Examiner: Massimo Bongiorno, Dept of Electrical Engineering
Supervisors: Mebtu Beza, Dept of Electrical Engineering, and Mattias Persson, RI.SE
Opponents: Dinesh Raju and Prajwal Kuduvalli Srikanth
With the increasing penetration of renewables in the grid, different ways of integrating them are being researched for efficient and stable system operation. The DC microgrid is one such system in which Distributed Energy Resources (DER) of DC in nature can be parallelly connected to a common DC-bus. However, since many power converters are involved in such a system, it might lead to interaction among the different converters resulting in system instability. Thus, the main purpose of this study is to investigate the stability issue in a converter dominated DC microgrid. In this thesis, the topology and operation principle of boost converter, bi-directional DC/DC converter and two-level Voltage Source Converter (VSC) is first studied and their input or output impedance models are derived. Moreover, simulink models of the converters are created and the analytical models are verified with the simulation ones by frequency-domain approach. Finally, the stability of the total system derived analytically is studied using stability analysis methods such as Nyquist criteria and Passivity analysis.
Findings show that, when the load in the DC system is increased by 40% from 50 kW to 70 kW, the system stability decreases due to the decrease of passivity in the overall system. In addition to this, when the voltage and current controller bandwidths of the VSC is increased it is found that the voltage controller parameter has more effect on the system stability than the current controller. Moreover, addition of a battery storage system to the DC grid shows that it integrates well during the charging process, but with a reduced stability margin. During the discharging of the energy storage, the system works in an islanded mode and becomes stable. Furthermore, when a capacitor bank of 7 mF capacitance each is added to the system it is observed that the system is more stable, however, the response becomes slower. Finally, it is also investigated that the droop controller of the VSC has little effect on the system stability.
Keywords: DC microgrid, stability analysis, DER - Distributed Energy Resources, PV - Photovoltaic, Nyquist criteria, Passivity analysis, VSC - Voltage Source Converter, controller bandwidth, battery storage, islanded mode, capacitor bank, droop controller.
Student project presentation
17 September, 2020, 13:15
17 September, 2020, 14:15