Research On The Hybrid Dynamic Var Compensation Technology For Power Distribution Systems

The application of electric energy, which acts as a pillar energy and economic artery in modern society, is one of the most important symbols of the level of development and comprehensive national power of a country. Reactive power is a crucial factor for the design and operation of AC power systems, and is closely bound up with the safety, stabilization, and economical operation of the power system. With the development of electric power industry, the requirement of reactive power becomes much stricter. It is unreasonable if all the reactive power is provided by generators and transmitted through long distance, which is usually impossible. So the reactive power should be supplied at where it is needed. Dynamic reactive power compensation is an economic and effective measure for improvement of voltage stability. At present, it is imperative under the situation to develop static dynamic reactive power compensators, which has important practical significance.The static var compensator (SVC) and the static synchronous compensator (STATCOM) are two kinds of important dynamic var compensators in power systems. This paper presents a hybrid static var generator (HSVG) from the view of nowaday technological economics of power systems in China. The proposed topology employs the STATCOM to replace the thyristor controlled reactor (TCR) generally used in an SVC. The HSVG integrates the advantages of the SVC and the STATCOM and offsets respective disadvantages. So it can not only meet different demands of reactive power, but also keep the system loss, cost, and occupied floor space at an optimal level, expecting superiority in the performance and practical operation of the device.This paper introduced some basic principles of dynamic reactive power compensation in transmission and distribution systems from the aspects of improving system voltage, increasing the transmission line capacity, and preventing voltage instability. The mathematic description of the dynamic reactive power compensation unit of the HSVG in dq0-coordinate under power system balance and imbalance is given, and the characteristics of voltage-current, stability, and harmonics of the HSVG are theoretically analyzed, which laid the foundation for the research of the control strategy and design of the experimental prototype.A coordinated control method for reactive power output is presented. The TSC is used to meet the steady state demand of reactive power, while the STATCOM is used to deal with the dynamic demand of reactive power in good time and keep enough controllable hot spare for reactive power against a possible transient disturbance. When the system reached a new stable working point, a certain number of TSCs should be switched on or switched off, and the STATCOM unloads the steady-state demand of reactive power in order to cope with a next potential disturbance.The hardware and its development platform of the digital control system of the HSVG are the key part of the HSVG where the whole control strategy is put into practice and play a vital role in the performance of the HSVG. Considering the acquisition and processing of large numbers of data and implementation of more complex arithmetic, this paper designed a digital control system based on the digital signal processor (DSP) and the Field Programmable Gate Array (FPGA). In the system mentioned above, the DSP is only used for the acquisition of data and control calculation, while the other tasks are done by the FPGA, which ensures the CPU resource well and makes the whole system working under quasi-parallel state. The DSP plus FPGA system is featured with good flexibility and real time performance, high integration, and strong universality. These advantages make it suitable for modularized design and easy to maintenance and extension, shorten the development schedule, and reduce the cost.A 380V/15kVA HSVG experimental prototype and its experimental system is designed. The main circuit and its peripheral circuit of the HSVG is studied, including the choice of the main circuit switch device and the design of relevant drive and protection circuits. The selection of the DC-link capacitor and the AC-link reactor of the voltage source converter (VSC) is presented through theoretical analysis, simulation, and experiment. Finally, some experiments are conducted on the prototype and the experimental results are presented verifying the compensation performance of the proposed topology and the feasibility of this paper.