System Adequacy Indices Research Articles

Adequacy Evaluation of Composite Power Systems Using an Evolutionary Swarm Algorithm

The generation and transmission capacities of many power systems in the world are significantly increasing due to the escalating global electricity demand. Therefore, the adequacy evaluation of power systems has become a computationally challenging and time-consuming task. Recently, population-based intelligent search methods such as Genetic Algorithms (GAs) and Binary Particle Swarm Optimization (BPSO) have been successfully employed for evaluating the adequacy of power generation systems. In this work, the authors propose a novel Evolutionary Swarm Algorithm (ESA) for the adequacy evaluation of composite generation and transmission systems. The random search guiding mechanism of the ESA is based on the underlying philosophies of GAs and BPSO. The main objective of the ESA is to find out the most probable system failure states that significantly affect the adequacy of composite systems. The identified system failure states can be directly used to estimate the system adequacy indices. The proposed ESA-based framework is used to evaluate the adequacy of the IEEE Reliability Test System (RTS). The estimated annualized and annual adequacy indices such as Probability of Load Curtailments (PLC), Expected Duration of Load Curtailments (EDLC), Expected Energy Not Supplied (EENS) and Expected Frequency of Load Curtailments (EFLC) are compared with those obtained using Sequential Monte Carlo Simulation (SMCS), GA and BPSO. The results show that the accuracy, computational efficiency, convergence characteristics, and precision of the ESA outperform those of GA and BPSO. Moreover, compared to SMCS, the ESA can estimate the adequacy indices in a more time-efficient manner.

An Efficient Framework for Adequacy Evaluation through Extraction of Rare Load Curtailment Events in Composite Power Systems

With the growing robustness of modern power systems, the occurrence of load curtailment events is becoming lower. Hence, the simulation of these events constitutes a challenge in adequacy indices assessment. Due to the rarity of the load curtailment events, the standard Monte Carlo simulation (MCS) estimator of adequacy indices is not practical. Therefore, a framework based on the enhanced cross-entropy-based importance sampling (ECE-IS) method is introduced in this paper for computing the adequacy indices. The framework comprises two stages. Using the proposed ECE-IS method, the first stage’s purpose is to identify the samples or states of the nodal generation and load that are greatly significant to the adequacy indices estimators. In the second stage, the density of the input variables’ conditional on the load curtailment domain obtained by the first stage are used to compute the nodal and system adequacy indices. The performance of the ECE-IS method is verified through a comparison with the standard MCS method and the recent techniques of rare events simulation in literature. The results confirm that the proposed method develops an accurate estimation for the nodal and system adequacy indices (loss of load probability (LOLP), expected power not supplied (EPNS)) with appropriate convergence value and low computation time.

Hybrid stochastic/robust scheduling of the grid-connected microgrid based on the linear coordinated power management strategy

It can be predicted that the future power system, especially microgrids (MGs) will consist of many different sources and storages. In this regard, many researches have been conducted on different energy management strategies to improve power system adequacy indices. This paper presents a hybrid stochastic/robust coordinated power management strategy (CPMS) to simultaneously improve the flexibility, reliability and security indices of MG in the presence of electric vehicles (EVs), energy storage system (ESS), distributed generation (DG) and demand response programming (DRP). At first, this paper models the original problem which minimizes the difference between MG operation and reliability costs and its flexibility and security benefits considering MG optimal power flow constraints. This problem is non-linear programming (NLP) that is generally resulted in the local optimal, hence, this formulation is converted into linear programming (LP) using the first-order expansion of Taylor’s series for linearization of power flow equations and a polygon for linearization of circular inequalities. In addition, the active and reactive load demand, energy price, active power of renewable energy source (RES), different parameters of EVs, and availability/unavailability of MG equipment are considered as uncertain parameters. Hence, the hybrid stochastic/robust optimization with coupling the bounded uncertainty-based robust optimization (BURO) and scenario-based stochastic programming (SBSP) is used for the presented problem for modeling of uncertain parameters. Finally, the proposed method is applied to 32-bus MG using GAMS software. The numerical results show the capabilities of the proposed strategy to improve power system adequacy indices determining the optimal power scheduling for MG devices.

Adequacy Assessment of Generating Systems Incorporating Storage Integrated Virtual Power Plants

This paper investigates the impacts of incorporating energy storage devices (ESD) into a virtual power plant (VPP) model and subsequently using it for performing generating system adequacy studies. A two-step linearized optimization formulation is proposed for developing the overall VPP model featuring the aggregated characteristics and capacities of several distributed generators (DG), ESDs and loads together with distribution network information like topology and constraints. While the first step determines the (charging or discharging) operating mode of the batteries, the second step outputs the optimal dispatch schedules for the DGs and ESDs in accordance with a fair energy allocation based operating strategy adopted in this paper. The VPP is modeled using an augmented IEEE 69-bus distribution network, and the overall generation adequacy studies incorporating the VPP are carried out on the IEEE Reliability Test System using a sequential simulation algorithm. Several indices are presented for quantifying the impacts of the ESDs’ incorporation on the VPP model’s performance. Finally, sensitivity studies conducted for investigating the effects of variations in the ESD rated capacities and charging/discharging periods on the VPP performance and system adequacy indices are also described.

Aggregation and Bidirectional Charging Power Control of Plug-in Hybrid Electric Vehicles: Generation System Adequacy Analysis

With increasing penetration of plug-in hybrid electric vehicles (PHEVs), it is necessary to introduce a two-way interactive mechanism between PHEVs and the power system to improve system reliability and satisfy the PHEV charging requirements. To address these requirements, a bidirectional charging power control method, based on the time series model of PHEV charging, is developed. The control strategy ensures that PHEVs achieve full-charge before the users’ desired disconnecting time, and allows the spare time during users desired charging horizons to be used to manage PHEV charging power and to respond to the generation capacity shortage signal. Grid-to-vehicle (G2V) and vehicle-to-grid (V2G) control strategies are defined to determine whether PHEVs draw power from the grid or inject power to the grid. Generation system adequacy and PHEV charging behavior are analyzed for various scenarios under this interactive mechanism, and the costs and benefits to vehicle users and utilities are analyzed based on several new generation system adequacy indices. The simulation results illustrate how charging power control can effectively improve generation system adequacy, particularly for higher PHEV penetration and when users are incentivized to extend charging horizons.

Impact of Energy Storage and Variability of PV on Power System Reliability

The variability of renewable sources has high impact on power system reliability, e.g. photovoltaic (PV), and energy storage (ES) is one of several options for better grid reliability [1–3]. This paper serves to establish power system reliability of hierarchical level 1 (HL1) using analytical techniques. The IEEE Reliability Test System (IEEE-RTS) generation model and load model are applied to convolute a system risk model. Incorporating PV improves system reliability but the variability of PV power output compromises on PV capacity credit. ES is included into system risk model to enhance system reliability performance. System adequacy indices are investigated to show the system reliability performance. Impact on system generation cost, via variation of PV and ES capacities are presented. Actual Singapore PV irradiance data and Energy Market Company price information are incorporated in this study. This analytical technique can help Independent Service Operators (ISO) to evaluate the potential of PV to benefit system reliability and find ways to improve its potentials.

Reliability and cost–benefits of adding alternate power sources to an independent micro-grid community

Interest in alternative energy resources such as wind, solar energy and fuel cell (FC) has been on the increase due to improved public awareness of the high energy cost and adverse environmental impacts of conventional energy sources. Therefore, the rapid growth and potential future demand for these energy sources suggest a need to consider both reliability and cost–benefits of the supply for each case. This paper presents a simulation methodology for reliability and cost assessment of these energy sources in an independent micro-grid (IMG) system, which is a distribution system with distributed energy sources such as micro-turbine, photovoltaic and fuel cells. A systematic technique and a computer program for reliability and cost assessment of the IMG system containing FC, photovoltaic (PV) and wind energy (WE) have been developed. The adequacy of the IMG is evaluated in three steps: (i) atmospheric data is generated for PV and WE in addition to the development of a 50 kW PEM FC generation and energy conversion model, (ii) the power delivered by these energy sources is calculated, and (iii) system adequacy and energy indices are calculated based on the system load balance equation, which is the combination of generated power and system load demand. The suggested technique can then be used to help system planners to provide objective indicators for suitable installation locations, operating policies, and energy type and size selection for IMG system containing alternative energy sources.

Composite system adequacy assessment using sequential Monte Carlo simulation with variance reduction techniques

Estimation of composite system adequacy indices using sequential Monte Carlo simulation approach with time-varying loads at each load bus is computationally quite expensive. Variance reduction techniques can be used together with the sequential simulation process to enhance the efficiency of the simulation. The paper discusses two commonly used techniques - control variates and antithetic variates with reference to the sequential simulation method used for adequacy analysis. The paper also utilises optimum control coefficients associated with the regression term in the control variate method to further improve the correlation between the control variate and the variable to be estimated. Case studies are conducted on two test systems.

Impact of power wheeling on composite system adequacy evaluation

One of the many possible uses and benefits of interconnection between neighbouring electric power systems is the opportunity to wheel energy through the transmission facilities of one system in order to serve another system. Wheeling of energy can also occur within a system when an independent power producer serves a load located at some other point in the system. iower wheeling transactions are recognized to have a definite impact on the utility's system losses depending upon the system topology, the amount of power/energy wheeled and the wheeling distance involved. These factors currently form the basis for determining service charges associated with power wheeling. Quantification of the reliability impacts of power wheeling is also important in order to understand fully the impacts of wheeling options. This paper discusses the effects of power wheeling on the composite power system adequacy indices of two test systems. The impacts associated with both intra-system and inter-system power wheeling are illustrated. The results presented were obtained using a basic computational tool for composite power system adequacy assessment.

Effect of rotational load shedding on overall power system adequacy indices

Reliability evaluation of a complete electric power system including generation, transmission and distribution facilities is an important objective in overall power system planning and operation. Due to the enormity of the problem, reliability analysis is not usually conducted on a complete power system, and reliability evaluations of generating facilities, transmission systems and of distribution system segments are usually conducted independently. There are many benefits in developing the ability to perform overall system reliability evaluation. The most obvious is the allocation of utility resources to optimise the overall system operation. The paper considers the influence of component outages in all parts of an electric power system to obtain a comprehensive appraisal of the overall system adequacy. Different selection procedures for dropping customer loads in the event of energy deficiencies arising from outages in the composite generation and transmission system are included in the calculation of overall power system adequacy indices.

Effects of nonutility generators on composite system adequacy evaluation

A significant component of the overall system electrical energy requirements of many power utilities is now being provided by independent power production facilities in the form of nonutility generators (NUGs) and cogeneration facilities. The authors discuss the impacts of NUGs on the composite power system adequacy indices of two test systems. They illustrate the effects on the calculated adequacy indices of using different system load curtailment philosophies in composite system adequacy assessment. Results obtained using an analytical approach and using a Monte Carlo simulation program as the computational tools are illustrated and presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Incorporation of a DC link in a composite system adequacy assessment — composite system analysis

The paper illustrates a procedure for adequacy evaluation of a composite generation and transmission system containing a direct current (DC) link. The transportation model option of the composite reliability (COMREL) program, developed at the University of Saskatchewan, is used to evaluate the composite system adequacy indices. The IEEE Reliability Test System (RTS) is augmented by adding a DC link and is used as the test system. The composite system load point and the overall system indices are calculated to quantify the adequacy of the composite system. The basic COMREL program was modified to accept a remote generation source connected to a composite system through a DC link. The analysis involves the construction of a generating capacity model at the remote bus and the movement of each generating capacity state through the DC line contingency state, without violating the carrying capability of the transmission line state or the generator state capability restriction. This type of analysis can be used to evaluate the benefits associated with design changes and capital expenditures in the DC facility. The paper presents the techniques employed for the composite system study and the results obtained for the modified RTS.

Hybrid approach for reliability evaluation of composite generation and transmission systems using Monte-Carlo simulation and enumeration technique

A hybrid approach using Monte-Carlo simulation and an enumeration technique for the reliability evaluation of large scale composite generation-transmission systems is presented. The Monte-Carlo method is based on combining the basic random sampling technique with a direct analytical approach for system analysis and the utilisation of a minimisation model for load curtailment. The method presented is suited to the analysis of large systems and can be used to include multi-state representation of generating units. Studies presented illustrate that derating-adjusted two-state models for generating units can lead to pessimistic appraisals of composite system adequacy. Variability in system load can be incorporated by using a chronological load representation or by an aggregated load model. The accuracy of the load representation and the required computation time increases with the utilisation of more steps in the annual load duration curve. The adequacy indices of different composite systems have different sensitivities to the number of steps in the load curve. A procedure for establishing a set of ratios of generation-transmission adequacy indices which can be used to judge the sensitivities of composite system adequacy indices to the number of steps in the load duration curve is presented. It is only necessary in the case of sensitive composite systems to use many steps in the load model to obtain accurate annual indices and fewer steps can provide sufficiently accurate annual indices in the case of non-sensitive composite systems. A non-uniform load increment step model can effectively improve the accuracy of the calculated annual indices for sensitive composite systems. These conclusions are supported by application of the concepts proposed to two test systems.

Indices for use in composite generation and transmission system adequacy evaluation

The adequacy of a composite power system involving both generation and transmission components can be expressed by computing a set of appropriate indices for both the individual load points and the entire system. A wide range of indices have been proposed in adequacy studies by power utilities, research organizations and educational institutions. The selection and definition of these indices are very dependent upon the intent behind the studies and the methodologies used. The absence of a uniform set of meaningful indices is a significant obstacle in any comparison of the different methodologies used for adequacy evaluation. This paper describes a set of load point and system indices that can be used in adequacy studies. These indices have been computed using the COMREL program developed at the University of Saskatchevan. The application of the indices using COMREL is illustrated by evaluating the adequacy of a small test system. The indices proposed and illustrated in this paper provide a basic framework for quantitatively assessing the adequacy of a composite generation and transmission system. It is expected that the indices presented in this paper will make a major contribution to the development of a standard set of overall system adequacy indices.

Effect of Station Originated Outages in a Composite System Adequacy Evaluation of the IEEE Reliability Test System

The reliability evaluation of a bulk power system normally includes the independent outages of generating units, transformers and transmission lines. Station originated outages, can, however, also have a significant effect on the composite system adequacy indices. Terminal station related failure events such as failures of breakers, transformers and bus sections are a major cause of multiple outages of major cxxponents (generators and/or transmission lines etc.). This paper illustrates the effects of terminal stations in composite system reliability evaluation by using the IEEE Reliability Test System (RTS). The paper also provides a valuable extension of the original RWS by including switching configurations at each bus.