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Stability enhancement of utility-scale renewable energy farms in weak grids

Australia has a national grid with many weak areas. It is currently undergoing a major transformation where fossil fuels are being replaced by renewable energy resources such as solar and wind. Many of these renewable resources are located in weak areas of the grid and are prone to various stability issues. Additionally, with the current trend in increasing the penetration of asynchronous generation, in future years, strong points in the grid are expected to be considerably weaker.

Project summary

This project will assist both network owners and operators to ensure customers get the maximum value of these renewable farms located in weak parts of the grid. It will also increase the reliability and security of the grid in such areas.

The project comprises of three tasks.

  • Task 1 | Weak grids classification and test-bed development

    This task will explore and propose new measures to classify weak networks and will identify grid scenarios and value-range under which weak-grid-connected wind/solar farms will experience instability issues. Additionally, this task will develop a testbed, based on the North-western Victorian network (in collaboration with and based on the data provided by AEMO and AusNet Services). This testbed will be used in the following tasks to validate their findings.

  • Task 2 | Grid-strengthening solutions

    This task will propose two main grid-strengthening methods, i.e., SynCons and grid-forming inverters, and will explore their optimal allocation and sizing in weak networks. Additionally, alternative control strategies for these grid-strengthening assets with a focus on ultra-weak networks will be devised. Finally, the black-start capability of grid-forming inverters will be explored and outlined.

  • Task 3 | Wind/Solar farms controls and their interactions with other PEC-connected assets

    In this task, two internal controllers (PoC voltage controller and PLL) for grid-following converters with a focus on ultra-weak grids (SCR less than one) will be developed to guarantee robust performance and stability over a range of grid strength scenarios and specifically when the grid is very weak. Additionally, this task will investigate the interaction of various power-electronic converter (PEC)-connected assets in the network, with a focus on the existing grid following inverters, and will identify grid scenarios and value-ranges that lead to oscillatory modes.

Knowledge Sharing

One significant outcome of this project is high-quality papers presented in prestigious journals. A list of published papers with a brief description and their link to the project tasks are provided here. This list will be updated as future papers are published.

  • Grid Forming Inverter Modeling, Control, and Applications

    This paper surveys the current literature on modelling methods, control techniques, protection schemes, applications, and real-world implementations pertaining to grid forming inverters (GFMIs). This paper presents a comprehensive review of GFMIs covering recent advancements in control technologies, fault ride-through capabilities, stability enhancement measures, and practical implementations. Moreover, the challenges of adding GFMIs into existing power systems, including a seamless transition from grid-connected mode to the standalone mode and vice versa, are also discussed in detail. Recently commissioned projects in Australia, the UK, and the US are taken as examples to highlight the trend in the power industry in adding GFMIs to address issues related to weak grid scenarios. Research directions in terms of voltage control, frequency control, system strength improvement, and regulatory framework are also discussed. This paper serves as a resource for researchers and power system engineers exploring solutions to the emerging problems with high penetration of IBRs, focusing on GFMIs. This paper mainly contributes to Task 2 of the project. GFMIs are one of the leading solutions for grid strengthening, and this paper studies various aspects of these assets in detail.

  • Power-Synchronized Grid-Following Inverter Without a Phase-Locked Loop

    This paper proposes a power-synchronized control strategy for grid-following inverters (GFLIs) to regulate their power exchange with the grid without any need for sensing/regulating the point of connection voltage. Contrary to conventional GFLIs, which rely on phase-locked loops for grid synchronization and have difficulties in weak grid conditions, the proposed strategy is power synchronized and utilizes the inverter terminal voltage for power control leading to its seamless performance in ultra-weak grids. Additionally, since the proposed approach does not require any voltage regulation at the point of connection to the grid, contrary to grid-forming inverters, it can reliably operate in stiff and/or series-compensated grids as well. The proposed approach benefits from a decoupling control structure that is tuned using a loop-shaping method. Compared to conventional GFLIs, this approach does not need any extra hardware; hence, it can easily be retrofitted into the existing large fleet of GFLIs. This paper mainly contributes to Task 2 of the project. The proposed power-synchronized GFLI is an ideal solution for incorporating the inverter-based resources in weak grids, and this paper provides a detailed description of this structure.

  • Optimal Allocation and Sizing of Synchronous Condensers in Weak Grids With Increased Penetration of Wind and Solar Farms

    SynCons, being synchronous machines without a prime mover, provide several benefits in weak power systems, such as frequency support, system strength, and voltage regulation. Although SynCons are widely utilized to mitigate the weak grid integration challenges, their installation /operation costs make them a costly solution. Additionally, their lead-time can be more than a year, which means their initial sizing and allocation must be optimal. In this paper, a method for the optimal allocation and sizing of SynCons is proposed. The main objective of this method is maintaining Short Circuit Ratio (SCR) in the system greater than pre-defined values while the investment and operation costs of SynCons, and voltage deviation in the system are minimized. Three meta-heuristic optimization algorithms are used to implement the proposed method, and its performance is evaluated via Electromagnetic Transient (EMT) time-domain simulation in a modified IEEE 39-bus system. This paper mainly contributes to Task 2 of the project. SynCons are one of the leading solutions for grid strengthening, and this paper studies their optimal allocation and sizing, especially in weak grids with high penetration of inverter-based resources.

  • Nonlinear Transient Stability Analysis of Phase-Locked Loop Based Grid-Following Voltage Source Converters Using Lyapunov’s Direct Method

    In this paper, the transient stability conditions for a grid-following Voltage Source Converter (VSC) are found using Lyapunov's stability theorem. These conditions take into account both the grid specifications and the VSC dynamics. The derived conditions are based on a well-known nonlinear model of the VSC Phase-Locked Loop. To evaluate the stability of the nonlinear system, Lyapunov’s direct method is employed. To this end, a new Lyapunov function is proposed, and its characteristics are analysed. Using this Lyapunov function, the domain of attraction of the system equilibrium point is calculated. Additionally, a novel system strength index based on the domain of attraction of the system is proposed. The privilege of this index over the conventional indices are absoluteness, VSC dynamics consideration, and comparability of different VSCs with each other from a stability point of view. In the end, the correctness of the proposed stability analysis is validated via simulation in Matlab/PLECS and experiment. This paper mainly contributes to Task 1 of the project. The derived conditions and the novel system strength index for evaluation of transient stability of the grid-following VSCs are new measures for classifying weak networks and can be used for identifying grid scenarios and value ranges under which weak-grid-connected inverter-based resources experience instability issues.

  • Stability Analysis and Nonlinear Control of Phase Locked Loop of a Weak-grid Connected Voltage Source Converter

    Voltage Source Converters (VSCs) connected to weak grids may face difficulties in synchronization, leading to their instability. As the heart of the system, Phase Locked Loop (PLL) unit is responsible for the synchronism of VSCs. In this paper, using the nonlinear state-space model of the system composed of the grid and the PLL, stability and domain of attraction of the equilibrium point of the system is investigated. Afterwards, using Lyapunov's theorem, a nonlinear controller that extends the domain of attraction of the equilibrium point is designed. Using phase portraits, the system stability with and without the designed controller while the VSC is connected to a very weak grid is evaluated. The results confirm that the system with the proposed controller remains stable, whereas the conventional system cannot maintain stability. Additionally, the domain of attraction of the compensated system is calculated. Finally, the results from phase portraits are validated by simulating the grid-connected VSC. This paper mainly contributes to Task 2 of the project. Upgrading the conventional PLL-based resources to extend the domain of attraction of their equilibrium points is a step forward for increasing the inverter-based resources' stability margin in weak spots of the grid, and this paper proposes a novel nonlinear technique to satisfy this requirement.

  • A Robust Exciter Controller Design for Synchronous Condensers in Weak Grids

    Weak grid scenarios and low-inertia systems are emerging issues in power systems, leading to voltage and frequency instabilities. Synchronous Condensers (SynCons) have recently drawn renewed attention as a promising solution to provide system strength and inertia support. Even though the exciter control of SynCons is a well-established technology, further developments are required to guarantee the stability of post-fault operations, particularly in weak grids. This paper proposes a data-driven approach for designing higher-order optimized exciter controllers to meet this requirement. A pseudo-random binary sequence (PRBS)-based system identification method is used to obtain frequency response data of the power system from the exciter point of view, which is then fed into the proposed optimal control design procedure. The proposed exciter controller is tested for voltage ride-through and fault scenarios in a single machine infinite bus (SMIB) case and the IEEE 39-bus test system to assess its performance compared to the conventional AC1A exciter controller. This paper mainly contributes to Task 2 of the project. SynCons are one of the leading solutions for grid strengthening, and this paper proposes a novel approach for designing SynCons' exciter controllers so that the stability of their post-fault operation, especially in weak spots of the grids is guaranteed.

  • Adaptive Virtual Resistance for Post-fault Oscillation Damping in Grid-forming Inverters

    Post-fault oscillations in active power and voltage responses of grid-connected Voltage Source Converters (VSCs) have been reported in the literature. They are caused by non-ideally-tuned controllers and the implemented current limitations used for protecting the VSCs from over-currents. These oscillations become more significant when the VSC is connected to a weak grid, deteriorating the recovery process of the VSC. This paper presents a method for damping the post-fault oscillations using an adaptive virtual resistor (VR). Even though these oscillations are observed with both grid-following and grid forming inverters (GFMIs), this paper focuses on droop-based GFMIs. The proposed method dynamically integrates a VR into the VSC control and removes it in the normal operation mode of the VSC. This method is implemented in the Synchronous Reference Frame (SRF), which is commonly used due to its decoupled active and reactive power control. The amount of virtual resistance used for oscillation damping is adaptive to the recovery rate of the VSC. Hence, the proposed method is robust against changes in grid strength. Finally, the performance of the method is evaluated in PSCAD/EMTDC and also experimentally validated. This paper mainly contributes to Task 2 of the project. GFMIs are one of the leading solutions for grid strengthening, and this paper proposes a method for damping their post-fault oscillations through an adaptive VR in their control structure.

  • Linear Parameter-Varying Control of A Power-Synchronized Grid-Following Inverter

    This paper proposes a Linear Parameter-Varying loop-shaping controller for a power-synchronized grid-following inverter (PSGFLI). This control strategy regulates the inverter output active and reactive powers at the terminal instead of the point of connection and does not require a phase-locked loop (PLL) for extracting the voltage phase angle. Hence, the prevalent stability issues exhibited when GFLIs are connected to weak grids are not present, and the proposed PSGFLI control strategy can work under both very weak and strong grid conditions without being prone to instability. In this approach, the controller parameters are functions of the operating point and are changed during the real-time operation such that the closed-loop performance is preserved in all operating points. Furthermore, since the grid impedance is a factor in the design process, a robustness analysis against grid impedance estimation error is conducted, and it is shown that discrepancies in estimated and real grid impedances are unlikely to make the system unstable. The performance of the proposed control design is validated in MATLAB/PLECS and experiments for both strong and weak grids. This paper mainly contributes to Task 2 of the project. PSGFLI is an ideal solution for incorporating the inverter-based resources in weak grids, and this paper provides a novel control scheme for enhancing its performance.

Chief Investigators

Project partners

The project control committee consists of representatives from the following organisations: