Mechanism of Conversion Between AC and DC Electrical Variables in LCC Main Circuits During Grid Dynamics Using Time-Varying Amplitude/Frequency Rotating Vectors

With the increasing challenges posed by line-commutated converter-based high-voltage direct current (LCC-HVDC) in the era of widespread renewable energy utilization, AC/DC dynamic interaction problems under controller regulations appeared more often. However traditional approaches often fall short in identifying the features of signals involved in AC/DC conversion in grid dynamics, and the characteristics of the conversion process in the LCC main circuit are overlooked. Consequently, this paper aims to fill this gap to help reveal the mechanism of grid dynamic problems dominated by controller regulations. Based on the recognition that the amplitude/frequency of AC electrical variables has time-varying characteristics in practice during grid dynamics, this paper focuses on the conversion mechanism between AC and DC electrical variables and then the basic features of electrical variables in LCC main circuits during grid dynamics are expressed using time-varying amplitude/frequency rotating vectors. First, the six-pulse firing pattern is expressed using the rotating vector with time-varying frequency. Then the time-varying amplitude/frequency rotating vectors are utilized to discuss how AC/DC voltage and current converse, including the power produced from them during grid dynamics. Moreover, simulations are performed to validate the efficacy of the mechanism, alongside elucidating its potential applications to benefit grid dynamic analysis. Finally, the salient contributions of this proposed framework are clarified.