| Some status are widespread in the distribution networks of our country, such as insufficient reactive power compensation capacity, lots of harmonic loads, serious no-load losses of distribution transformer, and so on, which cause a great deal of energy losses. The power supply reliability and power quality problems of the distribution networks also have attracted people’s great attention. With the rapid development of modern power electronics technology, control technology and energy storage technology, the static synchronous compensator (STATCOM) and the energy storage devices play increasingly important roles in energy saving and power quality controlling in modern power system. Based on the National863Project and the National Natural Science Foundation of China, this thesis focus on the energy storage and inverter control technologies in distribution network applications, in order to improve power quality and reduce energy loss in the distribution networks.First, the structure of the novel energy saving compensation system of STATCOM integrated with distribution transformer (DT-STATCOM) was studied and the characteristics of several DT-STATCOM structures were carefully analyzed. It’s pointed out that in the structure of DT-STATCOM with multi-group taps in Dyn-type transformer, STATCOM has larger access voltage range, so we can choose an appropriate connected voltage to realize the target of simplify the main circuit structure of STATCOM. This thesis mainly analyzed the compensation mechanism of the DT-STATCOM with multi-group taps in Dyn-type transformer. The internal current distribution in the transformer windings was analyzed and an appropriate tap voltage level was selected, ensuring that the distribution transformer windings would not be over current.Second, the mathematical model of three-level DT-STATCOM was established, and the double-loop direct current control strategy of STATCOM system was built, including an improved ip-iq current detecting method and a three-level hysteresis current control method with variable hysteresis-band. In the improvedip-iq current detecting method, both voltage phase angles of the detecting point and the compensation point were used, extending the traditional ip-iq reactive current detecting method and solving the current detecting problem when the detecting point and the compensation point were different. The hysteresis current control method with variable hysteresis-band overcame the shortcoming of large scale of switching frequency in the traditional hysteresis current control method.Third, the output filter of STATCOM plays an important role in filtering high frequency harmonics and improving grid current waveforms. This thesis analyzed the characteristics of LCL output filter of D-STATCOM mainly from these aspects:the attenuation amplitude of harmonic currents, the fundamental voltage drop of inductor and the reactive power compensation ability of D-STATCOM. Based on this, a generally applicable method to design the parameters of LCL filter in D-STATCOM was proposed. Besides, an improved control method is proposed for STATCOM with LCL filter: calculating the network-side current using the inverter-side current by an adjustment coefficient, then the calculated network-side current is compared with the command current in the control system. By this method, the impact of the filter capacitor current is reduced, and the reactive power compensation ability of D-STATCOM is improved.Fourth, combine energy storage device with STATCOM to form an energy storage STATCOM system, which has flexibly bidirectional four-quadrant operation ability and can further improve the power quality and power supply reliability of distribution networks. This thesis analyzed the structure of super-capacitor-based energy storage STATCOM system and proposed a corresponding control strategy. The primary objective of energy storage STATCOM system is to maintain the stability of DC bus voltage and compensate reactive power for the grid. At the same time, the energy storage STATCOM system can provide certain active power to the grid according to the actual situation. Based on this, a method using the energy storage STATCOM system for urban rail train regenerative braking energy absorption and utilization was put forward in this thesis, which combines the advantages of the two regenerative braking energy absorption modes—energy storage mode and inverter feedback mode. The method can effectively absorb train regenerative braking energy and maintain the stability of the DC bus voltage. Besides, it can reduce the design capacity of each regenerative braking energy absorption mode.Fifth, on the basis of theoretical studies, simulation analyses were conducted for the proposed system structure and control strategy. Furthermore, a multi-function experimental platform of DT-STATCOM system was built, experimental schemes for two types of DT-STATCOM structure were designed—DT-STATCOM with multi-group taps in Dyn-type transformer and DT-STATCOM with central point taps in Dyn-type transformer, and experimental tests were conducted. The experimental results show that DT-STATCOM system has significant compensation effects for both reactive and harmonic loads.Finally, the full-text work and innovation points were summarized, and the future research work was prospected. |
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