Search

Abstract When a severe fault occurs in the distribution network, all or parts of it may be disconnected from the upstream network. Partitioning of these islanded areas is a solution to supplying the affected loads. Due to the variable nature of loads and renewable distributed generation (DG), the static model of partitioning with a fixed nature during island operation cannot be suitable. Therefore, in this article, considering the variable nature of loads and renewable distributed generation, a dynamic model is presented for the island partitioning to restore more valuable loads, which is suitable for quick decision‐making in emergencies. Also, a method for deciding on the mode of charging and discharging storage systems in emergencies is proposed. This model considers time limitation, uncontrollable DGs, controllable DGs and their control, controllability, and priority of loads, tie‐switches, storage systems, simultaneous faults, different situations of unintentional islanding of the distribution network, position of switches, and variable nature of loads and distributed generations. So, it is more comprehensive than the previous methods. Applying the proposed model to the modified IEEE 69‐bus system with controllable and uncontrollable generation and storage systems assuming different scenarios shows the effectiveness of the proposed scheme.

When a severe fault occurs in the distribution network, all or parts of it may be disconnected from the upstream network. Partitioning of these islanded areas is a solution to supplying the affected loads. Due to the variable nature of loads and renewable distributed generation (DG), the static model of partitioning with a fixed nature during island operation cannot be suitable. Therefore, in this article, considering the variable nature of loads and renewable distributed generation, a dynamic model is presented for the island partitioning to restore more valuable loads, which is suitable for quick decision‐making in emergencies. Also, a method for deciding on the mode of charging and discharging storage systems in emergencies is proposed. This model considers time limitation, uncontrollable DGs, controllable DGs and their control, controllability, and priority of loads, tie‐switches, storage systems, simultaneous faults, different situations of unintentional islanding of the distribution network, position of switches, and variable nature of loads and distributed generations. So, it is more comprehensive than the previous methods. Applying the proposed model to the modified IEEE 69‐bus system with controllable and uncontrollable generation and storage systems assuming different scenarios shows the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

Abstract A systematic definition of an aircraft generator's “available power” (used to supply loads on its DC bus) is given, which is defined in the context of constraints on transient and steady‐state performance. Using a geometric, data‐driven approach, such a characterisation has been achieved, and a new method, called Power availablE Estimation Tool (PEET), has been developed to determine in real‐time whether a given load can be fulfilled at a given time while maintaining power quality. This is an important problem, especially for safety critical electrical systems such as more electric aircraft, for which it is imperative to know a priori whether an added load will result in voltage variations outside of allowed values. PEET is introduced and its conceptual framework is formalised. Lastly, it provides simulation results that illustrate its performance. The results show that the PEET method produces reliable a priori estimates of power availability, and that this can be achieved within time frames that make it applicable in a real‐time implementation.

A systematic definition of an aircraft generator's "available power" (used to supply loads on its DC bus) is given, which is defined in the context of constraints on transient and steady‐state performance. Using a geometric, data‐driven approach, such a characterisation has been achieved, and a new method, called Power availablE Estimation Tool (PEET), has been developed to determine in real‐time whether a given load can be fulfilled at a given time while maintaining power quality. This is an important problem, especially for safety critical electrical systems such as more electric aircraft, for which it is imperative to know a priori whether an added load will result in voltage variations outside of allowed values. PEET is introduced and its conceptual framework is formalised. Lastly, it provides simulation results that illustrate its performance. The results show that the PEET method produces reliable a priori estimates of power availability, and that this can be achieved within time frames that make it applicable in a real‐time implementation. [ABSTRACT FROM AUTHOR]

Abstract As the ratio of newly installed power generation utilizing power electronic converters increases, it results in a decrease in the overall inertia and damping of the power system, as well as the decrease in its ability to resist interference. Aiming to address this issue, the authors propose an adaptive optimal control approach for the parameter in the virtual synchronous generator (VSG). This strategy considers the power angle curve and analyzes the principles of inertia and damping within the oscillation period and its range of values. By coordinating the values of parameters, it achieves the optimization of these parameters. At the same time, in order to take into account the demand for fast regulation response under normal operation control conditions and the demand for synchronization support under disturbance fault conditions, a multi‐mode switching control strategy including virtual synchronization control and conventional PQ control is further proposed. Based on MATLAB/Simulink software, the strategies mentioned above have been proven through simulation. The findings testify the proposed method can significantly enhance the regulation response characteristics of the VSG by optimizing its parameters. Additionally, it enables smooth switching of the control mode independently, based on the specific requirements of different control scenarios. This approach fully leverages the virtual synchronous support and fast regulation characteristics of the power electronic converter, and has practical engineering value.

Abstract Frequency security is the premise of realizing the net‐zero transition. A novel assessment scheme is proposed to quantify the frequency security levels of transmission systems under different inverter‐based resources (IBRs) and their control parameter combinations. A novel system frequency dynamic model is proposed as a parametric optimization method, where the saturation of generators, energy storage systems, and renewable energy sources is incorporated as differential algebra equations. This problem is further reformulated as a mixed‐integer linear programming problem to generate a sufficient amount of data under different IBR integration and control parameters. The frequency security assessment problem is formulated as a data‐driven multivariate classification problem, which is solved by the optimal classification tree (OCT) algorithm with better interptretionability. Simulations are conducted on a transmission system. Numerical results indicate that the proposed system frequency dynamic model can capture the frequency dynamic under different IBR reinforcement plans and the OCT can realize accurate classification of the synthetic data regarding frequency security.

This paper proposes a method for generation expansion planning that incorporates full-year hourly multiple load levels balance constraints, providing sufficient flexibility to address load fluctuations and intermittency associated with variable renewable energy sources. Typically, ensuring that the generation system possesses enough flexibility to manage this intermittency involves considering the operational characteristics of generators within unit commitment constraints. However, to mitigate the substantial computational burden caused by the number and type of variables, various approximation techniques are often employed. Unfortunately, these techniques can introduce unrealistic elements into the problem. Instead of considering the operational characteristics of generators, this approach classifies the system’s demand into three levels: base load, intermediate load, and peak load, using the proposed load classification method. The multiple load-level balance constraints are then applied to ensure that the capacity of generation units in each level is sufficient to meet their corresponding demand, with particular emphasis on matching fast-response generation units and their corresponding demand. The resulting generation expansion plan can be obtained with significantly reduced computational effort. The proposed load classification method and generation expansion planning approach have been tested using the latest power development plan of Thailand. Compared to another method that is not taken flexibility into account, 5 Gigawatts of fast-response generation capacity are selected instead of base load generation units. With the improved computational time achieved by the proposed generation expansion planning method, it can account for input data uncertainty by solving multiple generation expansion planning problems with varying input data and distinct individual probabilities.

A typical power system consists of a network of generation, transmission, and load spanning a wide area, with varying spatial characteristics crucial for realistic problem-solving. Traditional generation expansion plans (GEP) often treat the generation system as a single area, specifying types and sizes of new power plants to meet future demand without designating specific locations. This approach leads to generation-demand imbalances, unnecessary transmission expansions, and other issues. Incorporating regional constraints like local demand, tie-line capacity, and available resources is essential in GEP. Considering the entire transmission network can address spatial characteristics but presents challenges due to extensive data preparation and computational complexity. This paper proposes a GEP approach that accounts for spatial characteristics such as primary energy sources, renewable energy potential, and feasible locations for future units. With the proposed method, the power system is divided into multiple zones, each represented by a single bus connected by interzonal transmission lines. This zonal approach simplifies the transmission model by focusing on interzonal data, making it more practical for actual power systems. An area-based reliability index is used to evaluate each area’s reliability level, aiding in the suitable placement of future generation units. The proposed method was tested using Thailand’s latest power development plan, PDP2018 revision 1. Results show that accounting for spatial characteristics alters the generation expansion plan. Additionally, new units are distributed across the system to maintain area reliability. Improved computational efficiency of this proposed method allows for addressing uncertainty by solving multiple scenarios with varying input data and probabilities.

Frequency security is the premise of realizing the net‐zero transition. A novel assessment scheme is proposed to quantify the frequency security levels of transmission systems under different inverter‐based resources (IBRs) and their control parameter combinations. A novel system frequency dynamic model is proposed as a parametric optimization method, where the saturation of generators, energy storage systems, and renewable energy sources is incorporated as differential algebra equations. This problem is further reformulated as a mixed‐integer linear programming problem to generate a sufficient amount of data under different IBR integration and control parameters. The frequency security assessment problem is formulated as a data‐driven multivariate classification problem, which is solved by the optimal classification tree (OCT) algorithm with better interptretionability. Simulations are conducted on a transmission system. Numerical results indicate that the proposed system frequency dynamic model can capture the frequency dynamic under different IBR reinforcement plans and the OCT can realize accurate classification of the synthetic data regarding frequency security. [ABSTRACT FROM AUTHOR]

As the ratio of newly installed power generation utilizing power electronic converters increases, it results in a decrease in the overall inertia and damping of the power system, as well as the decrease in its ability to resist interference. Aiming to address this issue, the authors propose an adaptive optimal control approach for the parameter in the virtual synchronous generator (VSG). This strategy considers the power angle curve and analyzes the principles of inertia and damping within the oscillation period and its range of values. By coordinating the values of parameters, it achieves the optimization of these parameters. At the same time, in order to take into account the demand for fast regulation response under normal operation control conditions and the demand for synchronization support under disturbance fault conditions, a multi‐mode switching control strategy including virtual synchronization control and conventional PQ control is further proposed. Based on MATLAB/Simulink software, the strategies mentioned above have been proven through simulation. The findings testify the proposed method can significantly enhance the regulation response characteristics of the VSG by optimizing its parameters. Additionally, it enables smooth switching of the control mode independently, based on the specific requirements of different control scenarios. This approach fully leverages the virtual synchronous support and fast regulation characteristics of the power electronic converter, and has practical engineering value. [ABSTRACT FROM AUTHOR]

Abstract The transition to renewable energy‐based power systems is fast progressing. One of the main challenges in keeping a power system with high operational reliability is to maintain the system frequency. As synchronous generator units are being replaced with power‐electronic converters, the rotating mass and the system inertia are decreasing. Virtual synchronous machine (VSM) control is a modern control technique that aims to compensate for the reduction in inertia. The usage of power electronic‐based converter units equipped with VSM control has to be managed and scheduled by system operators. An assessment of the operational frequency reliability is used to evaluate different service usages. A method is proposed that allows the comparison of different frequency management strategies. The proposed method uses fuzzy logic to evaluate the system risk for abnormal frequency and the system effort in the form of frequency control usage. This allows to quickly compare different frequency management strategies whilst keeping in mind many different reliability indices. The proposed method is validated with a modified IEEE Reliability Test System with integrated wind power capacity.

DC microgrid provides the horizontal infrastructures to integrate distributed generation (DG) and loads. Unlike traditional AC systems, DC systems cannot survive or sustain high magnitude fault currents. It makes locating faults very difficult. The conventional protection techniques completely de-energies the DC link in the DC microgrid. A new protection scheme for multi-terminal DC microgrid against line-to-line fault and the low resistance earth fault is presented in this study. The scheme isolates the faulted section from the DC microgrid. Healthy sections are operated without any disturbance and supply continuity is maintained in a ring main DC bus system. The current sensors are mounted at DC bus segments to monitor the entering and outgoing current at different nodes. Further, the current sensors are also mounted at both ends of service mains to monitor their current difference at both ends of the service mains. The controller detects this current difference and opens circuit breakers. To meet the requirement of fast interrupting time and high short-circuit current withstanding capability, insulated-gate bipolar transistors used as circuit breakers. The fault location scheme gives the fault location in various sections (service mains) and faults resistance in the microgrid. The proposed concepts have been verified by computer simulation.

With the large-scale configuration of distribution automation and the widespread implementation of distributed generation (DG) and flexible load (FL), the distribution network can achieve active regulation, which makes the traditional fault restoration strategy and distribution network reliability evaluation more complicated. Given this background, a method of reliability evaluation for the active distribution network considering ‘supply-grid-load’ is proposed. On the supply side, a distribution network islanding model considering DG is established, with the objective of supplying power to the maximised amount of equivalent power load. On the grid side, the influence of distribution network automation on the distribution network reliability is elaborately analysed and the time-varying failure rate model of equipment which considers the life cycle characteristics is presented. On the load side, an incentive-based FL model is proposed based on the elastic coefficient matrix. Finally, simulation results of RBTS-BUS6 system demonstrate the effectiveness of the proposed model and method.

Islanded microgrids (IMGs) are embedded power networks with distributed energy resources (DERs) providing a reliable and flexible energy option for off‐grid customers. This work addresses the planning model of renewable‐based IMGs feeding multi‐energy demands considering investment and emission related objectives. The proposed solution is to determine the optimal mix and sizing of various energy sources in IMG, including renewables; for multiple energy demands. This study also presents a hybrid‐scenario and Monte Carlo approach to gauge the uncertainty involved in multi‐energy demands, i.e. electrical, heating, and cooling loads; together with correlation among wind and solar generations. The spatial interdependence among renewable generations is implemented using copula; that generates a synthetic set of stochastic correlated data. The combined load scenarios for multi‐energy demands and renewable samples are implemented with the proposed hybrid approach in the formulated stochastic planning model. In this work, the formulated problem is proposed to solve using meta‐heuristic multi‐objective ant lion optimiser algorithm, that is validated on the test system. The superiority of the proposed approach is highlighted in comparison with other multi‐objective optimisers. The multi‐energy dispatch between associated sources and loads were simulated to show how the obtained capacity can suffice the seasonal multi‐energy demands of a typical day considered. [ABSTRACT FROM AUTHOR]

Sub‐synchronous oscillation (SSO) causes significant damage and performance degradation in wind farms (WFs). The root causes and mitigation methods of SSO have been identified as a result of many studies. An overview that analysed and summarised those findings in different perspectives helps optimise the existing solutions and to find alternative approaches to mitigate SSO. Therefore, a comprehensive overview of the SSO analysing techniques, mechanisms, and mitigation strategies are presented in this study. This overview is focused on the WFs based on type 3 and 4 wind turbine generators and the WFs connected to high‐voltage DC transmission systems. The dominant SSO modes in each application have been presented along with the identified root cause, influencing factors, and the mitigation methods. Besides that, a comprehensive comparison between the existing mitigation strategies is presented to identify the alternative approaches and improvements. The identified improved methods are presented along with the time and frequency domain simulation results to validate their applicability. Finally, an insight into the future direction of this research is presented along with the conclusions. [ABSTRACT FROM AUTHOR]

This study examines experiences of grid operators to successfully integrate very high penetrations of wind and solar photovoltaic (PV) resources. The variability of these resources creates challenges in balancing the system generation and demand, and ensuring resource adequacy and essential reliability services. The inverter‐based nature of wind and solar PV leads to challenges in frequency, transient, and small‐signal stability. In this study, seven system operators demonstrate the ability to manage these challenges in a variety of power systems, from stand‐alone island systems to larger island systems that are interconnected to neighbours, to balancing authorities that are strongly interconnected within very large synchronous systems. They operate within a variety of market constructs, from full regional transmission operators to vertically‐integrated utilities. All are experiencing increases in the penetration of inverter‐based, variable energy resources and finding creative solutions to these challenges. [ABSTRACT FROM AUTHOR]

This study presents a trajectory tracking control strategy that modulates the active power injected by geographically distributed inverter-based resources to support transient stability. Each resource is independently controlled, and its response drives the local bus voltage angle towards a trajectory that tracks the angle of the centre-of-inertia. The centre-of-inertia angle is estimated in real time from wide-area measurements. The main objectives are to stabilise transient disturbances and increase the amount of power that can be safely transferred over key transmission paths without loss of synchronism. Here the authors envision the actuators as utility-scale energy storage systems; however, equivalent examples could be developed for partially-curtailed photovoltaic generation and/or Type 4 wind turbine generators. The strategy stems from a time-varying linearisation of the equations of motion for a synchronous machine. The control action produces synchronising torque in a special reference frame that accounts for the motion of the centre-of-inertia. This drives the system states toward the desired trajectory and promotes rotor angle stability. For testing, a reduced-order dynamic model of the North American Western Interconnection is employed. The results show that this approach improves system reliability and can increase capacity utilisation on stability-limited transmission corridors. [ABSTRACT FROM AUTHOR]

The operation of a standalone microgrid is often optimised by the central controller that decides the set-points of the local controllers of the distributed generators. However, the optimised set-points may not remain optimal for a longer period due to the variability of loads and renewable generations. This study proposes a technique for readjusting the dispatch of the suitable generation units, between the optimisations carried out by the central controller, to support load changes. To this end, the potential field concept has been suggested to be adopted by the loads to select the appropriate generation units. The decision is made based on different criteria, such as their cost, reliability, emission, and power loss. This process requires low computational efforts, and thus, can act and make decisions immediately. This is valid as far as the change in the loads is not substantial. For a change above a predetermined level, the microgrid's central controller steps in to optimise the system. The central controller also performs the regular optimisation periodically to retune the whole system and reconfirm the optimal operation. The performance of the proposal has been evaluated by numerical analysis and validates the success and efficacy of the proposal. [ABSTRACT FROM AUTHOR]

In this study, a passive islanding detection technique based on the variation in the angles of superimposed sequence components of voltage and current is proposed. The pre‐ and post‐islanding voltage and current superimposed components are derived using the corresponding voltage and current phasors. The angle between the positive and negative sequence voltage and current and the superimposed components of these quantities are derived to take further action. Furthermore, decisions obtained from the angles of different superimposed components are combined using a voting technique to take the final decision with more reliability and accuracy. The scheme does not require the establishment of any threshold and completely relies on the rule base obtained through a simulation study. A microgrid with multiple connection points is simulated using a real‐time digital simulator (RTDS). A hardware‐in‐the‐loop test bench is arranged using a dSPACE DS1104 and RTDS. Numerous test cases are simulated to assess the performance of the proposed technique. [ABSTRACT FROM AUTHOR]

Decline in photovoltaic (PV) output power is observed due to aging factors such as solder bond failure, corrosion of busbars, formation of cracks in solar cell, failures of bypass diode etc. Furthermore, these happenings reflect changes in PV module parameters such as increment in series resistance/reduction in open‐circuit voltage and/or decrease in fill factor etc. However, these parameter variations can be easily examined with the module level I–V curve. This study proposes a new approach of exploring the I–V curve of the PV module using an inverter pre‐startup condition, i.e. just before sending the PV power to the grid. From this pre‐startup I–V curve, one of the important parameters, i.e. module series resistance is estimated. The proposed method is investigated through simulations in MATLAB/Simulink and is experimentally tested on a PV output coupled to a grid‐connected micro‐inverter. The key advantage of the proposed method is that it does not require any additional circuitry/sensors to extract the I–V curve. Furthermore, there is no need to disconnect the PV from its normal operation. [ABSTRACT FROM AUTHOR]

Preservation of wind turbines (WTs) grid‐connectivity during grid faults and grid‐code (GC) compliant reactive power injection at PCC during voltage drops is an imperative task to perform in modern WTs. This is known as the low‐voltage ride‐through (LVRT) capability of WTs, emerging as an integral GC requirement. Despite current provision of LVRT in PMSG‐based WTs, there is still likelihood of grid voltage drops leading to adverse effects in wind power plants. In this research, a peak current limiter has been designed for machine‐side converter (MSC) of the PMSG‐based WT to execute GC requirements in a reliable manner. This scheme is capable of preventing over‐voltage across the dc link of back‐to‐back (BTB) converter and over‐current in the grid‐side converter (GSC). A dual current controller is utilised for regulating GSC positive‐ and negative‐sequence components. A prominent feature of the proposed controller is its simplicity and applicability to available BTB control systems. On the other hand, the WT mechanical system operates as a storage device during voltage drops, eliminating the need for installing external apparatus such as energy storage systems and braking choppers across the dc‐link. Simulation results conclude the reliable operation of the WT equipped with MSC current limitation scheme during grid faults. [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]

To achieve clean and sustainable energy, the demand for renewable energy has been increasing day‐by‐day. As it is known the conversion efficiency of PV cells is very less, which motivates further research in the development of PV systems. Incorporating the power converters of less cost, more life‐time, compact size, and preferably low complexity will address the above‐mentioned limitation. To achieve optimum performance from PV systems for different applications especially in interfacing the utility to renewable energy sources, choosing an appropriate grid‐tied inverter is crucial. The different types of PV inverter topologies for central, string, multi‐string, and micro architectures are reviewed. These PV inverters are further classified and analysed by a number of conversion stages, presence of transformer, and type of decoupling capacitor used. This study reviews the inverter topologies for all PV architectures, which is new of its type. All the parameters such as merits, demerits, complexity, power devices of the aforementioned PV inverter are drafted and tabulated at the end of every classification. Different control strategies for balanced and unbalanced grid integration such as dq, αβ, abc, fault ride through, and unified power flow control are discussed. This review would be helpful for researchers in this field to select a most feasible inverter for their application, as this study reviews considerable number of PV inverters on one platform. [ABSTRACT FROM AUTHOR]

Different energy systems are generally planned and operated independently, which result in the low energy utilisation, weak self‐healing ability and low system reliability. Therefore, an adaptive clustering‐based hierarchical layout optimisation method is proposed for a large‐scale integrated energy system, considering energy balance, transmission losses and construction costs. First, an adaptive clustering partition method based on energy balance and load moments is proposed to determine the optimal location of energy hubs and to allocate each distributed generation and load to different energy hubs, forming multiple regional integrated energy systems adaptively. Then, the proposed hierarchical layout optimisation model is formulated as to find the modified minimum spanning tree of the regional integrated energy system and multi‐regional integrated energy systems, respectively, to construct an economical and reliable interconnection network. Finally, the effectiveness of the optimisation model and strategy is verified by simulations. [ABSTRACT FROM AUTHOR]

Standalone DC microgrids (SDCMGs) are emerging as prominent solutions to remote customers. As these SDCMGs mainly depend on renewable energy sources to curb carbon emission, reliability of the system is lessened due to their intermittent nature. The battery could offer a solution to this problem as it is inherently enclosed by DC grid for power balancing, but the purpose may not be served completely due to high capital cost and maintenance. Thus, the diesel generator (DG) becomes a major alternative to overcome this issue because of low investment, high compatibility, and flexibility. However, DG inhibits with cranking delay, sluggish response and low fuel efficiency under frequent switching and variable loading scenarios. To suppress the issues, a new power management strategy (PMS) is developed to ensure proper coordination between different sources, storage devices and loads in SDCMG. In addition to this, an effective control scheme is also proposed to achieve seamless regulation of DC bus voltage even under extreme conditions. In this study, different SDCMG configuration is considered for testing and simulation of system is carried out in real‐time digital simulator to prove their viability. A scaled prototype is developed to validate simulation results and authenticate proposed PMS and control scheme. [ABSTRACT FROM AUTHOR]

Security‐constrained unit commitment (SCUC) is one of the most important optimisation problems for operation planning of power systems. Indeed, with the fast expansion of power grids and the increasing growth of heterogeneous market participants in electricity markets, the day‐ahead SCUC continuously encounters significant performance challenges. To improve the computational performance of SCUC and achieve solutions of good quality, an integrated framework which combines a data‐driven approach and a variable‐aggregation method is presented in this study. The variable‐aggregation method effectively approximates the original network security constraints with a reduced number of variables. Moreover, as aggregated constraints could reduce the feasible region of the original SCUC and potentially degrade solution quality for certain SCUC instances, it is preferable to only apply such an aggregation approach towards difficult SCUC instances. Therefore, a data‐driven classification method, by adopting relevant SCUC input data as features, is integrated to first predict whether a SCUC instance is 'easy' or 'hard'. To this end, the integrated framework could improve the overall performance of SCUC instances, by and large, in terms of computational efficiency and solution quality. Numerical results illustrate the effectiveness of the proposed integrated framework. [ABSTRACT FROM AUTHOR]

With the capacity increase of large‐scale DC renewable energy and DC loads, the reliability issues of hybrid AC/DC networks become increasingly prominent. On the other hand, a modern distribution network should also deliver power to individual customers based on their priority levels and the traditional reliability indices are not suitable for modern networks. Flexible reliability (FR) is a new concept that considers the loads' importance in the reliability evaluation of distribution grids. In this study, a novel comprehensive FR index is introduced for hybrid AC/DC networks. To determine the proposed index, power networks are modelled as a topological combination of edges and nodes. An axiomatic design concept which is a systematic binary matrix‐based approach is then employed for topology analysis. The proposed index is based on the systematic counting of feasible paths from generators to loads, considering loads' importance. The effectiveness of the proposed index is demonstrated through an illustrative example and IEEE 33‐bus distribution system. [ABSTRACT FROM AUTHOR]

This work deals with the implementation, control and operation of a reconfigurable small hydro‐solar photovoltaic (PV)‐battery energy storage (BES)‐based distributed generation system (DGS) working in isolated mode (IM) and grid connected mode (GCM). The DGS consists of a hydro generation‐based on a PMSG integrated with a PV array supported by the BES. It operates in an IM during the grid outage and when the grid is available, it synchronizes and operates in GCM. A frequency adaptive comb filter‐frequency locked loop (FLL)‐based control is adopted to estimate errorless phase angles of the hydro generator and grid voltages for quick and accurate grid synchronization. A third‐order band pass filter‐based control technique is used to extract fundamental component of load currents and determine switching pulses to VSC. The overall multi‐objective control is used to maintain voltage and frequency of the system at the point of common coupling during IM and GCM. The power quality of grid and generator currents are maintained at non‐linear loads. The modified perturb and observe‐based control technique is used to extract maximum power from the PV array and it provides drift‐free operation during the change in insolation with de‐rating capability to protect the BES from overcharging in IM. [ABSTRACT FROM AUTHOR]

The generated power of the Sea‐wave slot‐coned generator (SSG), as one of the high capacity wave energy converters, is dependent on the period and height of the waves. The generated power of this energy‐converter changes due to the variation in these parameters. Thus, different aspects of the power systems may be affected that must be investigated by increasing the share of these converters for electricity production in the power‐systems. In this paper reliability assessment of composite power‐systems containing large‐scale SSG is performed. The Xie and Beni (XB) index is calculated for obtaining the optimum number of states in the reliability model of the SSG and fuzzy c‐means clustering technique is proposed for reducing the number of states of the model. The contingency analysis method is proposed for calculating the reliability indices of the composite power‐system incorporating large‐scale SSG. DC load‐flow method, which considers line capacity, is used to perform load‐flow in the composite power‐system including generation and transmission networks. In the proposed technique, a load‐shedding program with the goal of minimizing the value of lost load is solved using of the linear programming method based on the interior‐point‐approach for contingency states leading to the load curtailment. [ABSTRACT FROM AUTHOR]

Optimal placement of protective devices and switches in the distribution system improves the system reliability at the cost of increased investment. In this study, a new bi‐directional analytical model has been developed to solve the problem of optimal placement of protective devices and switches in various zones of a distribution system with distributed generation (DG). The model determines the optimal number and locations of reclosers, switches, 'fuse‐save fuses' and 'fuse‐blow fuses' for increasing the system reliability while reducing the investment and outage costs, considering system data uncertainties. For considering uncertainties in load data, failure rates (temporary and permanent) and repair rates, the model utilises three‐point estimate method. The formulated problem has been solved for 69‐bus distribution system and radial distribution system connected to bus 4 of the reliability busbar test system, using genetic algorithm optimisation technique. After analysing the test results, it is concluded that considerable profit to the utility is achieved by optimal placement of protective devices and switches in various zones of the distribution system with DG units. [ABSTRACT FROM AUTHOR]

Traditionally, power system's inertia has been estimated according to the rotating masses directly connected to the grid. Due to the significant penetration of renewable generation units, the conventional grid inertia is decreasing, subsequently affecting both reliability analysis and grid stability. As a result, concepts such as 'synthetic inertia', 'hidden inertia' or 'virtual inertia', together with alternative spinning reserves, are currently under discussion. Under this new framework, an algorithm to estimate the minimum inertia needed to fulfil the European network of transmission system operators for electricity requirements for rate of change of frequency values is proposed and assessed. Both inertia and additional active power can come from different sources, such as storage solutions, renewable sources decoupled from the grid, interconnections, or a combination of them. The power system under consideration includes thermal, hydro‐power plants, and renewable generation units, in line with the most current and future European power systems. More than 700 generation mix scenarios are simulated, varying the renewable integration, the power imbalance, and the inertia constant of conventional power plants. The solutions studied here provide important information to ease the massive integration of renewable resources, without reducing the grid capacity in terms of stability and response to contingencies. [ABSTRACT FROM AUTHOR]

The operation and maintenance activity of off‐shore wind turbines (WTs) increases the cost of the generated energy. Although significant efforts have been made to improve the reliability of the mechanical subassemblies, electrical and electronic subassemblies fail more frequently, causing undesirable downtimes and loss of revenues. Since off‐shore WTs and wave energy converters (WECs) share the electrical and electronic subassemblies, the reliability of WECs is expected to be affected by the same causes. This study presents a robust model predictive control for a WEC consisting of an oscillating water column (OWC) installed in a point absorber. The control system is capable of dealing with open switch faults in one or two insulated‐gate bipolar transistors of the same arm in any of the voltage source converters (VSCs), or even in both VSCs at the same time. The system allows the OWC WEC to generate energy, although under certain restrictions, thereby reducing the urgency of repair and loss of revenues. The performance of the proposed approach is tested for several cases of open switch faults, experimentally in the laboratory using an OWC WEC emulator. [ABSTRACT FROM AUTHOR]

The droop control strategies can realise an autonomous power allocation among virtual synchronous generators (VSGs) that can provide the recompenses of reduced system complexity and enhanced reliability. The stability of a low‐inertia microgrid can be improved by implementing a VSG, designed for the coordination between multiple variable electric vehicle (EV) based charging loads. This study mainly investigates two control schemes. Firstly, the instantaneous contribution from EV charging control for any disturbance that can provide adequate damping and inertia to a low‐inertia microgrid without degradation of the battery. Secondly, the adjustment of Q –V droop control is suggested by correcting the excitation voltage of a VSG. This method can reduce the influence of line impedances and power ratings on reactive power sharing in a multi‐VSGs system. Additionally, four different active and reactive power control modes of VSG are discussed to emphasise EV charging and discharging control through the VSG controller. These modes are explained through circuit and vector diagrams in direct and quadratic coordinates. The efficacy of the proposed strategies and their influence on power sharing is theoretically demonstrated and analysed. Finally, the theoretical results are validated through extensive simulation and experimental verification. [ABSTRACT FROM AUTHOR]

This study is interested in optimisation of both sizing and energy management system of a hybrid storage system (HSS) associated with photovoltaic panels. The battery (BT) considered as the principal storage organ and a super‐capacitor used as the secondary storage system to improve the BT life span makes up the HSS. The main purpose of this study is to explore a novel optimisation approach to jointly optimise the sizing and the fuzzy logic energy management system (FLEMS). In fact, an optimisation function based on sequential quadratic programming algorithm is proposed. The optimisation methodology has been performed jointly and successfully for the sizing of the BT storage system and the membership functions parameters of the FLEMS in order to decrease the levelised cost of energy with a violation time by 5% of mean absolute percentage error score <1.5% throughout the year. According to the simulations results, a benefit analysis has been done to assess the associated financial impact. [ABSTRACT FROM AUTHOR]

Distributed generators bring in promising benefits as well as question the efficacious relaying operation of the conventional radial distribution systems on the penetration. Among different types of interface machines, synchronous generators upon increased penetration create major disruptions to the existing operation of relays and circuit breakers due to their high fault current capability. Hence, this work employs reconfiguration as a strategy to mitigate the protection issues imposed by a highly proliferated DG network. As restructuring also has the capability to create relay miscoordinations, a vigilant identification of optimal structure is carried out. 22.9% loss reduction, 24.3% reduction in the sustained interruption hours faced by an average consumer and 35.8% decrease in the maximum electricity price paid by the consumer at a load point are achieved for the practical 52-bus microgrid while attempting to reconfigure in view of addressing the protection challenges. 21.84% drop is observed in total size of fault current limiters as well. 33 and 69 bus benchmark systems transformed to a microgrid are also employed to validate the proposed reconfiguration strategy. New tie-line combinations are searched for each test system in a view to look for a superior structure giving out best results in the context of study. [ABSTRACT FROM AUTHOR]

Hybrid renewable energy systems (RESs) with storage are gaining significant attention for off‐grid communities. Planning of reliability constrained social‐economic analysis is a combinatory optimisation problem. This work focuses on the reliability constrained optimisation of the levelized cost of energy (LCOE) of RES with the storage considering the cost of emission. Hybrid energy systems consist of photovoltaic (PV), wind turbine (WT) and diesel generator (DG) with storage. In this work, a systematic approach for determination of reliability constrained optimal combination of generating and storage technologies based on techno‐socio‐economic criteria has been developed. To evaluate the reliability and the expected generation from the individual unit, probabilistic production costing simulation has been performed using an analytical technique. The optimal sizing of RES with storage has been evaluated using particle swarm optimisation technique and tested on ten different cases. The combination of generation/storage technologies that fetch minimum LCOE while satisfying the reliability standards is chosen as the optimal configuration. Results show that a combination of PV, WT and pumped storage hydro is the best option to meet the energy requirements, with the least LCOE of 0.268 $/kW while satisfying reliability criteria. Further, an optimum generation expansion plan has also been determined for the aforementioned optimal configuration. [ABSTRACT FROM AUTHOR]

With the large-scale configuration of distribution automation and the widespread implementation of distributed generation (DG) and flexible load (FL), the distribution network can achieve active regulation, which makes the traditional fault restoration strategy and distribution network reliability evaluation more complicated. Given this background, a method of reliability evaluation for the active distribution network considering 'supply-grid-load' is proposed. On the supply side, a distribution network islanding model considering DG is established, with the objective of supplying power to the maximised amount of equivalent power load. On the grid side, the influence of distribution network automation on the distribution network reliability is elaborately analysed and the time-varying failure rate model of equipment which considers the life cycle characteristics is presented. On the load side, an incentive-based FL model is proposed based on the elastic coefficient matrix. Finally, simulation results of RBTS-BUS6 system demonstrate the effectiveness of the proposed model and method. [ABSTRACT FROM AUTHOR]

With increased penetration of converter interfaced generation (CIG) in power systems, the suitability of modelling approaches and models dependent on the system conditions has to be studied. However, their suitability has yet to be systematically analysed. In this study, the authors first develop a methodology for identifying the maximum penetration of aggregated rooftop sources that can be represented as negative loads in both the steady state and dynamic analysis. Then they verify the suitability of a positive‐sequence performance‐based model and a detailed electromagnetic transient (EMT) model for utility‐scale photovoltaic (PV) plants under different fault conditions. Based on the results, they perform dynamic simulations on a large practical system with both positive‐sequence transient stability (TS) and EMT‐TS hybrid simulation. This study shows that positive‐sequence performance based PV plant model is more suitable for study cases with faults relatively far away from the point of connection. In addition, the results indicate that positive‐sequence models and TS simulation approach can produce comparable results with reference to the EMT‐TS hybrid simulation with EMT detailed models for normally‐cleared single‐line‐to‐ground faults, but the inadequacy of positive‐sequence based model and simulation approach becomes obvious for prolonged‐tripping of unbalanced faults in the proximity of the PV plants. [ABSTRACT FROM AUTHOR]

This study proposes a two‐stage robust optimisation framework to model uncertainties in construction cost, load and renewable power generations (RPGs) in the coordinated transmission and reactive power infrastructure expansion planning problem. The proposed planning model, based on the network AC model and its voltage constraints, is represented as a mixed‐integer linear programing problem with the aim of minimising: (i) the construction costs of lines, (ii) installation cost of reactive power resources and (iii) active load shedding as RPGs curtailment. Using the suggested robust optimisation approach, the best expansion plan is obtained even in the worst case of the defined uncertainty sets and the plan robustness level is controlled via parameter definition for the individual uncertainty sets. The Benders decomposition algorithm is used to solve the two‐stage problem where the plan security level is also taken into consideration using the N−1 contingency criterion. The standard Garver 6‐bus system and the IEEE 24‐bus reliability test system are used to validate the proposed robust framework. [ABSTRACT FROM AUTHOR]

Renewable energy generation exhibits a distributed development trend, and has changed the way distribution networks operated. Due to the integration of large‐scale distributed photovoltaic (PV) generation cluster and the access of electronic devices, distribution networks are suffering from more and more serious voltage risks. By using appropriate methods, PV inverters can autonomously regulate reactive power output in a distributed manner to improve voltage profile in networks. In this paper, a distributed Newton‐based voltage control method for large‐scale PV generation cluster in distribution networks is presented to realize distributed coordination of PV inverters, which is based on matrix splitting and approximate Newton iteration, and can fast respond to reactive power mismatch and realize voltage profiles optimization. Exhibiting a more efficient, reliable and flexible performance than existing decentralized methods in IEEE test systems with different PV penetration, the proposed method is demonstrated to be effectively to organize and control PV cluster to realize fast voltage optimization in distribution networks. [ABSTRACT FROM AUTHOR]

The operation of multiple microgrids (MGs) with distribution system enhances reliability and resiliency of the power supply significantly by enabling high penetration of locally available distributed energy resources (DERs). However, there are several issues in the proper operation of multiple MGs that need to be addressed. Providing proper protection coordination is one of the most important issues in the coordinated operation of networked MGs (NMGs). In this study, an efficient protection coordination scheme for NMGs is proposed by utilising the commonly used numerical directional overcurrent relays (DOCRs) with single and dual settings. Issues related to the operation of NMGs such as the change in network topology, connection status of the point of common coupling and low voltage fault ride through of DERs have been considered. The protection coordination problem has been formulated as an optimisation problem and solved using the interior point method. The proposed protection coordination scheme of NMGs has been tested on the 69-bus radial distribution system with the integration of three MGs, which is not reported in earlier literature. The suitability and effectiveness of the proposed DOCRs coordination scheme have been demonstrated. [ABSTRACT FROM AUTHOR]

This study presents a new methodology to determine the optimal Offshore Wind Farms connection point in a power system, with the purpose of maximising the wind power penetration, considering a Benders decomposition approach to represent the security constraints related to the ' $N-1$N−1 ' contingency criterion. The proposed algorithm uses a mixed-integer non-linear programming approach associated with a non-linear programming, and it is evaluated in a tutorial system (IEEE 14-Bus) and two networks from the literature: a small case (IEEE RTS-79), and a medium and real case (Brazilian Birds System). In each case, three points geographically close to each other are selected as candidates in the onshore grid for the wind power injection. The experiments conducted clearly show that the connection decision from the proposed approach led to the maximum safe wind power penetration among the candidate options, which proves its applicability and effectiveness, even for more complex networks, and considering all single transmission line contingencies for safe operation. [ABSTRACT FROM AUTHOR]

The short‐circuit current characteristics fed by the renewable energy source are quite different from those of the synchronous generator, which may lead to the phase selection error or failure of the traditional fault phase selection component, and then affect the operating reliability of the distance protection and the automatic reclosing. This study analyses the existing issues in the application of traditional phase selection components to the tie‐line of renewable energy power, according to the fault characteristics of different renewable energy sources such as photovoltaic power source, direct‐drive wind turbine and doubly‐fed wind turbine. Furthermore, the sequence voltage expressions in the case of different types of faults, which takes account of the influence of the renewable energy type and the transition resistance, are theoretically derived. Hereby, a novel phase selection method by using sequence‐voltage phase comparison and phase‐voltage amplitude comparison is proposed. This method is applicable to the tie line of different types of renewable energy power station, and possesses the superiorities of high reliability and strong tolerance towards large fault transition resistance. Digital simulation results verify the favourable performance of the proposed method. [ABSTRACT FROM AUTHOR]

The desire to modernise electrical infrastructure, and the increasing number of extreme weather events, has led to an increased interest in deploying microgrids as resiliency resources. While there are times when a microgrid is operated to increase the resiliency of critical end‐use loads, there are also other operational goals, such as efficiency, that may be considered. However, it can be challenging for microgrid operators to coordinate assets in real time to balance operational objectives. This study presents a slider‐based multi‐objective control that enables microgrid operators to change the operational set points of microgrid assets to respond to rapidly changing system conditions. [ABSTRACT FROM AUTHOR]

This study proposes a composite generation and transmission expansion planning (CGTEP) with high renewable penetration in Iran power grid. The goal is to minimise the generation and transmission investment and operation costs, start‐up/shut‐down costs, and the cost of energy not served during a 10‐year planning horizon. The problem is formulated in a mixed‐integer linear format, considering power flow limits, ramping rate limits, minimum up/down times of the generation units, uncertainty of wind and solar generations, and deterministic and probabilistic reliability constrains including reserve margins for generation capacities and expected energy not supplied boundary. In addition, according to the specific geographic characteristics of Iran, import and export of the electricity, as well as the limitation of water resources for power generation, is modelled in the CGTEP problem. The proposed CGTEP method is implemented in business as usual and renewable target cases without considering the uncertainty, and RT case with uncertainty considerations. In RT cases, 30% renewable penetration is targeted to reduce the CO2 emission by 10%. In addition, the capability of Iran power grid in providing green energy for the neighbouring countries is investigated in this study. [ABSTRACT FROM AUTHOR]

A microgrid (MG) is a cluster of small‐scale sources known as micro‐sources, the energy storage devices, and the loads. MGs are promising entities that allow a considerable amount of renewable energy penetration into the system. A stochastic optimisation model is presented in this study to find the optimal size of the distributed energy resources (DERs). The uncertainties are considered with the help of multiple scenarios of random variables viz. wind output, solar output, and load demand data. A particular case of Beta distribution is used to create these variations. A cost‐based, multi‐variable constrained non‐linear objective function is formulated. The considered test MG system is more generalized with solar and wind energy penetration. The results show the effect of operational as well as the planning aspects together under various reliability conditions for an isolated MG. The formulated problem is solved to obtain a global optimal solution using a sequential programming approach and a comparative assessment of results with a hybrid particle swarm optimisation (PSO) approach has also been presented. It is found that the results obtained from the sequential programming approach are better compared to the hybrid PSO approach. [ABSTRACT FROM AUTHOR]

The increasing penetration of distributed generations enables an innovative operation paradigm that allows islanded operation to enhance the resilience of the distribution grid. In this study, a cyber‐physical oriented islanding strategy is proposed by coordinating centralised and distributed control to achieve seamless islanding transition and operational flexibility in emergency conditions. A cyber‐physical control structure is developed to mitigate various disturbances (e.g. emergencies or fluctuations) according to different operation conditions. Specifically, the distributed fault isolation and seamless islanding transition are coordinated to mitigate the outage caused by unplanned islanding, while a secondary control is proposed to support primary control by reducing the power fluctuations during islanded operation. With a rapid response speed, the local cyber‐physical devices are coordinated to accomplish islanding separation by selecting a feasible islanded area even under an unplanned islanding situation. A field test was conducted on a practical distribution network in China, and the results demonstrated the effectiveness and feasibility of the proposed islanding strategy. [ABSTRACT FROM AUTHOR]

The connection of renewable energy sources to local low‐voltage networks is becoming more accepted as electrical power networks progress to higher renewable penetration. Renewable energy resources, for example, wind and solar are highly dynamic and intermittent compared with more traditional generation sources, which imposes increasing challenges to the electrical network operator in terms of effectively managing the resource to maximise energy transfer and maintaining system stability. Therefore, transient energy storage systems (TESSs), for example, electrochemical batteries with fast charging/discharging capabilities are suitable candidates to improve the availability and reliability of connected renewable systems. In this study, two potential TESS technologies are presented, the lithium‐ion (Li‐ion) and sodium–nickel chloride (NaNiCl2) battery, and their feasibility to improve power systems in terms of power delivery and frequency fluctuations are compared. Experimentally validated battery models are presented and used to investigate the TESS performance in terms of state‐of‐charge, terminal voltage variation, peak current, power, energy and efficiency. The models and general design procedure may be applied to systems of different ratings and duty variations. [ABSTRACT FROM AUTHOR]

The nature of wave resources usually requires wave energy converter (WEC) components to handle peak loads (i.e., torques, forces, and powers) that are many times greater than their average loads, accelerating equipment degradation. Moreover, due to their isolated nature and harsh operating environment, WEC systems are projected to possess high operations and maintenance (O&M) cost, i.e., around 27% of their leveled cost of energy. As such, developing techniques to mitigate these costs through the application of condition monitoring and fault tolerant control will significantly impact the economic feasibility of grid connected WEC power. Toward this goal, models of faulty components are developed in the open source modeling platform, WEC‐Sim, to estimate the performance and measurable states of a WEC operating with likely device and sensor failures. Two types of faulty component models are then applied to a point absorber WEC model with basic controller damping and spring forces. Resulting changes in device behavior are recorded as a benchmark, and a graph‐theoretic approach is proposed for fault detection and identification utilizing multivariate time series. Simulation results demonstrate that these faults can greatly affect the WEC performance, and that the proposed method can effectively detect and classify different types of faults. [ABSTRACT FROM AUTHOR]