Frequency Recovery Research Articles

Resilient distributed control of islanded microgrids under hybrid attacks

In this paper, a resilient control strategy is proposed to improve the stability of frequency and voltage recovery for the islanded microgrid (MG) under hybrid cyber attacks. To deal with the common false data injection attacks (FDI) and denial of service attacks (DoS) in MGs, the proposed resilient control strategy utilizes the observers to accurately estimate the potential FDI signals on both the sensors and actuators of each distributed generation unit (DG) and reconstruct the unavailable states in the system to enhance the system’s ability actively. The ultimate uniform boundedness (UUB) of the system under hybrid cyber attacks is proved by the Lyapunov stability theory. Finally, an islanded MG system is established in MATLAB/SIMULINK, and multiple scenarios are simulated to verify the effectiveness of the method.

Recovery time and strategy of DFIG speed based on a high-proportional hydropower grid

Large-scale wind power access reduces the equivalent inertia of a power system. The wind turbine has a considerable reserve of rotational inertia, which is important for reducing the risk of low-inertia operation in a grid and for improving frequency reliability. In this work, a doubly fed induction generator (DFIG) inertia frequency regulation strategy is studied. The secondary dip in frequency caused by speed recovery after the wind turbine participates in frequency regulation is an important factor that limits regulation capability on the DFIG. To address such problem, the power dip and recovery generated by the DFIG after active speed recovery is analyzed and modeled. Then, a linearized frequency response model of a hydropower single machine is established to investigate the secondary disturbance effect of wind turbine active speed recovery moment on the frequency of the high-ratio hydropower system.A conclusion is drawn that the moment that corresponds to the first recovery of system frequency to the quasi-steady-state frequency is the optimal recovery moment of the turbine in the linear SFR model. Combined with the frequency control characteristics of the frequency limit control(FLC)’s “large dead zone, ” an active speed recovery strategy of the turbine based on frequency deviation judgment is designed.

Optimizing the Location of Frequency Regulation Energy Storage Systems for Improved Frequency Stability

The installation of battery energy storage systems (BESSs) with various shapes and capacities is increasing due to the continuously rising demand for renewable energy. To prepare for potential accidents, a study was conducted to select the optimal location for installing an input BESS in terms of frequency stability when the index assumes the backup input of the BESS. This study builds on the premise that installing a BESS on a bus in an area where active power absorption and transmission are the most active can significantly contribute to increasing the frequency recovery of the power system. Based on this premise, the magnitude of the active power flow and the proportional characteristics of the phase difference between buses were mathematically confirmed. This study also calculated the effective power sensitivity index of a bus with 13 FR-ESSs installed in a domestic system and reviewed the frequency output by establishing a table for each failure scenario. The results indicated that the effect of frequency rise can be estimated at the level of tidal current calculation. Thus, the study suggested a direction for subsequent studies to improve the sensitivity index.

An implementation of inertia control strategy for grid-connected solar system using moth-flame optimization algorithm

As the solar power system power system grows rapidly, inertia control strategy (ICS) becomes crucial to enable stable grid integration. However, the existing ICS lacks of dynamic weather analysis with maximum power point tracking (MPPT) and fault-ride through (FRT) capabilities such as low voltage ride-through (LVRT) and high voltage ride-through (HVRT). In this work, an inertia weighting strategy and the Cauchy mutation operator are introduced to improve the moth-flame optimization (MFO) algorithm to support vector machine prediction of photovoltaic power generation. In this paper, the proposed adaptive VICS with variable moment of inertia (J) and damping factor ( D P ) demonstrates its effectiveness with faster frequency recovery, less overshooting and continuous stable operation under grid fault and dynamic weather. The MFO algorithm is used to implement inertia control strategies for grid-connected solar systems. Accurate simulation results confirm the inertia control of the emulsion and the control of the solar system. The results of the simulation show a significant improvement in frequency with the designed MFO and compared to Horse Herd Optimization (HHO). The proposed method contributes to improve photovoltaic energy prediction, reduces the impact of photovoltaic power penetration into the grid and maintains the system reliability.

Postischemic Infusion of Apigenin Reduces Seizure Burden in Preterm Fetal Sheep

Seizures are common in preterm newborns and are associated with poor neurodevelopmental outcomes. Current anticonvulsants have poor efficacy, and many have been associated with upregulation of apoptosis in the developing brain. Apigenin, a natural bioactive flavonoid, is a potent inhibitor of hyaluronidase and reduces seizures in adult animal models. However, its impact on perinatal seizures is unclear. In the present study, we examined the effect of apigenin and S3, a synthetic, selective hyaluronidase inhibitor, on seizures after cerebral ischemia in preterm fetal sheep at 0.7 gestation (98–99 days, term ~147 days). Fetuses received sham ischemia (n = 9) or ischemia induced by bilateral carotid occlusion for 25 min. Immediately after ischemia, fetuses received either a continuous infusion of vehicle (0.036% dimethyl sulfoxide, n = 8) or apigenin (50 µM, n = 6). In a pilot study, we also tested infusion of S3 (2 µM, n = 3). Fetuses were monitored continuously for 72 h after ischemia. Infusion of apigenin or S3 were both associated with reduced numbers of animals with seizures, total seizure time, and mean seizure burden. S3 was also associated with a reduction in the total number of seizures over the 72 h recovery period. In animals that developed seizures, apigenin was associated with earlier cessation of seizures. However, apigenin or S3 treatment did not alter recovery of electroencephalographic power or spectral edge frequency. These data support that targeting brain hyaluronidase activity with apigenin or S3 may be an effective strategy to reduce perinatal seizures following ischemia. Further studies are required to determine their effects on neurohistological outcomes.

Development of Energy Storage Systems for High Penetration of Renewable Energy Grids

As the proportion of renewable energy generation systems increases, traditional power generation facilities begin to face challenges, such as reduced output power and having the power turned off. The challenges are causing changes in the structure of the power system. Renewable energy sources, mainly wind and solar energy cannot provide stable inertia and frequency regulation capability. Ultimately, the power system’s emergency response capability to face an N-1 is reduced, which leads to a reduction in system stability. Therefore, the application technology of the battery energy storage system is used to support the impact of changes in the new power system structure. This paper designed control technologies based on the WECC second-generation generic model, namely, dynamic regulation, steady regulation, and virtual inertia regulation. The models and control strategies are verified on Taiwan’s 2025 power system target conditions, which consider the expected capacities for battery energy storage systems, and renewable energy sources with different load and N-1 fault levels. According to the simulation results, the capabilities of the RoCoF limitation, frequency nadir, frequency recovery, and system oscillation regulation are evaluated in the proposed strategies. Finally, the analysis results can help power operators make informed decisions when selecting and deploying battery energy storage systems.

Closed-Loop Synthetic Inertia Control for Wind Turbine Generators in Association With Slightly Over-Speeded Deloading Operation

Following a frequency event in a power system, synthetic inertia control (SIC) of a wind turbine generator (WTG) can improve the frequency nadir by instantly releasing the stored kinetic energy in the rotating masses. However, SIC might lower the settling frequency below the maximum steady-state frequency deviation, thereby disabling automatic generation control (AGC). This delays frequency recovery to the nominal value. This paper proposes a closed-loop SIC scheme for a WTG in association with slightly over-speeded deloading operation (SODO) that can improve both the frequency nadir and settling frequency. To achieve this, prior to an event, the proposed SIC scheme performs SODO, which can store more releasable energy into the rotating masses of a WTG while minimizing the energy loss. The proposed scheme can improve the frequency nadir by releasing more energy, improve the settling frequency, which enables AGC, and avoid over-deceleration of the rotor speed of a WTG in the low wind speed region. Simulation results clearly demonstrate that the proposed SIC scheme can improve both the frequency nadir and settling frequency under various wind conditions, event sizes, and penetration levels, thereby supporting the frequency stability in a power system that has a high penetration of wind power.

Effect of age and sulfate chloride environment on the Self-Healing performance of the desert sand engineered cementitious composite materials

This study applied pre-damage of 1 % tensile strain and 75 % failure loads to tensile and prismatic specimens of desert sand polyethylene fiber (PE) engineered cementitious composite (ECC) specimens at different ages (3 and 28 days), respectively. Self-healing was applied to the pre-damaged PE-ECC under 5 % Na2SO4 and 3 % NaCl, 5 % Na2SO4, and water environments. The mechanical properties of the PE-ECC after self-healing, the changes in the number and width of cracks, tensile strength and resonance frequency recovery rate were analyzed. Then, the self-healing products and self-healing process of the PE-ECC were further examined by SEM, EDS, and XRD microscopy. Results showed that self-healing PE-ECC could complete the process within 10 days in different environments and found the self-healing process involved the continuous generation and consumption of self-healing products. The 3-day-old self-healing PE-ECC was stronger than its 28-day-old counterpart. The self-healing performance was strongest in 5 % Na2SO4, whereas it was weakest in water. The main self-healing product of the PE-ECC was CaCO3, which was the result of the coupling effect of carbonization, secondary hydration, and erosion product fillings.

Adaptive Active Inertia Control Strategy of MMC-HVDC Systems for Flexible Frequency Support

The Modular Multilevel Converter High Voltage Direct Current Transmission (MMC-HVDC) technology is considered to be the most feasible choice for high-voltage and high-power transmission systems, and its flexibility and high controllability provide a new solution for renewable energy grid integration. The MMC topology contains a large number of capacitors, which enables it to provide a certain active inertia support for the connected AC system. Different from a synchronous machine, the active inertia control of an MMC can flexibly adjust a system’s inertia-supporting power by changing the control parameters. By introducing a variation of the Sigmoid function with amplitude-limiting capability, this paper proposes an adaptive active inertia control strategy for the MMC-HVDC system. The proposed scheme adjusts the inertia constant adaptively according to the frequency change rate of the AC system, which can better respond to the frequency recovery performance. Finally, the MMC-HVDC simulation model is established in PSCAD/EMTDC to verify the effectiveness of the proposed control strategy.

Frequency and voltage regulation control strategy of Wind Turbine based on supercapacitors under power grid fault

As the penetration rate of wind turbines gradually increases, the fluctuation of system voltage and frequency significantly increases under power grid fault. In severe cases, wind turbines will disconnect from the grid, leading to large-scale power outages. In order to improve the stability of wind turbine grid connection, a large number of literature has studied the frequency and voltage regulation strategies. However, the existing fault ride through control methods mainly solve the problem of reactive power support during faults, while frequency regulation schemes focus on the frequency changes caused by load fluctuations. There is little research on methods that simultaneously consider voltage ride through and frequency regulation under power grid fault. This paper proposes a voltage and frequency regulation control strategy of wind turbines based on supercapacitors to address the above issues. By switching the functional modes of the energy storage device at different stages, wind turbines achieve the fault through and frequency recovery, thereby enhancing the fault tolerance ability. In the strategy, control process is divided into power grid fault period and fault recovery stage, and it adjusts the auxiliary priority level of energy storage devices based on the electrical response degree of different stages. During the fault, energy storage device is in voltage regulation stage because the voltage drop degree is greater than the frequency fluctuation, and it can ensure the wind turbine does not run off the grid by providing dynamic reactive power support to power grid. In the fault recovery stage, energy storage device is in the frequency regulation stage because of the large frequency deviation, it output active power to accelerate the system frequency recovery rating. Finally, the effectiveness of the control strategy proposed in this paper is verified through simulation.

Cooperative Primary Frequency Regulation Strategy of Wind Storage System Based on Variable Inertia Coefficient

Direct-drive permanent magnet synchronous wind turbine-generator (PMSG) is connected to the grid through the rotor side and grid side converter. The rotor speed is decoupled from the grid frequency, so PMSG output power cannot respond to the change in system frequency. However, with the gradual increase of wind power penetration, the static stability of system frequency is weakened, so the new grid-connected wind farm should have the ability of active frequency support. This paper proposes a cooperative primary frequency regulation (PFR) strategy for wind storage systems based on variable inertia coefficient. PMSG provides inertia based on the distribution of rotational kinetic energy and modifies the polarity of the inertia coefficient at the frequency recovery stage to speed up the frequency recovery. Droop control is realized by energy storage and variable power tracking control (VPPT). Energy storage and VPPT respectively suppress the frequency decrease/increase and only participate in the down/up single-side PFR, which can reduce the energy storage configuration capacity. And a safety discharge coefficient considering the state of charge (SOC) is introduced to prevent over-discharge of energy storage devices when outputting energy. Finally, the PFR capability of PMSG under the proposed control strategy is verified by simulation results.

A robustness-enhanced frequency regulation scheme for power system against multiple cyber and physical emergency events

Emergency events, e.g., accidental generator outages and abrupt steeps in electric load in physical layer, and unexpected communication time delays in cyber layer, are raising increasing concerns in modern power systems, and can cause system frequency deterioration, thus threatening power energy security. However, currently there is few discussions on dealing with multiple emergency events in power system comprehensively and simultaneously. In order to counter the adverse effects caused by these various emergency events and maintain the system frequency, a robustness-enhanced frequency regulation scheme is proposed in this article based on the coordinate transformation technique and Lyapunov theorem. First, to analyze the dynamic process of the power system in contingency conditions, the model of the power system frequency regulation is reconstructed considering multiple emergency events. Furthermore, to guarantee the power system frequency stability, a virtual auxiliary surface is constructed based on the coordinate transformation technique, which can provide the control reference for frequency regulation. On this basis, a robust controller is developed to drive the system’s trajectory to the proposed virtual auxiliary surface, thus guaranteeing the robustness of power system frequency, e.g., minor frequency deviation and faster recovery speed, even with emergency events. In addition, the stability of the system with the proposed controller is rigorously proved by Lyapunov theorem. Finally, the effectiveness of the proposed scheme is verified by case studies. The results show that by using the proposed scheme, the maximum frequency deviation and recovery time can be improved to about 61.11% and 46.40% of the original value under multiple emergency events, respectively. Therefore, the proposed scheme can counter well against emergency events and contribute to the power system’s frequency regulation and energy security.

Multi-Mode Active Inertia Support Strategy for MMC-HVDC Systems Considering the Constraint of DC Voltage Fluctuations

The Modular Multilevel Converter based High Voltage Direct Current Transmission (MMC-HVDC) systems can provide active inertia support for the connected ac grid for fast frequency recovery. However, the submodule (SM) capacitance voltages will be affected by the transient power flow and there exist dc voltage fluctuation issues during the active inertia support process. This paper proposes a multi-mode active inertia support strategy for MMC-HVDC systems considering the constraint of dc voltage fluctuations. The analytical relationship of dc voltage fluctuations and ac frequency variations when MMC adopts the virtual inertia control is firstly established, and then different inertia support operation modes are divided according to the allowable dc voltage fluctuation ranges. Next, the multi-mode active inertia support strategy for MMC-HVDC is proposed, which can provide flexible active inertia support on the premise of reasonable dc voltage fluctuations according to different frequency variations and recovery requirements. Besides, the inertia support process of MMC is equivalent to the zero-input response of first-order circuit to calculate the maximum sustainable supporting time, which ensures that the dc voltage fluctuation constraint can be well satisfied in different operation modes. Finally, several simulation results verify the effectiveness of the proposed control strategy.

Dynamic Event-Triggered-Based Integral Reinforcement Learning Algorithm for Frequency Control of Microgrid With Stochastic Uncertainty

The intermittency of renewable energy and the uncertainty of load put forward higher robustness requirements for the frequency recovery of microgrid (MG). The energy storage equipment provides an idea for frequency control because of its fast response and strong controllability. In this paper, an on-line adaptive frequency control method is proposed to control the governor and energy storage to realize the frequency recovery of MG under stochastic uncertainty. First, the MG system with external disturbances is constructed as a zero-sum differential game model to obtain a robust optimal control scheme. Then, considering the system parameter uncertainty, an improved integral reinforcement learning (IRL) algorithm is designed, in which the reinforcement signal contains a non-quadratic function to solve the energy storage control constraints. Furthermore, a novel dynamic event-triggered control (DETC) is developed to reduce control update times of energy storage. The dynamic variable in DETC coupled with static trigger includes not only the past triggering information but also the disturbances. DETC has a larger trigger threshold than static event-triggered control (SETC). Meanwhile, the proposed algorithm is implemented by an action-critic network structure, in which the action network of energy storage is updated aperiodically. Finally, simulation results show that the proposed control algorithm can realize the frequency recovery of MG, and it has good robustness compared with other algorithms.

Coordinated Predictive Control of Offshore DC Collection Grid and Wind Turbines for Frequency Response: A Scheme Without Secondary Frequency Drop

From both technical and economical perspectives, all-dc offshore wind farm (OWF) is a trend for future offshore wind energy applications. Furthermore, in an all-dc OWF, both the capacitors in dc collection grid and offshore wind turbines have the potential for onshore grid frequency response. In light of this, this paper presents a model predictive control scheme, which coordinates the operation of offshore dc collection grid capacitors (dc-grid capacitors) and offshore wind turbines for the onshore frequency support. With this scheme, offshore dc-grid capacitors provide fast inertia support right after the frequency event, while offshore wind turbines provide primary frequency response for a longer period. A key feature of the proposed scheme lies in the ability to suppress the secondary frequency drop issue, which is caused by sudden power reduction of offshore dc-grid capacitors during the frequency recovery phase. An analytical framework is derived to validate this ability. Finally, simulation results verify the effectiveness of the proposed scheme.

Pulsar glitch in a strangeon star model – III. The recovery

ABSTRACT Strangeon star model has passed various observational tests, such as the massive pulsars and the tidal deformability during binary mergers. Pulsar glitch, as a useful probe for studying the interior structure of pulsars, has also been studied in strangeon star model in our previous papers, including the recovery coefficient, the waiting time of glitches, and glitch activity. In this paper, the recovery process of a glitch is described in the strangeon star model, based on the starquake picture established before (in Paper I). After the starquake, the inner motion of the stellar matter would reduce the tangential pressure in the cracked places at the equatorial plane. The recovery (increase) of the tangential pressure would be achieved by a viscous flow towards the cracked places at equatorial plane, which leads to the exponential recovery of the spin frequency. A uniform viscous flow can reproduce the single exponential decay observed in some glitches, and the viscous time-scale τ and the depth h of the cracking place below the surface can be fitted by the recovery data. It is found that h increases with glitch size Δν/ν, which is expected in the glitch scenario of strangeon stars. The magnitude of the recovery predicted in this recovery model is also consistent with that derived from observations. The single exponential decay reproduced by a uniform viscous flow can be generalized to two or more exponentials by the multicomponent of viscous flows.

Distributed Discrete-Time Secondary Cooperative Control for AC Microgrids With Communication Delays

This paper presents a droop-based distributed discrete-time secondary cooperative control scheme for AC microgrids with communication delays, which can regulate the voltage and frequency of all distributed generators (DGs) and ensure that each DG obtains active power sharing accurately. Unlike the ideal communication among DGs, the delays caused by low-band-bandwidth communication networks generally lead to potential oscillation and endanger the system stable operation. To this end, the proposed control protocol aims to compensate the general communication delays for cyber-physical microgrids actively by using predictive control theory while realizing voltage and frequency recovery as well as active power sharing precisely. Morever, each DG only needs to share information with its neighbors in the proposed scheme, and sufficient conditions for achieving stability of the closed-loop control systems are derived. The effectiveness of the control strategy is also verified by simulation studies and some experiments on OPAL-RT platform.

Stochastic unit commitment to determine frequency response ramp rate including wind turbines with synthetic inertia and virtual synchronous generator

In this paper, the dynamic frequency constraints are derived for power systems incorporating full-converter wind turbines. These wind turbines could be equipped with a virtual inertia by either synthetic inertia control or virtual synchronous generator concept. In the former scheme, deviations in the wind turbines rotor speed remain in synchronism with the system frequency. However, the wind turbines start to restore their rotor speed before the system frequency recovery initiation, in virtual synchronous generator scheme. The restriction on the virtual inertia are determined considering a limit variable on wind turbines rotor speed drop. The frequency response requirements are derived in terms of governor’s base power, ramp rate and reserve power. A linear frequency nadir constraint is also proposed for the synthetic inertia strategy. Then, the constraints are integrated into the stochastic unit commitment to be consistently and economically met over 24-hour period. As such, the contingency size management are coordinated with the virtual inertia provision. The unit commitment results of IEEE 118-bus system including multiple wind farms reveal that the virtual synchronous generator concept can be an optimal solution for future low-inertia grid’s challenges. Finally, accuracy of the unit commitment results is validated through time-domain simulations of the detailed system model.

The permanently rotating wind turbines: a new strategy for reliable power system frequency support under low and no wind conditions

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

Distributed quantum multiagent deep meta reinforcement learning for area autonomy energy management of a multiarea microgrid

This paper presents a distributed area autonomy load frequency control (DAA-LFC) method capable of balancing the interests of different grid operators and achieving fast frequency recovery. The method treats each area controller in a multiarea microgrid as an agent. During offline training, the agents enter into gameplay with each other to obtain a global optimization policy. The agents involved in this method are capable of independent decision-making and need not communicate with each other during online operation. In addition, this paper presents a distributed quantum multiagent deep meta-deterministic policy gradient (DQMA-DMDPG) algorithm, which employs both large-scale learning and meta-learning to achieve collaborative multitask learning by setting reasonable exploration parameters under different tasks. A quantum method is used to set the exploration action noise as a set of superposition states to obtain richer samples. These innovations deliver better performance in terms of frequency deviation and total generation cost, thus satisfying the requirements of different grid operators. A simulation based on a four-area microgrid of the China Southern Grid (CSG) demonstrates that the proposed method can simultaneously reduce the frequency deviation and power generation costs and balance the interests of multiple operators.