Effects of field forcing on fault currents 2

[Bung]

Does field forcing in generators (50-100MW size) act to increase the fault currents? I have had somebody tell me that it can push the fault current contribution by a generator well above (2pu)the subtransient value. A search around the web has give me lots of interesting stuff on generator control, but not much on how the fault contribution behaviour is affected. I would be particularly interested in modelling techniques.

thanks
Bung
Life is non-linear...

 

[jbartos]

Suggestion: The generator subtransient reactance, Xd", is essentially determined by the generator amortisseur windings. Visit

http://www.hq.usace.army.mil/cemp/e/Et/nolinear.pdf

http://www.usace.army.mil/publications/eng-tech-ltrs/etl1110-2-317/chap-6.pdf

http://www.usbr.gov/research/results/High Voltage.pdf

for the generator amortisseur winding, and

http://www.easypower.com/help/manual14.html

for generator modeling data

 

[Bung]

I don't have too much of a problem with the subtransient reactance, my question is more about what happens to the voltage behind the reactance when you start messing with the field, and how does that affect the short circuit current. My (admittedly limited) understanding of the subtransient, etc reactances is that they are just a device to account for the changing internal voltage of the machine. It is easier to assume a fixed internal voltage and vary the machine reactance (in simple terms - it probably isn't quite that simple in reality) than try to cope with a fixed reactance and varying internal emf. But field forcing brings an additional variation to the internal emf which needs to be taken into account somehow. Or maybe it doesn't - I don't know.
Bung
Life is non-linear...

 

[ijl]

The occurrence of a short circuit (somehow) induces an additional DC current in the field winding, and eventually other rotor windings. This current again induces an additional emf in the stator windings. This phenomen is described using the subtransient reactance, as you correctly state. If I understand you right, the term "field forcing" is another way of saying that the control system tends to keep the output voltage at a desired value. In order to model the "field forcing", you have to model the control system. In the simplest case, the control system just increases the DC voltage driving the magnetization current. There should be no need to modify the generator model itself, assuming that the model is sufficiently realistic, of course (including rotor windings, flux linkages etc.)
ijl

 

[tolin]

Field forcing is used in case the generator is taking its excitation power from its own line side. In case of a short circuit near the generator, the voltage will decrease to zero, the field too and by that the short circuit current. In order to maintain short circuit current at a high level, typical three times rated current for some seconds, field forcing or boosting is used giving the protection time to act and trip the circuit breaker. Except once,I have never seen field forcing beeing taken into account in calculating short circuit currents. The exception is from GEC Alstom Protective Relays, Application Guide, Section 17.2 where field forcing is considered when calculating earthing transformers (fault voltage multiplied by 1.3).

 

[BJ001]

I have not seen anywhere the suggestion of field forcing of the machine has an impact upon the inital fault current which is limited by the sub transient reactance of the machine.

The field forcing is a measure to ensure that sufficient curret is available to operate the protection.
As the duration of the fault progresses the sub transient reactance has less of an effect.
Eventually after sufficiect time (say in the order of 5 seconds) the fault current is IL/xd (xd=direct axis synchronous reactance of the machine).

As 1/xd is equal to the short circuit ratio of the machine the final current would be the SCR x IL.

This would result in a fault curret being half the value of the normal line current if the machine had a short circut ratio of 0.5 hence the field forcing to keep the current at a higher value.

 

[Bung]

I agree field forcing has no effect at initiation of the fault - the current will be determined by the subtarnsient X, and the field forcing will only be initiated by the resulting voltage collapse. The question is, can this result in an increase in fault current over the next few hundred milliseconds as this increasing field current forces up the internal emf of the machine (ie in effect decreases the machine transient reactance to a value below the subtransient reactance)?

Bung
Life is non-linear...

 

[jwerthman]

Field forcing generally has minimal effect on subtransient and transient fault currents because of the time constant of the field circuit. The field current just cannot change fast enough to really affect the fault current during the first few cycles. However, field forcing does have a substantial affect on the steady-state fault current.