Research On New Advanced Reactive Compensation Equipments And Their Control Technique

With the rapid development of national economy and technology, new trends have taken place in the modern power grid and load structure. The operation of large amount of impulse loads and fluctuant loads, such as electric arc furnaces, large rolling mills and electric locomotives, etc., makes voltage fluctuation, flicker and three-phase unbalanced even worse, which seriously weaken the economic and reliable operation of the grid. Besides, their social impact and economic losses are very serious. So, there is an urgent need for additional dynamic reactive power compensation device to improve the power quality.Compared with the traditional reactive power compensation equipments, SVC and D-STATCOM, as dynamic reactive power compensation equipments in medium and low voltage distribution grid, have significant advantages in controllability and response speed.Based on the analysis of three-phase unbalanced system's instantaneous power, this paper proposes a method that by installing SVC compensator with a filter to make the imbalance power caused by negative sequence and harmonic power flowing into the SVC and achieve the ultimate objective of a balanced system. A feedforward-feedback composite control method on the unbalanced load compensation of SVC is also given in this paper, which take the voltage loop control and three-phase unbalance control integrated as well as make power factor loop combining with the three-phase unbalanced control. It is proved that this method can meet the needs of various working conditions.The influence of the D-STATCOM under the imbalance power grid voltage is studied in this paper. When the three-phase power grid voltage is unbalanced, the output voltage of D-STATCOM has negative sequence content and 3rd harmonic,which can cause the over-current or burn the device in serious situation. Therefore, an improved switching function modulation is proposed in this paper, which can effectively improve the voltage output performance of D-STATCOM and suppress the 3rd harmonic and negative sequence. Meanwhile, a method which adds a negative sequence voltage feedforward control to the dual-loop voltage control of D-STATCOM to suppress the over-current caused by the negative sequence voltage, is proposed in this paper.Finally, a SVC equipment with the capacity of±6000kVar and a D-STATCOM equipment with the capacity of±50kVar are developed in this paper based on the relevant theory and simulation studies on SVC and D-STATCOM. The relevant experiments are also given in this paper. The results show that the SVC and D-STATCOM devices developed in this paper can meet the requirements of unbalance compensation and voltage control requirements, and the proposed control method is accurate, effective and feasible.