Predictive Current Control and DC-Link Capacitor Voltages Balancing for Four-Leg NPC Inverters V. Yaramasu, B. Wu Dep. of Elec. and Comp. Eng. Ryerson University Toronto, ON M5B 2K3 Canada Email: vyaramas@ee.ryerson.ca M. Rivera Dep. of Industrial Technologies Universidad de Talca Curico, CHILE Email: marcoesteban@gmail.com J. Rodriguez Dep. of Elect. Eng. Universidad T´ ecnica Federico Santa Mar´ ıa Valpara´ ıso, 2390123 Chile Email: jrp@usm.cl Abstract—This paper presents a finite control set model pre- dictive strategy to control the three-phase four-leg neutral-point- clamped (NPC) inverters. The four-leg NPC inverter delivers power to the unbalanced/nonlinear three-phase loads, and it can produce three output currents independently. The proposed method uses the discrete-time model of the inverter and load to predict the load current and capacitor voltages behavior for each valid switching state of the inverter. The control method chooses a state which generates the minimum error between the output currents and their references and also between the capacitor volt- ages. The neutral-leg switching frequency reduction algorithm is also proposed to improve the efficiency of the converter. Through computer simulations, the feasibility of the proposed predictive control scheme is verified; it performs well, showing a clear capacity to compensate disturbances while balancing the DC-link capacitor voltages and reducing the average switching frequency. Index Terms—Predictive control, Four-leg converters, Three- level converters, Diode-clamped, NPC inverter. I. I NTRODUCTION Recently, most utilities in North America have adopted three-phase four-wire systems as their medium voltage distri- bution systems [1]. Multilevel inverters are most suitable for medium voltage, high power applications, because of many attractive features, such as high voltage capability, reduced common mode voltages, near sinusoidal outputs, low dv/dt’s, and smaller output filters. Conventional three-leg neutral- point-clamped (NPC) inverters [2] are not suitable for four- wire systems with unbalanced/nonlinear loads for the follow- ing reasons: (i) insufficient DC-link utilization, (ii) high ripple on DC-link capacitors, and (iii) the problem of balancing the DC-link capacitor voltages. The four-leg NPC is a promising topology compared to three-leg NPC in four-wire systems; it offers full utilization of the DC-link voltage and less stress on the DC-link capacitors [3]. Moreover, reliability of the system and the control of the neutral-point voltage during faulty operation can be improved by using a four-leg NPC inverter [4]. A great variety of mod- ulation methods are available, such as pulse width modulation (PWM) [5], [6] and 3D space vector modulation (SVM) [7]– [9]. In contrast to PWM, by using SVM, the converter capacity can be fully utilized to deliver output voltage. Predictive control is a class of controllers that have found recent application in power electronics [10]. This control has become an attractive alternative to classical modulation methods, due to its fast dynamic response and simple concept. Because power converters have a discrete nature, the applica- tion of predictive control constitutes a promising and better suited approach than the standard schemes that use the mean value of the variables. The finite set model predictive strategy with a prediction horizon of one sample time is a simple and very flexible control scheme that allows easy inclusion of the nonlinearities and constraints in the design of controller [11]– [13]. Moreover, this scheme does not require internal current control loops and modulators. In this paper, a predictive control technique is proposed to handle the output currents and as well as the balancing of the capacitor voltages for the four-leg NPC inverter, in an easy and intuitive manner. This control scheme uses the load, output L f filter, and DC-link model to predict the load current and capacitor voltage behavior for each valid switching state of the converter. The neutral-leg switching frequency reduction algorithm is also proposed to reduce the losses associated with the corresponding phase. To validate the feasibility of the proposed control method, simulations are carried out using MATLAB/Simulink software for different conditions of references and loads. This paper is organized as follows: in section II, the math- ematical model of the NPC converter and load is presented, followed by the explanation of the proposed control strategy in section III. In section IV, simulation results are presented, verifying the feasibility of the proposed method and finally in section V conclusions are drawn. II. THREE-PHASE FOUR-LEG NPC I NVERTER TOPOLOGY The three-phase four-leg NPC inverter with an output L f filter considered in this paper is shown in Fig. 1. This converter presents a connection format similar to the conventional three- phase NPC converter, with an additional leg connected to the neutral point of the load. The voltage in any leg x of the NPC inverter, measured from the negative point of the DC-link (N ), is expressed in terms of switching signals and DC-link capacitor voltages as follows: v xN = v dc1 S 1x + v dc2 S 2x , x = u, v, w, n, (1)