The growing integration of large-scale renewable energy sources (RESs) has significantly reduced power system regulation capability, increasing the risk of power imbalance and frequency insecurity. This paper presents a regulation capability evaluation method for coordinating net-load and frequency regulation capability (FRC) requirements. To balance feasibility and model complexity, an equivalent regulation capability quantification model incorporating spatial-temporal distribution is introduced for net-load requirements under normal-state. For credible contingencies, frequency regulation amplitude, rate, and an FRC quantification model are formulated to accurately evaluate the FRC requirements. In addition, a data-driven adaptive bound selection and fitting algorithm is developed to handle nonlinear frequency constraints, substantially enhancing computational efficiency. Case studies on the modified HRP-38 system and a large-scale Chinese power grid validate the effectiveness of the proposed model and algorithm.

The volatility of wind and photovoltaic (PV) power poses significant challenges to grid stability, where energy storage systems (ESSs) serve as critical flexibility resources. However, the existing ESS configuration methods lack the source-grid coordinated optimization considering the distinct frequency characteristics between wind and PV fluctuations. In this paper, a source-grid coordinated optimization method for ESS siting and sizing is proposed. Based on empirical mode decomposition (EMD), the renewable energy outputs are decomposed into grid-connected component and source level energy storage allocation tasks. An improved multi-objective particle swarm optimizer (IMOPSO) is utilized to site and size grid level energy storage. Simulation results demonstrate the effectiveness of the proposed approach.

This paper investigates the fault current characteristics of doubly-fed induction generators (DFIGs) in weak grid scenarios, addressing the limitations of existing methods that assume a predefined voltage dip and neglect the influence of DFIGs on post-fault voltage. A transient fault voltage model is proposed to accurately capture the interaction between DFIGs and weak grid impedance, which significantly affects fault current calculation. The model introduces an additional pole to account for fault voltage transients, addressing computational errors in low grid strength conditions. Furthermore, a fault current expression for DFIGs under severe faults, particularly after crowbar activation, is derived, revealing the impact of external grid impedance on transient fault currents. Case studies with expanded fault scenarios and hardware-in-loop experiments demonstrate the superiority of the proposed method over existing approaches in weak grid applications.

Setting a reasonable carbon reduction plan in coastal metropolises is the key part to reach the global carbon target. Carbon reduction will change urban climate and influence energy demand, both of which affect the optimization results of carbon reduction pathways. Current generation expansion optimization models consider direct abatement contribution and solve most problems of planning for long-term carbon emission reduction in energy systems. However, the construction of new type power systems also indirectly impacts carbon emissions by changing microclimate factors such as heat island intensity. By combining generation expansion with carbon emission prediction model, the proposed approach in this paper considers the hidden mechanism of carbon and heat emission change on air-conditioning loads and dynamically optimizes the carbon reduction pathways in coastal metropolises. Taking Pudong Area in Shanghai as an example, the estimated cost of carbon reduction is reduced by the proposed approach. Some suggestions for the carbon reduction in coastal metropolises are made according to the simulation results.

As the penetration rate of renewable energy in the power grid continues to rise, the reserve criteria for traditional power grids dominated by synchronous generators (SGs) have difficulty meeting system frequency security requirements. This study proposes a frequency security-constrained optimization approach for the allocation of reserve capacity in high-penetration renewable energy grids that utilize multitype reserve resources, including SGs and nonsynchronous units, to address the frequency security issue. First, strategies and models for expanding the sources of frequency regulation reserves are analyzed, including various types of renewable energy generation, such as wind turbine (WT) curtailment and the combination of photovoltaic (PV) cells and battery storage. A refined reserve criterion is then proposed that considers multidimensional evaluation indices from both operational economy and frequency security aspects. Finally, a bilevel optimization model for reserve capacity allocation on multiple timescales that considers frequency security is constructed. The rationality and effectiveness of the proposed reserve allocation scheme were verified using a practical power grid in Southwest China.

Fast and accurate analysis of the short-circuit characteristics of large wind farms has important engineering application value, and the short-circuit characteristics of wind farms under the influence of the wake effect vary greatly. Therefore, it is necessary to establish a wind farm short-circuit fault time equivalence model. A wind farm short-circuit fault dynamic equivalence method considering the effect of wake effect is proposed. First, the wake effect factor is defined to reflect the degree of the unit affected by the wake effect. Then, the wake effect factor is used as the grouping basis to reduce the variability of operating state of the units within the group under the influence of the wake effect. A positive- negative- zero-sequence network equivalence method is analyzed to improve the effectiveness of the equivalence model in asymmetric short-circuit faults. An equivalence method suitable for zero-sequence network is proposed and a platform is built for verification. The simulation results show that the dynamic short-circuit fault equivalence model proposed can accurately reflect the active and reactive short-circuit output characteristics of wind farms under the influence of the wake effect.

Renewable energy plays a key role in reducing carbon emissions. The electric power system is widely considered a platform to accommodate renewable energy because of the high efficiency in transmitting energy through electricity tie lines. However, most renewable energy generation is weather-dependent, bringing difficulties in balancing generations and electricity demands. There are two major solutions to address the imbalance problem. The first is to balance the fluctuation of renewable energy in time scales by deploying energy storage, but with huge costs and security risks. The other is to balance the fluctuation in spatial scales by enhancing the interconnection among electric power systems, which however results in a sharp increase in the fault current in many countries. The high level of fault current threatens the security of power system. In other words, the overrating fault current of electric power systems is one of the key bottlenecks for carbon neutrality. To mitigate the highlevel fault current because of the interconnection, we propose a novel mechanism, namely fault current release. It achieves new functionalities with proven equipment at low cost and will create no impact on the electric power systems in normal operation. The simulation selected six representative electric power systems, including those in the USA, the UK, China, Brazil, Nigeria and IEEE reliable test system. Results show that the fault current release method enables an increase of renewable energy accommodation from 19.2% similar to 47.2% to over 90%, much higher than the existing methods can do. Correspondingly, the emissions of CO2, NOX and SOX in these systems decrease by 67.84% similar to 88.07%, 61.17% similar to 88.24% and 58.25% similar to 88.33%, respectively. The proposed method is also economical in that it can accommodate renewable energy at about one-tenth the cost of energy storage. Our findings hence suggest a promising way to build a carbon-free energy system.

The fault current characteristic of doubly fed induction generators (DFIGs) is determined by both the external grid voltage and the internal rotor voltage. However, there are multiple fault-ride through strategies employed by DFIGs. In this paper, a fault current calculation method for DFIGs under complete fault-ride through strategies is proposed. The transient behavior of the DFIG is described by the state space model in which the stator voltage and rotor voltage are selected as input signals. The internal dynamics of the DFIG is modeled by the rotor voltage which is controlled by multiple strategies in sequence. Comparative case studies are carried out to demonstrate the accuracy of the proposed method.

The shunt compensated grid (SCG) connecting with doubly-fed induction generator (DFIG) system easily suffers from high-frequency resonance (HFR). Existing studies reshape the DFIG based on a pre-calculated frequency to suppress the HFR. When facing voltage disturbance, shunt capacitors are added to the grid which greatly changes the resonance frequency, probably resulting functional failure of HFR suppression. In this article, a parallel impedance-reshaping control (PIRC) strategy for the rotor side converter (RSC) and grid-side converter (GSC) control is proposed to suppress the HFR of the DFIG-SCG system when the shunt capacitor varies in a wide range. A high-order coefficient is combined with the original impedance of the RSC-induction generator (RSC-IG) side, and a damping controller is designed to reshape the impedance of the DFIG system. Based on the parallel structure of the equivalent impedance of the GSC side and the RSC-IG side, the impact of the shunt capacitor to the controller can be eliminated, thus providing effective HFR suppression in a wide frequency range. Case studies demonstrate the advantage of the PIRC in HFR suppression and harmonic suppression.

Recently, many distributed fault-tolerant (Di-FT) secondary controls have been designed to improve the robustness of the islanded AC microgrids (MGs) to actuator faults/attacks. However, some problems still exist among the existing research: (a) the convergence speed of the faulty MG's states has not been discussed, which is important for reducing the inter-harmonic component around the fundamental frequency thereby improving the power quality; (b) large computing and sampling resources are required by the relatively high control gain for dealing with the possible serious actuator faults. Therefore, a distributed event-triggered fixed-time (Di-ET-FT) fault-tolerant secondary control of the faulty islanded AC MG is designed in this paper to release the communication burden, save the computing resources, and achieve the fixed-time convergence speed for improving power quality simultaneously. The Di-ET-FT fault-tolerant secondary control has a two-layer control framework, which are the observer layer as the upper layer and the fault-tolerant control layer as the lower layer. Meanwhile, two event-triggered strategies are designed in these two layers to decrease sample/control updating numbers and communication numbers, respectively. Zeno behavior of the Di-ET-FT fault-tolerant secondary control can be evaded effectively by proving that the interevent time is lower bounded by a positive value. The abovementioned advantages of the designed secondary control are validated by the real-time simulation based on NI-PXI simulator.

This paper proposes a hierarchical event-triggered model predictive control (HEMPC)-based voltage control strategy to coordinate the reactive power (Var) outputs among all WTs for improving the high-voltage ride-through (HVRT) and post-fault voltage restoration capability of the large-scale wind farms (WFs), while reducing the heavy computational task of the WF controller. The event-triggering condition and optimal controller are designed to activate the proposed HEMPC strategy based on the system state deviation and input-to-state stability (ISS) mechanism, which can substantially decrease data communication in a non-periodic sampling fashion. A hierarchical solution method based on the alternating direction method of multipliers (ADMM) is developed, aiming to further improve the computing efficiency of the voltage optimization problem of the large-scale WF. Two time-domain WF simulation cases in Matlab/Simulink corroborate that the proposed HEMPC strategy is more efficient in minimizing the WT terminal voltage deviations and reducing the computational burdens of the WF controller compared with traditional control strategies.

In AC microgrids (MGs), the required high control gain for dealing with control faults would reduce the robustness of the secondary control to communication constraints. To tackle with this problem, the subarea physical infrastructure of AC MGs and the two-layer secondary control framework are designed. For the physical infrastructure, the whole AC MG is divided into several areas, where each area is composed of a distributed generator (DG) and several geographical-close loads. Furthermore, by utilizing the advanced metering infrastructure (AMI), loads transmit the local load change information to the DG in the same area. Based on this physical infrastructure, a distributed event-triggered fixed-time fault-tolerant (ET-FTFT) secondary control framework is proposed in this paper to improve the resilience of the secondary control to sensor and actuator faults/attacks, and communication constraints, simultaneously. The adopted two-layer control framework is to decouple the communication constraints and control faults. The event-based strategy is adopted in the upper layer to reduce the communication burden and Zeno behavior can be evaded. Real-time simulations based on the NI-PXI real-time simulator validate the advantages of the distributed ET-FTFT secondary control framework, which are the better robustness to control faults, and the fixed-time convergence to improve the power quality.

Many countries have set ambitious targets to achieve zero-carbon electricity systems by the Mid-21st Century. In their pathways, the renewable mix and the energy storage mix have been considered as two important facets. Current literature mostly focuses on how the storage mix is affected by the renewable mix, but few studied the inverse impact and the dynamic interaction between the storage and renewable mixes. We, therefore, developed an electricity system optimisation model with hourly resolution to investigate how the interaction between renewable and storage mixes could accelerate the decarbonisation in future 30 years. This study considered the decarbonisation roadmap in the UK designed by the National Grid with variable factors such as cost structure of renewables and storages, annual investment budget, and load growth. Our research finds that short-duration energy storages with duration time at 6-8 h are preferred for providing cheap and rapid ramping power to meet the daily fluctuation in the early stage (2020-2030) of the decarbonisation process. In the late stage of retiring fossil fuels (2040-2050), high-share wind energy plus with long-duration storages (with duration time longer than 38 h) can solve the problem of great-quantity and long-lasting energy shortage caused by renewables, thereby achieving high-renewable penetration.

With reductions in the pressure ratio of the fan and increases in bypass ratio of the turbofan in recent years, the engine windmilling and relight capability have attracted more and more attention in aviation safety. This paper proposes a combination of the speed model and CFD to examine the internal flow evolution of the wide-chord fan under the windmilling condition. Firstly, we establish a semi-empirical speed model for predicting the fan rotational speed of a high-bypass ratio turbofan at any flight Mach number and altitude based on the flow characteristics of a windmilling fan. The model can provide an initial windmilling airflow at the specified speed for subsequent CFD calculations to reduce the number of iterations. Then, a three-dimensional CFD model is proposed to obtain the operating boundary of the windmilling fan according to the direction of energy transfer between the fan and the airflow. The mechanisms of fan internal flow evolution and tip leakage flow when it operates at a constant speed line were analyzed. The results show that the work distribution profiles along the blade span exhibited self-similarity. The fan blade operated in both spanwise and chordwise mixed configuration. The flow moved gradually from compressor-like regions near the hub to turbine-like regions near the shroud, where the stirrer mode was the transitional stage between them during windmilling. When the rotor entered the windmilling, it operated at a significantly off-design angle of attack, that caused a large area of flow separation to occur near the pressure surface (PS) instead of on the suction surface (SS). The flow separation and loss gradually increased from the hub to the shroud. Meanwhile, the direction of tip leakage flow also changed, a streamwise vortex core was observed on the PS near the blade tip. The core mixed with the low-energy fluid on the PS, resulting in flow blockage, that changed the working profile of the rotor. As the mass flow rate increased to the turbine operating point, the intensity of interaction and influence area between the leakage flow and the main flow further increased, and the aerodynamic loss increased significantly.

In this paper, a coordinated impedance-reshaping control (CIRC) strategy for the rotor side converter (RSC) and grid-side converter (GSC) control is proposed to suppress the HFR of the DFIG-PCG system. The impedance of RSC-induction generator (RSC-IG) is reshaped by introducing a high-order coefficient into the original impedance, while the impedance of the GSC is reshaped by considering the grid-side current control. A damping controller is designed to reshape the impedance of the DFIG system by the cooperative control of RSC and GSC. Compared with the conventional HFR suppression method relying on DFIG's electricity parameters, the proposed CIRC strategy can effectively suppress the HFR of the DFIG-PCG in a wide range frequency based on the determined coefficients of impedance reshaping, which makes the reshaped impedance of DFIG is calculated no matter the DFIG parameters loss, thereby avoiding the strategy failure when some electricity parameters are losing. Finally, a time-domain simulation model of the DFIG-PCG is established on MATLAB/Simulink to verify the effectiveness of the proposed CIRC strategy.

To mitigate the threat from increasing fault current level in high-voltage transmission systems, a “dredging theory” based on the fault current splitter (FCS) was proposed to help the over-duty circuit breaker clear the fault. However, detailed practice of this theory is not fully studied. This paper investigates the feasibility of suppressing the fault current by FCSs from the perspectives of operation and planning. The operating scheme of FCSs is firstly qualitatively analyzed, and an iterative formula is derived to calculate the fault current in a system including multiple FCSs. Furthermore, the voltage sag caused by FCSs is evaluated by the Specified Computer Business Equipment Manufacturer Association (SCBEMA) curve and is minimized by the proposed optimization model considering maximum fault current limitation. A repeat-searching particle optimization (RSPSO) algorithm is developed to solve the model efficiently. This solution is validated to be effective and acceptable through two case studies.

Clean renewable energy is given high priority to generate power in power system dispatching. Unfortunately, the objective to minimize wind power curtailment may results in additional costs, since the inverse peak regulation characteristic of wind power probably result in redundant power qualities curtailed to ensure the balance of the electric supply and demand in real time. In this paper, considering the cost of deep peak regulation of thermal unit and wind power curtailment, unit commitment comprehensive optimal model is proposed to minimize total cost by searching the optimal wind power curtailment. In order to achieve an economic strategy among the wind power curtailment and the deep peak regulation, two indexes including the amount of hourly optimal wind power curtailment and the critical value of comprehensive wind power curtailment are developed. Particle swarm optimization algorithm is employed for solving the proposed model. Afterwards, the relationship between the degree of deep peak regulation and the amount of wind power curtailment is researched, the derived conclusions can provide valuable information for system operators.

The current 25% of global renewable energy among electric grids generation is far from the stated goal of 60% in 2040 and 100% in 2050. One of the biggest dilemma to cost-effectively increase this percentage, through grids interconnection, is that the traditional fault blocking methodology has reached its safety protection ceiling. In contrary to the traditional wisdom that any form of faults must be avoided, here we proposed a fault dredging methodology by constructing human “faults” to create artificial fault current release channels to solve high-level fault current bottleneck during grids interconnection. Using actual region electric grids from globally representative countries across six continents for demonstration, our results show that human “faults” can enable the percentages of renewable energy in USA, UK, China, Brazil, Australia, Nigeria and IEEE RTS system increase from 19.2%~47.2% to over 90%, which are much higher than all of conventional technologies. Correspondingly, the emissions of CO2, NOX and SOX in these systems respectively decrease by 67.84%~88.07%, 61.17%~88.24% and 58.25%~88.33%. Our findings suggest a new approach to reach the global renewable energy goal and promote the progress toward achieving carbon-free energy system.

Biogas, as the product of anaerobic fermentation, is one of the main routes to utilize bioenergy. The biogas production rate is determined by lots of factors, of which temperature determines most. However, when operating in the cold climate or cold areas, the low digesting temperature often results in a relatively low efficiency or even a suspension in the fermentation process. This paper proposes a self-excited heating method to increase the digesting temperature in cold weather. It utilizes the self-produced biogas as fuel to generate heat for heating the biogas digester. The influence of this method on biogas production rate and digesting temperature are verified in this study. The results show that the digesting temperature increases about 3%–9% with a self-excited heating method for both household-scaled biogas digester and industry-scaled digester.

The large-scale wind power introduces the challenge of the power demand and generation balancing. Energy-intensive load (EIL) is a promising option for peak shaving since it can change its production time and power demand without affecting its overall production. However, EIL which is discretely adjustable is unable to track the net load in real time. A two-stage complementary peak shaving strategy of EILs with the aid of battery energy storage systems (BESSs) is proposed to address this issue. This paper establishes an optimization model with the minimum system operation costs and wind curtailment costs as the objective function, in which EIL operation constraints and BESS power and energy balance constraints are added to the unit commitment model. And the neural network algorithm is used to solve this optimization problem. Finally, a system with a high proportion of wind power is adopted to analyze the functions of EIL and BESS in the method. It is verified that the proposed strategy can effectively reduce the amount of wind curtailment and the operation costs of the system.