Surge Caps and Lighting Arrestors on 5kV motor 2

[rockman7892]

I have an installation of a new 350hp 4.16kV motor in which the motor termination box contining the surge capacitors and lighting arrestors is too large to fit in the location next to the motor where it was intended.

Because of this several people here have proposed some solutions.

Solution 1 was to eliminate the Surge Caps and LA's all together and therefore eliminate the large terminatation box thus allowing a smaller termination box to be used. I dont feel as if it is a good idea to get rid of these components but dont have enough knowledge to defend my thought. Can someone help me understand why they are needed and point me to some info to back my thought.

Solution 2 was to keep the Surge Caps and LA's in this large box, however mount it elsewhere away from the motor where there is room. We would then come from this box to a much smaller box next to the motor itself to pick up the motor leads. My qustion and concern is, what is the effect of locating the surge caps and LA's away from the motor itself. If this is allowable, what is a safe or acceptable distance or location?

 

[rockman7892]

Pete

I've been out of town the past week and haven't had time to catch up on this tread. I finally got a chance to read it all the way through and must say you did an excellent job of researching this subject. Thanks alot for your references and calculations. I'm going to sum up what I think I learned from this thread to approach my project manager regarding the proposed relocation of these caps and arrestors.

1) The primary function of these caps and arrestors at MV and above motors are to protect the motor from voltages surges that are created by the "inductive-kick" from opening and closing both vaccum and bottle breakers. The motors here are all fed from vaccum contactors. Not much was mentioned in the way of vaccum contactors but I'm assuming that all the same effects and results apply?

2) The reason that the caps and arrestors should be placed close to the motor (as cited several times) are primarilly because of two reasons:

a) Inductance in the cable that is created when
locating the caps far away. This inductance limits
the effectiveness of the capacitor in smoothing the
surge waveform.

b) Reflection phenomenon when locating the arrestor far
from the motor. Witht the arrestor located away
from the motor the reflection pehnomenon can cause
the surge to be twice as large at the motor as
opposed to if the arrestor was located at the motor.

These are the main reasons, backed up by many of your citations and calculations for locating these devices as close to the motor as possible.

3) The debate still seems to be open as to weather or not these devices are absolutely necessary at ALL motors. However with that said, if they are used in order to be 100% effective they should be located as close to the motor as possible. Since locating them away from the motor limits their effectiveness, I would be inclined to say in my case that if the only option that we have is locting them away from the motor then we should just do away with them all together realizing that we are comprimising complete protection either way. What do you think?

 

[electricpete]

I agree with your summary. There is some difference in practice, but an IEEE guide is the closest thing to an industry standard practice in the US.

I would be more inclined to place them as close as practical to the motor, rather than to do away with them.

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[Marke]

Just to put my two cents worth in, my understanding is:
1. The problem is due to the inductive kick back when the current is interrupted extremely quickly. V = L x di/dt When an air breaker is opened, there is an arc and the effective di/dt is much slower than with a vacuum bottle.
2. If the current can be diverted as the bottle opens, then the di/dt is reduced and the magnitude of the voltage is also reduced.
3. If capacitors are used, then it is important that there is no additional impedance in series with them as this will reduce their effectiveness if the current through them changes.
4. The location of the capacitors can be sited anywhere along the current path from the contactor to the motor, so it can be at the contactor end or at the motor end or half way between. If the capacitors are located at the motor end of the current path, then the interruption of the current flow in the feeder cable between the motor and the contactor will result in a voltage transient at the contactor end relative to the motor end.
5. The issue of additional series impedance only applies to the distance between the existing current path and the capacitors etc. If the capacitors are mounted in a separate enclosure and there is a cable between the motor terminal box and the capacitors, there is an additional impedance. If the main cable routes through the capacitor box on the way to the motor and connects directly to the capacitors on the way through, this will work MUCH better.
6. There are many rules that are applied to situations such as this because it is the easy way to ensure that a working compromise is achieved. This thread illustrates what can happen if you say that it is permissible to site the capacitors away from the current path. - make a simple rule that can not be misinterpreted and stick to it and there will not be problems. This does not always mean that it is the best or the only solution.
Many of these fast switching situations only make sense to engineers experienced in working with High Frequency energy. RF engineering. This is particularly true in the EMC field where a conductor has inductance as well as capacitance and resistance.

If you connect your surge protection at the motor terminals, you will not go wrong, therefore that is a good rule to apply.
If you connect your surge protection so that it is on the current path, it will also work. If you connect your surge protection so that it connects to the current path but at a distance, it will not be nearly as effective.

Best regards,

Mark Empson
L M Photonics Ltd

 

[Marke]

I reread the OP section 1 where the suggestion was to remove the surge capacitors altogether and I wanted to reinforce that I do not consider that to be an option if vacuum contactors are used. This does not suggest that you should only use them when vacuum contactors are used, simply that to use vacuum contactors without the surge capacitors is a high risk.

I also believe that the voltages normally associated with closing transients are much lower in magnitude and energy than the opening transients, but agree that under some conditions, these may need to be considered as well. - it is a case of risk management and where the perceived risks are.
Referring back to the original questions:
1. remove the surge capacitors? - answer No. I think that is the unanimous answer of this thread.
2. move them away from from the motor? - answer yes provided that they are on the current path and not "tee" connected away from the current path with the consideration that there may be additional increasing risk of closing transients causing issues as the connection point is moved away from the motor terminals.

Best regards

Mark Empson
L M Photonics Ltd