Understanding the Short Circuit Characteristic of a Synchronous Generator

[maverick280857]

Hi everyone

Books mention the Open Circuit and Short Circuit tests on a synchronous generator in order to determine its synchronous impedance. One observes that the Short Circuit Characteristic (plot of short circuit armature current versus field current) is a straight line all the way, whereas the Open Circuit Characteristic (plot of open circuit terminal voltage versus field current) droops due to magnetic saturation.

My question is: why does the Short Circuit Characteristic (SCC) not exhibit saturation?

I would be grateful if someone could offer a detailed explanation. I suspect this is due to armature reaction, but I am not very clear about this.

Thanks in advance.

Cheers
Vivek.

 

[dlgoff]

I think it has to do with the fact that the generator still has inertia when the short occurs.

Generators are driven by turbines, water wheels, diesel engine or other types of prime movers. When a short circuit occurs on the system powered by a generator, the generator continues to produce voltage at the generator terminals as the field excitation is maintained and the prime mover drives the generator at normal speed. The generated voltage causes a large magnitude fault current flow from the generator to the short circuit. The flow of fault current is limited only by the generator impedance and the impedance of circuit between the generator and short circuit. In case of a short circuit at the generator terminals, the fault current is limited by generator impedance only.
Synchronous Motors


Synchronous motor design is very similar to generator design. Both have a magnetic field produced by direct current and stator carrying alternating current. During normal operation, synchronous motors consume ac power from power line and convert the electric energy into mechanical torque. When a short circuit happens on the system feeding a synchronous motor, the voltage at motor terminals drops drastically. As a result, motor stops delivering mechanical torque to the load and starts to decelerate. Still, the inertia of the load and motor wheel drives the synchronous motor. The synchronous motor turns to generate and deliver fault current for prolonged period of time after the initiation of short circuit. The fault current is limited by motor impedance and impedance of the system from short circuit to the motor terminals.

http://www.arcadvisor.com/faq/faultcurrent.html#Generators"

 

[maverick280857]

Thanks.

 

[m.s.j]

Excuse me;
I think the question is about open circuit and short circuit characteristics of generators not short circuit fault current phenomenon.

The open-circuit saturation curve is obtained when driving the SG at rated speed, on open circuit, and acquiring the SG terminal voltage, frequency, and field current. The aggregated core, friction, and windage losses may be measured as the input power for each open-circuit voltage level reading. As the speed is kept constant, the windage and friction losses are constant. Only the core losses increase approximately with voltage squared.

The Short-Circuit Saturation Curve is obtained when the SG is driven at rated speed with short-circuited armature, while acquiring the stator and field currents values should be read at rated 25%, 50%, 75%, and 100%. Data at 125% rated current should be given by the manufacturer, to avoid overheating the stator. The high current points should be taken first so that the temperature during testing stays almost constant. The short-circuit saturation curve is a rather straight line; because the machine is unsaturated during steady-state short-circuit.

Indeed, in open circuit test, the primary mmf of generator (exciting flux) shouldn't be compensated by any armature fluxes, so saturation of core due to over exciting (saturation) can be occur, but in short circuit test, armature fluxes is operating against rotor fluxes and level of flux density can be stay constant approximately.


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[uart]

Hi everyone

Books mention the Open Circuit and Short Circuit tests on a synchronous generator in order to determine its synchronous impedance. One observes that the Short Circuit Characteristic (plot of short circuit armature current versus field current) is a straight line all the way, whereas the Open Circuit Characteristic (plot of open circuit terminal voltage versus field current) droops due to magnetic saturation.My question is: why does the Short Circuit Characteristic (SCC) not exhibit saturation?

I would be grateful if someone could offer a detailed explanation. I suspect this is due to armature reaction, but I am not very clear about this.

Thanks in advance.

Cheers
Vivek.

It's a good question Vivek.

The reason why the open circuit voltage "saturates" as field current increases is simply because the magnetic field strength is a non-linear function of the field current (due to the non-linear B/H characteristics of the iron). This part I'm pretty sure you already understand, the voltage is just proportional to the rate of change of magnetic flux (and hence rotational speed times flux density) so it's fairly straight forward that it should follow the same non-linearity as the flux density.

When considering the short circuit current however you should note that (neglecting resistance) if v=0 then the rate of change of total flux must also be zero. So the short circuit current must in fact rise to whatever is required to exactly oppose (that is, to demagnetize) the main field. When you think of it this way it's acting rather like a current transformer, the current required to demagnetize is simply equal to the main field winding current multiplied by the effective turns ratio. Thus its a very simple linear relationship.

 

[uart]

ok m.s.j beat me to it while I was typing :)

 

[maverick280857]

Thanks msj and uart.