One was 6k.

 

[ScottyUK]

CNC machines suggests lots of spindle drives, which almost certainly means 6-pulse rectifiers. Likely the source of the problem. You can either replace the drives with newer designs with active front ends, which will be hugely expensive, or investigate either a passive harmonic trap or perhaps an active filter. The choice depends to some extent upon where you are suffering the worst effects - on individual circuits, or at load centres where one load interacts with others.

You really do need to identify whatever is responsible for the "sub cycle events above 1000 amps". Fast, high current switching can cause impulse voltages which might explain the high casualty rate among your power supplies. Do you have any 'difficult' loads such as a large welder or similar which might be the origin of these transient events?

As others have suggested, a half-decent power quality consultant will be able to make some rapid inroads into identifying the causes of the problems. Fixing them is sometimes a little harder.

 

[pwrengrds]

Did this happen when the transformer was replaced? If your having a low voltage issue it could be as simple as your on a low tap on the transformer (which most likely has 5% taps). What is your unloaded voltage. As a transformer is loaded it will drop in voltage about 7% (depending on the impedance) from no to full load. On a 480 system that would result in going from 480 to 446 volts. If your starting at no load of 470 volts instead of 490 volts it could be your problem.

The transient's is another issue that Scotty has given you some pointers on.

 

[DaveMeharg]

There are some good ideas people have suggested. One thing I would encourage you to do is to set up some monitoring of the power at various points in your system. You will need to have meters that can capture power quality information such as voltage sags and surges, harmonic containt of the voltage and current and the kW, KVA and PF of the load. The meter needs to hold at least a weeks worth of data so you can down load it and printout the information. It sounds like the utility may already have this information on the line side of your facility. Maybe you can ask them for a copy of this. I would set up metering on the main incoming switchgear and then at various critical down stream points in your system. This will help form the basis for deciding where the problem is and how to mitigate it.

The utility is saying that the voltage is well within limits. I would ask them for what the limits are and ask them for a printout or trend of the voltage and load readings. This will help you decide if you need to adjust taps on your downstream tranformers etc. Based on the informatin you gave of 467 volts this represents about a 2.7% vcltage drop which is a little high if its at your main incoming bus. It would be better to be closer to 2% or less. One way to compensate for this on the incoming side is to have load tap changers on your incoming transormers to keept the voltage within a tight limit. One concern is you say on your secondary circuits you are going down to 98 volts wich represents almost 14% drop. this is much more then the 2.7% drop on the primary. If this is a sustained situation you might either have a down stream transformer that is overloaded or the taps are not set properly. If its a momentary sag then you may have a large piece of equipment that is starting up and pulling the voltage down. You could attack this by looking into soft start methods of starting this equipment as well as putting ciritical 120 volts loads on UPS equipment to keep it from being affected by the momentary start. There are recommendations by IEEE in their red book on voltage regulation, voltage flicker etc. that you might find helpful. They have charts that based on the size of a motor you are starting what size transformer you would need to prevent voltage flicker when the unit starts across the line.

It sounds like your harmonics are a little high but not too bad. I would speak to the vendor of your VFDs or other equipment to see if they can recommend local line reactors or filters at the pieces of equipment that are generating the harmonics. They can do some quick modeling what the affect of adding line reactors to their equipment would due to improve the overall harmonic content at both your incoming as well as down at the secondary level.

The other issue could be your distribution. If you find from your metering that your incoming switchgear voltage is staying fairly stable around +/- 2% but your down stream readings are fluctuating much higher you may need to add more down stream distribution rather then have everything fed off of one or two feeders that are overloaded or connected to the heavy starting equipment.

I know I threw out a lot of ideas. I hope this helps you get started.