Hot Restart 4

[marada93]

In the event of a boiler trip (MFT), under certain conditions, the boiler fan train will trip as well via interlock (extreme negative furnace draft, for instance).

My question is this...

Is there a requirement (code?) that prevents attempting restart on a fan (900 HP) unit it coasts down to zero speed?

My way of thinking if the fan is still rotating in the correct direction, it should be OK to restart even though it's still spinning. Seems to me if you go for a restart in this case, the inrush would be reduced as it would come to speed much quicker than if it was at rest. Therefor there should be no harm to the motor OR fan. This would reduce the hot restart time because boiler temps would not have fallen off much.

Thanks in advance!

 

[davidbeach]

If you try to catch a spinning motor with anything other than a VFD you are risking trouble. When the motor goes off line, the magnetic field takes a while to fully collapse. Until it does collapse, there is a voltage on the motor terminals and closing back in out of sync with that voltage can cause trouble. The voltage takes much longer to collapse than the time the motor can supply fault current; with no excitation it won't produce current to speak of, but there will still be voltage.

Most modern VFDs have the capability of catching a spinning motor, including spinning in the reverse direction, and taking the motor to the correct speed (and direction) without the nasty stuff associated with trying to do so across the lines.

 

[RRaghunath]

I agree with davidbeach and would like to add that inspite of the risk of application of momentary over voltage at motor terminals, the power plant equipment are designed (can withstand voltage of 1.5pu) to be restarted / reaccelerated that way (Power plant auxiliary power supplies changeover scheme).

raghunath_n00@rediffmail.com

 

[unclesyd]

How do the 1000 HP motors that I've familiar with restart with a "ride through" in power supply circuits. I've been in control room when there has been a complete drop out of the supply voltage, O on the historian and the RPM starts falling, and the "ride through" picks up as the supply voltage is restored the motor never stopped..

As I've stated before this system worked too good when someone moved the "ride through" in circuit to where the "E Stop" didn't work as the motor restarted at around 400 RPM when I took my finger off of the E stop. Wrecked a SS rotary Screw Compressor.

 

[buzzp]

You can bet they are watching the voltage, frequency, and zero crossing (or peak for that matter) to determine when the spinning motor can be connected to the supply again.

 

[electricpete]

There are a number of ways to safely allow re-energization of a motor.

One is to wait a safe time after deenergization. That time is given by 1.5 times the motor open-circuit time constant. The motor open-circuit time constant is

Toc = (Lmag + L2)/R2 = (Xmag + X2) / (2*pi*F *R2)
where
Toc = open circuit time constant.
R2 = rotor resistance
Lmag = magnetizing inductance
L2 = rotor leakage reactance
Xmag = magnetizing reactance at power frequency
X2= rotor leakage reactance at power frequency
F = line frequency

This time constant gives the decay of the flux. Residual voltage is proportional to flux times speed. Even if we don't take credit for decay of the speed (usually more difficult to predict) we can calculate conservative time looking only at decay of the flux as above. With 1.5 times the open circuit time constant the flux should be below 33% of it's original value which will ensure even if there is worst-case reclosing out-of-phase between residual and incoming power, the total volts/hz will not exceed 133% which limits the destructive torque to manageable levels. This approach is in accordance with NEMA MG-1.

For most motors below 500hp I am fairly certain Toc will be 1 second or less.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[davefitz]

I am not sure this is relevant , but there was a fatal accident at a British combined cycle plant in 2000 which took 2 lives from a similar attempt to re-energize prior to spin down. IN that case , it was a steam turbine generator that was still spinning, and the 2 engineers had entered the breaker room and had re-energized the windings. The breaker exploded .

 

[davefitz]

Correction:
the Accident occurred in Aug 2001 at Teeside, and 3 workers were killed when they were shortcircuiting a 16 kV X former prior to spin down of the steam turbine.

 

[ScottyUK]

davefitz,

Corrections to the correction:

8th August 2001.

Transformer in question was a 16/11kV 48MVA unit auxiliary transformer on a steam turbine generator. The transformer was being put back in service as part of plant reconfiguration. The transformer LV voltage did not match the live 11kV network in the plant and the decision was made to change tap.

The transformer was equipped with an off-circuit tapchanger intended for operation with the transformer isolated. Through a gross procedural breakdown, the tapchanger was operated with the transformer energised. As the tapchanger operated it formed a short circuit between two adjacent tapping windings, causing a colossal fault current to circulate in those windings and leading to disintegration of the tapchanger and evolution of a significant volume of gas. A rising bubble of ionised gas is thought to have caused an interphase flashover in the immediate vicinity of the tapchanger, directly across the terminals of a 365MVA generator under full load. This second fault, involving much more energy than the first, caused a serious over-pressure of the transformer tank which was far beyond the capability of the PRD to relieve. The tank ruptured the bolted lid flange along two sides, ejecting jets of burning oil and gas and enveloping the four workers.

I had the sad task of leading the team of electrical engineers who repaired the damage and replaced the transformer. The images and smell will stay with me forever.

The above notes are my understanding of the results of the official enquiry by the HSE - any errors are mine.



----------------------------------

If we learn from our mistakes,
I'm getting a great education!