INVESTIGATION OF DC MICROGRID STABILITY FOR SOURCE DISTURBANCES

Abstract

The DC power distribution systems have been drawing significant attention and have shown the potential to compete with conventional AC systems. DC microgrids (DCμGs) are based on direct current technologies and consist of distributed energy resources (DER) and loads operating in a controllable and synchronized way either in a grid-connected or islanded manner. Most energy sources such as solar photovoltaic (PV), fuel cell, and battery energy system are generating DC in nature. Nowadays, DC operated electronic loads are developing that eliminates further power conversion stages, reducing costs and losses. Furthermore, DCμG is more preferred than its AC counterpart because of its benefit, like; the direct connection of DC loads with renewables generating DC natively, more comfortable control system compared to AC counterparts, higher reliability, and efficiency. Power electronic converter plays a vital role in the efficient energy conversion system and integration of the DER units for excellent performance and efficiency of the power network. It can also be used to regulate the power flow between the source and the load subsystem. However, the generation output disturbance of DGs, sudden load changes, and impedance mismatch among the source and load converters subsystem cause the stability problem of the DC microgrid. The hybrid DCμG with multiple DG sources and ESSes have varying characteristics and fluctuating voltage levels due to the changing of the wind speed and solar irradiation. The micro sources are linked in parallel to the central DC grid system through power converters. The DCμG has typical interconnected distributed power topologies in which the power converter interconnect between the portions of the policy with various voltages levels. In such structures, instability in the system occurs due to the tight control of load side converters that act as constant power loads (CPLs) in which its small-signal model contains negative input resistance. This negative incremental impedance causes the system poorly damped and can cause unstable poles in the frequency domain and worsen the system stability. The intermittent character of the input renewable energy sources and the continually changing power consumption are the variables that challenge the DCμG power management and hence, DC bus stability. The decentralized control scheme is proposed to manage the integrated system for proper and stable operation with the multiple sources interfaced in parallel to the primary DC grid using a single topology of the DC microgrid. Each source converters are controlled autonomously and ii interconnected in a harmonized way to the point of a typical DC bus grid for reliable and flexible operations. The decentralized control method realizes the different operating modes using the DC bus voltage signal (DBS) control. This control method is a combination of voltage droop control and voltage level signaling. Voltage level signaling is a method that allows multiple source operations programmed in a prioritized manner. The dominant source is used to regulate the intermediate DC voltage. The control methodology offers an autonomously controlled service of each terminal without communication. MPPT and CVC (Constant Voltage Control) schemes are used in the DG's interface converter control to regulate power and voltage fluctuations due to changing input conditions. Furthermore, the control strategy used the DC bus voltage as a control parameter. Each power source converters are prompted by monitoring the change of DC bus voltage to keep the DCμG power balance. Accordingly, system reliability and flexibility operation is maintained. A DCμG composed of wind, solar, BESS with and without grid-connection that enable flexible and reliable performance proposed in this thesis. The control method uses the DC bus voltage level as a communication signal, and the self-controlled scheme implemented at each terminal without communication. The developed DCμG decentralized control method is simulated in real-time simulations with software control-in-the-loop (CIL) to verify the controller accuracy and performance through OPAL-RT real-time simulator. On the other hand, the input filter employed to mitigate disturbances due to switching devices and avoid electromagnetic interferences—the optimum damping resistance designed to eliminate instability problems in the feedback control loop of the POL converter. The stability problem is due to the dynamic filter interactions with sources. The stability condition verified using the Nyquist stability criterion.