Disconnection times on a system if a stand by generator is used

On a fully designed power distribution system, clearly the fault levels, earth loop impedances and ultimately fuse disconnection times are determined by the system parameters Transformer %Z etc one or two in parallel for min and max fault levels levels. What needs to be considered when bringing on a stand-by generator, clearly all Fault levels, Earth loop impedances and consequently fuse disconnection times would change because of the Generator parameters. How would you design in these new parameters, bearing in mind that disconnection times on a large system may no longer be achievable 

Parents
  • Generators have a very different behaviour under fault than the almost limitless street supply - initially the frequency may fall, and once it drops out of voltage regulation, the effective impedance rises dramatically, and may be near infinite if it stalls. It is not safe to assume that there will be enough 'oomph' to clear a large fuse  or breaker (i,e, one near the full power rating )reliably.  The solution as others have alluded is a suitably sized earth fault relay to ensure ADS against low impedance earth faults,  (LN overloads are allowed to take their time.,, ) and if required then co-ordinated RCDs of varying delay on any sub-mains whose ADS would not reliably  clear in time for safety on earth fault.

    Mike

Reply
  • Generators have a very different behaviour under fault than the almost limitless street supply - initially the frequency may fall, and once it drops out of voltage regulation, the effective impedance rises dramatically, and may be near infinite if it stalls. It is not safe to assume that there will be enough 'oomph' to clear a large fuse  or breaker (i,e, one near the full power rating )reliably.  The solution as others have alluded is a suitably sized earth fault relay to ensure ADS against low impedance earth faults,  (LN overloads are allowed to take their time.,, ) and if required then co-ordinated RCDs of varying delay on any sub-mains whose ADS would not reliably  clear in time for safety on earth fault.

    Mike

Children
No Data