The microgrid has more flexibility and reliability to fulfill the system stability, power quality requirement, etc. However, the voltage unbalance might occur when the unbalanced loads including single-phase loads between two phases or between one phase and the neutral are connected in the microgrids. The unbalanced voltages occurred in the microgrids may cause problems with power quality such as power system instability, additional power losses, and adverse effects on the motors and power electric converters.
In order to provide highly reliable and good quality power to the distributed loads, the power-converter based DGs can be used as the distributed active filter to compensate the voltage imbalance, for which the imbalance power should be shared among the distributed generations in the islanded microgrid.
In the islanded microgrids, the droop control method has been widely used for power sharing, but the mismatched line impedance was not considered in most literature for accurate power sharing. With the help of the low-bandwidth communication, the centralized control method was proposed to compensate the voltage imbalance and to share the imbalance power. However, the single point of failure feature results in limited flexibility and low reliability.
In recent years, consensus-based distributed control schemes have been studied for distributed voltage imbalance compensation with accurate sharing of the negative-sequence current among the DGs. However, this method may cause differences in negative-sequence voltage among the DGs to incur the circulating negative-sequence currents in the microgrid.
Kim Sung Hyok, a researcher at the Faculty of Electronics, has proposed a method whereby virtual impedance is adopted to regulate the DG equivalent impedance at the fundamental negative-sequence for sharing the imbalance power in proportion to the DG power rating with consideration of mismatched line impedance. In that case, the line equivalent impedance is defined as the serial combination of the line impedance and the virtual impedance controlled by the DG unit at the fundamental negative-sequence.
In order to ensure the accurate imbalance power sharing regardless of the impact of mismatched line impedance, the discrete form of the consensus algorithm is considered to design the communication law for the global average of per unit imbalance power discovery.
The global average of the imbalance power is used to find the virtual negative-sequence impedance correction term by the designed consensus algorithm. Here, the per unit imbalance power mismatch is fed to a proportional-integral (PI) controller to discover the virtual negative-sequence impedance correction term.
The virtual negative-sequence impedance is regulated adaptively to eliminate the imbalance power sharing mismatch. Accordingly, the load imbalance power is accurately shared among the DGs.
The virtual negative-sequence impedance regulation based on the consensus algorithm is developed to accurately regulate the line equivalent impedance regardless of the detection of line impedance.
The effectiveness of the proposed method has been verified by the simulation and experiment results.
You can find more information about this in his paper “Distributed virtual negative-sequence impedance control for accurate imbalance power sharing in islanded microgrids” presented to the SCI Journal “Sustainable Energy, Grids and Networks”.
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