Study On Modulation Technique And Relating Problem Of Novel AC/DC/AC Voltage Source Converter

AC/DC/AC voltage source converter (VSC) widely used in civil, industry or military territory becomes research focus of scholars and experts from home and abroad. With the demand of power energy and the request of consumers to high quality and reliable power increasingly getting higher, AC/DC/AC VSC is starving for improvement in input and output performance. However, its high capacitance DC capacitor brings negative effect which is noticed today. The equipment that uses high capacitance electrolytical capacitor has disadvantages, such as bulk, high price, heavy harmonic pollution and relating power quality problem. In many cases, the lifetime of the capacitor is main factor limiting lifetime of the converter. So, novel topology and modulation technique of AC/DC/AC VSC is a difficult task to scholar engaged in this field. Due to above-mentioned problem, a series research work on modulation technique and overvoltage protecting method for novel AC/DC/AC VSC under different loads is carried out.A novel AC/DC/AC VSC main circuit topology is proposed and system mathematic model including DC side and inverter-bridge is estabished and fluctuating situation of DC side voltage and harmonic of line side current are detailedly analyzed. Foundation of mathematic model provides important theoretic basis for harmonic analysis and suppression of DC side overvoltage and research of modulation strategies. Analysis results show that DC side voltage fluctuation can be estimated by summing fluctuations due to the rectifier and inverter-bridge and the higher of DC side capacitor capacitance, the larger of THD for line side current.Producing high quality voltage is one of novel AC/DC/AC VSC's important functions. However, uncontrolled rectifier for new topology outputs ripple DC voltage. As ripple DC voltage, low harmonics of output voltage are largely increased by using traditional moudlation technique so as to produce adverse effect to load. Due to above-mentioned problem, an advanced reconstruction modulation wave SPWM (ARMW-SPWM) technique is proposed. Investigation results show proposed technique can eliminate harmonics caused by ripple DC voltage for output voltage under resistance inductance load and can greatly reduce DC capacitor so that main circuit are simplified. The ARMW-SPWM technique theory analysis is verified by simulation.It is very importance of producing correct voltage vector for converter under variable speed drives system of scalar control. Because of the constant DC side voltage assumption during the switching period leads to an error in produced voltage for novel AC/DC/AC VSC so that the electromotor cann't be accurately controlled. So the new modulating principle, named Flux Linkage Differential Space Vector Pulse Width Modulation (FLDSVPWM), is proposed in the basis of Space Vector Pulse Width Modulation (SVPWM). Investigation results show FLDSVPWM technology used is able to produce the correct voltage vector. On the other hand, Due to DC side voltage fluctuation, the linear modulation region ends earlier compared with a high capacitance DC-link capacitor VSC. So, Amplitude Constant Overmodulation (ACOM) technique is proposed that can make converter's output modulation index achieve higher.It is very importance of suppressing overvoltage and resonance for novel AC/DC/AC VSC to improve its reliability and line side performance. When motor is light load or braking, power direction is changed from converter to motor so as to make DC side voltage raise intolerably high. Traditionally, the DC side overvoltage protection has been carried out by ideal nonripple DC side voltage. But DC side voltage for novel AC/DC/AC VSC is by far not constant and fluctuates rapidly so that traditonal overvoltage protecting method cann't be used. To solve this problem, a new DC side overvoltage protection method, named PFDTC, based on power factor dynamic tracing control has been introduced. The method can ahead judge braking state of electromotor and suppress the energy back to avoid the overvoltage. Otherwise, due to small DC side capacitor and samll line side impedence, the resonance frequency between DC side capacitor and line side inductance becomes significantly high getting close to or even exceed the switching frequency that easily result in resonance. The approximate margin to avoid the resonance is decided by simulation and experiment so that adverse effect from resonance is suppressed.Novel AC/DC/AC VSC is designed and experimented. The experimental prototype of novel AC/DC/AC VSC is discussed in detail. Especially, multiple configuration controller buildup with high performance DSP system and C51 platform is developed according to the character of control tasks. Hence high speed real-time control and friendly human interface is combined. Finally, the results obtained from experimental system prove the validity and feasibility of novel AC/DC/AC VSC, which gives important theortical and practical foundation for further research.