Fault current Contribution from induction motors

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. 

Whichever method is used it is not usually necessary to include the contribution of fault current from induction motors, because such current will have decayed to almost zero at the fault clearance time. If there are large motors connected to the main switchboard then their contribution will be similar to a generator and should be included, see sub-section 7.2.7 and Reference 3 therein, and sub-section 11.8.5. 

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

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