Research On Three-level PWM Rectifiers

In recent years, high-voltage and high-power power electronics devices have been widely used.In the application fields of these devices, diode-clamped three-level PWM rectifier is an effective and practical solution since it has many advantages. For example, it can provide sinusoidal input current which contains low harmonic components, its power factor can be very high and it provide bidirectional flow energy and so on. In this paper the control system of the diode clamped three-level PWM rectifier will be studied in detail.According as the relations between topologies of the three-level PWM rectifier in different switching instants, the mathematic model in ABC three-phase stator coordinate is set up. Using coordinate transformation, the mathematic model in dq two-phase rotating coordinate is established, and by this means, the mathematic model is predigested. Base on the mathematic model, the controller is designed and stability is analyzed later. In the analysis, the load is treated as disturbance.Three-phase symmetric sinusoidal voltages can synthesize a rotate space voltage vector. In this paper, the problem of how to attain space voltage vector by SVPWM modulation strategy in three-level PWM rectifier is discussed. By this means, three-phase input sinusoidal currents can be obtained. And a modulation strategy that can reduce switching loss is introduced, tooNeutral-point balance problem is an inherent problem of three-level PWM rectifier. Without neutral-point control, the harmonic components of input current will greatly increase, and the DC-link capacitors and the switching devices may probably be destroyed. The influences of every switching state on neutral-point are analyzed. Then coordinate transformation is used, by this means, the influence of small-vector and medium-vector on neutral-point is quantified. Based on the theory, two control strategies of neutral-point are provided. One is Bang-Bang control method, and the other is accurate control method. In the first method, neutral-point is controlled by choosing positive and negative vectors. In the second method, the instants of positive and negative vectors is accurately controlled and neutral-point is controlled accordingly.In this paper, the direct current control strategy, which combines the current state feedback decoupling method and two closed-loop control strategy is studied in detail. After being decoupled, the model of three-level PWM rectifier is very similar to the DC motor governor system, so the idea of controller design in the DC motor governor system can be used for reference, engineering design method, optimal parameter confirming method and natural surging method, are used in the design process of two closed-loop control system. Also, in order to improve the dynamic characteristic of the rectifier and lighten the burden of the controller, compound control strategy is used.A three-level PWM rectifier prototype is constructed, which uses TMS320F240 DSP as the heart of control system. Experimental results demonstrate that, using proposed neutral-point control method, the neutral-point ripple can be restrained effectively. Also the control system can provide satisfied steady state and dynamic characteristic for the sinusoidal input current and output DC voltage.