The technology of multilevel converters has emerged as a very important alternative in the area of a high power medium voltage energy control. Due to the series connection of semiconductors, it is possible to reach medium-high voltages with standard components.
Multilevel converters offer several advantages compared to their conventional counterparts. By synthesizing the AC output terminal voltage from several voltage levels, staircase waveforms can be produced, which in their turn approach the sinusoidal waveform with low harmonic distortion, thus reducing filtering requirements. However, the several sources on the DC side of the converter make multilevel technology difficult to control by the need to balance the several DC voltages. Thus, linear controllers, designed on the basis of the small-signal linearization, are not effective, and stability can not be ensured as large-signal disturbance occur.
Up to now, the topologies for high power multilevel converter are classified in three main types: flying capacitor converter (FC), diode clamped converter (NPC) and cascaded H-bridge converter (HB). These topoligies are described in a paper by J. Rodríguez, et al, “Multilevel Inverters: A survey of topologies, controls and applications,” IEEE Trans. on Ind. Electr., Vol 49, No. 4, pp. 724-738, August 2002; a paper by R. Teodorescu, et al., “Multilevel Converters: A Survey,” in Proc. EPE'99, Lausanne, 1999; and a paper by J. S. Lai, et al., “Multilevel Converters—A New Breed of Power Converters,” in Proc. IEEE-IAS Conf., 1995, pp. 2441-2548.
Even if the cascaded H-bridge converter has the disadvantage of needing separated DC sources, this topology is an attractive option due to its several advantages, such as, modularity, simplest composition, and reduced number of components (they do not have the need of extra clamping diodes, nor balanced capacitors). Additionally, the structure of multicells in cascade H-bridge allows the nourishment of different charges in DC when it is used in an active rectifier application. See U.S. Pat. No. 6,005,788 titled “Hybrid topology for multilevel power conversion” by Lipo, et al., which is incorporated by reference.
The use of cascaded H-bridges has been successfully implemented in commercially available large drives and some static VAR compensators as reported in F. Z. Peng, et al., “A Multilevel Voltage Source Inverter with Separate DC Sources for Static VAR Generation,” in Proc. IEEE-IAS'95 Conf., pp. 2541-2548.
However, H-bridge converters have a major challenge regarding their control. It must guarantee a current almost sinusoidal and in phase with the line voltage in the AC-side and for each phase. Simultaneously, the controller must regulate and stabilize the voltages levels of every single capacitor on the DC-side. This means that the number of available controllers is inferior to the controlled variables. For instance, in a three-phase H-bridge converter of 2n+1 levels (n H-bridge converters in cascade), the control problem consists in controlling n+3 state variables (three currents plus n DC voltages) with only n switching functions. This is due to the fact that every H-bridge cannot be considered as an independent structure to control, as they interact with other cells. For instance, in a branch of a series connection of H-bridges they share the same current and the effective injected voltage is the sum of voltages in every cell.
Generally, an active filter application involves the compensation of harmonic distortion and reactive power, i.e., periodic disturbances, caused by a distorting nonlinear load. Control schemes based on the introduction of a bank of harmonic oscillators (resonant filters) is perhaps one of the most appealed techniques to guarantees rejection of periodic disturbances, thanks to its simplicity and effectiveness. This type of schemes is based on the internal model principle. This principle states that the controlled output can track a class of reference commands without a steady state error if the generator, or the model, of the reference is included in the stable closed-loop system. Therefore, according to the internal model principle, if a periodic disturbance has an infinite Fourier series (of harmonic components), then an infinite number of resonant filters are required to reject such a disturbance. For a detailed description of internal model principle, reference is made to B. Francis and W. Wonham, “The internal model principle for linear multivariable regulators,” Applied Mathematics and Optimization, Vol. 2, pp. 170-194, 1975, which is incorporated by reference. Applications of this principle to power electronics systems are disclosed in U.S. Pat. No. 6,940,187 titled “Robust controller for controlling a UPS in unbalanced operation” by Escobar, et al., and also in U.S. Pat. No. 60,265,727 titled “Adaptative controller for D-STATCOM in the stationary reference frame to compensate for reactive and harmonic distortion under unbalanced conditions” by Escobar, et al.