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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.

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.

Due to advantages such as being clean and safe, photovoltaic (PV) generation has become an effective way to solve the energy crisis and environmental pollution problems. However, the PV inverter does not have inertia and damping characteristics. With the increased penetration of the PV generation into the power system, large-scale PV grid-connection has posed a great challenge to grid frequency stability. The Cascaded H-bridge (CHB) inverter has become a promising candidate for the PV generation system by virtue of advantages such as modular structure and direct grid-connection without power frequency transformers. An inverter control strategy with grid frequency support function is proposed in this paper for the three phase CHB based PV generation system. With the PV string active power reserve, grid frequency dynamic characteristics are improved without energy storage devices. In order to solve the common module power unbalance problem for the CHB topology, the PV string output power is controlled to be balanced with the reserved power distribution control between PV strings proposed in this paper. The H-bridge inverter input power balance is maximized, and balanced three phase grid-connected current output is achieved. The over-modulation risk of H-bridge modules is reduced. At last, simulation models are built in Matlab/Simulink, and a 10kW experimental platform is also built. The effectiveness of the control strategy proposed is validated by simulation and experimental results.

This work proposes the Hybrid Control Converter (HCC), a new kind of VSC control. This method aims to have one single converter mimic the behavior of two individual, virtual converters working in parallel (one with grid-following and the other with grid-forming control). This way, the hybrid converter can show behavioral characteristics of both kinds of converter control at the same time. Two possible applications for the HCC are presented. The first one is related to synthetic inertia capability of wind turbines. Results show that the HCC presents superior performance when compared to conventional techniques based on grid following converters, resulting in a smaller grid frequency drop during transients. The second application shows that the HCC allows for autonomous connection of multiple distinct AC systems using a meshed DC transmission system. This enables automatic frequency support and generation and load sharing between the AC grids without requiring any specific action by the system operator. [ABSTRACT FROM AUTHOR]

Flexible power point tracking (FPPT) aims to regulate the output power of photovoltaic (PV) systems to a predefined value to enable grid support functionalities, such as virtual-inertia provision. Even though partial shading is a common condition in PV systems, especially in small-scale residential PV systems, the current literature, only investigates FPPT control under uniform shading conditions. This article proposes a novel global FPPT (GFPPT) algorithm to provide two functionalities in PV systems under partial shading conditions: first, constant power control (CPC), in which the PV power is regulated to predefined reference value, and second, power reserve control (PRC), under which a constant power reserve is kept in the PV system under steady state to enable grid support under transients. The conventional global maximum power point tracking (GMPPT) algorithm is also modified to adapt with the proposed GFPPT algorithm. The aim of such modification is to obtain faster dynamics after the detection of any environmental changes. Experimental results under dynamically changing partial shading conditions verify the effectiveness of the proposed algorithm in providing CPC or PRC functionalities in PV systems. [ABSTRACT FROM AUTHOR]

A rapid deployment of renewable generation has led to significant reduction in the rotational system inertia and damping, thus making frequency control in power systems more challenging. This paper proposes a novel control scheme based on Model Predictive Control (MPC) for converter-interfaced generators operating in a grid-forming mode, with the goal of exploiting their fast response capabilities to provide fast frequency control service to the system. The controller manipulates converter power injections to limit the frequency nadir and rate-of-change-of-frequency after a disturbance. Both centralized and decentralized MPC approaches are considered and compared in terms of performance and practical implementation. Special attention is given to the decentralized controller by generating an explicit MPC solution to enhance computational efficiency and reduce hardware requirements. Simulation results obtained from a detailed dynamic model of the IEEE 39-bus system demonstrate the effectiveness of the proposed control schemes. [ABSTRACT FROM AUTHOR]

In this paper, a reliable methodology is proposed in order to implement and validate a Model Predictive Control (MPC) scheme on an actual Voltage Source Converter (VSC) integrated in a scale-down multi-terminal DC grid. The objective of the investigated MPC controller is to enable AC frequency support among two asynchronous AC areas through a High Voltage Direct Current (HVDC) grid, while considering physical constraints, such as maximum and minimum DC voltage. A systematic and accurate implementation strategy is proposed, based mainly on the Hardware In the Loop (HIL) and Power Hardware In the Loop (PHIL), leading to the real-life testing on VSC, controlled by a classical microcontroller. The technical problems during the implementation process, as well as the proposed solutions, are described in detail through this paper. This procedure is deemed valuable to bridge the gap between offline simulation and the actual implementation of such advanced control scheme on experimental test rig. [ABSTRACT FROM AUTHOR]

Energy storage systems (ESSs) are being utilized to improve wind farms’ (WF) frequency support capability due to their high reliability, fast response and the dual role of energy users and suppliers. Nevertheless, the problem of how much capacity should each ESS possesses in order to better serve the WFs has never been investigated. With this perspective, this paper proposes an optimal ESS allocation (OEA) scheme for doubly fed induction generator (DFIG) based WFs to mitigate the impact of wake effect in frequency support. Firstly, the synchronous stability of wind turbines under frequency dips is analyzed with the concept of frequency support margin (FSM), and the detrimental impact of the wake effect is also investigated. Subsequently, the role of ESSs to improve wind turbines’ synchronous stability is demonstrated. To make the OEA scheme practical, the wind turbines in a WF are segmented into different clusters based on the received wind speed. Afterwards, the OEA problem is formulated, where the objective is to optimize the coherency of the wind turbine clusters’ FSM level. The simulation results show that ESS can provide secondary frequency support under major grid frequency drops, and the proposed OEA scheme can reduce the risk of loss of synchronous stability. [ABSTRACT FROM AUTHOR]

The penetration of renewable energy sources on a large scale in conventional electric networks increases exponentially for environmental preservation. However, the increase in RESs leads to some technical problems that include (a) fluctuations in power production, (b) less or no generation units for power balancing, (c) reduced inertia due to RESs decoupling from the conventional grid using converters. Micro-hydro power plants (MHPPs) are emerging as a mature balancing technology and a great alternative to large hydropower plants as they encounter population displacement and many environmental problems. Modern pump storage MHPPs uses a power converter between the grid and machine to control the consumption of power during pumping mode. This power electronic interface loses the ability of the rotating mass of the machine to contribute to the grid inertia by making it independent of the grid frequency. This problem is solved through the control of power converters in such a way that the inertia effect is synthesized termed as synthetic inertia. MHPPs can also be operated in a standalone mode where they are not connected to the grid. This paper reviews control schemes applied in literature for the frequency, voltage, and inertia control, in both the grid-connected and standalone modes. This study starts with the standalone MHPPs, covering the literature review and control structure for voltage and frequency control of standalone MHPPs. Then it presents a detailed operation, control principle, synthetic inertia concepts, and architecture of grid-connected MHPPs. The mathematical formulation of the grid, permanent magnet synchronous generator, synthetic inertia inclusion, and control structures, including direct torque control, virtual synchronous machine, and model predictive control, is also presented. Finally, the paper presents the concluding remarks with the comparative analysis of various control structures to include synthetic inertia, its suitability, and future scope for MHPPs.

The injection of a significant amount of wind power tends to increase the difficulty of the grid frequency control. Therefore, wind farm (WF) has to have the ability to participate in system frequency support. However, during the frequency control process in a large WF, individual wind generators may prone to instability due to possible over-deceleration. In addition, the deeply-intertwined cyber physical power system (CPPS) has gradually replaced the traditional grid due to the development of information and communication technology. However, CPPS also introduces cyber uncertainties that have a significant negative impact on the real-time control of power system. To address above issues, this paper considering possible cyber uncertainty, and proposes a grouping-based hierarchical frequency support scheme for WF. In the proposed scheme, the variable speed wind turbines (VSWTs) are clustered into several groups, according to their wind profiles at first, by dispatching the same control commands to the VSWTs belonging to the same group. As a result, the number of the VSWTs and control variables reduces significantly. Then, a hierarchical control scheme based on virtual inertial control is proposed with consideration of possible cyber uncertainty. The top-layer controller determines whether cyber uncertainty exists, and calculates power increment based on frequency deviation and cyber uncertainty. The bottom-layer controller coordinates the power increment distribution on the basis of ensuring the safe operation of all VSWTs. By do this, it is expected to achieve not only optimal frequency response but also wind generator stability. The effectiveness of proposed scheme is verified on a simulation platform built using MATLAB.

The advent of renewable energy has posed difficulties in the operation of power systems whose net inertia is becoming critically low. To face such challenges, grid-forming power has been one of the potential solutions pursued by the industry and research community. Although promising, grid-forming power converters are still immature for mass deployment in power systems. In the meanwhile, an enormous amount of grid-following power converters has been underexploited when it comes to grid-supporting functionalities. Therefore, this article proposes an external inertia emulation controller (eIEC) for grid-following power converter to provide frequency support to the grid. For the purpose of minimizing installation efforts and resources, the controller is designed in such a way that it can be implemented in an external controller communicating with the grid-following power converter via an industrial communication link. This article also investigates the effect of communication delay on the stability performance of the proposed controller. In addition to the detailed analysis, hardware-in-the-loop experiments are also carried out to validate the proposed eIEC. [ABSTRACT FROM AUTHOR]

The rotational inertia and primary frequency response capabilities of power systems are declining as synchronous generators (SGs) are replaced by inverter-based variable renewable energy (VRE) resources. Therefore, when a high VRE proportion is present, it becomes essential to include frequency security constraints in power system planning. Under this circumstance, the decline in SGs would deteriorate the system frequency response performance; thus, inverter-based resources are supposed to provide frequency response support. This paper presents a frequency-constrained stochastic planning method for high VRE share systems. Frequency response support from wind farms is considered in the planning process for the first time, and the unique characteristics of wind power support are addressed. Specifically, the uncertainty of and variations in wind power support capabilities are modeled from the field-measured data, and the optimal combination of various support schemes can be determined by introducing binary variables with novel linear constraints into the planning process. Moreover, the nonlinear frequency constraints are transformed and embedded in the planning model by the proposed computationally efficient adaptive piecewise linearization method. The planning is formulated as a stochastic optimization model to consider the VRE power uncertainty. Finally, case studies on the IEEE 30-bus system validate the proposed method, with results indicating time savings of 22%. Further, compared with the case that only SGs provide frequency support, the results based on the East China system demonstrate that considering wind power support can improve the VRE share from 49.5% to 79.7%. [ABSTRACT FROM AUTHOR]

As wind power generation increases, there is an increasing need for frequency support from wind turbines. There are many advantages to connect offshore wind farms with the electricity grid via a high-voltage direct current (HVdc) system. HVdc transmission systems allow wind farms to provide frequency regulation and voltage control, making them more flexible compared with HVac transmission systems. Several frequency-support control methods have been developed previously. However, few works have compared various control methods for HVdc-connected offshore wind farms. This work presents a detailed comparison of the main frequency-support control methods that can be used for HVdc-connected offshore wind farms. These control approaches are mainly based on the communication-based schemes or coordination-based methods. The communication-based scheme uses communication devices to directly transfer the grid frequency information to an offshore wind farm. The coordination-based method transfers the dynamic dc voltage information from the HVdc via generator emulation control and then enables the voltage-source converter on the wind-farm side to supply frequency regulation. In this work, we considered the synthetic inertial control of wind turbines for our analysis, including inertia and droop controllers. Furthermore, the capacitors in the HVdc system can release the stored energy to help wind farms regulate frequency during operation. [ABSTRACT FROM AUTHOR]

Using VSC-MTDC to provide frequency support for interconnected power systems is an attractive solution. Considering the disadvantages of traditional methods in power shortage or surplus absorption into the VSC-MTDC from AC systems requiring active power support (RAPS system) as well as flexible and effective power balance in AC systems providing active power support (PAPS system), this paper proposes a novel frequency support control method for interconnected power systems using VSC-MTDC. The main principle of the proposed frequency support control method is firstly to make the active power shortage or surplus of the RAPS system be fully and naturally absorbed into the DC grid with the AC slack bus mode control proposed for RAPS converters; in addition, the smooth switching control and the power limiting control are further proposed to realize smooth and safe power shortage or surplus absorption. Secondly, according to the proposed flexible PAPS systems matching strategy, one or more appropriate PAPS systems with adequate capacity are selected to effectively and accurately balance the unbalanced power via the corresponding PAPS converters using the active balancing power control. The theoretical analysis and simulation results using PSCAD/EMTDC have verified the validity, flexibility and universality of the proposed frequency support control method. [ABSTRACT FROM AUTHOR]

In the future power systems, a large number of offshore wind farms will be connected to the AC grids through high voltage DC (HVDC) and multi-terminal DC (MTDC) grids. As wind power penetration level increases, complex grid codes and regulations will be imposed on wind turbines for frequency support. To follow any grid code and requirement for frequency support, two important features should be included in the wind turbine frequency support: i) It should be able to adjust the maximum additional power that the wind turbine temporarily provides for frequency support; ii) It should be capable of adjusting the time interval in which the wind turbine provides additional temporary power. The first feature is mainly important for reducing rate of change of frequency (RoCoF) and improving the frequency nadir while the second one is mainly important for fast frequency recovery from its nadir and improving the second frequency drop. This paper indicates that the conventional method cannot offer both of the two aforementioned features. To address this issue, two approaches are proposed for frequency support by wind turbines. The first one uses P-ωr and P-ƒWF droops in each wind turbine controller, where P, ωr, and ƒWF represent the wind turbine power, rotor speed, and wind farm frequency. The second method employs P-ωr and P-ƒWF droops in each wind turbine controller. Performance and effectiveness of the proposed methods are evaluated by time-domain simulation studies on an MTDC grid in the PSCAD/EMTDC software environment. [ABSTRACT FROM AUTHOR]

Multiple strategies have been proposed to exchange frequency support amongst asynchronous AC areas through Multiterminal DC (MTDC) grid. Majority of these strategies rely on basic droop control mechanisms involving either inertial or primary frequency droop, or both. This paper addresses the requirements on those droop coefficients, which ensure small-signal stability. To that end, a stability criterion is hypothesized based on certain observations. Using a reduced Nth-order model, the hypothesis is analytically proved for the cases when either inertial droop or primary frequency droop control is active. Moreover, implication of this hypothesis on a recently proposed ratio-based frequency support method is analyzed to derive new stability constraints over and above the existing performance requirements. To support the stability hypothesis, numerically constructed stability boundaries in the droop coefficient space are studied. The stability boundary obtained from small-signal analysis of the full-order nonlinear model is shown to approximately match the boundary obtained from the Nth-order model, which further strengthens the hypothesis. Finally, various time-domain simulation studies are performed on full-order model to validate the stability hypothesis. [ABSTRACT FROM AUTHOR]

This paper proposes a variable proportion coefficient based control scheme for the doubly-fed induction generator based wind turbines (DFIG-WTs) to implement frequency support by regulating rotor speed. To reduce the adverse impact caused by regulating DFIG-WT on the dynamic characteristic of the grid frequency, a two-stage switching control scheme is employed for the proposed method. In the first stage, the variable proportion coefficient is designed for emulating the virtual inertia of the DFIG-WT to provide inertia response. In the second stage, a fuzzy control scheme is employed to design the variable proportion coefficient for both quickly restoring the maximum power point tracking (MPPT) operation of DFIG-WTs and avoiding the secondary frequency drop to system. Case studies are undertaken based on WSCC 9-bus and IEEE 39-bus power systems, respectively. Simulation results show that the proposed control scheme can not only make grid-connected DFIG-WTs provide the friendly frequency support, but also help them to fully use the frequency regulation ability of synchronous generators for quickly restoring the MPPT operation. [ABSTRACT FROM AUTHOR]

A torque limit-based (TLB) method was proposed in literature in order to emulate inertial response of variable speed wind turbines (VSWTs). In this paper, this conventional TLB scheme is firstly modified by considering a finite ramp rate for inertial power of the VSWT. It is exposed that the maximum values of the VSWT's inertial power and kinetic energy released by its rotor have a non-linear relationship with its operation point. Then, a linear TLB scheme is proposed to make the inertia emulation more flexible by customizing its key parameters based on the VSWT's operating point. Accordingly, the released kinetic energy and power ramp rate can be selected in proportion of the VSWT's power, rotor speed and/or its reserved kinetic energy. The derived scheme offers a significant reduction of the mechanical tensions on the turbine compared to the conventional one. In addition, when the parameters of the proposed strategy is designed according to the VSWT's power, the inertial response of the corresponding wind farm can be exactly estimated only by deploying its total generation, regardless of its wind turbines’ installed capacities and operating points. Furthermore, a new approach is projected to estimate the VSWT's inertial response during the deceleration period using an analytical closed-form function. This facilitates large scale system studies. Finally, the efficiency of derived linear inertia emulation is evaluated through a typical grid with various levels of the wind power penetration. [ABSTRACT FROM AUTHOR]

Grid frequency support is one of the most challenging problems, since minor variations in it may lead to huge financial losses. This problem becomes even more challenging with the horizontal and vertical expansion of modern cloud data centers (DCs). In the past, several efforts have been made to manage frequency deviations using flywheels, commercial buildings, electric vehicles, and renewable energy resources. However, these are not adequate due to their complex operations. To fill these gaps, in this paper, we propose the usage of cloud DCs and uninterruptible power supply (UPS) units for the effective frequency regulation. This is achieved by designing a 2-layer hierarchical control scheme for optimal segregation of the regulation signals amongst the DCs and UPS batteries. The proposed scheme determines the scheduling policy for jobs at DCs; along with the charging and discharging cycles of the UPS batteries. The job scheduling is carried out with respect to MapReduce tasks in accordance with the regulation signals with minimal service level adherence violations. Additionally, the sustainability of DCs is also supported through active participation of UPSes during peak hours. The overall frequency support problem in the considered setup involves multiple objectives under multiconstraint environment. Thus, it has been divided in two subproblems, which are addressed individually using either integer linear programming or mixed integer linear programming. In a nutshell, the proposed scheme serves dual purposes, i.e., manages frequency fluctuations and sustains DCs during peak hours. It has been validated using data traces taken from OpenCloud Hadoop cluster and Pennsylvania New Jersey Maryland. The results obtained prove the effectiveness of the proposed scheme for frequency support and DC's sustainability. [ABSTRACT FROM AUTHOR]

High penetration of renewable energies through fast-response power converters results in a considerable displacement of conventional synchronous generators and losing of system inertia for frequency control. Modular multilevel converters (MMCs) can be employed serving as an interface between the large-scale renewable generation and power grids. In a microgrid with high shares of renewables integrating through MMCs, submodule (SM) capacitors can be used as energy storage to provide a degree of synthetic inertia for system frequency support. This paper quantitatively exploits the MMC synthetic inertia in a microgrid with flexible renewable penetration levels. An MMC inertia coefficient concept is proposed. It is mainly affected by the penetration ratio, the SM capacitance, and the modulation index of an MMC, with the system operation constraints. According to the analysis, a substantial portion of system inertia can be provided by properly designed MMCs. Detailed MMC frequency control loops are presented. The capacitor average voltage is proportionally linked to the frequency deviation, in order to flexibly adjust the capacitor energy during frequency events. The MMC output active power is deliberately and simultaneously regulated according to the capacitor energy change rate. By controlling the MMC capacitor voltage, dc side power, and output active power, an amount of energy can be released or absorbed by SM capacitors to improve the system frequency response during the frequency event transients, while the renewable generation is scarcely influenced. The proposed concept is experimentally verified and the results show that, with proper system parameters and control loops, the line frequency deviation and the rate of change of frequency can be significantly reduced. Good agreements of the system frequency characteristics have been achieved between the theoretical calculation and experimental results. [ABSTRACT FROM AUTHOR]

Frequency regulation is one of the most crucial ancillary services that strives to maintain the demand and supply in smart grid setup. The deviations in grid’s frequency can be managed efficiently by adjusting the power generation and consumption of supply and demand sides, respectively. Traditionally, frequency support is provided using conventional generators but their usage leads to the emission of harmful gases, degraded heat rate, and associated wear and tear. However, electric vehicles (EVs) can play a significant role in managing demand and supply imbalances in the near future; with their penetration expected to reach 400 billion by 2020. Moreover, EVs have large charging and discharging capacities due to which they can provide instantaneous frequency support. Motivated by these factors, in this paper, a power management scheme has been presented to leverage the participation of EVs for secondary frequency regulation. The proposed scheme uses a 2-level hierarchical control mechanism to attain the following objectives: 1) to minimize the frequency deviations at the grid level; 2) to support bi-directional vehicle-to-grid in accordance with users’ power requirements; 3) to generate an optimal schedule for EV’s charging and discharging needs; 4) to reduce battery degradation; and 5) to maximize EV’s revenue. Using these objectives, the problem of frequency support has been formulated as a mixed integer linear programming problem. The proposed scheme has been evaluated using Mosek solver on real-time data acquired from PJM and CAISO. The results obtained demonstrate the effectiveness of the proposed scheme for providing frequency support in comparison to the existing scheme. [ABSTRACT FROM AUTHOR]

In the last few years, there has been an exponential increase in the penetration of electric vehicles (EVs) due to their eco-friendly nature, and ability to support bidirectional energy exchanges with the smart grid. Besides serving transportation needs and reducing the carbon footprints in the environment, EVs are widely used for instantaneous grid frequency support. However, the existing research proposals have concentrated majorly on unidirectional vehicle-to-grid (V2G) support using fleet of EVs, which in turn leads to reduced frequency regulation and reserve capacity of participating EVs. Motivated from these facts, in this paper, an “aggregator-based hierarchical control mechanism” for secondary frequency regulation (SFR) using a fleet of EVs has been presented. In the proposed solution, EVs’ scheduling problem has been formulated to provide optimal SFR, while satisfying EVs’ energy demands under battery degradation constraints. This multiobjective primal problem (Mo-PP) under multiple constraints is solved using an approximation approach. This task is achieved by decomposing the complex Mo-PP into four different subproblems (SPs), corresponding to controllers deployed at different layers. The designed SPs are then iteratively solved using interior point method. In summary, the tradeoff between SFR and EV's bidirectional energy demands has been investigated in this paper. Moreover, battery degradation issues induced due to frequent charging and discharging cycles of EVs are also explored. Optimal dispatch of regulation signals among the aggregators and charging stations also takes into account the advantages of conventional droop mechanism. Lastly, widely accepted Pennsylvania–New Jersey–Maryland and ERCOT regulation data have been used to perform extensive simulations. The results obtained demonstrate that the proposed scheme achieves 22.6% and 6.8% better performance in comparison with the existing schemes based on colored petri net and proportional integral derivative controller, respectively. [ABSTRACT FROM AUTHOR]

The increasing penetration of Renewable Energy Sources (RES) and, more in general, of converter connected power generation is progressively reducing the overall inertia of the electricity system introducing new challenges in the network Frequency Support (FS). Therefore, FS strategies for RES are becoming an important aspect to be developed in order to en-sure the secure operation of the electricity power grid. The aim of the present paper is to propose an innovative approach for the inertial emulation of Wind Turbine Generators (WTGs) in order to ensure a positive effect on the system frequency avoiding unstable operations of the WTG and reducing the negative impact of the rotor speed recovery on the secondary frequency drop. Moreover, the paper provides a detailed definition of the triggering logics adopted to activate and deactivate the FS making it suitable for implementation in available industrial controllers. The proposed controller is firstly tested on a simplified configuration to show its enhanced performances with respect to previously developed approaches. Then, simulations are carried out in a more realistic network to assess the positive impact of the controller on the system frequency. Results highlighted a 40% reduction of the system Rate of Change of Frequency while the frequency deviation at the transient nadir is improved by 35%. [ABSTRACT FROM AUTHOR]

The possibilities of selectively providing inertial and primary frequency support as an emergency support service to asynchronous AC areas through multiterminal direct current (MTDC) grid with minimal-communication are explored. Under nominal condition, only voltage droop is considered to allow the AC systems operate asynchronously as they are meant to. When an AC system needs support, the corresponding converter transmits basic information via a distress signal through the DC lines or existing communication channels used for common DC voltage-droop control. Following AC-side disturbances, the frequency droop is activated with values that are predetermined based on our proposed design procedure, which ensures primary frequency support through selective converter participation. Model predictive control, which aims to achieve the best possible frequency dynamics with a single-point actuation is also implemented at the converter station connected to the affected area to provide inertial support. Following a converter outage, MTDC power references are modified to ensure minimal impact on the AC-side frequencies. Simulation studies showing the effectiveness of the proposed strategies are presented. [ABSTRACT FROM AUTHOR]

Provision of inertial and primary frequency support in a controlled manner through multiterminal direct current (MTDC) grid connecting asynchronous AC areas and offshore wind farms (OWFs) is explored. Under nominal condition, only voltage droop is considered to allow the AC systems operate asynchronously as they are meant to. Following an AC-side disturbance, the corresponding converter transmits basic information via a distress signal through the DC lines or existing communication channels used for common DC voltage-droop control. Upon receiving the distress signal, a new decentralized inertial-droop-based controller and the traditional decentralized frequency-droop controller are activated in preselected participating converters with values that are predetermined based on our proposed design procedure. The procedure ensures a new ratio-based performance while providing frequency support. The proposed scheme is based on an $N{\text{th}}$ -order model of $N$ -asynchronous-area MTDC system. Furthermore, to extract frequency support from an OWF connected to an AC-MTDC system, its power reference is modified such that the whole of the AC-MTDC system with OWF is emulated as an $N$ -asynchronous-area MTDC system. The proposed strategies are first implemented in the $N{\text{th}}$ -order model for theoretical validation and later, in full-order models of study systems (with and without OWF) for a more rigorous verification. [ABSTRACT FROM AUTHOR]

This paper proposes a frequency support control scheme for large-scale wind farm (WF) by considering distributed layout of wind turbines (WT) and wake effects inside the WF. The frequency support of WF is formulated as optimization problems. A novel clustering-based optimization method is proposed to coordinate all the WTs so that the whole WF can work at the optimal operating states. In large-scale WF, number of WTs is numerous. With the proposed method, the WTs are first clustered into groups according to their wind profiles. By dispatching the same control commands to the WTs belonging to the same group, each group can be further seen as one single WT. As a result, the number of the WTs and control variables reduces significantly and hence the optimization problems are highly simplified. A case study is carried out to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The islanded scenarios of voltage source converter (VSC) high-voltage DC (HVDC) directly supplying power to islanded loads or connecting to wind farm are becoming popular, in which some new issues are required to be further addressed. While several control strategies have been studied, to the authors’ best knowledge, few have investigated the potential capability of VSC HVDC using the typical phase-locked loop (PLL)-synchronized vector control (PLLSynVC) to address islanded operation issues. To explore the potential capability of the typical PLLSynVC in islanded system, this paper proposes a frequency regulation scheme (FRS), including inertia and primary frequency control, for the PLL-synchronized VSC HVDC based on the motion equation concept. By adding the FRS, the VSC HVDC using the PLLSynVC, similar to a synchronous generator from the view of the phase motion equation, not only can self-maintain the frequency stable in an islanded system, but also naturally avoids the transition of control operation modes without changing the basic control structure. Furthermore, based on the developed phase motion equation, the inertia characteristics of VSC HVDC with the FRS is studied, which synthetically considers the impact of different controllers and PLL, and makes VSC HVDC's manifested inertia clear. The effectiveness of FRS and inertia analysis results is verified by time-domain simulations. [ABSTRACT FROM AUTHOR]

The changing inertia profile caused by the increased penetration of inertialess renewable energy sources has raised concerns about power system frequency control. The kinetic energy of the turbines in wind turbine generators (WTGs) can be utilized to support power system frequency during contingencies. In this paper, we investigate the frequency support capability of WTGs operating at the maximum power point (MPP). The requirements to prevent secondary frequency dips, provoked from switching between normal operating mode and the frequency support mode, are formulated. A time-variable droop characteristic is proposed for frequency support from WTGs, which is quite effective in preventing large frequency excursions and facilitates smooth recovery of the kinetic energy of WTGs. The performance of the proposed method is examined in different operating conditions of WTGs in a single bus model of power systems, as well as a regional 14-generator model of the Australian National Electricity Market. The results show that with the time-variable droop characteristic, the frequency nadir following a contingency can be largely improved and simultaneously, the WTGs can smoothly regain their kinetic energy and continue operating at the MPP. [ABSTRACT FROM PUBLISHER]

When wind power accounts for a large portion of the islanded microgrid power, it may need to support the ac bus frequency regulation. The increasing penetration of variable speed wind turbines (WTs) in microgrids leads to a lower inertia, as the rotational speed of the turbine and the grid is decoupled by power electronic converters. Lower system inertia results in a larger and faster frequency deviation after occurrence of abrupt variations on generation and load. It is possible to implement control loops in the WT converters to provide a virtual inertia and support frequency regulation in the microgrid. This paper investigates the variables related to the frequency compensation capability of WT, such as kinetic energy, dc-link capacitance, turbine size, wind penetration, number of turbines, operating region along the power curve, power reserve and droop control gain, etc. The analysis is structured as a design of experiment (DOE) to have a clear and organized comparison of multiple system configurations. An optimal control technique is applied to provide fair comparison among the system variables. A flowchart explaining how the DOE and controllers gains were defined is provided. [ABSTRACT FROM AUTHOR]

Synthetic inertial frequency response has attracted increasing attention due to the paradigm shift in power generation from synchronous generators toward wind turbines. This paper presents a novel nonlinear controller to enable inertial response of a variable speed wind turbine (VSWT). The VSWT has a nonlinear model which cannot efficiently be dealt with by linear control techniques as it may experience large deviations from its operating point during the frequency support period. To design the controller, a nonlinear model for the wind turbine together with power system is utilized. The output power of the wind turbine is expressed in terms of the state variables, and its exact Taylor series expansion is used in control design procedure. A regular controller is first designed based on the input–output feedback linearization approach. Then, the regular controller is replaced by an adaptive one to accommodate parameter variations of VSWT. The proposed controller is then embedded in the detailed model of the VSWT in MATLAB/Simulink environment. Extensive investigations, including the effects of different controller parameters, are carried out. Impacts of the most important practical operational constraints in the VSWT inertia response are also analyzed. Results verify the superiority and appropriateness of the proposed controller. [ABSTRACT FROM PUBLISHER]

The ever-growing dependency of modern life on electricity may impose huge burden on smart grids (SGs). This dependency affects the demand-supply gap and may lead to undesirable frequency fluctuations. In the worst case, these fluctuations may result in blackouts. In this direction, fleet of electric vehicles (EVs) may play a crucial role in reducing these fluctuations to a great extent. So, this paper proposes a novel scheme for efficient frequency support in SG environment by utilizing fleet of EVs. These EVs act as controllable loads and work in close coordination with aggregators and charging stations. Aggregators play a crucial role in regulating charging and discharging rates of EVs while meeting their energy requirements with the help of the proposed colored petri net based controller. The proposed scheme has been evaluated with respect to publicly available frequency regulation data acquired from PJM and ERCOT. In addition to this, the scheme has also been compared with an existing approach and results clearly depict that the proposed scheme is more scalable in comparison to the existing schemes in V2G environment. [ABSTRACT FROM PUBLISHER]

Modern large wind farms are required to provide frequency regulation service like conventional synchronous generation units. The frequency support capability of modern wind farms has been widely investigated and implemented. Remotely located large offshore wind farms are probably connected to the onshore system grid through voltage-source converter-based–high voltage direct current (VSC-HVdc) transmission systems. Due to the decoupling of VSC-HVdc and signal transmission delay, offshore wind farms may not be able to respond to the onshore grid frequency excursion in time and, consequently, the stability and security of the power system will be put at risk, especially for those with high wind penetration. This paper proposes a coordinated control scheme to allow VSC-HVdc link to contribute to the system frequency regulation by adjusting its dc-link voltage. By means of this approach, the dc capacitors of VSC-HVdc are controlled to absorb or release energy so as to provide frequency support. To further enhance the system frequency response, the frequency support from VSC-HVdc is also finely coordinated with that from offshore wind farm according to the latency of offshore wind farm responding to onshore grid frequency excursion. The control scheme is evaluated for both underfrequency and overfrequency events, and results are presented to demonstrate its effectiveness. [ABSTRACT FROM AUTHOR]

With the increasing penetration of wind power in power systems, it is desirable for wind turbines to have similar characteristics as conventional synchronous generators. Conventional generators provide frequency support to the grid through the methods of inertial response and primary and secondary frequency regulation, whereas variable-speed wind turbine generators (WTGs) do not have those desired abilities because they are integrated into the power grid via power electronic converters. Although many different control strategies have already been published to enable WTGs to temporarily support the transient frequency, the published strategies may bring various negative effects to the system. This paper proposes a new control strategy to compensate for inertia of the wind farm. It can improve WTGs' temporary frequency support based on the coordinated control of the WTGs and the energy storage (ES) system. The simulation results show that this strategy could provide better performance of temporary frequency support and overcome problems such as system frequency oscillation and a secondary frequency drop. The proposed control strategy can be realized in all wind speed conditions with a small-scale ES system. [ABSTRACT FROM PUBLISHER]

Inertial response from remote offshore wind farms (WFs) connected through point-to-point DC links relies on emulation of the onshore AC system frequency variation at each offshore wind turbine generator (WTG). This is not straightforward for a DC grid interconnecting WFs to multiple onshore AC systems. To address this problem, this paper builds up on a communication-less approach previously reported for provision of frequency services from offshore WFs connected through a DC grid. A weighted frequency scheme is adopted which relies on the fiber optic link embedded within the sub-sea cables for fast communication of onshore frequency variations up to the offshore converters. The effectiveness of the proposed approach is shown analytically and also through a case study on a 4-terminal DC grid interconnecting an offshore wind farm and two onshore AC systems. [ABSTRACT FROM PUBLISHER]

This paper investigates an improved active power control method for variable speed wind turbine to enhance the inertial response and damping capability during transient events. The optimized power point tracking (OPPT) controller, which shifts the turbine operating point from the maximum power point tracking (MPPT) curve to the virtual inertia control (VIC) curves according to the frequency deviation, is proposed to release the “hidden” kinetic energy and provide dynamic frequency support to the grid. The effects of the VIC on power oscillation damping capability are theoretically evaluated. Compared to the conventional supplementary derivative regulator-based inertia control, the proposed control scheme can not only provide fast inertial response, but also increase the system damping capability during transient events. Thus, inertial response and power oscillation damping function can be obtained in a single controller by the proposed OPPT control. A prototype three-machine system containing two synchronous generators and a PMSG-based wind turbine with 31% of wind penetration is tested to validate the proposed control strategy on providing rapid inertial response and enhanced system damping. [ABSTRACT FROM PUBLISHER]

Control of the converter stations in a multi-terminal DC (MTDC) grid to provide frequency support for the surrounding AC systems is the subject matter of this paper. The standard autonomous power sharing control loop for each converter is modified with a frequency droop control loop. The objective is to minimize the deviation from nominal AC system frequency and share the burden of frequency support among the converter stations of the MTDC grid. The effectiveness of the frequency support is demonstrated through nonlinear simulation of a test system consisting of three isolated AC systems interconnected through an MTDC grid with four converter stations. An averaged model of the MTDC grids is developed to carry out modal analysis of combined multi-machine AC-MTDC systems. Modal analysis is used to characterize and substantiate the time domain behavior in presence of frequency droop control. It is established that appropriate droop control loop for the MTDC grid converters could be effective in reducing the deviation from nominal AC system frequency provided the sensitivity of the system eigen-values to changes in control parameters (e.g., droop coefficients) is accounted for a priori through modal analysis. [ABSTRACT FROM PUBLISHER]