Load Hunting Problem in 25 MW STG

The signal to hunt comes from its governor, but what is influencing the governor has to be determined first.

On old units with hydraulic governors it can be air in the control oil. Look for a blocked bleed orifice in the piping at the highest point. On mechanical governor look for loose linkages.

On electronic governors look for conflicting inputs. it could be a number of things.

You should describe your plant and what it's feeding into. Island mode or grid connected with other machines? Does it feed passout steam to other systems? All influence the governor.

Do NOT touch the system during operation. Fiddling with a governor can destroy the machine instantly. Observe proper procedures.

I'm surprised your system handled swings that large without tripping.... but that's another topic. Ask yourself "why" until the root cause is found. It is a great troubleshooting technique.

for example... the STG output swings +/- 12MW when running at a 22MW setpoint. WHY? The governor was opening the inlet valve and the limiter was closing it. WHY? The set point was too high above the parameter, it over shot into a limiting region, closing the valve.... So the solution would be to reduce the ramp rate on the turbine.

I'm also interested to know how a turbine of 25MW delivers 34Mw on the up-swing. What is the overload rating of the m/c? What happens to the steam plant feeding it?

Or is it just false power meter indication.

I was thinking that myself. The units I've worked with could run above design output by 10%, nowhere near 150%. And as you say... the steam plant ... eeek... must have swung drum levels something fierce.

I think that the questioner is not sure of the facts. This forum cannot solve 'perpetual motion' type of problems.

penttijp

There are tons of reasons for a piece of equipment to hunt. I don't think it has anything to do with perpetual motion.

I agree wholeheartedly with both of the 2 other posters. A 36% overload (25 to 34KW) should have tripped the unit probably by the time it hit 28kw let alone 30. The boiler water level controls would have been screaming for more water in order to maintain a 34kw rate and the safety's would probably popped when it dropped to 12kw(I assume that these were a fast drop and a quick rise).

I would question whether most boilers could handle a fast 36% increase in demand assuming that it is rated properly unless of course it normally serves 2 TG sets and only one was up and running.

Check your meters and observe your RPMs and hertz output since most governors couldn't handle those swings without some speed variations.

Please excuse the typing errors. Should be MW not kw.

If this is a mechanical governor you need to start looking for looseness or tightness in the system. Either one will cause the system to wind up and overshoot. This can occur in any joint or lever.

Regardless of what type of governor you have, the fact that the machine governs well until a load set of 22 MW indicates that the governor is capable of governing, If you were troubleshooting the system one of the first things you would do is when the unit started hunting you would bring in the load limit until the unit stabilized that would tell you that the unit could govern.

The fact that you use the term load set point indicates to me that you probably have an electrohydraulic governor, In this case I would have a good look at the valves and servos to see if there is said looseness or tightness.

Have you plotted the governor movement against steam supply/pressure?

It rather sounds like when you up the output, you exceed the supply.

E.g. the governor gos full open as the supply exhausts, the out put crashes, supply recovers and the governor is slow to respond, so it goes over - exceeds and re-crashes.

Whatever happened to Seshu0971? No answer? Gobbled by the gov'nor?? Big bad brute. Gambling with that type is bad. If you win you wear a grin, but if you lose its costly.

At one plant I've worked with, while the STG governor had the capability to run in MW control it wasn't used. When I inquired as to why, the response in short was that it was only quasi-stable. Small fluctuations in the plant would cause small swings that would continue to resonate until the drum level swings threatened to trip the plant. So they opted to operate in "steam pressure control" (note this was constant) and let the generator put out whatever power that equated to for a the given steam flow. On a more detailed analysis, it made perfect sense.

I don't know your situation exactly, but there may not be an equipment problem!

Hi ChaoticIntellect,

Such cases as you mention are not rare. In my experience few vendors really bother to understand the customer requirements. they tend to supply standard fare but not quite fit for requirement. ST gov would be ok but overall system tends to instability. One new plant could not even make the thirty day trial period without a trip.

In the above case--who knows--.Maybe the request for info was only a teaser.