Analysis and Improvement with Noise Immunity of dq Frame Based Open-loop Phase Detection Scheme

The measurement accuracy and rapidity of phase detection block are the crucial factors for the higher performance control of grid-connected converters under high-frequency noise condition. Open-loop phase detection (OPD) in the dq frame is characterized by fast grid synchronization, but its noise immunity has been unevaluated quantitatively, including interference principle and influence law of phase accuracy. To remedy this problem, a detailed noise immunity analysis model is developed in this paper, which shows that the OPD behaves different noise immunity when the dq frame is in different positions. In case of the voltage coincides with the d-axis, the noise has the smallest interference on the phase accuracy, and OPD has the strongest noise immunity. On the contrary, when the angle between voltage vector and d axis is close to \pi/4, the phase error is the largest. Inspired from this, a novel OPD to improve the phase-measuring accuracy is proposed. By fine-tuning the rotation angle of dq frame, the angle between voltage vector and the d axis is always close to 0, which can enable the phase error to be acceptable. In addition, the enhanced algorithm can reduce the bandwidth requirement of the filter and shorten the dynamic response time in the phase-detecting process. The correctness of the noise immunity analysis and the effectiveness of the improved algorithm are verified by the real-time simulation.