Influence Mechanism of PLL on Active/Reactive Voltage Excitation and AC current Amplitude/Frequency Response Relationship of LCC-HVDC station

With increasing penetration rate of renewable energy in the weak power grid, the large-scale application of Line Commutated Converter based High Voltage Direct Current (LCC-HVDC) Transmission has led to dynamic issues threatening the stable operation of the system, such as overvoltage at the sending end and wide-range oscillations. Within these dynamic issues, the converter station regulates the direct current via AC-DC conversion, thereby affecting the amplitude of the AC current, meanwhile the firing angle command and phase-locked loop (PLL) position establish the frequency of the AC current. This implies that the converter station no longer serves solely as a load for power transmission but follows the mechanism to regulate active and reactive power by adjusting the amplitude/frequency of the output AC current. Especially the PLL control approach is vital in understanding this operational mechanism in the weak power grid. However, current research primarily focuses on mathematically describing the converter station based on physical connection relationships, neglecting the necessity to comprehend the operational mechanism to understand how devices influence the stable functioning of the system. Actually, the relationship between the output AC current's amplitude/frequency and power excitation represents a generalized description of the converter station's operational mechanism. Consequently, researchers fail to establish the influence mechanism of the PLL approach on this excitation-response relationship. Therefore, this paper reveals the operational mechanism of the converter station in adjusting the output AC current's amplitude/frequency based on active/reactive voltage (representing the meaning of active/reactive power) as well as the excitation-response relationship. Then, it focuses on the process where active/reactive voltage regulates the frequency of the AC current through phase-locked error, clarifying the influence mechanism of PLL on AC current frequency regulation. Finally, the simulation is added to further explain the influence mechanism of PLL.