Industrial Electrical and Power Engineering
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.
Research in the "High Voltage Lab" at Leicester is on electrical insulation and dielectrics - materials that respond (either in a useful or detrimental way) to electric fields. The main applications are in high voltage or high field systems, such as polymeric power cables and transformer bushings, capacitors and printed circuit boards.
Research in the Electronics, Machines, Propulsion Systems and Renewable Energy Labs is related to many aspects of practical electronics, including the generation and utilisation of electrical power. The Group has an established reputation in electrical and hybrid propulsion, including the development of electric racing engines. A common factor in the research is the increasing use of power electronics and micro-controllers for the efficient conversion of electrical power, such as in spacecraft power supplies. Extensive facilities are available for simulation, design, prototyping and experimental verification of new circuits, machines and systems. Practical electronics expertise is also a key area of industry engagement and consultancy.
Following a £1M refurbishment, the Group is now accommodated in a large laboratory suite, comprising three dedicated laboratories. There are a wide range of laboratory resources and specialist equipment including:
- High voltage testing equipment up to 400kV DC, 80kV AC 50Hz
- Broadband dielectric spectroscopy (mHz to MHz)
- Space charge measurement using pulsed electroacoustic (PEA) techniques
- High sensitivty electrical treeing and electroluminescence equipment with application in electrical treeing detection and partial discharge interpretation
- Equipment for the development of electronically controlled drives and generators
- 50 kVAp PV array
- Equipment for the development embedded generation systems, including: photovoltaic, wind power and variable speed generation and a large scale wind turbine simulator
- Electrostatic and magnetodynamic modelling and CAD facilities
- Large and small electrical machines and drives test facility with a 200kVA 3 phase supply
- 3000 amp pulsed power generator
- large battery charge/discharge unit designed for fork-lift truck and submarine batteries.
The group has access to a wide range of other facilities around the University including those of the Advanced Microscopy Centre, and engine and propulsion test facilities.
Acting Head of Group: Dr Stephen Dodd
Honorary Visiting Professor Professor John Fothergill
Teaching Fellow: Dr Nikola Chalashkanov
Teaching Fellow: Dr Robert Entwistle
Senior Experimental Officer: Andy Willby
Technician: Luigi Alessandro
Currently we have 8 PhD students.
Staff in the High Voltage lab include:
Areas of work include:
- Dielectric Spectroscopy
- Advances in Understanding Electrical Breakdown in HV Insulation Systems
- Theoretical understanding of electrical ageing leading to diagnosis and prognosis
- Space Charge Measurement
Dielectric spectroscopy is a powerful technique, which we use:
We use a Solartron 1296A Dielectric Interface coupled to a frequency response analyser, and we have also developed high-voltage time domain techniques and ultra-sensitive bridge techniques for low-loss materials such as cross-linked polyethylene.
Measurements can be made:
At Leicester we pioneered the first experimental work (reported in Nanotechnology) showing that insulation materials (e.g. polyethylene) filled with nanometric (e.g. 50nm diameter silica) particles have the potential to enhance dielectric properties properties significantly.
The graph shows how space charge accumulated in epoxy filled with both "micro-particles" and nano-particles" led to increased fields under high voltage DC conditions. The field, which may lead to enhanced ageing and breakdown, was much higher for conventional materials filled with micro-particles than those with nano-particles.
Advances in Understanding Electrical Breakdown in HV Insulation Systems:
We have led the way in understanding electrical degradation and breakdown of polymeric and composite insulators. For example, work, supported by the National Grid Company, has led to the development of novel physical models and computer simulations of electrical tree growth, an important electrical breakdown mechanism in polymeric insulation.
The high voltage laboratory activities include:
Leicester has played a prominent role in the development of quantitative physical models for electrical ageing. This can serve as a basis for the identification of ageing markers and the prediction of service life.
Models have been developed for
Under high field conditions, particularly under DC, charge may be accumulate inside an insulating material. This can be detrimental as it increases and distorts the electric field, which may lead to premature ageing and failure.
The laboratory collaborated with Dr John Alison and Prof. Robert Hill at King's College London to develop a range of pusled-electro-acoustic measurement systems for measuring space charge distributions in films and slabs of solid insulation.
The apparatus has a resolution of around 1pC and 25 microns. Fast measurements can be made with one of the systems, allowing, for example, the observation of the movement of charge with time.
The graph shows charge moving in "packets" through a 0.15mm film cross-linked polyethylene under a field of 120kV/mm.
A common factor in the research is the increasing use of power electronics and micro-controllers for the efficient conversion of electrical power. Extensive facilities are available for simulation, design, prototyping and experimental verification of new circuits, machines and systems.
Members of staff in this area include:
There are currently 8 post-graduate research students.
In addition to fundamental research the activities in this area have strong links with industries.
Specific areas of current and recent research are:
While new topologies and designs of electrical machines and power converters are being developed continuously as part of this research it is equally important to consider the use of existing products where possible. To successfully integrate and control a variety of generating and storage equipment in a system it is important to understand intimately the characteristics of each system element as well as the interaction between them. For that reason commercial products are often operated under operating conditions that may not been envisaged by the manufacturer. Using laboratory tests the suitability of such equipment has been established and, where necessary, adapted in collaboration with manufacturers.
- Integration of wind power in diesel networks
- Network interface for flywheel energy buffer
- Modular PV/grid interface with common DC busbar
- Control of stall-regulated variable speed wind turbines
- Advanced maximum power tracking of PV systems
- Integration of high-penetration PV generation in electrical distribution systems
- High-efficiency DC-DC converters for fuel cells and ultra-capacitors
- Operation and control of micro-grid systems
For information about potential areas of post-graduate and collaborative research please contact Dr Bleijs.
As part of the PV research two systems have been installed on University buildings. The first PV system, designed and part-funded under the EU UNIVERSOL project, is a 11.2 kWp research and demonstration system installed on an adjacent laboratory roof , consisting of 64 mono-crystalline modules, flexibly arranged into 7 sub-arrays of different sizes and topologies. This system is extensively instrumented to monitor meteorological and electrical variables, using a dedicated data acquisition and data storage system. Click here to download further information. The second PV system of 38.2 kWp is integrated in the new David Wilson Library building and uses 3 different silicon cell technologies (mono- and poly-crystalline and amorphous thin film) for different locations. This system is also being monitored to analyse the performance of each technology.
In addition to the operational PV systems the research group has available a wide range of facilities and equipment for academic and industrial research, such as:
- Wind Turbine Simulator (software-driven hardware-in-the-loop system)
- Module-size Halogen Solar Simulator
- Programmable PV Array Simulator (including partial shading)
- Motor/generator sets of various power ratings
- DC and AC variable speed drives and inverters
- Ballard Fuel Cell
- Battery Test Cell (including multiple charge/discharge unit)
- Instrumented Diesel Generator Set
- Ultra-capacitor Bank
- Programmable Power Supplies and Load Banks
- DC and AC machines (cage and slip-ring induction, PM and wound-field synchronous)
- Power Analysers
In remote locations such as rural areas and island communities the cost of electricity supplied from diesel generators (DG) is often very high due to the costs of fuel transportation. In many cases such locations have an excellent wind regime, making the use of wind turbine generators (WTG) an attractive option. Due to the variability of the wind, parallel operation of a DG and a WTG may affect the voltage and frequency stability of the local grid. Studies have been carried to identify equipment and control methods that can assure stable and economic operation of wind-diesel systems. It has also been shown that small amounts of energy storage can eliminate or reduce the need for frequent start-stop operation of the diesel generator with the associated fuel and wear penalties.
A flywheel is eminently suited as an energy buffer to filter the variable output power of wind turbines, connected to a weak grid or isolated diesel generators. To exploit the energy storage capacity of a flywheel a bi-directional variable-speed interface between the grid and the flywheel is required. Continuously variable transmissions (CVT) and power-electronic variable speed drives (VSD) have been investigated, including the design of suitable controllers for power smoothing.
Both current-source and voltage source inverters have been tested for this application; using back-to-back voltage source inverters with vector control and dedicated power controllers, fast and accurate control of active and reactive power flow has been achieved.
When medium size PV systems are integrated in buildings the sub-arrays often experience different levels of insolation due to their orientation and shading from nearby objects, resulting in sub-optimal performance and loss of energy yield. A novel PV/grid interface has been developed, consisting of modular current-fed DC-DC converters, a common high-voltage DC link and an industrial 3-phase inverter with adjustable power factor and grid supervision. Each converter contains a fast and accurate maximum power point (MPP) tracker and the system control includes power feedforward to minimise DC bus voltage fluctuations. Excellent MPP tracking over a wide input voltage range has been demonstrated.
To make best use of the available energy wind turbines must operate at variable speed related to the prevailing wind speed. In wind speeds below the rated value the control must be aimed at extracting maximum energy, while above the rated wind speed the rated power or torque must be maintained. Because direct measurement of the wind, as seen by the turbine rotor, is not possible the control system must use other information to achieve the control objectives. Various algorithms have been tested through simulation and are being implemented on a hardware-in-the-loop wind turbine simulator consisting of software-controlled DC motor drive and an industrial bi-directional AC cage generator drive.
Maximum power tracking algorithms for PV systems may fail to operate correctly under rapidly changing insolation conditions due to the array capacitance. Under partial shading conditions where multiple maxima are present these algorithms are often also unable to find the global maximum. Advanced algorithms have been developed that are able to recognise such situations and find the true maximum power point. These algorithms have been implemented in a fast DSP controller and tests using our solar simulator have confirmed correct operation under adverse conditions.
As the cost of PV cells and modules continues to fall the penetration of PV generators from domestic and industrial installations will increase, in particular in regions of the world with high solar radiation. The integration of a large numbers of PV generators in the local distribution system may lead to operational problems related to voltage level, stability and power quality. Investigations are undertaken to establish the relation between network configuration, penetration level and load pattern on these issues, based on models of existing local networks, and to find appropriate measures to eliminate or reduce these effects.
Fuel cells produce a DC voltage that depends on the load current and cannot be used directly to supply a DC or AC load. For application in automotive propulsion systems or in micro-grids in combination with other generators and loads it is necessary to convert the output voltage to a constant (and preferably higher) DC voltage level. In view of the high costs of fuel cells and the hydrogen fuel it is essential that this conversion process is highly efficient. Similar requirements apply to the use of ultra-capacitors, with the additional need for very fast response. New power-electronic circuit topologies and control methods are under investigation using a combination of detailed modelling, simulation, hardware construction and micro-processor control implementation.
Micro-grid systems consist of a number of generators (both dispatchable such as diesel generators and fuel cells, and non-dispatchable such as wind generators and PV generators) and loads (including deferrable), together with storage devices (batteries, flywheels and ultra-capacitors), that operate in low-voltage AC or DC networks. Micro-grids can be operated interconnected to the power grid or in islanded mode. The key function of micro-grids is to ensure stable operation under fault conditions and network disturbances. While the micro-grid offers technical and economic benefits the increased penetration of distributed generators may lead to technical challenges such as over or under voltages at the point of connection, protection malfunction, increase in short circuit levels and power quality problems. The power flows inside the micro-grid and through the interconnection to the power grid must be accurately controlled to avoid frequency and voltage control problems. The aim of this research is to investigate the role that the use of power-electronic converters and micro-controllers can play in ensuring stable operation under all conditions.
- A selection of relevant publications is shown below.
- J A M Bleijs: 'Wind turbine dynamic response – difference between connection to large utility network and isolated diesel micro-grid'. IET Renewable Power Generation, Vol.1, Issue 2, June 2007, pp.95-106
- J A M Bleijs, F Hardan & A J Ruddell: ‘Flywheel Energy Storage System for Wind Power Smoothing in Weak and Autonomous Networks’. Proc ‘Wind Power for the 21st Century’ Conference, Kassel, Germany, Sept 2000, pp.270-273
- A G Dutton, J A M Bleijs, H Dienhart, M Falchetta, W Hug, D Prischich & A J Ruddell: ‘Experience in the Design, Sizing, Economics, and Implementation of Autonomous Wind-Powered Hydrogen Production Systems’. International Journal of Hydrogen Energy, 25, no.8, May 2000, pp.705-722
- J A M Bleijs & J A Gow: ‘Fast Maximum Power Point Control of Current-Fed DC-DC Converter for Photovoltaic Arrays’. IEE Electronics Letters, 37, no.1, pp. 5-6, Jan. 2001
- J A M Bleijs & A L U Mayere: ‘PV Systems at the University of Leicester: Performance and Developments’. Proceedings 22nd European Photovoltaic Conference, Milan, Sept. 2007, pp. 3089-3093
- Y Liu & J A M Bleijs: ‘Intelligent MPPT Control of PV Panels using a DSP Based DC/DC Converter, tested with a Flexible Solar Emulator’. Proceedings 22nd European Photovoltaic Conference, Milan, Sept. 2007, pp. 3059-3063
- J A M Bleijs: ‘Continuous Conduction Mode Operation of the Three-Phase Diode Bridge Rectifier with Constant Load Voltage’. IEE Proc on Electric Power Applications, Vol.152, No. 2, March 2005, pp.359-68
- O A Ahmed & J A M Bleijs: ‘High-efficiency DC-DC Converter for Fuel Cell Applications:Performance and Dynamic Modelling', Proceedings IEEE ECCE 2009, San Jose, September 2009, 8p.
- For further information on solar energy:
- For wind power:
Director - Dr. Paul Lefley
Recent work in the field of electrical machines has lead to the development of a new type of electric motor that is energy efficient, electronically controlled and of a low cost to manufacture, with the intention to make OEM manufacturers aware of an alternative to the world’s reliance on inefficient single phase induction motors. These energy efficient motors are based on new patented designs and are essentially permanent magnet based brushless DC motors with very low electronic component count. The new designs have been developed for low torque ripple and quiet operation. The new motors are attracting growing commercial interest worldwide.
Traditionally Dr. Lefley has developed electric motors for electric and hybrid vehicles. However, recently has also developed new bespoke motor/generator technologies for aerospace applications where high power densities at very high speeds are essential. Such machines are designed for directly coupled gas-turbine applications, where a combined high speed starter coupled with a generator capability is required. This is a new field and is of considerable interest not only in aerospace but in military applications as well. Further work on energy efficient motors for industrial fan and pump applications is also on-going and is currently being spun-out through Synchropulse Ltd (see below). Dr. Lefley has developed in conjunction with colleagues from the Electrical and Electronic Power Engineering Group a complete fuel cell based electric vehicle drive system using a Nexa fuel cell, ultra-capacitors for energy storage, and an energy efficient permanent magnet brushless DC motor.
Shell Springboard Winners, SynchroPulse - Lord Wade, Dr Paul Lefley and Andrew Hogbin, SynchroPulse Ltd., and James Smith, Shell UK Chairman.
Power Electronics and Pulsed Power
Dr. Lefley has had considerable experience in development of power electronic based systems including recently, the application of pulsed power both at very high currents and high voltages.
The work on ultra-rapid battery recharging stemmed from fundamental work on how a rechargeable battery may accept charge at a high rate without causing deleterious effects to the battery such as overheating, gassing, active material shedding, etc. The purpose of this research was 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 lead-acid 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 until a level of almost 90% state of charge is reached. A 24 kW charger has been developed to rapidly recharge large battery installations of between 24 to 72 volts at 800 Ahr capacities in under an hour.
The very high current power electronics (up to 2,500 amps) is applicable to all large scale energy storage media, and interested parties are encouraged to contact Dr. Lefley. Much of the ultra-rapid battery recharging work is currently seeing commercial interest. The work on electrostatic precipitation was an application of modern high frequency switched mode power electronics to replace the traditional low frequency transformer/rectifier set, but also to improve the dust collection efficiency in the precipitator by applying a controllable pulsed waveform from the new power supply. This system was implemented in a pilot project at Didcot B power station in Oxfordshire, UK.
• Optimal Design of a Novel Single Phase PM BLDC Motor Using Genetic Algorithm, Lefley P, Ahmed S, EPE-PEMC 2012, Novi-Sad, September 2012.
• Cogging Torque Minimization in the Double Stator Cup Rotor Machine, Diryak E, Lefley P, 4th Symposium on Applied Electromagnetics SAEM'12, 3rd to 6th June 2012, Sopron Hungary.
• High Voltage, High Frequency Transformer Design, Lefley P, Devine P. Transformers Analysis Design and Measurement, CRC Press, Monograph Ch 21, ISBN 9781466508248.
• Synthesis and Analysis of a High-performance Low-cost Permanent Magnet Brushless DC Motor, Lefley P, International journal for Computation and Mathematics in Electrical and Electronic Engineering (COMPEL).
• Fault Detection of a Series Compensated Line during the Damping Process of Inter-area Mode of Oscillation, Lami F, Lefley P. IET DPSP 2012 - Protecting the Smart Grid. The 11th International Conference on Developments in Power System Protection. 23-26 April 2012.
• Rechargeable batteries – Part 4: Battery graveyard, Energize April 2012 Lefley P, Soge A, Starkey J
• Rechargeable batteries – Part 3: Lithium-ion batteries, Energize April 2012 Lefley P, Soge A, Starkey J
• Rechargeable Batteries Part 2: Nickel based batteries, Energize March 2012 Lefley P, Soge A, Starkey J
• Rechargeable Batteries – The Evolution and Beyond, Energize Jan/Feb 2012 Lefley P, Soge A, Starkey J
• A New Design of Low Cost Energy Efficient Single Phase Brushless DC Motor, Lefley P, Ahmed S, Journal of Electrical Research Review, ISSN 0033-2097, 2/2012.
• Static Characteristics of a Novel Low Cost Brushless DC Permanent Magnet Motor, Lefley P, Journal of Electrical Research Review, ISSN 0033-2097, R88 NR 1a/2012.
• A Novel Three-Phase Buck-Boost Power Quality Converter, Lefley P, Starkey J, Seventh Mako/CIGRE Conference October 2 – 4, 2011.
• Synthesis and Analysis of a High Performance Low-Cost Permanent Magnet Brushless DC Motor L. Petkovska, P. Lefley, G. Cvetkovski XV International Symposium on Electromagnetic Fields in Mechatronics, Electrical and Electronic Engineering – ISEF'2011 Funchal, Madeira September 2011.
• Optimisation of the Design Parameters of an Asymmetric Brushless DC Motor for Cogging Torque Minimisation, Lefley P, Petkovska L, Cvetkovski G, European Power Electronics Conference EPE 2011, Birmingham, September 2011.
• From Dynamic Modelling to Experimentation of an Induction Motor Powered by a Doubly-Fed Induction Generator by Passivity based Control, Electrical Machines and Drives, InTech, Monograph Ch 7, ISBN 978-953-307-548-8.
• Design and Control of the Brushless Doubly Fed Twin Induction Generator (BDFTIG) - Part 2, Bensadeq A, Lefley P, IEEE 14th International Power Electronics and Motion Control Conference 6th to 8th September 2010, (EPE-PEMC 2010), Ohrid.
• Finite Element Analysis of a Novel Single Phase Permanent Magnet Brushless DC Motor, Lefley P, Ahmed S, IEEE 14th International Power Electronics and Motion Control Conference 6th to 8th September 2010, (EPE-PEMC 2010), Ohrid.
• Study of the Impact of Asymmetrical Stator Pole Arc on the Cogging Torque for Single Phase Permanent Magnet BLDC Motor, Ahmed S, Lefley P, IEEE International Conference on Electric Power and Energy Conversion Systems 10th to 12th November 2009, (EPECS09), Al Sharjah.
• Design and Control of the Brushless Doubly Fed Twin Induction Generator (BDFTIG), Bensadeq A, Lefley P, IEEE 11th Spanish Portuguese Conference on Electrical Engineering 1st to 4th July 2009, (11CHLIE), Zaragoza.
• Development of a Single Phase PM BLDC Motor from a Novel Generic Model, Ahmed S, Lefley P, IEEE 11th Spanish Portuguese Conference on Electrical Engineering 1st to 4th July 2009, (11CHLIE), Zaragoza.
• Energy Efficiency Advantages of a Brushless DC Motor for a Variable Speed Compressor. Lefley P , Currington I, International Rotating Equipment Conference, Dusseldorf, 27th to 29th October 2008.
• Transformers in Practice, Lopez-Fernandez, Turowski, Kazmierski, Lesniewska, Ertan, Lefley, et al, monograph ISBN 978-84-609-9515-9.
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