Reverse Power and Turbo Generators

The normal way of removing a generator set from the system is by reverse power, this means that the Governor control is pulled back until the set cannot produce enough power to supply to the system, because it is under powered the system then tries to drive the generator as a motor, as soon as this occurs it soon after opens the circuit breaker. The reason for doing this is, if you opened the circuit breaker while suppling the system, the Governor setting could possible cause the set to over speed, and cause possible damage, so reverse power is the safe way of removing a set from the system, because the Governor setting is below that of the system.

Regards JD.

jd,

I have agreed with nearly every post I've seen from you, and many have given me new insight, but I have to disagree with you on this one. In 30+ years as an operator and engineer in marine, industrial and utility generation, I've never used a protective relay to perform a normal shutdown of a unit unless I'm testing that relay. The negative consequences if the reverse power relay fails to operate, or the circuit breaker fails to open, can be severe.

We recently experienced a mechanism failure in a 69KV generator main circuit breaker which prevented the breaker from tripping. The turbine was still receiving a minimal amount of steam, at near zero load, so we simply left the unit on-line while investigating the breaker problem. Unfortunately, the boiler operator had a serious case of brain-cramp and tripped the turbine stop valve. The caused the unit to motorize at up to 75% of full load current. As the turbine blades began to "push" the steam, the axial thrust on the turbine rotor reversed direction. As the thrust bearing is unidirectional, that bearing was severely damaged. Additionally, the field breaker is interlocked with the stop valve, so when the stop valve tripped, the generator became a very large, extremely inefficient induction motor. This caused localized arcing and flash-over at the rotor end turns, and overheating in the rotor slots. Repairing the turbine and generator cost over US$1.5M, with significantly more in lost revenue because the unit was unavailable for several months.

In every automated system I've worked with, the governor control system is set to ramp the unit down at a controlled rate. At the same time, the AVR setpoint changes to maintain unity power factor. When the load drops below 0.5% of nameplate (1 MW max), the generator main breaker is opened. The unit will not overspeed because the ramp-down sets the governor to just above no-load sync. speed. The governor program is set to maintain the generator at 0.25% load if the breaker fails to open.

For manually operated systems a similar process is performed by the operator. This is also the method provided by General Electric in the operation/maintenance manuals for their turbine generators. Westinghouse and De Laval steam turbine manuals have similar instructions, as do those for Neyrpic hydroelectric units.

jdretired hit it on the head as to the proper way to remove a machine from the line. Some stations don't adhere to this good advice. Most sequential tripping systems are set up with the breaker taking out the main stop valve. If you don't use this scheme and experience problems with valves, reducing load by whatever means possible and having the reverse current relay take out the stop valve might not give you a warm fuzzy feeling but it certainly increases the odds of one line of defence functioning.

Your reply tweaked my interest. We have had experience with a runaway turbine (disintegrated) and I fully understand the methodology you describe. I am curious as to why General Electric would then set up controls for both of our machines to only control to +1 MW, no lower. Following your advice would require placing machine in fully manual control to remove the last 1 MW. Also, upon opening the breaker, both machines leave the stop valve open and rely on the 3 different stages of overspeed protection. This is at GE's recommendation. My understanding is that this was the industry norm for non-utility generators.

If I was operating a machine that was in warranty I would tend to follow the manufacturers recommendations to the letter.

If I am correct in assuming that you are calling triple redundancy, 3 different stages of overspeed protection, and assume that you have either Mark 5 or 6 controls probably without a mechanical overspeed device.

I am surprised that GE changed their thinking on sequential tripping. In the past the stop valve tripped the breaker and the breaker tripped the stop valve. In most cases the boiler also takes out the stop valve, if any of the devices didn't work there was a huge potential for damage.

From a manufacturers point of view it is probably entirely adequate to bring the unit to 1 MW and open the breaker. Realizing that if the breaker opens at full load and the control valves are functioning properly the governor will go to 105% speed by design, it is a fairly good bet that 1 MW of load probably calculates out to something less than unit overspeed.

You mentioned that you had a runaway turbine that disintegrated, there are so many things that can cause this. The leading cause of destructive overspeed almost has to be stuck or inoperable valves in combination with high machine loading, In the case of industrial units non-return valves are a source of problems. I can't comment on your failure.

What would you do if you reduced load to 1 MW, flipped the switch and the breaker didn't open? I would hope that you wouldn't be thinking in terms of making a little money with MW's while you looked for the problem. It is like all things in operation that aren't working properly, at this point you probably are now below the minimum flow that you need in the turbine, and you have decisions to make.

Back to the original theme of this discussion, reverse current relaying is a form of generator protection. Would the OEM find you too conservative if you tested this relay each time you removed the machine from the line--who knows? Certainly not if you go on with the intent of coming off when something fails or the next scheduled overhaul.

These are Mark 5 machines, but what I meant by the 3 stages of overspeed protection are quite different from triple redundancy.

• Control valves will close completely on 5% increase above synchronous speed when the generator breaker opens
• Electronic trip device to open on 9% increase above synchronous speed to trip main stop valve
• Mechanical trip device to open on 10% increase above synchronous.

Granted, this last option uses the same final control device (stop valve), but this is fairly robust emergency overspeed protection, therefore, reliance on tripping the stop valve with the breaker is not neccessary (but very much so in the opposite scenario).

This is an industrial process where the boiler is not tied to the turbine, there is a header system feeding turbines and processes, therefore, there is no tie between boiler and turbine trips.

The runaway turbines was entirely our own fault, attempting a controlled shutdown of the refinery following a large process leak that contaminated boiler water with saturated brine. Stop valve and control valves siezed in place. When load was removed (open breaker), steam did not cease. kaboom.

When I first started this I thought this is off topic, then after going through the tale as I remember it, it is pertinent.

There was a paper mill in the early 70's, whose name shall remain anonymous, that had problems with either the black liquor or green liquor, I'm not a paper maker or a boiler type, but at any rate they wound up in the same predicament that you were in. In their case there were multiple units on line, when they realized that they couldn't reduce load they tried hydraulic tripping of the units. This was not successful. They confirmed that the units had tripped hydraulically.

The station had an electrical tie with the system so they had speed control as long as they were tied in. The Chief Engineer let it be known that the only way he wanted any breakers open was by the reverse current relays until he figured out how to get out of this fix.

He had the boys break out some porta powers and jacked the valves shut, he confirmed that he was safe by operation of the reverse current relays. He got all of his units off without any excursions into overspeed.

I am pretty sure that wasn't his concept of why he had the reverse current relay but it seems to have worked for him.