Research Of Static Synchronous Compensation Based On Three-level Direct AC-AC Converter

At present,the compensation of reactive power and harmonics in power grid mainly adopts the device based on the structure of inverters.The energy storage element on the DC side is a large electrolytic capacitor,which is expensive and easy to damage.So the cost of the device is greatly increased.This paper proposes an inverter-less static synchronous compensator(STATCOM)based on Buck three-level AC-AC direct converter with a small-rated transformer,realizing the compensation of reactive power and harmonics without energy storage elements.Firstly,this paper introduces the virtual quadrature source and even harmonic modulation technology to solve the problem that AC-AC direct converters can not synthesize the output voltage with any frequency and phase.The circuits which are suitable for the virtual quadrature source technology are analyzed.Buck three-level AC-AC direct converter is selected as the research object of inverter-less STATCOM.The working principle,working mode and control strategy of the converter are introduced in detail and its limitations are improved.Secondly,the principle of reactive power and harmonic compensation of single-phase inverter-less STATCOM is introduced.The detection methods of reactive power and specific harmonic currents of single-phase inverter-less STATCOM are deduced.The closed-loop control strategy of single-phase inverter-less STATCOM is also proposed.The simulation results verify the feasibility of reactive power and harmonic compensation of single-phase inverter-less STATCOM.Then,the structure of three-phase inverter-less STATCOM system is proposed and its input and output characteristics are analyzed.According to the characteristics of harmonic contents in three-phase three-wire system,the coupling problem between adjacent even harmonic modulation waves is solved by using the non-adjacent even harmonic modulation wave technology.This paper deduces the detection methods of reactive power and specific harmonic currents of three-phase inverter-less STATCOM.This paper also puts forward the successive harmonic compensation control strategy.The simulation results prove the correctness of the current detection methods and the reactive power and harmonic compensation ability of three-phase inverter-less STATCOM.Finally,the hardware and software circuits of single-phase inverter-less STATCOM are designed.The experimental results further verify the feasibility of reactive power and harmonic compensation of inverter-less STATCOM and the correctness of correlative theories.