| Theoretical and application of DC micro-grid have been attracting much more attention in recent years, in order to satisfiy the requirement of distributed renewable energy utilization, especially with the improvement of DC technology and steady increasement of DC load. Parallel operation technology of DC micro-grid converters is one of the hot research topics, and the power sharing and control strategy has always been the key for parallel converter operation. This paper has studied the control strategy of multiple three-level Buck-Boost bidirectional converters parallelly running in a bipolar DC micro-grid, and the achievments can be concluded in five aspects as follow.To solve the problem of neutral point potential unbalance in three-level converter, the mechanism of neutral point potential unbalance is theoretically analyzed, and an adaptive neutral point potential control strategy is developed to ensure the potential equilibrium of positive and negative bus versus zero bus, which can stably maintain the bus voltage in the DC micro-grid.For the performance of voltage controlled mode (VCM) converters and current controlled mode (CCM) converters in parallel system, the transfer functions of the command value and value of load current to the output voltage are derived respectively. The stability and dynamic performance of VCM converters and CCM converters are analyzed and compared according to the pole-zero maps of the transfer functions.For the parallel operation of VCM converters in the micro-grid, I-U droop control is adopted and its operation principle is analyzed. A secondary regulation aldorithm is proposed for larger scale DC micro-grid system to overcome the line resistance influence on bus voltage, which includes both voltage and current compensation control strategy, thus realizing the power sharing between VCM converters.For the parallel operation of CCM converters in the micro-grid, U-I droop control is adopted and its operation principle is analyzed. A secondary regulation aldorithm is proposed for larger scale DC micro-grid system to overcome the line resistance influence on bus voltage, which includes both voltage and current compensation control strategy, thus realizing the power sharing between CCM converters.For the system that VCM and CCM converters are employed parallelly, I-U droop control is adopted for VCM converters and the U-I droop control is adopted for CCM converters. The values of two instruction are satisfied with certain relations that can realize the power distribution between the converters. For the larger scale of DC micro-grid system, secondary regulation including a voltage compensation control and a current compensation control is proposed to realize the power distribution between the converters.On the basis of theoretical research, the validity of the theoretical analysis and the effectiveness of the control strategy are verified by the hardware-in-the-loop simulation platform based on dSPACE1104. |
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