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]

Gunnar you are saying that there are two different cases involved with these protection elements. Since there is only one set of protection elements wouldn't these elements have to meet whatever the minimum requirements of these two cases are which in this case sounds like the transients entering the system which is what the cited manual is referring to?

 

[Skogsgurra]

I don't rest my case. Just waiting for someone else to chime in.

You did ask if it was OK to put the protective elements away from the motor. I say Yes. But not with a separate cable, you will have to "land" on the caps/surge arresters with your motor cable and then continue from there to the motor. Or just accept things as they are.

I say that because the transients you get from a vacuum breaker is totally different from a lightning transient. Other impedance, other energy contents.

Also a lightning transient is very unlikely that deep in your installation. You probably have taken care of that at an earlier point.

Gunnar Englund

http://www.gke.org

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[electricpete]

I do think it is a very complex subject – certainly beyond my understanding. And in the end, some of the answers depend on opinions (Practices are not completely standardized).

Here are some excerpts from IEEE 62.21. The whole document is long and complicated and I can't say I have read the whole thing or understand the whole thing. After each excerpt, I will suggest some conclusions that you MIGHT draw from the excerpt, but of course without reading the whole thing we need to be careful.

In addition, a survey of several thousand motors in industrial service showed that few were equipped with surge protection, and there was almost no evidence of failure due to absence of surge protection. A survey by WG 3.4.9 of Surge Protective Devices Committee found (from a small sample of utility installations) that surge protective capacitors were failing at about the same rate as those motor insulation failures that were not caused by overheating. It was also recognized that capacitor leads as usually installed, and even when of quite short lengths, have sufficient inductance to prevent the capacitor from protecting the machine from steep-front surges. Motor starting surge fronts as short as 200 nanoseconds had been measured.

From this quote along, we might conclude that surge protection is not particularly important. The number of surge device failures from adding surge protection is roughly equal from number of motor failures eliminated by adding surge protection.[/QUOTE]

For a particular machine installation a quantitative evaluation such as is presented in this guide is required to determine whether protective coordination with the insulation withstand is achieved.

This quote is probably the most relevant one. IT TELLS US WE CANNOT MAKE SWEEPING GENERALIZATIONS. WE NEED TO EVALUATE THE SPECIFIC INSTALLATION. THE GUIDE GIVES VERY DETAILED EVALUATION METHOD.

capacitor internal inductance plus the inductance of leads as long as one meter can isolate the capacitor from the motor during steep-front starting surges, and may not be effective in wavefront sloping [B42]. Surge arrester lead length is not as critical when machine protective arresters are applied together with short lead length capacitors, because the capacitors will lengthen the rise time applied to the arrester lead inductance.

It seems more acceptable to extend the ARRESTER lead length than the CAPACITOR lead length.

[Example of using the "look-up method" in section 6.3.2]
Using Figure 9—To limit the 85 ns surge peak that is caused by inductance of the capacitor and capacitor leads to 2.0 pu, the ordinate value on an interpolated 85 ns curve must be no greater than (2.0 / 2.6) = 0.77 pu. This occurs at about 2.0 ìH. (For interpolation, see next paragraph.) Subtracting the 0.5 ìH of the capacitor, the leads must have an inductance of 1.50 ìH or less. At 1.25 ìH/m, the capacitor lead length should not exceed 1.2 m (1.50ìH / 1.25ìH/m = 1.2 m). To provide protective margin, shorter is better. [FOR THIS SPECIFIC EXAMPLE]

Again, this refers only to one particular analysed situation. For this situation, a max capacitor lead length of 0.5m was calculated. Shorter capacitor lead lengths are better. I would suspect that the shorter is better part generalizes accross a wide range of examples, and I think historical practice is simply to get them as close as practical, without detailed analysis of lead length requirements.

One other factor about the ability of the motor to withstand surges - In addition to the "dedicated turn insulation" mentioned above, there are 2 flavors of large motors that you can purchase under NEMA standards: 2.5 p.u. surge withstand and 3.5 p.u. surge withstand.

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[electricpete]

two corrections in bold:

"From this quote alone, we might conclude..."

"For this situation, a max capacitor lead length of 1.2m was calculated.."

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[electricpete]

Gunnar - I am not sure I understand your points.

I think you are saying that the lead length requirements are associated with protection from incoming transients rather than breaker switching? If so, I would disagree. The protection at the motor is to protect from transients associated with breaker cycling. Other arresters upstream will protect from transients from the grid coming into the plant.

You mentioned that the source of the transient is inductive kick. But there is a surge applied to the motor even when the breaker closes (not interrupting a current). It is because you have a very sudden application of voltage. That sudden increase in voltage represents a travelling wave with high dv/dt that will go to the motor and stress the first few turns. Now my simplification is that the reason high dv/dt stresses the turn insulatin: the voltage changes so fast that the voltage wave has reached one turn but not yet the next turn... so we see voltage difference between turns.

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[rockman7892]

Just to understand some of the terms we have been talking about here I drew up a typical circuit to help visualize some of these terms and make sure that I am understanding them. Hopefully I have labeled this circuit correctly.

I think I answered my own question regarding the effect of the capacitor under steady state. The equation for the current going through the cap is i=C* dV/dT. Because the value of C is such a small value (microfarads) normal voltage levels of 5kV will have little effect and thereore very little current to ground. However during a surge the there is a high dV/dT as a result of the surge and therefore current passes throught the capacitor to ground. Hopefully I understand correctly.

In some previous posts it was mentioned that there was internal inductance in the surge capacitor? I was not aware of this, is there a quick explanation?

 

[electricpete]

We have a Tee or joint or tap where the surge devices tap off the main line. The surge devices are connected between the Tee and ground. The length of lead between the Tee and ground is the surge cap lead length and arrester lead length we have been talking about.

There is also a distance between the Tee and the motor. This distance includes the motor leads. It has some importance to the IEEE analysis, but it is not what we have been talking about. (we have been talking mostly about the cap lead length).

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[electricpete]

I think I answered my own question regarding the effect of the capacitor under steady state. The equation for the current going through the cap is i=C* dV/dT. Because the value of C is such a small value (microfarads) normal voltage levels of 5kV will have little effect and thereore very little current to ground. However during a surge the there is a high dV/dT as a result of the surge and therefore current passes throught the capacitor to ground. Hopefully I understand correctly.

Yes, absolutely correct.

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[rockman7892]

Pete

I have redrawn this circuit based upon your description. Hopefully it is now correct.

So what we are saying is that is the cap lead distance between the "T" and the cap that can add inductance and therefore minimize effects of the surge cap. The distance between the "Tee" (cap) and motor has no relevance in this discussion. Although in another article as I referenced above it states that distance to motor is important when only using an arrestor, but because a cap is used in combination I dont think that is a factor here.

Does the inductance on the capacitor leads come from the dI/dT caused by the surge?

 

[electricpete]

You have redrawn it correctly (surgecircuit2.pdf) to match the terminology I used.

What I discussed was the cap lead length. We can very easily analyse the effect of that lead inductance (tends to reduce the effective value of the capacitance since the inductive reactance subtracts from the capactive reactance). It is the length that was discussed in several of the quotes above.

The other distance between Tee and motor is also important, but the reasons are not as obvious for me. Here is a quote from that same IEEE guide that tells us that this other distance is also important:

Although surge withstand capability levels must be specified for the windings, it is desirable, because of the
unpredictable nature of the surge magnitudes and rise times, that for critical applications surge protective
capacitors and gapless metal-oxide surge arresters (MOSA) be installed at or very close to the motor terminals.

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[Skogsgurra]

The fast voltage rise when switching on is a fact, no doubt about it. But it is a very infrequent event and it will not affect the insulation very much. All machines are built to withstand such surges - even if it results in a transient being up to twice the peak voltage of the applied voltage.

The problem with uneven voltage distribution in motor windings (the "first turns effect") is a problem when the voltage steps are applied repeatedly. Many thousand times per second as they are in a PWM frequency inverter application. The failure mode then is not an insulation "puncture" but the accumulation of ozone in the windings. The ozone attacs the insulation and breaks it down. Typical time to failure is many months to years.

I have not said that the protective elements can be added anywhere and with arbitrarily long leads. I said, in my first post, that "a protective element can be put any place between breaker and motor". I didn't think that anyone could interpret that as using a stub to connect the caps and arrestors.

I simply wanted to tell rockman that, if he wanted to move the protective elements away from the motor, he could do so. And that is regardless if the transients are from lighning, closing or breaking the motor current. The resulting discussion has surprised me.



Gunnar Englund

http://www.gke.org

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[electricpete]

I am not sure how you discount the importance of closing transient on the basis of it being infrequent. The opening transient is equally infrequent to the closing transient, last I checked.

That motors are designed to withstand a transient of 2*V seems to be a statement based on false comparison. It is not the voltage but the rate of change that is important.

As I understand your opinion is: use surge protection for vacuum interrupters and don't use it without vacuum interrupters. I have no objection to people presenting opinions and it might in fact be a very valuable and practical rule. However, the IEEE standard does not make such generalizations.



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[Skogsgurra]

Pete. If there is a will - there can be understanding.

I have not said that the peak voltage as such is the problem. I said that the fast voltage rise is a fact and that machines are built to cope with it, even if it reaches twice the peak voltage.

There is a very big difference between the closing of a breaker and the interruption of a breaker. Voltages can, in the later case, easily reach ten times the system voltage or more. It does not when closing the breaker.

Vacuum breakers interrupt very quickly and do not dissipate energy in an arc as other breakers usually do. That produces higher voltages and is why protection is needed when vacuum breakers are used.

I would appreciate if you read what I am saying instead of trying to counter with "witty" remarks like closing and opening being equally frequent. I already knew that they tend to be equally frequent.

Gunnar Englund

http://www.gke.org

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[davidbeach]

Gunnar is right on, the chop associated with premature interruption creates a much more significant transient than closing ever will. It doesn't take much chop, even chopping at 10A will produce notable results. Nothing of the sort can be created by closing at even the worst possible phase angle.

 

[electricpete]

5.8 Full-voltage motor starting, prestrike voltage A primary source of steep-fronted surges that a motor must repeatedly withstand is the closing of a breaker or contactor to energize the motor. When electrical conduction is established between the closing line and load contacts in an energizing breaker or contactor, the voltage across the contacts just prior to electrical conduction causes a traveling wave or surge to propagate toward the motor at the instant of conduction. The pre-conduction voltage across the contacts is referred to as the prestrike voltage.

6. Motor surge protection
Steep-fronted surges appearing across motor terminals may be caused by lightning strikes, circuit breaker prestrikes and re-strikes, motor starting, aborted starts, bus transfers, switching windings (or speeds) in twospeed motors, or switching of power factor correcting capacitors. Turn insulation testing itself also imposes a high stress on the insulation system

The quotes above will establish I believe that the inductive kick during opening of a vacuum circuit breaker is not the only problem to be considered, although it may very well be among the most severe transient. Whether or not lesser transients pose a threat to the motor that merits surge protection is a matter of opinion - there is no one right answer.

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[electricpete]

I believe the IEEE quotes above establish that the location of the surge protection is important for the surge protection function. That includes both the lead length and the distance from the motor.

If I am to understand gunnar's comments - the reason we can put surge protection anywhere is because it is not needed?

If we felt that surge protection were not needed, wouldn't it be smarter to eliminate it rather than putting it somewher that it does nothing. That would not only save money during construction, but would also eliminate a failure mode from unnecessary equipment.

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[electricpete]

And maybe I have misunderstood why you said we don't care about distance Gunnar. Maybe you are just saying that the location of the surge protective equipment does not compromise its fuction as long as it is between the breaker and the motor? In that case, there is a discrepancy between your comments and the IEEE guide.

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[Skogsgurra]

Well - Pete.

That wouldn't be the first time


Gunnar Englund

http://www.gke.org

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[electricpete]

And when I have re-read all your comments, I see you are in fact suggesting we can put in the caps at the breaker end of the cable, rather than the motor end.

This conflicts with IEEE 62.21-2003, Section 6.1:

Although surge withstand capability levels must be specified for the windings, it is desirable, because of the
unpredictable nature of the surge magnitudes and rise times, that for critical applications surge protective
capacitors and gapless metal-oxide surge arresters (MOSA) be installed at or very close to the motor terminals.

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[Skogsgurra]

Well - Pete, again.

I have seen many, many installations with 6 and 11 kV motors where the surge arrestors are placed at the breaker, running for decades with no problems. Perhaps those installations weren't so "critical" as those in your IEEE 62.21-2003, Section 6.1?

Gunnar Englund

http://www.gke.org

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