A Study On DC Capacitor Voltage Control Strategy Of Cascade STATCOM

With the rapid development of the power system and the increasing of power loads, the grid reactive power loss is increased too. At present, the power grids are faced many problems, such as the deficiency of reactive power compensation capacity, inefficient on the power transmission and use, increasingly serious pollution on harmonic. Especially, the loss of power grid is increased and power quality is affected, because of the impact reactive loads. Therefore, foreign scholars pay more and more attention on the impact of reactive power on the grid voltage. Harmonic suppression and reactive power compensation have become a major issue in the field of power electronics and power system. In order to improve level of safe operation and power quality of the grid, we must have advanced conditioning control means except the reasonable grid structure. It is a very important practical significance that made the real-time fast reactive power compensation when reactive power is deficiency. It is difficult to meet the requirements of the compensation system, because the technologies of traditional compensation devices are lagging behind. Static Synchronous Compensator (STATCOM), as a new type of dynamic reactive power compensation device, has much superiority, such as a fast response, high precision control, the filter performance and can effectively suppress the voltage fluctuation and flicker. So the STATCOM becomes an important member of the Flexible AC Transmission Systems (FACTS).This paper firstly describes the existing reactive power compensation devices, and analyzes the superiority of STATCOM comparing with other compensation devices. Secondly the working principle of the STATCOM is analyzed.Lastly, comparing several main circuit structures of the large capacity STATCOM, chose the superior chain STATCOM as the main study object. In this paper, Combined with the development of STATCOM, the common DC bus voltage control structures are enumerated, their advantages and disadvantages are analyzed and compared. The mechanism of DC voltage is explained and the various measures to control the balancing of the DC voltages are analyzed. Due to the randomness and uncertainty of the wind, the wind farms request the fans to have a certain low voltage ride through capability, especially in the event of failure or imbalance, STATCOM can improve the low voltage ride through capability. Therefore, research objective will be to analyze this phenomenon for voltage-drop in wind farms occasions.The DC voltage control strategy is based on the structures of the mathematical model of the device. Depending on actual operating conditions, firstly set up the ideal chain STATCOM model. According to the differences in physical model and the ideal model, this paper analysis of the reasons leading to imbalance phenomenon, lastly building a model with losses. For the widely used STATCOM devices in the wind, this paper describes the three-level DC voltage control strategy to balance the DC bus voltage. In order to overcome instability of the wind farm and the defects of the traditional control methods, the paper proposes a new control strategy-DC voltage control strategy with additional circuit. And the proposed control method is studied with simulation on the PSCAD model. The simulation and experimental results show the correctness and the feasibility of the mathematical model and the proposed control method.The research result of this paper can be helpful to the STATCOM designers, and provides the feasibility basis for the practical application of the chain STATCOM devices. With the continuous development and improvement of the power electronics and control strategy, the chain STATCOM will have a broader application.