An Improved NLM Strategy for MMC Emergency Power Control

This paper proposes an improved nearest level modulation (NLM) strategy for the modular multilevel converter (MMC) station to realize emergency power control. The sending-end MMC station whose ac side is connected to the islanded renewable power plant must control the ac voltage amplitude and frequency. The power flowing into the MMC station is uncontrolled. Power surplus situation (ac input power is greater than dc output power) will occur and dc voltage will rise to a damage value when the receiving end station blocks. This paper analyzes the working principle of MMC and introduces the traditional NLM strategy. Then proposes an improved NLM strategy. With the target of stabilizing dc voltage, the proposed strategy reduces the number of working MMC submodules according to the surplus power. The submodule capacitors absorb the surplus power by increasing their voltage within a proper range. The surplus energy is stored in MMC itself. The simulation results shows that this strategy can balance the input and output power of the MMC, and stabilize the dc/ac voltage. This strategy can buy time for the islanded renewable power plant decreasing output power without extra hardware addition. It’s convenient and effective to apply in MMC.