Current contribution Transient fault

Calculation of fault current - rms symmetrical values Calculate the Sub-Transient symmetrical RMS Fault Current Contributions 11.6.1 Calculate the sub-transient peak fault current contributions... 

Tables 5.1-5.4 show the approximate resistance and reactance values in per-unit for two-pole and four-pole low voltage induction motors that are generally of the Design D type. Tables 5.5-5.8 show the approximate resistance and reactance values in per-unit for two-pole and four-pole high voltage induction motors that are of the reduced starting current type. In the absence of exact data from a manufacturer these data can be used for system studies such as starting motors, transient stability and fault current contribution. The data from a manufacturer should be used for calculations and system studies that are to be carried out during the detailed design phase of a project.

In some cases it is also necessary to consider the fault current contributed by motor consumers, particularly if large synchronous motors are fed from the same busbars as the main generators or main transformer infeeds, see Chapter 11. Induction motors contribute fault current during the sub-transient period and so extra allowance must be made when calculating the making duty. 

CALCULATE THE SUB-TRANSIENT SYMMETRICAL RMS FAULT CURRENT CONTRIBUTIONS... 

Calculate the Sub-Transient Peak Fault Current Contributions... 

The sub-transient impedance determines the initial decay, i.e. in the first cycle or so. Therefore the emfs E" and E, together with the reactances Xj and X j, need to be used for calculating the fault currents. In a similar way to induction motors, the synchronous motors will contribute to fault-making dnty reqnirements. However, they will also contribute towards the fault-breaking duty because of the transient effects. 

Significant contribution of sub-transient fault current from induction motor consumers. 

Some of the literature use transient notation, e.g. References 3, 22 and 28. Others use sub-transient notation particularly in relation to fault current contribution in power systems, e.g. Reference 24. 

In the above simple example some of the margin between 60 kA and 80 kA will be taken up by the sub-transient contributions from the motors. It can be noted at this point that if the transformer is subsequently increased in rating by the addition of forced air fans, then the fault current passed by the transformer will be unchanged. It is advisable to specify the rating of the transformer in its forced air-cooled mode of operation, if such cooling is considered likely to be needed in the future. This would ensure that the incoming circuit breakers and busbar normal rated currents would be correctly matched to the transformers. 

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