In the traditional virtual flux estimation for a three-phase PWM rectifier, the integration element causes problems for the initial value and DC bias, and the unstable grid voltage induces a non-constant flux amplitude. To address these issues, an improved direct power control (DPC) scheme based on an adaptive sliding mode observer (ASMO) is proposed in this work. The observer employs a sigmoid function as switch function to estimate the grid side source voltage. Meanwhile, an adaptive compensator instead of pure integral element is also designed to dynamically adjust compensation. The stability of this observer is proved by the Lyapunov function; moreover, simulations and experimental results indicate that this new virtual flux observer substantially improves the observation accuracy based on voltage sensorless control. The application of this strategy successfully suppresses the fluctuation of the dynamic voltage response in the DC bus, eliminating high-frequency noise from the grid side, whilst simultaneously boosting the power quality.
1. PWM rectifier
2. Virtual flux
3. Adaptive sliding mode observer
Fig. 1. System structure of traditional voltage sensor-less (VF-DPC)
EXPECTED SIMULATION RESULTS:
Fig. 2. Comparison of steady-state curve under four methods.
Fig. 3. Dynamic simulation I of saturation suppression and adaptive sliding mode.
Fig. 4. Dynamic simulation II of saturation suppression and adaptive sliding mode.
Fig. 5. Dynamic simulation of traditional voltage sensorless control.
Fig. 6. Dynamic simulation of adaptive sliding mode.
Fig. 7. Comparison of bus voltage during load step.
Fig. 8. Comparison of phase current and voltage during load step.
This paper introduces sliding mode control in a virtual flux observer, based on the three-phase PWM rectifier model under virtual flux DPC; moreover, the systematic design of an orthogonal feedback compensation method to calibrate the flux estimation has been proposed. An improved sensorless control algorithm with an adaptive sliding mode observer has been simulated and experimentally verified. Results show that the combination of sliding control and virtual flux observer has improved dynamic response over traditional control strategies. This scheme can significantly improve the observation accuracy and dynamic response performance of the observer, and suppress the dynamic fluctuation and harmonic disturbance, increasing the overall power quality and delivery.
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