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Surge Caps and Lighting Arrestors on 5kV motor
2

Surge Caps and Lighting Arrestors on 5kV motor

Surge Caps and Lighting Arrestors on 5kV motor

(OP)

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?

RE: Surge Caps and Lighting Arrestors on 5kV motor

If your breaker is a vacuum type breaker, then keep the surge protection. If you have any other type of breaker, the protection should not be needed. At least not if the insulation system is built for 5 kV. Best is to ask the manufacturer.

Re distance. It is not any problem to put the protective elements away from the motor. The surge that occurs is a result of interrupting the inductive motor current very suddenly. And, since the interruption is in the breaker, a protective element can be put any place between breaker and motor. Make sure that the ground, if connected to the protective elements, is solid and full area also where you put the caps and arresters.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge Caps and Lighting Arrestors on 5kV motor

(OP)

Gunnar

Thanks for the response.  We do not have a breaker but rather a Siemens 5kV drawout contactor which uses vaccum bottle contactors.  I guess that since these contactors are still vaccum type then we should keep the protection elements?

Can you explain more or point to references as to why these should be used with vaccum contactors and how the surge is created as a result of interrupting the inductive motor current?

I'm assuming the lighting arrestors are stricly for lighting strike protection on the line or motor itself?

When you reference the ground connection above I'm assuming you are taking about the equipment ground conductor coming from the starter to the motor?

RE: Surge Caps and Lighting Arrestors on 5kV motor

Firstly, it is not about lightning protection. Surge protectors are not effective for direct hits (a direct hit will destroy your plant - more or less) and induced or capacitively coupled lightning transients get damped in your transformer.

Secondly, the contactor is just as bad as a breaker. It is all about the vacuum bottles used. They do not create much of an arc that slowly reduces current to zero, but interrupt almost instantly. That causes a very high kick-back voltage, which is known to destroy motor windings. I had my first case in 1974 (Narvik ore harbour in Norway). We had three motors for ore conveyors destroyed within a couple of days and things haven't changed since then.

References? I would google vacuum, overvoltage, motors and then refine the search. There was a thread in Eng-Tips not very long time ago. I shall try and find it - or someone else will.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge Caps and Lighting Arrestors on 5kV motor

(OP)

I was doing some more research about the location of surge arrestors in a circuit and came across this in another post.

I came across and article that stated that the parallel cable length between the arrestor and the device being protected plays a factor in contributing to the overall overvoltatge surge level.  

The voltage arcoss the leads is determined by the length of the leads and the result of the induced voltage given by di/dt of the increasing surge current.  

The lead voltage is summed with the voltage across the arrestor to determine the total voltage seen across the proteced device.

Seeing this it leads me to believe that location of the arrestor does play a factor in location of these protective devices.

 

RE: Surge Caps and Lighting Arrestors on 5kV motor

One thing for sure - long leads on surge caps are bad.  i.e. tapping off the T-leads and running 10 meters to surge caps,.  Those cap leads add inductance which limits the effectiveness of the surge cap  (X = -1 /[wC]  + w*L... small L has big impact at high frequency).    Also think about the timing aspects if a traveling wave originating at the breaker has to travel further to get to the surge cap than to the motor.... not a good thing.

Adding a surge cap with very short lead length tap off at some ponit further upstream of the term box MAY be ok.... I'm not sure.  Take a look at IEEE 62.21
 

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RE: Surge Caps and Lighting Arrestors on 5kV motor

pete

Aren't cables were capacitive ?

RE: Surge Caps and Lighting Arrestors on 5kV motor

Cables would present parallel capacitance to ground as well as series inductance. It is the series inductance that is relevant, because it is series with the surge capactor. Total impedance of the series combination:

Z = j*w*L - j/(w*C)

Since the capacitve reactance is larger, the  net capacitve reactance is
Xc = 1/(wC) - wL

Normally we consider the L small.  But as the frequency w increases (surges have very high frequency content), that series L becomes more important.

Or in the time domain, think of it this way:  As soon as a sudden dv/dt is seen, the capacitor would normally respond immediately with high current. But the current thru the series conductance cannot change immediately, so limits the effetive ness of the capacitor.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Got it pete. Series inductance vs parallel capacitance to ground.

But how serious is the series cable inductance as compared to the sure diverter capacitance ? Is there any OEM specs restricting the cable length to the surge diverter ?

I have seen all motor/generator surge diverters only right at the terminals. So locating the surge diverter away from the terminals has never crossed my mind.

RE: Surge Caps and Lighting Arrestors on 5kV motor

An excerpt from EPRI Power Plant Electrical Reference Manual Volume 6 - Electric Motors - By Richard Nailen:

"Capacitor and arresters combined for a complete surge protection system.  At the higher voltages, they occupy a lot of space, and users sometimes ask that they be located in a separate cubicle nearby instead of in the motor terminal box itself.  This practice should be avoided.  If the circuit distance between the protective devices and winding is more than 3 ft, the cable inducatance tends to nullify the action of the capacitor, so that protectionwill be reduced.  A common ground connection should be made for the motor and the capacitor.  It shoudl be located so that the incoming voltage spikes hits the capacitor first before reaching the motor winding."

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Thanks pete. I knew I could count on you. A LPS.

OP, I think pete answered your question beyond doubt.

RE: Surge Caps and Lighting Arrestors on 5kV motor

One thing I would note is it seems that practices vary widely.

At our plant, we have surge caps on 13.2kv motors, but no arresters.  We have neither arrester nor capacitors at 4kv motors. We have arresters remotely at the plant incoming power.

I have heard from one fairly respected name that we don't need our 13.2v surge caps on our circ water motors which have maybe 1500 feet of cable between switchgear and motor.... he says that cable will filter anything from switchgear or upstream.

I have heard one very large utility that uses absolutely no surge protection on any motor, but they have very strict requirements that motors have dedicated turn insulation.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

pete

Is there web reference to that EPRI manual ? I would like buy a copy.

RE: Surge Caps and Lighting Arrestors on 5kV motor

edison123, be prepared for a shock when you find the price.  EPRI publications tend to be expensive for members and outrageous for non-members.  Like possibly $1000 plus.

RE: Surge Caps and Lighting Arrestors on 5kV motor

davidbeach

Would that $ 1000 be for one publication or a collection of them ? If it is later, I wouldn't mind since I love collecting tech journos, papers, mags, books etc.

But if pete can score a free copy for me, that would be fine too. :)

RE: Surge Caps and Lighting Arrestors on 5kV motor

You can get "Managing Motors" by Richard Nailen from Bark's publications for somewhere under $100.  I have both publications and they are almost the same.   Both are very handy references for motor maintenance and operation.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

I think that the discussion has diverted (sic) somewhat. The capacitors and surge arresters are not there to protect from external transients, like lightning overvoltage, but from the inductive kick caused by interrupting motor current.

Putting the protective elements close to the motor is good practice mostly because that guarantees that they stay connected. It has very little with cable inductance or capacitance to do.

The arresters and capacitors are there to offer an alternate path for the current being interrupted by the vacuum. If that path is situated close to the breaker, close to the motor or somewhere in-between does not really alter the way the caps and arresters work and what voltage the motor windings are exposed to.

If we look at the first option, putting them close to the breaker, the parallel path is there when the breaker starts to open, current immediately finds its way through capacitors and arresters and the overvoltage is kept within limits.

If we now look at the other option, putting them close to the motor, the inductive current will also find its way through capacitors and arresters as soon as the breaker starts to open. Keeping overvoltages within limits.

If the capacitors are selected in the tens - hundreds of nanofarads, there will not be any excessive du/dt and rise-times will be moderate in comparison to cable length. Hence no travelling wave or transmission line effects.

The above reasoning leads to the conclusion that caps and arresters can be put anywhere between motor and breaker, including motor or breaker terminals. But they shall not, as Pete says, be connected via stubs.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge Caps and Lighting Arrestors on 5kV motor

Thank you pete. Just orderered the book. S84.95 including shipping.

RE: Surge Caps and Lighting Arrestors on 5kV motor

(OP)
Great Discussion!

Gunnar I see your point however it differs from what was cited by pete's reference manual?

Do the surge caps have any effect during steady state conditions, such as leakage to ground, phase shift, voltage waveform dampening etc...?

RE: Surge Caps and Lighting Arrestors on 5kV motor

I don't think it differs. It is about two different things. Transients entering a system shall have low inductance paths to ground (the cited manual) while the suppression of the forming of kick-backs is quite another thing. All that is needed for that is to keep a reasonable low impedance (in comparison to the motor winding) path active for the short time needed to divert the current that flew in the ciecuit breaker just before it opened.

These two things are very often confused. I think that may be the case here as well.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge Caps and Lighting Arrestors on 5kV motor

(OP)

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?

RE: Surge Caps and Lighting Arrestors on 5kV motor

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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge Caps and Lighting Arrestors on 5kV motor

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.

Quote (IEEE62.21):

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]


Quote (IEEE62.21):

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.

Quote (IEEE62.21):

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.

Quote (IEEE62.21):


[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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RE: Surge Caps and Lighting Arrestors on 5kV motor

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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RE: Surge Caps and Lighting Arrestors on 5kV motor

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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RE: Surge Caps and Lighting Arrestors on 5kV motor

(OP)

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?

RE: Surge Caps and Lighting Arrestors on 5kV motor

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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RE: Surge Caps and Lighting Arrestors on 5kV motor

Quote:

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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RE: Surge Caps and Lighting Arrestors on 5kV motor

(OP)
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?

RE: Surge Caps and Lighting Arrestors on 5kV motor

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:

Quote:

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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RE: Surge Caps and Lighting Arrestors on 5kV motor

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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge Caps and Lighting Arrestors on 5kV motor

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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RE: Surge Caps and Lighting Arrestors on 5kV motor

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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge Caps and Lighting Arrestors on 5kV motor

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.

RE: Surge Caps and Lighting Arrestors on 5kV motor

Quote (IEEE62.21):

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.

Quote (IEEE62.21):

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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RE: Surge Caps and Lighting Arrestors on 5kV motor

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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RE: Surge Caps and Lighting Arrestors on 5kV motor

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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RE: Surge Caps and Lighting Arrestors on 5kV motor

Well - Pete.

That wouldn't be the first time  smile
 

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge Caps and Lighting Arrestors on 5kV motor

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:

Quote:

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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RE: Surge Caps and Lighting Arrestors on 5kV motor

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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge Caps and Lighting Arrestors on 5kV motor

Quote:

All machines are built to withstand such surges [closing] - even if it results in a transient being up to twice the peak voltage of the applied voltage.
What is the basis for this statement?
 

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Sorry, I did not intend to ignore your prior post.  That there are capacitors located remotely and it is different than what the IEEE says supports the common theme I have promoted throughout this thread:  surge protection is a complex subject.  Practices vary.  There may be thumbrules that work for some people in some situations but these are not universally agree. It is difficult to make generalizations.  

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RE: Surge Caps and Lighting Arrestors on 5kV motor

The attached analysis represents my simple understanding of why it would be preferred to place the cap at the motor vs the breaker.

It assumes a fairly simple model with the breaker represented as the voltage source, and lumped impedances for the capacitor, the upstream inductance, and the downstream inductance.

Solving simple lumped circuit equations to determine the transfer function Vmotor/Vsource, under some assumptions of high frequency, we find that the motor votlage increases as we increase the downstream inductance and decreases as we increase the upstream inductance.

I would be glad to hear any comments on the assumptions or the conclusion.  I am by no means claiming this is a perfect analysis  (as I said before I think surge analysis can be much more complicated than I understand... and I am also a little leery of relying heavily on lumped elements to predict wave phenomena).  

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Whoops - I see an error in the sign of my Vc term in KCL.  Let me correct it and try again.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Here is revised version. The resulting expression for Vmotor is a little simpler - no longer depends on downstream inductance from this revised analysis.

Vmotor is still minimized by maximizing the upstream inductance (between the cap and the breaker).  Suggests that cap at the breaker would not be a good idea.

Once again I'll be glad to hear any comments.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

OK - another possible error is my voltage divider to determine Vc.  It is correct if we have no current drawn from the terminal labeled Vm.  Probably not a good model.  Will try again maybe tomorrow.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Attached is yet another revision.  I have added the motor impedance into the model.  I have assumed the motor behavior is dominated by the capacitance to ground at the high frequencies of interest.  

With this model, the result is again very similar to predicted from my first model:
* the motor votlage increases as we increase the downstream inductance
* the motor voltage increases as we increase the upstream inductance

 

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RE: Surge Caps and Lighting Arrestors on 5kV motor

typographical correction:
"* the motor votlage increases as we increase the downstream inductance
* the motor voltage decreases as we increase the upstream inductance"

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RE: Surge Caps and Lighting Arrestors on 5kV motor

The hard to follow math file can probably be replaced by a simpler discussion:

Let's say I have to build a filter to protect the motor from surges.  I am given a single parallel capacitor and a single series inductor.  Should I put the capacitor in front of or behind the inductor?

The answer depends in part on how you model the breaker.  If you model it as anything resembling a voltage source, then putting the capacitor behind the inductor (cap next to motor) makes a better filter and will protect the motor better.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Pete.

You have now moved the question from that of permissible practical changes to an inductive kick-back protection scheme to that of the topology of a general low-pass filter.

In my opinion, you have arrived at an answer that is valid for an LC filter. An answer that we, and all engineers, knew was right from the beginning. It is, after all, one of the classical examples we all met in engineering school - in the earlier grades.

The main problem with your math is that you start with the wrong assumption, that of the surge coming from a voltage source with zero internal impedance. In fact, the source is a current source that is short-circuited up to t=0 and then applied to the cable, caps/arresters and motor.

I would like to see a new analysis starting from those correct assumptions and then see if moving the protective elements along the cable really makes such a catastrophic difference as you seem to think.

I do not have the time right now, but you seem to have it. I will check in later, shall we say in 15 hours? to see what you have found.

 

Gunnar Englund
www.gke.org
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RE: Surge Caps and Lighting Arrestors on 5kV motor

Sticking with your approach:

If we represent the source as current source Is with parallel Zs, we can replace it with voltage source Vs = Is * Zs and impedance Zs in series with Vs.

It looks just like the previous model, except instead of ZLup, we have Zseries = ZLup + Zs.

It suggests an easy way to determine the solution without repeating the analysis.

Define a new parameter Leq =Zseries/(j*w). = (ZLup + Zs.)/(jw) = Lup + Zs/jw.

Now we can see the effect on the solution for any source impedance simply by substituting Leq instead of Lup in the solution.

So let's examine the possible values of Zs

Case 1 – ASSUME that  Zs is pure inductive. Zs = Ls*jw.   
Leq = Lup + Zs/jw = Lup + Ls*jw/jw = Lup + Ls.
Now substitute into the Vm solution instead of Lup:  Leq = Lup + Ls.
The conclusions remain the same.

Case 2 – ASSUME that Zs is pure capacitive.  Zs=1/<jwCs>
Leq = Lup + Zs/jw = Lup + 1/(jwCs)/jw = Lup – 1/(Cs*w^2)
Now substitute into the Vm solution instead of Lup:  Leq = Lup – 1/(Cs*w^2)
Making  the usual high frequency assumptions, Leq>0, the conclusion remains the same.

Case 3 – ASSUME that Zs is pure resistive.  Zs = Rs.
Leq = Lup + Zs/jw = Lup + R/jw
Now substitute into the Vm solution instead of Lup:  Leq = Lup + R/jw
Note the numerators  has not been affected, so the numerator conclusion stays the same (increasing Ldown increases Vmotor).    
The effect on the denominator is to add an imaginary component where there was none before.  The magnitude of the denominator is sqrt(Real^2+Imag^2).  If we look at changes to Lup it will affect only the real part and the results are again the same.

So, even with your suggested changes to the model, the results are the same.  

With all that said, I don't think current source is the right approach.  Look at any textbook showing a surge traveling down a line and you'll see it modeled as a voltage source on the front end of the line and a voltage output at the end of the line.   The voltage spike travels down the line from one end to the other.   There is current flow associated with that voltage spike, but it is primarily through the distributed capacitance to ground of the cable, not end-to-end through the cable.   

 

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Take a look at attached excerpt from "The Vacuum Interrunter, Theory, Design, and Application" by Paul G. Skide  ISBN 978-0-8493-9091-3

It is "TABLE 5.6 - The Effect of Surge Suppression when Switching Motors with Vacuum Circuit Breaker Compared to an SF6 or Air Circuit Breaker"

These results are based on typical values.  Some things to note from the table:

1 – In the absence of surge protection, the switching-on transient seen at the motor is the same regardless of whether we use vac interuppter or air breakers.  The switching off transient (still in absence of surge protection) is only slightly worse with vacuum interrupter  (4 – 5 p.u. in .2-.5 usec)  than with air  breakers (4 – 4.5 p.u. in .2-.5 usec).   It would suggest that presence or absence of a vac interrupter may not be appropriate for use as the sole criteria for determining if surge protection is required.  As I have said before, it may very well be a practical and useful thumbrule and as such is a valuable contribution.  But it is not necessarily the end of the story.  We can exceed motor specification levels even without vacuum interrupter **

2 – As others have said, the switching on transient is not as severe as the switching off transient.   And I will point out that I have never said otherwise.  What I did say was that switching off is not the sole purpose of surge protection as others had implied.  I believe that my point is supported by this chart which shows not a huge difference between the unprotected switch-on (4 pu in 0.2-0.5usec)  and an unprotected switch-off  (4-4.5 or 4-5 pu 0.2-0.5 usec).       Again it supports the conclusion that the switching on transient is also important (switching on transient can exceed motor speicification).   **


3 – For the protection scheme including MOX and R/C, better protection is afforded when the protection is placed at the motor than at the panel.   The preferred location at the motor is consistent with the excerpt from the IEEE Guide.  (although I can't say I have ever heard of R / C surge protection on a motor).

4 – The author analysed several other protection schemes at the motor, but didn't even bother to analysi other protection schemes located at the panel.   I can't speak for the reason for this omission, but it is certainly possible that  he doesn't envision that protection at the panel is a common approach.

** On points 1 (vac interupter vs air breaker) and point 2 (switching off vs switching on), we were faced with a question of whether a change from at most 4 to 5 pu in 0.2-0.5 usec is significant.   A very relevant number to bring into this analysis is the motor specification for surge withstand.  NEMAMG-1 and IEEE522 specifies that new motors be tested to 2.5 pu or 3.5 p.u. with  the same 0.2-0.5usec rise time.  So, at 4 pu,  the closing transients on air breakers are still beyond what we tested the motor to when new (at the most stringent test level of 3.5).   This particular fact does not support the notion that we can ignore the closing transient and does not support the notion that we can ignore the any transient associated with air breakers.   This fact does lead us again to the question:  what is the basis for the claim that ALL motors are designed to withstand the  closing transient ?

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Sorry, my last paragraph was in error.  The 3.5 p.u. test increases to 3.5 pu in a period 0.1 usec to 0.2 usec per IEEE522, so the p.u. values are proabaly not directly comparable in terms of the associated dv/dt.  To be honest, I have no idea how we can allow such a large variation in the rise time numbers quoted by both sources since 100% variation ion rise time causes 100% variation in dv/dt.

At any rate, ignore the very last paragraph of my last post.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

A transition to the simple world of electricpete - what is the importance of dv/dt vs Vpeak?

Going back to my simple model:  the wave travels through the winding.  If it is traveling fast enough, then the voltage difference is seen between adjacent turns.

So let's say applied voltage step increaes 0 to Vmax in Tstep

Case 1 - time to travel through 1 loop is Tloop = 2*Tstep.
The voltage between adjacent turns is Vmax/2.

Case 2 - time to travel through 1 loop is Tloop = 1*Tstep.
The voltage between adjacent turns is Vmax.

Case 3 - time to travel through 1 loop is Tloop = 0.5*Tstep.
The voltage between adjacent turns is still Vmax.

And of course as we decrease Tloop further, we can never get a difference more than Vmax.

So there is a threshhold effect of rise time.  At relatively slow rise times, the voltage between turns depends on the rise time.  At faster rise times, we hit the maximum, and the stress is no longer dependent on the rise time.

I am not sure where the 0.1-0.2use and 0.2-0.5sec would be in relation to the threshhold.  I guess I could use typical coil dimensions, make some assumptios about mu and epsilon to determine speed of the wave, and try it out.  A project for another day.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Correction in bold:
And of course as we decrease Tstep further, we can never get a difference more than Vmax.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

As we are converned with inductie surges when breaking a circuit, am I oversimplifying to consider a simple DC inductive/resistive circuit? When voltage is applied to an induction, the current increases exponentially depending on the induction and the resistance of the circuit. The voltage never exceeds applied voltage. When the current is interrupted, the voltage is determined by the inductance and the resistance of the circuit.  As a vacuum contactor interrupts the current very rapidly and presents an extremely high resistance, the "inductive kick" on de-energization may be many times the applied voltage. The addition of a shunt capacitor provides a path for the current and presents an impedance to a steep wave front that decreases as the wave front becomes steeper.
When considering inductive kick and interuption of current is it valid to ignore the AC and consider the current as fast decaying DC? Am I on the right track here, Gunnar?

Bill
--------------------
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Jimmy Carter

RE: Surge Caps and Lighting Arrestors on 5kV motor

That simple model does not seem to match up too closely with what we see in my attachment 23 Aug 08 17:32.  Unprotected motor with air breaker sees  4 pu closing , 4-4.5 pu opening (rise time  0.2-0.5used).  Maybe the data is flawed?  The author doesn't explain a lot where the chart came from. Anyone else wants to present a reference... please do.

Returning to my quest to compare the p.u. values in the attachment to the surge test levels.

I mentioned two levels:  2.5 and 3.5 for testing.  Actually it is 2.0 or 3.5

NEMA MG-1 specified that the 2.0 is "standard" and the 3.5 is "special"

So, let's just use the standard motor at 2.0 instead of the 3.5 special.

If the thresshold rise time is somewhere up around 5 usec, than rise time is not relevant, clearly the 2.0 that we specify the motor tested to is less than the 4.0.  The 4.0 pu stresses seen during closing with air breaker are higher than the test.

But where is the real threshhold time?  I would venture to say it must be above 2.0usec.  Otherwise why on earth would IEEE522 specify a wide range 0.1-0.2usec (only makes sense if you are below the threshhold where rise time is not important).

But even if we are absurdly generous and use the other assumption that the threshhold rise time is somewhere down below 0.1usec (in which case the IEEE limit makes no sense whatsoever), then  the turn stress is inversely proportional to the rise time.  So this 2 pu in 0.1-0.2usec gives a voltage stress equivalent to to 4 pu in 0.2-0.4 usec.

Our brand new standard new motor is tested to withstand a stress of 4 pu in 0.2-0.4 usec.  And we are going to subject it 4pu in 0.2-0.5us during switching on with an air circuit breaker.  And 4-4.5 during switching off with an air circuit breaker.  We have exceeded the test stress during switching off and gone right up to the limit when switching on. That doesnt' give me a lot of confidence, considering the normal test strategy (for example hi-pot) is supposed to test beyond the operating envelope when the motor is brand new. so that we have some margin to operate reliably in the field for many years, including allowance for degradation from aging.   And let's remember the unrealistically generous assumption of threshhold below 0.1usec.

All things considered, I would say this analysis does support the assertions of the last paragraph of my 23 Aug 08 17:32 post qualitatively (although the quantitative conclusion is different).  i.e. The typical levels of stress created even by closing (vs opening) of air circuit breaker (vs vacuum contactors) in an unprotected environment (ie 4.0 pu in 0.2-0.5usec) IS at or above the stress level created during testing of new motors at the NEMA MG-1 "standard" level 2.0 pu.

By the way, my real world experience is that most motors do fine without surge arresters (all of the 4kv motors at our plant). But again, I am just cautioning against generalizations.  

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RE: Surge Caps and Lighting Arrestors on 5kV motor

1 -  http://www.nepsi.com/msp.htm

Quote:

Due to the wavelength and travel time of voltage surges, the MSP [Motor Surge Protector] is most effective when placed as close as practical to the motor terminals with the ground leads being as short as possible. This will limit the surge voltage seen by the motor to the discharge voltage of the arrester. This should be done for all critical and large medium voltage motors. Where there are many small motors or explosion proof motors in hazardous locations, a single MSP at the motor control center is recommended.
This particular manufacturer of motor surge protective devices clearly indicates that locating the surge protector as close as possible to the motor gives the best protection and therefore is preferred for critical and large motors.   They mention some  special situations when the surge protector would be located at the MCC... for multiple smaller motors protected by one surge protector (not clear to me how this would work) or explosion proof motors in hazardous locations.
2 - http://books.google.com/books?id=V1SXEnhCN88C&pg=PA580&amp;dq=%22Surges+generated+during+a+switching+%E2%80%98ON%E2%80%99%22&amp;sig=ACfU3U2znF1siLAvlTIb0JMkMktws7pXgg

Quote:

(ii) Surges generated during a switching 'ON' operation
Field data collected from various sources have revealed failure of a motor's windings, even during an energizing process. It has been shown that a motor may be subject to an overvoltage of 3-5 p.u. with a front time 't1' as low as 1 us or even less (signifying the steepness of the TRV) during switching ON.
The above link goes on to explain the reasons.  It has to do in part with timing of the closure of the contacts on the three phases.    

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RE: Surge Caps and Lighting Arrestors on 5kV motor

pete

As a rewinder, I agree with you on many motors failing during switching on rather than on switching off or during running. Based on my clients' feedbacks, I would roughly categorize the percentage failures as 80% switching on, 18% during running and only 2% on switching off.

And I have never seen a surge protector located away from the motor or generator.

And I appreciate all your time and efforts in this thread. Wish I could give another lps.

RE: Surge Caps and Lighting Arrestors on 5kV motor

Thanks for the comments edison. Good point about motors failing on start rather than stop.   Also glad you took the plunge to order the Nailen book. I'll bet you won't be disappointed.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Yeah, I think that book will be worth the price even with additional $ 23 for the shipping.

RE: Surge Caps and Lighting Arrestors on 5kV motor

I will have to sit down and sort through these postings. But some comments can be made immidiately:


First. Bill's simple model is very valid when it comes to overvoltage when breaking an inductive circuit. The need for protection wasn't really a need until the vacuum breakers appeared. And the OP was about vacuum breakers. The simple model does, however, not take the transmission line effect on the wavefront into account. I still maintain, and I have got good reasons for it, that such effects are a minor problem and that no protection is needed because of them.

Second. Edison's observation "As a rewinder, I agree with you on many motors failing during switching on rather than on switching off or during running. Based on my clients' feedbacks, I would roughly categorize the percentage failures as 80% switching on, 18% during running and only 2% on switching off"  is a very common false conclusion. A motor that was destroyed by inductive kick-back at switch-off will remain undetected until energized again. It will then blow fuses or trip breakers. The observation that it failed when switching on is, therefore, one of the most common false conclusions in industry.

Third. "And I have never seen a surge protector located away from the motor or generator" As a rewinder, you probably only see the motors. And then you will see protection at the motor, if there is any. What you do not see is all protection left in the cabinets. I have seen them - many of them.

Pete. All references have wordings like " as close as possible to the motor". Of course! But, in the OP's case, "as close as possible" is obviously a bit away from the motor - but still rather close. It is also very natural that a producer of protective equipment recommends mounting that makes his equipment perform as good as possible. Also, I do not think that you have given us a quantative answer as to how bad more remote (say 10 or 20 feet, but still connected directly to the motor cable) mounting really is. Does it mean that the stresses on the motor winding will be 1 percent more? 10 percent? 100 percent? If the answer is 1 or 10 percent - even 20 percent, then I think that most of this thread has been totally unnecessary.




 

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Gunnar

I rewind not only motors but lotsa utility generators up to 250 MW, both at my shop and at sites. Like you, I also visit various sites in many countries. I have never come across surge arrestors located away from the terminals in my nearly quarter century in this line. None. Never.

Your suggestion the motors could have failed unnoticed during switch-off goes not gibe with many of my clients telling me that they had tested (at least an IR) the motors/generators before energizing.



 

RE: Surge Caps and Lighting Arrestors on 5kV motor

We have different experiences here. Why did they test a motor before energizing? OK if it was a newly installed motor. But such motors do not fail very often.

Most motors that fail do so unexpectedly. It is not very common to test a motor before every routine start.

I am a wee bit reluctant to accept such talk.

The different experiences regarding installation of protection may be because of different parts of the world? You know the saying about Heidelberg and Jena?

I will be seeing ABB Motors in Västerås within short. I am not so sure they know about this either, but shall talk to the designers directly.

BTW, the protection in Narvik (see previous post) was installed in the cabinet. Not at the motor site. Has worked ever since. A recent generator in a co-gen plant was destroyed because of vacuum breakers (after 840 starts) There was no transient protection in the cabinet or the motor. The company that deleivered the motor control panel installed protection in the panel. Works very well. There was no discussion as to where to put the protection. Have we been doing this wrong for decades? And why doesn't it show?

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RE: Surge Caps and Lighting Arrestors on 5kV motor

I will have to agree that there are probably many factors that contribute to the 80% failures during start.  Motors might become moist or contaminated during shutdown period under non-ideal conditions.  High mechanical and magnetic forces during start.  High temperautres in parts of the motor.   But I do believe that starting surges are one factor that can attack weak motor turn insulation and push it over the edge to failure.  

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One small quote which may or may not be relevant.  "Electrical Transients in Power Systems", 2nd ed by Greenwood ISBN 0-471-62058-0, page 541

Quote:

There is an issue which must be discussed in this application and that is the need to place the arrester as close as possible to the transformer it is protecting.  [b]Indeed, installing all protectors as close as possible to the protected object is a cardinal rule of protection.  Let us see why this is so...
What follows are some figures and analysis that I don't understand.  Looks like waves reflecting back and forth from the terminals of the protected device.  Will try to study it when I get more time.

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Here is another very interesting thing:
http://books.google.com/books?id=V1pepm-1m-cC&pg=PA574&;dq=The+length+of+the+interconnecting+cables+plays+a+vital+role+in+containing&amp;sig=ACfU3U3aubBWByzKaQLravBCrtWcFcyX6Q

Quote:

Note
The length of the interconnecting cables plays a vital role in containing or enhancing the severity of the incidence wave. After the interrupter, the surge enters the cable and propagates ahead. As it propagates, it rises in amplitude, at a rate of Vt/t, (Figure 17.15 [at the bottom of this page]) until it reaches the far end of the cable. The longer the cable, the higher will become the amplitude of the incidence wave which will be more severe for the terminal equipment (refer to protective distances, Section 18.6.2). The length of the interconnecting cable is therefore recommended to be as short as possible. The manufacturer of the interrupting device can suggest a safe length for different sizes of cables, depending upon the voltage of the system and the equipment it is feeding.

I think they are saying that the voltage spikes apparently grow in magnitude as they travel down the cable, so shorter cable is better.   

That is just werid.   It does ring a bell from the discussions on VFD cable length.  I vaguely remember for VFD's that too short a cable length or too long a cable length can cause the higher spikes at the motor.   Is it a similar type phenomenon?   I am not sure how to interpret that chart at the bottom of the page which shows voltage increasing then decreasing with time where time related to length.

Even though I don't understand it, I suspect  it is probably right.   It doesn't agree wtih my circuit analysis, but that doesn't mean much -   I have said all along that my circuit analysis didn't include wave effects.

If it is true that the voltage spike gets bigger and bigger as it goes down the line, then we have found a very good reason that the protective device should be located at the motor end.   Certainly for an arrester – a spike which was below the threshhold for conduction at the breaker end might grow much bigger at the motor end.   For a cap, the implications are not as clear but still seems to make sense to put the protection at the location you want protected if magnitudes can increase going along the line.

==============================

Quote (skogsgurra):

Also, I do not think that you have given us a quantative answer as to how bad more remote (say 10 or 20 feet, but still connected directly to the motor cable) mounting really is.
I did provide in my attachment 23 Aug 08 17:32 results showing that for one type of protection, the motor surge increased from 1.5 pu. to 3.0 pu when we moved the protection from the motor to the panel.  Take it for what it's worth.  In my part of the world, we don't need detailed justification/analysis to follow standard and manufacturer recommendations, we need justification/analysis when we deviate from them.  

=============================

Quote (skogsgurra):

22 Aug 08 16:14
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.

Quote (skogsgurra):

24 Aug 08 3:16
then I think that most of this thread has been totally unnecessary.
You have advised me when you didn't like my tone inappropriate and I will do the same.  On the surface, it sounds as if you are suggesting that you had provided the definitive input in your very first post, and no-one else should have dared to provide any discussion after yours.  I don't really think that's what you meant and I chalk it up to heat of the moment.  
If you can shed any light on my questions about the thumbrules for cable length between VFD /motor and how/whether it might shed some light on the passage showing longer cable creates higher surge, I'd be interested to hear.
 

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Pete.

Yes, you did get me right when you think that I thought I had delivered a definitive answer.

The whole thing is about the kick-back when opening an inductive circuit. And there, I think that my answer was very valid.

What you and several others have discussed are transients coming from the outside. Transients that have a very low source impedance, i.e. same as the cable wave impedance. I have, obviously in vane and before a completely blank auditorium, tried to explain the difference and why there is a difference.

Sorry that this seems to have developped into something personal. I did not intend that.

Gunnar Englund
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RE: Surge Caps and Lighting Arrestors on 5kV motor

To summarize one point:  There are differences in surge withstand capability of motors, both in the specification world and the design world.

In the specification world, we can test to 2.0 pu or 3.5 pu  (0.1-0.2 usec) for NEMA large motors.  I'd hope you'd agree that's a pretty big difference.

In the design world, I would group it into three categories of turn insulation from worst to best:
Worst - no dedicated turn insulation.  No mica strand insulation.  Turn insulation funciton is performed simply by film on the strands.
Middle  - no dedicated turn insulation.  Strand insulation includes Mica tape.
Best -  Dedicated mica-paper tape turn insulatio.

You will find all of these varieties in service.  I did failure analysis of a motor with shorted turns (tripped during start...I'm not making any claim when it failed).  We segregated the non-failed portion of the winding and surge tested it - some sections didn't even come close to meeting the reduced maintenance level surge test requirement.  We cold stripped one full top/bottom coil section of the windings (to avoid damaging them in a burnout) and did an inspection.  There was no dedicated turn insulation and the strand insulation was just film. You would be amazed how brittle and fragile that strand insulation was.  I have a video somewhere showing how it crumbled in my hands if anyone is interested.  

The point being, there are big differences among motors.  Do you know the specification or construction of the turn insulation of the motors whose service history you are citing Gunnar?   If not, there is reason for caution because the genralization may not apply to motors with lower strenght turn insulation.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Quote:

What you and several others have discussed are transients coming from the outside.
That is not what I intended to address.  If there is a particular statement or argument you think applies exclusively to external surges, please let me know.  Note he attachment to my post 23 Aug 08 17:32 is labeled "The Effect of Surge Suppression When Switching Motors..."  Clearly, this particular reference addresses the motor switching transients.  And in one table, it describes a fair number of the items we have been talking about (vacuum vs air breakers, opening vs closing, protection at motor vs switchgear).

If you are inclined to agree to disagree, that's fine with me.  I think if you read my comments carefully, you will find that I have never asserted a strong conclusion... just brought additional references, analysis, and questions into the discussion.

But there is still a question I am really interested in.

The link in my post 24 Aug 08 13:55 to the document that described increasing surge voltage magnitude as the cable gets longer.  

I had several questions about that including how it relates to vfd cable lenght which is somethign I remember you and the others on this board have discussed  (it's not something I'm familiar with... we don't have vfd's).  

What are the thumbrules for selecting vfd cable length?
Is there any explanation for those thumbrules?
Does it shed any light on the idea that surges can grow larger as they travel down the cable?

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RE: Surge Caps and Lighting Arrestors on 5kV motor

My post 24 Aug 08 14:42 - the description of the middle category was a little weak and narrow.  There can be a variety of strand insulation which incoporates Mica or some new materials whose names I don't know which perform a lot better than simple film on strand described in the weakest category.  All of these would be included in the middle category.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Pete.

I am not in any mood to continue this thread. There are several sites where you can find the data you are asking about. One very early paper is the classical "Riding the Reflected Wave" by the guys at Allen Bradley (now Rockwell automation). It can be accessed at http://ieeexplore.ieee.org/iel3/4210/12267/00564866.pdf?tp=&arnumber=564866&isnumber=12267 if you have an IEEE account, which I believe you have.

There are several installation guides where cable length is discussed. The series of ABB Technical Guides, notably #5 is also highly recommended.

Do not expect any further posts from me in this thread.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Surge Caps and Lighting Arrestors on 5kV motor

Thanks gunnar. It was an interesting discussion for me anway.  Sorry if any of my comments rubbed you the wrong way.

I will take a moment to explain one reason why this thread has really captured my interest.

The most critical motor in our plants are the reactor coolant pump motors.  Failure of any one of them would cause a plant transient and would likely cost $10 million (plant must be cooled down and depressurized for replacement).

The motor is located deep inside the reactor coolant building in an area which is completely inaccessible during power operations and still difficult to get to during shutdown conditions (must dress up in yellow anti-contamination clothing, go up and down many ladders, accumulate radiation dose, etc).

The PWR design from this supplier provides the surge caps directly in the motor terminal box.  And these surge caps are film type caps, not oil-filled caps (the quantity of oil used in minimized for fire-loading reasons) .  They have 51 series elements.   The supplier tells us to trend the capacitance reading every 18 months and replace when it shows evidence of a single shorted cap (which we see periodically).

You can imagine, it is a lot of work to go to the inaccessible location every outage to do these measurements.  But we do it.  And it is even more work when we have to replace those caps since they weigh around 150 pounds and have to be rigged into this location.

How much easier it would be if those capacitors were located in the switchgear which is a very accessible location.

So the question comes.... if it doesn't make any difference, why did the supplier put them on the motor and make our life so difficult?    It could be that he just didn't know any better?   If that's the case, maybe we can get the moved.  It's an idea I'm tossing around  as a result of this thread.

Also you have mentioned there is no need for surge suppression in absence of vacuum interrupters.  Our motors have air breakers, but yet we still have surge caps on all of our 13.2kv motors, including these reactor coolant pump motors.  So maybe another alternative would be to just get rid of those surge capacitors (instead of relocating them) which not only reduces maintenance requirements but eliminate potential failure (cap failure) for a component which MIGHT not be needed.  Also we have long cable length which may help attenuate surges but I'm not so sure anymore.  At any rate, removal of surge caps if it is an acceptable approach would be a pretty darned easy modification (just lift the leads of the existing caps).  Another idea to toss around.
 

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Here is my simplified attempt at a calculation of the effect of cable between arrester and motor.  Effect of the associated capacitor is neglected (I don't know how to model that part).

As we know, when the wave first hits the motor, it can almost double, depending on the relative impedances of motor and cable.   The ratio is 1 +  (Zmotor-Zcable)/(Zmotor+Zcable)
For simplicity, let's assume that the it doubles.

So let's say we have an incoming wave with rise time  0.8 usec and peak value 3 pu.  
Slope rho = 3/0.8 = 3.75 pu/usec

The doubling effect means that the rate of increase of voltage at the motor location is 2*rho ie 7.5 pu/usec

Let's say the arrester MCOV is 1.5 pu.
Let's say the speed of the wave in the cable is u = 140,000 km/sec (just under half the speed of light in a vacuum).
Let's say the distance from arrester to cable is L = 28m
Time to travel that distance = T = L / u ~ 0.2 usec

At the arrester, the incoming wave looks like the following:
t=0,  Va = 0
t = 0.2,  Va = 0.75
t = 0.4,  Va = 1.5.  At this time, the arrester shorts and also sees reflection from the motor.  But none of this will become apparent at the motor until t=0.6

Here's what the motor looks like:
t=0,  Vm = 0
t = 0.2,  Vm = 0
t = 0.4,  Va = 1.5   (remember it's increasing twice as fast)
t = 0.6, Va = 3.0

So with 28 m of cable, the result is that our 1.5 pu arrester alows 3.0 pu voltage at the motor.   It is probably overly-conservative based on the fact that we have neglected the capacitor.  You could perhaps use a similar calculation as a bounding calcualtion if you are trying to justify a short run of cable.

Anyone please check or comment on  my assumptions and calculation.

Fwiw, I based my calc on my understanding of the attached excerpt from Handbook of Power System Engineering by Yoshihide Hase (he talks about the slope, but doesn't carry the calculation through to find the peak).  Note also the discussion of the effect of separation distance upon oscillations on the last page.
 

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Actually, my example problem could have had the incoming wave peak magnitude at 1.5pu (vs 3.0pu) , provided the slope is still maintained at 3.75 pu/usec.   In this case, the result is that our lightning arrester does absolutely nothing.  It allows the incoming 1.5 pu wave to double exactly as it would have in absence of surge arrester.    In contrast if the surge arrester were very close to the motor, the voltage would be limited very close to 1.5 pu.

At least, that's my understanding.  I'm open to comments.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

"An Update On Surge Protection Of Medium Voltage Motors:
A Comparison Of The Standards And Applications"
Lanphier, M. Sen, P.K. Nelson, J.P.
NEI Electr. Power Eng. Inc., Arvada;
Electrical and Instrumentation Applications in the Petroleum & Chemical Industry, 2007. PCIC Europe 2007. 4th European Conference on
Publication Date: 13-15 June 2007
On page(s): 1-8
ISBN: 978-3-9523333-0-3
INSPEC Accession Number: 9693240
Digital Object Identifier: 10.1109/PCICEUROPE.2007.4353996
Date Published in Issue: 2007-10-22 11:34:05.0


Quote:

The rate-of-rise of a surge can be reduced by selecting proper values of (L) and (C) and maximizing the time period (T). The series (L) is usually given by the cable. The shunt  capacitance (C) can therefore be changed to limit the risetime. Surge capacitors are most effective when placed as close to the motor as possible. The time constant associated with a capacitor placed at the motor end of the cable is larger than if the same capacitor were placed at the source end of the cable since (ZcC) > (RsC). A common practice is to use a lumped capacitance per the table below.
[Note Zc = Cable Surge Impedance while Rs = source impedance.]

 

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RE: Surge Caps and Lighting Arrestors on 5kV motor

Here is the article "Riding the Reflected Wave" that Gunnar had mentioned:
http://www.ab.com/drives/techpapers/ieee/pcic.pdf

After reviewing that and some other references, the effect of cable length seems a lot simpler and less mysterious:

A very long cable causes an amplification at the motor upon reflection of the wave.

The amplifiation factor depends on ratio of impedances. Worst case is 2.0 when Zmotor >> Zcable.

Starting with a short cable, as we increase cable length, we transition from no wave effects (no amplification) to full wave effects (amplification based on impedance ratio).   The transition point depends on the wavelength of the surge
(shorter rise time => transition at lower cable lengths.)

Knowing this, the passage that I cited 24 Aug 08 13:55 was misleading:  "After the interrupter, the surge enters the cable and propagates ahead. As it propagates, it rises in amplitude, at a rate of Vt/t, (Figure 17.15 [at the bottom of this page]) until it reaches the far end of the cable".     The surge doesn't increase magnitude "as it propagate" down the cable... we just have the potential to see more amplification due to reflection when it hits the receiving end for longer cables.   So the conclusions that I speculated about in that particular post based on that description of growing pulses were wrong.   In particular, the doubts I had expressed in that post about using my circuit model ( for predicting effects of changing cap location ) based on that strange description are eliminated.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

I have spent awhile reviewing various references and google.  I found numerous additional refernces recommending to put the protection close to the motor or at least analyse the situation:  For example:

Quote:


IEEE Std 141-1993 -" IEEE Recommended Practice for Electric Power Distribution for Industrial Plants: (The Red Book)

6.7.3.9 Rotating machine protection
Incoming surges can be transferred through transformers by electrostatic and electromagnetic coupling. Therefore, surge voltages can be experienced on the transformer secondary as well as the generator terminals as a result of surge-voltage impulse on the transformer primary terminals. This can occur even though the transformer is protected with arresters at the primary terminals.
When high-voltage surges are internally generated, the standard protective circuit for rotating machines consists of arrester and capacitor located near the machine terminals. The function of the arrester is to limit the magnitude of the voltage to ground, while the capacitor lengthens the time to crest and rate of rise of voltage at the machine terminals.
.....

6.7.3.9.2 Rotating machine surge protection practice
Much documentation exists relating to the surge protection of rotating machines. An ideally protected installation requires the following:
a) A strictly effectively shielded environment
b) Arresters at terminals of machine
c) Surge capacitors at terminals of machine
d) Strict adherence to good grounding practices

.......................

6.7.3.9.3 Special care required for proper installation of surge capacitors
Exploratory observations confirm the presence within shielded environments of voltage transients that approach arrester sparkover magnitudes and have exceedingly steep fronts (0.1 µs front-time). Although lightning does not usually entail such steep fronts, certain switching events do; for example, insulation breakdown, capacitor switching problems, or discharge of high lightning current-to-ground. As established previously, separation distance between protective equipment and apparatus to be protected invokes (sometimes serious) depreciation of protection. This is particularly true when steep wavefronts are involved. Surge capacitors, and preferably arresters also, should be connected directly to the machine terminals so that added inductance of the power cable circuit and of the surge capacitor lead will not interfere with their action. This limits the arrester and capacitor total lead lengths to one or two feet, thus requiring extreme care in the motor terminal box equipment arrangement.

Each application should be reviewed on its own. If several machines are fed from a common bus, for example, it may be suffcient to connect arresters on the line side of the feeder circuit breaker, placing only the capacitors at the machine terminals. Such practice generally requires that the insulated conductors of each motor feeder circuit are continuously enclosed in a grounded metallic raceway and that more than one feeder will be closed at the same time, along with a careful analysis of the arrester-protective level and the capacitor wave-shaping action as a function of the feeder length involved. A direct, low-impedance path between machine winding and surge-protective devices must exist on both line and ground sides of the circuit. A good ground connection to the machine frame is essential.

=================================
The only thing I did find that mentioned or recommended putting protection at the breaker terminals was Siemens literature about a device called a Surge limiter.    I have attached some info.  Since I need two attachments and this is a new topic, I will continue in the next post.
 

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RE: Surge Caps and Lighting Arrestors on 5kV motor

The only information I found about placing any motor surge protective device at the switchgear was some Siemens literature about a device called a "surge limiter".  

First, a quote from the attachment to my PREVIOUS post 29 Aug 08 16:30:

Quote:


Siemens Tech Topics #3
Limiters and arresters differ in several fundamental respects. The type 3EF surge limiter can absorb the trapped energy associated with a vacuum interruption, whereas a surge arrester has a greater energy absorption capacity to deal with system phenomena, including lightning strikes and switching surges from all sources. The 3EF surge limiter has a lower (i.e., better) protective voltage level than an equivalent surge arrester....


In capsule form, our recommendations for vacuum circuit breaker application (vacuum contactor application recommendations differ) are as follows: ....
    2. For transformers of reduced BIL rating, add some form of protection. (Either surge limiters at the switchgear, or surge capacitors at the transformer, or surge arresters at the transformer).
    3. For motors with locked rotor current under 600A, add surge protection (surge limiters at the switchgear, or surge capacitors at the motor, or surge arresters at the motor).

Notice they don't treat a surge arrester and a surge limiter as  interchangeable parts.  If we use limiter we put it at the switchgear... if we use arrester we put it at the motor.   And they give a hint of the differences between these devices – limiter has a lower (better) protective level..

Now let's explore the differences a little more by looking at the attachment to THIS post.

Quote:


Type 3EF surge limiters

* Lower protection level than in the case of common arresters
* The protection characteristic is relatively insensitive to steepedge impulse waves
* Especially suitable for limiting switching impulse overvoltages
...
The housing is made of plastic. Inside the housing there are gaps and non-linear resistors connected in series.

The resistors are Siemens SIOV metal-oxide varistors...
As a result of these characteristics, the surge limiters respond very quickly to switching surges and restrict them to low values.
So in contrast to the typical surge arrester (gapless Zn O), the surge limiter is a series gapped arrester with Si O element.

These are completely different devices.  The limiter is optimized to provide better protection for switching surges than a standard arrester (clamps to a lower value at the arrester terminals) and to provide a smaller footprint (for installation in switchgear), at the espense of ability to handle other types of surges.  The only device they allow putting at the breaker is the limiter which offers a lower (better) level of protection.  I would assume that this is needed to compensate for the distance effects which make arresters less effective as we remove them away from the protected device.

So we have found only one group of references that allows us to put the protection at the switchgear (the Siemens literature), but the protection is not an arrester, it is a limiter.   And the Siemens recommendations are careful to identify they are not interchangeable – the arrester is to be used at the motor and the limiter is used at the switchgear (one or the other, not both).

Gunnar – is it possible that the cases you are citing involve installation of limiters at the switchgear, rather than arresters?  From looking at the attachment, we see that limiters have a smooth plastic case, while arresters typically have the rippled porcelain external surface.

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RE: Surge Caps and Lighting Arrestors on 5kV motor

pete

Just got the Nailen book today. First glance - very informative. Thanks for your tip.

RE: Surge Caps and Lighting Arrestors on 5kV motor

(OP)

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?  

RE: Surge Caps and Lighting Arrestors on 5kV motor

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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RE: Surge Caps and Lighting Arrestors on 5kV motor

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

RE: Surge Caps and Lighting Arrestors on 5kV motor

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

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