UPS DC intermediate circuits without AC line separation

In the larger present-day UPS (uninterruptible power supply) systems, the DC voltage lies in the region of 400 V. In such cases, transformation of mains AC voltages of 3×380 V to 3×440 V will not be necessary. DC rectification equipment of such UPS devices is therefore designed without any rectifier transformers. The advantages presented by this concept are smaller dimensions and weights, smaller losses, and lower costs. Safety aspects in such devices require the same safety precautions as do normal AC systems. Special attention must be paid to all possible faults in the DC circuit. The possibility of short-circuit to earth must be countered by additional monitoring and shut-down facilities. In the event of earthing of the neutral line in the AC system, the battery voltage will have superimposed on it an alternating voltage which will prohibit the earthing of the battery mid-point. This superimposed AC potential will change along with the phase control angle imposed on the rectifier set. Because of these superimposed potentials, alternating voltages may be transmitted capacitively to metal battery-stand frames, which may cause noticeable electrification on being touched. Typical values and corrective measures are described and discussed