Synchronous Generator In Isolated Mode Basics

Yes..."Negative MVArs are "generated" ("produced") when excitation is decreased below that required to make the generator terminal voltage equal to grid voltage. Sometimes this is referred to as "under-excitation." In the power generation industry, most (but not all) generators are not made to be operated for extended periods of time in an under-excited condition (negative MVArs) because of unwanted heating which can damage the generator. The amount of negative MVArs the generator is capable of "producing" is also usually much less than the amount of positive MVArs.The generator manufacturer usually provides some kind of "reactive capability" graph or curve which depicts how the generator can be operated with repect to negative MVArs, positive MVArs, and MW (megawatts)."

http://www.control.com/thread/1026239446

Hi,

If you think my attempted guess of what should happen to an isolated generator if we slightly over/under excite is correct, then you might say my simulation result below should be wrong.

Please have a look.

Why would the voltage increase? The only thing increasing would be available current...

When the excitation is reduced below the level to achieve nominal no load voltage, rather than the voltage decreasing, the reactive current (vars) flowing to thegenerator from the load increases resulting in a leading pf condition on the generator.

Hi,

The as far I know, generated emf E = k * N * d(phi)/dt

In our case, phi is constant, but with the rotor rpm its rate of change should increase, and hence the generated EMF must be directly proportional to rotor RPM.

what do you think?

This might help...

http://en.wikipedia.org/wiki/Faraday%27s_law_of_induction

With a resistive load, running alone, there are no VARs.(neglecting the inductance of the generator windings)

It depends what is driving it, a diesel engine at fixed throttle, a diesel engine with a speed governor, a waterwheel, an induction motor?

If you increase excitation in stand alone you increase output volts, if you increase excitation in parallel or against the mains you generate VARs. If you increase volts to a resistive load it follows ohms law, if the attempted increase in output power slows the engine then it depends on the power characteristic of the engine.

For a fixed input power, the output power must be fixed and hence the output volts, so with changes of excitation the rpm will adjust to a value where the power is the same, but rpm and torque have changed.

Bottom line, changing excitation with fixed power input and fixed resistive load changes rpm and torque.

Get the basics clear in theory before you rely on matlab.

Hi engineertony,

I was in the exact same thought as your description, i.e. I was thinking that adjusting excitation with fixed power input and fixed resistive load (as is the case in my simulation) should adjust the RPM.

That was my basics.
I then tried to simulate it (see Post #2), but the simulation went counter to me.
I can mail you my simulation model, perhaps you can spot something.

While operating the generator under leading power factor (under excitation) the armature reaction is having magnetizing effect. This leads to change in magnetic flux distribution pattern in stator end zone. This causes end support structure like core press ring, stator casing etc. to link the end leakage flux thereby resulting in high eddy current losses. These parts are not designed to carry heavy magnetic flux. These parts are not made from laminated sheets as the stator core. They are solid steel structures. Therefore higher flux linkage would cause heavy heating in these parts and may cause heavy damage to the generator.

The generators are designed to operate at"knee point" of the magnetizing characteristic of the rotor. So increasing the excitation to 120% may not increase the voltage by 20%. It would be less than 20%. But over fluxing may cause the flux to take path through core support structure (as the saturated core will cause higher magnetic reluctance and flux will take a path of lower reluctance provided by core support structure). This would also cause heavy eddy current losses in core support structure and may cause fusion of core with support structure like core bars, apart from core melting due to its own higher eddy losses.

"While operating the generator under leading power factor (under excitation) the armature reaction is having magnetizing effect."

Just the first line.

This generator never operates under a leading power factor, it has a coil, a core, and a resistive load. It doesn't matter how you excite it you cannot get the current to lead the generated voltage.

"The armature reaction is having a magnetising effect" The armature is the rotating bit and is magnetised by the excitation current, that's all. This magnetic field passes by the stator windings and induces a voltage. If anything else gets affected by this rotating field then it's neglible. And how can reducing this excitation, and hence the strength of the rotating field lead to more induced voltages in the surrounding metalwork!!

Beware the bull****er