Papers in the Special Issue:
11 papers were accepted for this special issue covering a wide range of subject matter expertise encompassing academia and research organisations, networks owners, system operators, original equipment manufacturers and consultancies. Of the proposed topics in the call for paper the following were addressed by the authors.
Below provides a description of each of the accepted papers.
An Overview of Stability Challenges for Power-Electronic-Dominated Power Systems: The Grid-Forming Approach (Shakerighadi et al.) focuses on power system stability aspects brought by the introduction of IBR and in particular the evolution from GFL to GFM control concepts. In doing so, the authors not only discuss the possible advantages of GFMs but also the concerns raised by several transmission system operators (TSOs). The paper includes a summary of Class 3 GFM control strategy and discusses the suitability of four main GFM control schemes to align with those requirements. Reduction of system inertia holds the first position in the ranking of stability challenges identified by various European TSOs followed by a few other technical challenges such as resonances due to cables and reduction of transient stability margins. Some of the developments to address these challenges are discussed. This is followed by several simulation results from different GFM control strategies where the use of the exact system frequency information is shown to play a crucial role in improving GFM performance.
Grid Forming Inverter and its Applications to Support System Strength – A Case Study (Zhou et al.) proposes hardware-in-the-loop testing of a GFM with a virtual synchronous generator (VSG) control with configurable inertia contribution. The objective is to investigate its capability to actively suppress sub-synchronous oscillations and to provide an inertial response. The effectiveness of GFM controls with battery energy storage systems (BESS) in improving system strength and transient stability have been demonstrated using electromagnetic transient (EMT) modelling of the transmission network in the Queensland, Australia. The study shows that with proper tuning and coordination with other generation in the area, the GFM BESS can address low system strength related stability issues including sub-synchronous oscillations, voltage stability and transient stability.
”Improving grid strength in a wide-area transmission system with grid forming inverters” (Mayer et al.) presents GFM as a viable option for system strength reinforcement using a wide-area EMT model of Australia’s National Electricity Market (NEM) power system. This model is already used for practical decision makings on power system operation, planning and generator connection. Vendor-specific GFM models are then integrated into the wide-area model and their performance is compared to that achieved from already connected synchronous condensers. Besides indicating promising results, the paper also highlights some limitations with respect to GFM current limitations and oscillatory control characteristics. Studies demonstrate the stabilising properties of centralised and decentralised or spatial application of the GFM as a credible technical alternative to synchronous condensers in mitigating sub-synchronous control interactions previously experienced due to the interaction of multiple GFLs under low system strength conditions.
An Adaptive Multi-Mode Switching Control Strategy to Improve the Stability of Virtual Synchronous Generator with Wide Power Grid Strengths Variation (Liu et al.) proposes an adaptive multi-mode switching control strategy for use in a VSG-type GFM. The shortcomings of the two existing types of implementations for VSGs are discussed. The authors point out that the power support approach has good dynamic performance in strong grids but poor performance in weak grids, and the voltage support type has good stability in weak grids but with oscillatory power output in strong grids. The shortcomings of the single control structure are therefore attributed to grid strength variations. An adaptive control with switching between the two operating modes is suggested to address this limitation. Identifying the grid impedance is the first important task. To avoid frequent switching between the two modes, a hysteretic boundary characteristic is added. The performance of the proposed adaptive switching algorithm is then assessed using simulation studies indicating a good response in power control mode for a strong grid and in voltage control mode for a weak grid.
An Application of Four-Wire Grid-Forming Power Inverter in Unbalanced Distribution Network (Döhler et al.) presents a detailed model for a GFM control based on two loops: an inner current control with a proportional integral (PI) controller and an outer voltage loop with a proportional (P) controller. The topology suggested is a three-phase four-leg four-wire topology, capable of operating in grid-connected or islanded mode if there is a need to provide the reference voltage and frequency for the AC bus. The model used includes three grid-connected photovoltaic (PV) systems connected via GFL inverters operating in parallel with the electric utility grid to emulate the dynamics of a power system with high share of distributed generation. Simulation studies indicate stable operation of the GFM with a sinusoidal voltage and frequency. Furthermore, the proposed GFM strategy exhibits effective voltage balance control where during the islanded mode of operation the unbalance factor (the ratio of negative to positive phase sequence voltage) is well below the 2% acceptable level.
Asking for Fast Terminal Voltage Control in Grid Following Plants Could Provide Benefits of Grid Forming Behavior (Ramasubramanian et al.) discusses that in IBR dominated power systems, it is possible to observe GFM inverter behaviour from a conventional IBR with a control structure based on a phase-locked-loop (PLL). This is demonstrated by analysing the stability behaviour of a conventional IBR by switching its control strategy from reactive power to voltage control. Analytical time domain results are further shown with small signal analysis including identification of participation factors. A key observation is the efficacy of fast voltage control to ensure system stability. The analysis also demonstrates similarities between various GFM control strategies with regard to fast voltage control characteristics.
Impedance Modeling and Stability Analysis of MMC with Different Types of Grid-Forming Control Schemes (Guo et al.) investigates the impedance-based stability characteristics of two GFM control schemes for modular multilevel converters (MMC): power synchronisation control (PSC) and direct power control (DPC). Using the harmonic state space, the equivalent impedance models of PSC-MMC and DPC-MMC are developed accounting for all controllers and MMC internal dynamic characteristics. Impedance shaping effects of main controllers are further analysed to identify the frequency bands where the MMCs have negative resistive impedance characteristics. Finally, potential instability phenomena in interconnected power systems are discussed using simulation case studies, and the virtual impedance method is introduced as a stabilisation control scheme. EMT simulation results demonstrate the accuracy of impedance models, and the positive impact of the stabilisation control scheme.
An improved damping adaptive grid-forming control for black start of PMSG wind turbines (Meng et al.) investigates an improved damping method for GFM control in permanent magnet synchronous generator (PMSG)-based wind turbines. The proposed control aims to suppress system frequency fluctuations and achieve a smooth black start from the wind turbine. The paper also presents detailed black start process in a microgrid with two PMSG-based wind turbines and energy storage. It is shown that, compared to the conventional GFM controls, the proposed control scheme can not only suppress frequency fluctuations better, but also achieve a smooth formation of the grid. Hardware-in-the-loop tests are also carried out to validate the proposed control strategy for achieving a smooth black start.
Damping torque coefficient analysis of PMSG-based WT with VSG control considering wind turbine dynamics (Qu et al.) uses the damping torque coefficient analysis (DTCA) for studying the small-signal stability of PMSG-based wind turbines under VSG control with consideration of wind turbine dynamics. Authors discuss that wind turbine dynamics can result in system instability, and propose that the minimum value of VSG damping control parameter shall be maintained to ensure system stability under all loading conditions below the rated wind speed. Simulation results from a single PMSG connected to a large grid with and without consideration of wind turbines dynamics are compared to verify the accuracy of the proposed analytical models and to evaluate the impact of control and system parameters on the system stability.
Assessment and Enhancement Control for Small-Signal Voltage Stability of VSC-HVDC Systems Supplying Passive Industrial Loads (Xue et al.) conducts small-signal voltage stability analysis for voltage source converter (VSC)-HVDC systems supplying industrial induction motor loads. The authors also propose stability enhancement control for the VSCs. Eigenvalue analysis is carried out to determine key factors influencing voltage stability. The analysis demonstrates that decreasing either the proportional gain of the VSC’s PI controller or the electrical distance between the VSC and induction motor is detrimental to voltage stability. The proposed control strategy adopts virtual reactance and virtual admittance control for the VSC to improve the voltage control capability. The effectiveness and robustness of the proposed enhancement control is validated with time-domain simulations in a multiple induction motor system supplied by VSC-HVDC.
Hierarchical control scheme for proportional power sharing and robust operation in multiple virtual synchronization-based DC/DC converters (Ji et al.) proposes a hierarchical control scheme to achieve higher efficiency and superior anti-disturbance ability for multiple DC/DC converters with virtual synchronization-based control. In the primary control layer, the power sharing among different converters can be equalized with the additional regulation signals from neighbouring weak communications to eliminate the influence of line parameter mismatch among the converters. In the secondary control layer, the extended disturbance-observer-based backstepping control is proposed to eliminate external power fluctuations on the whole system. Two control levels are integrated as the hierarchical control architecture to realise different control objectives in different time scales. Theoretical analysis and simulations are presented to show the benefits of the proposed hierarchical control scheme.