Active capacitor voltage control of flying capacitor multilevel converters L. Xu and V.G. Agelidis Abstract: A novel control strategy for voltage balancing of flying capacitor multilevel converters controlled with phase shifted SPWM is presented. Based on the polarity of the phase current, a relatively small square wave is added to the nominal modulation signals (typically sinusoidal) controlling the converter switches and in this way the flying capacitor voltages can be directly regulated. A simple algorithm is developed to explore an optimal solution for multiple capacitor voltage regulation. The impact of the proposed control strategy on the flying capacitor voltage variation and converter output voltage is analysed. The effectiveness of the control strategy is validated by simulation results of a five-level converter. 1 Introduction High power applications such as flexible alternating current transmission systems (FACTS) and high voltage direct current (HVDC) require a large number of power devices connected in series in order to block the high voltage in such systems. Traditionally, thyristors or fully controlled gate turn-off thyristors (GTOs) have been used. More recently, insulated-gate bipolar transistors (IGBTs) have emerged as strong candidates for these applications mainly due to their advantages that include ease of control and capability to operate at relatively high switching frequency, which leads to low harmonic distortion and fast dynamic response. However, the need for a relatively large number that must be connected in series remains a drawback. Moreover, the issue of switching losses in the converter also needs to be addressed. One solution to high-power applications is the use of multilevel Voltage Source Converters (VSCs), which allows higher power-handling capability with reduced harmonic distortion and lower switching power losses when compared with their two-level counterparts. Among these multilevel converter topologies, the most popular ones are the cascaded multimodular VSC [1] , the neutral-point-clamped (NPC) multilevel VSC [2] and the flying capacitor (FC) multilevel VSC [3, 4]. While the Cascaded multimodular VSC is only suitable for handling reactive power, the other two multilevel topologies have found wide applications in high-power FACTS, HVDC and motor drives [5] . How- ever, a common problem associated with multilevel VSCs is the capacitor voltage balancing. For the NPC multilevel converter, the mean current drawn from the neutral point over a modulation cycle is zero and the neutral point potential remains balanced within each fundamental period. However, during transient operation or if there is any unbalance in the switching pattern, the neutral point voltage will vary. There have been a number of possible solutions to this problem based on the use of either redundant switching states or auxiliary balancing circuits [2]. For the FC converter, it has been shown in [6, 7] that, the capacitor voltage is balanced with each switching period provided that the control signals have the same duty cycle, the power devices have the same characteristics and the load currents are symmetrical. However, these conditions cannot be met in a real system, and consequently, the voltages of the flying capacitors will vary. Appropriate methods must therefore be integrated into the control to ensure the balancing of such voltages. In [8] , a filter circuit of the R-L-C type tuned at the switching frequency and connected in parallel with the load is proposed to achieve voltage balancing. The main idea is that any unbalances of capacitor voltages will result in voltage harmonics at the switching frequency being generated at the output. A harmonic current at the switching frequency and in phase with the harmonic voltage flows through the filter circuit and the converter, which indeed automatically corrects the unbalanced flying capa- citor voltages. This method is reliable and has the advantages of achieving satisfactory balancing even when the fundamental AC current is relatively small. However, the extra filter increases the cost of the overall system, especially for high-voltage applications, as it is tuned at a relatively low frequency, i.e. switching frequency. In addition, it introduces extra power losses. In [9] and [10], the proposed methods for balancing capacitor voltages are based on modifications of the control strategy. Specifically, the approach adopted in [9] forces the flying capacitors either to charge or to discharge by short- circuiting the capacitors for a very short-period (typically a few microseconds). The short-circuit current is limited only by the loop stray inductances. However, modern circuit design is based on minimisation of the loop inductances. Consequently this method could lead to extremely high short-circuit currents and in this way could have an undesirable impact on the power devices. In [10], it is proposed to change the pulse position of the converter control pulse to cause a variation in the voltage applied to the load. Such voltage variation will then L. Xu is with the School of Electrical & Electronic Engineering, Queen’s University of Belfast, Stranmillis Road, Belfast BT 9 5AH, UK V.G. Agelidis is with the Department of Electronics and Electrical Engineering, University of Glasgow, 72 Oakfield Avenue, Glasgow G12 8LT, UK r IEE, 2004 IEE Proceedings online no. 20031051 doi:10.1049/ip-epa:20031051 Paper first received 2nd April 2003 and in revised form 19th September 2003. Originally published online: 13th February 2004 IEE Proc.-Electr. Power Appl., Vol. 151, No. 3, May 2004 313