To meet the rising need for clean energy, distributed generation (DG) such as photovoltaics, wind turbines, fuel cells, and microturbines has been quickly promoted in modern power distributed systems. To aggregate and utilize several DGs, DC microgrids (MGs) have been frequently adopted due to its reliability and adaptability. Moreover, DC MGs can operate effectively in either grid-connected or islanded mode to provide economical operation and more dependable electricity. In islanding operations, it is crucial to manage the desired power across DG units, and the droop control method is commonly used to run DG units in a decentralized manner. However, due to absence of communication system, it is impossible to meet the power sharing accuracy and voltage quality requirements with the typical droop controller. Therefore, numerous distributed control methods based on communication networks have been proposed to overcome the problem of droop control. In particular, to achieve low-cost operation, consensus-based distributed control schemes are widely implemented in which a sparse communication system is used instead of a fully connected network. Unfortunately, because of communication time delays, conventional consensus-based distributed control schemes fail to obtain accurate power sharing and voltage compensation in DC MGs. Therefore, this thesis presents advanced consensus-based distributed control schemes to achieve accurate power allocation and correct voltage compensation in DC MGs regardless of heterogeneous time delays.
Firstly, to address the problem of time delays, an anti-heterogeneous time delay estimator is proposed based on scattering transformation and a PI consensus algorithm, and proper power allocation is achieved along with voltage compensation in a distributed fashion. In the proposed estimator, transmitted/received signals are modified by means of the scattering transformation, and average consensus is achieved successfully by the PI consensus algorithm. Consequently, proportional power sharing and voltage compensation are achieved concomitantly by voltage shifting method despite heterogeneous temporal delays, mismatched line resistances, and load variation.
Secondly, the secondary control level is simplified by using only one PI controller which avoids decoupling secondary control loops. Furthermore, the proposed control strategy can be applied directly without any prior knowledge of the detailed microgrid configuration or the required load power measurement, which reduce the overall complexity and cost of the system.
Finally, outstanding performance of the proposed method is guaranteed with nonzero initial condition and different time delay conditions such as uncertain time delays, intermittent change in time delays. Thanks to these important characteristics, the proposed method can be applied directly with any practical communication system.
All control strategies are confirmed via digital simulation with PSIM and laboratory testing with scaled-down microgrid prototypes. For validation with actual time delays caused by practical communication without any approximation, a CAN bus is implemented by using CAN protocol version ISO11898-1 with a bit rate of 500 kb/s. These results demonstrate the viability and efficiency of the proposed control methods. The concluding section of the thesis draws conclusions and suggests future study directions.