Design, Control, Operation and Stability of Grid-connected DC Microgrid with Constant Power Load

The paper investigates the design, control, operation, and stability of grid-connected DC microgrids. A DC microgrid consists of PV generation, a Li-ion battery for high-energy density storage, a supercapacitor for high-power density storage, an electric vehicle load, and a constant power load (CPL). A three-level neutral-point clamped (NPC) converter facilitates the interface between the utility AC grid and the DC microgrid. A DC-DC boost converter and two DC-DC bidirectional converters respectively connect the PV array, battery, and supercapacitor to the common DC bus. Battery and supercapacitor are coordinated to operate as a composite energy storage system (CESS). In this, batteries supply DC and low-frequency components of load power, whereas supercapacitors supply transient and high-frequency components. DC-DC upper stream converters feed to electronically controlled load, which behaves as CPL and causes instantaneous instability. The proposed CESS scheme performs three functions simultaneously, viz., energy-time shift, proper load sharing among various resources, and active stabilizer. This scheme improves power quality at the DC side as well as the AC side continuously against both AC side fault and DC side disturbance caused by source and load. In addition, it works as active stabilization to mitigate the instability caused by constant power load. A single controller for these two bidirectional converters is designed and developed to serve the specified functions. In this research, a suitable grid-connected DC microgrid is designed, and AC side faults, DC side disturbances, and instability due to CPL with their mitigations are simulated and verified. The proposed scheme is simulated in MATLAB Simulink software, and the results are validated with OPAL-RT (RT-4510) based Hardware In Loop (HIL) real-time simulator.