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Inertial and droop control are common methods for improving grid frequency control. Using the first‐order derivative of the frequency in the inertial control loop only investigates the rate of frequency variation, while other useful information is not taken into consideration. Accordingly, in this study, the application of a fractional‐order derivative (between 1 and 2) in the inertial control loop is proposed to have more useful frequency‐based information. To achieve this goal, firstly continued fraction approximations and secondly, the discretization is adopted by using the Tustin Method with Prewarping. Inertia loop and droop loop gain have a significant impact on the operation of the wind turbines and the power system frequency control. Therefore, in this paper, a novel method for the adaptive adjusting of droop and inertia control loops gains in DFIG is proposed. To utilize more frequency error information, fractional‐order adaptive law is proposed to update the loop gains and the inertia control loop. In order to have a robust system, which is permanently stable for every fractional order (FO), is used parameter space method. Simulation results demonstrate the proper performance of the FO‐based adaptive approach to increase the frequency nadir (FN) and decrease the frequency variations. [ABSTRACT FROM AUTHOR]

Here, a novel photovoltaic power integration system based on LLC resonant converters is proposed for medium-voltage DC distribution network, which exhibits a feasible and efficient solution for large-scale solar energy integrating into DC grids. A control system with the voltage signal generation part and the frequency control part is designed to realise the current and voltage sharing of the sub-module, while achieving the maximum power point tracking (MPPT) of the PV arrays. To verify the practicability of the system with many switching devices, this paper proposes an optimisation model to accelerate the simulation speed of this system without losing accuracy. Finally, the feasibility of the proposed system is validated by the simulation based on PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

Large-scale wind farms integrated with multi-terminal direct current (MTDC) system decouples the frequency between the wind farms and the receiving end, resulting in weakened frequency regulation ability of the onshore AC grid. Recently a new topology is proposed, called diode rectifier based HVDC, which recouples the wind farm and onshore AC system due to its characteristics, where offshore AC voltage changes with the DC voltage of HVDC system. A coordinated power control strategy for frequency regulation participation is proposed in this paper for offshore wind farms connected with diode-rectifier based high voltage direct current (HVDC) system. Considering the primary frequency and inertia response control, variable power supplementary control is implemented in wind turbines, and a telecommunication-less method of frequency event detection via detection of offshore AC voltage is proposed, avoiding communication delay and communication failure. Simulations using EMTDC/PSCAD show that frequency dip can be accurately detected by the AC voltage of offshore wind farm, and wind farm can quickly compensate active power. [ABSTRACT FROM AUTHOR]

Low-inertia power systems suffer from a high rate of change of frequency (ROCOF) during a sudden imbalance in supply and demand. Inertia emulation techniques using storage systems, such as flywheel energy storage systems (FESSs), can help to reduce the ROCOF by rapidly providing the needed power to balance the grid. In this work, a new adaptive controller for inertia emulation using high-speed FESS is proposed. The controller inertia and damping coefficients vary using a combination of bang–bang control approaches and self-adaptive ones, to simultaneously improve both the ROCOF and the frequency nadir. The performance of the proposed adaptive controller has been initially validated and compared with several existing adaptive controllers by means of offline simulations, and then validated with experimental results. The proposed controller has been implemented on a real 60 kW high-speed FESS, and its performance has been evaluated by means of power hardware-in-the-loop (PHIL) testing of the FESS in realistic grid conditions. Both simulations and PHIL testing results confirm that the proposed inertia emulation control for the FESS outperforms several previously reported controllers, in terms of reducing the maximum ROCOF and improving the frequency nadir during large disturbances. [ABSTRACT FROM AUTHOR]

With the increasing penetration of renewable energy sources in the power system, the power electronic inverters are widely used to interface with the grid, which will reduce the inertia of the power system. This study proposes ancillary inertial service from single‐phase rooftop solar photovoltaic (PV) based inverter to the grid. The inertia emulation control technique transforms the behaviour of inverter like a synchronous generator under power imbalances. A hybrid energy storage system consisting of battery and supercapacitor (SC) has been connected at the DC bus to take care of the variability in PV output power and load fluctuations. The SC absorbs/injects the fast‐varying power and provides the inertial response to arrest the frequency deviation and battery charges/discharges to bring back the frequency to the nominal value. Real‐time simulation is carried out to test the system behaviour for different operating conditions using OPAL‐RT 5700. [ABSTRACT FROM AUTHOR]

Increasing penetration of wind power has a negative effect on system frequency deviation due to the fast variability and uncertainty characteristics of wind. In this study, a wind power variation prediction‐based automatic generation control (AGC) feedforward control method is applied within the conventional AGC framework to minimise system frequency and tie‐line deviations. Firstly, the variogram function is introduced for analysing the characteristics of wind power variations. A relationship between the variogram of wind power and the trend component of wind power is found and then a three‐parameter power‐law model is established. Then, the generation rate constrains of AGC generators is considered as the main constraint factor that influences wind power smoothing. By predicting the wind power variations in AGC control time‐scale, a coordination AGC feedforward control strategy is proposed. The control strategy makes different types of AGC generators act in advance to improve the ability of the power system, corresponding to the forecasted ramp variations in wind power generation. Finally, the performance of the proposed coordination control strategy is verified and tested using the actual data from China. Simulation results show that with this new strategy, frequency deviation under wind power variations can be effectively decreased. [ABSTRACT FROM AUTHOR]

With the proliferation of intermittent renewable energy sources, power systems need to withstand an increasing number of disturbances that may affect system frequency. In this paper, we focus on the effects of high penetration level of wind turbine‐generators (WTG) on the power system operational planning from the frequency point of view. Specifically, an exact formulation for the minimum frequency calculation is presented to increase accuracy and speed of analyses. Then, the effect of WTG's frequency response on the power system frequency stability is investigated to compute the maximum acceptable generation outage as a function of penetration level. Finally, as an application to power system operational planning, the obtained frequency dynamics of the power system and the wind turbine frequency response are integrated into the unit commitment problem. Simulations are performed on the IEEE 39‐bus test system to study the effects of the WTGs on the frequency stability of the system. Results illustrate a more secure and reliable operation of the power system when the wind turbines participate in the frequency regulation task. Moreover, including the frequency response of the wind turbines in the unit commitment problem can reduce the ancillary cost, meanwhile increase the frequency stability of the system. [ABSTRACT FROM AUTHOR]

The use of inertia emulation in a microgrid has been growing as a promising control approach to maintain the bus voltage and frequency. An inertia emulation‐based control technique is proposed for a DC microgrid supplying AC loads in this study. The proposed work includes: (i) the DC bus voltage regulation using virtual inertia control and economic power management by a current sharing algorithm in cascade with a restoration control; (ii) AC load bus voltage and frequency regulation using virtual excitation emulation and inertia emulated control that utilises the emulated inertia and DC bus voltage. Further, it also provides inertia support to the grid. The coordinated current sharing algorithm manages the current (power) sharing between the grid and battery pack of electric vehicles based on electricity price and state of charge of the battery. The cascaded restoration control eliminates the steady‐state error in the DC bus voltage. Finally, the DC microgrid is simulated under various operating conditions to verify the performance of the proposed control approach. The simulation results reveal that the objectives are achieved successfully. [ABSTRACT FROM AUTHOR]

With higher penetration of converter‐connected renewable energy sources (RES) into power systems, the successful operation of the system is challenged by significant reductions in system inertia. Presently, given the dominant share of the conventional synchronous power plant, RES power plants are not demanded to provide ancillary services. However, as RES connections increase, RES power plants will play a major role in power system operation, contributing to frequency control. This study demonstrates that photovoltaic power plants (PVPPs) can provide effectively different types of frequency support based on a power reserve and an offline maximum power point tracking (MPPT) technique. An innovative method to de‐load the PVPP without significantly increasing the MPPT complexity is proposed. Results from different PVPP frequency support methods, under varying levels of photovoltaic penetration, are presented which demonstrate their capability to provide inertia support comparable to that of synchronous generators. A new variable droop control method, which releases maximum power during the inertial response and returns to fixed droop gain value after a specified time is also presented. The results from using the variable droop show that the frequency nadir and the rate‐of‐change‐of‐frequency can be significantly reduced and some power reserve still maintained after a frequency event. [ABSTRACT FROM AUTHOR]

The rapidly increasing penetration of renewable energies has introduced severe challenges to power system frequency controls due to the highly intermittent and uncertain power output of renewable energies. This study proposes a proactive frequency control method to control traditional synchronous generators in advance in anticipation of sudden power fluctuation. Therefore, the frequency deviations can be well mitigated by such early‐acted control signal. In order to obtain this new control reference, an ensemble‐forecasting model based on the extreme learning machine algorithm is designed to predict ultra‐short‐term power fluctuations, which serves as an extra signal for automatic generation control. The proposed method was verified on an equivalent model of the Singapore power system with various types of generations and loads. The simulation results clearly demonstrate the accuracy of the forecasting model and the advantages of the proposed control method. The proposed method can also reduce the frequency charging/discharging of energy storage systems, which can effectively extend their lifetime. [ABSTRACT FROM AUTHOR]

The maximum power point tracking (MPPT) mode is widely applied in photovoltaic (PV) power generation system. In this article, instead of MPPT, a novel control strategy of grid-connected PV power supply for frequency regulation is proposed. First, the static frequency characteristic curve of PV plant is proposed, and a method of synthesising the PV power output power–voltage characteristic curve and static frequency characteristic curve into frequency regulation curve of PV power supply is introduced. So the PV plant can participate in frequency regulation of the system. Through simulation analysis in power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC), PV plant realises the function of frequency regulation when load changes, so the variation of system frequency can be reduced and the frequency stability of power system can be improved, and the effectiveness of the strategy is verified. [ABSTRACT FROM AUTHOR]

High‐voltage direct‐current (HVDC) grids may provide fast frequency support to ac grids with the aid of supplementary control algorithms and synthetic inertia contribution from offshore wind farms. However, when all converters within the HVDC grid are fitted with these supplementary controllers, undesirable power flows and reduced power transfers may occur during a power imbalance. This is due to simultaneous frequency oscillations on the different ac systems connected to the HVDC grid arising during the support operation. To prevent this adverse effect, an auxiliary dead‐band controller (ADC) is proposed in this study. The ADC modifies the dead‐band set‐point of the fast frequency controllers using measurements of the rate of change of frequency and frequency deviation. A four‐terminal HVDC integrated with an offshore wind farm is modelled to analyse and study the effectiveness of three different supplementary fast frequency control algorithms. Results show that the proposed ADC scheme improves the performance of fast frequency control algorithms. For completeness, a small‐signal stability analysis is carried out to confirm that a stable system operation is maintained. [ABSTRACT FROM AUTHOR]

The uncertain demeanour from wind generators and loads adversely affect the grid operational stability. Various control approaches have been explored to remedy the system uncertainties while maintaining generation and load demand balance. This study proposes a fuzzy‐based proportional–fractional integral–derivative with filter controller to sustain frequency stability in wind integrated power systems having different configurations. The controller parameters have been tuned using a recently developed coyote optimisation algorithm (COA). The proposed control approach is executed and validated on three distinct configurations of two‐area power systems. All test models are integrated with a doubly fed induction generator (DFIG) type wind turbine units (WTUs). Different case scenarios have been considered to analyse the efficacy of the proposed control strategy in the presence of WTU. Furthermore, the impact of inertial support delivered by the DFIG‐WTU and higher penetration of wind energy in the power system has been studied. The analysis reveals that the control scheme in coordination with WTU support reduces the stress on a wind turbine during the inertial control scheme and maintains the grid frequency stability under unexpected load disturbances. Stability and robustness analysis are also conducted to verify the validity of the introduced control approach. [ABSTRACT FROM AUTHOR]

The system equivalent inertia of the power grid is gradually decreased with the increasing penetration of renewable energy resources, which leads to a higher risk of frequency fluctuation after a major fault. For the frequency emergency control in the scenario of a high-voltage direct current (HVDC) line block fault, a novel criterion for enabling the fast response of user-end distributed loads is proposed in this study based on voltage feature identification and frequency drop detection. The reactive power to voltage sensitivity coefficient is combined with the Hausdorff algorithm to increase the accuracy of the calculation. The proposed criterion can realise the HVDC block fault identification at the end of the distribution grid, so as to achieve fast decentralised decision-making for distributed loads. This method may greatly reduce the need for high-speed communication and take actions faster than the primary and secondary frequency controls of the traditional units. The criterion makes it possible for low voltage small load in the user-end to be controlled precisely and selectively to fill the large power imbalanced caused by an HVDC block fault. The validity and superiority of the criterion are verified by simulation experiments using multiple test systems. [ABSTRACT FROM AUTHOR]

The proposal focuses on the role of data centres (DCs) and electric vehicle (EV) energy storage systems (ESSs) for frequency regulation and it provides a new opportunity for different resources to participate in the power sector. Frequency fluctuations are introduced with the emergence of inverter‐dominated renewable energy sources (RESs) as it does not provide rotational inertia to the grid, such as synchronous generators. In this study, the virtual synchronous generator (VSG) controlled inverter compensates for the lack of inertia. Particularly, this research work analyses the involvement of DC and EV batteries as ESSs in designing an integrated technical virtual power plant (VPP) to support frequency variations using the VSG concept. Microgrid (MG) simulations are performed in MATLAB/ Simulink platform. Case studies are carried out to validate the grid export and grid import performance of the model by using variations in the load and the solar irradiance of the MG. It is verified that bidirectional power flow takes place between the proposed VPP components and the grid that enables the grid with high penetration of RESs. The work provides a conceptual framework for future contributions towards the smarter usage of assets such as ESS and a greener future. [ABSTRACT FROM AUTHOR]

Power grids all over the world are nowadays facing high penetrations of renewable power generation. The converter‐based generation units play a major role in system behaviour and operation. Thereby, also the reliability of the power system is impacted in many different aspects. It has to be verified whether the frequency reliability assessment, which is often studied by simulations, is still accurate, every time a new control structure is considered. For this, the number of simulations should not be further increased if possible, in order to keep the computational efforts low. In this study, the authors verify that the assessment outcome will be accurate when the load duration curve (LDC) and wind power curve (WPC) of a system are described well enough. This is shown to be true for different frequency controls, implemented in wind power plants in the system. Besides, the effects of different LDCs and WPCs are analysed, as they can change over time. The used methodology is validated on the IEEE reliability test system (IEEE RTS). [ABSTRACT FROM AUTHOR]

Using inverter-based topologies and lack of rotational masses can lead to a noticeable reduction in the inertia of modern systems and have detrimental effects on the resiliency, stability and strengths of microgrids. Effective frequency control ancillary services and modern adaptive control mechanisms can be proposed to resolve the mentioned challenges practically. From this perspective, several flexible and intelligent control approaches have been recently introduced to create a balance between generation and load demand during various operational conditions in low-inertia power systems. This study suggests a supportive collaboration between two distributed generations including virtual inertia of wind turbine generator and fast speed micro-turbine based on an adaptive optimal model predictive control (AOMPC). To demonstrate the effectiveness of the proposed framework, the results are compared with the previous controllers like optimal proportional–integral, optimal fractional order proportional–integral–derivative (PID), optimal fuzzy PID, the optimised membership function of fuzzy and adaptive MPC controller during multiple load variations, changes in the weather patterns, unwanted time-varying uncertainties and collapse of power generation units. [ABSTRACT FROM AUTHOR]

The low and variable inertia due to the integration of large-scale wind generation poses severe challenges to the frequency control of the power grid. In this study, a novel model predictive control (MPC)-based load frequency controller (LFC), which can adapt to the changes in system inertia, is proposed to improve the frequency control effect. First, the inertia estimation scheme based on measurements is designed. Then, the state-space model, which is the key that determines the control effect of MPC, is corrected in time based on the estimated inertia. The proposed method in this study can avoid the problem of MPC-based LFC failure caused by inaccurate model parameters. The performance of the proposed strategy is demonstrated in two scenarios, i.e. wind power fluctuation and load multi-step, using a two-area interconnected power system with time-variant inertia. [ABSTRACT FROM AUTHOR]

To guarantee the frequency stability of renewable energy sources (RESs) generation integrated into the power system, primary frequency regulation (PFR) for RES generation controlled by virtual synchronous generator (VSG) has been concerned extensively. This study mainly focuses on the dynamic and steady-state characteristics of VSG in the PFR dead-band. Based on the typical structure of VSG, small-signal model of the conventional VSG is established to analyse the issue of the sharp change of angular frequency. Consequently, a novel modified VSG (M-VSG) controller is proposed, which can ensure a stable angular frequency and synchronise with the grid without the phase-locked-loop. The inertia and damping properties of M-VSG are studied theoretically by eigenvalue analysis, and the design procedure of control parameters is presented. To enhance the practicability, a flexible PFR strategy of M-VSG is added considering inertia mode, bidirectional mode, and unidirectional mode according to the operation state of RES generation. Moreover, after PFR is added, the proposed controller is analysed with eigenvalue loci to guarantee the stability and dynamic characteristics outside the dead-band. Finally, hardware-in-the-loop experiment results based on RT-LAB demonstrate the validity of the theoretical analysis and the superiority of M-VSG in participating in flexible PFR. [ABSTRACT FROM AUTHOR]

With the large-scale wind power, centralised access to high-voltage direct current (HVDC) asynchronous interconnected sending system, the random fluctuation of wind energy increases the uncertainty of frequency regulation. A frequency coordinated control (FCC) strategy based on situation awareness is proposed to make full use of the frequency regulation potential of wind turbines and HVDC transmission. Firstly, the influence of wind power permeability and HVDC power ratio on frequency regulation capability is analysed, and the mathematical model of FCC is established. Secondly, based on the information of FCC obtained by wide-area measurement system and data acquisition system, a non-linear mathematical prediction model is constructed, and the frequency response in finite time domain is predicted according to the frequency deviation and its change rate. According to the results of the prediction, the parameters of FCC are dynamically adjusted to realise the whole process variable objective optimal control on the time scale of primary frequency regulation. Finally, the simulation results show that the maximum frequency deviation of the proposed control method is reduced by about 46.6% and the frequency callback time is reduced by about 11.6% compared with the traditional control method with fixed parameters under the same disturbance. [ABSTRACT FROM AUTHOR]

Traditional way to enable frequency support provided by MTDC system for the connected weak AC systems is to modify the power reference of each converter under standard power‐sharing droop control based on the weak grid's frequency deviations. However, this scheme artificially couples all asynchronous AC systems to tackle with the frequency disturbances of any connected AC systems, which might result in broader frequency disturbance propagations. To overcome this, this study proposes a decoupled frequency control (DFC) scheme to improve weak AC systems frequency stabilities. First, the relationship between the active power and power set points of each converter is derived by the linearization of MTDC power flow equations. Then, the frequency coupling properties of each AC system with traditional frequency control are derived, and it shows that the frequency disturbance will be propagated to all the connected AC systems with traditional control. Furthermore, the control law of each converter in the proposed DFC is developed through the defined decoupled matrix, and the independent frequency regulation characteristic of each converter can be obtained. Finally, numerical simulations have demonstrated the effectiveness of the proposed DFC scheme in events of load changes and converter loss in the main and weak AC grid. [ABSTRACT FROM AUTHOR]

A multi-microgrid (MMG) system comprises of a group of self-sufficient microgrids capable of operating in grid-connected, islanded, or interconnected modes. A three-microgrid-based MMG system consisting of a solar photovoltaic system, doubly-fed induction generator-based wind generation, and static synchronous compensator as distributed energy sources are considered. Each microgrid includes a battery interfaced with a bidirectional DC–DC converter. As there are multiple battery units in the MMG system, different states-of-charge (SoC) are evident in the batteries. Hence, there is a need to charge/discharge by following the SoC of the batteries. In this study, an intelligent scheme employing fuzzy logic is applied to monitor the SoC of the batteries and determine the reference current for current control through the bidirectional DC–DC converter in an MMG system. Microgrid frequency control in the MMG system is carried out using the proposed five-membership function-based fuzzy logic control. During sudden transient events, the proposed intelligent frequency control strategy shows an improved transient response as compared to conventional PI control. A multi-input and multi-output fuzzy logic-based power management algorithm is proposed to handle multiple sources, batteries, and loads in the system. The MMG system with the proposed control is simulated using real-time digital simulator OP4510 from OPAL-RT. [ABSTRACT FROM AUTHOR]

This study proposes a novel frequency regulation strategy based on a power reserve control method. Unlike existing strategies, irradiance information and complex model estimation are not required, except for the maximum power curve. The suboptimal power point at any irradiance and reserve ratio can be automatically converged with the proposed tracking method. Furthermore, an adaptive step-size tracking method is proposed to improve the output power fluctuation around the suboptimal power point. The simulation results with several scenarios are used to verify that the proposed strategy can quickly and robustly track a given power reserve ratio under the irradiance and reserve ratio variation conditions. The proposed strategy can significantly improve the frequency response of power systems with photovoltaic (PV) generation. [ABSTRACT FROM AUTHOR]

Gotland is a Swedish island that is connected to the grid only by two high voltage direct current (HVDC) cables. The growth of installed wind power on Gotland has been temporarily stopped due to concerns regarding the reliability of the operations. This study investigates the capability of a centralised energy storage system along with or without wind curtailment to support the growth of installed wind power capacity. The energy storage system is tested for maintaining frequency stability during unintentional islanding through dynamic studies using power system simulator for engineering (PSS/E). The results assess the ability of energy storage to prevent frequency instabilities and provide primary frequency response albeit of the absence of any rotating inertia. The analysis determines the requirements for the power and energy capacity of the energy storage system in relation to the exported power from the HVDC cables at the instant of fault, which eventually relates to the wind power capacity. Moreover, the study examines wind power curtailment as primary and/or secondary frequency response and the impact on the energy capacity. Finally, the study did not explore the effect of fast power ramps on the energy storage to leave the door open for all types of inverter‐based energy storage technologies. [ABSTRACT FROM AUTHOR]

Tidal power plant (TPP) is an emerging and fast‐growing addition to clean energy technologies. Potentially, it may bridge the gap of energy scarcity and lower environmental impacts. Therefore, a meticulous study on it in providing support to various ancillary services, especially, in load frequency control (LFC), need to be carried out. Penetration of the intermittent unit like TPPs into the power system causes capacity addition but at the cost of a decrease in inertia and primary frequency response owing to the power electronic interface. Due to this, following disturbance, both the maximum frequency nadir and the rate of change of frequency deviation deteriorate, which poses a threat to LFC service. So, in this study, the contribution of the deloaded TPPs, with different level of penetration, in frequency regulation employing control strategies such as inertia control and droop control is analysed. Performance of these control strategies has been successfully demonstrated through a series of simulation‐based experiments. Further, the impact of virtual inertia contribution from capacitive energy storage systems (CESSs), in case of insufficient inertia support from TPPs on LFC performance of the system is investigated. Lastly, the impact of a combined response of inertia and droop control strategies with CESSs on frequency nadir is also explored. [ABSTRACT FROM AUTHOR]

In this study, a proportionate power sharing among the parallel inverters operating in an islanded microgrid is achieved using droop and virtual inductance control. The same droop coefficients are used to achieve the frequency regulation as well. The frequency changes which are inevitable in droop control are measured and used to emulate the behaviour of damping and inertia to the DC link voltage using hybrid energy storage system consisting of battery and supercapacitor units. The proposed DC link voltage regulator restores the DC link voltage quickly by providing power corresponding to the rate of change of frequency and frequency deviation. This reduces the impact of voltage variations on the DC‐load and keeps modulation index within the linear range for voltage source inverter. The design aspects of DC link voltage regulator, damping and inertia constants, selection of battery and supercapacitor units based on rating of the DC link voltage are discussed. The proposed decentralised droop control and DC link voltage restoration methods are validated through detailed simulation and experimental studies. [ABSTRACT FROM AUTHOR]

In this study, a control algorithm based on non‐linear adaptive second‐order Volterra filter (NLVF) is utilised to operate the voltage‐source converter (VSC). The main role of VSC is to supply compensating current to improve the power quality in wind‐based off‐grid distributed power generation system. This off‐grid system consists of three‐phase induction generator, VSC and non‐linear loads. The proposed control algorithm is adaptive as well as non‐linear in nature. The characteristic of Volterra series is exploited to generate the reference source current for VSC operation. The NLVF estimates the active and reactive weights of fundamental load current for reference current generation. It mitigates the power quality problems created by disturbances in non‐linear load or wind oscillations on off‐grid system. The solution of power quality problems such as the presence of harmonics in supply current, load unbalance and reactive power compensation is obtained. Nevertheless, voltage and frequency control are also achieved. The VSC is used as a shunt compensator in the system. The entire off‐grid system is designed and validated through the laboratory prototype development. The objective of the system is achieved and the steady state and dynamic performance is found satisfactory. [ABSTRACT FROM AUTHOR]

Nowadays, power system inertia is changing as a consequence of replacing conventional units by renewable energy sources, mainly wind and photovoltaic power plants. This fact affects significantly the grid frequency response under power imbalances. As a result, new frequency control strategies for renewable plants are being developed to emulate the behaviour of conventional power plants under such contingencies. These approaches are usually called 'virtual inertia emulation techniques'. In this study, an analysis of power system inertia estimation from frequency excursions is carried out by considering different inertia estimation methodologies, discussing the applicability and coherence of these methodologies under the new supply‐side circumstances. The modelled power system involves conventional units and wind power plants including wind frequency control strategies in line with current mix generation scenarios. Results show that all methodologies considered provide an accurate result to estimate the equivalent inertia based on rotational generation units directly connected to the grid. However, significant discrepancies are found when frequency control strategies are included in wind power plants decoupled from the grid. In this way, authors consider that it is necessary to define alternative inertia estimation methodologies by including virtual inertia emulation. Extensive discussion and results are also provided in this study. [ABSTRACT FROM AUTHOR]

Active load variations in uncertain dynamical power system environments affect the energy exchange and efficiency in multi‐area power systems, which could compromise the stability of power grids. Hence, model‐free load frequency control mechanisms are needed in order to sustain proper performances under such conditions. An online model‐free adaptive control scheme based on integral reinforcement learning is proposed to regulate load frequency deviations in multi‐area power systems. This scheme takes into account the generation rate constraints of the power generation units and the optimal control decisions do not employ any knowledge about the dynamical model of the power system. This approach reformulates Bellman equation and approximates the associated solving value functions and model‐free control strategies using neural networks. The adaption mechanism uses value iteration processes to evaluate the underlying modified‐Bellman equation and model‐free control strategy in real time. The performance of the adaptive learning scheme is compared with other control methodologies using challenging validation scenarios. [ABSTRACT FROM AUTHOR]

Growing trends in the deployment of inverter‐based renewable energy will decrease the inertia and frequency control capability of electric power systems by replacing conventional power plants; thus, the frequency of future power systems might be dynamic. This study proposes a capability‐coordinated frequency control (CCFC) scheme of a virtual power plant (VPP) including adjustable‐speed pumped storage hydropower (AS‐PSH), a wind power plant (WPP), and an energy storage system to support the frequency nadir and reduce the steady‐state error of system frequency. The CCFC scheme is based on a hierarchical‐control structure in which a CCFC organises the output of local frequency control units. To support the frequency nadir, the CCFC dispatches weighted frequency errors that are proportional to the available headroom of the units; thus, the errors are forwarded separately with a system frequency error to the primary control loop of each unit and thereby arrest the frequency nadir at a higher value than a system without the CCFC. To reduce the steady‐state error of the system frequency, the CCFC determines a partial active power command by additionally feeding an integrator of the CCFC with a modified frequency error that depends on the unit with the largest control. [ABSTRACT FROM AUTHOR]

The replacement of a conventional synchronous generator (SG) with higher wind penetration greatly reduces the inertial support available for the system. Therefore, a new consideration – the dynamic frequency changes in a wind energy system using a super‐capacitor (SC) – is proposed here to provide faster, limitless, inertial response during load disturbances. By considering the dynamic frequency change in defining the power exchange between SC and network, this proposed method not only provides inertial and primary frequency response but is also able to avoid a second frequency dip. To avoid deep charge and discharge of SC, a simple energy management system (EMS) which considers the system frequency deviation and the state of charge (SOC) of SC is adopted in this technique. The effectiveness of the proposed technique is modelled in MATLAB/Simulink software. Comparative analyses between the proposed virtual inertial support (VIS)‐based SC and existing VIS control strategies are performed. The simulation results obtained prove that the proposed VIS greatly reduces the frequency nadir and peak frequency of the system during sudden load increase and decrease, respectively. [ABSTRACT FROM AUTHOR]

Recently, the virtual synchronous generator (VSG) has drawn considerable attention for its function of simulating the electromechanical transient of SG to make converters equip with the characters of inertia, damping, voltage regulating, and frequency regulating. Energy storage systems (ESSs), which are connected to the converter by a DC/DC circuit, are the basis of VSG. The output active power of VSG is restricted by the capacity margin of the grid-side converter and DC/DC circuit, and the output energy is bounded by the energy margin of ESS, which means that VSG is restrained by treble constraints: DC side power, DC-side energy, and AC-side power. A small signal model of VSG is built, and the method of getting the treble constraints is proposed. Through the analysis of the dynamic response characteristics of the VSG, the operating zone of VSG under constraints is confirmed. Simulations in power system computer aided design (PSCAD)/electromagnetic transient including DC (EMTDC) verified the theoretical analysis and the transient characteristics of VSG when the constraints are broken and simulated. [ABSTRACT FROM AUTHOR]

Existence PV generation mostly operates under the Maximum Power Point Tracking (MPPT) mode and has no ability to increase or decrease active power, accordingly, PV generation does not participate in grid frequency regulation. Due to an increasing penetration of grid-connected PV generation, when a frequency contingency event occurs, grid may have neither enough inertia response nor governor support and frequency deviation may have a risk of exceeding the limit. Therefore, a novel adapted de-loading control strategy without MPPT for grid-connected inverters is proposed here. This control strategy provides a reserve power in PV system to meet the frequency control demand and an online output Active Power-Voltage Matching (APVM) technique, which is generated as a control flowchart, is implemented in this control strategy to make it possible to adjust PV output voltage performance matching different active power demand with different operation modes. The proposed control strategy of PVs is simulated using PSCAD/EMTDC Software. Results verify the correctness and effectiveness of the proposed adapted de-loading control strategy under several operating conditions. [ABSTRACT FROM AUTHOR]

Connecting large offshore wind farms using high‐voltage direct current (HVDC) transmission systems based on the modular multilevel converter (MMC), the offshore alternating current (AC) voltage and frequency are regulated by the offshore MMC station and are important for the stable wind power generation and transmission. This study proposes an enhanced AC voltage and frequency control strategy of the offshore MMC for wind farm integration, where an additional frequency loop is used to improve its AC voltage and frequency controllability. A fault current injection control is also proposed, where the offshore MMC station actively provides fault currents during an offshore AC fault to enable overcurrent protection for the network. To ride‐through onshore AC faults, a direct current (DC) voltage‐dependent AC voltage controller is introduced to actively reduce the offshore AC voltage during an onshore AC fault to alleviate the DC overvoltage of the HVDC system. Simulation results in normal operation and during offshore and onshore faults confirm the feasibility of the proposed control scheme. [ABSTRACT FROM AUTHOR]

High‐voltage direct current (HVDC) can speed up the recovery of a power system. However, both rectifier and inverter sides of HVDC are weak systems during a black start. To coordinate frequency characteristics of both sides within the frequency deviation constraint, a frequency regulation strategy based on variable‐parameter frequency limit control (FLC) during a black start is proposed. First, the mathematical models of asynchronous interconnected AC–DC system are established, and the optimal FLC parameters leading to minimum frequency deviation of both sides are obtained. Subsequently, based on the characteristics of the optimal FLC parameters with the parameters of the equivalent generator and the capacity of the AC system, a frequency regulation strategy is proposed to adapt to the continuously changing parameters and configurations of the AC power system during a black start. Finally, the simulation model is established using PSCAD/EMTDC software to verify the control strategy and the correctness of the optimal FLC parameters. [ABSTRACT FROM AUTHOR]

In recent years, high penetration of distributed generations based on wind energy, solar energy and so on in the existing power system network has been noticed. However, due to their stochastic behaviour, operations under autonomous mode as well as in grid‐connected mode are not an easy task. This has forced the power utilities to re‐define frequency regulation criteria to enhance the overall system stability and reliability. In line with the same, dynamic performance analysis of load frequency control (LFC) of an autonomous hybrid power system model (HPSM) consisting of tidal power plant (TPP) and diesel power plant is explored in this study. A concept of deloaded TPP is adopted in the studied HPSM to utilise the available reserve power for the frequency support. Apart from this, the studied model also incorporates frequency regulation through inertia and damping control and supplementary control strategies. These control strategies are realised through conventional controllers whose gain values are optimised using quasi‐oppositional harmony search algorithm (QOHSA) for the optimal dynamic performance of LFC. The efficacy of the proposed QOHSA is corroborated by comparing the results with those yielded by few other existing state‐of‐the‐art algorithms. [ABSTRACT FROM AUTHOR]

Power flow calculation is a basic tool for power systems operation and control. Considering static frequency regulation characteristics of power systems, dynamic power flow (DPF) gives more precise results of system operation with frequency deviation than conventional power flow. The increasing penetration of wind energy into power grid makes it necessary to take wind power into consideration in DPF calculation. The operational traits and frequency regulation characteristics of major types of wind turbines are discussed, including fixed speed wind turbines and variable speed wind turbines with integrated control strategy combining over‐speed control and droop control. With the primary frequency regulation characteristics of wind turbines, a simplified DPF algorithm is proposed in this study for power systems integrating wind power generation. The IEEE 30‐bus system is modified to verify the proposed method considering different levels of wind power penetration. [ABSTRACT FROM AUTHOR]

An integrated controller of wind turbines with both inertial response and primary frequency regulation (PFR) to provide complete dynamic frequency support for the grid with high wind power penetration is investigated. The wind turbine control governor contains two cross‐coupled controllers: pitch controller and maximum power point tracking (MPPT) controller. First, as a precondition for the PFR, a de‐loading pitch control scheme is proposed to reserve capacity required for frequency regulation. Then, by optimizing the MPPT scheme, the rapid virtual inertia response is achieved even under de‐loading operation condition. Based on the analysis of the steady‐state characteristics of wind turbines with frequency droop control, the primary frequency control strategy, which enables the adjustment of frequency droop coefficient, is further proposed through pitch angle changes. Thus, the PFR and inertial response can be both achieved by the proposed de‐loading pitch controller and optimized MPPT controller. A three‐machine prototype system containing two synchronous generators and a Doubly Fed Induction Generator (DFIG)‐based wind turbine with 30% of wind penetration is implemented to validate the proposed integrated control strategies on providing inertial response and subsequent load sharing in the event of frequency change. [ABSTRACT FROM AUTHOR]

This study investigates a novel operating method of the interconnected power grid to improve dynamic stability using variable inertia control of large wind farms. In this scheme, the system inertia is considered as an adjustable operating parameter which can be regulated for inter‐area power oscillation control and frequency support. Based on the linear state equations derived from a two‐area interconnected power system, the effect of the controlled inertia on power oscillation damping is evaluated theoretically considering the different cases of power feeding and receiving networks. The respective wind turbine variable inertia control strategies for the feeding and receiving regional networks are proposed to enhance both inter‐area power oscillation damping and frequency stability. A typical two‐area interconnected power system with a high 30% wind penetration is simulated, which consists of four conventional power plants and two doubly fed induction generator (DFIG)‐based wind farms. The results demonstrate that by using the proposed control scheme, the frequency stability and the inter‐area oscillation damping are both significantly improved for the more effective inertia support in the interconnected power system. [ABSTRACT FROM AUTHOR]

This research presents a model of a utility‐scale photovoltaic unit (USPVU) enhanced with an embedded hybrid energy storage system (HESS), suitable for stability studies in transmission systems. The main goal of this model is the simultaneous provision of primary frequency control and dynamic grid support. The primary frequency control includes both droop response (achieved by the frequency sensitive mode [FSM] operation) and inertial response (IR). To obtain these grid support functions, the research designed a suitable voltage and frequency (V–f) control, which coordinates the photovoltaic (PV) maximum power point tracking control, HESS converter control, and PV inverter control. Firstly, a midterm assessment of energy requirements in the sized HESS, based on frequency data, validated the energy availability of the enhanced USPVU for primary frequency control, according to new prequalification rules for energy‐constrained units. Then, transient stability assessments were performed on a representative transmission system to check the performance of the added FSM and IR in USPVUs with dynamic grid support. The results of the frequency phenomena in the IEEE 39‐bus system showed that the enhanced USPVU shared primary frequency control responsibilities with the conventional generation. This was achieved with two criteria to dispatch and commit conventional units by USPVUs. [ABSTRACT FROM AUTHOR]

An improved control methodology for off‐grid, three‐phase induction machine working as a single‐phase generator is presented in this study. A three‐phase machine made to work as a single‐phase generator has advantages of lower size and cost than their single‐phase equivalent in almost all ratings except in the lower fractional kilowatt (kW) ranges. The three‐phase windings are configured as isolated two equivalent phases in the proposed system for this purpose. The machine uses photovoltaic (PV)‐assisted single‐phase inverter to one of its two phases for variable excitation requirement to cater dynamic loads at different rotor speeds besides having a fixed capacitor to provide bulk excitation. A storage battery is used along with the PV panels to improve the reliability of the scheme during low or no insolation periods. A stator‐flux‐oriented control is implemented for regulating the terminal voltage within specified limit at a fixed frequency. The control strategy ensures fixed load voltage and fixed frequency even with perturbations of load or rotor speed. Different simulation results, verified by suitable experiments using a 1 kW, 415 V laboratory prototype, confirm the suitability of the proposed concept. [ABSTRACT FROM AUTHOR]

Energy storage systems have established their capability to overcome the problems caused by intermittent nature of renewable sources when integrated to existing grid. Voltage and frequency control, as well as load shifting can be done using grid level storage systems incorporated with renewable sources. They can effectively serve the system as an energy sink and source. Operational use of energy storage for grid level support of a wind electric generator (WEG) is demonstrated in this study. The battery storage supported WEG along with controllers is modelled. Artificial neural network controller, which is having inherent learning capability, is developed to regulate the power flow between wind generator and utility grid. The proposed algorithm and the corresponding controller are simulated in MATLAB/Simulink and implemented in the DSP processor. The real time data exchange between Simulink and the floating point DSP processor TMSF32028335 is realised using on‐board JTAG emulator. The hardware implementation using DSP processor presented in this work establishes the efficacy of the proposed control strategy for real time applications. [ABSTRACT FROM AUTHOR]