The use of modular and multicell topologies enables to minimize the time and production costs in the design of one-type converters. In addition, this approach is well suited to reducing the higher harmonics of current and voltage and it can also be used for reservation. In general, this approach can be used for the following purposes:

• Increase in the system power with limited parameters of the available components

• Decrease in the design period of new power-electronic devices, with distinctive voltage and current characteristics

• Reservation of devices and their components with the output parameters continuity

• Reduction of higher harmonics in input and output current or voltage

• Matching of the input and output currents and voltages • Unification and standardization of components

Modular and multicell converters were first used in secondary power sources for autonomous devices, primarily airplanes (Rozanov et  al., 2007). This approach is based on the theory of structural-algorithmic synthesis of secondary power sources (Mytsyk, 1989). The same design principles are widely used in the power engineering for the design of dc power systems (Sood, 2001). Various approximations of the harmonic signal were analyzed in Mytsyk (1989). In particular, a signal with N stages was considered (Figure 9.1). Obviously, the distortion in this case depends on two variables: the sampling interval θn and the quantization level An.

Complete minimization of the distortion depends on two variables and entails numerical solution of a system of transcendental equations. The solutions obtained do not lend themselves to technical realization. Therefore, in practice, the relative level is assumed to be a multiple of the minimum level of the first stage. A better method is to combine pulseamplitude modulation with pulse-width modulation (PWM). In that case, the modulation frequency is minimized, and the number of stages is selected so as to have the best utilization of the components or is determined in accordance with technical and economic optimization.