Loss of Excitation on 600 MW Power Plant

Hi All,

We have 600 MW power generator. Two days ago when our power is 150 MW, our DC excitation circuit breaker is suddenly open, and our unit is not tripped. Our output power was still 120-130 MW, so we can produce MW without excitation and then we stopped unit by manual.

When our loss of excitation our reactive power went up 390 Mvar and our excitation system limited this value.

Could you help me? How can be possible, we can produce MW without excitation system?

So, a synchronous generator can become a non-synchronous (induction) generator when the excitation power is lost and the generator breaker is not tripped. HOWEVER, when that happens, the generator rotor usually gets very, Very, VERY hot in a pretty short period of time. And, usually, the rotor gets damaged in the process. (Non-synchronous (induction) generators do not operate at synchronous speeds, and require slip to function properly. It's the slip that generates the heat on the rotor which a synchronous rotor isn't designed for. You can look up induction machines (generators and rotors are the same, just like synchronous generators and motors are the same) and slip if you're really curious.)

Most <b>EVERY</b> synchronous generator I have ever worked on has a loss-of-excitation relay, which was "backed-up" by a status contact from the AC and DC excitation breakers/contactors, so that if DC power to the slip rings and field were ever lost the generator breaker would be tripped to protect against the damage which can pretty quickly result from operation as an induction (non-synchronous) generator.

It's pretty serious to be running without excitation; I hope it wasn't for too long a period of time. In addition to having someone check out the generator rotor (field) someone needs to determine why the generator was not tripped when excitation was lost. Now, there may be some generator somewhere in the world which might be "designed" to operate for a brief period of time in non-synchronous (induction) mode, but I've never worked on one. Or really even heard of one. The leading reactive power magnitude means LOTS of MVArs were coming in from the grid the unit was synchronized to, and I'll bet if you look at some historical data from the event you will find the turbine-generator speed was NOT at rated, too.

Hope this helps! It's never a good idea to operate (most) synchronous generators as non-synchronous (induction) generators, not even for a short while. (Time is dependent on many factors, but we're generally talking less than a few minutes to prevent damage caused by heating.)