Research On Control Strategy Of Cascaded H-Bridge Multilevel STATCOM

With the grid connection of distributed energy sources such as photovoltaic and wind power and the continuous increase of non-linear loads,the research on power quality of power system is also gradually deepening.Static Synchronous Compensator(STATCOM)can compensate reactive power while suppressing grid harmonics,which is the focus of research in enterprises and universities.At present,the key point of cascade STATCOM research is DC side voltage balance.In the case of asymmetrical grid voltage,it also faces the same challenge.In this paper,the control strategy of cascade H bridge STATCOM is studied and analyzed.The main circuit topology of cascaded H bridge STATCOM is multi-level inverter,so the traditional two-level modulation strategy cannot be directly applied.Therefore,this paper studies multilevel modulation technology,including multi-carrier PWM modulation and specific harmonic cancellation technology,among which carrier phase-shift modulation is able to achieve in-phase sub-module unit power equalization and is the modulation strategy chosen in this paper.In order to study the control strategy of cascaded H bridge STATCOM,the working principle of cascaded H-bridge STATCOM with multilevel is used to establish its mathematical model.On this basis,the current control structure of voltage feedforward active power and reactive power decoupling is constructed.For reactive current detection,an i_p-i_q detection method is used to accurately detect the reactive power of the grid,so as to achieve effective reactive power compensation.Under the voltage balance condition of the power grid,aiming at the unbalance of capacitor voltage on the DC side,the mechanism of unbalance capacitor voltage on the DC side is analyzed from the Angle of active power flow direction,and a three-stage voltage equalization control structure is adopted to realize the voltage equalization control on the DC side,including global voltage equalization,phase voltage equalization and phase voltage equalization.Finally,it is verified by simulation that the cascaded H-bridge STATCOM control strategy studied in this paper can realize bidirectional reactive power compensation of the grid,and make the DC capacitor voltage balance of each sub-module on the DC side under the balanced condition of the grid.In the case of unbalanced grid voltage,the equalization control in the balanced condition can no longer play an effective role.In order to realize the voltage equalization control under the condition of power grid voltage imbalance,the double-order generalized integrator phase-locked loop and positive and negative sequence voltage separation are studied.In the condition of power grid voltage imbalance,negative sequence voltage especially affects the voltage equalization control between phases.Therefore,feedforward zero-sequence voltage injection method is adopted to achieve voltage equalization between phases.Finally,through simulation results,it is verified that the sampled scheme can achieve DC side capacitor voltage equalization.Figure 70 Table 6 Reference 65...