Analysis and control of unified active power filter




Muthu, Subramanian

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The combined series and shunt active filters have been proposed to alleviate the power quality problems at the demand-side power systems. They provide compensation for load reactive power, load harmonics, unbalanced loads, utility harmonics, utility disturbances and utility imbalance. However, the conventional approach for the control of the combined active filter systems have resulted in large operating capacity of the shunt active filter because reactive power compensation involves only the shunt active filter. Furthermore, the harmonic mitigation problems are handled mainly by indirect harmonic compensation schemes rather than direct harmonic isolation schemes. This thesis presents the analysis and control of Unified Active Power Filter(UAPF) and proposes a novel concept of load reactive power compensation involving both the series active filter and the shunt active filter. This new control strategy of the combined active filter is to achieve the reduction in KVA rating of the shunt active filter and the optimal operation with increased efficiency. The thesis also applies discretetime sliding-mode control technique to enhance the performance of the combined active filter system in terms of fast dynamic response and effective solution to harmonic mitigation problems. The thesis also presents simulation and experimental results to provide verification of the proposed UAPF concept. In this thesis, the involvement of series active filter for reactive power compensation is achieved by controlling the phase difference between the load voltage and the utility voltage. The complete steady-state operating characteristics of UAPF are analyzed with the identification of the different operating modes of UAPF and the analysis of active and reactive power handled by the active filter components. The results of the analysis are shown to provide an insight about the load reactive power compensation by the series and shunt active filter. The reduction in ratings of the shunt active filter is demonstrated by an apparent power analysis of active filter components. The results of the analysis are also useful to design and select the optimal operating point for UAPF. The performance of UAPF to meet the stringent power quality standards are realized by applying discrete-time sliding-mode control schemes for the load voltage regulation and the active power factor correction. Various voltage and current control techniques used for three-phase voltage-source inverters are surveyed to identify the discrete-time sliding mode control technique as the suitable one. A generalized design procedure is derived for the control of power converter systems and the control scheme is extended to the load voltage control of shunt active filter and the line current control of series active filter. The control algorithms are developed to track a given load voltage and line current reference signals respectively. The effect of computational delay in DSP implementation is studied extensively and the control law is designed with the consideration for the computational delay. The systematic approach for the design of DC link voltage regulation is also presented in this thesis. A prototype experimental setup including the power circuit for UAPF and DSP based control circuit is built to implement the control and to verify the performance characteristics of UAPF. A real-time control algorithm is developed and is implemented on a DSP TMS320C40 system with PWM implementation by DMA without the intervention of CPU. The steady-state operating characteristics of UAPF is investigated by experiments. The operation of UAPF at the optimal operating point is shown to reduce the ratings of the shunt active filter and to improve the efficiency. The steady-state operation and the dynamic response of discrete-time sliding mode load voltage control and utility line current control are examined by simulation and experiments. The invariance property and the robustness property of the discrete-time sliding mode control are also demonstrated by the experimental results. With the discrete-time sliding mode control, the compensation characteristics of UAPF are shown to meet the stringent power quality standards.



Electric filters