Research On Key Technology Of STATCOM Based On MMC Structure

All walks of life cannot leave the electricity in current society.With the continuous development of economic society,the grid is becoming more and more big.Since the electricity load is not purely resistive,it will generate a lot of reactive power and harmonics,so it is necessary to improve the power quality.Static synchronous compensator can dynamically compensate system reactive power and suppress harmonic,improving the power quality significantly.STATCOM is an important part of the smart grid construction.Therefore,it is meaningful to study the key technology of STATCOM.Firstly,compare and analyze the main circuit of STATCOM's converter,selecting the modular multilevel converter as the main circuit of STATCOM.Modular multilevel converter has obvious advantages in high voltage and high power field.MMC has the advantages of good output waveform,four-quadrant operation,easy modular design,redundant design and high efficiency.MMC's modulation strategy selects CPS-SPWM which is simple and the equivalent switch frequency is high.Considering the disadvantages of slow speed and high switching frequency,the improved voltage balance control is introduced which include adjusting factor,lower limit of voltage and grouping sorting.The PI-IR regulator is used as the bridge arm circulation controller,which can effectively inhibit the double frequency component of the bridge arm circulation.Secondly,the instantaneous reactive power theory that can detect reactive power and harmonic current is introduced.The principle of compensation for reactive power and harmonic suppression is analyzed.This paper presents a phase-locked loop called DSOGI-PLL that can work well in a complex condition such as harmonic interference,frequency mutation and unbalanced power grid.An indirect compensation method based on feedforward voltage decoupling is presented.Theoverall control strategy of the system is obtained by combining all of above control methods.Finally,the system is simulated and verified in simulink software.On this basis,the software program and hardware circuit of the system are designed and the small experimental platform is built.The simulation and experiment show that the proposed topology and control strategy have good reliability and validity.