Medium Voltage Contactor Dropout Time

[EP007]

Medium voltage (4.16 kV system) vacuum contactor dropout time is important for fuse-contactor coordination. Some vendors have values between 15-25 ms, and others, several cycles, which I have been more used to seeing in the past. Compare that to a 3 or 5 cycle vacuum breaker, and 15-25 ms is extremely fast. Doe anyone have some insights on this?

 

[dpc]

You are confusing a circuit breaker fault clearing time with contact parting time. The 3 to 5 cycle breaker clearing time includes arc time and includes possible restrikes. Contact parting time is much shorter.

A 1-cycle contactor opening time is typically what I've seen and used for approximation. This is the amount it takes the contactor to open its contacts, not the for current to go to zero. Also, a contactor is not going to be interrupting fault current - at worst it must handle motor locked rotor or inrush current. Medium-voltage contactors may have delayed opening to allow the fuse time to clear faults.

 

[EP007]

What you say makes sense. However I am finding a very wide range of stated opening times, with closer to 100 ms (6 cycles) very common in product specs.

http://www.joslynclark.cn/up_files/myupload/mvc series(databinuser-2011-12-7-18-39-22).pdf

http://download.schneider-electric.com/files?p_File_Id=303390307&p_File_Name=NRJED111143EN(web).pdf

 

[dpc]

Those times seem longer than I remember for the old air contactors. I'd contact the manufacturer. Those times may include arc time, or it could be that I was mistaken. Vacuum contactors don't have to move very far when opening, so 90 ms seems like a heck of a long time for contact parting.

 

[podobing]

EP007,
While the contactor drop out time is important, so is the overload setting, which depends on motor full load amperes. You can select a fuse that works well with the contactor, but crosses up with the overload. Many of these fuses do not like to trip on overload (low level) currents, and will fail catastrophically. All three items must be considered, so as DPC suggests, contacting the contactor manufacturer would be a good place to nail down one of the variables. Motor HP will determine the overload setting, and then you must "simply" select a fuse that fits between the two.
Dave

 

[EP007]

podobing, for the overload region, on the 4.16 kV, we use a motor protection relay, with the contactor, but only in the overload region. Above that level, in the short circuit region, the fuse should open, not the contactor, and that is achieved by proper coordination. However, if during the fault, the contactor drops out before the fuse clears the faulty, then it can open higher current than it's capability. Another coordination issue.

 

[dpc]

It's not simply a coordination issue, the contactor has an intentional drop out time delay to allow the fuse to clear.

 

[LionelHutz]

The Joslyn Clark fuse co-ordination is simple. They have the tested series rating listed right on the spec sheet for any R rate fuse up to 38R except the 36R fuse.

Most contactors use a DC holding coil so it will take a little time before they begin to open after the power is removed. The dc current in the coil typically has to decay by passing through a freewheeling diode.

 

[EP007]

Lionel, for example, 90 ms opening and 4.62 kA, 5 kV. I see 38R listed, but when I plot that point with a Ferraz A480R 18R, the point is not to the right of the fuse total clearing. Doing it that way, an 18R wouldn't work. Either the opening time or interrupting current capability should be higher or the fuse lower such as 12R.

 

[LionelHutz]

But it does work...

 

[EP007]

At what short circuit and X/R levels is the test? Is the fuse in current limiting region?

 

[LionelHutz]

The E2 fuse co-ordination lists 400 MVA, 45.7KA RMS, 38R fuse max for a 5kV voltage. If you don't believe their data then call an application engineer and ask about it or simply don't use it.

 

[EP007]

No one talked about believing or not. I'm sure it does work, and relies on the fuse. I prefer the approach shown in the eaton Consulting Application Guide, part 10, motor starting, fuse contactor coordination. If the point is to the right we have ensured coordination.