Surge Capacitors & Arrestors for Motor... 3

[kssschsekhar]

Dear Folks,
Greetings...In one of the projects, surge arrestor and surge capacitors are located in MV motor terminal box. As soft-starter has been provided for this motor, is it recommended to change the location of surge protection devices to switchgear end (before soft-starter)? The power supply to motor is feeding from Vacuum Circuit Breaker. What are the consequences in shifting surge protection devices to switchgear end? Appreciate your discussion and suggestions...Thank you..

 

[RRaghunath]

Surge protection is at times recommended when motors are switched directly using vacuum switchgear. The surge capacitors could pose problem to soft starter and I think you should consult the soft starter vendor as well as motor vendor. The recommendation could be to remove the surge capacitors!!

 

[electricpete]

There was a thread here with a lot of discussion of surge cap location:
237-224515

There were a lot of different aspects covered there. Buried within was a discussion about whether surge cap provides comparable protection when remote from motor at the switchgear. I think it highlighted one thing: there certainly are varying practices with respect to surge protection of motors in general. In retrospect, I think surge cap will usually do almost as good at the switchgear as at the motor.

In your case, I'm pretty sure that putting surge caps at the motor would not be an option. Putting them just downstream of the vacuum contactor would make sense to me in that situation fwiw.

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[electricpete]

Correction:
237-224515
should've been:
Thread237-224515

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[electricpete]

As mentioned, installing caps at motor is probably not an option in your situation.

However, I found another datapoint on the general question of preferred location of caps (motor or switchgear).

The authors below advocate that placing caps at the motor is far preferable to the switchgear. One reason is that apparently that caps near the breaker are more likely to interact with the breaker during interruption to cause problems such as reignition and current chopping.

http://www.ipst.org/TechPapers/2003/IPST03Paper5c-3.pdf

“Simulation of Protective Schemes of High Voltage Utility Motors”, by Archana soni , Gayatri Agnihotri, Ganga A., presented at International Conference on Power System Transients – IPST 2003 in New Orleans, USA

In accordance with industry practices and standards surge capacitors should be located at the load and as close to the load terminals as possible. This minimizes series inductance effects, which reduce the effective surge impedance of the load circuit. All of these factors are important in reducing the probability of degradation of solid insulation, which could result from the non-uniform distribution of fast transient voltages in machine windings.

It is important to locate the surge capacitors at the load and not at the breaker. If the capacitor is located at the load, the high frequency current resulting from re-ignition is limited in amplitude by the surge impedance of the cable located between the capacitor and the breaker. This limitation of the amplitude of high frequency current reduces the probability of virtual current chopping. In addition, location of capacitors at the load minimizes series inductance effects. If, on the other hand, the capacitor is located at the breaker, re-ignition can cause the capacitor to discharge bulk through the breaker. In that case, the high frequency current could have a higher frequency and amplitude than if the capacitor were located on the load side of the cable at the load.

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[jraef]

I think it's important to separate the issues of surge capacitor location from having capacitors on the load side of a solid state soft starter. You really have no choice, if you leave them in the motor, you will damage the soft starter SCRs, the capacitors or both. It's a race to see which one goes first.

So once you accept that, then you have to look at the PURPOSE of the surge caps. Reducing inductive kickback to prevent chopping currents, especially in vacuum contactor starters, is their main function. So it really becomes a moot point as far as SURGE caps is concerned because you no longer have a straight vacuum starter. Even though the RVSS starter may have a vacuum bypass contactor and a line isolation contactor, all of them that I am aware of will still turn the motor off with the SCRs, not the contactors. So there really is no purpose for the surge caps any longer.

When the RVSS mfr says to move the caps to the line side, they are referring to Power Factor Correction capacitors, not surge caps. PFC caps have a different purpose and may be very necessary, warranting their retention in the circuit. I always recommend that PFC caps in an MV soft starter circuit be controlled by another VC so that they are only on-line AFTER the soft starter is done ramping and the Bypass Contactor is closed.

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[Skogsgurra]

I second Jeff. When the contactors do not break inductive load, you don't need kick-back protection.

Having a soft-starter in the circuit changes a lot of practices.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

I assume the line side vacuum breaker has some protective trips, so it seems possible that breaker may trip at some point during its lifetime while on bypass. Since it will not be a regular occurrence, it makes sense that surge protection would be less important.

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[kssschsekhar]

Thanks Jeff for hitting on Nail's Head...As surge capacitance value is much less than PFC, is it recommended to keep the Surge capacitor at motor terminal box, where motor is being started by RVSS with bypass contactors....Installing surge capacitor at motor terminal is analogus to using a long cable between RVSS and motor...!!!

 

[RRaghunath]

A long cable between RVSS and motor poses problem with regard to the RVSS (dv/dt control more specifically). Hence, better to check with the RVSS vendor if you like to keep the surge capacitor at the motor terminals!

 

[kssschsekhar]

I have just now read one article that "if motor starting current is more than 200A for 11kV motor, surge capacitor is not required to consider"...I am thinking that how come this is link to current chopping surges???

Source, section-V C of the link:

http://www2.schneider-electric.com/...e_protection_PCIC08_formate_v3_IEEE_CTbis.pdf