A Low Capacitance Cascaded H-Bridge Multi-Level StatCom
Abstract
This paper introduces a cascaded H-bridge multilevel converter (CHB-MC) based StatCom system that is able to operate with extremely low dc capacitance values. The theoretical limit is calculated for the maximum capacitor voltage ripple, and hence minimum dc capacitance values that can be used in the converter. The proposed low-capacitance StatCom (LC-StatCom) is able to operate with large capacitor voltage ripples, which are very close to the calculated theoretical maximum voltage ripple. The maximum voltage stress on the semiconductors in the LC-StatCom is lower than in a conventional StatCom system. The variable cluster voltage magnitude in the LC-StatCom system drops well below the maximum grid voltage, which allows a fixed maximum voltage on the individual capacitors. It is demonstrated that the proposed LC-StatCom has an asymmetric V-I characteristic, which is especially suited for operation as a reactive power source within the capacitive region. A high-bandwidth control system is designed for the proposed StatCom to provide control of the capacitor voltages during highly dynamic transient events. The proposed LC-StatCom system is experimentally verified on a low-voltage 7-level CHB-MC prototype. The experimental results show successful operation of the system with ripples as high as 90% of the nominal dc voltage. The required energy storage for the LC-StatCom system shows significant reduction compared to a conventional StatCom design.
EXISYTING SYSTEM:
The three-phase CHB converter is constructed from three single-phase converters connected via a common star-point. This implies that each phase-leg must buffer the per-phase variations in instantaneous power that occur within each fundamental cycle, which is in contrast to monolithic multilevel converters which have a common dc-link. To buffer the energy variations, while still maintaining the necessary capacitor voltage to control the converter current, significant energy storage is required within each CHB phase-leg. This dictates the use of large H-bridge capacitance values, which has the following drawbacks: High direct cost of large dc capacitors. The indirect cost on reliability of the system due to the tendency of electrolytic capacitors to fail before other system components . The indirect cost on cell protection due to the very large energy that is dissipated in the event of shootthrough of the dc-link. The significant weight and volume of the converter, which can make it difficult to containerise highpower StatComs.
PROPOSED SYSTEM:
Recently, the high reliability of film capacitors has become a major driving force in replacing electrolytic capacitors in power converters. However, the relatively low capacitance values achievable with film capacitors, compared to electrolytic capacitors with the same volume, has limited the application of film capacitors in CHB-MC applications. In typical CHB converters, the H-bridge capacitance values are chosen to limit variations in capacitor voltages to 10% of the nominal dc voltage . There are two reasons why voltage variation must be limited. The first is that the peak voltage on each capacitor must be limited to avoid destruction of the associated semiconductor devices and to avoid lifetime reduction due to cosmic-ray failure rate implications. The second reason is the lower limit on capacitor voltage variation, which is dictated by the need to maintain sufficient cluster voltage such that effective current control can be maintained In this paper, to complement the research done , the issue of minimizing the maximum capacitor voltages is addressed. Furthermore, it has been shown previously in (for typical CHB StatComs) that controlling the square of capacitor voltage results in a decoupled and linear cascaded control system. This paper utilizes the same concept to significantly reduce the computational power required to implement the analytic filtering scheme proposed. The use of complex capacitor voltage filtering is avoided as the ripple on the square of the capacitor voltages is sinusoidal (with a frequency equal to twice that of the grid frequency). Estimation of one sinusoidal waveform per-phase requires much less computational power compared to directly estimating mixed-frequency capacitor voltage signals.
CONCLUSION:
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