Space Vector Based Hybrid Random Pulse Width Modulation Technique for a 3-Level Inverter David Solomon George, M. R. Baiju Power Electronics Research Laboratory Dept. of Electronics & Communication College of Engineering, Trivandrum Kerala, India david@rit.ac.in, mrbaiju@gmail.com Abstract - This paper proposes a Space Vector based Random Pulse Width Modulation technique (RPWM) for a 3-level inverter. Randomization is achieved by random placement of active vectors in fixed switching time period. Flexibility in random placement is limited when the duration of active vectors is comparable with switching time period. In such case, this paper proposes a hybrid operation where, in addition to random placement of active vectors, switching frequency is also varied randomly for every PWM cycle. In hybrid operation, switching frequency is limited in the range 2.5±1KHz by limiting the bounds of the randomization. The proposed scheme uses a mapping technique, by which, randomization is effected to the vectors of a 2-level inverter. From random vectors of 2-level inverter, random switching vectors for the 3-level inverter are derived. The proposed scheme does not require any external circuitry and it can be implemented for any 3-level inverter topology. The algorithm is implemented in TMS320LF2407 DSP platform for a 2-hp induction motor driven by a 3-level inverter in open-end winding configuration and experimental results are presented. I. INTRODUCTION Pulse Width Modulation (PWM) techniques provide easy and efficient control of adjustable speed drives and are widely used in industrial applications [1], [2]. The energy of output voltage of the inverter is concentrated in the harmonics of switching frequency. This results in acoustic noise, mechanical vibration and electromagnetic interference. Random Pulse Width Modulation (RPWM) techniques are used in the case of 2-level inverters for the mitigation of these effects [3]-[11]. These techniques distribute the energy concentrated in the harmonics of switching frequency into the continuous spectrum [3]-[11]. The RPWM can be implemented by techniques using fixed switching frequency [3]-[6] or random switching frequency [3], [7]-[10]. Random Pulse Position Modulation is the commonly used fixed switching frequency technique in which, the positions of PWM pulses are randomly varied within the fixed switching period [3]-[6]. A triangular carrier with randomly varying slope can generate a random switching frequency RPWM signal with spread spectrum characteristics [3], [8]. A carrier based hybrid random PWM scheme has been implemented for a 2-level inverter in which random PWM signals are obtained by using a random frequency triangular carrier and a Pseudo Random Binary Sequence (PRBS) [11]. The scheme requires frequency modulator and logic circuits as external circuits for the generation of random carrier and PRBS [11]. Random modulation techniques have also been implemented for 3-level inverters [12]-[15]. RPWM signals can be generated by comparison of reference signal with random numbers within each sampling period [12], [13]. Approximating the instantaneous reference space vector by switching among different possible combinations of inverter voltage space vectors, can result random PWM for 3-level inverter [14].This scheme results in increased number of switching in each sampling period [14]. Randomization for a 3-level inverter can be achieved by varying the switching frequency along with voltage balancing of link capacitors [15]. This paper proposes a Hybrid Random Pulse Width modulation scheme for a 3-level inverter which is based on the random placement of the active vectors within the switching period. Whenever the duration of the active vector becomes comparable with the switching time period, the proposed scheme employs a variable switching frequency technique. This space vector based scheme does not need any sector identification and the inverter voltage vectors are automatically generated in the optimum sequence from the instantaneous amplitudes of 3-phase reference signals. The proposed method does not need any external hardware circuitry for generation of the random sequences. The proposed scheme can be implemented for any 3-level inverter topology. In the present work, the scheme is implemented and experimentally verified for a 3-level inverter using open- end winding configuration of the induction motor. II. THE PROPOSED SCHEME A. Principle of the proposed scheme The proposed Hybrid Random Pulse Width Modulation scheme is based on the principle that, when the instantaneous reference space vector is approximated by switching the inverter voltage vectors in Space Vector PWM (SVPWM) method, the accuracy of approximation depends on the duration of active vectors and not on the position of the active vectors in the switching period. Therefore by placing the active vectors randomly in every PWM switching period, non periodic switching signals are generated, introducing randomness in the pole voltages. The flexibility IEEE PEDS 2011, Singapore, 5 - 8 December 2011 978-1-4577-0001-9/11/$26.00 ©2011 IEEE 1074