Integrating renewable energy and energy storage system provides a prospective way for power supply of remote areas. Focused on the isolated grids comprising renewable energy generation and energy storage, an energy storage sizing method for taking account of the reliability requirement and a bi-level control strategy of the isolated grids are presented in this paper. Based on comparative analysis of current energy storage characteristics and practicability, Sodium-sulfur battery is recommended for power balance control in the isolated grids. The optimal size of the energy storage system is determined by genetic algorithm and sequential simulation. The annualized cost considering the compensation cost of curtailed wind power and load is minimized when the reliability requirement can be satisfied. The sizing method emphasizes the tradeoff between energy storage size and reliability of power supply. The bi-level control strategy is designed as upper level wide area power balance control in dispatch timescale and lower level battery energy storage system V/f control in real-time range for isolated operation. The mixed timescale simulation results of Nan'ao Island grid verify the effectiveness of the proposed sizing method and control strategy. (C) 2014 Elsevier Ltd. All rights reserved.

Microgrid is commonly regarded as.an efficient way for integration of distributed generation (DG) in low voltage network. However, the integration method of microgrid in power system for maximum benefit needs to be further promoted. In this paper, a stochastic bidding strategy of microgrid in a joint dayahead market of energy and spinning reserve service is proposed taking into account of uncertainty of renewable DG units' output power and load. The stochastic bidding strategy is modeled as bi-level optimization problem and can be divided into two steps. First, Latin Hypercube Sampling (LHS) is utilized for generating microgrid uncertain net power scenarios according to day-ahead uncertain power scenario models, and then reduced by backward scenario reduction technique for less computation. Second, the upper level total bidding profit including bidding revenue, expected imbalance and operation cost is optimized by interior point algorithm in MATLAB for making optimal bids. The expected imbalance and operation cost is calculated by iteratively invoking lower level deterministic unit commitment under each microgrid uncertain net power scenario. The lower level deterministic unit commitment is coded and solved by mixed integer nonlinear programming (MINLP) solver DICOFT in GAMS. Finally, the optimal energy and spinning reserve bids are given by solving the bi-level bidding model. The model is applied to a modified typical low-voltage microgrid and the effectiveness and excellence of proposed strategy is proven by comparing simulation results with traditional deterministic bidding strategy. (C) 2014 Elsevier Ltd. All rights reserved.

Considering the environment protection and fuel cost in remote or island area,renewable energy generation and energy storage systems are utilized to form an isolated grid for power supply.Focus on isolated grids with high penetration and large scale wind generation,a control strategy of isolated grid consisting of wind generation and energy storage is presented in this study.The control strategy contains upper wide area power balance control and lower energy storage V/f control.According to ultra short-term wind power and load forecasts,optimal switching control quantities of wind turbines and loads in next control period are decided by wide area power balance control for minimization of control cost.And then grid power variation can be adjusted within power and capacity range of energy storage system.By real-time detection of voltage and frequency deviation,active and reactive current references are generated for power compensation in V/f control.Guangdong Shantou Nan’ao Island is given as a specific example for illustration.Finally,Matlab/Simulink software is applied to verify the effectiveness of the proposed control strategy in isolated grid.

Due to unit outage and fluctuation of renewable power generation and load, scheduling reasonable amount of spinning reserve is important for maintaining safe and economical operation of microgrids. Based on day-ahead forecast errors models of wind power, photovoltaic power and load, the uncertainty distribution models of wind power and photovoltaic power are built by total probability formula that comprehensively considers forecast errors and unit outage. After discretizing wind power, photovoltaic power and load uncertainty distributions, the discrete uncertainty distribution model of microgrid power is generated combining with the outage probability distributions of dispatchable units. Then taking minimized microgrid total social cost as objective, a computing model of optimal spinning reserve in microgrids is presented considering uncertain factors of microgrid power and supplying spinning reserve to the main grid. Finally an example confirms the feasibility of the proposed model after optimizing the spinning reserve by mixed-integer genetic algorithm. This work is supported by National Natural Science Foundation of China (No. 50837003).

The undispatchability of wind farm brings great challenge to safe operation of power system. Designing energy storage system for dispatchability of wind farm is an effective integration solution. This paper presents a novel sizing method of energy storage system for dispatchability of wind farm at specified confidence level, which is for the compromise between dispatchability of wind farm and energy storage size. The persistence method was used to generate the forecast data of wind power, and then based on forecast data a dispatch strategy was proposed for the minimization of energy storage size. The confidence level of dispatchability was defined as constraint and constraint functions of energy storage rated power and capacity were established separately with non-parametric estimation and curve fitting. The investment cost model considering lifetime of energy storage is proposed as objective function with the constraint functions. For obtaining optimal energy storage size, genetic algorithm was used to solve the non-linear objective function. Finally, the sizing method of lithium-ion battery energy storage system is illustrated as an example.

The defects of present analysis models for power system cascading failures are summarized, and a correlation model is proposed according to the characteristics of cascading failures. The link of cascading failures is defined and the correlation function is constructed to describe the relationship between the links, moreover, by analyzing the various influencing factors of cascading failures, the correlation function is quantified. The proposed model is not only useful to reflect the relationship between the links of cascading failures timely, but also quick to predict the trend of cascading failures and determine the main factors and minor factors, which provide a basis for prevention and control of cascading failures. Finally, IEEE 39-bus test system and actual power system are shown that the developed correlation model is correct and efficient for cascading failures in power system.

Human reliability analysis (HRA) is necessary for system safety assessment as well as equipment reliability analysis. The cognitive reliability and error analysis method (CREAM) as a representative HRA method provides nine common performance conditions (CPCs) to represent the contextual conditions under which a given action is performed. With a scarcity of empirical data, a high uncertainty in analyzing results is produced by subjective judgment. To obtain more objective and effective HRA results, this paper presents an optimized quantification method to evaluate the human error probability (HEP) according to CREAM, and its linguistic variables. The starting point for the quantification is an introduced fuzzy version of CREAM. However, many fuzzy rules are redundant, and occupy extensive computing time. The clonal selection algorithm combined with the fuzzy model is proposed to optimize the rule set. The evaluation method using the fuzzy-clonal selection algorithm is aimed to support possible applications for prospective and retrospective studies in the domain of safety assessment of power systems. The simulations are carried out on four presumed contexts, and a practical power system. The conclusions illuminate the feasibility of the proposed quantitative method.

Vulnerable transmission lines or sections have great impacts on large-scale blackouts and cascading failures in power grids. A comprehensive scheme to study transmission vulnerability based on the fault chain theory of security science is proposed in this paper. In this scheme, the cascading failure process and its generic features are described according to a fault chain. Power flow transfer, sequent overload of lines, and damage of transmission sections are modeled and normalized to predict all the fault chains. On the basis of the fault chains, a new vulnerability assessment index is presented to identify critical lines and sections contributing to failure extension and system instability. The effectiveness of the proposed assessment scheme is demonstrated via numerical examples on three power systems. Considering operation properties and transient impact, the proposed scheme can be executed in an efficient way.

In order to satisfy the needs of plug and play feature of distributed resources (DRs),class extension of microgrids is proposed to build the microgrid common information model (CIM) according to CIM extension guideline and microgrid's characteristics. For the established microgrid CIM,the changes to the existing microgrid CIM due to the addition of new DRs to a microgrid with fixed network structure is investigated. Referring to previous studies on application of power system CIM,the method of automatic implementation is explored to modify microgrid CIM after adding new DRs with plug and play feature. The research results can meet the needs of plug and play feature of DRs in a microgrid with fixed network structure.

In view of the plug-and-play requirements for a large number of smart devices in a smart grid,which results in the difficulties in setting up communication channels,a self-organizing communication mode in the smart grid based on geographical information is proposed.In this self-organizing communication mode,addressing is performed by all smart devices using geographical information.Routing and forwarding for data packets is implemented using geographical information.Depending on the information type,information fusion is realized in the process of forwarding data packets.A query algorithm is presented that automatically generates routing information in the process of enquiry and transmits information using the greedy algorithm.The test results verify the feasibility of self-organizing communication mode based on geographical information in a smart grid.

The defects of present methods for power system cascading failures have been summarized. According to the concept of preventive control of large scale blackout, a new risk asseessment method based on fault chain and fuzzy comprehensive evaluation is proposed for power system cascading failures. The model based on fault chain for cascading failures is built up, and then the consequence of cascading failures on the concept of transient security is quantified. Furthermore, the membership function is used to obtain the risk level which is described by fuzzy language, and then defensive suggestions are given by analysing the assessment results. Finally, IEEE 39-bus test system and actual power system simulations are shown that the proposed risk assessment method is utilizable and efficient to assess the risk of cascading failures in power system quantitatively and to establish defensive measures. © 2010 Chinese Society for Electrical Engineering.

Based on the graph theory,a new search method for power system cascading failures was proposed.According to real-time information of networks topology and load flow,the state graph of power system was established with divided substations service areas and transmission passages.Through the initial fault line included in transmission passage,the next key transmission line,which was suffered greatly from the outage line,was quickly found out by using the graph-theory-based approach.In the searching process,the behaviors of protective relays were taken into account,and the power flow and stability calculations were alternatively conducted to quickly find out the kind of power system cascading failures modes,which were easy to occur and lead to serious consequences.Using the proposed method,the cascading failures modes were quickly found out and the searching process was rational by means of adding the behaviors of protective relays.At the same time,wihout searching again,the cascading failures modes with the same state graph of power system only needed to be revised partly based on the changed power flow,which was not sensitive to the power flow.IEEE 39-bus test system shows that the developed method is correct and efficient for searching cascading failures modes in power system.

A real-time initiation approach of power system emergency control is proposed in this paper. In this approach, the system risk is defined and obtained by virtue of the start-up probability and the loss of system. The former can be provided by the fuzzy logic inference taking the imbalance power and the uncertainty of generator relays into account, and the latter is associated with the control cost. And then, based on the system risk, the start-up criterion is presented to decide the control time for system stability and low control cost. The approach is performed on the Hubei Grid under interconnection of North China Grid and Central China Grid. Comparing with the conventional approach, it can be validated that the proposed approach making the most of steady-state information for fast computation can avoid system instability caused by control delay or cascading blackout and ensure low control cost.

Cascading failure should be modeled before accident in order to preventing large scale blackout. Based on the probability analysis model of fault chain of security science, the prediction index of middle segments of fault chain based on flow of network is presented to establish the fault chains, in which the cascading failure probability is quantified. The method is simple to compute and is ended by the system collapse. The results can be used not only for on-line analysis of cascading failure, but also for risk assessment and preventive control. The simulation on IEEE 39 buses system testifies the validation of the proposed method.

System characteristics of smart grid such as autonomy,cooperation,hierarchy,evolvement and periodic stability etc.,are analyzed.The concept is put forward that the smart grid is a system of systems composed with the interconnected multiple autonomous systems.Accordingly,a conceptual layered cooperative system of systems model for the smart grid is developed.With this model,the autonomous systems distributed at different layers can be implemented via any one of the totally distributed structure,distributed plus concentrated structure and totally centralized structure,and has the periodic stability characteristic.Each autonomous information system or automation system can set one or more boundary nodes if necessary for communicating between each autonomous system.Moreover,the multiple autonomous systems jointly fulfill the periodic objectives of smart grid.At last,the principle and idea for smart grid development are illustrated under the guidance of the conceptual hierarchical cooperative system of systems model.

It is mentioned in this paper that the fault analysis for transformers with the method of fuzzy fault tree based on fuzzy theory,which turns the fault rate into fuzzy math fault rate and processes probability estimation on the fault using expert graded method without statistical information.The way of changing into fuzzy math is expatiated and an applied example is also been offered.The result shows that the analysis method of fuzzy fault tree provided good flexibility and adaptability.It is valuabe to transformer fault analysis and fault prevention in practice.

Treating voltage controllers as autonomous control agents, a novel secondary voltage regulation (SVR) strategy based on multi-agent theory is presented for improving the performance of secondary voltage control actions under different system operating states. According to the principle of the multi-agent technology, the framework of this new type of control scheme is described and the optimal coordination methods are investigated in the paper. Under normal conditions, secondary voltage controller (treated as Coordination Agent) is responsible for coordinating primary voltage controller (treated as Execution Agent) based on the optimization of the quadratic programming objective. In system contingencies, the agent-based cooperation methods are presented for enhancing the ability of fast and coordinated voltage and reactive power control. The concept of virtual control agency is proposed to adapt the emergency dynamic control environment. Contract net protocol (CNP), a widely used protocol in multi-agent system, is introduced to realize optimal coordination and cooperation among control agents in the agency. The simulation results show that the proposed scheme is very effective for maintaining an appropriate voltage profile under normal operating conditions and settling voltage violation problems in system contingencies, which is helpful in keeping voltage stability in case of large disturbances.

To improve the indeterminacy and imperfection of distribution networks fault information that make it difficult to obtain accuracy fault diagnosis results,a new fault diagnosis algorithm based on the rough sets and decision tree theory is proposed,which can extract diagnosis rules directly from reduced decision table.The rough sets theory as a new mathematical tool is used to deal with inexact and uncertain knowledge for pattern recognition.The target is mainly to remove redundant information and to seek for reduced decision tables by use of discernible matrix.As a quickly learning theory and classification tool,according to the value of the information entropy,the decision tree is used to extract diagnosis rules directly from reduced decision table.The rules from decision tree can be with clear relationship and easy to explain.With the proposed method the blindness and redundancy of ruling can be avoided and the space for decision table is evidently diminished.According to the value of information entropy,the priority of different characteristic information can be conducive to acquiring quick and satisfactory results for distribution networks without looking up the whole rules.In addition when the fault information is imperfect,the results can be still acquired according to the remained information, so the proposed method possesses strong tolerant ability.This method is thus developed to ensure diagnosis precision and speed up the implementation of distribution fault diagnosis system.Finally,examples are given to verify its effectiveness,and the comparison with the former method shows its advantages.