Power hardware-in-the-loop testing for multiple inverters with virtual inertia controls

The large-scale deployment of sustainable energy sources has become a mandatory goal to reduce pollution from electricity production. As photovoltaic (PV) plants replace conventional synchronous generators (SGs), their significant inherent rotational inertia characteristics are reduced. The high penetration of PV results in reduced system inertia, leading to system frequency instability. Virtual inertial control (VIC) technology has attracted increasing interest because of its ability to mimic inertia. Adoption of the energy storage system (ESS) is hindered by the high cost, although it can be used to provide virtual inertia. The determined forecast gives PVs the ability to reserve power before shading and compensate the power when a system power drop occurs, which can increase system inertia. Nevertheless, it has forecast errors and energy waste in a stable state. To improve the stability of the microgrid and improve the ESS efficiency, this study proposes an adaptive forecasting-based (AFB) VIC method using probabilistic forecasts. The adaptive power reserve and virtual inertia control are proposed to reduce energy waste and increase system inertia. The simulation results reveal that the proposed method has adaptive system inertia to reduce the reserved power, required ESS power capacity, and battery aging.

Low frequency nadir and high rate-of-change-of-frequency (ROCOF) due to system disturbance e.g., generator tripping, are the most challenging issues for power system with high penetration of inverter-based resources (IBRs) since most of IBRs operate at grid following mode, injecting required power to grid. Solving these issues allows further penetration of IBRs in power grid. Recently, a new operational mode named as grid-forming (GFM) is introduced as a promising method to solve these problems since IBRs at this mode will behave as voltage sources, contributing virtual inertia or fast frequency control. This paper shows the methodology to realize droop-type GFM mode in type IV wind turbine then allows it to provide inertia control. Control effectiveness from GFM wind farm is verified in PSCAD software with various simulation conditions.

To mitigate voltage transients caused by power fluctuations in microgrid systems, this study investigates model predictive control and virtual inertia control for the voltage regulation strategy of energy storage unit converters. By drawing an analogy with the virtual synchronous machine equation in AC systems, the virtual capacitor inertia equation is derived for DC systems. Subsequently, model predictive control (MPC) is integrated with virtual inertia (VI) control, leading to the development of an MPC-VI cooperative control method. The reference value for the inner control loop is computed in real time using model prediction, enabling the injection of a counteracting signal opposite to the direction of DC bus voltage fluctuation during disturbances. This approach effectively suppresses rapid voltage variations and enhances system inertia. Furthermore, by incorporating a threshold-based mechanism, the issue of prolonged dynamic response time is mitigated. Simulation and experimental results demonstrate that, compared to conventional control strategies, the proposed MPC-VI method significantly attenuates instantaneous and severe voltage fluctuations, allowing for a more gradual voltage transition during transient events. Additionally, with the implementation of the threshold equation, the system returns to steady state without notable delay, preserving the droop characteristics of the control scheme.

Increasing levels of renewable energy sources (RES) in the form of inverter-based resources (IBRs) affect the overall inertia of the power system. Grid-forming (GFM) inverters have the potential to contribute to synthetic inertia and enhance the oscillatory stability of the system. This paper presents a comparative analysis of the performance of GFM inverters and conventional synchronous generators (SGs). The GFM inverters are implemented using a virtual synchronous machine (VSM) control model, whereas conventional SGs use the GENROU model. PowerWorld Simulator and DSATools are employed for steady-state and dynamic (time-domain) simulations. The Single-Machine Infinite-Bus (SMIB) system and the benchmark two-area Kundur system are used to investigate the damping performance of GFM inverters under varying grid strengths. The performance of system damping is evaluated using conventional eigenvalue analysis and the matrix pencil method. The simulation results show that GFM inverters with VSM control provide acceptable damping performance compared to conventional SGs and have the potential to play a key role in future low-inertia power grids.

This paper investigates virtual inertia control of doubly fed induction generator (DFIG)-based wind turbines to provide dynamic frequency support in the event of abrupt power change. The model and control scheme of the DFIG is analysed. The relationships among the virtual inertia, the rotor speed and the network frequency variation are then investigated. The hidden kinetic energy that can be released to contribute to the grid inertia by means of shifting the operating point from the maximum power tracking curve to a virtual inertia control curve is investigated. The virtual inertia control strategy based on shifting power tracking curves of the DFIG is proposed and the calculation method for determining these virtual inertia control curves is presented. A three-machine system with 20 percent of wind penetration is used to validate the proposed control strategy. Simulation results show that by the proposed control strategy, DFIG based wind farms have the capability of providing dynamic frequency support to frequency deviation, and thus improving the dynamic frequency performance of the grid with high wind power penetration. (6 pages)

In modern power systems, there is a greater penetration of renewable sources, resulting in the expansion of microgrid. By implementing RES instead of conventional synchronous machines, the system’s overall inertia is significantly reduced. Modern and future power systems must reduce system inertia and keep the frequency at its nominal value since frequency oscillation impacts on the functionality, stability, and resilience of the system. Therefore, creating a stable, scalable, and reliable virtual inertia control system is crucial to effectively minimize the deviations during significant contingencies. Consequently, taking into account the probable issues, the foremost objective of this research work is to improve the dynamic security of an island microgrid through the implementation of a frequency control concept based on virtual inertia control. In the research study, the proposed microgrid system comprises of photovoltaics, wind-generating units, thermal power units, storage units, electric vehicles (EVs), and loads. A cascaded PIDFN controller is optimally designed using a novel metaheuristic modified differential evolution (MDE) algorithm that regulates the frequency based on virtual inertia control. In the MATLAB®/SIMULINK environment, various operating situations such as load variations, RES, and EVs disconnection are investigated, and the performance of the VIC-based MDE controller was compared to that of other controllers based on evolutionary optimization algorithms such as DE and TLBO. The results validate that the recommended virtual inertia control strategy enhances the reliability of the system.

The continuous increasing share of power-converter-based renewable energies weakens the power system inertia. The lack of inertia becomes a main challenge to small-scale modern power systems in terms of control and stability. To alleviate adverse effects from inertia reductions, e.g., undesirable load shedding and cascading failures, three-phase grid-connected power converters should provide virtual inertia upon system demands. This can be achieved by directly linking the grid frequency and voltage references of dc-link capacitors/ultracapacitors. This paper reveals that the virtual inertia control may possibly induce instabilities to the power converters under weak grid conditions, which is caused by the coupling between the d- and q-axes as well as the inherent differential operator introduced by the virtual inertia control. To tackle this instability issue, this paper proposes a modified virtual inertia control to mitigate the differential effect, and thus, alleviating the coupling effect to a great extent. Experimental verifications are provided, which demonstrate the effectiveness of the proposed control in stabilizing three-phase grid-connected power converters for inertia emulation even when connected to the weak grid.

A dc microgrid is a low inertia system dominated by power converters. As a result, the change rate of the dc voltage is very fast under power variation. In this study, a distributed virtual inertia control is proposed to enhance the inertia of the dc microgrid and decrease the change rate of the dc voltage. The inertia of the dc microgrid can be enhanced by the kinetic energy in the rotor of the permanent magnet synchronous generators (PMSG)‐based wind turbine, the energy stored in batteries and the energy from the utility grid. By introducing a virtual inertia control coefficient, a general expression of the inertial power provided by each controllable power sources is defined. The proposed inertia control is simply a first‐order inertia loop and is implemented in the grid‐connected converter, the battery interfaced converter and the PMSG interfaced converter, respectively. The small‐signal model of the dc microgrid with the proposed inertia control is established. The range of virtual inertia control coefficient is determined through stability analysis. Finally, a typical dc microgrid is built and simulated in Matlab/Simulink, and the effectiveness of the proposed control strategy and correctness of the stability analysis are verified.

Inverter-based resources (IBRs) possess dynamics that are significantly different from those of synchronous-generator-based sources and as IBR penetrations grow the dynamics of power systems are changing. This article discusses the characteristics of the new dynamics and examines how they can be accommodated into the long-standing categorizations of power system stability in terms of angle, frequency, and voltage stability. It is argued that inverters are causing the frequency range over which angle, frequency, and voltage dynamics act to extend such that the previously partitioned categories are now coupled and further coupled to new electromagnetic modes. While grid-forming (GFM) inverters share many characteristics with generators, grid-following (GFL) inverters are different. This is explored in terms of similarities and differences in synchronization, inertia, and voltage control. The concept of duality is used to unify the synchronization principles of GFM and GFL inverters and, thus, established the generalized angle dynamics. This enables the analytical study of GFM-GFL interaction, which is particularly important to guide the placement of GFM apparatuses and is even more important if GFM inverters are allowed to fall back to the GFL mode during faults to avoid oversizing to support short-term overload. Both GFL and GFM inverters contribute to voltage strength but with marked differences, which implies new features of voltage stability. Several directions for further research are identified, including: 1) extensions of nonlinear stability analysis to accommodate new inverter behaviors with cross-coupled time frames; 2) establishment of spatial–temporal indices of system strength and stability margin to guide the provision of new stability services; and 3) data-driven approaches to combat increased system complexity and confidentiality of inverter models.

The future of the machine-controlled power system is advancing to an inverter-based power system with a huge integration of renewable energy sources (RES) like photovoltaic (PV) and wind energy. The penetration of RES leads to instability of frequency due to the lack of inertia. Recent development has been made in inverter’s based virtual inertia control for frequency regulation in virtual synchronous generator-based systems. Inverters connecting to the microgrid are provided with virtual inertia and droop control by the virtual synchronous generator (VSG) system. In this study, three parallel VSGs based PV systems are used to investigate the frequency response and voltage stability. An advanced genetic algorithm (GA) optimization method is utilized to attain the optimum parameters of the droop control used in the VSGs. Furthermore, two different case studies with respect to load change are analyzed in detail with comparisons with the classical droop controller. The simulation results observed many improvements of the proposed controller over the classical controller.

The integration of the significant number of virtual inertia control systems could create new challenges in the implementation of virtual inertia control systems in today and future power systems. Since the large amounts of RESs/DGs have been integrated by the electrical industry into power systems, considerable effort is required to effectively manage the installed virtual inertia control units. This chapter presents the technical challenges and further research needs regarding the virtual inertia control system. The key aspects of the challenges are how to manage changes in system topology created by various virtual inertia control systems as new control units in the network along with the resulted change in system dynamics and how to make the robust power grid by taking the advantage of the potential flexibility of the decentralized/distributed virtual inertia control units. The challenges such as the optimization of the virtual inertia control system and real-time inertia estimation are emphasized for supporting the reliable operation of virtual inertia control units for their application in low inertia power systems with high penetration of RESs. Finally, some important challenges related to the market for inertia service and the regulation of the virtual inertia control system are discussed for further research.

With the conventional power supply being replaced by a large number of wind power and the investment of ultra‐high‐voltage transmission channels, the operating characteristics of the power system have undergone profound changes, and the characteristics of power electronics have become increasingly evident. The dynamic characteristics of DFIG (double‐fed induction generator) with virtual inertia control are in a coupling relationship with the dynamic characteristics of the power system, which leads to significant changes in the frequency oscillation characteristics of the wind turbine. The intrinsic mechanism and external performance of the sub and super‐synchronous oscillating systems are essentially different from those of conventional wind turbines. Based on the analysis of oscillation mechanism and stability of traditional wind turbines, this study reviews the literature and summarises the existing key technical problems. Secondly, the eigenvalue analysis and simulation are used to demonstrate the influence of virtual inertia on the sub and super‐synchronous oscillations of wind turbines.

The high-level penetration of intermittent renewable power generation may limit power system inertia, resulting in system frequency instability in increasing power converter-based energy sources. To resolve this problem, virtual inertia control using distributed gray wolf optimization (DGWO) method in a synchronous generator is simulated under a distinct output fluctuation condition. First, the DGWO algorithm was established to achieve a local and global balance solution, and standard test functions were employed to verify the model convergence. Second, the key parameters that determine the effect of the virtual inertia controller in the power grid were analyzed. A DGWO-based optimization strategy to stabilize inertia was also developed. Finally, simulation results using step and random loads under a high permeability level are provided to verify the effectiveness of the proposed model. In the step load disturbance, the system can recover from the disturbance point to the stable point after 3 s under the regulation of the proposed control strategy, which is reduced by 18 s compared with the traditional control method. In the random load test, it takes only 12 s, 63 s less than the traditional one. Accordingly, the power system frequency can be stabilized more quickly from a disturbance state to a stable stage.

As a typical grid-forming control method, virtual synchronous generator (VSG) control enhances system inertia but introduces frequency oscillation issues. This problem becomes particularly severe in multi-VSG parallel systems when inconsistent virtual inertia exists among power sources, significantly compromising the security and stability of power system operation. Virtual inertia control can directly regulate the rate of change of frequency (RoCoF) under constant torque difference conditions to suppress frequency oscillations, offering faster response characteristics. Therefore, this paper proposes a distributed virtual inertia control strategy for multi-VSG parallel systems. First, a small-signal model of the multi-machine parallel system is established, and its small-signal stability is demonstrated. Second, a neighbor-communication-based distributed virtual inertia coordination control method is proposed. Through neighbor information exchange and local decision-making, this method enables dynamic adjustment of each unit's virtual inertia, driving frequency synchronization among all units in the system. This effectively suppresses post-disturbance frequency oscillations and enhances the dynamic performance of low-inertia power systems. Furthermore, the stability of the proposed control strategy is rigorously proven through the construction of a Lyapunov energy function. Finally, MATLAB/Simulink simulations verify that the proposed virtual inertia control strategy can effectively mitigate frequency oscillations while reducing their settling time.

Because of the control mode of the doubly-fed fan itself, its own frequency is decoupled from the grid frequency. When the load is disturbed, the system frequency changes, but the energy stored in the fan rotor does not respond to the frequency change as quickly as the synchronous generator. Therefore, large-scale access of fans will reduce the inertia level of the power grid, which is not conducive to the frequency support of the power grid. Based on the investigation of the control methods to improve the fan inertia, this paper sets up the virtual inertia control model and the integrated inertia control model respectively, and finds that both can improve the inertia of the system, and the latter can effectively make up for some deficiencies of the virtual inertia control. In addition, the inertia of fan is quantitatively identified by using the motion equation of fan rotor under disturbance.