Improved particle swarm optimization-based selective harmonic elimination and neutral point balance control for three-level inverter in LVRT operation

In high-power applications, selective harmonic elimination pulsewidth modulation (SHEPWM) method is widely used to eliminate low-order harmonics in three-level inverters with low power losses. However, it faces the challenges that the conventional optimization methods for solving harmonic elimination equations in SHEPWM are easy to be trapped in local optimum. Moreover, the neutral point (NP) voltage of three-level inverter cannot be balanced under low-voltage ride-through (LVRT) condition with conventional SHEPWM method. To address these problems, a SHEPWM scheme is proposed, which is on the basis of an improved particle swarm optimization (IPSO) algorithm to solve the harmonic elimination equations and an enhanced NP balance control method in LVRT operation. First, an IPSO algorithm is proposed to calculate the switching angles of the transcendental equations for harmonic elimination. The IPSO algorithm defines a nonlinear negative exponential inertia weight considering the current and optimal fitness value, which can dynamically adjust the local and global searching speed and step size according to the actual situation, greatly improving the convergence speed and solution accuracy and avoiding falling into the local optimum. Then, by increasing the degree of freedom of the output current direction and combining with the NP voltage deviation, an enhanced NP voltage balance control method of SHEPWM in LVRT operation is proposed. The validity of the proposed method is proved by simulation and experimental results.