Research On Key Techniques Of Cascaded H-bridge Static Var Generator

With the widely use of non-linear and impact load, m any power quality problem s appearerd in transmission and distribution networks of power system. Among those, problems such as low power factors and severe harmonic pollution are the m ost prominent. Static reactive power generator has become a resear ch hotspot in the field of reactive power compensation because it could not only solve the aforem entioned problems but also with many excellences, including fast compensation and higher accuracy. However, compensation capacity of a single power module is generally lim ited by the switching frequency of the power electronic devices, as a result it can’ t meet the requirem ent of the power syste m capacity. As a novel high-capacity reactive power compensation device, cascaded H-bridg e SVG has quickly attracted m uch attention, a nd the key techniques to designed it include reactive current detection, modulation technique, dynamic reactive power control, DC side voltage balance control, cont rol and protection unde r the unbalanced conditions, etc. This paper focus on the follo wing three aspects: dynamic reactive power control, DC side voltage balance control and control under the unbalanced conditions.Keeping the DC side voltage stability and bala nce is the basis of reliable operation. On the basis of the mathematical model and operation characteristics of cascaded H-bridge SVG, causes for imbalance of DC side voltage are an alyzed, the mathematical model is simplified by decoupling control mothed, and a three-level balancing control method of DC side voltage is presented, which is based on zero sequence voltage control and in cludes global DC side voltage balancing control, phase to phase DC side voltage balancing control and balancing control of DC side voltage w ithin a phase, thu s the problem of DC side voltage balancing control is well solved. In th e end, through simulation anal ysis, the proposed m ethod and relevant balance control m ethod are com pared to verify that the proposed m ethod can effectively maintain the dynamic balance of DC side voltage.The reactive power dynamic control is the most crucial control algorithm for the device, which directly affects the reactive power compensation effect. In view of the traditional PI control has a series of inherent defects, such as dif ficult to se t parameters and lack of adaptability, etc. A fuzzy PI con trol algorithm combined by PI control and fuzzy control is introduced, the PI control param eters are optimized and setted by fuzzy controllor. On the basis, a direct current control m ethod which is based on fuzzy PI control is put forward, and the reactive power compensation perform ance is im proved greatly. Through sim ulation analysis, the proposed mothed is verified to be practicable and has some advantages, such as fast compensation and higher accuracy.The reactive power com pensation performance is serio usly affected under the unbalanced condition, even the device m ay be fo rced out of service for over current. The operation characteristic of th e device is derived under unbala nced system, and the positive and negative sequence com ponent of unbalan ced voltage are separated by sym metrical component method and coordinate transform. On the basis, a negative sequence voltage feedforward control m ethod and a com pensation control m ethod for reactive power and negative sequence component of load are pres ented, thus the problem s of over current and reactive power compensation caused by unbalanced system are well solved, and the proposed method is verified to be efficient by simulation analysis.