pre shortcircuit
TRANSCRIPT
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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 meansrequired 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.
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Short Circuit Calculations
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Short Circuit Calculations
The protection device must clear
the short-circuit current within atime that is compatible with the
thermal stresses that can be
withstood by the protected cable.
Permissible limit under steady-state
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Short Circuit Calculations
Circuit protection using a Circuit Breaker
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Short Circuit Calculations
Short-Circuit Types
80%
15%5%
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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 forcesii. 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
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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.
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Short Circuit Calculations
Short circuit current near a generator
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Short Circuit Calculations
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Short Circuit Calculations
Standardised short-circuit voltage for public distribution system
Conductor resistivity values
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Short Circuit Calculations
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Short Circuit Calculations
EXAMPLE: