Arc flash level at synch. gen terminals
Arc flash level at synch. gen terminals
(OP)
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
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






RE: Arc flash level at synch. gen terminals
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.
RE: Arc flash level at synch. gen terminals
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?
RE: Arc flash level at synch. gen terminals
RE: Arc flash level at synch. gen terminals
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.
RE: Arc flash level at synch. gen terminals
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?
RE: Arc flash level at synch. gen terminals
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.