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Copyright of Zhejiang Electric Power is the property of Zhejiang Electric Power Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

To enhance frequency and active power control performance, this research proposes a decentralized robust optimal tuning approach for power grid frequency regulation support using energy storage systems (ESSs) as the primary source of grid-forming (GFM) converters. The proposed approach employs the robust Kharitonov theory to find a family of stabilizing sets of a proportional-integral (PI)-based supplementary controller, which is used in the outer control layer of the GFM control system. A family of stabilizing parameter sets is found in the presence of system uncertainties and disturbances that are common in power grid operation. Then, using a developed Bayesian optimization algorithm, an optimal set of parameters is determined among the mentioned family member sets. The proposed sophisticated combination of a robust control theorem and an optimization algorithm provides a promising solution for the robust and optimal tuning of control system parameters in ESS-based GFM converters. The efficacy of the proposed method is demonstrated via simulation and laboratory real-time experiment results for a given detailed case study. [ABSTRACT FROM AUTHOR]

A recently proposed droop controller design achieved approximate ratio-based frequency support through Multiterminal High-Voltage Direct Current (MTDC) grids connecting asynchronous AC areas. The design was performed via a reduced-order model, which neglects system losses. In this paper, to achieve improved tracking, a model-reference-estimation-based corrective control approach is presented, which estimates the differences between the reduced and actual models and compensates for the same. The sufficient condition for perfect ratio tracking by the proposed controller in the presence of modeling uncertainty is established. It is shown that a continuous sliding-mode controller that is robust to bounded modeling uncertainty takes the form of the PI compensator in the proposed corrective control. The region of stability of the proposed controller is established through root locus and numerical eigenvalue analyses. Finally, the effectiveness of this corrective strategy is illustrated through time-domain simulations performed in a full-order model, which is a detailed phasor-based differential-algebraic representation of the AC-MTDC grid. [ABSTRACT FROM AUTHOR]

Wind turbines can contribute to frequency regulation by releasing rotor kinetic energy， which is an effective way to ensure frequency stability in power systems with high wind power penetration. However， the rotor speed recovery process may cause a secondary frequency drop （SFD） in the system. To address this issue， a wind turbine speed recovery strategy based on the asymptotic characteristics of an exponential function is proposed， following both qualitative and quantitative analysis of the frequency support and speed recovery. First， during the recovery period， the rotor speed is restored according to the power reference value defined by an exponential function. After the speed is recovered， the control switches to maximum power point tracking （MPPT） to manage the turbine's output power. Second， the parameters of the exponential decay coefficient are tuned based on various constraints. Finally， simulation results validate that the proposed strategy effectively mitigates SFDs while ensuring a smooth rotor speed recovery process.

Grid-forming （GFM） converters， known for their superior inertia damping characteristics， have emerged as a center research focus in the renewable energy sector. As a result， the damping characteristics of GFM wind power converters are first examined. The analysis centers on their equivalent external characteristics from an impedance standpoint， and the Bode stability criterion is utilized to study their stability during grid connection. Subsequently， the frequency support characteristics of GFM wind power converters are examined in terms of inertia. A comparative investigation is undertaken via simulation analysis to gauge the supportive capability of GFM and grid-following （GFL） wind power converters during alterations in grid frequency and power fluctuation on the wind turbine side. The findings reveal that the external impedance characteristics of GFM wind power converters not only display inductive properties but also maintain commendable stability throughout grid integration. In response to a grid malfunction， GFM wind power converters can furnish a significant measure of power support， equating to a rise in system inertia， thereby indicating favorable grid integration characteristics.

The integration of high-penetration renewable energy sources is playing an increasingly important role in the frequency regulation of the power system. However, due to the varying output characteristics and response speeds of different renewable distributed energy resources (DERs), the overall response from the collective output of these distributed energy resources may not meet expected requirements and could have adverse effects on the grid. One drawback of traditional distributed coordinated control is its high communication requirements. This paper proposes using digital twin (DT) technology for the coordinated control of distributed energy resources, which can minimize the communication needs between various distributed energy resources while achieving coordinated control. Verification of the frequency support coordinated control strategy based on digital twin technology shows that it can effectively enhance the individual output characteristics and overall response of each distributed energy resource, providing effective support for grid frequency. [ABSTRACT FROM AUTHOR]

Coordinated control technology attracts increasing attention to the photovoltaic–battery energy storage (PV-BES) systems for the grid-forming (GFM) operation. However, there is an absence of a unified perspective that reviews the coordinated GFM control for PV-BES systems based on different system configurations. This paper aims to fill the gap by providing a comprehensive review of coordinated GFM control strategies for PV-BES, considering various system configurations. Typical configurations of PV-BES systems are explored, followed by a detailed discussion of conventional GFM control methods used in the PV-BES systems. Furthermore, coordinated GFM controls are analyzed in PV-BES systems based on different configurations, providing the common DC bus configuration as a widely adopted configuration due to its more control degrees of freedom and ease of expansion. Moreover, the mode division and switching coordinated control based on the system power is the most widely used. Furthermore, challenges in the coordinated GFM controls for the PV-BES system in future applications are briefed and emphasized before the conclusion. [ABSTRACT FROM AUTHOR]

Copyright of Zhejiang Electric Power is the property of Zhejiang Electric Power Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The increasing proportion of new energy in the power system leads to a decrease in system inertia and weakens the frequency stability of the system. The use of grid-forming (GFM) control for inverters is an effective method to improve frequency stability in new energy generation systems. Due to uncertain disturbances in microgrids, it can cause changes in system frequency. A control strategy for linear active disturbance rejection control combined with model information (LADRC-MI) is proposed in this manuscript. In this method, the inertia level and damping of the system are improved by using virtual synchronous generator (VSG)-based GFM control. Then, the obtained part of the model information is written into the coefficient matrix of the linear extended state observer. The LADRC-MI is constructed to improve the immunity of the controller. Finally, the effectiveness of the strategy in improving the immunity and frequency stability of the converter is analyzed and verified. [ABSTRACT FROM AUTHOR]

With the increasing penetration of wind power into the grid, its intermittent and fluctuating characteristics pose a challenge to the frequency stability of grids. Energy storage systems (ESSs) are beginning to be used to assist wind farms (WFs) in providing frequency support due to their reliability and fast response performance. However, the current schemes mostly focused on the sole operation of either WT or ESSs for frequency support, lacking in economy and reliability. To address these issues, this work proposes a sliding mode consensus control (SMCC) method for ESSs to assist the frequency response of WTs. First, a coordination strategy for the WTs and ESS is proposed to reinforce the frequency support capability of a WT–ESS by determining the optimal capacity of ESSs. Meanwhile, a novel SMCC is proposed to achieve proportional power sharing among ESSs and increase the total power output of the entire WFs during wind energy shortages. Moreover, an event triggered communication mechanism is utilized to reduce the information exchanges among ESSs. Subsequently, a Lyapunov stability analysis is conducted to prove the stability and finite time consensus of the proposed method. Simulation case studies are conducted in IEEE 39-bus system to validate the effectiveness of the proposed framework. [ABSTRACT FROM AUTHOR]

Copyright of Electric Power Automation Equipment / Dianli Zidonghua Shebei is the property of Electric Power Automation Equipment Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Renewable energy resources emerge as a sustainable alternative to augmenting the energy supply of floating production storage and offloading (FPSO) platforms. However, the increased generation at FPSO based on converter-interfaced energy decreases the system-equivalent inertia constant, which becomes more susceptible to frequency deviations. This paper proposes and evaluates the combined frequency-voltage-var control performance to mitigate frequency variation in a typical FPSO unit with penetration of floating wind energy generation. The control functions are communication-free and embedded in the active front-end variable frequency drives (AFE-VFDs), which are installed on the FPSO and have the primary function of controlling the speed of water injection pumps. The FPSO electrical power system model is developed in MATLAB/Simulink®. Comparative results obtained from the AFE-VFD equipped with volt-var, freq-var, and combined freq-volt-var functions are shown to highlight the proposed solution merits. The results have shown a conflicting behavior with the frequency and voltage deviation improvement associated with absorption and injection of reactive power, respectively. Accordingly, the frequency-volt-var prioritizes frequency deviations during heavy transient events and voltage deviations during regular operation.

Constraining the virtual inertia of variable-speed wind turbines within the variable frequency limit time will be the key to improving the wind power's reliability in actively supporting the system frequency. First, this paper analyzes the controllable characteristics and evaluation methods of the virtual inertia of the wind turbine. Second, it quantifies the variable frequency limit time of the grid with high-proportion wind power and considers the wind turbine speed and power margin to restrict the virtual inertia of the wind turbine during the period of frequency drop or increase to provide reliable inertia support to ensure the safety of the system frequency. On this basis, the inertia demand under the frequency safety of the system was analyzed, and the virtual inertia control strategy of the variable-speed wind turbine in the variable frequency limit time was proposed using the speed tracking performance of the wind turbine. Finally, the grid with a high-proportion wind power simulation system is built to verify that the variable- speed wind turbine can reliably satisfy the inertia demand of system frequency modulation under the proposed control strategy and significantly improve its active support performance to the power grid. [ABSTRACT FROM AUTHOR]

In this paper, a simplified model of voltage source converter-based high-voltage direct current (VSC-HVDC) link is proposed that is effective in the analysis of multi-machine systems, even when crucial applications of the link are involved. The model is derived by eliminating the DC dynamics, including the converter-related impedances as a part of the AC system transmission network and obtaining the converter currents in a straightforward manner. Case studies are conducted on 4-machine, 10-bus and 16-machine, 68-bus systems to prove the accuracy of the model. The study clearly indicates the model's ability to reproduce the influence of VSC controllers, impact of variable power levels and effects of multiple HVDC links in a system. It is further verified for significant VSC-HVDC applications. The model is effective at handling frequency support of asynchronous systems and can be applied to VSC-HVDC connected offshore wind farms feeding multi-machine systems. It is demonstrated that the proposed model can be efficiently used for analysis of large AC systems embedded with VSC-HVDC links with lesser modeling complexity and computation time. [ABSTRACT FROM AUTHOR]

With the continuous increase in the installed capacity of new energy systems, the impact of power shocks on grid frequency is becoming more significant, seriously affecting the stability of the grid and thermal power units. For this reason, a mixed variable parameter energy storage-assisted frequency support control method is proposed. This method introduces an integral control mode based on the existing control mode and forms a co-ordinated control mode. The impact of the switching point of the mixed control mode is analyzed, and a new mode switching method is used. The model adjusts the frequency regulation parameters according to the different states of the grid frequency, making the frequency regulation more efficient. Finally, a frequency regulation model that integrates various improvement methods is built, and a 10 min continuous load disturbance is applied to compare and verify the frequency regulation performance of the model as a whole. The results indicate that the proposed control method can significantly improve the quality of grid frequency regulation, reduce the frequency deviation amplitude to 0.00096 p.u., reduce the participation time of thermal power units to 190.8393 s, and reduce the peak output to 0.1934 MW, demonstrating its engineering feasibility. [ABSTRACT FROM AUTHOR]

With the rapid increase of wind energy integrated into power systems, wind turbine generators (WTGs) are required to provide frequency support to maintain the system frequency stability. However, the frequency regulation is achieved by employing temporary energy reserves from WTGs at the initial stage of a disturbance. Therefore, a second frequency dip (SFD) may occur, if no other energy reserve is available to compensate the power deficiency as WTGs have to recover their operating points and rotor speeds back to the initial operating points. To deal with this problem, this paper proposes a consecutive power dispatch scheme to reduce the SFD and prevent WTGs from over-deceleration. All WTGs are divided into two groups with in a wind farm: Group 1 (G1) WTGs operating at maximum power point tracking (MPPT), Group 2 (G2) WTGs operating at deloading power. If a frequency contingency occurs, the proposed scheme aims to release an amount of kinetic energy (KE) stored in the rotating masses of G1 WTGs to improve the frequency nadir (FN). Following this, energy reserves are released from G2 WTGs to compensate the power shortage during the period when G1 WTGs rotor speeds have to be recovered. The simulation results show that the scheme causes a small SFD while improving the first FN and preventing the rotor from over-decelerations in various wind conditions, contingency sizes, and wind penetration levels.

The massive integration of renewable energies into the grid using fast-response converters without inertia generates issues such as inertia reduction, temporary voltage violations, and power quality reduction. The system inertia reduction is a critical problem that could lead to grid frequency exceeding the acceptable range, resulting in undesirable load-shedding or even large-scale blackouts. To overcome these issues, the use of electric vehicle bidirectional chargers (EVBCs) implementing functionalities such as distributed virtual inertia (VI), long-term frequency support, voltage support by reactive power, and harmonics compensation, has been proposed as a possible solution. This article proposes a novel control strategy to manage a hybrid energy storage system (HESS) composed of dc-link capacitors and battery, through an isolated two-stage ac–dc converter (composed of a dual active bridge resonant type dc–dc converter cascaded to a voltage source inverter), intended for off-board EVBCs. The HESS management allows decoupling of the active power dynamic response since dc-link capacitors supply the fast dynamic response for VI support whereas the battery supplies the slower dynamic response for long-term frequency support, respectively. Hence, the VI support does not affect the battery lifetime. Simulations and experimental results are presented for a 2.5 kW prototype to validate VI, frequency-voltage support along with harmonics compensation.

To enhance frequency and active power control performance, this research proposes a decentralized robust optimal tuning approach for power grid frequency regulation support using energy storage systems (ESSs) as the primary source of grid-forming (GFM) converters. The proposed approach employs the robust Kharitonov theory to find a family of stabilizing sets of a proportional-integral (PI)-based supplementary controller, which is used in the outer control layer of the GFM control system. A family of stabilizing parameter sets is found in the presence of system uncertainties and disturbances that are common in power grid operation. Then, using a developed Bayesian optimization algorithm, an optimal set of parameters is determined among the mentioned family member sets. The proposed sophisticated combination of a robust control theorem and an optimization algorithm provides a promising solution for the robust and optimal tuning of control system parameters in ESS-based GFM converters. The efficacy of the proposed method is demonstrated via simulation and laboratory real-time experiment results for a given detailed case study.

The large-scale offshore wind power integrated into the onshore power grid through voltage source converter-based high voltage direct current (VSC-HVDC) system is unable to provide inertia response and frequency support to the onshore power grid. To improve the frequency characteristics of the receiving-end power grid, a coordinated frequency control strategy combining VSC-HVDC and offshore wind power is proposed. The onshore converter adopts virtual inertia control, which uses DC capacitors to absorb or release energy for inertia support after the receiving-end grid is disturbed. Wind-farm-side VSC (WFVSC) obtains the frequency signal of the receiving-end power grid by detecting the local DC voltage. The offshore wind farm (OWF) transfers the frequency deviation into an additional power signal and sends it to the power controller to adjust the output, thereby performing inertia and primary frequency response. In addition, a secondary frequency regulation strategy for wind farms has been designed to achieve non-difference frequency regulation of the receiving-end power grid. Finally, a simulation model of VSC-HVDC integrated OWF system is constructed to demonstrate the proposed coordinated frequency control strategy for VSC-HVDC and OWF. The results indicate that the proposed control strategy can effectively enhance the frequency support capability of the receiving-end power grid. [ABSTRACT FROM AUTHOR]

Copyright of Electric Power Automation Equipment / Dianli Zidonghua Shebei is the property of Electric Power Automation Equipment Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

As the world grapples with the energy crisis, integrating renewable energy sources into the power grid has become increasingly crucial. Microgrids have emerged as a vital solution to this challenge. However, the reliance on renewable energy sources in microgrids often leads to low inertia. Renewable energy sources interfaced with the network through interlinking converters lack the inertia of conventional synchronous generators, and hence, need to provide frequency support through virtual inertia techniques. This paper presents a new control algorithm that utilizes the reinforcement learning agents Twin Delayed Deep Deterministic Policy Gradient (TD3) and Deep Deterministic Policy Gradient (DDPG) to support the frequency in low-inertia microgrids. The RL agents are trained using the system-linearized model and then extended to the nonlinear model to reduce the computational burden. The proposed system consists of an AC–DC microgrid comprising a renewable energy source on the DC microgrid, along with constant and resistive loads. On the AC microgrid side, a synchronous generator is utilized to represent the low inertia of the grid, which is accompanied by dynamic and static loads. The model of the system is developed and verified using Matlab/Simulink and the reinforcement learning toolbox. The system performance with the proposed AI-based methods is compared to conventional low-pass and high-pass filter (LPF and HPF) controllers. [ABSTRACT FROM AUTHOR]

Energy storage systems (ESS) and permanent magnet synchronous generators (PMSG) are speculated to be able to exhibit frequency regulation capabilities by adding differential and proportional control loops with different control objectives. The available PMSG kinetic energy and charging/discharging capacities of the ESS were restricted. To improve the inertia response and frequency control capability, we propose a short-term frequency support strategy for the ESS and PMSG. To this end, the weights were embedded in the control loops to adjust the participation of the differential and proportional controls based on the system frequency excursion. The effectiveness of the proposed control strategy was verified using PSCAD/EMTDC. The simulations revealed that the proposed strategy could improve the maximum rate of change of the frequency nadir and maximum frequency excursion. Therefore, it provides a promising solution of ancillary services for frequency regulation of PMSG and ESS. [ABSTRACT FROM AUTHOR]

Constraining the virtual inertia of variable-speed wind turbines within the variable frequency limit time will be the key to improving the wind power’s reliability in actively supporting the system frequency. First, this paper analyzes the controllable characteristics and evaluation methods of the virtual inertia of the wind turbine. Second, it quantifies the variable frequency limit time of the grid with high-proportion wind power and considers the wind turbine speed and power margin to restrict the virtual inertia of the wind turbine during the period of frequency drop or increase to provide reliable inertia support to ensure the safety of the system frequency. On this basis, the inertia demand under the frequency safety of the system was analyzed, and the virtual inertia control strategy of the variable-speed wind turbine in the variable frequency limit time was proposed using the speed tracking performance of the wind turbine. Finally, the grid with a high-proportion wind power simulation system is built to verify that the variable-speed wind turbine can reliably satisfy the inertia demand of system frequency modulation under the proposed control strategy and significantly improve its active support performance to the power grid.

New ancillary services and additional requirements for the grid integration of variable renewable energy (VRE) are being defined worldwide, in response to the technical challenges caused by increasing levels of VRE utilization in electric power grids. Currently, the use of wave energy is still limited to a few applications or demonstrations, where the level of penetration of wave power into the grid is not significant. To anticipate the future requirements for wave power integration, and the possibilities for provision of services, this paper considers the lessons being learned through the challenges caused by high penetration levels of other VRE sources into the grid, particularly wind power. On this basis, this paper presents an overview of grid support services that wave power plants can be expected to provide in power systems dominated by converter-interfaced generation, i.e. low inertia systems. Specifically, the focus is on services that support the active power balance in the power system. Then, the current capabilities and future perspectives for the provision of frequency support by wave farms are discussed.

Coordinated control technology attracts increasing attention to the photovoltaic–battery energy storage (PV-BES) systems for the grid-forming (GFM) operation. However, there is an absence of a unified perspective that reviews the coordinated GFM control for PV-BES systems based on different system configurations. This paper aims to fill the gap by providing a comprehensive review of coordinated GFM control strategies for PV-BES, considering various system configurations. Typical configurations of PV-BES systems are explored, followed by a detailed discussion of conventional GFM control methods used in the PV-BES systems. Furthermore, coordinated GFM controls are analyzed in PV-BES systems based on different configurations, providing the common DC bus configuration as a widely adopted configuration due to its more control degrees of freedom and ease of expansion. Moreover, the mode division and switching coordinated control based on the system power is the most widely used. Furthermore, challenges in the coordinated GFM controls for the PV-BES system in future applications are briefed and emphasized before the conclusion.

The increasing proportion of new energy in the power system leads to a decrease in system inertia and weakens the frequency stability of the system. The use of grid-forming (GFM) control for inverters is an effective method to improve frequency stability in new energy generation systems. Due to uncertain disturbances in microgrids, it can cause changes in system frequency. A control strategy for linear active disturbance rejection control combined with model information (LADRC-MI) is proposed in this manuscript. In this method, the inertia level and damping of the system are improved by using virtual synchronous generator (VSG)-based GFM control. Then, the obtained part of the model information is written into the coefficient matrix of the linear extended state observer. The LADRC-MI is constructed to improve the immunity of the controller. Finally, the effectiveness of the strategy in improving the immunity and frequency stability of the converter is analyzed and verified.

The large-scale offshore wind power integrated into the onshore power grid through voltage source converter-based high voltage direct current (VSC-HVDC) system is unable to provide inertia response and frequency support to the onshore power grid. To improve the frequency characteristics of the receiving-end power grid, a coordinated frequency control strategy combining VSC-HVDC and offshore wind power is proposed. The onshore converter adopts virtual inertia control, which uses DC capacitors to absorb or release energy for inertia support after the receiving-end grid is disturbed. Wind-farm-side VSC (WFVSC) obtains the frequency signal of the receiving-end power grid by detecting the local DC voltage. The offshore wind farm (OWF) transfers the frequency deviation into an additional power signal and sends it to the power controller to adjust the output, thereby performing inertia and primary frequency response. In addition, a secondary frequency regulation strategy for wind farms has been designed to achieve non-difference frequency regulation of the receiving-end power grid. Finally, a simulation model of VSC-HVDC integrated OWF system is constructed to demonstrate the proposed coordinated frequency control strategy for VSC-HVDC and OWF. The results indicate that the proposed control strategy can effectively enhance the frequency support capability of the receiving-end power grid.

Synchronous generators provide an inherent inertial response to frequency deviations because of their huge revolving mass that is electro-mechanically tied to the electrical network. Contrariwise, the power converters isolate the revolving mass of variable-speed wind turbines from the electric network. Therefore, they are not able to provide an inherent inertial reaction to frequency events on the electric network. This reduces the effective network inertia, which is essential for marinating the power system's frequency. To address this problem in cases of using a wind energy doubly fed induction generator, this study introduces a kinetic energy recovery controller to the rotor-side converter. [ABSTRACT FROM AUTHOR]

This paper presents the topology and control of a photovoltaic inverter with an internal battery storage system in conjunction with droop control designed to perform ancillary services such as frequency and reactive power support (voltage regulation), active power dispatch through a proposal to control the charging and discharging of batteries and harmonic current compensation (active filter) in a microgrid connected to an IEEE 13-bus system. The converter in question consists of two stages, a 3-level NPC DC/AC converter and Boost and Buck-Boost DC/DC converters. The system in question is capable of helping to dampen frequency oscillations, as well as regulating the PCC charging curve by charging or discharging the battery bank. In addition, the proposed system is capable of supplying reactive power to the grid on a permanent basis, thus helping with voltage control. It is also capable of compensating for harmonic currents caused by non-linear loads connected to the PCC. In this context, we can see the multifunctionality of the photovoltaic inverter in helping to mitigate disturbances associated with the power quality, with the differential of charge and discharge control to preserve the useful life of the energy storage system. For this study, Matlab/Simulink software was used to implement and validate the proposed topology and control systems. In the computer simulations carried out, satisfactory results were obtained in relation to the execution of the ancillary services mentioned above, thus demonstrating the viability of the proposed strategy. [ABSTRACT FROM AUTHOR]

Increasing penetrations of renewable-based generation have led to a decrease in the bulk power system inertia and an increase in intermittency and uncertainty in generation. Energy storage is considered to be an important factor to help manage renewable energy generation at greater penetrations. Hydrogen is a viable long-term storage alternative. This paper analyzes and presents use cases for leveraging electrolyzer-based power-to-gas systems for electric grid support. The paper also discusses some grid services that may favor the use of hydrogen-based storage over other forms such as battery energy storage. Real-time controls are developed, implemented and demonstrated using a power-hardware-in-the-loop(PHIL) setup with a 225-kW proton-exchange-membrane electrolyzer stack. These controls demonstrate frequency and voltage support for the grid for different levels of renewable penetration (0%, 25%, and 50%). A comparison of the results shows the changes in respective frequencies and voltages as seen as different buses as a result of support from the electrolyzers and notes the impact on hydrogen production as a result of grid support. Finally, the paper discusses the practical nuances of implementing the tests with physical hardware, such as inverter/electrolyzer efficiency, as well as the related constraints and opportunities.

Copyright of Electric Power Automation Equipment / Dianli Zidonghua Shebei is the property of Electric Power Automation Equipment Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper proposed a unified active power optimization control of wind farms under wake effect. It takes the sum of the kinetic energy variation and pitch angle variation as the optimization objective and used the particle swarm algorithm to achieve the optimization results. The main feature of the proposed method is that it unifies the kinetic energy optimization under a low wind speed area, the pitch angle and kinetic energy trade-off optimization under a medium wind speed area, and the pitch angle optimization under a high wind speed area. Combined with the de-loaded power constraint, it can flexibly reach various optimal operating states of the wind farm. The simulation results show that the proposed method optimizes the rotor speed and pitch angle in different wind speed areas, and releases kinetic energy and/or increases the output power of the wind farm to provide frequency support by switching the operating states. [ABSTRACT FROM AUTHOR]

Under virtual synchronous control, the photovoltaic energy storage grid-connected system can realize synchronous grid connection. However, the power coupling relationship between units needs to be analyzed to reduce additional operation risks and improve the support performance of virtual synchronous control. In this paper, the definition of virtual inertia of the energy storage device is described, and the power coupling relationship between the virtual synchronous generator and synchronous generator is analyzed by establishing a small-signal model of photovoltaic energy storage grid-connected power generation system. The influence of the power coupling control coefficient on the static and transient stability of the grid-connected system is analyzed by the extended equal area rule and eigenvalue method. Based on this, a power coupling control strategy is proposed to improve inertia and damping support performance. Finally, a grid-connected power generation simulation system with a high proportion of photovoltaic energy storage is built to test and verify. The results demonstrate that the proposed control can reduce the risk of power oscillation induced by virtual inertia and improve the grid-connected system’s frequency support and power oscillation suppression ability.

Due to their fast response and strong short-term power throughput capacity, electric vehicles (EVs) are promising for providing primary frequency support to power grids. However, due to the complicated charging demands of drivers, it is challenging to efficiently utilize the regulation capacity of EV clusters for providing stable primary frequency support to the power grid. Accordingly, this paper proposes an adaptive primary frequency support strategy for EV clusters constrained by the charging-behavior-defined operation area. First, the forced charging boundary of the EV is determined according to the driver's charging behavior, and based on this, the operation area is defined. This ensures full utilization of the available frequency support capacity of the EV. An adaptive primary frequency support strategy of EV clusters is then proposed. The output power of EV is adaptively regulated according to the real-time distance from the EV operating point to the forced charging boundary. With the proposed strategy, when the EV approaches the forced charging boundary, its output power is gradually reduced to zero. Then, the rapid state-of-charge declines of EVs and sudden output power reductions in EV clusters caused by forced charging to meet the driver's charging demands can be effectively avoided. EV clusters can then provide sustainable frequency support to the power grid without violating the driver's charging demands. Simulation results validate the proposed operation-area-constrained adaptive primary frequency support strategy, which outperforms the average strategy in terms of stable output maintenance and the optimal utilization of regulation capacities of EV clusters.

As the trend towards the decentralization of grids continues, new typologies such as microgrids have gained popularity in recent years. Microgrids offer greater reliability as they can operate both connected to the main grid and in islanded mode. However, the transition to island mode can be even more challenging when a microgrid has a high penetration of intermittent sources like Doubly-Fed Induction Generators (DFIGs). In this context, designing new controller algorithms is essential to improve frequency support in microgrids with DFIGs. Therefore, this paper proposes a novel Filtered Repetitive Model Predictive Control (FR-MPC) applied to the frequency support in a microgrid. The control structure is applied to the Grid Side Converter (GSC) of a DFIG, while the DC bus voltage is supplied by a Battery Energy Storage System (BESS). The FR-MPC prediction model uses a state-space model with an embedded repetitive action, which has the capacity to mitigate harmonics of the microgrid. However, the inclusion of the repetitive loop degrades the closed-loop robustness. To overcome this limitation, the prediction model features a robustness low pass filter. Furthermore, this paper presents an analysis of the frequency response and pole map of the key components of the controller: the robustness filter, the repetitive loop, and the control law. Finally, simulation results demonstrate that the proposed controller outperforms both the pure Repetitive MPC (R-MPC) and the Generalized Predictive Control (GPC) in terms of performance.

The integration of converter-based renewable energy resources in a power system leads to reduced inertia, creating a multitude of challenges relating to stability and reliability. On the positive side, the faster response of converter-based renewable energy resources to frequency disturbances, compared to the conventional synchronous-based generators, reduces the required minimum level of inertia in future low-inertia power systems. Accordingly, grid support from distributed photovoltaic (DPV) systems is one of the emerging solutions to overcome the challenges of these systems. This paper demonstrates how adaptive power system frequency support, which modifies the dynamic of frequency support in DPV systems according to the available level of power system inertia, improves overall system operation. In this way, for lower values of power system inertia, a higher amount of power reserve is maintained across DPV systems. Additionally, a faster frequency–Watt response is implemented to provide a larger amount of grid support under frequency disturbances. Simulation results on a composite load model of distribution feeder show that applying for adaptive frequency support from DPV systems improves the frequency nadir during under-frequency events. Experimental results show the effectiveness of the proposed approach in providing a faster dynamic response and higher amount of power support from DPV systems under low inertia conditions. • Proposing an adaptive approach for frequency support with distributed photovoltaic systems. • Obtaining faster frequency response with injection of higher amount of power to grid during under-frequency. • Demonstration of improved frequency response using the composite load model of a distribution feeder. [ABSTRACT FROM AUTHOR]

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Large synchronous generators are of high importance for the stability of power systems. They generate the frequency of the system and stabilize it in case of severe grid faults like trips of large in-feeders or loads. In distributed energy systems, in-feed via inverters will replace this generation in large parts. Modern inverters are capable of supporting grid frequency during severe faults by different means on the one hand. On the other hand, higher Rates of Change of Frequency (RoCoF) after incidents need to be accustomed by future systems. To be able to analyse the RoCoF withstand capability of synchronous or induction generators, suitable models need to be developed. Especially the control and excitation system model need enhancements compared to models proposed in standards like IEEE Std 421.5. This paper elaborates on the necessary modelling depth and validates the approach with example results.

The problem of frequency stability becomes more concerning as the presence of converter-interfaced units increases and conventional generators are suppressed. A decrease in total system inertia, inherently delivered by synchronous generators, results in abrupt frequency changes and jeopardizes power system stability. Therefore, securing sufficient flexible resources with frequency support capability is necessary. The rotational masses of wind turbines (WTs) are a significant and economical source of flexibility in power systems. However, the available kinetic energy (KE) of the WTs’ rotational masses depends on wind conditions and can only be exploited when the wind speed is sufficient for their rotation. When the wind speed is low, the WT is stopped and cannot support the frequency recovery. In this paper, a new concept of WT operation is proposed, which enables the permanent rotation of the WT under low and no wind conditions, making them reliable flexible resources that can continuously provide frequency support. Due to its widespread presence, the doubly-fed induction generator (DFIG) type of machine was considered. The variable-speed WT’s converter management allows rotational speed control, fast power injection, and release of the turbine’s stored KE even when no wind energy is available. The estimated accessible KE in the WT justifies the proposed concept, and the energy consumption due to motoring operation under low and no wind conditions is shown to be acceptable. A case study is performed for the South Banat region in Serbia to demonstrate the presented management concept. Additionally, a dynamic simulation was implemented to illustrate the permanent operation strategy’s impact on frequency stability in a low-inertia system under low and no wind conditions. Besides virtual inertia continuous capability, the proposed concept provides reduced wear of the WT mechanical components due to a lower number of on/off events.

In a world where the energy crisis is becoming overwhelming, demand for integrating renewable energy sources is increasing and forming microgrids is becoming an essential solution. The new microgrid systems, which depend mainly on renewable energy sources instead of conventional synchronous generators, come with a low inertia concern. This paper proposes a virtual inertia controller based on a high-pass filter (HPF) to support the frequency of the AC microgrid while maintaining the DC voltage of the DC microgrid within the nominal ranges in cases of contingencies. The proposed system encounters an AC-DC microgrid with a renewable energy source on the DC microgrid alongside constant power load and resistive loads, while on the AC microgrid side, a synchronous generator is used to present the low inertia of the grid with dynamic loads and static loads. The state-space linearized model of the system is developed and verified using Matlab Simulink. The dynamic response of the proposed controller is compared to the low-pas filter (LPF)-based controller. Moreover, the effect of changing the system's parameters on eigenvalues is investigated. [ABSTRACT FROM AUTHOR]

This paper proposes an innovative inertial response imitation (IRI) and rotor speed recovery (RSR) control scheme of a doubly-fed induction generator (DFIG, Type 3 wind turbine generator) to provide better frequency support response and RSR services for a high wind power penetrated electric power grid. To achieve the first benefit, the coupling relationship between the control coefficient of DFIGs and the frequency deviation was established by using the exponential function so that the control coefficient becomes large with the increasing frequency deviations and sizes of disturbance. After supporting the system frequency, the exponential function was employed to schedule the dynamic control coefficient to alleviate the negative effects of RSR on the instantaneous system frequency. The benefits of the proposed IIR and RSR strategy were investigated in a test system under various scenarios of sizes of disturbance and wind speed conditions. Test results clearly demonstrate that the proposed IIR and RSR strategy is capable of boosting the maximum system frequency excursion and reducing the negative influences on the system frequency during the speed recovery period. [ABSTRACT FROM AUTHOR]

Grid frequency must be regulated in its nominal range to guarantee the stable operation of an electric power grid. Excessive grid frequency excursions result in load shedding, grid frequency instability, or even synchronous generator damage. With the growing wind penetration, there is an increasing issue about the reduction in inertia response. This paper addresses a self-adaptive inertial control strategy for improving the frequency nadir and smoothly regaining the rotor speed to the initial working condition without causing a second frequency drop (SFD). The first objective is achieved by determining the incremental power considering the maximum rate of change of frequency; the secondary goal is realized by smoothly decreasing the power reference based on the decreasing function. Simulation results verify that the proposed control strategy not only boosts the frequency nadir but also guarantees the smooth rotor speed recovery with a negligible SFD. [ABSTRACT FROM AUTHOR]

—With a high proportion of renewable energy, the issue of grid frequency fluctuations is becoming increasingly prominent. To tackle this challenge, wind farms can enhance frequency regulation response by increasing rotational kinetic energy through overspeed control. However, this approach may result in energy losses due to the wake effect. This paper proposes an optimized configuration strategy for wind farm frequency regulation reserve (FRR), utilizing pitch angle cooperative control to reduce wake losses. The objective is to explore the optimal configuration for maximizing wind energy utilization under FRR conditions. A genetic algorithm is used to find the optimal control parameters in the established FLORIS-based wind farm power reserve wake model, which involves rotor speed and pitch angle control, to maximize output power and rotational kinetic energy. Based on this, a lookup table is constructed to map wind conditions to the optimal control parameters for different FRR level. According to the reserve power schedule, optimization control is applied to the wind farm. Simulation results show that, compared to the traditional equal distribution strategy for FRR power, the proposed control strategy can enhance the total kinetic energy reserve of the wind farm by optimizing wake loss, thereby improving frequency support capability. [ABSTRACT FROM AUTHOR]

Synchronous generators provide an inherent inertial response to frequency deviations because of their huge revolving mass that is electro-mechanically tied to the electrical network. Contrariwise, the power converters isolate the revolving mass of variable-speed wind turbines from the electric network. Therefore, they are not able to provide an inherent inertial reaction to frequency events on the electric network. This reduces the effective network inertia, which is essential for marinating the power system’s frequency. To address this problem in cases of using a wind energy doubly fed induction generator, this study introduces a kinetic energy recovery controller to the rotor-side converter.

This paper presents the topology and control of a photovoltaic inverter with an internal battery storage system in conjunction with droop control designed to perform ancillary services such as frequency and reactive power support (voltage regulation), active power dispatch through a proposal to control the charging and discharging of batteries and harmonic current compensation (active filter) in a microgrid connected to an IEEE 13-bus system. The converter in question consists of two stages, a 3-level NPC DC/AC converter and Boost and Buck-Boost DC/DC converters. The system in question is capable of helping to dampen frequency oscillations, as well as regulating the PCC charging curve by charging or discharging the battery bank. In addition, the proposed system is capable of supplying reactive power to the grid on a permanent basis, thus helping with voltage control. It is also capable of compensating for harmonic currents caused by non-linear loads connected to the PCC. In this context, we can see the multifunctionality of the photovoltaic inverter in helping to mitigate disturbances associated with the power quality, with the differential of charge and discharge control to preserve the useful life of the energy storage system. For this study, Matlab/Simulink software was used to implement and validate the proposed topology and control systems. In the computer simulations carried out, satisfactory results were obtained in relation to the execution of the ancillary services mentioned above, thus demonstrating the viability of the proposed strategy.