This PLECS demo model illustrates a neutral-point clamped (NPC), three-level voltage-source inverter. The NPC topology has been adopted for high power applications as it can achieve better harmonic reduction than traditional two-level voltage source inverters and the associated control strategies help to minimize semiconductor losses. This model is designed to deliver power to a 50 Hz, 130 VRMS grid from a dynamic DC source.
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.
Control
An outer voltage controller implemented as a PI regulator provides a current setpoint to an inner dq current controller. The voltage controller measures the DC voltage and calculates the error compared to the 450 VDC reference value. The current controller has separate PI regulators for direct and quadrature currents that produce a Vdq reference. The AC phase voltages and currents are measured and fed to the current controller. A phase-locked loop generates the reference phase angle for the abc to dq transformations. A low pass filter is inserted into the feedback path from the Ia,b,c measurement to represent the limited bandwidth of the current sensor. Modulation indices are generated for each of the three separate modulators for the three phase legs.
Simulation
The DC voltage is regulated to 450 VDC after the transients settle. The perturbation caused by the step change in current from the PV array is regulated out in approximately 50 ms. The switching pattern clearly shows a three-level pulse train with the fundamental waveforms phase-shifted. The real and reactive power delivered to the grid can also be observed.
Try it
This model is available in the PLECS Demo Model library provided in both versions of PLECS.