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The_Specialist gravatar image
RedR

I have consulted with a colleague since I am no expert, but he advises: I would use the motor kVA value rather than current for sizing generators, which summates to the 1035kVA value. The worst case condition for the generator on starting is an on-load value of 840kVA with a 195kVA motor starting. The impact on the generator will be dependent on the method of motor starting (direct-on-line, star–delta etc) which can vary the starting current and resulting voltage drop or dip expected at the generator – under worse cases the generator may fail to start. Usually this is analysed by the generator supplier via appropriate sizing software as a 195kVA motor full load current will be around 370A, which could be multiplied by 8 to reflect starting at direct-on-line ie. nearly 3000Amp.

I think an increased safety margin would be useful and therefore suggest a standby generator size of 2000kVA which is double the 1035kVA load. A 2000kVA unit is usually readily available although this needs checking for the local location, if not then two 1000kVA units may be considered and even preferable if the eight motor loads can be spilt into two separate load centres. In this case the maximum use of the installed capacity is made if the largest motors are started first.

An additional factor is the loading on the diesel engine. If the genset is too large for the running load, there can be serious maintenance problems due to engine 'slobber' (!). I think this percentage value is below 30% loaded for continuous use you can suffer these problems. Ideally a generator should operate at 70% of its rated value.

To minimise the effects of running the engine on low load, it is beneficial to run the engine up to load using an external load for one to two hours, at least once a month. Another measure may be for engines where slightly load or extended idle periods of engine operation are mandatory, that the objectional effects of engine slobber can be avoided by loading the engine to at least 30% load for approximately 10 minutes every 4 hours.