IT Service Management and Systems Audit
Which of the following describes a double-conversion (online) UPS (uninterruptible power supply) appropriately?
- AEven in normal operation it converts the utility power to DC and then back to AC with an inverter before supplying it, so there is no momentary interruption from switching during a power outage, and it is less affected by voltage and frequency fluctuations
- BIn normal operation it supplies the utility power directly and switches to supply from the inverter after detecting an outage, so a brief interruption can occur at the moment of switching
- CAs long as fuel keeps being supplied, it can continue to supply power no matter how many days an outage lasts
Show answer and explanation
Answer: A. Even in normal operation it converts the utility power to DC and then back to AC with an inverter before supplying it, so there is no momentary interruption from switching during a power outage, and it is less affected by voltage and frequency fluctuations
A double-conversion (online) UPS converts utility power to DC with a rectifier and, while charging its battery, always supplies stable AC produced by its inverter. Even during an outage, the inverter simply continues to be fed from the battery, so there is no momentary interruption from switching, and disturbances in voltage or frequency are hardly passed on to the equipment; for this reason it is widely used for servers. On the other hand, because it is always converting power, its losses are larger than those of a standby (offline) UPS. Long outages are covered by a generator that starts while the UPS is supplying power.
- A. Correct. Power always passes through the inverter, so there is no interruption during an outage.
- B. This describes a standby (offline) UPS.
- C. This describes a generator. The time a UPS can supply power is limited by the capacity of its battery.
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