Neutral Point Clamp Inverter

Introduction

MATLABSolutions demonstrate In this particular task, This model simulates two identical circuits representing a 50kW, 380V, 50Hz, three-phase, three-level inverter. The IGBT inverter employs the SPWM technique with an 8kHz carrier frequency to convert DC power from a +/-200Vdc source to 220V AC at 50Hz. The inverter supplies a 50kW resistive load through a 75kVA 220/380V transformer. L-C filters are implemented at the converter output to eliminate harmonic frequencies (Fh) primarily generated around multiples of the 8kHz switching frequency (Fh = n*8000 +/- k*50Hz). The PWM Generator block produces the 12 inverter pulses required for operation. The system functions in open loop mode with a constant modulation index.

Power circuit

The circuit model is a bidirectional three-level voltage-source inverter (VSI), with three legs, one per phase, each containing two series-connected high-side switches and two series-connected low-side switches. Often, IGBTs with anti-parallel diodes are used as the switches for an NPC converter, but other two-quadrant switch configurations can also be employed. In this case, the IGBT 3-Level Half Bridge power module components are used, which each implement a single leg for an NPC converter. The power module has two configurations: a switched configuration where ideal switches represent the semiconductors, and an averaged configuration that uses controlled voltage and current sources. The averaged configuration is particularly well suited for real-time simulations with high switching frequencies, such as for hardware-in-the-loop testing.

The DC source, e.g., photovoltaic panels feeding a solar inverter, is modeled as a controlled current source. It provides 10 ADC for the first half of the simulation and 15 ADC for the second half of the simulation, corresponding to a sudden increase in received solar energy. This current charges the DC bus, which is split into two series-connected capacitors, with the mid-point connected to each of the three IGBT legs. Clamping diodes are placed between the capacitor mid-point and the one-quarter and three-quarter points of each leg. The mid-point of each leg is then fed to the respective phase of the AC grid via an inductor. The grid is modeled as an ideal three-phase 50 Hz, 130 VRMS voltage source. The DC capacitors have initial voltages of 200 VDC, though the setpoint for the DC bus is 450 VDC, so they will charge up at the start of the simulation.