Research On Reactive Power Compensation Of Chained STATCOM Under Unbalanced Operating Conditions

The unbalanced load in the distribution network produces a large amount of reactive power,negative sequence and harmonic current,resulting in a large loss of power in the power system.Therefore,it is necessary to study the compensation of the reactive power compensator in the unbalanced environment.As the STATCOM of the chain structure has many advantages,including: dynamic response speed,good economic performance,good harmonic performance,so in the unbalanced conditions of reactive power compensation plays an important role.The structure of the chained STATCOM device has two types of wiring,one is star and the other is triangular.For the reactive power compensation under unbalanced operating conditions,in order to better realize the power balance between the commutation chain,it is usually solved by injecting the zero sequence current.The chained STATCOM of the triangular structure has its own zero Sequence current flow channel,so usually use the triangle chain STATCOM as a reactive power compensation device.The work done in this paper is as follows:Firstly,the basic principle of STATCOM and the main circuit structure are introduced.Then,the mathematic model of the STATCOM device is established.Finally,in order to design the controller of STATCOM device,we choose the method of coordinate transformation to get the order mathematical model The In order to control the STATCOM device,it is necessary to complete the feedforward decoupling control in the positive sequence mathematical model and the negative sequence mathematical model in advance.Aiming at the modulation strategy of H bridge cascade STATCOM,the single pole frequency doubling SPWM modulation method and the bipolar SPWM modulation method are analyzed.By comparing the output level of the two methods and the output voltage harmonic component,Frequency CPS-SPWM modulation method.Secondly,the accurate detection of reactive current is the premise of STATCOM device to complete reactive power compensation.Firstly,two kinds of reactive current detection methods based on instantaneous reactive power theory are introduced: dq reactive current detection method and reactive current detection method.These two methods can be in a specific occasion to achieve accurate detection of reactive current.Finally,a method to detect the reactive current under unbalanced conditions is proposed,and the relevant simulation model is established to verify the accuracy of the method.Based on the analysis of the principle of unbalanced DC voltage of the cascaded H bridge STATCOM device,a hierarchical control method is proposed to control the DC side capacitor voltage balance.The control method is derived from the idea of sequential control: in order to control the DC side of the overall voltage to maintain balance,select the control device by the positive sequence current to achieve,the same can be obtained,in order to control the phase of the DC side of the voltage to maintain Balance,choose by injecting zero sequence current method to achieve.For the control of the device to make the capacitor voltage within the balance method,select the control by the various modules of the output voltage amplitude and phase size to achieve.Finally,the model of 3k V / 5MVA is established accurately in Matlab / Simulink simulation environment,and the accuracy of the method is verified by simulation.