# A Fast, Decentralized, Self-Aligned Carrier Method for Multicellular Converters +
## 1 Introduction  
In recent years, the studies of multicellular converters built with parallel or serial switching-cells have been intensively developed [1; 2; 3; 4; 5; 6; 7]. These topologies allow managing high voltage or high current outputs whilst using low voltage and low current rating switches. Further, they can help to achieve high efficiency improvement with low harmonic distortion and can minimize the size and the volume of external passive filters. Figure 1 shows the configuration of such converter types. The switching-cells can be connected in parallel in a multiphase converter (Figure 1a) to increase its current output and to reduce the output current ripple [1,2] or in series in a multilevel converter (Figure 1b) to increase its voltage output and to obtain lower total harmonic distortion [3,4]. In case of large numbers of switching cells that need to be managed, the control system is the main issue of these converter topologies for applying the centralized control structure [8, 9, 10]. Generally, there are two controller types for implementing carrier-based PWM methods: the centralized one and the decentralized one. In the first type, the control system needs to compute a set of phase-shifted or level-shifted carriers to generate appropriate interleaved PWM signals for all converter switching cells. If the system is dynamically reconfigured, e.g., an active switching cell is removed or added either to increase the overall efficiency or to provide a solution in case of a cell failure, the centralized controller has to reconfigure all the carriers. Furthermore, this centralized approach requires many wires to setup communications and controls between the controller and the converter cells. On the other hand, the decentralized control approach uses a separated controller for each switching cell, and this helps to significantly reduce the complexity of system wiring. In the literature, research on the structure and the method of decentralized control for multicellular converters can be classified into three directions as follows. According to the first research direction, the decentralized control is characterized by a hierarchical architecture that has two control levels, such as primary-secondary controller [9,11] and master-slave or central-local controller [12, 13, 14, 15, 16, 17, 18, 19], and the information exchange between these levels is conducted by communication links. The system level controllers, namely secondary/master/central, are responsible for general information management and for performing tasks such as voltage balance, current balance, and power exchange. Meanwhile, the controllers of the lower level, namely primary/slave/local, are in charge of creating PWM control signals. In a case of using carrier-based PWM, the inter-carrier phase-shift angle or the level-shifted carrier for cell controller is calculated by using the information from the system level controller. With this control structure, the system easily achieves global optimization; however, its reliability is reduced due to high dependence on the main controller. In the second research direction, the local controller of each converter cell operates independently based on its own current and voltage information, without exchanging information with neighboring cells. This structure is easy to connect, but the carrier phase-shifted angle of each module is calculated through a complex algorithm, therefore, processing time is an issue with increasing numbers of modules [20,21]. In the third research direction, the cell local controller exchanges information with its neighboring cells. The exchanged information can be the cell position [22], the phase angle [10,23], or the amplitude [24] of the cell carrier. The system becomes stable after some iterations of the algorithm and stabilizes with a permissible error [25]. This structure  
Figure 1: Multicellular converters, (**a**) multiphase converter [1,2]; (**b**) multilevel converter [3,4].