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 r educe harmonics, reduce pulsewidth modualtion (PWM) switching frequency and increase available output voltage or frequency. The type of parallel struc ture 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 paralle l structures presents areas for investigation encompassing analysis, simula tion, control, and design. This 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 to ind ependently regulate each inverter's current is employed. Experimental resul ts show a loss of current control that is similar to a random event. The in stability between the parallel inverters and the common motor can result in large uncontrolled currents. Simulations established that the reduction in controller gain, as the regulator enters the PWM pulse-dropping or overmod ulation region, results in a loss of current control. Experimental results show the loss of current control is the result of an interaction between th e parallel inverters through the dual-wound three-phase motor. Modification s were made to the modulator and a two-phase discontinuous controller was e mployed; the gain characteristic of the two-phase modulator in the overmodu lation region extends the dynamic range of the motor drive.