27 November 2010

Short Circuit Study Tutorial

Which are the parameters considered for short circuit study?
  • Transient(Xd') or sub-transient (Xd'') reactance is considered for +ve sequence.
  • -ve sequence reactance X2 approximately equal to Xd''
  • Zero sequence reactance is 0.1 to 0.7 times Xd''

Transmission Line
  • +ve seq impedance Z1 equal to -ve seq impedance Z2
  • Zero seq impedance Z0 depends upon return path, ground wires and earth resistivity.
  • X0 is 2 to 2.5 times +ve seq reactance X1
  • R0 is 5 to 10 times +ve seq reactance R1
  • B0 is 0.6 to 0.8 times +ve seq susceptance B1
Why Generator MVA rating, transformer MVA rating are not appear in short circuit report?
In the load flow study the transformer, Generator MVA rating are relevant as the loading on the equipment are compared with the rating.In short circuit study the fault level should be compared with breaker rating . Hence only breaker ratings are given in short circuit study.
In short circuit study R / X ratio of short circuit path is mentioned. What does it signify and how is it calculated?
R/X ratio of short circuit path is computed in case of 3 phase fault. The value is used in the selection of asymmetrical braking capacity from the symmetrical braking capacity. The asymmetrical braking capacity should be selected based on the R/X ratio ( i,e resistance to reactance ratio of driving point impedance and the breaker operating cycles. Please refer Chapter 10, Section 10.1 & 10.6 of "Elements of Power System Analysis", Fourth edition, McGraw Hill publication by W.D Stevenson, for further details.

Source: /www.prdcinfotech.com/faq.html


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