Control Research Of 3-Phase 4-Leg Inverter

The traditional three-phase inverter has a structure with three arms. when bearing with the three-phase symmetrical load, the power supply three-phase output voltage is symmetrical. but when with the three-phase asymmetric load, the load neutral potential point will drift, so the output three-phase is voltage no longer symmetrical. Aiming at this, scholars have proposed a inverter topology form with 3-phase and 4-leg.3-phase and 4-leg structure can solve the problem of neutral point drift, and maintains the output voltage symmetry, when the load is asymmetric.Firstly,the paper researched the mathematical model of 3-phase and 4-leg in a-b-c coordinate system,α-β-0 coordinate system and the d-q-0 coordinate system,and analysed the principle of three-phase unbalance, established bisynchronous rotating coordinate with a-b-c and d-q-0 coordinate. Then the paper introduced the principle of 3-D SVM in a-b-c coordinate system for 3-phase and 4-leg inverter, and applied it to the design control, in order to improve the quality of three-phase voltage power output and to reduce the harmonic components.In the experiment simulation, a 3-phase and 4-leg inverter simulation system was built, which adopted the dual-loop control with inner loop using capacitive current and outer loop using capacitor voltage. In the bisynchronous rotating coordinate, the positive sequence component, negative sequence component and zero sequence component of the output voltage were respectively controlled. Simulations were carried out in various load conditions, such as empty-load, full-load, half-load, sudden-load, voltage fluctuations at load terminal, non-linear load and three-phase asymmetric load. In order to reflect the advantage of the 3-phase and 4-leg inverter system with asymmetric load, this paper established a three-phase and three-arm inverter system with the same simulation parameters, and focused on comparison between the two at the situation with asymmetric load. Experimental results show that the established 3-phase 4-leg inverter, which was controled through positive sequence, negative sequence and zero sequence, has many advantages, such as quick response, high accuracy, little THD, anti-interference ability, non-linear load capability, in particular, the inverter has outstanding capacity with asymmetry load. All of above verified the superiority of the whole system.