Undervoltage protection on motors 350 kW, 400 kW 1

[SA07]

Hi
We have some motors 350 kW, 400 kW 690 Vac. We are a power plant exporting power on utility grid. Sometimes during disturbance on utility grid, our motors trip by under voltage.
These are critical motors so the whole plant trip resulting in penalties from the utility company. Motors are protected by Siemens Simocode. We already contacted Siemens. They recommended to reduce undervoltage setting. Problem is still occurring. The motors are boiler fans. They are started by soft starter with bypass contactor.

There is overload protection on the motors.
Undervoltage setting is 176 V 4s.

Can we disable the undervoltage protection? Is there any risk on doing this?

 

[RRaghunath]

176V corresponds to ~25% of 690V. This is low even for the bypass contactor to remain in picked up condition. Is the contactor powered by DC or AC UPS supply??
Voltage dip causes fan to slow down and when the voltage recovers, fan draws high starting current to run up to full speed. Since your system is not designed for DOL start of the fan, tripping is inevitable.
It seems there is issue in design here. I feel a critical drive should have been designed for DOL start or the drive should have been a VFD (not soft starter). VFD can be programmed to have voltage dip ride-through feature for short duration dips and flying start feature for longer duration dips.

 

[davidbeach]

Yes, you can disable the protection. And, yes, there is a risk in doing so. Figure out your ride-through requirements and make sure the motors stay on line for that. Depending, that may mean replacing a bunch of AC control wiring with DC. Not difficult, but not trivial.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[SA07]

All control voltage is from an AC UPS.

Plz see printscreen of fault recorder. 10-13 s after fluctuations in voltage 647.98 V (instead of 690 V), the plant tripped.

 

[SA07]

Our ID fans are supplied by a VSD. These also are sometimes affected by network disturbance. We had to program 3 automatic restart after faults in the VSD and put some delay on boiler PLC to prevent the ID fans from tripping.

There are other panels (400 Vac) where there were voltage protection relays. We had to bypass them since sometimes the relay tripped and causing the whole plant to trip as well.

 

[itsmoked]

Are you sure the trip is occurring because of sensing? Not something unexpected happening like a contactor dropping out?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

... Motors are protected by Siemens Simocode. We already contacted Siemens. They recommended to reduce undervoltage setting. Problem is still occurring. The motors are boiler fans. They are started by soft starter with bypass contactor.

Simocode is a solid state OL relay. Is it a Siemens Soft Starter too? If so, the Soft Starter will have it's OWN protection schemes separate from the Simocode. If the Simocode is on the Bypass Starter, and the Bypass Starter is controlled by the Soft Starter, then it might be the Soft Starter that is dropping out the Bypass Starter.

176V is a VERY extreme voltage drop for a 690V motor. Peak torque of the motor would drop to just 6% of normal, so your motor would go into a stall condition. I don't see a way to make this work under those conditions unless you use a VFD, and even then it would need to be one that can stay alive under those extreme conditions. That would be difficult to find as well. An Active Front End drive could possibly do it because some of them will pull rotational energy off of the motor to support the DC bus level and keep the drive operating under fairly extreme voltage dips. I've never seen one that will go THAT low, but it's possible someone could modify something. One that I just looked up will survive down to 185V, so even that once would not hold in.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

"Modify" yah!

Like using a battery bank along with the usual caps on the DC link.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[RRaghunath]

You said the voltage dipped to 647.98V which is hardly 6%. If so, why did the Undervoltage protection (that is set at 176V) operate??
It seems you are missing something here.

 

[FacEngrPE]

Your fault recorder appears to be showing RMS voltage. A waveform capture of an event would be more revealing to the experts here. This fault recorder does not indicate the instantaneous lowest transient voltage which might be what is triggering your undervoltage voltage protection.

If your house bus really drops to 176V (.25 pu), the house bus is probably feeding energy to the offsite fault. If that is the case, either something is needed to keep the bus voltage higher, or all of the loads need to tolerate riding through loss of power.

If you actually have loss of power I would expect the boiler to trip for additional reasons. The fans may just be tripping first.

 

[SA07]

Hi

Please see attached wiring diagram of soft starter and simocode.
Whenever there is voltage fluctuation, can the soft starter detects an undervoltage and sends a signal to the simocode which then becomes faulty?
Or can a small fluctuation in voltage cause the simocode to trip by undervoltage?
As mentioned previously, on other panels we had voltage protection relays which tripped during voltage fluctuations. We had to bypass them.

How can we track the first cause of tripping of the boiler?