Research On Digital Control Technique For Three-Phase Inverters

The three-phase inverters, which behave as the main AC power supplies, have played an important role in many areas such as official automatic system, medicine, telecom, national defense, and so on. Fast response and little distorted output voltage of the three-phase inverters depend on proper design of control system. The realization of full-digital controlled system makes both possible.Supposing load as disturbance, mathematic models of three-phase inverters in abc three-phase stator coordinate,αβ0 stator coordinate and dq 0 rotating coordinate are established and their difference is compared. Further more, the state space average models of three-phase inverters are given.This thesis has presented increasing-state feedback control schemes both in analog field and digital field for a single-phase inverter. And based on this analyzed the possibility of extending these control strategies to three-phase inverters. Further more, it has proposed the increasing-state feedback schemes in the dq 0 rotating coordinate both in analog field and digital field and pointed out the increasing-state feedback scheme is more suitable for digital control. And the design of controller is based on the pole-assignment method. Theoretical analysis and simulation verify this control strategy can restrain disturbance quickly and high quality output waveform can be obtained. In the digital control system, one-step-delay control is always adopted because the delay which arises from the sampling and calculation reduces the maximum modulation ratio. The influence of one-step-delay control on increasing-state feedback three-phase inverter system is analyzed through simulation and the analysis shows the effect of the one-step-delay control should be considered.Based on all the above researches, the increasing-state feedback digital control scheme in dq frame is applied on a 50Hz three-phase inverter experimental equipment. The control algorithm is implemented using a TI TMS320F240 DSP. Experimental results prove the increasing-state feedback digital control scheme in dq frame for three-phase inverters can provide not only low THD during steady-state operation but also good transient response subject to load step change.