An essential part of any UPS's role is to isolate and protect its critical load from any DC components. The output transformers within traditional 'transformer' based UPS are sometimes mistakenly believed to provide galvanic isolation. However this is not so, because a static bypass switch failure could feed a mains generated dc component directly to the load. There are also two other potential DC component sources within any UPS configuration - the battery and the internal DC circuit. Therefore both traditional UPS and modern transformerless designs such as PowerWAVE UPS must incorporate other protection measures.
Galvanic isolation of batteries is very rarely considered necessary or included nowadays, since the advent of maintenance-free lead-acid or nickel-cadmium batteries.
A UPS inverter IGBT failure can impose an output DC component. A traditional UPS's transformer would successfully provide load isolation, whereas a transformerless UPS with no inverter output transformer has no natural isolation for this condition. Instead, the transformerless UPS uses hardware and software regulation and control to prevent DC components from any source reaching the load. With advanced PowerWAVE UPSs this comprises a fully-redundant EDCP (electronic DC protection) system, which reduces the probability of DC component propagation to practically zero.
The PowerWAVE UPS EDCP system comprises three parts. Firstly, redundant DC-component regulation continuously detects and regulates any DC component to within ±10mV, compared with a normal mains supply tolerance of ±300mV. Secondly, if the DC component exceeds 4V the DC-component control circuit will automatically and instantaneously transfer the load to bypass. Thirdly, if an inverter fuse blows and generates a DC component, the EDCP system automatically removes it by blowing the remaining fuse. An additional EDCP system can protect the bypass circuit, transferring the load to the inverter within 2 to 5 ms if a bypass SCR fails.
Because the EDCP system operates at all times, with redundant detection, regulation and control circuits, PowerWAVE UPS systems are very safe and secure. Although tens of thousands of EDCP protected three-phase units are currently operating, none has ever passed a DC component through to their load.
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