The conventional control method of shunt active power filter(SAPF)can only achieve the control goal when the power grid is balanced

and the compensation effect is not satisfactory when the load changes. In order to solve these problems

this paper chooses the neutral-point clamped(NPC)three-level SAPF for the three-phase four-wire system as the object of study

then proposes a nonlinear control method based on Lyapunov function. Firstly

the mathematical model of the controlled object in the dq0 coordinate system is established. Then

the nonlinear control strategy based on Lyapunov function is proposed for the current inner loop. From the stability point of view

the appropriate control parameters are selected to ensure the steady-state and dynamic performance of the system under balanced or unbalanced conditions. Thirdly

for the problem of the unbalanced DC side voltage

a control method based on the principle of charge balance is used to change the time between positive and negative small vectors in space vector pulse width modulation(SVPWM)algorithm

to maintain DC capacitor voltage stability and balance. Finally

the feasibility and superiority of applying nonlinear control method based on Lyapunov function to SAPF is verified by the simulation on Simulink software and the experiment. Compared with the conventional PI control

the proposed control method has a faster response speed and better compensation effect.