Transients generated within a facility

[rockman7892]

We have been experiencing and battling voltage transients for some time now at our facility. We have seen these transients in the form of VFD DC bus overvoltage, damaged electronics, etc.. Although we have verified that most of these transients are coming from the utility outside the plant I was curious how equipment inside the facility can cause transients?

We do not have any capacitor switching inside our facility so I was more or less referring to motor loads, breakers, etc.. How can some of these items create transients?

We have several vaccum breakers and contactors which I know can cause transients at the motor or load, but can they cause transients upstream as well?

 

[marks1080]

You mentioned you use VFDs. These will add a harmonic component to your system from the switching. The switching in the VFD can also cause voltage transients.

When you switch any component (like your breakers and contactors) you will create a transient response. This shouldnt last more than a few cycles. The transient will go both up and downstream through the system.

If you are battleing voltage issues constantly it probably isn't related to a transient. More likly there is a permenant fault or maybe some coupling between AC and DC or the harmonics from the VFD are causing a DC offset. You can use a filter to help eleminate harmonic distortion.

How have you verified that some of your issues are on the utility side?

 

[gattie]

Your best plan of action is to carefully log all incidents &
circumstances when the damage occurred.
Recordings on DCS systems can play an invaluable role in this type of problem solving.
eg: starting or stopping large motors or switching activities.
I have always used this logic with great success.
Vacuum switches or contactors are often culprits & can generate huge overvoltages which destroy electronics & switchgear.

 

[sibeen]

Quite simply it's the old V = L (di/dt) equation coming into play.

If you have a current flowing and attempt to break it, the inductance of the cabling will come into play and the voltage generated by the above equation will come into play.

 

[wareagle]

Quote "Although we have verified that most of these transients are coming from the utility outside the plant"
I would doubt this statement. What makes you think this is true? Studies have shown that most problems are caused by equipment inside the plant.

 

[jack6238]

Mr.Rockman7892,
I agree with wareagle-sounds like we both have been there befor. Do you honestly believe that serious electrical transients are coming form the system into your VFD units and causing problems??? There must be at least two transformers, (typical but it may be one) between you and the VFD units and that is a lot of impedance. (Calculate the impedance value and you will see it the majority), so how could these transients come in "from the utility outside the plant"? I do not think so!!

Take look at the switching transients in your system. Here are some good resouces.

1. Electrical Transients in Power Systems-Alan Greenwood. This is the bible on this subject.
2.High Voltage Circuit Breakers-Ruben Garzon.This discuses the transients associated with switching the various manfacturer's breakers. Some vacuum breakers create more voltage transients than others.

Have a good day,
Jack

 

[rockman7892]

I guess I stand corrected and can now confirm that only a handfull of these transients have been caused by the utility as has been documented here.

So I guess that leaves me with trying to figure out what in the facility is causing these issues. It sounds like as others have said that it is not necessarily motors starting/stopping that are causing the issues, but rather the circuit breakers associated with these motor and the di/dt from these motors that are causing the issue. Is there a good whitepaper out there that discusses this topic?

Even with the several transformers and impedance between the utility and the VFD's I dont think it would be unlikely that this could still be a cause of the problem. Jack6238, mentioned that you could calculate the impedance value to see the effect. What is the calculation that relates impedance to transient effects.

I plan on trying to install meter(2) in my facility to capture such transients. The fluke meter I have cannont see transients below a 1/2 cycle in duration and I'm not sure what the capability of my Multilin PQMII's are. I am investigating other meters.

 

[jghrist]

http://www.copper.org/applications/electrical/pq/homepage.html

http://www.electrotek.com/pqconsult.htm

http://www.pqsoft.com/

are some good places to start.