Topologies and Control Strategies of Multilevel Converters

Multilevel converters have been continuously developed in recent years due to the necessity of increase in power level of industrial applications especially high power applications such as high power AC motor drives, active power filters, reactive power compensation, FACTS devices, and renewable energies [1-9]. Multilevel converters include an array of power semiconductors and capacitor voltage sources which generate step-waveform output voltages. The commutation of the switches permits the addition of the capacitors voltages and generates high voltage at the output [8, 10, 11]. The term multilevel starts with the three-level converter introduced by Nabae [12]. By increasing the number of levels in the converter, the output voltage has more steps generating a staircase waveform which has a reduced harmonic distortion [13]. However, a high number of levels increases the control complexity and introduces voltage unbalance problems [10]. The Neutral Point Clamped (NPC) converter, presented in the early 80’s [12], is now a standard topology in industry on its 3-level version. However, for a high number of levels, this topology presents some problems, mainly with the clamping diodes and the balance of the dc-link capacitors. An alternative for the NPC converter are the Multicell topologies. Different cells and ways to interconnect them generate several topologies which the most important ones, described in Section II, are the Cascaded Multicell (CM) and the Flying Capacitor Multicell (FCM) with its sub-topology Stacked Multicell (SM) converters [11-14]. The CM converter is the series connection of 2-level H-bridge converter, that several configurations have been proposed for this topology [13]. Since this topology consists of series power conversion cells, the voltage and power levels may be scaled easily. As other alternative topologies, the FCM converter [15, 16], and its Arash Khoshkbar Sadigh EECS Department, University of California-Irvine, Irvine, CA, 92617, US email: [email protected] S. Masoud Barakati Department of Electrical and Computer Engineering, University of Sistan and Baluchestan, Zahedan, Iran email: [email protected] 312 A.K. Sadigh and S. Masoud Barakati derivative, the SM converter [17-19], have many attractive properties for medium voltage applications [15-22]. To control the multilevel converters, there are several modulation methods which can be classified to high and low switching frequency. High switching frequencies based methods have several commutations during one period of the fundamental output voltage. The pulse width modulation (PWM), sinusoidal pulse width modulation (SPWM) and space vector PWM are common methods for the high switching frequency. Low switching frequencies based methods have one or two commutations during one period of the fundamental output voltage, generating a staircase waveform. The multilevel selective harmonic elimination (SHE) and the space vector control (SVC) are common use for the low switching frequency. The mentioned topologies of multilevel converters as well as their several control methods are discussed in this chapter. 1 Multilevel Converters Topologies 1.1 Cascade Multicell (CM) Converter The CM converter was introduced in the early 90s [23, 24]. This topology is based on the series connection of units known as cells with three-level output voltage, as shown in Fig. 1. Structure of each cell is based on an isolated voltage source, Fig. 1(a). When only one dc voltage source is available, a bulky and complex multi-secondary input transformer is required, Fig. 1(b). Therefore, the cost and size of the converter is increased. Since this topology consists of series power conversion cells, the voltage and power levels may be scaled easily and a maximum of output voltage levels is obtained. The total output voltage, corresponding to the sum of each cell output voltage, is: (1) where n is the number of cells connected in series. An additional advantage of this topology is that when an internal fault is detected and the faulty cell is identified, it can be easily isolated through an external switch and replaced by a new operative cell without turning off the converter [25]. However, while the replacement is done, the maximum output voltage in the faulty leg is reduced to: (2) where f is the number of faulty cells. As the 2-cell-5-level CM converter is controlled by phase shifted-SPWM (PSSPWM) and operated with a modulation index equal to 0.8 ( ), its control strategy, switches states as well as the output voltage are shown in Fig. 2. The 1 2 + n 1 n out i i v v = = 1 f out f v v n = −      