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Harmonics are often overlooked when a generator is supporting a UPS system.
Harmonics that are generated by the UPS can be completely different to those generated by the load that the UPS system is supporting. This is particularly the case when the UPS system is a true double conversion unit.
As a result it is common for the generator to overload if the load that the UPS system is supporting transfers to its reserve supply (the generator) during a utility mains failure situation.
How and why does this happen?
The complex current and voltage waveforms that are generated by the load harmonics can cause the control systems on some generator installations to detect this situation as a fault.
For example; harmonics can cause more crossover points within the fundamental waveform which the generator control can see as a generator frequency error and try and alter the generator speed to compensate. If the error is outside of the generator specification it will shut down causing loss of load.
Often with standby power installations the actual problem of the load seen by the generator is not taken into account within sizing calculations, the generator is only matched with the UPS at the time of installation.
This oversight might not be immediately be a problem, but with time as more equipment is added to the UPS load the generator can fail if the UPS system is taken out of circuit.
The situations mentioned above become a problem when the generator system is approaching its maximum load rating.
This situation can be monitored by carrying out a regular service and maintenance of the generator and UPS, every six to twelve months depending upon the growth of load, controlled mains failure tests. These force the UPS system to carry its full load on battery and then for the generator to take over when it has come up to speed. By doing this test as a controlled exercise, first, the site personnel are prepared for problems in case of failure and second, in the event of a utility mains failure there is confidence that the standby generator will support the load.
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