Unbalanced Load Correction Of Cascaded DT-STATCOM

As an effective means of the power system reactive power compensation and power quality regulation,D-STATCOM has been widely studied and used.Cascaded multilevel distribution transformer STATCOM(DT-STATCOM),which integrated the cascaded multilevel converter and the distributed transformer,has combined the advantages of both high-voltage high-power STATCOM and D-STATCOM.By selecting proper integrated points,the power electrical devices could withstand lower voltage and converter output large compensation power.And the converter adopts cascaded multilevel topology which has a large compensation capacity and lower output harmonic component.Based on the topology of cascaded multilevel DT-STATCOM,theoretical reach and experiment verification of the reactive power compensation and unbalanced load correction is developed.Firstly,the structure scheme of cascaded multilevel DT-STATCOM is explained detailed,as well as the basic compensation principle.The topology of cascaded multilevel converter,output filter and the way of integration are emphatically analysised.With the change of the tap position,the relationship between the tap voltage and the high voltage side of the transformer is studied.Based on the principle of magnetomotive force balance,the distribution of the compensation current in the transformer windings is studied.Then,zero sequence voltage injection method is introduced to balance the interphase DC capacitance voltage,and related control strategies and principles are researched.The mathematical model of positive and negative sequence of the DT-STATCOM system is established,and the unbalanced power of each phase link are calculated and analyzed by calculating the negative sequence component and the zero sequence voltage.The unbalanced power produced by zero voltage is used to make up the unbalanced power produced by negative sequence component,which helps us to solve the required zero voltage.Based on the constraint condition that the control system does not occur over modulation,the unbalanced compensation ability of DT-STATCOM is deduced.MATLAB/Simulink simulation model is built to verify the control algorithm.Next,the comprehensive control strategy of reactive power compensation and unbalanced load correction is analysised in detail.Due to the different electrical quantities of the compensation point and the load point,amplitude and phase compensation are used to detect the current of positive and negative component.The feedforward decoupling controlling strategy has been established to fulfill the current tracking.And the hierarchical control strategy is used to realize the DC capacitors voltage at the around of the reference value.And the simulation is builted to verify the algorithm.Finally,a scale-down prototype was developed.The hardware and software process were illustrated.Further more,dynamic simulation experiments are carried out to verify the algorithms.