Abstract:Providing a reliable and economical supply of electricity to customers has always been of paramount importance of electric power systems. The failures of electric power systems can have enormous economic costs and serious negative social impacts. For example, half of India's population was affected by the massive blackout of India's power grid in 2012, which was the largest power system failure in history. Therefore, risk issues have been extensively and continuously recognized and studied from viewpoints of both power system planning and operation for decades. Moreover, recently the high renewable energy resources (RES) penetration of existing and future power systems will also have significant impact on system security and reliability due to fast fluctuation and unpredictable characteristics of RES. It can have either positive or negative impacts on the risk of power system operation and planning. The reliable and economical operation of future electric power systems requires development of new concepts, models, and methodologies for risk evaluation and management of power systems. This special issue is dedicated to address this need. Thirteen original papers are brought together to discuss various aspects of risk evaluation and management of future power system operation and planning

Abstract:The Indian power system underwent two separatemassive blackouts on 30th and 31st of July 2012.These blackouts exposed a series of inherent problems underlying the power system of India, such as its generation planning, transmission planning, grid infrastructure,management systems, primary energy resources, environmental constraints, electricity market, and informationtechnology, which together aggravate the massive blackouts.These factors which affect the adequacy and securityof the power supply in a similar manner as the transmission congestion of physical grids does, can be called by ageneric term as generalized congestion. Competitive manipulations and ‘‘game-playing’’ between variousinterested parties further intensifies these congestions.Here, from the perspective of generalized congestions, the paper reflects the inevitability of India’s massive blackouts,discusses occasional effects of trigger events, inherentevolution law of blackouts and the enlightenment on power industry in China, and explores risk control measures to defend blackouts in China. The positive role of strongsmart grid in insuring energy and environment security ishighlighted.

Abstract:As a consequence of competition in electricity markets, a wide variety of financial derivatives have emerged to allow market agents to hedge against risks.Electricity options and forward contracts constitute adequateinstruments to manage the financial risks pertainingto price volatility or unexpected unit failures faced bypower producers. A multi-stage stochastic model is described in this tutorial paper to determine the optimal forward and option contracting decisions for a risk-aversepower producer. The key features of electricity options toreduce both price and availability risks are illustrated byusing two examples.

Abstract:A short-term reliability evaluation is used for power stations, where each power generating unit is presentedby a multi-state Markov model. The main obstaclefor reliability evaluation in such a case is a "curse ofdimensionality"—a great (huge) number of states of entire power station that should be analyzed. A modern approachis proposed based on using Lz-transform that drastically simplifies computation. The proposed approach is useful for power system security analysis and short-term operating decisions. In order to illustrate the proposed approach,the short-term reliability evaluation for a power station with different coal fired generating units is presented.

Abstract:Ice flashover, lightning flashover and birddamage are the main reasons that cause transmission facility failure. The impact of these environmental factorson the operational risk levels of power systems should be taken into account in power system maintenance schedulingand operation planning. This paper studies the midshort-term risk assessment methodology considering theimpact of the external environment. The relationship model between natural disasters and transmission lines is presented.The conditional outage rate model and the sampling technique are then proposed considering the correlatedoutage of multiple transmission lines when a disaster happens. The framework of the mid-short-term riskassessment model is outlined. A test case of Jiangxi provincialpower grid validates the proposed model. The results show that the model can quantify the impact of disasters on the forced outage rate of transmission componentand their outage correlation, and thus effectively revealing the mid-short-term risk of power systems. The model can facilitate a more strategic decision-making on maintenance scheduling and operation planning of power systems.

Abstract:The modal analysis method is utilized to studythe influence of doubly-fed induction generator (DFIG) onelectromechanical oscillations. On this basis, the smallsignal stability region (SSSR) of power systems with DFIGin injection space is evaluated and the corresponding relationship between SSSR boundary and electromechanicaloscillations is analyzed. The effects of the locations ofDFIG on SSSR are considered. It is found that the boundary of SSSR consists of several smooth surfaces,which can be approximated with hyper-planes in engineering application. With the integration of DFIG, SSSRbecomes smaller, thus indicating the deterioration of the small signal stability of the system. The 11-bus system with four generators is used to illustrate the proposed method.

Abstract:Large-scale wind farm integration has brought several aspects of challenges to the transient stability of power systems. This paper focuses on the research of the transient stability of power systems incorporating with wind farms by utilizing risk assessment methods. The detailed model of double fed induction generator has been established. Wind penetration variation and multiple stochastic factors of power systems have been considered. Theprocess of transient stability risk assessment based on theMonte Carlo method has been described and a comprehensive risk indicator has been proposed. An investigation has been conducted into an improved 10-generator 39-bus system with a wind farm incorporated to verify the validity and feasibility of the risk assessment method proposed.

Abstract:Based on a new perspective in coordinating with the traditional ‘‘N-1’’ criteria and system risk, a real-time electricity market model is presented, in which the system risk is employed to model the system’s overall security level.This new model is called the risk-based security-constrained economic dispatch (RB-SCED). Relative to the security constrained economic dispatch (SCED) used in the power industry today, the RB-SCED finds more secure and economic operating conditions. It does this by obtaining solutions that achieve a better balance between post-contingency flows on individual branches and the overall system risk. The method exploits the fact that, in a SCED solution, some post contingencybranch flows which exceed their limits imposelittle risk while other post-contingency branch flows whichare within their limits impose significant risk. The RB-SCED softens constraints for the former and hardens constraints for the latter, thus achieving simultaneous improvement in both security and economy. In this work, the basic concept and the mathematical formulation of the RB-SCED model are system atically described. Experimental results on a 9-bus systemand the ISO New England actual system have demonstrated the advantages of RB-SCED over SCED.

Abstract:With the high penetration of renewable energy sources, the reliability of power systems becomes morevulnerable than ever because of the greater uncertainty and intermittence in power generation. Reactive power plays animportant role in the power system reliability, because it isclosely related to the system voltage stability and voltagecollapse. However, reactive power-related reliability issuesare seldom emphasized in conventional power reliability evaluations. This article investigates power system reliabilityof real and reactive power. Real and reactive power shortages and the associated voltage violations due tosystem failures are considered on reliability evaluation ofpower systems. A three-stage load-shedding technique forpost contingencies is implemented to determine the contributionsof real and reactive power on the system reliabilityand to find an optimal way to release networkviolation. The results provide the detailed information on power system planning and operation for system plannersand operators from real and reactive power aspects.

Abstract:Contingency analysis (CA) requires fast executiontime for real-time power system operations. Because CA problems can naturally be divided into separate subtasks,parallel computing helps to speed up the computationtime. This paper proposes a master/slave parallel computingarchitecture and studies the computation of CA in alarge-scale power system through high performance computing,adopting a message passing interface for implementation.In particular, although the execution time of CAvaries, there is a tradeoff between having an imbalanced workload and ‘‘paying’’ a synchronization penalty for parallel computing: either factor blocks the progress of scalability. The proposed layered dynamic scheduling method is effective to tackle the challenge of high synchronization cost and work load imbalance and have the potential to further scale for the N - 2 contingency analysis.

Abstract:Solar energy plays an important role in the global energy framework for future. Comparing with conventional generation systems using fossil fuels, the cost structure of photovoltaic (PV) systems is different: the capital cost is higher while the operation cost is negligible.Reliabilities of the PV system can also influence the costfor producing electricity. Investors, planners and regulators require deep insight into the return and cost of a PV project.A reliability based economical assessment of largescale PV systems has been conducted utilizing Universal Generating Function (UGF) techniques. The reliability models of solar panel arrays, PV inverters and energy production units (EPUs) are represented as the corresponding UGFs. The expected energy production models for different PV system configurations have also been developed. The expected unit cost of electricity has been calculated to provide informative metrics for making optimal decisions. The proposed method has been appliedto determine the PV system configuration which provides electricity for a water purification process.

Abstract: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 togeneration 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 wellas an appropriate reliability assessment model are essentialfor 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 thefollowing factors: running time, operating environment,operating conditions, and wind speed fluctuations. A multistatemodel for wind farms is also established. Numerical results illustrate that the proposed model can be well applied to power system reliability assessment as well assolving a series of reliability-centered decision-making problems of power system scheduling and maintenance arrangements.

Abstract:Universal Generating Function (UGF) techniqueshave been applied to Multi-State System (MSS)reliability analysis, such as long term reserve expansionof power systems with high wind power penetration.However, using simple steady-state distribution models for wind power and large generating units in reliability assessment can yield pessimistic appraisals. To more accurately assess the power system reliability, UGF techniques are extended to dynamic probabilistic simulation analysis on two aspects of modelling improvement.Firstly, a principal component analysis (PCA) combined with a hierarchal clustering algorithm is used to achieve the salient and time-varying patterns of wind power, then a sequential UGF equivalent model of wind power outputis established by an apportioning method. Secondly, other than the traditional two-state models, the conventional generator UGF equivalent model is established as a fourdiscrete-state continuous-time Markov model by Lztransform.In the construction process of such a UGF model, the state values are transformed into the integralmultiples of one common factor by choosing proper common factors, thus effectively restraining the exponential growth of its state number and alleviating the explosion thereof. The method is suitable for reliability assessment with dynamic probabilistic distributed randomvariables. In addition, by acquiring the clustering informationof wind power, the system reliability indices, such as fuel cost and CO2 emissions through different seasonsand on different workdays, are calculated. Finally, the effectiveness of the method is verified by a modified IEEE-RTS 79 system integrated with several wind farmsof historical hourly wind power data of Zhangbei wind farm in North China.

Abstract:Greenhouse gas emission regulation and renewable energy promotion policies have been implemented in many countries. Yet these two kinds of regulation policies have complex interactions between each other, and can either enhance or reduce the overall emission reduction efficiency. If not well tuned, these regulation policies may deviate from their original intention and lead to unnecessary social cost. Hence, the policy effectiveness,cost effectiveness, and dynamic efficiency of different policy mixtures between emission trading and renewable energy subsidy are studied based on a novel dynamic simulation platform of power economy and power system.Simulation results show that these two kinds of regulationpolicy can coexist, but a good coordination between theemission trading and the renewable energy subsidy can achieve better emission reduction outcomes.