The in house machine which is a 34.5 MW Gas turbine tripped on negative reactive power flow while conducting the load throw off test. While testing, the in-house power requirement was minimal as a result of which power was being exported through grid line. What could be the reason?

Kindly respond.
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When did this problem start? Had the in-house machine been working properly and then it started tripping? Was new in-house load added to the facility, perhaps with a different power factor?

Was the reverse reactive power relay recently "calibrated"?

Reactive power is a function of the load when a facility is operating independently of a grid.

Or, it may be the AVR was recently "calibrated" or misadjusted.

Or were power factor correction capacitors recently added to the facility?

 

Dear CSA,

This problem occurred all of a sudden when we were conducting the load throw off test to test the stability of the machine. The machine had been running without any interruption till that time. The reactive power relay and the AVR were calibrated well before. What I infer is that the Automatic power factor of the AVR was switched ON which might have caused the negative power to flow, which would be even greater when the load is thrown off quickly. Please respond.

 

rak11226,

Ah, the infamous "load throw-off" test. Always loved performing those (NOT!!!).

There's just too much about the circumstances of the testing that we just don't know. Did you open a breaker supplying the grid from the plant? Did you open one of the turbine-generator circuit breakers? Which turbine's AVR was in VAr/Power Factor mode--the one that tripped? Was the plant connected to the utility at the time the negative reactive power flow occurred? Is there a Power Management System, or Load-sharing System, or Load-Shedding System sending signals to one or more of the turbine-generators during normal operation?

Really, it's going to be next to impossible to help without understanding a LOT more about the configuration of the plant and how the generators are connected together (or not), and how they are connected to the grid, and how the infamous "load throw-off" test was performed.

About the only time it's necessary to perform a "load throw-off" test (called load rejection testing most places in the world) is for a steam turbine to prove it won't overspeed (hopefully), and for gas turbines it's used when there is a contractual requirement for the generator to be able to re-synchronize very shortly (a few minutes or less) after a non-turbine trip event. So, for a gas turbine that means that when the load is suddenly lost because of a breaker-open event that the unit must not lose flame (and not overspeed) so the unit can quickly be re-synchronized to the grid. And, that's the ONLY reason to perform a load rejection test on a gas turbine (a single-shaft, heavy duty gas turbine, that is). The axial compressor acts like a very big brake on the turbine (remember, nearly two of every three horsepower produced by the turbine is used to drive the axial compressor!) and today's control systems are VERY good at controlling fuel flow-rates during load rejection so "stability" isn't a problem.

Now, if you're talking about a larger plant with multiple turbines, then stability might be a problem when a grid separation or large load rejection occurs, but, in general, if the facility was configured properly and if there's a well-programmed PMS or Load-Sharing/Load-Shedding system in place, then stability is not normally an issue too much these days.

But, without a LOT more information, it's virtually impossible to say what might have happened, or more importantly, how to prevent it from happening again. Sorry; it would have been nice to have been able to be of help, but not this time.