Integration and Interfaces (WP3)

This package of work concentrates on the engineering issues associated with implementing a Highly Distributed Power System. The control of such a system must enable load tracking and optimal dispatch of generation resources whilst ensuring stable operation of multiple DERs under transient conditions. Central to this is the device network interface which incorporates the control, communications and power electronics necessary to connect the DER to the power supply system.

M. Ristic, D. Brujic and K. Thoma, "Optimal Operation of Distributed CHP Systems for Participation in Electricity Spot Markets", EUROCON, 2007. The International Conference on 'Computer as a Tool', pp.1463-1469, 9-12 Sept. 2007

An optimisation tool is proposed to determine optimal operation schedule for distributed combined heat and power (CHP) plants which participate in electricity spot markets. It was tested and analysed using realistic data for a heat and electricity demands and their corresponding prices. The results show substantial cost savings compared to the benchmarked alternatives.

K.H. Ahmed, S.J. Finney, and B.W. Williams, "Passive Filter Design for Three-Phase Inverter Interfacing in Distributed Generation", Compatibility in Power Electronics, 2007. CPE '07, Page(s):1-9, May 29-June 1, 2007

With the growing use of inverters in distributed generation, the problem of injected harmonics becomes critical. These harmonics require the connection of low pass filters between the inverter and the network. This paper presents a design method for the output LC filter in grid coupled applications in distributed generation systems. The design is according to the harmonics standards that determine the level of current harmonics injected into the grid network. Analytical expressions for the maximum inductor ripple current are derived. The filter capacitor design depends on the allowable level of switching components injected into the grid. Different passive filter damping techniques to suppress resonance affects are investigated and evaluated. Simulated results are included to verify the derived expressions.

A. M. Massoud, S. J. Finney, A. Cruden and B. W. Williams, "Three-Phase, Three-Wire, Five-Level Cascaded Shunt Active Filter for Power Conditioning, Using Two Different Space Vector Modulation Techniques", Power Delivery, Vol.22, No. 4, pp.2349-2361

The three-phase, three-wire, five-level cascaded Inverter is used as a medium-voltage shunt active power filter. The capacitor voltage-control technique used as a harmonic current extraction method for the two-level inverter is extended to the five-level shunt active power filter, with a technique proposed for balancing capacitor voltages. Predictive current control based on the supply current (not the active filter current) is employed. Two different space vector modulation (SVM) techniques viz., phase-shifted SVM and hybrid SVM, are used for multilevel inverter pulsewidth-modulation generation. The proposed five-level shunt active power filter is validated by simulation and practically for both modulation techniques. The proposed technique results in the same software and hardware requirements for any m-level inverter.

A. M. Massoud, S. J. Finney, A. Cruden and B. W. Williams, "Mapped phase-shifted space vector modulation for multi-level voltage-source inverters", Electric Power Applications, IET Volume 1, Issue 4, July 2007, pp 622 - 636

A new multi-level space vector modulation (SVM) technique, termed mapped phaseshifted SVM, is proposed. The SVM technique enables the use of two-level SVM with multi-level inverters without introducing any carrier level modulation or phase shifts. The proposed technique is theoretically analysed for three-, five- and seven-level inverters and then generalised for the m-level inverter, validated by simulation using Matlab/Simulink, and confirmed practically for a five-level, shunt, active power filter.

A. Salamah, S.J. Finney and B.W. Williams, "Three-phase phase-lock loop for distorted utilities", Electric Power Applications, IET, Nov. 2007, pp937 - 945

A novel three-phase phase-locked loop (PLL) structure suitable for phase and angular frequency derivation from distorted ac utility voltages is presented. The proposed PLL has a simple structure; a conventional three-phase PLL followed by a proportional-integral (Pl)-controlled moving average filter together with a phase-locking algorithm. The objective of the proposed technique is to capture the fundamental phase angle and angular frequency of three-phase clean, distorted, balanced or unbalanced ac utilities. The PLL gives fast, accurate angular frequency and phase locking and is robust to utility distortion such as line notching, random noise, voltage imbalance, phase loss, phase imbalance, harmonics, dc offsets and frequency variation. The analysis presented substantiates the immunity of the proposed PLL to unbalanced and distorted utility conditions. The PLL technique is simulated and digital signal processor (DSP)-implemented for a three-phase system to verify the analytical results. The simulated and experimental results, for numerous utility conditions, demonstrate its phase-tracking ability, whereas the conventional technique fails to lock accurately in highly distorted, three-phase grid-connected operation.

H.B. Zhang, A. M. Massoud, S. J. Finney and B. W. Williams, "Operation of an active power filter with line voltage SVM under non-ideal conditions", 2007 Compatibility in Power Electronics, Page(s):1-7, May 29 2007-June 1 2007

A shunt active power filter based on line voltage space vector modulation is presented and analyzed. In three-phase three-wire system, by employing line voltage vectors instead of phase voltage vectors which are inaccessible with non-ideal mains voltages, this control scheme extends the application of space vector modulation into non-ideal conditions, and operates under both ideal conditions and non-ideal conditions. Experimental results are presented to demonstrate the validity of the approach to compensate reactive power and harmonics.

H.B.Zhang, A. M. Massoud, S. J. Finney, B. W. Williams, T. C. Lim and H. Hotait, "Operation of a wye connected three-level active power filter under non-ideal conditions", 2007 Power Electronics and Drive Systems, pp.62-67, 27-30, Nov. 2007

A three-level wye shunt active power filter based on a predictive current controller and line voltage space vector modulation is presented and analyzed. This control scheme treats the non-ideal load as a pure resistive load and compensates the reactive power and harmonics under both ideal and non- ideal conditions, thus the power quality at point of common couplings is improved. Experimental results are presented to demonstrate the effectiveness of this approach, and also reveal the limitation of wye structure APF.

K.H Ahmed, A.M. Massoud, S.J. Finney and B.W. Williams, "Modified current control of three-phase inverter interfacing in distributed generation", 2008 4th IET Power Electronics, Machines and Drives Conference, pp.401-405, 2-4, April 2008

This paper presents a modified current controller technique for three-phase inverter based grid interfaced distribution generation systems. The measurements and digital processor calculation delay is compensated by using a modified Kalman filter. For system robustness two current loops are implemented. Also, the states of the control variables can be observed and the number of the measurements sensors can be decreased. Stability and dynamic performance are analyzed. Simulation and experimental results confirm the validity of the proposed approach.

K.H Ahmed, A.M. Massoud, S.J. Finney and B.W. Williams, "Autonomous control of a PWM inverter in islanded distributed generation with different feedback signals", 2008 4th IET Power Electronics, Machines and Drives Conference,pp.573-577, 2-4 April 2008

A control strategy for PWM inverters in distributed generation system to operate in islanded mode is presented in this paper. The control system must provide sufficient: active and reactive power to match the connected load, therein providing high power quality. The multiloop controller contains regulators for filter capacitor current, filter capacitor voltage, and real and reactive powers. The system modelling and controller description details of the proposed inverter based distributed generation system are given. The control system in this mode is based on voltage and frequency droop. A detailed analysis, based on the concept of harmonic impedance establishes the appropriateness of capacitor current feedback versus other feedback control signals with respect to inverter power quality. To compensate the DSP calculation delay, a modified Kalman filter design is discussed. Simulation and experimental results are provided to demonstrate the validity of the proposed control method.

K.H Ahmed, A.M. Massoud, S.J. Finney, B.W. Williams, "Optimum selection of state feedback variables PWM inverters control", 2008 4th IET Power Electronics, Machines and Drives Conference, pp.125-129, 2-4 April 2008

Different control schemes for voltage source inverter interfacing are presented and compared. Different state feedback control approaches are investigated, therein identifying a generalized optimum control variable selection criterion that is appropriate to the control of inverter systems using an LCL passive filter. A standard synchronous proportional plus integrator (PI) controller is added into the outer loop control to achieve zero steady state error. A (P) proportional inner loop control is used to provide fast dynamic compensation. Finally, system harmonic impedance expressions are derived and used to study the effects of supply voltage distortion on the harmonic performance. The presented optimum variable selection criterion has been verified through the similarities between the different inverter system waveforms and experimental implementation.