Design of Adaptive Protection Schemes for Microgrids

Abstract

With the integration of small energy sources that can feed loads independently constitutes a microgrid. If those small energy sources or distributed generators (DGs) are of different nature like photovoltaic or wind or any other types of distributed energy source; then that microgrid is termed as hybrid microgrid. After the integration of DGs into the existing system, the conventional protection schemes may fail to provide reliable operation. The dynamic behavior of microgrid system under faulty conditions makes adaptive protection a general necessity for reliable microgrid operation. In design of adaptive protection, the grid-connected and islanded modes have immense importance including grid-connected mode without DGs in microgrid. In this thesis, a new adaptive protection scheme is proposed based on the above-mentioned modes of microgrid operation. The proposed method considers nature of DGs connected, fault location detection and fault nature identification based on quadrature and zero sequence components of fault current considering impact of X/R ratio of DGs. The proposed methodologies for adaptive protection schemes are verified in Matlab-Simulink environment and the results are found to be satisfactory while various faults are simulated at different nodes of the microgrid model. At the time of verification of effectiveness of the proposed methodologies, the time derivative of quadrature and zero-axis components of fault current are considered sufficient to instantaneously detect the fault location and fault nature in microgrid system. Types I, III and IV wind distributed generators have a different and wide range of current sharing capacity during the fault occurrence. For the design of the proper protection scheme in a hybrid microgrid, it becomes important to study different wind distributed generators. In a hybrid microgrid consisting of single and doubly-fed induction generators and photovoltaic distributed generators, the fault current in a feeder shows different behavior which changes as per the type of distributed generators and grid/islanded connection of microgrid operation. Based on the type of wind and photovoltaic distributed generators, a provision of a new adaptive protection scheme should be the primary concern for updating the relay settings as per the change like distributed generators, a distance of the fault from the point of common coupling and nature of the loads in the microgrid system. The q0 components of fault current are used for detecting the low X/R ratio of distributed generators, modes of operation, transient reactance during the series and shunt faults in a hybrid microgrid. The novel contribution in this part of research work is the implementation of a fuzzy-based adaptive protection scheme through analysis of the q0 components. In addition, the relay current shared by different distributed generators is derived for the q0 components in terms of the transient's component is another contribution. Considering q0 components and transient reactance, a new relation between the relay current settings and modes of operation has been identified for adaptive relaying. The effectiveness of the proposed adaptive protection scheme for the hybrid microgrid is verified through a simulation case study using Matlab–Simulink software. In a Hybrid microgrid, the overcurrent relays sense the changes in the fault currents while the microgrid switches from the grid-connected to islanded mode of operation. Further, for the different types of distributed generator, such as PV, Wind turbines of types I, III, and IV; the variation in fault currents are detected by the relays. This leads to delays and inappropriate coordination in conventional protection schemes. In this thesis, an adaptive protection scheme with optimal settings is proposed for phase and earth fault detection. It also takes care of different nature of distributed generators (DGs), all feasible operating modes of hybrid microgrid with only q component of fault current while zero component is used to differentiate between earth and phase faults. Also, a new strategy is proposed that optimizes the coordination time of fuses as a backup to primary and backup relays with new coordination time interval constraints. A differential evolutionary algorithm is proposed for determination of optimal settings for the directional overcurrent relays.