Abstract—This paper describes a theoretical and experimental study on a control strategy for the parallel operation of three-phase voltage source inverters (VSI), to be applied to UPS. The proposed control system for each inverter consists of two main loops, which both use instantaneous values. The first (parallelism control) employs the feedback of the inductor currents from the output filter to modify the input voltages of the same filter and thereby control the power flow of each VSI to the load. Additionally, the second loop (voltage control) is responsible for controlling the output voltages of the LC filter, which coincides with the output voltages of the VSI. The proposed control strategy ensures the proper sharing of the load current and avoids current circulation among the inverters during transient and steady-state operation. The VSI and the proposed control strategy are analyzed in an orthogonal stationary frame, and as a result, simple and effective models were achieved. The proposed control system was digitally implemented in a TMS320F2812 DSP and was verified through experimental results with a 10 kVA prototype, which has the parallel operation of two three-phase VSIs.
II. INTRODUCTION
ININTERRUPTIBLE power supply (UPS) devices are employed to feed critical loads which, at high power values, utilize a three-phase system. Moreover, inmany applications the total load consists of a set of single and three-phase loads, which requires the employment of three-phase UPS, capable of feeding all types of load. In addition, critical loads also require a power supplywith high-reliability and redundancy that can be obtained with the parallel-connection ofUPSs.As iswell-known, the parallelism of UPS is a problem related to the parallel operation of voltage source inverters (VSIs). The parallel operation of three-phase VSIs has a greater complication due to the complexity and the greater number of variables involved in a three-phase system. Recent publications in the literature [1]–[3] broach this problem and they point out that there is still a need for new solutions to the parallelism of three-phase VSIs. It is also well-known that the parallel operation of VSIs requires a control system to ensure proper operation of the structure. A traditional solution is the strategy based on the frequency and voltage droop [4]–[18]. This strategy controls the average active and reactive power flow from the VSI to the load and it does not require communication among the inverters. It provides increased reliability and redundancy but it has errors associated with load sharing, poor transient response, reduced voltage regulation and it does not control the division of the harmonic currents [1], [3]. In the literature there are interesting studies reported [8], [9], [11], [18]–[22] which minimize the disadvantages, in most cases addressing single-phase systems. On the other hand, there are strategies related to communication, such as central control [23]–[25], master-slave control [26]–[29] and distributed control [2], [3], [30], [25]. These strategies are most effective in terms of load sharing, but high reliability and redundancy are not available due to the communication between units. In recent years, these strategies have been based on the instantaneous current control [2], [3], [24], [34], [35], in which the parallelism control receives information on the instantaneous current supplied by all units. Good transient response and the appropriate load sharing among VSIs, including the harmonics of load current, are advantages introduced by instantaneous control. The evolution of these strategies is associated with the use of microcontrollers, DSP and FPGA in power converters [36]–[44]. The digital control provides the means to propose new solutions to complex problems as in the parallel operation of three-phase voltage inverters. In this context, this paper proposes a distributed control strategy for the parallel operation of three-phase VSIs. Each VSI has its own control unit, responsible for regulating the output voltages and ensuring its parallel operation.
