Practical guidance to calculation of prospective short-circuit currents on T&D systems

Faults In T&D Systems

When a fault occurs on the transmission or distribution system, the current which flows into the fault will be derived from a combination of three sources: (1) Major generating stations via the T&D networks, (2) Embedded generators connected to the local network and (3) Conversion of the mechanical inertia of rotating plant equipment.

Calculation Of Fault Levels

Fault Level Calculation Fundamentals

The management of prospective fault level conditions in terms of circuit breaker make and break duties requires an assessment of the fault current contributions from all potential sources.

The methodology described in IEC 60909 allows for the calculation of short circuit currents using sequence components.

The methodology makes certain assumptions about the nature of the fault for the purposes of defining the network impedance conditions.

1. For the duration of the short circuit there is no change in the type of short circuit involved, that is a three phase short circuit remains three phase and a line-to-earth short circuit remains line-to-earth fault for the duration of the fault.
2. For the duration of the short circuit, there are no other changes in the network.
3. The impedance of the transformers is referred to the tap-changer in nominal position. This is admissible because an impedance correction factor KT for network transformers is introduced.
4. Arc Resistances are not taken into account.
5. All line capacitances and shunt admittances and non-rotating loads, except those of the zero sequence system, are neglected.

Simple Fault Level Calculations

The method of calculating system fault levels are covered in detail in many text books and it is not the intention of this document to reproduce that material. However, some practical guidance may be of assistance.

The basis of fault level calculations can be either:

1. Ohms at a chosen Standard Voltage
2. Percentage impedance drop at a chosen standard MVA base
3. Per unit method (an adaptation of (b))

Many planners prefer option (b) and the most convenient and normal standard base is 100MVA.