Modeling and small signal stability analysis of synchronverter based distributed generators in islanded microgrid

Abstract

The growing trend in energy demand has led to the wide adoption of Distributed Generators (DGs) because of their environmental advantages, renewable sources, and economic feasibil ity due to the recent advancements in power electronics. Generation of electricity using DGs is in the form of either DC or a variable frequency AC. Hence, Power Electronic Converters (PECs) are needed to integrate DGs into the grid, also known as Inverter Interfaced Dis tributed Generation (IIDG). A group of these DG units with a cluster of local loads together forms a small power system called a microgrid. Microgrids are the tiny structure of a centralized power grid containing DGs, transmission lines, and various loads that can be self-controlled and managed. A microgrid can operate in grid-connected mode or islanded mode. For grid-connected mode, voltage and frequency reference is provided by the grid, but in the case of islanded mode, DG should maintain its voltage and frequency output within a permissible range. Therefore, stability is a crucial concern in an islanded mode of operation. RES-based DGs have very low or no rotating mass, resulting in low or no inertia. Increas ing penetration of these DGs reduces the overall inertia of the power grid. Due to this, the effect of disturbance in the power grid will result in faster and more considerable variation in system frequency. Inertia plays a very important role in control of power system, since it is one of the essential factors in maintaining the stability of power system. Also, it slows down the effect of disturbance on the variation of frequency. Hence, to provide virtual inertia to the system, an inverter control scheme is designed to emulate virtual inertia. A Virtual Synchronous Generator (VSG) is one of the proposed solutions for the emulation of virtual inertia. Synchronverter is one of the many different models of VSG. The idea is to implement the synchronous generator equations in the control part of the converter. The focus of this thesis is to analyze the stability of IIMG in island mode of operation with synchronverter as the local controller for DGs. This is accompanied by developing a linearized small-signal model for individual sub-modules and then combining all the sub modules on a common reference frame to obtain the complete model of the microgrid. This methodology can be extended to a microgrid with n number of DGs. In a microgrid, the network dynamics can affect the microgrid’s stability, so the network modeling is also in cluded in this thesis. Small-signal modeling is formulated for multiple IIDGs connected to the transmission line network along with various passive loads. Analysis of the small-signal state-space model is conducted using MATLAB. Further, the control scheme is simulated in RSCAD within substep environment. To analyse the performance of the synchronverter during various disturbances, various cases have been considered for both equal ratings as well as unequal rating of DGs. Besides, the comparison of the performance of the synchronverter control with the droop control scheme is presented.