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?

 

[Skogsgurra]

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

http://www.gke.org

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

 

[rockman7892]

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?

 

[Skogsgurra]

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

http://www.gke.org

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

 

[rockman7892]

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.

 

[electricpete]

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


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[edison123]

pete

Aren't cables were capacitive ?

 

[electricpete]

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[edison123]

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.

 

[electricpete]

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."

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[edison123]

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

OP, I think pete answered your question beyond doubt.

 

[electricpete]

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[edison123]

pete

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

 

[davidbeach]

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.

 

[edison123]

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.

 

[electricpete]

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[Skogsgurra]

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

http://www.gke.org

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

 

[edison123]

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

 

[rockman7892]

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...?

 

[Skogsgurra]

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

http://www.gke.org

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