Published in IET Electric Power Applications Received on 28th October 2010 Revised on 29th January 2011 doi: 10.1049/iet-epa.2010.0260 ISSN 1751-8660 Spread spectrum modulation scheme for two-level inverter using vector quantised space vector-based pulse density modulation B. Jacob M.R. Baiju Power Electronics Laboratory, Department of Electronics and Communication, College of Engineering, Trivandrum, India E-mail: mrbaiju@ece.cet.ac.in Abstract: A Space Vector based Pulse Density Modulation scheme for a two level Voltage Source Inverter is proposed in this paper. The proposed scheme employs a first order Sigma Delta Modulator. The principle of Vector Quantization is used for quantizing the reference voltage Space Vector in the Sigma Delta Modulator. For the spatial quantization, the inverter voltage vector space is divided into seven Voronoi regions. In this paper, a method is proposed to code these Voronoi regions using instantaneous reference phase amplitudes without using lookup table. The proposed scheme naturally selects the switching vectors in the over-modulation condition and hence results in a smooth transition from linear to over-modulation region. In Pulse Density Modulation scheme, the switching frequency varies randomly, resulting in the spreading of harmonic spectra. The minimum pulse width that can occur in the proposed scheme is equal to the sampling time period. Therefore, the occurrence of pulse width less than the trun on time of the switching devices is avoided. The proposed scheme is implemented for a 11.5 kVA two level inverter driving 2-HP three phase induction motor. Experimental results of proposed scheme are compared with s Space Vector PWM scheme and Random Space Vector PWM scheme. 1 Introduction THE adjustable speed drives, based on inverter-fed induction motors, need efficient control of both frequency and voltage. In pulse width modulated (PWM) inverters, frequency and voltage control is achieved by varying the duty ratio of inverter switches [1–3]. The PWM schemes with constant switching frequency will generate prominent harmonic clusters in the output of voltage and current spectra [3, 4]. The output frequency spectra of inverter will determine the electromagnetic interference characteristics of inverters and acoustic noise generated by electric machine [3–9]. In random PWM schemes, the width of the pulses is varied over a wide range to obtain variation in switching frequency [5–7]. The randomised switching times results in high- frequency switching and the associated minimum pulse width problem [10]. In this paper, a pulse density modulation scheme, with variable switching frequency similar to that used in the case of analogue-to-digital converters, is proposed for two-level inverters. The motivation for adopting the principle of pulse density modulation in the case of two-level inverter is that the switching converters can be viewed as analogue-to- digital converters [11–13]. In over sampling analogue-to- digital converters, sigma – delta modulators are used to reduce quantisation noise by spreading the spectra of the quantisation noise [14, 15]. Sigma – delta modulation with scalar quantiser is applied to two-level inverters for power control [16–22]. Hexagonal quantiser can also be used in sigma – delta modulator to control the voltage source inverters [11–13]. For efficient quantisation in digital communication and data compression, the concept of vector quantisation is used instead of scalar quantisation [23, 24]. The present work proposes to adopt this principle of vector quantisation in the quantiser of sigma – delta modulator. In the present work, a space vector-based pulse density modulation is presented for two-level voltage source inverter. The first-order sigma – delta modulator is used to obtain pulse density modulation. In the proposed scheme, the principle of vector quantisation is used for quantising reference space vector as part of the sigma–delta modulator. The scheme has been experimentally verified for 11.5 kW, 415 V two-level inverter driving 2-HP induction motor. The experimental results of the proposed scheme are compared with the space vector PWM scheme and random SVPWM scheme. 2 Principles of sigma – delta modulation and vector quantisation as used in the proposed scheme Sigma –delta modulators are widely used in over-sampling analogue-to-digital converters to reduce quantisation noise. The principle of vector quantisation is utilised in image and audio compression for efficient quantisation. This paper uses the principle of sigma–delta modulation to control the inverters and the principle of vector quantisation to realise the quantiser in sigma – delta modulator. Basic principles of IET Electr. Power Appl., 2011, Vol. 5, Iss. 7, pp. 589–596 589 doi: 10.1049/iet-epa.2010.0260 & The Institution of Engineering and Technology 2011 www.ietdl.org