| The performance of a converter, if the topology structure of the converter has been determined, mainly depends on the control strategy designed on it. so it is important to study the control strategy for a nonlinear converter system. Although there have been a variety of control strategies for controlling a converter,the control performance of those strategies need to be further improved. Fortunately, the emergence of passivity-based control (PBC), which gives a new idea for the controller design of a nonlinear system.Compared with other control strategies, PBC is a compound control technology,and it has the advantages of lesser control parameters, better dynamic performance, stronger robustness and ensuring an object can stabilize in wide range. However, there is a fly in the ointment, its steady-state performance depends on the model accuracy of a control object. In practical engineering, due to the accurate model of a control object can hardly be obtained, so it cannot achieve zero steady-state error tracking. Especially when the traditional PBC is adopted to track the harmonic signal on frequency domain, the steady-state tracking error caused by model error will be more obvious. Therefore, the study of extending the traditional PBC to frequency domain is not only important for practical application, but also can provide a research idea for multi-target control scheme on frequency domain of the power electronic device. In addition, the previous studies about designing the multi-frequency PBC and designing the PBC for a high-order converter systems have not been reported, however with the high order of converter system, and researching the PBC of a high-order system also has the important practical significance.In view of this, a representative device that unified power quality controller (UPQC) is chosen as the control object in this paper, of which the purpose is that in-depth studying the PBC on frequency domain.In section II, a multi-frequency modeling method for UPQC is proposed by using Fast Fourier Transform (FFT) technique. Due to this method can be used to achieve the frequency domain decomposition for AC model of a converter system, so the selective frequency control and the DC control can be achieved. In addition, this method is not only suitable for three-phase three-wire system, but also for three-phase four-wire or single-phase system. And the coupling between phases will not be caused during the transformation for multi-frequency modeling, and the model parameters (such as output filter inductance, capacitance, wire resistance, etc.) of each phase are not required to be strictly symmetric with using FFT. These advantages can overcome the shortcomings of the use of instantaneous power transformation for the converter modeling. This chapter also proves that the multi-frequency DC model of UPQC on frequency domain can satisfy the passivity condition, so the UPQC can be controlled by using PBC.In section ?, a design method of zero steady-state error multi-frequency PBC is proposed for a low-order converter system. Firstly, traditional PBC theory is extended to the frequency domain to design the multi-frequency PBC for controlling the shunt converter of UPQC, namely the selective frequency control for output currents of the converter can be realized on frequency domain. At the same time, the selection rule of control parameter and its dynamic and static performance are also studied. Secondly, the relationship between the steady-state performance and modeling accuracy is analyzed in detail, based witch the modified multi-frequency PBC is proposed. At last, in the experimental part, firstly, a method of controlling DC voltage of the converter in the case of the non-sinusoidal and unbalance AC voltage is proposed, and then the performances of the traditional and modified controller are verified on experimental prototype with inaccurate model of the converter, and experimental results show that the modified multi-frequency PBC can achieve zero steady-state current error control at selective frequency, and has the excellent dynamic performance.In section IV, a design method of zero steady-state error multi-frequency PBC, which is also called cascade multi-frequency PBC, is proposed for a high-order converter system. Firstly, traditional PBC theory is extended to the frequency domain to design the cascade multi-frequency PBC for the series converter of UPQC (essentially it is a high-order system), namely the selective frequency control for the capacitance voltage of the converter is realized on frequency domain. After analysis, it is found that steady-state performance of the controller designed by the traditional theory also depends on the modeling accuracy of the series converter, then a modified cascade multi-frequency PBC is proposed. At last, the performances of the traditional and modified controller with inaccurate model are verified by experimental prototype, and experimental results show that the modified control has the excellent dynamic and static performance.In addition, in section IV, another zero steady-state error multi-frequency PBC method for the high-order converter system is also proposed, which is called high-order multi-frequency PBC. Similar to the above research methods and steps, the theoretical analysis and experimental verification show that the proposed controller can achieve zero steady-state voltage error at selective frequency, and has the excellent dynamic performance. The two design methods have their own advantages, the former can achieve modular design for a high-order converter system, and the latter can simplify the hardware design of the controller.In section V. firstly, hardware design and software control and instruction signal extraction of the UPQC are studied in detail. Then a large number of experiments are carried out on the UPQC device consisted of T-type neutral point clamped (T-NPC)converter, including a variety of voltage quality (swell, sag, imbalance, background harmonics, imbalance with background harmonics) and current quality control experiments. Finally, the experiment results validate the feasibility of the proposed control strategy based UPQC. |
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