Industrial plant incoming voltage regulation and voltage drop

[pittengineer]

We are having an issue where an 120V branch circuit is dropping to 112V during the starting of a motor and causing problems with electronics. This voltage drop is causing several problems and we are searching for answers as to what we are seeing and where to make corrections.

Our system has a utility transformer with a secondary of 4.16kV controlled by an automatic tap changer. 4.16kV is distributed to main switchgear which is then distributed to different sections of plant through feeder breakers. The area where I am seeing this drop is off one of the feeder breakers which serves a second 4.16kV switchgear bus which has a 5kV motor and a 4.16kV - 480V transformer. On the 480V bus there is a 480V - 240/120V transformer that has the 120V issue that I am referring to which is a dip lasting about 1s in duration. Looking at this I have a few questions.

1) Should we look at our main incoming utility voltage to verify that it is within limits as a starting point? What is the range of acceptable or ideal voltages for this incoming voltage?

2) When looking at meters on incoming switchgear I see voltages of about 4035V, 4079V, and 4122V. However when I look at the voltages on the second downstream switchgear I notice that they are all upwards of 4130V. I dont believe that the voltages downstream could be larger than the upstream ones. Do you think this could just be an error in measurment? Both meters are connected in an open Delta arrangement to the PT's.

3) Should this momentary voltage drop be a concern for re-evaluating my system and possibly re-tapping some of these transformers. We have no other problems with motors or anything, and just this one electronic piece of equipment seems to be the problem?

4) Would it be more logical to focus on the downstream 480V and 120v circuits then looking way upwards in the system? Wont each one of these transformers present a voltage drop of some sort due to impedance.

Thanks in advance for any advice on this.

 

[alehman]

1. Yes.
2. This is probably measurement error, unless you have capacitors connected downstream.
3. Adjust transformer taps won't reduce the amount of voltage dip. It will only make the minimum higher. This could solve the problem but if the resulting voltage is too high, that could lead to overheating of motors, etc. This is only a patch and not a real solution.
4. You need to look to the motor that is causing the dip and the circuitry that is upstream of that and your sensitive load. If the 5kV motor is causing the dip, the problem is not in the 480V circuitry.

Voltage drop is caused by large current passing through impedances in the circuitry. Starting motors draw large amounts of current leading to voltage dip. One way to reduce that is use a reduced-voltage motor starter, but analysis would be needed to determine if it's appropriate to your system. Too much impedance and/or too much current causes too much voltage dip. The problem may lie with the circuitry in your facility, the utility, or both. This commonly leads to finger-pointing between the customer and utility. The utility may have rules about the maximum permissible motor size. Above that size, they may require reduced-voltage starting.

I would strongly recommend hiring a qualified engineer to evaluate the situation.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[waross]

I had a similar problem. Starting the chiller motor crashed the computer. A constant voltage transformer feeding the computer solved the problem nicely.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[racobb]

"2) When looking at meters on incoming switchgear I see voltages of about 4035V, 4079V, and 4122V. However when I look at the voltages on the second downstream switchgear I notice that they are all upwards of 4130V. I dont believe that the voltages downstream could be larger than the upstream ones. Do you think this could just be an error in measurment? Both meters are connected in an open Delta arrangement to the PT's."

Check to see if the PT ratios are different on the incoming and downstream meters.


Alan

 

[pittengineer]

We've also noticed that this drop is occuring plant wide and even measuring minimums of 109V in office receptacles during certain times. Would this be an indication that we should raise or incoming voltage to some amount?

I'm going to look into the two measurment errors mentioned. Might be PT ratios as mentioned. There are power factor caps in the plant, but there are none downstream of the particuar feeder breaker and feeder ciruit I mentioned.

 

[TurbineGen]

What type of electronics are having the problem with the lower voltage? Perhaps using a UPS to power them would be an option? 112V is not that low. Most AC 120V systems are designed for voltages as low as 105V.

Is the motor being started with a load on it? If not, a soft start will likely minimize the problem as well. If it is loaded on start up you'll need to do some analysis to see if you can still start the motor on reduced voltage.

When you start this motor, is there an appreciable drop in the line voltage at the 4.16 kV switchgear?

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[PHovnanian]

We've also noticed that this drop is occuring plant wide and even measuring minimums of 109V in office receptacles during certain times. Would this be an indication that we should raise or incoming voltage to some amount?

How high does it go during periods of light load? If the no load set point for your LTCs regulator is set low, you might be able to raise it somewhat. The down side to this may be increased operation of your LTC and higher maintenance costs.

 

[alehman]

measuring minimums of 109V in office receptacles during certain times

If 109V refers to the motor starting dip and it's normally around 120, it is not necessarily an indication to raise the voltage. As I said, that can lead to other problems. If you raise it only one or two volts, it would probably do not harm other than increasing your energy use, and might be just enough to stop the electronics malfunction.

Dips to 112 or even 109 volts really shouldn't upset the electronics and they should be able to operate at 109 volts continuously.

As others have said, a voltage conditioner may be the simplest "fix" but this much voltage dip is really not desirable if it happens very often. 6% is commonly considered the maximum tolerable dip for occasional lighting flicker.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[magoo2]

You might want also to take a look at upsizing the 480 V/120 V transformer. What is the kVA size of the transformer and how many HP is the motor?

Otherwise you can consider using a constant voltage transformer or multiple constant voltage transformers to specific electronics equipment.

 

[JMatt]

Are you checking the voltages at the two switchgear simultaneously by looking at a real-time SCADA system? It's not impossible for you to check the voltage at one switchgear, then have your load profile change during the time that it takes you to get to the next switchgear.

4035V to 4130V is not too big a difference.

It sounds like you have a heavily loaded MV system, but not too bad that it causes MV contactors to drop out. Your low voltage system on the other hand is loaded heavily and loads drop when the MV source drops.

Cheap fix #1: Put a UPS on your computer.
Cheap fix #2: Upgrade your 480/120V transformer.

After that, it's more complicated. See IEEE 141 - Red Book concerning acceptable voltage drops.

 

[alehman]

Upsizing the 480/120 transformer will not reduce voltage dip, but a constant voltage transformer might.

The real problem lies upstream on the medium voltage system. Correction there will probably be expensive. If the plant-wide voltage dips are otherwise tolerable, a UPS, constant-voltage transformer or other solution specific to the electronic equipment might be viable. However I suspect there will be other problems in your plant with that much voltage dip.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[Kiribanda]

pittengineer,

1)4.16 kV is the open circuit voltage of the utility transformer.
2)4.16 kV is the NOMINAL voltage of the MV bus.

Based on above, how a 5 kV motor is connected to a network with 4.16 kV nominal voltage. The rated voltage of the motor should be 4 kV.
What is the rated (name plate) voltage of the motor?

Since 4.16 kV is the NOMINAL bus voltage, the measured voltage may or may not be equal to 4.16 kV.

Transformer LTC does not make any help in this type of motor starting problems.

Solutions may be

(a)to review the limitations of Utility for DOL motors?
(b)to use a soft starter
(c)to raise one tap on 480 V transformer and make sure that its open circuit voltage is with in the equipment utilization voltage range.

 

[busbar]

ANSI C84.1-2006 voltage ranges for 120V systems is 106-127V and for 4160V systems is 3600-4400V.

 

[pittengineer]

Spoke with utility and they tell me that the are regulating secondary voltage (tap changer) to 120V on the secondary of the 2400V to 120V PT's with the PT connected L-N on 4.16kV secondary. They said that there is a 2V bandwitdh on tap changer.

This means that they are regulating secondary to 4.16kV.

Management complain that they measure 117V on 120V system during normal operation and measure the 109V drop during motor starting transients. They insist that we get the utility to increase secondary voltage by 5% to 4368V.

I see that in IEEE Red Book 4370V is listed as maximum service voltage for a 4.16kV system with 4050 or so being minimum.

I question the approach to raising incoming voltage to near maximum level. I feel a concern here and dont know if this is correct solution to problem? What negative effects are we possibly going to face with this increase? Wouldn't this require possibly re-tapping unit subs to cloer match secondary voltage?

Does increaseing transformer size help with voltage drop in some cases, or is %impedance usually same for transformers?

 

[racobb]

There is nothing wrong with increasing the voltage to +5% over the nominal. It is done every day on distribution systems.

When people talk about a bandwidth you need to make sure you understand what they mean. Is a 2 volt bandwidth +/-2 or is it +/-1 volt. If it is 4 (+/- 2 volts which I doubt) then that is close to what management says they are mearuring. There is also a delay in the TC correction so it is possible for you to get that low before correction, depending on the delay.

Be careful of management using the wrong data or not understanding the data. I recently had a facility complain because their voltage dipped to 416 volts for 8 msec. An unreasonable whole 1/2 cycle....Distribution fuse probably blew and cleared the fault.

Lots of data without the ability to interpret and apply it is just as bad, if not worse, than no data!

Raising the voltage will not solve your motor starting problems though.

Replacing transformers probably won't help much, would need more info for that.

Sounds like it is a facility/management loading issue more than utility issue.

Alan

 

[TurbineGen]

I agree with Alan. Raising the voltage will not help. Why is 109V a problem during motor start? It is still inside the operating range of your 120V system. Raising the voltage will certainly give a higher voltage during motor start, but the voltage drop will still be the same.

It sounds to me like you have some sensitive equipment that cannot handle the transients caused by the starting of the motors. I would consider using a UPS as a filter. that is certainly less expensive than replacing your transformers.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.