problems with contactors

[Pafi]

I wonder if anyone has an idea about how to protect/ improve maintenance performance for three phase contactors used with shunt reactors. My circuit voltage is 3x660V and I'm using multiple steps with shunt reactors, switched ON and OFF by air type contactors in order to regulate the power factor for a wind turbine, km away from the grid (without this system, when the turbine does not work or the wind is low the PF is capacitive, long MV cables). It works but the electrical arc when the contactors switch is very high and I'm afraid that, in short time, the contactors will wear out quickly. Can I improve, to put some capacitors/ surge protection, etc.?

Thanks in advance!
Pafi

 

[waross]

What are the ratings of your contactors relative to the rated current of the reactors? The current rating of the contactors may be based on a mostly resistive load. You may want to consider using contactor ratings based on HP.
A motor at locked rotor conditions draws a very reactive current. A motor starting contactor must be capable of interrupting locked rotor current safely.
A contactor interrupting full load current to a normally running motor will be interrupting about 1/6 of the design capability of the contactor.
Even with properly sized contactors I would expect a bigger interrupting arc than when interrupting resistive currents.
Adding capacitors to reactors may be counter productive.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Check out the severe service contactors of the bar & shaft design. They're intended for plugging and inching duty, and similar applications. There will be an arc, but they're designed to handle it with extended arc chutes.

You might also consider using vacuum contactors provided the reactor current is sufficiently large to avoid current chopping. Siemens are one manufacturer of LV vacuum contactors; they are expensive.

 

[KuanYau]

Pafi (Electrical)
(OP)

1. In the IEC world, LV Contactors up to 1000Vac are covered under IEC 60947-X-X.

2.IEC 60947-4-1 stipulates the Utilization categories.
For alternating current:
AC-1 Non-inductive or slightly inductive loads, resistance furnaces loads,
AC-2 ..... up AC-8b Hermetic refrigeration compressor motor control.....,
AC-12...-15 for control relays.

3. Select a contactor with the correct Utilization category to suit the application and the number of operation required ensured by the manufacture, stated in their catalogue.

4. Contactor manufacturers provide graphs showing the same contactor can be used for different Utilization categories but with lower number of operations under higher Utilization categories .

 

[Frederick]

Have the contactors given a true cryogenic treatment as described in ASM Handbook Volume 4A. This has been shown to increase their lives.

 

[MatthewDB]

This would be a good application for solid state relays. They turn off at the current zero so there is no voltage spike like a contactor faces.

Omron makes SSRs for 660V, but I don't know if the current capability is high enough.

 

[MatthewDB]

Regarding IEC 60947-4-1:

AC3 is normal inductive motor use.
AC4 is inching and plugging.

Inching is briefly energizing an induction motor to move a short distance. Plugging is reversing an induction motor at full speed, either to bring it to a rapid stop or to reverse rapidly.

Both inching and plugging involve interrupting high current at low power factor.

If you stick with contactors, selecting one with an AC4 rating equal to the inductor operating current will give far longer life.

 

[Pafi]

I made some tests, inspired by the grading capacitors used to diminish - smooth - the TRV curve in HV breakers. I put AC capacitors, 3uF, between the poles of the same phase of the contactor(in parallel with the contact). And the switch-off electric arc has very much diminished. But I don't know if there will be other problems, don't want to risk for example higher overvoltages on reactors. I'm thinking to use a scope to see the voltage on the reactor phase, cannot simulate because I don't have a model for electric arc.

 

[Skogsgurra]

Closing the capacitor may be tough for the contactor. But 3 uF doesn't seem to be a problem. Larger capacitors need a current limiting resistor.

Gunnar Englund

http://www.gke.org

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