Large-scale renewable energy transmission via the voltage source converter (VSC) based high-voltage direct current (HVDC) is a crucial development direction for constructing a new-typed power system in China. However, renewable energy is characterized by volatility, intermittency, and randomness. When the sending-end modular multilevel converter (MMC) cannot adapt to the rapid fluctuations in renewable energy output, its energy balance will be disrupted by the active power difference between the AC and DC sides, causing issues such as wideband oscillations and exacerbated circulating currents. To solve the problem mentioned above, a novel energy balance-based control method for MMCs connected to wind farms is proposed in this paper, enabling the MMC to effectively adapt to fluctuations in renewable energy output and naturally maintain circulating current at a relatively low level. Firstly, the evolution principle illustrating topology decomposition and reconfiguration of the MMC is revealed. Secondly, the control method for AC internal voltage is proposed, which combines the energy balance between the half MMCs and voltage amplitude support. Thirdly, the DC internal voltage is defined, and its control method is proposed based on the MMC's overall energy balance. Then, independent control of each bridge arm is achieved by integrating the energy balance of the bridge arms with both the AC and DC internal voltages. Finally, an electromagnetic transient simulation model is built with PSCAD/EMTDC, and the efficacy and practicality of the proposed method are demonstrated through extensive simulation experiments.

To ensure their sound and continuous operation to the greatest extent, VSC-based DC girds have extremely stringent requirements for transmission line relay protection. In terms of guaranteeing their reliability, accurate identification of lightning strikes on DC transmission lines is one of the urgent key problems to be solved. An effective ultra-high-speed identification scheme of lightning strikes suitable for the VSC-based DC grid is proposed in this paper. First, an 1-mode reverse voltage traveling wave (RVTW) is constructed applying the pole-mode transformation theory. Next, fault traveling wave propagation characteristics along the DC transmission line are analyzed in depth utilizing Peterson's law. Then, differences of time-frequency electromagnetic transient characteristics of 1-mode RVTWs between disturbances and faults caused by lightning strikes are distinguished in detail by means of the classical wavelet transformation multi-resolution analysis theory. Finally, extensive simulations are carried out to evaluate the performance of the proposed identification scheme, and by which its excellent rapidity, reliability and robustness are validated.