VSD motor with Lightnin Arresters etc

[magneticted]

Client has asked for a particular medium voltage (3.3 KV )VSD motor and has called for lightnin arresters and surge capacitors to be housed in the motor terminal box. Is this possible.

 

[electricpete]

Anything is possible if you communicate the requirements to the motor supplier. For those who have any protection in the terminal box, it is typically only surge caps. Surge arresters or lightning arrestors are typically located further up in the power supply system.

Now a question for others on the forum - if you have high-speed differential or high-speed ground protection for the motor, would it trip if the arrestor conducts?

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

 

[dpc]

In a previous life when I was doing design of large power plants, we routinely put surge arrestors (and caps) on all medium-voltage motors located outdoors, such as circ water pumps.

We also put differential relays on the large motors. I don't recall any problems with tripping of the 87 for surge arrestor conduction, but it's an interesting question. These machine protection differential relays can be exceedingly fast, so I wouldn't be surprised if one might operate if a surge arrestor turned on.

 

[magneticted]

Hi DPC, did you put surge arrestors and caps on VSD motors?

 

[davidbeach]

If you have a surge arrestor in the zone of protection, either differential or ground, the protection would see the operation of the arrestor.

I don't think there is a way of having meaningful "high speed differential" with surge arrestors in the zone without the protection tripping on arrestor operation. High speed means you can't delay the protection to wait out the surge arrestor, and meaningful differential protection means you can't open up the settings enough to not trip on arrestor operation.

Ground protection, on the other hand, is a bit different. High speed, when ground is compared to differential, is probably slower. Ground protection has to allow for "false residual" transients on motor starting, that are much less likely to impact a differential relay (and most modern differential relays are designed to detect and block operation on transients) than they are to impact a ground relay. To allow for these starting transients, the ground relay is not going to be set for instantaneous operation on low residual currents.

If you don't want the protection to trip the motor on surge arrestor operation (that's a separate issue), the best bet would be to have the surge arrestors outside the differential zone of protection and inside the ground zone with sufficient time on the ground elements to allow motor starting transients and surge arrestor operation to happen without tripping.

 

[jraef]

Don't do it. Read the VFD manual, it should have very clear instructions on this issue.

In VFD fed motors, surge arrestors w/caps should be located UPSTREAM from the VFD (RVSS Soft Starters as well). The charging current flow of capacitors downstream from solid state swiching devices looks like a short circuit to the device, risking di/dt failure of the switching device. In addition, since there are high harmonics on the output of a VFD and the capacitor will be the lowest impedance device in the circuit, the cap will draw more current than it is supposed to, causing eventual failure of the cap (assuming a transistor or SCR didn't pop first).

The same holds true for Power Factor Correction capacitors by the way, but that would apply only to RVSS Soft Starters since VFDs don't need them.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more." Nikola Tesla
Read the Eng-Tips Site Policies at faq731-376
Member, [blue]P³[/blue]

 

[magneticted]

jraef,Okay if we inform the client he must put this equipment upstream where exactly would you advise to house the surge protection equipment, would be in the switchgear, or stand alone at some other point

 

[electricpete]

Sorry - I completely missed the "vfd" in the subject and post.

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

 

[jraef]

I'm no expert on lightning protection actually, I just know what you can't do with drives and soft starters. In most projects I have been involved in where it was used, we used Station Service Class lightning protection on the main switchgear, and we tend to use GE Tranquel series. That is not meant to be a design endorsement, just who we tend to work best with. We have also used ABB and Siemens on the incoming switchgear depending on who has the best package on a project. My personal preference is to protect as high upstream as possible, because you protect more equipment. I doubt that anything is really going to protect against a direct hit right at the drive anyway.

If you do a Keyword Search in this froum and in the Electric Power Engineering forum you will find some good discussions on this in the past.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more." Nikola Tesla
Read the Eng-Tips Site Policies at faq731-376
Member, [blue]P³[/blue]

 

[dpc]

I agree with jraef - you don't want arresters downstream of the VFD - I missed the VFD reference initially. I was talking about constant speed motors running across the line.