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This study, based on a novel control strategy, proposes a sizing method for battery energy storage systems (ESSs), which makes the wind power system more dispatchable. The main objective of the proposed control‐based sizing method is to facilitate robust unit commitment by smoothing the output power of wind according to a desired reference. Owing to the energy conversion loss, the controller closely monitors the battery state of charge (SOC) to prevent the battery from being fully discharged during the smoothing procedure. The proposed controller can automatically revise the battery dispatching reference maintaining the SOC constant. In each dispatch interval, model predictive control is used in real time to ensure that the output power follows the generated reference. The ESS is sized according to the simulation results solved by the proposed control strategy. Meanwhile, the performance of the proposed control strategy is validated over actual wind power data. [ABSTRACT FROM AUTHOR]

As the progress of electrification for the public transportation sector is accelerated, it becomes more and more important to integrated planning charging infrastructure for public transportation, while emerging technologies such as distributed renewable generation and energy storage system should be accounted. Here, an integrated resource planning framework is proposed where both planning investment cost and operational cost are considered. An aggregation strategy is also proposed to optimise the charging decisions for electric bus on different routes, which could effectively improve the planning and operation efficiency. To address the uncertainties involved in this process, a scenario‐based chance‐constrained programming approach is adopted. A numerical case study is also included to demonstrate the effectiveness of this proposed resource planning model. [ABSTRACT FROM AUTHOR]

The non‐preventable ever‐increasing rate of wind power generation in market‐based power systems faces the operators with challenging situations for making optimal decisions. So, it is essential to equip the operators with applicable control strategies and further corresponding control facilities. Moreover, the high‐priority of cheap wind power utilisation increases the probability of transmission lines congestion. Therefore, different solutions such as transmission switching (TS) and demand response (DR) programs have been recently introduced to manage the intermittent wind power generations. Accordingly, this study addresses the social welfare maximisation problem with coordinated control of TS and DR facilities to handle the regarding uncertainties using yet another linear matrix inequality parser (YALMIP). In fact, rapid algorithm and powerful employed solvers as well as simplicity of use, make YALMIP a practical modelling and optimisation toolbox. In this respect, the MOSEK solver is preferred by YALMIP to solve the proposed mixed integer linear programming problem. In addition, wind power uncertainty is modelled using the discrete‐time Markov chain approach and optimisations are performed on the 8‐bus and the large‐scale IEEE 118‐bus test systems. Results show that the proposed control strategy is highly capable of maximising social welfare by determining the optimal control commands in a real‐time manner. [ABSTRACT FROM AUTHOR]

The modernisation of the world has significantly reduced the prime sources of energy such as coal, diesel and gas. Thus, alternative energy sources based on renewable energy have been a major focus nowadays to meet the world's energy demand and at the same time to reduce global warming. Among these energy sources, solar energy is a major source of alternative energy that is used to generate electricity through photovoltaic (PV) system. However, the performance of the power generated is highly sensitive on climate and seasonal factors. The unpredictable behaviour of the climate affects the power output and causes an unfavourable impact on the stability, reliability and operation of the grid. Thus an accurate forecasting of PV output is a crucial requirement to ensure the stability and reliability of the grid. This study provides a systematic and critical review on the methods used to forecast PV power output with main focus on the metaheuristic and machine learning methods. Advantages and disadvantages of each method are summarised, based on historical data along with forecasting horizons and input parameters. Finally, a comprehensive comparison between machine learning and metaheuristic methods is compiled to assist researchers in choosing the best forecasting technique for future research. [ABSTRACT FROM AUTHOR]

Prediction of the transient stability in real‐time can be a boon in preventing the occurrence of catastrophic events in a particular area that could potentially affect the entire grid and cause blackouts. Synchronised voltage angle measurements procured from the installed phasor measurement units in the network have been used to devise an index for assessment of transient stability in the system. The behaviour of the trend of the acceleration at the instant of the critical clearing angle has been used in this study as the basis for differentiating between a stable and an unstable power swing. Upon detection of an unstable condition, the generator, which is most likely to be going out of step is detected, and appropriate control action is suggested for mitigating the predicted out of step condition. Moreover, the distance relay maloperations caused during a stable power swing can also be a reason for triggering cascading outages. Suitable remedial measures to avoid such maloperations of the distance relay have been proposed. The efficacy of the proposed philosophy has been tested on a single machine infinite bus system, WSCC‐9 bus system and New England 39 bus system modelled in Power Factory/DIgSILENT. [ABSTRACT FROM AUTHOR]

This study aims to present an architecture for the planning of microgrids (MGs) in order to support system operator decision. In short, the proposed strategy is an iterative procedure that tries to find the optimal size of distributed energy resource (DER), which attends the necessities of stakeholders. The architecture has five distributed and correlated stages named MG coordination, MG operation optimisation, reliability assessment, contingency assessment, and searching mechanism. Since the DER selection involves multiple criteria and interests of different parts, it requires a multi‐attribute decision system providing a list of possible configurations based on their relative importance as denoted by the stakeholders. Owing to that, the particle swarm optimisation is used to create the multidimensional space of search in which the optimal solution will be selected by means of Pareto front decision criteria. As a result, the architecture provides a candidate solution to optimal size (optimal rated power) of each DER, that must be installed in the MG in order to have an optimal balance between technical, economical, social, and environmental aspects. To have realistic results, such a strategy is performed on a case study of a potential campus MG program. [ABSTRACT FROM AUTHOR]

This study scrutinises the impacts of demand‐side resources (DSRs) on the power system reliability via a novel multi‐target generation maintenance (GM) model. The nominated model uses the lexicographic preferences to hierarchically consider the environmental issues, economics, and reliability of power systems. In this regard, the produced emission, which reflects the per unit produced pollutant values in different locations, is minimised. Taking into account the environmental constraints, the total incurred expenditures, including operating and maintenance costs, reserves costs, and total incentives are also minimised. Subsequently, the GM problem considering the correlation constraints is handled while the reliability index, the average net reserve value, is maximised over the scheduling horizon. The DSRs improve the system reliability such that the total costs, and emission level, do not exceed the situation in which DSRs are not available. The GM scheduling is a highly complicated problem and considering DSRs makes it even more complicated. To handle this problem more efficiently, appropriate linearisation technique is used, while the proposed model is formulated in GAMS modelling language. To evaluate the capability of DSRs in system reliability improvement, the modified 24‐bus IEEE‐RTS is conducted. Results indicate that by selecting proper location and incentives, significant improvement is obtained. [ABSTRACT FROM AUTHOR]

This article addresses a critical issue on the robust unit commitment (RUC), which is the construction of an accurate and reliable uncertainty set for solar photovoltaic (PV) systems. The classic robust unit commitment (CRUC) provides an inaccurate and unreliable model for the highly intermittent solar generations. However, the authors' proposed uncertainty set utilises the forecast models that rely on the clear‐sky and overcast solar forecasts, which are more accurate and reliable. The uncertainty set for solar generations is constructed based on the type of day, levels of daily uncertainty index (DUI) and daily energy index (DEI), and the uncertainty level of solar ramps. The test results on the IEEE 118‐bus test system demonstrate that (i) using DEI and DUI reliably and efficiently manages and reduces the cost for highly uncertain and overcast day compared to the CRUC and (ii) the RUC cost is much sensitive to the selected level of DUI and DEI than the uncertainty level of the solar ramps. [ABSTRACT FROM AUTHOR]

In the background of the energy Internet, the number of data centres is increasing, aiming at the shortcomings of power supply and reliability in power supply system of traditional data centre and the development trend of green data centres. This article proposes an AC & DC hybrid power supply system with high-proportion renewable energy. The system includes photovoltaic generation, wind power generation, photothermal power generation, thermal utilisation system, power storage system, and different types of loads. Therein, a multi-port power electronic transformer is used to realise AC & DC multi-level mixing system with high proportion of variety of distributed renewable energy. Thereafter, the power supply, power grid, and the load can efficiently complement each other by using the integrated energy storage system such as electricity storage and heat storage. In detail, the rationality and functionality of the AC & DC hybrid power system are analysed through the system simulation and power supply reliability calculation. Moreover, the results show that the proposed hybrid power supply can effectively improve the impact of high-proportion renewable energy access to the grid and provide a new design scheme for the green data centre. [ABSTRACT FROM AUTHOR]

The main cause for cascading failure is large-scale overload caused by localised abnormal flows. With the increase of wind power penetration, the complexity of flow distribution is strengthened. Therefore, it is necessary to study influences of site selection and permeability of wind power generation on cascading failure. An analysis method based on electric betweenness of nodes was put forward, and influences of permeability change on cascading failure on different location selection for wind power generation were researched. According to the study, it was found that when the electric betweenness was low, the overload of wind power led to declining uniformity of power flow distribution, resulting in transfer of flows to other lines. As a result, the heavy load of lines was further aggravated to promote the development of cascading failure. Simulation results of the IEEE30 system showed that with the increase of wind power penetration, the cascading failure was aggravated; and under the same permeability, the lower the electric betweenness of location selection was, the severer cascading failure would be. [ABSTRACT FROM AUTHOR]

As the environmental problems caused by the consumption of the non-renewable fossil energy are getting increasingly serious, the new energy power generation has received wide attentions. Multi-terminal high voltage DC (HVDC) transmission is one of the key technologies for improving the reliability and stability of new energy power generations. Besides, HVDC circuit breakers (CBs) become the bottleneck of the development and application for multi-terminal HVDC transmission power systems. Here, the active DC vacuum circuit breaker (VCB) is taken as the research object. Meanwhile, the optimisation design of the commutation circuit for the DC VCB is realised. According to Kirchhoff's law, the genetic algorithm is used to calculate the commutation parameters under different optimisation objectives, to obtain the lowest cost and the shortest sheath development time. Based on the software package MATLAB, the simulations and analyses of different optimisation objectives and parameters are carried out. Through the comparative analyses of the two optimisation objectives, the relationships between the development time of the sheath gap and the cost have been obtained. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC)-based high-voltage DC (HVDC) transmission technology has become an ideal solution for large and remote wind farms integration. Oscillation or instability phenomena might occur due to the rapid control of power electronic converters in both wind farms and MMC-based HVDC system. In order to reveal oscillation characteristics of wind farms and MMC-based HVDC interconnected system, the system equivalent damping stability criterion is deduced theoretically, and a system damping analysis platform is built in this study. On this base, the impact on 1–1000 Hz oscillation characteristics from many factors including controller parameters of wind turbines and MMC-based HVDC, wind turbines power and wind turbines number is analysed, and mechanism of instability under different conditions is explained physically in terms of system damping characteristics. Time-domain simulations in RT-LAB are presented to validate the stability study. [ABSTRACT FROM AUTHOR]

The continuing growth of an offshore wind farm integrated into a power grid via high-voltage DC has posed great challenges for automatic generation control (AGC) of the power system. To address these challenges, a new concept of 'dynamic dispatch of AGC units (DDA)' under control performance standards from the view of economic dispatch (ED) has been proposed in the previous work, and proved to be an effective technique to co-operate the AGC units with different ramping rates and to fill the gap between ED and AGC. However, the existing DDA model is deterministic in nature, which can hardly deal with the uncertain forecasting error of the offshore wind power output. A novel stochastic DDA model based on chance-constrained programming is proposed considering a random offshore wind power forecasting error, and a hybrid algorithm combining the evolutionary programming algorithm and point estimate method is then developed to solve the stochastic model. Numerical results from a two-area test system with additional offshore wind power generation demonstrate the accuracy and computation efficiency of the hybrid algorithm and the benefits offered by the stochastic AGC dispatch method. [ABSTRACT FROM AUTHOR]

The hybrid AC/DC microgrid is considered to be the more and more popular in power systems as increasing DC loads. In this study, it is presented that a hybrid AC/DC microgrid is modelled with some renewable energy sources (e.g. solar energy, wind energy), typical storage facilities (e.g. batteries), and AC, DC load, and also the power could be transformed smoothly between the AC and DC sub-grids by the bidirectional AC/DC converter. Meanwhile, coordination control strategies are proposed for power balance under various operations. In grid-connected mode, the U–Q (DC bus voltage and reactive) or PQ method is adopted for the bidirectional AC/DC converter according to the amount of exchange power between AC and DC system in order to improve the DG utilisation efficiency, protecting the converter and maintain the stable operation of the system. In islanded mode, V/F control is applied to stabilising the entire system voltage and frequency, achieving the power balance between the AC and DC systems. Finally, these control strategies are verified by simulation with the results showing that the control scheme would maintain stable operation of the hybrid AC/DC microgrid. [ABSTRACT FROM AUTHOR]

Energy management system (EMS) is able to coordinate the generation and load to ensure economic and reliable operation and improve performance metrics of microgrids (MGs). However, most the previous studies have focused mainly on the optimal dispatch of the gas turbines and/or energy storage systems inside a normal MG. Very few works have been carried out to address the potential power outage in MGs. In this study, a novel optimal approach in the EMS based on online forecasts is proposed to minimise the system-wide outages of MG, specifically through pre-emptive load shedding. Online distributed generations' output forecasts are updated to predict the state of system and characterise potential upcoming outages. Proper control strategies based on receding horizon methods are introduced in this study. Outage mitigation actions using pre-emptive load shedding are then performed to avoid power outages or minimise the duration of unavoidable outages. The effectiveness of proposed approach can be demonstrated in the presence of various fluctuations, which can be easily integrated in the EMS for both islanded and grid-connected MGs. [ABSTRACT FROM AUTHOR]

With the integration of the ultra-high-voltage direct-current (UHVDC) of large capacity, the scheduled outputs of the power plants are limited, and the direct power purchase (DPP) by large consumers is influenced. The negative impact caused by the failure modes of the UHVDC on the economic dispatch (ED) and the security of the power system is to be studied. Here, the derated capacities of the UHVDC are described together with their state probabilities. To maximise the profit of power plants, an ED model with the DPP is proposed. The relationship between the amount of the DPP and the power transmission by the UHVDC is decided based on two contract strategies for the DPP. The interior-point method is applied to solve the ED problem. The simulation results validate the feasibility and accuracy of the proposed model, which is beneficial to the development of the power market and the UHVDC in China. [ABSTRACT FROM AUTHOR]

The remarkable benefits of wind power as an environmentally friendly renewable energy resource have led to an increasing penetration of offshore wind energy in modern power systems. Since the DC series–parallel connection system does not need large capacity DC/DC converters, it can reduce the construction costs of wind farms to some extent. In this study, the models of wind speed, wind direction and the wind turbine output power characteristic are developed. Based on this, the output power of each turbine considering the wake effect can be estimated. Then, the reliability block diagrams are drawn according to component relationships in reliability. Moreover, indexes like the probability of all wind turbine string failure, system unavailability and the average available output power of the power system are proposed to assess the reliability of the DC series–parallel offshore wind farm. A case study is presented to calculate the reliability indexes of different situations, where the amount of wind turbines remains the same while the number of turbine strings varies. It can be concluded that the DC series–parallel offshore wind farm is more reliable when increasing turbine string number, and reference is provided for DC series–parallel wind farm structure optimisation. [ABSTRACT FROM AUTHOR]

Offshore wind power resource, which is relatively stable, abundant, and with modest impact on the environment, is a kind of clean and renewable energy suitable for large-scale development. Offshore wind farms have different topological structures of the collection system, such as AC parallel type, DC parallel type, DC series type, single-ring type, and double-ring type. The purpose of this paper is to analyse reliability and economy for various topologies and provide an insight into wind farm construction. In this study, the topology structure of various offshore wind farms is introduced firstly. Then, the reliability assessment methods are proposed. The reliability of the chain wind farm is analysed with the reliability block diagrams, while the reliability of the ring wind farm is analysed by using the classification method. In addition, the economy is evaluated considering investment costs. The investment cost for each type of component is calculated separately, which contributes to assessing the investment cost of the whole offshore wind farm. Detailed results from a case study, which includes 96 wind turbines, demonstrate that the DC series–parallel wind farm has superiority in reliability and economy, as well as in terms of its potential applications. [ABSTRACT FROM AUTHOR]

The dynamic behaviour of microgrid system under faulty conditions makes adaptive protection a general necessity for reliable microgrid operation. In design of adaptive protection, the grid‐connected and islanded modes have immense importance including grid‐connected mode without distributed generators (DGs) in microgrid. In this study, a new adaptive protection scheme is proposed based on the above‐mentioned modes of microgrid operation. The proposed method considers nature of DGs connected, fault location detection and fault nature identification based on quadrature and zero sequence components of fault current considering impact of X/R ratio of DGs. The proposed methodologies for adaptive protection schemes are verified in Matlab‐Simulink environment and the results are found to be satisfactory while various faults are simulated at different nodes of the microgrid model. At the time of verification of effectiveness of the proposed methodologies, the time derivative of quadrature and zero‐axis components of fault current are considered sufficient to instantaneously detect the fault location and fault nature in microgrid system. [ABSTRACT FROM AUTHOR]

Increasing share of distributed energy resources (DERs) in power networks results in dynamic network topology and short‐circuit fault levels. Low fault infeed in PV connected power network will rarely damage any power components, but detection of faults is of prime importance. The existing over‐current, voltage, and impedance‐based relays may fail to detect/clear the faults. However, it is observed that PV infeed networks are generally weak and experience a collapse in terminal voltage with increase in current during the system faults. In this article, variation in fault currents and voltages is utilised for formulating a new voltage‐current‐time inverse (VCTI) mathematical relay model. The proposed relay model enhances the operating time of the relays and maintains the protection coordination for weak power networks, especially powered with high PV generation. Using this relay model, a cluster‐based protection coordination scheme for PV‐dominant penetrated power network is presented. The performance of the proposed relay model and coordination schemes is compared with inverse definite minimum time (IDMT) relays and other relying models. Based on the proposed relay model, a hardware relay prototype model has been developed and it is found that hardware results are highly stable and are within 3% accuracy of simulated results. [ABSTRACT FROM AUTHOR]

With the integration of the ultra-high-voltage direct-current (UHVDC) of large capacity, the scheduled outputs of the power plants are limited, and the direct power purchase (DPP) by large consumers is influenced. The negative impact caused by the failure modes of the UHVDC on the economic dispatch (ED) and the security of the power system is to be studied. Here, the derated capacities of the UHVDC are described together with their state probabilities. To maximise the profit of power plants, an ED model with the DPP is proposed. The relationship between the amount of the DPP and the power transmission by the UHVDC is decided based on two contract strategies for the DPP. The interior-point method is applied to solve the ED problem. The simulation results validate the feasibility and accuracy of the proposed model, which is beneficial to the development of the power market and the UHVDC in China.

The continuing growth of an offshore wind farm integrated into a power grid via high-voltage DC has posed great challenges for automatic generation control (AGC) of the power system. To address these challenges, a new concept of ‘dynamic dispatch of AGC units (DDA)’ under control performance standards from the view of economic dispatch (ED) has been proposed in the previous work, and proved to be an effective technique to co-operate the AGC units with different ramping rates and to fill the gap between ED and AGC. However, the existing DDA model is deterministic in nature, which can hardly deal with the uncertain forecasting error of the offshore wind power output. A novel stochastic DDA model based on chance-constrained programming is proposed considering a random offshore wind power forecasting error, and a hybrid algorithm combining the evolutionary programming algorithm and point estimate method is then developed to solve the stochastic model. Numerical results from a two-area test system with additional offshore wind power generation demonstrate the accuracy and computation efficiency of the hybrid algorithm and the benefits offered by the stochastic AGC dispatch method.

The hybrid AC/DC microgrid is considered to be the more and more popular in power systems as increasing DC loads. In this study, it is presented that a hybrid AC/DC microgrid is modelled with some renewable energy sources (e.g. solar energy, wind energy), typical storage facilities (e.g. batteries), and AC, DC load, and also the power could be transformed smoothly between the AC and DC sub-grids by the bidirectional AC/DC converter. Meanwhile, coordination control strategies are proposed for power balance under various operations. In grid-connected mode, the U–Q (DC bus voltage and reactive) or PQ method is adopted for the bidirectional AC/DC converter according to the amount of exchange power between AC and DC system in order to improve the DG utilisation efficiency, protecting the converter and maintain the stable operation of the system. In islanded mode, V/F control is applied to stabilising the entire system voltage and frequency, achieving the power balance between the AC and DC systems. Finally, these control strategies are verified by simulation with the results showing that the control scheme would maintain stable operation of the hybrid AC/DC microgrid.

Generation maintenance scheduling (GMS) is one of the most important scheduling problems in the restructured power systems. The maintenance time interval of generation units is the crucial factor of GMS for an operation lifespan of generation units, particularly within the smart grid which needs high reliability. Accordingly, this study proposes a multi-objective-GMS (MO-GMS) optimisation model for maintenance scheduling of generation units based on the global criterion approach, adopting a suitable compromise function. The proposed MO-GMS model determines the maintenance intervals, aims to maximise both the generation company's (GenCo's) financial returns from selling electricity and the system reserve at every time interval from the independent system operator (ISO) standpoint. This method searches the optimal maintenance weeks for each generation unit, considering the objectives of both GenCo and ISO, simultaneously. The proposed MO-GMS model is formulated as a mixed-integer non-linear programming problem and examined on the IEEE 24-bus and IEEE 118-bus test systems. The success of the proposed multi-objective model is validated by comparing the obtained results with intelligent optimisation algorithms.

Distribution Asset Management is an important task performed by utility companies to prolong the lifetime of the critical distribution assets and to accordingly ensure grid reliability by preventing unplanned outages. This study focuses on microgrid applications for distribution asset management as a viable and less expensive alternative to traditional utility practices in this area. A microgrid is as an emerging distribution technology that encompasses a variety of distribution technologies including distributed generation, demand response, and energy storage. Moreover, the substation transformer, as the most critical component in a distribution grid, is selected as the component of the choice for asset management studies. The resulting model is a microgrid-based distribution transformer asset management model in which microgrid exchanged power with the utility grid is reshaped in such a way that the distribution transformer lifetime is maximised. Numerical simulations on a test utility-owned microgrid demonstrate the effectiveness of the proposed model to reshape the loading of the distribution transformer at the point of interconnection in order to increase its lifetime.

The main cause for cascading failure is large-scale overload caused by localised abnormal flows. With the increase of wind power penetration, the complexity of flow distribution is strengthened. Therefore, it is necessary to study influences of site selection and permeability of wind power generation on cascading failure. An analysis method based on electric betweenness of nodes was put forward, and influences of permeability change on cascading failure on different location selection for wind power generation were researched. According to the study, it was found that when the electric betweenness was low, the overload of wind power led to declining uniformity of power flow distribution, resulting in transfer of flows to other lines. As a result, the heavy load of lines was further aggravated to promote the development of cascading failure. Simulation results of the IEEE30 system showed that with the increase of wind power penetration, the cascading failure was aggravated; and under the same permeability, the lower the electric betweenness of location selection was, the severer cascading failure would be.

The remarkable benefits of wind power as an environmentally friendly renewable energy resource have led to an increasing penetration of offshore wind energy in modern power systems. Since the DC series–parallel connection system does not need large capacity DC/DC converters, it can reduce the construction costs of wind farms to some extent. In this study, the models of wind speed, wind direction and the wind turbine output power characteristic are developed. Based on this, the output power of each turbine considering the wake effect can be estimated. Then, the reliability block diagrams are drawn according to component relationships in reliability. Moreover, indexes like the probability of all wind turbine string failure, system unavailability and the average available output power of the power system are proposed to assess the reliability of the DC series–parallel offshore wind farm. A case study is presented to calculate the reliability indexes of different situations, where the amount of wind turbines remains the same while the number of turbine strings varies. It can be concluded that the DC series–parallel offshore wind farm is more reliable when increasing turbine string number, and reference is provided for DC series–parallel wind farm structure optimisation.

Modular multilevel converter (MMC)-based high-voltage DC (HVDC) transmission technology has become an ideal solution for large and remote wind farms integration. Oscillation or instability phenomena might occur due to the rapid control of power electronic converters in both wind farms and MMC-based HVDC system. In order to reveal oscillation characteristics of wind farms and MMC-based HVDC interconnected system, the system equivalent damping stability criterion is deduced theoretically, and a system damping analysis platform is built in this study. On this base, the impact on 1–1000 Hz oscillation characteristics from many factors including controller parameters of wind turbines and MMC-based HVDC, wind turbines power and wind turbines number is analysed, and mechanism of instability under different conditions is explained physically in terms of system damping characteristics. Time-domain simulations in RT-LAB are presented to validate the stability study.

Offshore wind power resource, which is relatively stable, abundant, and with modest impact on the environment, is a kind of clean and renewable energy suitable for large-scale development. Offshore wind farms have different topological structures of the collection system, such as AC parallel type, DC parallel type, DC series type, single-ring type, and double-ring type. The purpose of this paper is to analyse reliability and economy for various topologies and provide an insight into wind farm construction. In this study, the topology structure of various offshore wind farms is introduced firstly. Then, the reliability assessment methods are proposed. The reliability of the chain wind farm is analysed with the reliability block diagrams, while the reliability of the ring wind farm is analysed by using the classification method. In addition, the economy is evaluated considering investment costs. The investment cost for each type of component is calculated separately, which contributes to assessing the investment cost of the whole offshore wind farm. Detailed results from a case study, which includes 96 wind turbines, demonstrate that the DC series–parallel wind farm has superiority in reliability and economy, as well as in terms of its potential applications.

Four cascaded quasi‐impedance source inverter (qZSI) modules are required for achieving nine‐level output voltage waveform. In case of one module failure, number of levels in output voltage are reduced to seven. This leads to decrease in the output voltage magnitude and increased THD (higher than conventional seven‐level qZSI). This is due to the dominance of unwanted harmonic component introduced in the harmonic spectrum. To restore voltage magnitude and optimize THD performance, novel voltage balancing algorithm is proposed. To validate the control algorithm for off‐grid and grid connected system, simulation results of the multilevel qZSI are discussed in two categories: (i) DC voltage source powered multilevel qZSI for RL load subjected to module failure and (ii) solar powered multilevel qZSI connected to utility grid subjected to module failure and variable solar irradiation. Pre‐fault and post‐fault performance of the system with the proposed control algorithm is discussed for both categories, which validates the effectiveness of the algorithm. Hardware results for proof‐of‐concept are discussed for DC voltage source fed cascaded qZSI connected to RL load during pre‐fault and post‐fault conditions. FPGA Virtex‐5 is used for hardware implementation of the control algorithm. The results validate the improvement in output voltage both quantitatively and qualitatively. [ABSTRACT FROM AUTHOR]

Multi‐level voltage source inverter (VSI) fed doubly fed induction machine (DFIM) has gained prominence in large rated hydro‐generating unit (HU) since it provides part load operation with reduced power converter rating and high dynamic stability. Power converters connected in rotor side control real and reactive power of the unit. This study discusses the condenser operation of variable speed HU (250 MW), to be commissioned in Tehri plant (India), with the power flow diagram. Further, it provides dynamic behaviour and survivability status of the DFIM unit during power converter and control circuit faults where power converter redundancy is not available in large rated DFIM unit. Economic analysis of 250 MW HU under power converter and control circuit failures are also investigated. The present work also explores fault tolerant operation of a 250 MW DFIM unit at open switch fault in converters to increase the continuity of the unit operation which provides additional 2% of reactive power support to the grid compared to the DFIM without power converter redundancy operation. Open switch fault is detected through Park's vector phase currents technique and variation in DC link voltage. The computer simulation results are validated in an experiment carried on 2.2 kW DFIM in the laboratory. [ABSTRACT FROM AUTHOR]

As renewable energy sources (RESs) penetration increases in the power system, the transmission system operators face new challenges to ensure system reliability and flexibility while ensuring high utilisation of uncertain RES generation. Controllable transformers with on‐load tap changers and phase shifting capability are the promising flexibility tools to keep the system acceptable security and flexibility levels by controlling the voltage levels and energy flow. The AC optimal power flow (AC OPF) with detailed modelling considerations such as the bus voltage magnitude by including these devices is challenging. This study develops the AC OPF model to propose a robust flexibility optimisation framework for daily scheduling problem with uncertain wind energy sources. Nevertheless, the proposed formulation representation is an intractable mixed integer nonlinear programming (MINLP) while it includes AC grid constraints and the augmented modelling of the mentioned transformers. Accordingly, the proposed MINLP problem has been converted into a mixed‐integer linear program where a certain level of solution accuracy can be achieved for the available time budget. The effectiveness of the proposed method is demonstrated using a modified 6‐bus and IEEE 118‐bus test systems. [ABSTRACT FROM AUTHOR]

This study proposes a unique method of bidding strategy which is based on bi‐level optimisation model for analysing the profits of the generation companies (GENCOs) and distribution companies in the day‐ahead and real‐time competitive power markets. The bi‐level optimisation problem includes lower‐level and upper‐level problems. In the lower‐level problem, market clearing model is presented, in which market clearing problem is solved, locational marginal price is minimised at load buses, net generation cost is also minimised with a simultaneous increase in profit with optimal placement of wind generator by assessing the conditional value‐at‐risk (CVaR). In the upper level, the profit is maximised by selecting the proper bidding strategy based on double‐auctioned market mechanism. To check the effectiveness of the proposed strategy, Monte Carlo simulation and scenario‐based approach are used to generate the scenarios considering the random line outages, generator outages and load variations. The CVaR is used as a risk assessment tool for the uncertainties that would inadvertently affect the profit of the GENCOs. The proposed approach is applied to the modified IEEE 30‐bus system to show the effectiveness of the proposed model and its impact on the bidders and suppliers. [ABSTRACT FROM AUTHOR]

Load dynamics play an important role in the voltage stability. Grid‐side short‐circuit faults may induce severe voltage drops at motor load buses. Once the fault is cleared, depending upon its clearing time, the load bus voltage may either return to its permissible range or degrade gradually until it collapses. From the short‐term voltage stability viewpoint, the former is a stable phenomenon, while the latter is an unstable circumstance and the load should be shed in order to prevent voltage instability from being spread over the grid. To cope with this problem, this study presents a new under‐impedance load shedding (UILS) scheme. A quadratic distance relay is used as the load shedding element. The trajectory of load impedance is monitored by the proposed UILS relay. Following the grid‐side fault clearance, if the impedance trajectory enters the load shedding zone and remains inside it for a predefined time, voltage instability is detected and the load is curtailed by the UILS relay. The proposed approach is tested on various networks, including a real power system model. The obtained results approve the efficiency of the proposed UILS scheme. [ABSTRACT FROM AUTHOR]

This study determines the optimal capacity of various components of an AC mini‐grid hybrid power system in a remote area of South Australia. A range of generation and storage technologies, such as diesel generators, wind turbines, solar photovoltaic arrays, battery banks, and flywheels are considered. A minimum system operating reserve is maintained to ensure a certain degree of reliability. Four different configurations of hybrid power systems are analysed in hourly time‐steps, over a year, using real data for system load, solar insolation, ambient temperature, and wind speed. Particle swarm optimisation approach is used to optimise the capacity of various components of each hybrid system to obtain the minimum net present cost (NPC) over a 20‐year system lifespan. Suitable capital, operating, and maintenance costs in Australian context are considered in evaluating the system. Various simulation results, such as annual energy balances, NPCs, levelised cost of energy and cash flow analysis obtained for each optimised system are carefully analysed and discussed. The modelling and results confirms the feasibility and economical replacement of the diesel power system with those of hybrid power systems which use renewable energy supply. [ABSTRACT FROM AUTHOR]

This study describes a methodology to solve the multi‐period distribution expansion planning problem considering reliability, distributed generation (DG), capacitor and switch placement in the context of smart grids. The developed computational model is formulated as a mixed integer nonlinear optimisation problem and solved through the combination of meta‐heuristics, stochastic simulation methods (Monte‐Carlo simulation), and application of optimal power flow. The main objectives of this optimisation problem are defining the best choice to install power system components and determining the installation period and size of components to minimise investment cost while maximising system reliability. The proposed method was implemented in a modified version of the IEEE‐RBTS bus 2 and in a 90‐bus test system. The obtained results show a reduction of >20% of the operating cost and >70% of the average interruption time. In addition, it was performed scenarios not including DG, self‐healing, and reliability analysis, individually. The analyses show the impact in the results by including a new variable in the expansion planning, increasing up to 40% in the fitness function value. Furthermore, it is presented with a discussion of the impact of the method to estimate the reliability indexes on the optimisation process. [ABSTRACT FROM AUTHOR]

In the long‐term transmission system hardening, critical components in transmission infrastructure should be identified and hardened to reduce the load loss risk in the presence of scheduled and unscheduled outages. Generally, the probabilistic information of assets in power systems can be evaluated via historical statistics. On the basis of the prior probabilistic information, this study proposed a tri‐level optimisation model to deal with the problem of long‐term transmission system hardening, in which the risk assessment is taken into account in the objective function. Furthermore, the problem is formulated as a two‐stage robust optimisation model. To address the non‐linear problem in the inner model, logarithmic transformation and piecewise linearisation are utilised to exactly linearise the non‐linear terms in the model. Finally, the standard column‐and‐constraints generation algorithm is employed to solve the proposed model with a master–sub‐problem framework. The test results on a standard IEEE RTS‐96 system show the effectiveness of the proposed model. [ABSTRACT FROM AUTHOR]

In a microgrid, a rational network is necessary to improve the economy, reliability, adaptability, security, and coordination of the supply with the load demand. A flexible network optimal planning model of autonomy microgrid is proposed. Several uncertainties including the output power of distributed generation and load power are considered and each uncertainty factor is divided into several typical scenarios. The probability of each scenario is calculated by asymptotically optimal scenario analysis. The annual investment cost, the fault outage cost expectation, the annual cost regret value expectation and penalty function, the violating electrical constraints degree, and the source‐network‐load equilibrium degree are applied to represent the flexibility of the planning model. Allocation of energy storage system capacity is considered in the network planning model, which can reduce the negative effects on the microgrid in extreme scenarios. An energy management strategy is constructed to coordinate the distribution of source and load. The tree structure encoding is employed to ensure the connectivity of the network. Dynamic Pareto set‐based particle swarm optimisation is adopted for the solution of the network planning model. Finally, the calculation results of planning schemes are demonstrated by radar chart, and the optimal one is selected by its comprehensive evaluation value. [ABSTRACT FROM AUTHOR]

A huge motivation has recently made on microgrid (MG) financial issues, aimed to investigate the contribution of MG operator (MGO) and private investor to reach an optimal operational strategy. Motivating the private investors to contribute in an energy production, is a considering benefit sharing factor by MGO to satisfy both of MGO and private investor. In this study, a reliability‐constrained optimisation approach is presented to calculate the number and size of MG system components. To this aim, planning problem is solved in two cases; full available state and state with considering random outage of units. Furthermore, all uncertainties of generation units are considered in the problem formulation. Non‐sequential Monte Carlo method is used to generate all scenarios. The proposed model simultaneously optimises two objectives, namely the benefits of MGO. The two‐stage heuristic method is used to solve the objective function. In the first stage, by utilising genetic algorithm, the solution to form the Pareto optimal front is found. In the second stage, to select the trade‐off solution among obtained Pareto solutions, the fuzzy satisfying method has been used. Simulations are carried out in two cases, with and without considering the share of a private investor of MGO's benefit, i.e. β. [ABSTRACT FROM AUTHOR]

In this circumstance of global warming, energy market deregulation, and enormous load growth, distribution network entails a proficient strategy to maintain the reliability and efficiency of the power service. Incorporation of solar photovoltaic (PV) system and battery storage (BS) in coordination with distributed static compensator (DSTATCOM) is a competent and practical approach to alleviate the power quality and reliability concern. In this study, a comprehensive strategic model is presented to optimally deploy PV, BS, and DSTATCOM to maximise voltage profile improvement, reliability, economic, and ecological benefit of the network. An accurate and precise novel voltage profile improvement indicator namely network voltage profile improvement index is proposed. Benefit–cost ratio and environment benefit index are proposed to quantify economic and environmental benefits, respectively. Similarly, reliability indices such as expected energy not served are used to appraise the system reliability. A fuzzy based extended version of NSGA II is utilised for the optimal deployment of the devices considering security limits. The proposed method is tested on 33‐bus and 69‐bus distribution networks considering time variant practical load models and the obtained results validate the efficacy and efficiency of the proposed method when compared with other multi‐objective algorithms. [ABSTRACT FROM AUTHOR]

This study proposes a new application of micro‐phasor measurement units (µPMUs) for adaptive coordination of overcurrent relays in microgrids. Mis‐coordination of overcurrent relays usually arising from the variation of relays fault current and it can cause damage to equipment of network and raise operating costs. Fault current injection and direction to microgrid are highly dependent on network uncertainties; therefore, fault current is affected by line and power plant outages. This study proposes an algorithm to detect these uncertainties in online operation. Then, microgrid overcurrent relays coordination is optimised again. Uncertainties are line and power plant outages in transmission network and microgrid side and two distinct methods are used for each. For online detection of uncertainties in the transmission side, it is assumed that a µPMU is installed between transmission network and microgrid point of common coupling; so, the topology changes such as line outage is detected by monitoring of Thevenin impedance estimation that is obtained by µPMU measurements. Uncertainties detection in a microgrid is done by signals that are sent by µPMUs and installed all over the microgrid. All data are gathered and analysed in phasor data concentrators and then overcurrent relays coordination is updated with such changes. [ABSTRACT FROM AUTHOR]

System operators still rely on deterministic criteria such as the N−1 (i.e. the system would be able to withstand the outage of any single component without any load shedding) to hedge the system against contingencies. While simple and practical, this criterion miscalculates the actual amount of reserve required since it ignores the probability of contingency occurrence. Therefore, this criterion may lead to suboptimal reserve procurement and economic performance of the system. This study presents a multiperiod probabilistic security‐constrained unit commitment (UC) model that includes the probabilities of generation and transmission contingencies for optimal reserve sizing, sourcing, allocation, and timing. The ability of energy storage systems (ESS) to provide contingency reserve is explicitly modelled. Benders' decomposition and linearisation techniques are applied to solve the proposed probabilistic UC, which would be intractable otherwise. The impact of ESS on the contingency reserve procurement and deployment in post‐contingency states are analysed on a modified version of the IEEE One‐Area Reliability Test System. [ABSTRACT FROM AUTHOR]

This study presents a method to analyse the secure and optimal operation of transmission systems consisting of both HVDC (high‐voltage direct current) and high‐voltage alternating current interconnections under a scenario of high penetration of offshore wind and taking into account the system spinning reserves. Different reliability management strategies are assumed, accounting for a single AC element and/or a single DC element outage and taking into consideration the probability of failure of the different power system components. The cost of operation of the system and the wind energy curtailed over the operating time are determined. This method is applied to a study case, based on the CIGRÉ HVDC Test System. The system behaviour over a complete year is modelled using realistic wind generation and demand based on scaling data from the Belgian power system. [ABSTRACT FROM AUTHOR]

This study proposes a two‐stage stochastic optimisation model for jointly wind turbine (WT) allocation and network reconfiguration (NR) so as to increase the resiliency of distribution system in face of natural disasters. In this regard, in the first level, a possibilistic‐scenario method is proposed to select the line outage scenarios. The proposed model is capable with distribution systems and considers different failure probabilities for system components subject to the intensity of natural disaster in its associated zone. After selecting the line outage scenarios, in the second level, a multi‐stage optimisation framework is proposed for jointly NR and WT allocation in a multi‐zone and multi‐fault system, considering the uncertainty of system load and wind power generation. This strategy makes an interconnection between NR and islanded WTs to increase the resiliency of system and decreases the load shedding. Different economic objectives including, costs of load shedding and power generation are considered in the model. In addition, hardening budget is taken into consideration for the transmission lines, which is minimised during the optimisation process. The simulation results demonstrate the capability and necessity of proposed resiliency‐oriented method and prove the importance of hardening budgets. [ABSTRACT FROM AUTHOR]

High penetration of wind energy requires fast-acting dispatchable resources to manage energy imbalance in the power grid. Battery energy storage systems (BESS) are considered as an essential tool to decrease the power and energy imbalance between the scheduled generation (day ahead forecast) and the actual wind farm output. Control methodology or battery management greatly impacts performance of the ESS. Better performance of BESS reduces the minimum required size of batteries for wind variability mitigation. This paper proposes a novel control method for BESS to fulfill a production commitment. This method, called “predictive controller,” is based on updated forecast data to improve the performance of the energy storage and consequently reduce the required size of energy storage. The Sodium–Sulfur (NaS) type battery is selected for the simulation purposes. Results show that the predictive controller reduces the error (between scheduled generation and actual wind farm output) more than the simple method (also known as minute-by-minute method) and other proposed methods in the literature. Also, a new formulation for the battery lifetime estimation is introduced, and it is used to analyze the impact of the proposed method on the battery lifetime depreciation. [ABSTRACT FROM PUBLISHER]

This study recommends a stochastic optimization model for the security constrained unit commitment (SCUC), which incorporates the optimal transmission switching (OTS) for managing the uncertainty of wind power generation and equipment failures, i.e. unit/line outages. Also, this study presents a technique in stochastic SCUC model with the OTS action using the AC optimal power flow (AC OPF). The AC OPF provides a more accurate picture of power flow in the power system compared to the DC optimal power flow that is usually considered in the literature for the stochastic SCUC models and the OTS action. While the stochastic SCUC model with the OTS action based on AC OPF is a mixed‐integer non‐linear programming model, this study transforms it into a mixed‐integer linear programming (MILP) model. The MILP approach uses a piecewise linear model of AC OPF, which allows the reactive power and voltage to be considered directly in power flow model. The proposed stochastic SCUC problem is evaluated on the 6 bus, IEEE 118‐bus and 662‐bus test systems in pre‐ and post‐OTS action. Obtained results demonstrate the effectiveness of the proposed model. [ABSTRACT FROM AUTHOR]

Power systems are prone to cascading outages leading to large‐area blackouts with significant social and economic consequences. Intentional controlled islanding (i.e. the separation of the system into sustainable islands) is an effective strategy to mitigate these catastrophic events. To ensure a correct separation, nonetheless, it is crucial to define a suitable time to split the system (i.e. to answer the when to island question). To consider the probability of the event, the reliability of the system components, the reliability of the information and communication technologies, and the potential economic costs of the event, answering the above question within a risk‐based framework becomes critical. To date, however, this has not been done. This study proposes a risk‐based methodology to define, in an adaptive manner, a suitable time to split the system following an event. This methodology complements the well‐studied where to island question, resulting in an integral solution of the islanding problem. To illustrate the approach, the IEEE 118‐bus dynamic system is adopted considering realistic security criteria. Simulation results demonstrate the effectiveness and flexibility of the methodology in identifying a suitable time for the creation of islands, which, in turn, results in the prevention of blackouts that would otherwise be obtained. [ABSTRACT FROM AUTHOR]

For the reliability assessment of a distribution system with microgrids (MGs), considerable computation time has to be cost to ensure convergence of an applied Monte‐Carlo method (MCM) to estimating indices of interest, in the context of adopting multi‐state models of constituent components. Towards achieving an efficient and accurate reliability assessment as much as possible, multi‐state models of distributed generation resources, especially the battery energy storage system, are proposed. Based on the proposed models, effect analysis of a given sampled MG state is conducted in a proposed analytical manner. Moreover, an accelerated MCM nesting the proposed effect analysis methodology is also presented, dedicated to reliability evaluation of a resulting multi‐state distribution system from the multi‐state constituent components. Case studies on a modified IEEE‐RBTS Bus6 F4 system demonstrate the effectiveness of the proposed method, against the plain MCM. In addition, impact of an important parameter, termed total state number of state‐of‐charge, on the convergence and speed of the proposed method has been fully analysed. [ABSTRACT FROM AUTHOR]

This study presents a two‐stage competent and efficient approach for optimal operation of wind–pumped‐storage‐hydro (PSH)–solar–thermal‐storage hybrid power plant to get maximum system revenue and profit along with maintaining the grid frequency. The wind speed is predicted for a deregulated market and accordingly, the wind plants are committed to supplying the demand. The operation of PSH, battery and solar power are considered in order to minimise the adverse effect of imbalance cost which comes into the picture due to the mismatch between actual and predicted wind power. The proposed operating strategy for the complex hybrid plant helps to reduce the uncertainty of renewable power sources in an economical manner. Two new energy levels associated with pumped storage, i.e. PEopt and PElow and four energy levels associated with the battery, i.e. BEmax, BEopt, BElow and BEmin have been considered in this work to show the robustness of the proposed strategy. The proposed approach is implemented and compared using Mi‐Power, bat algorithm, particle swarm optimisation algorithm, genetic algorithm and cuckoo search algorithm. Modified IEEE 14‐bus system is used to validate the effectiveness of the proposed approach. The bilateral contracts with a double auction bidding model for the competitive power market are also considered for the implementation. [ABSTRACT FROM AUTHOR]

Probabilistic production simulation is an important tool used in conventional power systems to calculate generated energy and evaluate reliability. With the continuous expansion of the renewable energy plants, new characteristics such as intermittency and volatility of power are widely discussed, and it is necessary to study the application of probabilistic production simulation for renewable energy power generation. This study proposes a probabilistic production simulation method based on sequence operation theory (SOT) to simulate the operation of a wind/photovoltaic/energy storage power system. Both the uncertainty of renewable resources and the outage of wind turbines are considered in this study. Considering the complementary property of the renewable energy, pattern clustering is used to analyse the meteorological conditions and to assist in the probabilistic production simulation. Moreover, the output model of the energy storage device is developed using the Monte Carlo method and controlled using a smoothing strategy of the energy storage device. Ultimately, the simulation example shows the feasibility and the higher efficiency of the algorithm compared with Monte Carlo method and a production simulation method based on equivalent energy function. [ABSTRACT FROM AUTHOR]