Digital current control of a three-phase four-leg voltage source inverter by using p-q-r theory

In this study, balanced/unbalanced linear/non-linear current control in three-phase four-leg voltage source inverter (VSI) is carried out by using modified p-q-r method instead of the classical p-q-r method, digital proportional integral derivative (PID) control rule and three-dimensional (3D) space vector pulse width modulation (3D SVPWM). For this purpose, three-phase four-leg power inverter and PWM signal generation circuits are designed and modelled separately. Even if the power source voltage is unsymmetrical, the proposed modified p-q-r method leads to measurement and to generating the instantaneous harmonic and reactive components of load current accurately. The performance of the classical p-q-r theory under non-ideal mains voltages is improved and the accuracy of the proposed modified p-q-r theory is demonstrated both through simulation and real-time application. Each phase of the three-phase four-leg VSI modelled directly in abc reference frame is decoupled from each other This simplifies the digital PID controller design significantly. Modelling of the whole system, modified p-q-r measurement and digital PID design steps are given in detail. Real-time experimental results for the designed three-phase four-leg VSI in stand-alone mode are compared with the simulation results.