Research On NPC-based 400Hz Inverter And Its Parallel Technology

400Hz power supplies are widely used for aircraft supporting in airports and ships. It is very meaningful to research the modularize 400 Hz inverter and its parallel technology. This paper focus on the harmonic distribution analysis and switches loss analysis of the NPC-module-based single-phase 400 Hz inverter, the optimal design of single-phase 400 Hz inverter module's power circuit and the closed-loop control technology, parallel technology and phase control of single-phase inverter module.T-type neutral point clamped(T-NPC) two-leg topology and 3-level neutral-point-clamped(3L-NPC) two-leg topology using frequency doubling sine pulse width modulation(SPWM) are analyzed. The NPC-based topology has lower harmonic distribution comparing to the full-bridge topology using unipolar double frequency SPWM and that means the requirements of LC filter can be reduced. Meanwhile, analysis of the switches' power loss among full-bridge, T-NPC two-leg and 3L-NPC two-leg topology shows that the T-NPC circuit has the lowest switches' power loss. Reseach shows that single-phase 400 Hz inverter module which uses T-NPC two-leg topology can not only get better waveform with smaller filter, but can also get higher efficiency. Single-phase 400 Hz inverter module is designed based on topology research. The design results of LC filter, isolation transformer, DC bus capacitors are presented. The switching devices are chosen and a voltage equalizer is designed for balancing the serial-capacitors' voltages.The closed-loop control technology which is able to run the inverter in parallel condition is researched. The traditional double loop control is briefly introduced. The problem is that there is starting inrush current when the module sets up as a current source if the traditional double loop control is adopted. In order to reduce the impact, the output voltage feedback control is proposed, and the closed-loop regulators are designed. Comparing to the proportional-integral(PI) regulator, the quasi proportional resonant(PR) regulator has more advantages in both steady-state and dynamic performance. The correctness of the inverter design and the feasibility of closed-loop control strategy are validated by simulations.The parallel technology for modularized inverters is researched. The master-slave parallel technology can be realized easily and it has good performance on current sharing. But the traditional master-slave parallel technology has the disadvantages of complex external interconnections and poor reliability. This paper adopts appropriate logic to take the place of unnecessary interconnections. Only RS-485 communication bus is needed between modules and the phase control is based on CAN bus between system controller and modules. The logic of data control and automatic selection of master are researched. The sudden change of phase caused by error of crystal oscillator between system controller and module controllers is pointed out, and the corresponding self-adaption control of inverter module is proposed.Two single-phase 15 kVA 400Hz inverter modules are developped. The experiment results of single-phase 15 kVA 400Hz inverter module and the parallel operation results are shown. The experimental results have validated the feasibility of the 400 Hz inverter topology and its control strategy.