QUANTIFICATION, ANALYSIS, AND ASSISTANCE TO SYSTEM INERTIA

Abstract

The ever-increasing demand for electricity, combined with environmental degradation, necessitates maximizing renewable-based generator (RBG) penetration. As a result, synchronous generators are being replaced by renewable energy sources, which causes the power system to transition toward a converter-dominant system with lower synchronous inertia. The electrical system was built with enough leeway to accommodate changes. To some extent, the system has accepted renewable energy penetration without much ado. However, the increased penetration of RBG pushed the system’s stability boundary, with frequent threats to its robustness, recurring frequency excursion, and lowfrequency oscillation conditions. So, when system stability is at stake, the operator requires RBG to participate in the same way as the synchronous generator (SG). Being the pinnacle of stability provider, the SG has been rendering various frequency regulation support. In addition, it instantaneously supplied kinetic energy during frequency deviation as coupled with power system frequency. A similar scheme of frequency regulation from wind turbine generators is explored by coordinated operation of rotor speed control and pitch angle control. The parameters of the supplementary control loop for frequency regulation in WTG are set by checking the inertia distribution and disturbance responses on the nodes. Further a coordinated operation of both synchronous and renewable generators are performed by analyzing the grid’s local and global stability impression in order to provide system security, even in the case of a credible contingency. Overall, the thesis focuses on inertia and frequency-related issues by thoroughly analyzing the current state-of-the-art for dealing with the challenges of frequency stability. The critical indicator of its state of health for frequency stability is studied by knowing the inertia distribution conditions and the effect of disturbances propagation in the grid with high penetration of RBG. The dissertation is divided in two sections, the first Section aims to quantifying and analyzing system inertia and its effect on nodes in the network using the study of complex network science. There is a need to revisit the concept of inertia because it is becoming a time and space variable when provided by RBG. The study is performed by exploring complex network science (CNS) concepts, which has made tremendous progress in offering valuable insights into the characteristics of realworld systems. Utilizing CNS by exploring the theoretical graph techniques in the transmission grid is particularly appealing due to its mesh-like structure that highly influence structural variables on he power grid’s dynamics. The Section elucidates 1) The role of SG and RBG units’ contribution in handling the disturbances and their propagation in the network and 2)The behavior of dynamic response of the nodes in the presence of faster frequency regulation action from WTG. A novel online Nodal Inertia Index (NII) is proposed that considers the network’s structural features and generator dynamic response in the varying inertia system. NII investigates the inherent property of the distribution of inertia in the system by the graph theory convex combination of walks of generator nodes. Further, the Section explores the dynamic response of nodes (DRN) in terms of disturbance location from the generator bus and considers the generators’ internal interactions. DRN employs the Fiedler vector to observe the global response and locational impact. The system is more prone to disturbance when inertia is reduced on buses with nodes that correspond to the greater value of Fielder components than when the same quantity of inertia is reduced on buses with smaller Fielder components. The application of NII and DRN in selecting reserve percentages of the WTG is showcased. The next Section provides detailed information on the frequency assistance provided by wind turbine generators in the inertial, primary, and secondary frequency control time frame. The network responses due to the impact of disturbance are not uniform on the network and are dependent on topology and area stability. So the grid condition and its requirement play a key role in determining the response required from the WTG. The grid status quo (GSQ) parameters are defined to ensure reliable WTG frequency regulation that can respond to up and down frequency events. A novel dynamic nodal weight (DNW) of the nodes is defined using maximal entropy random walk that defines the dynamics of each node spreading power. Moreover, a modified weighted kmeans++ clustering technique is proposed using DNW to obtain the clusters as well as system’s spatial equivalent points. While using the spatial equivalent distance, the deloading percentage is decided followed by the frequency response support. The novel indices help minimize the deloading and, by considering the system requirement and the operating limits of the WTG, guide in deciding the frequency loop gain parameters. The impact of the proposed scheme is justified by simulating a modified IEEE 39 bus system with DFIG integration in the real-time digital simulator. Conjoint participation of wind generation with a conventional power plant also necessitates wind farms to participate in automatic generation control (AGC) for frequency regulation. This implicates the wind farm operator to monitor commands received from the transmission system operator (TSO). Consequently, advanced control techniques are deployed, which assuredly help in power tracking however increase pitch angle controller dynamics. This has resulted in mechanical stress on the equipment involved. In order to improve wind-farm response, a synergistic frequency regulation control mechanism (SFRCM) is proposed. The scheme considers the response time and reserve availability depending on forecasted wind data and examines load curve patterns to obtain the reference signal for the wind-farm controllers. The work provides a distinct solution to address the following: 1) Optimal pitch dynamics regulated operating point tracking with revised-pitch angle control (R-PAC), 2) Maximization of rotational kinetic energy viz attuned-rotor speed control (A-RSC), to increase stored kinetic energy in the rotor. However, in a real wind farm, all wind turbines do not experience the same wind speed due to the wake effect. The wake introduces different wind speeds to WTG, resulting in WTG variation in output power and, thus, non-uniform reserve. Consequently, controlling all the WTGs in windfarm with the same controller parameters results in the under-utilization of WTG power. So a novel active power dispatch mechanism (APDM) is proposed that decides the power setpoint reference for each WTG by taking wind power forecasted data and scheduled power dispatch command as inputs. APDM also includes the influence of the wake effect in the wind farm and ensures minimum pitch angle variation and maximized rotational kinetic energy in the rotor perpetually. The power reference tracking mechanism avails the command from APDM and operates the rotor angle control and pitch angle control to track the reference power command.