Short Circuit Analysis
(IEC 60909 Standard)
Extent & Requirement of SC Studies
❑
Planning Stage
❑
Short-circuit studies can be performed in order to help finalize the
system layout, determine voltage levels, and size cables, transformers,
and conductors
❑
Existing System
❑
Fault studies are necessary in the cases of added generation, installation
of extra rotating loads, system layout modifications, rearrangement of
protection equipment, verification of the adequacy of existing breakers,
relocation of already acquired switchgear in order to avoid unnecessary
capital expenditures, etc
Purpose of Short Circuit
❑
Short circuit study can be used to determine the following
conditions:
❑
❑
Evaluate the electrical equipment short circuit withstand
❑
Verify the protective device Making capacity
❑
Verify the protective device Breaking capacity
❑
Verify the protective device Thermal withstand capacity
Selecting settings for relay coordination
Type of Short Circuit
Type of Short Circuit
Short Circuit Input Data
Short Circuit Input Data
Short Circuit Input Data
Short Circuit Input Data
Short Circuit Input Data
IEC-60909 Short Circuit Important Terms
Calculation Methods
Calculation Methods
X/R for Peak Current (ip)
Calculation Methods
Calculation Methods
Calculation Methods
Short Circuit Phenomenon
Short Circuit Phenomenon
Thevenin Equivalent
AC Component of Fault Current
Near-To-Generator Short Circuit
This is a short-circuit condition
to which at least one
synchronous machine
contributes a prospective initial
short-circuit current which is
more than twice the generator’s
rated current, or a short-circuit
condition to which synchronous
and asynchronous motors
contribute more than 5% of the
initial symmetrical short-circuit
current ( I"k) without motors
AC Component of Fault Current
AC Component of Fault Current
Far-From-Generator Short Circuit
This is a short-circuit condition
during which the magnitude of
the symmetrical ac component
of available short-circuit
current remains essentially
constant.
AC Component of Fault Current
DC Component of Fault Current
❑
The DC component or shift in zero axis of current from reference
or true zero (voltage zero axis) results from the necessity to
satisfy two conflicting requirements
(A) Fault current wave must be maximum (-ve peak) when voltage wave
is zero, because of highly inductive source fault impedance (X/R
very high) 90 deg. lag of current.
(B) The actual instantaneous fault current at fault is the value
determined by the pre-fault network condition. It will be zero if
fault occurs on unloaded circuit. So instantaneous fault current at
t=0 should be zero.
DC Component of Fault Current
❑
Fault at zero point on the
voltage wave
❑
To satisfy (a) and (b) above the
AC current wave will have to be
shifted by Ip = √2. Iacsymrms,
when fault occurs at zero
point of the voltage wave.
The shifted current axis is DC
component of fault.
DC Component of Fault Current
❑
Fault at peak point on the
voltage wave
❑
For fault occurring at peak point
of voltage wave fault current is
already zero (as required from
(a) and (b) above) & Hence no
current shift is required & Hence
no DC component in fault current
Decay of DC Component
Decay of DC Component
IEC Short Circuit Study Case
Maximum Short Circuit Currents
❑
Voltage factor Cmax according to Table-1 (IEC 60909) shall be applied for
the calculation of maximum short-circuit currents
❑
Choose the system configuration and the maximum contribution from
power plants and network feeders which lead to the maximum value of
short-circuit current at the short-circuit location
❑
Motors contribution to the fault shall be included
❑
Resistance RL of overhead lines and cables are to be introduced at a
temperature of 20 °C
Minimum Short Circuit Currents
❑
Voltage factor Cmin according to Table-1 (IEC 60909) shall be applied for
the calculation of minimum short-circuit currents
❑
Choose the system configuration and the minimum contribution from
power plants and network feeders which lead to the minimum value of
short-circuit current at the short-circuit location
❑
Motors contribution to the fault shall be neglected
❑
Resistance RL of overhead lines and cables are to be introduced at a
higher temperature
Voltage Factor (c)
MV CB Short Circuit Duty
LV CB Short Circuit Duty
MV CB – IEC 62271 CB Test Model
Selection of Switchgear / CB Rating
Circuit breaker or switchgear should be selected to satisfy “MAKE‟,
“BREAK‟ & “THERMAL‟ short circuit duty.
❑
MAKE (kAp) = Peak making duty (at ½ cycle)
This is maximum instantaneous fault current magnitude seen by
CB/ switchgear during fault.
❑
BREAK (kArms) = Asymmetrical break current in kArms at instant
of break of a CB (opening time of CB + relay opening time).
❑
THERMAL = 1 or 3 sec short circuit duty
Device Duty Comparison