Arc flash level at synch. gen terminals 1

[SictEng]

Hi all,
When conducting an arc flash study involving synchronous generators, it's easy enough to determine the fault clearing time on the load side of the breaker, but for a fault on the line or generator terminal side, how would one determine the fault clearing time for arc flash purposes?

I imagine the arcing fault current from the generator would decay depending on the type of excitation, but other than using an arbitrarily conservative time (1 or 2 seconds), is there any way to determine the AF level for a gen breaker?

Thanks

 

[dpc]

Not unless you know exactly how the generator, governor, exciter and voltage regulator will respond to an arcing fault between the generator terminals and generator breaker. If you know the generator impedances and time constants, a generator decrement curve can be developed. But you also must factor in voltage regulator and exciter response. If you have an accurate generator decrement curve, the total arc energy, can, in theory, be calculated by integrating the arc power over time to determine the arc energy. You must also include system contribution (in on-line) until the generator breaker opens.

Some software programs (SKM & EasyPower that I know of) have an "Integrated" calculation method that will provide an approximation of the generator contribution decrement over time. But you will still want to assign a maximum time for the arc duration, for a lot of reasons. Generator terminals are high arc-flash hazard areas. This would include PT cubicles and other ancillary stuff connected to the main generator leads. Iso-phase bus not so much, since there is essentially no chance of a phase-to-phase fault in the iso-phase and assuming high-resistance grounding of the generator is used.

 

[ScottyUK]

The AVR will play a large part in this. A PMG-fed AVR will feed the fault, while a bus-fed AVR will tend to collapse when presented with a close-in fault. Once the protection detects the fault - which should be quickly if the machine has stator differential protection - then the field suppression takes time to collapse the field and bring the terminal voltage down. Don't assume that one or two seconds is especially conservative.

From a practical perspective, arc fault level on any utility-class generator will be so far off the charts that it won't matter. What size machine are you talking about?

 

[SictEng]

Roughly anywhere from 2 - 20 MVA. Stator differential protection is usually present.

 

[iwright]

One our gas turbines had a fault at its terminals which began as a single phase to ground fault and quickly developed into a three phase arcing fault. This was a 71MVA 13.8kV generator, the generator breaker opened in about 3 cycles and cleared the system contribution (~25kA) to the fault but the generator fed the fault for sometime afterwards. The relay record is cut off after half a second but in that time the current dropped from about ~18kA to around ~9kA. I've attached a couple picture of the terminal box after the event.

The differential protection will seperate you from the grid quickly and trip your exciter but your generator is still storing a lot of energy. As dpc mentioned if you have the model information there is software that can do calculations how long it will take for the fault current to decay. I have the generator and exciter models built for a 333MVA thermal unit with a static exciter and the dynamic simulation estimates about 1.7 seconds for the fault current to decay to zero.

 

[SictEng]

The generator in questions is 4.5MW at 4.16kV. The AVR is bus fed.

The study is being done in ETAP 12. I can run the analysis using the integrated decrement curve mentioned by DCP to calculated the fault current. My main concern is the accuracy of the decrement curve. You mentioned that it took about 1.7 for the fault current to decay to zero in your dynamic simulation, however, my ETAP decrement curve decays to the generator FLA, not zero.

I would also like to know how compound excitation comes into play. The excitation system is a Basler DECS-200. The manual say that it has a 10 second field forcing rating of 200%. Is this compound excitation?

 

[SictEng]

Is there any way for ETAP to consider different fault contribution times from other sources, even if the faulted generator bus is fixed at 2 second FCT?

The problem is that I need to declare the fault clearing time as it is on the generator side of the breaker, but ETAP seems to include contribution from other branches for the full 2 seconds, even though those contributions will be removed by other protection. This is resulting in a huge amount of fault current for the full two seconds.