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IEC 60909 Fault Calculations

To calculate system faults, we can use the guidance given in IEC 609096 "Short-circuit currents in three-phase a.c. systems. For faults far from the generator faults in three phase systems, each type of fault the symmetrical short-circuit current I"k is given by:

The Formulae

  Three-Phase System Single-Phase System D.C. System
Three phase short circuit I k " = c U n 3 Z 1 - -
Line-to-line short circuit I k2 " = c U n Z 1 + Z 2 = c U n 2 Z 1 = 3 2 I k " I k " = c U n 2 Z 1 I k " = c U n 2 Z 1
Line-to-line-earth short circuit I kE2E " = 3 c U n Z 2 Z 1 Z 2 + Z 1 Z 0 + Z 2 Z 0 = 3 c U n Z 1 +2 Z 0 - -
Line-earth short circuit I k1 " = 3 c U n Z 1 + Z 2 + Z 0 = 3 c U n 2 Z 1 + Z 0 I k " = c U n Z 1 + Z 0 I k " = c U n Z 1 + Z 0

Note:

  1. for most components (except synchronous machines), we can take Z2 as equal to Z1.
  2. for single-phase line to line is equivalent to line-neutral, and for d.c. positive to negative

Source Impedance

Source impedance Z, is given by:

ZQ=cUn3Ik"

 

Symbols

c                   - voltage factor
I"k                  - initial symmetrical three phase short-circuit current (r.m.s.)
                        - I"k1        line-to-earth
                        - I"k2        line-to-line
                        - I"kE2E     line-to-line-earth
Un             - nominal system voltage, line-to-line (r.m.s.)
                        - positive-negative voltage for d.c. systems
Z1             - positive sequence short -circuit impedance
Z2             - negative sequence short circuit impedance
Z0             - zero sequence short circuit impedance

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