Published in IET Power Electronics Received on 21st April 2008 Revised on 29th April 2009 doi: 10.1049/iet-pel.2008.0299 ISSN 1755-4535 Reduction of voltage harmonics using optimisation-based combined approach R.N. Ray 1 D. Chatterjee 2 S.K. Goswami 2 1 National Institute of Technology, Agartala, 799055, Tripura, India 2 Jadavpur University, Kolkata 700032, West Bengal, India E-mail: debashisju@yahoo.com Abstract: Voltage harmonics elimination problem in a micro-grid consisting of different types of distributed generation systems is addressed. In the proposed scheme, harmonic elimination is achieved by providing optimisation-based technique to some distributed generation-connected inverters and the sine pulse-width modulation (PWM) technique to other inverters connected to same micro-grid system. The lower order harmonics are eliminated with the help of particle swarm optimisation-based selected harmonic elimination technique applied to the PWM inverters. The higher order harmonics present in the micro-grid voltage waveform will be reduced by the dominant harmonics of same order generated in opposite phase by sine PWM inverter. This is achieved by varying the phase angle of the carrier wave of the sine PWM inverter, which generates the dominant harmonics with sidebands very close to the amplitude of prominent voltage harmonics present in the system but in opposite polarity. The same is validated by simulation as well as by suitable experiments. 1 Introduction A micro-grid [1] is a portion of a power system that includes one or more distributed generation (DG) units capable of operating either in parallel with or independent from a large utility grid, while providing continuous power to multiple loads and end-users. Distributed power generation system is emerging as a complementary infrastructure to the traditional central power plants. This infrastructure is constructed on the basis of decentralised generation of electricity close to consumption sites using DG sources [2]. Various forms of DG sources are wind, fuel cell, solar, biogas/biomass, tidal, geothermal, hydrogen energy, gas micro turbines and small hydro-power farms. The basic principle of DG relates to issues of sustainability, renewability and pollution reduction. In recent years, the fuel cell [3] power systems have attracted a lot of attention because of its increased efficiency than other various types of DG sources. Most of the DG sources generate DC power except wind energy, where AC power is generated by induction generator [4]. The AC power at variable voltage and frequency generated by wind generators is connected to the micro- grid system with the help of DC-link pulse-width modulation (PWM) inverter fulfilling the grid voltage and frequency requirement. Generally, PWM-based inverters are employed to interface different DGs with the AC micro-grid. A major issue with these inverters is eliminating harmonics from the output AC voltage so as to meet the specified total harmonic distortion (THD) limitations [5, 6]. The most commonly used technique for harmonic reduction from voltage waveform is the selective harmonic elimination (SHE) method [7, 8]. The SHE problem becomes complex and computation intensive with the increase of number of harmonics to be eliminated. Many approaches have been reported to modify the numerical process of solution for the transcendental equations [9–18]. The Walsh function method has been proposed in [12–14], whereas sine-wave approximation approach to form a set of non-transcendental equations is derived in [15]. Prediction of initial values for rapid convergence of Newton has been shown in [16], whereas in [17] a homotopy-based computation technique is used to find all possible multiple solutions for the bipolar switching pattern. An online computation method using neural 334 IET Power Electron., 2010, Vol. 3, Iss. 3, pp. 334–344 & The Institution of Engineering and Technology 2010 doi: 10.1049/iet-pel.2008.0299 www.ietdl.org