For balanced short circuit calculations, the positive-sequence impedance is the only relevant impedance.
However, for unbalanced short circuits (e.g. single phase to earth fault), symmetrical components The initial short circuit current for a single phase to earth fault is as per IEC 60909
ngle phase to earth short circuit current (A)
is the voltage factor that accounts for the maximum system voltage (1.05 for voltages <1kV, 1.1 for voltages
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are the reactance and resistance, respectively, of the equivalent source impedance at the fault
is a factor to account for the equivalent frequency of the fault. Per IEC 60909-0 Sec on 4.4, the following
sequence impedance is the only relevant impedance.
symmetrical componentscome into play.
The initial short circuit current for a single phase to earth fault is as per IEC 60909-0 Equation 52:
is the voltage factor that accounts for the maximum system voltage (1.05 for voltages <1kV, 1.1 for voltages
is the nominal voltage at the fault location (Vac) is the equivalent positive sequence short
is the equivalent negative sequence short circuit impedance ( is the equivalent zero sequence short circuit impedance (
Worked Example
System model for short circuit example
In this example, short circuit currents will be calculated for a balan
a simple radial system. Note that the single phase to earth fault currents will not be calculated in this example.
Step 1: Construct the System Model and Collect Equipment Parameters
The system to be modeled is a simple radial network with two voltage levels (11kV and 415V), and supplied by a single generator. The system model is shown in the figure to the right. The equipment and cable parameters were collected as follows:
is the nominal voltage at the fault location (Vac)
is the equivalent positive sequence short circuit impedance (Ω) is the equivalent negative sequence short circuit impedance (Ω) is the equivalent zero sequence short circuit impedance (Ω)
In this example, short circuit currents will be calculated for a balanced three-phase fault at the main 11kV bus of a simple radial system. Note that the single phase to earth fault currents will not be calculated in this example.
Step 1: Construct the System Model and Collect Equipment Parameters
s a simple radial network with two voltage levels (11kV and 415V), and supplied by a single generator. The system model is shown in the figure to the right. The equipment and cable parameters were
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phase fault at the main 11kV bus of a simple radial system. Note that the single phase to earth fault currents will not be calculated in this example.
Step 1: Construct the System Model and Collect Equipment Parameters
s a simple radial network with two voltage levels (11kV and 415V), and supplied by a single generator. The system model is shown in the figure to the right. The equipment and cable parameters were
Equipment
Transformer Cable C3 Length = 100m
Size = 3C+E 95 mm2
Step 2: Calculate Equipment Short Circuit Impedances
Using the parameters above and the equations outlined earlier in the method were calculated:
We will model a fault on the main 11kV bus, so all impedances must be referred to 11kV. The two low voltage motors need to be referred to this reference voltage. Knowing that the
calculate the winding ratio and apply it to refer the 415V motors to the 11kV side:
The 415V motor impedances referred to the 11kV side is therefore:
Equipment 415V Motor M2 415V Motor M3
Step 4: Determine Thévenin Equivalent Circuit at the Fault Location
Using standard network reduction techniques, the equivalent Thévenin circuit at the fault location (m bus) can be derived. The equivalent source impedance is:
= 0.85 pu
= 0.30 pu
Step 2: Calculate Equipment Short Circuit Impedances
Using the parameters above and the equations outlined earlier in the methodology, the following impedances
Resistance (Ω) Reactance (Ω)
We will model a fault on the main 11kV bus, so all impedances must be referred to 11kV. The two low voltage motors need to be referred to this reference voltage. Knowing that the transformer is set at principal tap, we can calculate the winding ratio and apply it to refer the 415V motors to the 11kV side:
The 415V motor impedances referred to the 11kV side is therefore:
Resistance (Ω) Reactance (Ω) 46.0952 146.5735 31.6462 100.6284
Step 4: Determine Thévenin Equivalent Circuit at the Fault Location
Using standard network reduction techniques, the equivalent Thévenin circuit at the fault location (m bus) can be derived. The equivalent source impedance is:
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We will model a fault on the main 11kV bus, so all impedances must be referred to 11kV. The two low voltage transformer is set at principal tap, we can
Step 4: Determine Thévenin Equivalent Circuit at the Fault Location
Using standard network reduction techniques, the equivalent Thévenin circuit at the fault location (main 11kV
Step 5: Calculate Balanced Three
Initial Short Circuit Current
The symmetrical initial short circuit current is:
Peak Short Circuit Current
The constant factor at the fault location is:
Therefore the symmetrical peak short circuit current is:
kA
Symmetrical Breaking Current
The symmetrical breaking current is:
kA
Computer Software
Short circuit calculations are a standard component of power
PTW, DIgSILENT, etc) and the calculations are far easier to perform with software than by hand. However manual calculations could be done as a form of verification to confirm that the software results are r
What Next?
The results from the short circuit calculations can be used to specify the fault ratings on electrical equipment (e.g.
switchgear, protective devices, etc) and also for protection coordination studies.
Step 5: Calculate Balanced Three-Phase Short Circuit Currents
The symmetrical initial short circuit current is:
kA
at the fault location is:
Therefore the symmetrical peak short circuit current is:
Short circuit calculations are a standard component of power systems analysis software (e.g. ETAP, EasyPower, PTW, DIgSILENT, etc) and the calculations are far easier to perform with software than by hand. However manual calculations could be done as a form of verification to confirm that the software results are r
The results from the short circuit calculations can be used to specify the fault ratings on electrical equipment (e.g.
switchgear, protective devices, etc) and also for protection coordination studies.
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systems analysis software (e.g. ETAP, EasyPower, PTW, DIgSILENT, etc) and the calculations are far easier to perform with software than by hand. However manual calculations could be done as a form of verification to confirm that the software results are reasonable.
The results from the short circuit calculations can be used to specify the fault ratings on electrical equipment (e.g.
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