DG Set Rating

Dear Mr Bharat,

When a D.G set has been designed for 500 KVA obviously it has to give an out put power of 400 KW at 0.8 pf provided its run in ideal condition of supply of adequate amount of cool air, complete exhaust of the hot air emanated both from the engine as well as the alternator. The D.G set has to run at the designed limits of ambient temperature, humidity and altitude. The engine is water cooled where as the alternator is air cooled. Therefore cool air has to be force blown to the fan side making the exhaust of hot air from the winding completely out such that the generator room temperature is same as if the generator is standstill. This condition is not possible practically. Therefore we take the safety aspect & derate it to 80% of the KVA rating.

Thank you for your guidance. As far as temperature is concern, there is a temperature rise test in standards and it has to be taken care that the values of this test are within the limits. And normally, in the manual of DG sets, only a 'well ventilated' room to be provided and forced ventilation does not of their demand. As you say, it seems that 80% loading on DG is only a safety aspect otherwise it can be loaded fully upto it's rating as per the prime rating calculations.

Most generator manufacturers provide guidelines for derating their equipment based on use as standby/prime supply, higher power factor, ambient temperature, altitude, etc. Usually the owner/operator manual includes such guidelines in the initial sections of the manual. There is some variation between recommendations of different manufacturers, so it is best to consult the manufacturer directly. I have encountered machines from some lesser known manufacturers with totally inadequate literature, but I do not expect this to be a problem in the 500 kVA range...Start with the manual, or consult directly with the manufacturer.