Study Of A Waveform Control Technique For Inverter Based On Realtime Harmonic Analysis

Uninterruptible Power Supply (UPS) systems are widely used for supplying critical loads which cannot afford utility power failure. A CVCF-PWM inverter is the core of a UPS system. High quality outputvoltage waveform is required for these inverters. To achieve nearly sinusoidal output voltage even withnonlinear loads, many waveform correction techniques have been proposed. This paper is focused onwaveform control technique based on real time harmonic analysis of CVCF-PWM inverters. Thefollowings are the main work: With the load current treated as a measurable disturbance input, a linear mathematical model isestablished for a PWM inverter. The mathematical model is given in form of transfet function and statesequations. In additions, it is indicated that these two forms of mathematical model can be chosen accordingto diffient control method. For surpressing disturb with no error, block diagram of this control technique is proposed and thecapability to surpress disturb with no error is demonstrated. For compansate disturb, FFT algorithm isused to analyze the harmonic of disturb. Although the FFT algorithm has dramasticly impoved calculationefficiency of the DFT algorithm, the calculation for real time control is still too complicated. A simplifiedFFT algorithm which dramasticly simplify the calculation of FFT is proposed. To analyze the disturb, adata window must be added to the original signal. This will affect the accuracy of the spectrum analysis.The affect caused by adding window is discussed, it is indicated that if the window length and sampelfrequency obey particular request, high precise result can be achieved. The transient performance is poor and has serious oscilization tendency, so transient paramentfeedback technique should be used to improve the performance. The effect to the performance of thiscontrol system caused by transient parament feedback is discussed in detail. It is indicated that transientparament feedback technique can not only improve transient performance but also make somecontribution to stable waveforme quality. The design problem for digital PD controller is also discussed, atlast, a useful set of design methord is proposed. The result of FFT algorithm is complex and can not be used directly, so it must be translated to directmagnitude and initial phase. Here, the calculation of degree is the most important problem, because thecalculation accuracy directly influences the performance of this control system. In this thesis, a kind ofcalculation algorithm based on seaching in harf table is discussed first. Though this algorithm has veryhigh lever accuracy, the calculation is very complicated. In order to improve the calculation efficiency, asolve algorithm based on neural network is proposed. This algorithm with satified accuracy dramasticlysimplify the calculation and the complication of programme implement. At last, the proposed control technique is tested. The control algorithm is implemented using a TITMS320F240 DSP. Experimental results prove that the proposed control technique can achieve not onlylow THD during steady-state operation but also good transient response subject to load step change.