To achieve the desired output power requirement, Uninterruptible Power Supplies (UPS) have relied on low voltage/high current capacity switching devices such as transistors and early MOSFETs and IGBTs. These devices were also relatively low speed when used in switching applications, and the use of transformers ensured that the output voltage was of a value suitable for the marketplace and also provided current limiting to prevent the destruction of the devices.
With the rapid development in the design and manufacture of the IGBT switching device used in all uninterruptible power supplies, the size of UPS system that can be reliably designed without an output transformer has increased over recent years. This has also been in part due to the increase of switching speeds, which means that the PWM carrier frequency has increased from typically 2kHz for the early switching devices up to in excess of 20kHz.
Note: – In order to produce a controlled sinewave output from the DC bus created by the rectifier output / battery the IGBT has to switch on and off numerous times at varying time periods to create the sinewave. For example, starting from the zero point of the waveform, the next time the IGBT switches on it will be for a very short duration, the following pulse will be on for longer etc. until the maximum pulse width is achieved at the peak of the sinewave. The reverse action will now occur with the pulses getting shorter in duration until the zero point is reached again.
The development of power switching devices used in uninterruptible power supplies has resulted in an increase in the output kVA of the UPS. Going back 5 or 6 years, the maximum capacity of Transformer-less UPS (as a single module) would have been 60kVA. Now it is in excess of 160kVA.
Without the need for an output transformer the footprint of the UPS has grown smaller, and in the instance of the smaller (up to 40kVA) UPS there is even enough have space to mount integral battery blocks to provide up to 10 minutes autonomy.
Another way manufacturers have maximised the output of Transformer-less UPS systems is to parallel multiple UPS modules in a rack. Therefore, the basic UPS module could be a 19 inch rack-mounted 25kVA module, but by the time 4 modules have been mounted in a rack we have either a 100kVA UPS or a 75kVA UPS with redundancy. These modules are also ‘Hot Swap’ modules, meaning that there is no load switching necessary to change a possible faulty module. One possible drawback with this method of achieving high UPS kVA capacities is that they can be forgotten about until extreme failure occurs. They still have the same uninterruptible power supply components, some of which have a recognised limited lifespan and although they are modular their weight means that they are not very engineer serviceable.
It is interesting to note that many UPS manufacturers offer an external isolation transformer as an optional extra!
If you want a ‘bullet proof’, ‘belt and braces’ power protection solution, we recommend your uninterruptible power supply choice should have a transformer to block any chance of ever getting a major spike into your critical systems. Technology may advance but for now we believe tried and tested is best!
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