A Study on Power Sharing Techniques with Enhanced Power Quality in DC Microgrid

Abstract

Recently, DC microgrid receives more attention in these days as it has no issues with reactive power flow and frequency regulation, which results in a notably less complex in control system. As droop control method is used to cooperate with various type of sources in microgrid, the DC bus voltage is swing according to the load power. Moreover, the line impedance affects the power sharing among distributed generations (DGs) in the system. In DC microgrid, there are many renewable energy resources, and their output power strictly depend on the resource condition. In order to utilize the DG sources effectively in DC microgrid, many kinds of power management methods are developed to improve the power quality and achieve accurate power sharing for DG microgrid system. In this thesis, the power sharing with enhanced power quality is studied for DC Microgrid based on the battery energy storage system including the supercapacitor (SC).

Firstly, this thesis introduces a new power distributed control method for a DC microgrid. This method is developed to share the load power proportionally to the rating of DG and restore the DC bus voltage when the load changes. For this purpose, a shifted voltage technique is developed based on the power rating and the instantaneous power of DGs. By adding the shifted voltage, the voltage drop caused by the droop controller is effectively compensated so that the DC bus voltage is constantly regulated regardless of the load change. To realize the proposed method, all the required information to determine the reference voltage is transmitted through a low-bandwidth communication link. The controller design process is presented in detail along with a system stability analysis in this thesis.

Secondly, this thesis presents a new control method for SC to compensate the transient for the DG in DC microgrid. The SC unit is controlled by modeling the SC as a constant voltage source in series with paralleled capacitor-resistor. The value of the resistor is adjusted according to the level of the SC voltage; it becomes negative to charge the SC when the SC voltage is lower than its nominal value; and become positive to discharge the SC when the SC voltage is higher than its nominal value. The power sharing between SC units is ensured by means of virtual impedance control method.

Thirdly, this thesis presents a new control method for the SC to compensate the transient load current and support the system under the pulsed load condition. Moreover, the cooperation of the multiple SC units in DC microgrid is also ensured by proportionally sharing the supporting current between the SC units. The current sharing between the SC units is achieved by means of the droop control. The conventional droop control is modified so that it can adaptively change its operation mode and parameters in order to compensate the load transient current and regulate the state of charge of the SC.

Fourth, this thesis presents a new configuration of a hybrid energy storage system (HESS) called a battery-inductor-supercapacitor HESS (BLSC-HESS), which is easily implemented to integrate the SC into DC microgrid. It splits the power between a battery and SC, and can operate in parallel. The power sharing is achieved between the battery and the supercapacitor by combining an internal battery resistor and output LSC filter, which consists of a SC and an inductor. The battery current is smoothed to supply and receive only the low-frequency current component under any disturbances and load conditions through cooperation with the SC. Complete guideline to design the parameters of the BLSC-HESS is also presented.

All proposed control methods and configuration are demonstrated by simulation and experiment with DC microgrid prototype in laboratory. Proper comparisons and discussion are provided to show its effectiveness of the improved power management techniques in DC microgrid.