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Published Papers

Co-Authored Paper Listings

Papers presented at a variety of conferences and published in peer reviewed journals.

List of Papers

Title: Modelling and control of a flying-capacitor inverter
Abstract: This paper presents the results of an investigation into the variation in the output voltage quality of a flying-capacitor inverter under staircase angle control. The trade-offs between capacitor size, power device voltage rating and output voltage quality are assessed through simulation. A general mathematical model for an N-level inverter is used for circuit operation analysis and its results are compared with those from a Simulink(R) circuit simulation. The computer model allows a thorough investigation of all possible switching pattern permutations that produce the desired sinusoidal output and maintain steady state capacitor voltage balancing. Results show that the total harmonic distortion in a sinusoidal synthesised output can be minimised by the correct selection of the switching mode sequence.
Authors: S.J. Watkins and L. Zhang
Source: Proceedings of EPE '01, 9th European Conference on Power Electronics and Applications, Graz, Austria, 27 - 29 August, 2001, CDROM Paper No. PP00439.
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Title: Influence of multilevel sinusoidal PWM schemes on the performance of a flying-capacitor inverter
Abstract: Multilevel inverters, including the flying-capacitor topology require more complex control due to the additional flexibility offered by the multiple output voltage levels. One such control method is based on the conventional sinusoidal pulse width modulation (SPWM) scheme and uses multiple triangular carriers. There are a number of SPWM variants that achieve the desired inverter control. This paper presents the results of an investigation into these different schemes and their influence on the power quality and efficiency of a flying-capacitor inverter.
Authors: S.J. Watkins and L. Zhang
Source: Proceedings of PEMD '02, the 1st International Conference on Power Electronics Machines and Drives (IEE Conference Publication No. 487), Bath, UK, 16 - 18 April, 2002, pp 92 - 97.
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Title: Multilevel asymmetric power converters for switched reluctance machines
Abstract: This paper presents a new family of multilevel asymmetric power converters which are suitable for unipolar current loads such as a switched reluctance motor drive. The fundamental operation of each topology is reviewed and some simulation results are presented showing the potential system performance improvements that can be realised by operating with intermediary voltages rather than the full dc-link.
Authors: S.J. Watkins, J. Corda and L. Zhang
Source: Proceedings of PEMD '02, the 1st International Conference on Power Electronics Machines and Drives (IEE Conference Publication No. 487), Bath, UK, 16 - 18 April, 2002, pp 195 - 200.
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Title: Analysis and control of a multi-level flying capacitor inverter
Abstract: The paper presents the results of a study into the optimal switch mode sequence for a multi-level flying capacitor inverter to synthesis a sinewave voltage -under staircase angle control. It is shown that a properly selected sequence will yield the voltage and loss balances in capacitor voltages and switching devices respectively and minimize the load voltage THZ) values. A general mathematical model for an N-level inverter is presented which can be conveniently applied for computer simulation of any operating mode.
Authors: L. Zhang, S. J. Watkins and W. Shepherd
Source: Proceedings of CIEP '02, 8th IEEE International Power Electronics Congress, Guadalajara, Mexico, 20 - 24 October, 2002, pp. 66 - 71.
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Title: An SR drive for a multi-megawatt high-speed application
Abstract: The paper gives an insight into the main features of an SR drive system from the perspective of a multi-megawatt drive application where the rated speed is typically above ten thousand rev/min and the operating voltage is in the range of several kV. The power electronic converter is based on a topology, which allows the application of multiple voltage levels to the machine's phase windings, while utilising lower voltage, fast switching IGBTs. The system's performance is illustrated by means of computer simulation which includes modelling of the electronics conduction and switching losses together with the copper and iron-loss in the machine.
Authors: S. J. Watkins and J. Corda
Source: Proceedings of EPE '03, 10th European Conference on Power Electronics and Applications, Toulouse, France, 2 - 4 September, 2003, CD-ROM Paper No. 0071.
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Title: Object-oriented model for inductance computation of a linear SRM
Abstract: The paper presents a method of computing the minimum and maximum inductance of a cylindrical linear switched reluctance motor based on an object-oriented model. This model is a useful tool for rapid computation of magnetic characteristics of the motor during the process of design optimisation. The key feature of the method is that the machine is decomposed into simple 3D segments with analytically derived equations, which are merged by applying an object-oriented programming approach to obtain the values of phase inductances for the extreme positions.
Authors: S. J. Watkins and J. Corda
Source: Proceedings of LDIA '03, the 4th International Symposium on Linear Drives for Industry Applications, Birmingham, UK, 8 - 10 September, 2003, pp. 45 - 48.
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Title: Multilevel space vector PWM control schemes for a flying-capacitor inverter
Abstract: The paper presents three different computational schemes for achieving space vector pulse width modulation, in the control of a multilevel flying capacitor inverter. Each scheme uses the same method of achieving cell-capacitor voltage balancing. Results of realistic simulations are used to compare and contrast these approaches. The timing control of the inverter switches and output performance of each method are presented and the most suitable one is highlighted.
Authors: S. J. Watkins and L. Zhang
Source: Proceedings of PEMD '04, the 2nd International Conference on Power Electronics, Machines and Drives (IEE Conference Publication No. 498), Edinburgh, UK, 31 March - 2 April, 2004, Vol. 1, pp. 12 - 17.
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Title: Control of multilevel flying capacitor inverters for high performance
Abstract: The paper presents a switching pattern selection scheme for the control of a multilevel flying capacitor inverter. The scheme reduces capacitor voltage fluctuation without using voltage feedback. The method is developed for sinusoidal voltage generation using the selective harmonic elimination (SHE) technique and was compared favorably with the result when voltage feedback was used. The result of the comparison is presented. Simulation and experimental results from a four-cell five level flying capacitor inverter validates the method.
Authors: L. Zhang, S. J. Watkins and D.-Q. Chang
Source: Proceedings of IPEMC 2006, the 5th International Power Electronics and Motion Control Conference, Shanghai, China, 14 - 16 August, 2006, Vol. 3, pp. 1 - 6.
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Title: A new low-cost hybrid switched reluctance motor for adjustable-speed pump applications
Abstract: This paper presents a new low-cost, hybrid SR motor intended for use in adjustable-speed pump drive systems. This motor is a single-phase motor, driven by a unipolar converter. This motor uses both the reluctance torque and the permanent magnet interaction torque. Compared with normal single-phase SR motors, it has an increased torque density. The cogging torque is properly used in this motor for reducing the torque ripple. It is demonstrated that such a motor drive system can be a suitable candidate to advantageously compete with the existing motor drive systems for low-cost applications. Finite Element models are used to analyze and predict the motor's performance. The proposed motor drive system has been fabricated and its performance tested in the laboratory. These experimental results are also presented.
Authors: K.-Y. Lu, P.O. Rasmussen, S.J. Watkins and F. Blaabjerg
Source: Proceedings of IAS 2006, 41st IEEE Industry Applications Society Annual Meeting, Tampa, FL, USA, 8 - 12 October, 2006, Vol. 2, pp. 849 - 854.
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Title: Nyt billigt laveffekt drivsystem?
Abstract:
Authors: P.O. Rasmussen, K.-Y. Lu, F. Blaabjerg and S.J. Watkins
Source: Teknisk Nyt Special, No. 48, 2006, pp. 24 - 29.
 

Title: Capacitor voltage balancing in multilevel flying capacitor inverters by rule-based switching pattern selection
Abstract: A rule-based scheme is investigated for capacitor voltage balancing in a multilevel flying capacitor inverter (MFCI). Without using voltage feedback, the scheme determines the best switching pattern for maintaining nil mean current in all capacitors, hence minimising the capacitor voltage fluctuation. The method is developed to work with the selective harmonic elimination (SHE) technique for sinusoidal voltage generation applied to control static VAR compensators. The method has been designed using a four-cell five-level MFCI as an example. The selected pattern has been shown to give superior performance in load-voltage total-harmonies distortion level and mean capacitor voltage fluctuation. The method is validated experimentally for this form of MFCI.
Authors: L. Zhang and S. J. Watkins
Source: IET Electric Power Applications, Vol. 1, No. 3, May 2007, pp. 339 - 347.
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Title: Active power filtering by a flying-capacitor multilevel inverter with capacitor voltage balance
Abstract: A new control strategy is developed for a four-level flying capacitor multilevel inverter to function as an active power filter (APF). This employs space vector modulation (SVM) scheme. However inverter switching vectors and sequences are determined according to the swings of capacitor voltage values at each level. A modified predictive current control law is developed This takes into account the derivative of the reference current in order to improve the performance of the conditioner when reference current changes quickly. Simulation study has shown that the proposed scheme leads to high performance harmonic current compensation for both steady and transient state operations and capacitor voltages at all levels are well balanced.
Authors: J. Hu, L. Zhang and S. J. Watkins
Source: Proceedings of ISIE 2008, IEEE International Symposium on Industrial Electronics, Cambridge, UK, 30 June - 2 July, 2008.
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Title: Direct power control of a doubly fed induction generator with a fixed switching frequency
Abstract: This paper presents the implementation of a direct active and reactive power control of a doubly fed induction generator (DFIG) applied to a wind generation system as an alternative to the classical field oriented control (FOC). The FOC is known to have complex control structure, which consists of current controller, power controller and frame transformations, and also depend highly on parameter variation such as rotor and stator resistances and inductances. The proposed direct power control (DPC) method produces a fast and robust power response without calling for complex structure and algorithms. However, it has high power ripple during steady state. In this paper, active and reactive power controllers and space-vector modulation (SVM) are combined to replace the hysteresis controllers used in the original DPC drive, resulting in the fixed switching frequency of power converter. The simulation results with FOC and DPC for a 3kW DFIG are given and discussed and experimental results for the same machine are shown to illustrate the feasibility of the proposed control strategy.
Authors: W.-S. Kim, S.-T. Jou, K.-B. Lee and S. J. Watkins
Source: Proceedings of IAS 2008, 43rd IEEE Industry Applications Society Annual Meeting, Edmonton, Alberta, Canada, 5 - 9 October, 2008.
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Title: A new low-cost hybrid switched reluctance motor for adjustable-speed pump applications
Abstract: This paper presents a new low-cost hybrid switched reluctance (HSR) motor intended for use in adjustable-speed pump drive systems. The motor is a single-phase motor, driven by a unipolar converter, which uses both the reluctance torque and the permanent magnet interaction torque. Compared with conventional single-phase switched reluctance motors, it has an increased torque density. The cogging torque is beneficially used in this motor for reducing the torque ripple. It is demonstrated that such a motor drive system can be a suitable candidate to advantageously compete with the existing motor drive systems for low-cost applications. Finite-element models are used to analyze and predict the motor's performance. The proposed motor drive system has been fabricated, and its performance has been tested in the laboratory. These experimental results are also presented.
Authors: K.-Y. Lu, P.O. Rasmussen, S.J. Watkins and F. Blaabjerg
Source: IEEE Transactions on Industry Applications, Vol. 47, No. 1, January/February 2011, pp. 314 - 321.
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Title: Modelling and control of an ultra-high-speed switched reluctance machine
Abstract: This paper presents some results of an ultra-high-speed switched reluctance motor operating in an electric assisted turbocharger on a diesel engine. The motor controller operates without a shaft-mounted position sensor and work has been undertaken to tune the sensorless algorithm parameters and improve the control stability. A motor model has been developed to more accurately simulate the machine operation in both motoring and generating in order to evaluate the accuracy of the position control and system performance.
Authors: S. J. Watkins and E. Winward
Source: Proceedings of PEMD 2016, the 8th International Conference on Power Electronics, Machines and Drives, Glasgow, UK, 19 - 21 April, 2016.
 

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