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Voltage fluctuation

2K views 14 replies 7 participants last post by  Handasee 
#1 ·
Industrial plant has a 600/347 volt service. During trouble-shooting a machine, I measured the following voltages between phases: 621V, 618V and 619V. The plant has a power meter that showed: 626V 616V and 624V about 1/2 hour after the initial reading. The local utilities condition of service specifies normal operating conditions of 550 to 625V. As I watched the power meter with the plant manager, we saw the meter go as high as 638 volts, and after about 5 minutes return to 619V (other phases proportional). We have asked the utility to check why the voltage is high and fluctuating. I am looking for some reasons to explain the fluctuation.
 
#3 ·
Most substation transformers have automatic tap changers. It could be that the utility needs to set theirs to respond a bit quicker.

But, if it isn't causing problems, I'd leave it be.

a 575 volt NEMA motor can be operated up to 632 volts without damage.

Plus, if the readings are taken at the main switchgear, the voltage will be lower downstream.
 
#13 ·
The local utility was at the plant in the morning. From what the plant manager told me, they will lower the tap ratio to bring down the voltage.
I've told the manager to wait until I can do some testing.
I agree with JRaef. I am going to investigate what capacitors are in the plant.
 
#14 ·
Is it high throughout the entire day? Or just at certain times during the day?
The utility I'm at long ago installed a boost xfmr on 3 feeders anticipating load growth that never happened, every once in a while we have to change taps for customers with high voltage complaints, they often call us back for low voltage when we have a heavier than normal load. The only PF correction we have at the sub is cap banks, no voltage regulators, no reactors, just caps. Some places are still running old equipment, your utility may be the same. Did they recently replace or maintain the XFMR feeding you?
 
#15 ·
I want to put a voltage monitor on the system to see when the voltages peak. I know this plant use to have a lot more machines, and I believe that the capacitor bank that they have is over-sized for their requirements. What I want to do is remove the capacitors from the system and get a reading so that I can calculate the correct size of capacitors.
 
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