Managing the power demand of leading power factor blade servers

As IT managers seek to extract ever more computing power from a limited floor space, they incorporate newer computer technologies, with powerful blade servers becoming increasingly popular. However this is pushing the load power factor from lagging towards unity and often, even into leading.

This creates a problem for legacy UPS installations. These mostly use pulse width modulated (PWM) technology which provides maximum power into lagging power factors.

As the load type changes from lagging to leading, these UPSs quickly approach their kW power limits or even go into overload. Replacing the existing legacy UPS with a higher output unit is expensive and usually disruptive, with changes in the power distribution and installation. Adding a second such UPS incurs even higher cost plus extra demand on premium floor space.

Instead, the best approach uses a modern transformerless UPS with adaptive inverter switching. This derates far less severely than a legacy double conversion system as the power factor moves to leading. For example a 300kVA transformerless UPS could deliver 232 kW into a 0.9 leading power factor load whereas a legacy double-conversion type would supply just 182 kW. In other words, the transformerless UPS has derated by just 3% with respect to its rated power at a power factor of 0.8 lagging, while the legacy system has derated by 24%.

This improved efficiency means that a lower power system can be specified by choosing transformerless technology. Eliminating the transformer makes the UPS considerably smaller and lighter as well. A transformerless UPS also imposes a reduced total harmonic current distortion on its input (THDi)

These factors create direct and indirect cost savings. Firstly, the transformerless UPS’s greater efficiency creates a smaller load on the incoming mains power and standby generator. The reduced THDi further cuts the demand on generator capacity. The higher efficiency UPS also generates less heat, saving air conditioning costs. The reduced size lowers installation costs, disruption to existing power systems and demand for data centre floor space.

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