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Short Circuit Calculations

Calculation method: IEC 60909 (International Electrotechnical Commisson) Standard for radial and mesh LV and HV network

Why Short Circuit Analysis: Short circuit currents must be calculated in the network for sizing an electrical installation and the required equipment, as well as determining the means required for the protection of life and property

Two values of short-circuit current must be evaluated: 1) The maximum short-circuit current. It is used to determine

i. The breaking capacity of the circuit breakers. ii. The making capacity of the circuit breakers. iii. The electrodynamic withstand capacity switchgear.

2) The minimum short-circuit current. It is required for selecting the time-current curve for circuit breakers and fuses. They are more important when

i. Cables are long and the source impedance is relatively high. ii. Protection of life depends on circuit breaker or fuse operation.

Short Circuit Calculations

Short Circuit Calculations

The protection device must clear the short-circuit current within a time that is compatible with the thermal stresses that can be withstood by the protected cable.

Permissible limit under steady-state

Short Circuit Calculations

Circuit protection using a Circuit Breaker

Short Circuit Calculations

Short-Circuit Types

80%

15% 5%

Short Circuit Calculations

Concequences of Short Circuit

1) At the fault location, electrical arcs i. Damage to insulation ii. Welding of conductors iii. Fire and danger to life

2) On the faulty circuit i. Electrodynamic forces ii. Excessive temperature rise

3) On other circuits nearby i. Voltage dips ii. Shut down of a part of network iii. Dynamic instability iv. Disturbances in control/monitoring circuits

Short Circuit Calculations

Short Circuit Calculations based on IEC 60909 (VDE 0102)

It can be applied to all networks (radial or meshed) up to 550 kV.

The method is based on Thevenin theorem. An equivalent voltage source and impedance at the short-circuit location is determined and short-circuit current is calculated from them. All network feeders as well as the synchronous and asynchronous machines are replaced in the calculation by their impedances (positive, negative, and zero sequence impedances). All line capacitances and the parallel admittances of non-rotating loads, except those of the zero-sequence system, are neglected.

Short Circuit Calculations

Short circuit current near a generator

Short Circuit Calculations

Short Circuit Calculations

Standardised short-circuit voltage for public distribution system

Conductor resistivity values

Short Circuit Calculations

Generator reactance values

Short Circuit Calculations

Short Circuit Calculations

EXAMPLE: