According to most grid codes, wind farms are required to inject reactive current into the connected power grid during fault. However, this requirement may lead to system instability and the failure of low voltage ride-through (LVRT), especially for wind farms connected to a weak grid. In this chapter, we suppose that the instability of wind farms during LVRT is mainly due to the instability of the phase-locked loop (PLL). Thus, a state-space model of the wind power system including the PLL under low voltage conditions is established for stability analysis. The mechanism of the system instability phenomenon during fault is investigated and revealed. To address this problem, an energy storage system (ESS)-based stability control strategy is proposed to maintain the stability of the wind power system during fault. The detailed electromagnetic model of the test system including three wind farms is installed in PSCAD/EMTDC environment for simulation studies. Simulation results verify the correctness of the obtained stability analysis results and the effectiveness of the ESS-based stability control strategy.