Operational reliability evaluation of restructured power systems with wind power penetration utilizing reliability network equivalent and time-sequential simulation approaches

In the last two decades,the wind power generation has been rapidly and widely developed in many regions and countries for tackling the problems of environmental pollution and sustainability of energy supply.However,the high share of intermittent and fluctuating wind power production has also increased the burden of system operator for securing power system reliability during the operational phase.Moreover,the power system restructuring and deregulation have not only introduced the competition for reducing cost but also changed the strategy of reliability evaluation and management of power systems.The conventional long-term reliability evaluation techniques have been well developed,which have been more focused on planning and expansion rather than operation of power systems.This paper proposes a new technique for evaluating operational reliabilities of restructured power systems with high wind power penetration.The proposed technique is based on the combination of the reliability network equivalent and time-sequential simulation approaches.The operational reliability network equivalents are developed to represent reliability models of wind farms,conventional generation and reserve provides,fast reserve providers and transmission network in restructured power systems.A contingency management schema for real time operation considering its coupling with the dayahead market is proposed.The time-sequential Monte Carlo simulation is used to model the chronological characteristics of corresponding reliability network equivalents.Asimplified method is also developed in the simulation procedures for improving the computational efficiency.The proposed technique can be used to evaluate customers’reliabilities considering high penetration of wind power during the power system operation in the deregulated environment.

Operational Reliability Modeling and Assessment of Battery Energy Storage Based on Lithium-ion Battery Lifetime Degradation

Battery energy storage(BES)systems can effectively meet the diversified needs of power system dispatching and assist in renewable energy integration.The reliability of energy storage is essential to ensure the operational safety of the power grid.However,BES systems are composed of battery cells.This suggests that BES performance depends not only on the configuration but also on the operating state over different lifetime durations.The lack of safety and reliability is the main bottleneck preventing widespread applications of BES systems.Therefore,a reliability assessment algorithm and a weak-link analytical method for BES systems are proposed while considering battery lifetime degradation.Firstly,a novel lithium-ion battery model is proposed to identify the degradation rate of solid electrolyte interphase film formation and capacity plummeting.The impacts of different operating conditions are considered in stress factor models.Then,a reliability assessment algorithm for a BES system is introduced based on a universal generating function.An innovative weak-link analytical method based on the reliability importance index is proposed that combines the evaluation results of state-oriented and state-change-oriented indexes through an entropy weight method.The model,algorithm,indexes,and the usefulness are demonstrated in case studies based on aging test data and actual bus operating data.The results demonstrate the effects of the battery status and working conditions on BES reliability.Weak-link analysis is also used to assist BES systems in avoiding short-board batteries to achieve long lifetimes and efficient operation.

Operational Reliability Model of Hybrid MMC Considering Multiple Time Scales and Multi-state Submodule

Environmental and electrical factors such as wind speed,air temperature and switching frequency have significant influences on the operational reliability of hybrid modular multilevel converter(MMC),which is commonly used for the wind power transmission.However,the existing reliability model of hybrid MMC based on statistics cannot accurately reflect the impact of these factors.In this paper,a new operational reliability model of hybrid MMC is presented.The reliability index of the hybrid MMC is coupled with its operation characteristics by calculating multi-term thermal cycling.In addition,an operation strategy of hybrid MMC is proposed to improve its reliability.The multi-state submodule(SM)is developed,which is capable of bypassing specific faulty power modules instead of the whole SM.Case studies show that the proposed operational reliability model could describe the impact of environmental and electrical factors.Also,the proposed operation strategy can improve the reliability of hybrid IMMC by extending the operation time of SMs.

Fundamental Research on Enhancing Operation Reliability for Large-Scale Interconnected Power Grids——An interview with Zhou Xiaoxin, chief scientist of "973 Program"

At the end of last year, the editors from Power and Electrical Engineers interviewed Zhou Xiaoxin on "Fundamental Research on Enhancing Operation Reliability for Large-Scale Interconnected Power Grids", a project of "973 Program". Mr. Zhou, the chief engineer of China Electric Power Research Institute(CEPRI) and an academician of Chinese Academy of Sciences, is the chief scientist in charge of this research project.

Wind power operation capacity credit assessment considering energy storage

Research on wind power capacity credit at the operational level plays an important role in power system dispatching.With the popularity of energy storage devices,it is increasingly necessary to study the impact of energy storage devices on wind power operational capacity credit.The definition of wind power operational capacity credit is given.The available capacity model of different generators and the charging and discharging model of the energy storage are established.Based on the above model,the evaluation method of wind power operation credible capacity considering energy storage devices is proposed.The influence of energy storage on the wind power operation credible capacity is obtained by case study,which is of great help for the power system dispatching operation and wind power accommodation.

A multi-state model for wind farms considering operational outage probability

As one of the most important renewable energy resources,wind power has drawn much attention in recent years.The stochastic characteristics of wind speed lead to generation output uncertainties of wind energy conversion system(WECS)and affect power system reliability,especially at high wind power penetration levels.Therefore,a more comprehensive analysis toward WECS as well as an appropriate reliability assessment model are essential for maintaining the reliable operation of power systems.In this paper,the impact of wind turbine outage probability on system reliability is firstly developed by considering the following factors:running time,operating environment,operating conditions,and wind speed fluctuations.A multistate model for wind farms is also established.Numerical results illustrate that the proposed model can be well applied to power system reliability assessment as well as solving a series of reliability-centered decision-making problems of power system scheduling and maintenance arrangements.

Optimal Energy Reserve Scheduling in Integrated Electricity and Gas Systems Considering Reliability Requirements

With the growing interdependence between the electricity system and the natural gas system,the operation uncertainties in either subsystem,such as wind fluctuations or component failures,could have a magnified impact on the reliability of the whole system due to energy interactions.A joint reserve scheduling model considering the cross-sectorial impacts of operation uncertainties is essential but still insufficient to guarantee the reliable operation of the integrated electricity and natural gas system(IEGS).Therefore,this paper proposes a day-ahead security-constrained unit commitment(SCUC)model for the IEGS to schedule the operation and reserve simultaneously considering reliability requirements.Firstly,the multi-state models for generating units and gas wells are established.Based on the multi-state models,the expected unserved energy cost(EUEC)and the expected wind curtailment cost(EWC)criteria are proposed based on probabilistic methods considering wind fluctuation and random failures of components in IEGS.Furthermore,the EUEC and EWC criteria are incorporated into the day-ahead SCUC model,which is nonconvex and mathematically reformulated into a solvable mixed-integer second-order cone programming(MISOCP)problem.The proposed model is validated using an IEEE 30-bus system and Belgium 20-node natural gas system.Numerical results demonstrate that the proposed model can effectively schedule the energy reserve to guarantee the reliable operation of the IEGS considering the multiple uncertainties in different subsystems and the cross-sectorial failure propagation.

Impact of Down Spinning Reserve on Operation Reliability of Power Systems

The development of renewable energy and the increasing peak-valley difference of load demand lead to an increasing requirement of spinning reserve(SR).However,the traditional operation reliability analysis of power system mainly focuses on the up SR and neglects the down SR.Therefore,the operation reliability of power system considering the impacts of down SR is investigated in this paper.Firstly,the constraints of down SR are incorporated into the day-ahead unit commitment(UC)model to obtain the generation scheduling and reserve allocation of power systems.Based on the dispatch results of UC model,the re-dispatched energy and load interruption can be determined using optimal power flow(OPF)model in the realtime operation in various contingency states.Operation reliability indices are calculated based on the load curtailment to represent the reliability performances of power systems.The proposed approaches are validated using the modified IEEE reliability test system.Case studies demonstrate that down SR can improve the operation reliability of power systems.

Mixed Aleatory-epistemic Uncertainty Modeling of Wind Power Forecast Errors in Operation Reliability Evaluation of Power Systems

As the share of wind power in power systems continues to increase, the limited predictability of wind power generation brings serious potential risks to power system reliability. Previous research works have generally described the uncertainty of wind power forecast errors(WPFEs) based on normal distribution or other standard distribution models, which only characterize the aleatory uncertainty. In fact, epistemic uncertainty in WPFE modeling due to limited data and knowledge should also be addressed. This paper proposes a multi-source information fusion method(MSIFM) to quantify WPFEs when considering both aleatory and epistemic uncertainties. An extended focal element(EFE) selection method based on the adequacy of historical data is developed to consider the characteristics of WPFEs. Two supplementary expert information sources are modeled to improve the accuracy in the case of insufficient historical data. An operation reliability evaluation technique is also developed considering the proposed WPFE model. Finally,a double-layer Monte Carlo simulation method is introduced to generate a time-series output of the wind power. The effectiveness and accuracy of the proposed MSIFM are demonstrated through simulation results.

Underwater minirobots actuated by hybrid driving method

Underwater minirobots have attracted significant interest due to their value in complex application scenarios.Typical underwater minirobots are driven mainly by a soft or rigid actuator.However,soft actuation is currently facing challenges,including inadequate motional control accuracy and the lack of a continuous and steady driving force,while conventional rigid actuation has limited actuation efficiency,environmental adaptability,and motional flexibility,which severely limits the accomplishment of complicated underwater tasks.In this study,we developed underwater minirobots actuated by a hybrid driving method(HDM)that combines combustion-based actuators and propeller thrusters to achieve accurate,fast,and flexible underwater locomotion performance.Underwater experiments were conducted to investigate the kinematic performance of the minirobots with respect to the motion modes of rising,drifting,and hovering.Numerical models were used to investigate the kinematic characteristics of the minirobots,and theoretical models developed to unveil the mechanical principle that governs the driving process.Satisfactory agreement was obtained from comarisons of the experimental,numerical,and theoretical results.Finally,the HDM was compared with selected hybrid driving technologies in terms of acceleration and response time.The comparison showed that the minirobots based on HDM were generally superior in transient actuation ability and reliability.

Direction Protection Based on Time-domain Travelling Wave for Flexible MTDC Grid

Fault direction identification is important for dc protection,which cannot only be used in directional pilot pro-tection,but also plays an important role in non-unit protection(for excluding backward faults).Direction criteria based on i and di/dt have been utilized in LCC-HVDC system extensively.However,the reliability and sensitivity of the protection are significantly influenced by fault transition resistance and line distribution capacitors.In a flexible MTDC grid,boundary char-acteristics of the DC inductor can be utilized to judge the fault direction.But when the DC inductor is only installed at converter outlet,this kind of direction protection cannot work.Therefore,this paper analyzes the frequency characteristic of the fault traveling wave(TW)first.On this basis,the direction protection based on the time-domain TW is proposed,which is immune from frequency-dependence line parameters.Compared with existing direction protection,it has higher operation reliability and sensitivity,and is independent of the line boundary.Finally,the feasibility and superiority of the designed direction protection are verified by simulation cases based on PSCAD/EMTDC.Index Terms-Frequency-dependent characteristic,operation sensitivity,operation reliability,traveling-wave based direction protection.