Simulation-based study of a single-phase cascaded H-bridge multilevel inverter for grid-connected photovoltaic systems

Abstract

In this thesis, the modelling and simulation of a single-phase seven-level cascaded H-bridge (CHB) multilevel inverter applicable in grid connected photovoltaic (PV) systems is discussed. A three-cell CHB inverter was modeled as a full MATLAB R2025b numerical simulation as a fully scripted model using forward Euler integration scheme with fixed time step of Ts = 0.5 µs. The model includes a custom single-diode PV module model with a solution based on a Newton-Raphson iteration, phase-shifted pulse-width modulation (PS-PWM), a single-phase phase-locked loop (PLL) to achieve grid synchronization, and a proportional-resonant (PR) current controller, which is based on a state-space form. The Tustin IIR recursion was replaced by the state-space realization since at Ts = 0.5 µs the discrete resonant poles are at z≈1, causing the IIR accumulator to round off numerically in milliseconds of initialization. The simulation was related to a 230 V, 50 Hz European household grid. The cells in the PV system have a DC link voltage of 115 V each, resulting in a cumulative bus voltage of 345 V and maximum inverter output is 327.75 V, which is higher than the grid peak of 325.27 V, and enables active power injection. Findings show that the seven level output voltage waveform realizes a total harmonic distortion (THDv) of about 2 percent and the grid current THD is about 1.8 percent, well below the IEEE Standard 519 limit of 5 percent. Power factor is over 0.99. The simulated values of THD are lower than the range of 5-8 percent in hardware implementations since the model assumes ideal switching devices with no dead time, an ideal DC source with no voltage ripple, and a complete decoupling feedforward which minimizes the control correction signal. The control system stabilized after three grid cycles after a 1000 to 600 W/m 2 step-reduction in irradiance, which validated good dynamic behavior of residential PV integration.