Multilin P4 relay with Soft starters

[tin2779]

Hi...
I am looking for a explanation in regards to this obsolte product P4 A relay from Multilin.
A 225 a breaker is feeding two 100 HP fans with independent soft starters. The application is ventilation fans.

The pitch of the fans has been changed, so they are not taking full 90 A as they should have been on 600 V.

The steady state current for both fans is 65 and 72 A respectively. Both fans have 30 KVAR cap banks which come online when the motor comes to speed and the soft starting circuit is out of the picture.

The fans are started after certain interval of each other.
After Fan#1 goes up to speed and the capacitor comes online, the main breaker trips.
I tried to find explanation with Multilin but not much help.

It looks like this relay has a single phase detection element that can be disable.This element works on based on non sinusoidal voltage and current unbalance.

Any suggestions, why this is happening. When they disconnect the capacitors everything works fine.

Thanks

 

[itsmoked]

Cap shorted?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[tin2779]

We have not checked that....But how relays being shorted can activate this relay to trip to current unbalance.

Thanks

 

[tin2779]

I forgot to add one more aspect of Multilin P4 A relay

"" Systems such as variable speed controllers may produce nonsinusoidal waveforms that may cause nuisance tripping of the single phase detection. If this is a problem, the single phase protection may be defeated by setting switch S305 off.""

Do you think, Soft starter might be creating a problem like this ?

 

[jraef]

Capacitor charging current? Caps may not even be shorted. If it was working before and now it isn't, the change in the fan pitch may mean that the lower load on the motor, which means lower power factor when you switch on the capacitors, which means a higher capacitor current.

You could use a capacitor switching contactor designed with an NTC resistor in series with the contacts to reduce the peak currents.
Click here to show an example of capacitor switching contactors

 

[LionelHutz]

I'm surprized you'd say the motor load affects the cap current. Voltage fluctuations will change the cap current. The motor running or motor load has nothing to do with capacitor current. You'd get the same cap current if the motor was off.

If the first soft-starter is done ramping and the second has not yet started it is not likely the starter causing the trip. Also, since you said the capacitor energizes and then the relay trips that would point to a bad capacitor. Look for an open fuse on the capacitor causing the current imbalance.

 

[tin2779]

Thanks all for your comments.

I got a feedback from the site and they have resolved the issue.

Just to recall the situation more correctly, Soft starter is feeding a 100 HP vent fan and after a little while when the capacitor will come on line, Multilin P4A relay will trip the breaker feeding the fans.

There is a single phasing protection on the relay that will trip the breaker in two situations Either if the current waveform is non sinusoidal or One of the phase is lost.

I don't see either of the thing happening. The dip switch in the relay was switched in the opposite direction and single phasing protection was turned of and later on when the fans were started again, and when capacitors came online, It never tripped.

The technician explaination was that because cap. bank is sized for full load of 90 A and the motor is pulling 73 A, So there is over compensation and which is distorting the voltage and was activating the single phasing protection on relay.

How can over volatage because of overcompanesation can distort the voltage. because the fan is up to speed and the softstarter circuit is out and a contactor is in picture, so no distorted waveform from soft starter.

Any comments, what is happening. I would appreciate a word.
Thanks

 

[LionelHutz]

Doesn't sound right but could be happening.

The capacitor bank should be about 25kVAR for a 100hp motor. I would not expect it to be rated for 90A.

At any rate, a motor will draw a fairly constant KVAR from the line when it is operating. A single capacitor can typically be used to compensate the motor at all loads. The amount of power factor correction will fluctuate as the load changes but not very much at all.

We were once asked to provide a switching capacitor bank to compensate a large motor as the load changed. It was just a waste of money but we still sold it because they just had to have it.

 

[tin2779]

LionelHutz
Thanks for your comments. Actually, as an engineer, I am just eager to find an answer. The issue is long resolved but I still want to find an answer. So, what was the Multilin P4 A doing when the cap. banks comes online and get the motor over compensated...which stopped happening when single phase protection was disabled ????????/

Strange some times though, these small things have some things which are not apparent.

 

[LionelHutz]

I don't know. I wasn't on site investigating this.

However, adding extra capacitors so you go to a leading power factor does not automatically cause a distorted voltage waveform. You would have to be saturating something magnetic, say the power transformer. Then, the saturated transformer would have a distorted voltage.

I misread your last post. You posted that the capacitor is sized for the full-load of the motor (90A) but you are running under full load (73A). You didn't post that the capacitors themselves were 90A. In that case, it is not even likely you are over-compensating the motor. Typically, a properly sized fixed capacitor will correct the power factor so it changes <5% as the motor operates from say 25% load to 100% load.