25 Oct TECHNICAL Q&A: RECTIFIER TECHNOLOGY
What is the purpose of a rectifier within a UPS system, and which technology would you recommend?
Our technical services manager, Jason Yates, has the answer:
A: The primary purpose of a rectifier is to provide the UPS with a clean and stable source of direct current (DC). When a UPS is operational, there are two distinct areas that require DC – the inverter, which is continuously supplying the load, and the battery charger, which is maintaining the charge status of the connected batteries.
The rectifier operates by converting the alternating current (AC) from the incoming mains supply into the DC as required. As with any electrical device connected to the mains supply, a UPS will generate current harmonics (THDi) and have an altering input power factor (PF). If not dealt with, a UPS could potentially cause severe electrical disturbances to the local electrical system.
The primary goal of a rectifier is to ensure that the harmonic currents remain low (e.g. less than 5% THDi at full load) and that the input power factor remains high (e.g. 1 or unity at full load). If these two conditions can be achieved, it will ensure that the UPS is operating effectively and efficiently.
The choice of rectifier technology employed in a UPS will dramatically affect the current harmonics produced and the operating power factor. The most common forms of technology are the 6-pulse, 12-pulse and Insulated Gate Bipolar Transistor (IGBT) style.
Although the 6 and 12-pulse versions are considered somewhat “old school”, there is still a market for their use within specific fields and applications. Typically, a 6-pulse will produce current harmonics in excess of 30% with an input power factor of around 0.6 to 0.8, whereas a 12-pulse will produce in excess of 10% with a power factor of around 0.8 to 0.9. That said, the 12-pulse provides a far superior performance, but it is still far from perfect.
The preferred solution is to use the IGBT style rectifier to achieve the highest level of performance. During typical operation the IGBT will be producing current harmonics no greater than 3% and have an input power factor of 0.99 (practically unity).
The IGBT rectifier provides a better performance due to one main factor: both the 6 and 12-pulse technologies operate using a power component called a Thyristor. For many years, this component has proven itself to be very rugged, high powered and reliable. However,
the Thyristor has limited controllability, so it is not possible to provide a fine level of detailed operation as required by the rectification process.
In recent years, the IGBT has become faster, more powerful, stable and cost effective. These improvements have meant that the IGBT has become the perfect component to replace the aging Thyristor, as they can be accurately controlled using the very latest digital signal processing techniques. Effectively, this means that the IGBT rectifier can provide the very highest levels of performance without any of the compromises.