This paper addresses the challenges posed by voltage fluctuations during the commutation process of Flexible Line Commutated Converters (FLCC), in which an MMC is in series between the transformer and LCC valve, and their impact on system insulation coordination. An optimized arrester configuration scheme is proposed based on voltage fluctuation analysis. During the commutation process, the voltage across the LCC valve and transformer is influenced by changes in commutation angle and MMC voltage, which may lead to high peak voltage on both rectifier and inverter sides, and high arrester reference voltage. Steady-state voltage simulations and calculations are conducted to analyze voltage peaks under different operating conditions. Based on this, a new arrester configuration is designed, in which the transformer side of the MMC is allocated a grounded arrester, and the valve side of the MMC is protected by valve arrester, to ensure minimum arrester number and reduce energy demands, in consideration of the energy distribution after fault and possible arrester combination scheme. The arrester ratio of CCOV and the reference voltage is optimized to enhance the low arrester protective level. Simulation results demonstrate that the optimized scheme effectively mitigates the impact of voltage fluctuations on equipment withstand voltage, improving the system's resilience and safety, thus providing theoretical support for the design and application of FLCCs.