| With the rapid development of the power & electron parts, PWM inverter is applied ,on a larger and larger scale ,to AC frequency control system﹑UPS﹑AC regulated power source﹑medium-frequency source and other power & electron installation ,but hazard of harmonic produced by inverter get more and more seriously to outside installation. According to summing up restraining harmonic effect of PWM techniques utilized in inverter , selective harmonic elimination technique(SHET) which has the best effect on restraining harmonic is profoundly studied in the paper ,and at the same time the harmonic problem of SPWM voltage source inverter which is applied on the largest scale at present is quantitatively and systematically analyzed . Based on SPWM basic principle and Fourier series theory , the mathematical model of output waveform of SPWM voltage source inverter is built .Through theoretical analysis and calculation ,distribution regularity of harmonic which inverter produces during variable frequency and variable voltage is obtained ,and harmonic problem existing in SWPM technique is found ,and these above provide theory reference for valid application of SPWM technique . The character of SHET is systematically summarized after expounding basis principle of SHET. In SHET ,equations which represent relation of fundamental component and preparatory eliminated harmonic components are a system of nonlinear , transcendental equations that require the iterative Newton-Raphson matrix method for its solution ,and convergence in solution depends on whether or not starting value given is right .By means of trial-and-error method of starting value in low order matrix ,assignment regularity of starting value is found ; Through calculation by means of programming , the track of switching angle changing with fundamental amplitude and value of switching angle under some fundamental amplitude are obtained .With regard of three-phase inverter ,two groups of solution are acquired . Acquiring value of switching angle settles a key problem which hampers wide-range application of SHET . In order to understand refraining harmonic effect of real output waveform in SHET ,the mathematical model of real output waveform in which the factors involved in turn-on time ﹑turn-off time ﹑added dead zone time of switching angle are taken into account is built ,and their effect on fundamental component and all order harmonics is analyzed ,and methods of adding dead zone time are studied ,and these above provide important theoretical basis for industry application of SHET . Through studying distribution regularity of switching angle in SHET and the problem found when improving quality and efficiency of output waveform , optimal strategies that double switching angles and minimum pulses are ignored are proposed , the goal that hardware circuit is convenient to be made and more harmonics are eliminated by using less loss switching transistor is reached .This plays pretty important role in the fact that advantage of SHET is exerted and it can be applied ,better and more quickly , to industry application. All design schemes and their advantage and disadvantage of on-line variable frequency and variable voltage of selective harmonic elimination inverter are studied ,and the possibility of on-line calculating switching angles is discussed ,and these above establish fundament for design of selective harmonic elimination inverter . Finally , the conclusions on output waveform and its spectrum diagram of selective harmonic elimination inverter drawn from experiments are shown to be in good agreement with theory . This research plays greatly theoretical and applicable role in extended industry application ; At the same time ,it will greatly promote the development of efficient and reliable supply electric in our country , and carve out a important way of utilizing power more efficiently and less pollutantly. |
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