Facing challenges in the war on power outages

Facing challenges in the war on power outages

By Jack Ward, MD of Powermode

In my previous blog I emphasised the need for generators and UPS (Uninterruptible Power Supply) systems to join forces in the war on power outages by being designed holistically – as a unified solution.

I said the key to success is compatibility.

One of the challenges facing system designers in their quest for compatibility between UPSs and generators is ’step loading’. When a generator turns on and the switch connecting it to the UPS closes, the immediate application of the load to the generator can cause sudden swings in frequency and voltage.

This can be avoided if the UPS has a ‘walk-in’ feature. This means the UPS rectifier has some means of controlling power flow, such as thyristors, so the power draw of the UPS can be applied gradually to the generator over a given period, usually 10-20 seconds. This prevents the protected load from varying.

Another problem is ‘voltage rise’. This occurs when a generator is sized too closely to that of the UPS and there is little or no other load on the generator. When a UPS is connected to the generator with a transfer switch, the UPS’ charger is turned off so that it may begin the power walk-in routine.

If the input filter is the only load on the generator, it may provide excess excitation energy. Most exciter controls have no way to deal with this excess energy, so the voltage wanders up without control to about 120%, limited only by some fundamental generator design constraint - usually magnetic saturation of the generator iron.

Most generator suppliers have preloads that can be attached to counter this effect. A UPS that disconnects its filter when its charger is off avoids this problem altogether. 

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