Synchronous Motor PF Trip Setting

[jbt260]

I have multiple 1750HP, 4160V synch pump motors. I am protecting them with a Multilin 469 relays. What is a typical setting for power factor trip? I currently have them set to trip at 0.95 lag after a 1 second delay. I am starting to have nuissance PF trips on the running pump when I try to start a second pump. The running pump trips on a PF. The second pump keeps running! The voltage on the running pump is around 4000V when it trips on PF. Can I lower the PF and/or increase the delay without damaging the motor?

Thanks

 

[Jraef]

I have multiple 1750HP, 4160V synch pump motors. I am protecting them with a Multilin 469 relays. What is a typical setting for power factor trip? I currently have them set to trip at 0.95 lag after a 1 second delay. I am starting to have nuissance PF trips on the running pump when I try to start a second pump. The running pump trips on a PF. The second pump keeps running! The voltage on the running pump is around 4000V when it trips on PF. Can I lower the PF and/or increase the delay without damaging the motor?

Thanks

The 469's PF trip function is not really meant for protecting a Synchronous motor if I remember correctly, because a Synchronous motor is intended to CORRECT the PF of other motors. There is a lot more to protecting a Synchronous motor than what a 469 is intended for. That is done with a different relay, an SPM.

 

[SG-1]

You have an interesting problem. When the second motor starts it causes a drop in the powerfactor which the running motor's relay interpets as a loss of excitation.

GE used to make a SlipGuard relay, it came with a default setting of .8 @ 1 sec.

The danger here is the running motor may slip a pole which can cause great mechanical damage. You need to find that point for your particular motors.

I take it that you cannot start them together.

 

[rcwilson]

It's possible that the second motor's starting current drops the bus voltage and the running unit makes up for it by delivering VARS to the starting motor. The flow of Vars out of the motor without a change in kW will change the power factor.

The pf number will go lower if the motor is operating over excited (leading power factor) or close to unity power factor. Var flow out of the machine is leading power factor operation.

The 469 really can be applied to a synchronous motor according to the IOM. But it sounds like your time delay is set too fast or your power factor for leading operation is set too high.

It is also possible that a CT is wired backwards making lead and lag opposite. Increase the excitation while watching the meters. If the Vars increase in the leading direction, polarity is correct. (Leading Vars increase = lagging Vars decrease).

IMHO, operating the motor under excited is more dangerous than over excited. But I would have to see your motors characteristic curves to make a good decision.

 

[jbt260]

Follow Up to OP

Follow Up to OP

We disabled the PF setting and set up a trend line on the running motor. While starting the second motor, we saw a disturbance in the PF of the running motor for approx 4 seconds then the running motor settled back to unity PF. We increase the time delay to 5 seconds and problem solved. Thank you for all your input.