Do I take this into consideration? If so how do I size wire in a building with a safety generator? What is my "Ze" when the supply switches from DNO to the safety generator? How do diesel generators behave during faults?
davezawadi (David Stone):
This is getting very confused. The engine will only produce its rated power assuming the frequency (RPM) is constant. All other associated power can only come from the rotating inertia of the engine and generator rotor. A complete short on most sizes of generators will stop the machine very quickly, perhaps 10 or 20 revolutions. The moment the engine speed falls so does its output power, power = torque x RPM, and the torque is limited at all speeds and gets less as the speed falls in most machines at limiting power output. Your document is probably not very accurate for most reasonable-sized generators say <500kVA. Also, the AVR is designed to reduce the excitation if the frequency falls, and usually to make it zero at 40Hz for a 50Hz machine to protect the mechanical parts from failure. The forces involved may be very large, sufficient to break generator shafts, or piston rods or the crankshaft with a low impedance short circuit. Such failures do occur in generator sets sometimes because those first few revolutions are critical, the short circuit current is not in any way "free", it is a real very large amount of power. Just because the short collapses the voltage does not reduce the real power in any way, the alternator is still making 230V or whatever, it is almost all lost in the windings and the external circuit as resistive loss, although perhaps 10% may be lost in the reactance.
I'm afraid Mike has made a slight slip, a synchronous alternator does not suffer "slip" as such, the output exactly follows the machine rotation at all speeds, in the same way that a separately excited motor does. Slip in an induction motor is simply to make the rotor magnetic field, in an alternator, this is externally applied DC and only controlled by the AVR. Small machines do sometimes operate as asynchronous machines but these have very poor overload and voltage control characteristics.
One further point is that most machines that are brushless depend on simulated transformer action to get the AVR excitation power to the rotor, where it is rectified and gives the rotor field. This is a very important part of our size of the machines under discussion. There are a number of various ways to do this, which are usually covered in Patents, it is easiest to just look on this as I suggest, although there may be a secondary alternator action or similar involved. It makes little difference to the operation.
How can the text extract not be accurate when coming from a reputable manufacturer? All the generator specs I'm looking at indicate that generators <500kw are capable of exporting short circuit current between 3x to 10x for some amount of seconds. For example, 300% for 10 seconds:
http://resources.kohler.com/power/kohler/industrial/pdf/g5438.pdf
I'm also confused when you say 10% will be lost to reactance when reactance itself is not real power. I'm also under the impression that reactance actually causes a generator speed up vs slow down.
davezawadi (David Stone):
<snip>... These are permanent magnet alternators and probably on very small machines with oversized engines. A short circuit on a conventional (synchronous) alternator will remove the voltage from the AVR (which is powered from the output terminals in most designs), thus removing most of the magnetic field from the rotor. This is a deliberate safety feature to prevent winding damage and mechanical failure. All the other details there suggest that these machines are intended to live a hard life outside, The data missing is how the voltage control works, although there could be some clue from the pilot-excited phrase suggesting that the permanent magnets only give some of the rotor field, the rest being from an AVR. <snip>
I suspect it is more like one of these.
The permanent magnet part is really only a big brother of a bicycle dynamo in that it provides the 'pilot power' that is AC that is then rectified to provide DC power to the electronics in the AVR controller that is setting the Exciter field, and perhaps to supply some ancillary electronics for engine management and cranking battery condition as well. The AVR senses voltage and current readings from the outbound phases to decide exciter level, but is not depending on it for the excitation power to the armature.
The fact that the AVR does not lose its supply when the output voltage collapses is both a benefit and a risk, as you point out, depending on the avr details it keeps going in fault state, but it also recovers more gracefully with spiky loads
The bigger machines by Brush all work on this basis.
It does also mean that clever microprocessor controls and telemetry stay up as well, and cannot be crashed by load side abuse, and that is essential in some more automated settings when either motoring in extra generation or generator shedding from afar.
Mike.