Power Electronics and Pulsed Power Research
The Group is proud to be accommodated in a large laboratory suite (comprising of three dedicated laboratories) that has recently seen a £1M refurbishment. There are a wide range of laboratory resources and specialist equipment including; a 400 kV test facility, a 3000 amp pulsed power generator, a large battery charge/discharge unit designed for fork-lift truck and submarine batteries, a large scale wind turbine simulator, a 40 kW solar panel installation, a range of electrical machine test facilities up to 100 kW, including a high speed turbo-generator, magnetic and electrostatic modelling and CAD facilities. In support of the battery recharging work, the Group has access to our in-house electron-microscopy suite. For further information relating to power electronics, machines and drives, please contact Dr. Paul Lefley.
High Powered and Ultra-fast Battery Recharging
The purpose of this research is to investigate and develop new ways of rapidly recharging batteries by taking into account the electrochemistry and physical effects of rapid recharging on the battery. The Group has had considerable success in developing a new ultra-fast charging technique using pulsed power electronics to enable batteries to be rapidly charged without overcharging, excessive gassing, or overheating. A real-time electronic charge management system prevents overcharging by controlling the rate at which the charge pulses are injected. This system substantially reduces gassing of an aqueous electrolyte battery until a level of almost 90% state of charge is reached. A 24 kW charger (See picture) has been developed to rapidly recharge large battery installations of between 24 to 72 volts at 800 Ahr capacities in under an hour. This work has been a useful foundation for on-going research into the ultra-rapid recharging of lithium batteries used in various transport applications.
The Electrical Power Group has a long history in electrostatic precipitation, primarily for cleaning up flue gas emissions from power stations. This research work includes the improvement in the design of the precipitator and the high voltage electrodes for greater dust collection efficiency by using electrostatic field modelling. The other area of research where the Department has made a significant impact in this industry is in the creation of a new high voltage, high power, power supply.
The conventional 50/60 Hz transformer/rectifier power supply design became the standard issue due to a lack of topological progress since their initial introduction by Frederick Cottrell in the early 1900s. However, this kind of unit has severe drawbacks as far as operation is concerned, including, but not limited to:
- Low quality input currents and low power factor
- Sluggish operating characteristics
- Low power supply efficiency
- Large size, weight and civil engineering costs associated with the oil insulated transformer.
The research work within the Group ultimately provided a step change in technology for the industry, by implementing a modern switched mode power supply topology to achieve a major improvement in the efficiency and effectiveness of electrostatic precipitators. The research prototype overcame design difficulties encountered with combining high frequency, high voltage and high power in a single power supply. It also served as a research tool to identify the usefulness of applying a variable waveform – including the addition of microsecond high voltage pulses – to the electrostatic precipitator for improved dust collection efficiency. In order to fulfil this technological achievement, a very special high voltage power transformer needed to be designed; one that could withstand very high voltages (up to 100 kV), operate at high frequencies (20 kHz) and at high power (up to 70kVA), had low leakage reactance for fast response times, was capable of withstanding short circuits, and was greater than 90% efficient.
The high frequency approach has numerous advantages over the traditional mains frequency rectification equipment in that, with a switching period of 100 microseconds compared with 8 or 10 milliseconds the output waveform approaches pure DC and the recovery time following a flashover is considerably reduced. This enables the precipitator to operate at a much higher (average) voltage and hence performance, since the dust collection efficiency of any precipitator is proportional to the square of the operating voltage. In addition, electrical safety is increased because when the precipitators arc and spark the arc is extinguished much more rapidly than with the conventional 50/60 Hz transformer/rectifier unit.
As part of the fundamental research work, methods of controlling the high voltage field in order to increase the dust collection efficiency were implemented. This included the injection of variable shaped voltage wave forms onto the high voltage field, not possible with a standard 50 Hz unit. Leicester’s prototype 70 kVA switched mode power supply was built, installed, and trialled at Didcot (A) power station in 1998. Since this pioneering research work at Leicester, switch mode power supply units are being installed all over the world today.