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Page 7

Thévenin equivalent circuit

is the transformer winding ratio (pu)

Conversely, by re-arranging the equation above, impedances can be referred to the LV side:

Step 4: Determine Thévenin Equivalent Circuit at the Fault Location

The system model must first be simplified into an equivalent

circuit as seen from the fault location, showing a voltage

source and a set of complex impedances representing the

power system equipment and load impedances (connected in

series or parallel).

The next step is to simplify the circuit into a Thévenin

equivalent circuit (http://en.wikipedia.org/wiki/Th

%C3%A9venin%27s_theorem) , which is a circuit containing

only a voltage source ( ) and an equivalent short circuit

impedance ( ).

This can be done using the standard formulae for series and

parallel impedances, keeping in mind that the rules of complex

arithmetic must be used throughout.

If unbalanced short circuits (e.g. single phase to earth fault) will be analysed, then a separate Thévenin

equivalent circuit should be constructed for each of the positive, negative and zero sequence networks (i.e.

finding ( , and ).

Step 5: Calculate Balanced Three-Phase Short Circuit Currents

The positive sequence impedance calculated in Step 4 represents the equivalent source impedance seen by a

balanced three-phase short circuit at the fault location. Using this impedance, the following currents at

different stages of the short circuit cycle can be computed:

Initial Short Circuit Current

The initial symmetrical short circuit current is calculated from IEC 60909-0 Equation 29, as follows:

Where is the initial symmetrical short circuit current (A)

is the voltage factor that accounts for the maximum system voltage (1.05 for voltages <1kV,

1.1 for voltages >1kV)

is the nominal system voltage at the fault location (V)

is the equivalent positive sequence short circuit impedance (Ω)

Peak Short Circuit Current

IEC 60909-0 Section 4.3 offers three methods for calculating peak short circuit currents, but for the sake of

simplicity, we will only focus on the X/R ratio at the fault location method. Using the real (R) and reactive

Short Circuit Calculation - Open Electrical http://www.openelectrical.org/wiki/index.php?title=Short_Circuit_Calc...

7 of 12 11-12-2012 14:46

Thévenin equivalent circuit

is the transformer winding ratio (pu)

Conversely, by re-arranging the equation above, impedances can be referred to the LV side:

Step 4: Determine Thévenin Equivalent Circuit at the Fault Location

The system model must first be simplified into an equivalent

circuit as seen from the fault location, showing a voltage

source and a set of complex impedances representing the

power system equipment and load impedances (connected in

series or parallel).

The next step is to simplify the circuit into a Thévenin

equivalent circuit (http://en.wikipedia.org/wiki/Th

%C3%A9venin%27s_theorem) , which is a circuit containing

only a voltage source ( ) and an equivalent short circuit

impedance ( ).

This can be done using the standard formulae for series and

parallel impedances, keeping in mind that the rules of complex

arithmetic must be used throughout.

If unbalanced short circuits (e.g. single phase to earth fault) will be analysed, then a separate Thévenin

equivalent circuit should be constructed for each of the positive, negative and zero sequence networks (i.e.

finding ( , and ).

Step 5: Calculate Balanced Three-Phase Short Circuit Currents

The positive sequence impedance calculated in Step 4 represents the equivalent source impedance seen by a

balanced three-phase short circuit at the fault location. Using this impedance, the following currents at

different stages of the short circuit cycle can be computed:

Initial Short Circuit Current

The initial symmetrical short circuit current is calculated from IEC 60909-0 Equation 29, as follows:

Where is the initial symmetrical short circuit current (A)

is the voltage factor that accounts for the maximum system voltage (1.05 for voltages <1kV,

1.1 for voltages >1kV)

is the nominal system voltage at the fault location (V)

is the equivalent positive sequence short circuit impedance (Ω)

Peak Short Circuit Current

IEC 60909-0 Section 4.3 offers three methods for calculating peak short circuit currents, but for the sake of

simplicity, we will only focus on the X/R ratio at the fault location method. Using the real (R) and reactive

Short Circuit Calculation - Open Electrical http://www.openelectrical.org/wiki/index.php?title=Short_Circuit_Calc...

7 of 12 11-12-2012 14:46