Current regulator instabilities on parallel voltage source inverters

Parallel inverters are often used to meet system power requirements beyond the capacity of the largest single structure. They have also been used to reduce harmonics, reduce PWM switching frequency and increase available output voltage or frequency. The type of parallel structure depends on the construction of the load motor; the most prevalent are dual three-phase machines, split-phase machines, six-phase machines and a standard three-phase machine with interphase reactors. Operation of parallel structures presents areas for investigation encompassing analysis, simulation, control and design. The paper reports on the commissioning of a 775 hp dual winding three-phase motor with parallel inverters. A simple method of paralleling structures with carrier-based PWM current regulators (CRPWM) to independently regulate each inverter´s current is employed. Experimental results show a loss of current control that is similar to a random event. The instability between the parallel inverters and the common motor can result in large uncontrolled currents. Simulations established the reduction in controller gain, as the regulator enters the PWM pulse dropping or overmodulation region, results in a loss of current control. Experimental results show the loss of current control is the result of an interaction between the parallel inverters through the dual wound three-phase motor. Modifications were made to the modulator and a two-phase discontinuous controller employed; the gain characteristic of the two-phase modulator in the overmodulation region extends over the dynamic range of the motor drive