Power System Protection
ECCE5302/Fall 2022
Abdelsalam M. Elhaffar
D. Sc. Tech., SMIEEE,
Office 2057, elhaffar@ieee.org
Calculation of short-circuit currents
Examples
EXAMPLE: FAULT CALCULATIONS ON
ATYPICAL SYSTEM
FIND THE S INGLE -PHASE-TO -GROUND FAULT CURRENT AT BUS G
conversion to a common base of 100 MVA is necessary
Sequence connections
for typical three-winding
and autotransformer
banks.
ZHM = ZH+ ZM
ZML = ZM + ZL
Z HL = ZH + ZL
From which
Positive- and negative-sequence networks and their
reduction to a single impedance for a fault at bus G
Zero-sequence network and its reduction to a
single impedance for a fault at bus G
This network is reduced for a fault at bus G by the first paralleling X0S ZH with ZL and
then adding ZM and X0GH;
SINGLE -PHASE-TO -GROUND FAULT AT BUS G
 Normally, the 3 I0 currents are documented in the
system, for these are used to operate the ground
(orearthfault) relays.
 Because X1 =X2, such that I1=I2, these reduce to:
Ib=Ic=–I1+I0 for the phase b and c currents, ( a+a2=–1).
 The currents shown are determined by adding I1+I2+I0
for Ia,
 –I1+I0 for Ib, and Ic and 3I0 for the neutral currents.
Phase and 3I0 current distribution for a singlephase-to-ground fault at bus G
The calculations assumed no load ; accordingly, prefault, all currents in the
system were zero. With the fault involving only phase a, it will be observed
that current flows in the b and c phase s. This is because the distribution
factor s in the zero -sequence network are different from the positive- and
negative-sequence distribution factor s.
Exercise 1
There is an incoming 11 kV supply to the
plant substation of a typical distribution
system. Two 11 kV lines to the pump house
substation where there are two 11/0.415kV
transformers.
The fault level at substation 11kV busbars is
250 MVA. The reactance of each 11 kV
interconnector line is 0.024 ohm.
The two 1.25 MVA transformers per unit
impedances are 6% each.
The interconnecting cables to the 415 V
switchgear are 0.00125 ohm.
Find:
a. Construct an impedance diagram to a
common base of 10 MVA.
b. Calculate the three phase fault current at
the 415 busbars when the BS breaker is
closed.
DG Exercise
A symmetrical three-phase fault occurs on the 11kV busbars of
the circuit below. Calculate the fault current when
a) the 30MVA generator is connected.
b) the 30MVA generator is disconnected.
Use base power =60MVA