Reliability analysis of photovoltaic systems

Reliability is an important parameter for the user of photovoltaic (PV) power systems. A methodology for the analytical treatment of the reliability of PV systems is proposed in this paper. The method depends upon the logic of the fault-tree technique. The reliabilities of the different components of a PV system are used to predict the reliability of the overall system. Today's most commonly known systems are considered and a reliability formula is developed for each system. The methodology presented is appropriate for a wide range of applications and system types.     

Availability analysis of heat recovery steam generators used in thermal power plants

The combined-cycle gas and steam turbine power plant presents three main pieces of equipment: gas turbines, steam turbines and heat recovery steam generator (HRSG). In case of HRSG failure the steam cycle is shut down, reducing the power plant output. Considering that the technology for design, construction and operation of high capacity HRSGs is quite recent its availability should be carefully evaluated in order to foresee the performance of the power plant.This study presents a method for reliability and availability evaluation of HRSGs installed in combined-cycle power plant. The method’s first step consists in the elaboration of the steam generator functional tree and development of failure mode and effects analysis. The next step involves a reliability and availability analysis based on the time to failure and time to repair data recorded during the steam generator operation. The third step, aiming at availability improvement, recommends the fault-tree analysis development to identify components the failure (or combination of failures) of which can cause the HRSG shutdown. Those components maintenance policy can be improved through the use of reliability centered maintenance (RCM) concepts. The method is applied on the analysis of two HRSGs installed in a 500 MW combined-cycle power plant.     

基于TPNT含风电场发输电组合系统可靠性评估

为克服含风电场可靠性评估中需已知风速分布函数的缺点,提出了一种基于三阶多项式正态变换(TPNT)的非序贯蒙特卡洛模拟法评估含风电场发输电系统的可靠性。在已知风速历史数据或风速分布函数的情况下,通过TPNT构建风速随机变量与标准正态分布变量的关系,进而利用标准正态分布函数的性质产生具有任意数量的具有指定相关性的风速样本,并应用于风电场接入的发输电系统可靠性计算中。通过算例分析验证了TPNT应用于发输电系统可靠性计算中的适用性。在此基础上,从风速相关性、额定容量、风资源强度和风电场位置四个角度分析了风电场接入对可靠性的影响。为含风电场发输电系统可靠性的评估提供了新思路。     

模糊故障树可靠性分析在三峡船闸的应用研究

介绍了模糊可靠性分析的基本概念，同时给出了基于故障率为模糊数的模糊故障树可靠性的分析方法。在大型复杂的三峡五级船闸监控系统故障树分析中，针对实际系统无法获取足够的可靠性原始数据信息，探讨应用模糊数学集合理论对系统进行基于故障率为模糊数的故障计算分析，通过计算结果分析，证明该方法可以应用于五级船闸监控系统的可靠性分析，并具有推广应用价值。     

A Technique for Reducing Computational Effort in Monte-Carlo Based Composite Reliability Evaluation

There is a growing need within the electrical utility industry to refine the methods used in the allocation of capital resources based on reliability considerations, and to determine what are the tradeoffs between cost and reliability. The evaluation of the steady state adequacy of bulk power (composite) systems is widely recognized as an important part of this process. The computational tools currently developed for bulk system reliability evaluation differ basically with respect to the method of selecting the system scenario (step (1)) and the network model used in the adequacy assessment (step (2)). For example, models such as SYREL, GATOR, RECS and COMREL, are based on the successive enumeration of severe/likely scenarios and use an ac power flow for adequacy assessment. In turn, models such as SICRET, MEXICO and CONFTRA are based on Monte-Carlo sampling of scenarios, and use a linearized power flow model ("dc" power flow) for adequacy assessment. One possible limitation of Monte-Carlo methods is the strong dependence of computational effort (proportional to the number of samplings) with respect to the desired accuracy of the estimates. For example, a sample size of 104 would be enough to estimate a LOLP of 10-3 with a relative uncertainty of 30%. However, the same estimate would require 106 samplings if the desired accuracy was 3%. Some methods have been proposed to reduce this computational effort, such as stratification, Importance Sampling and bounding methods. This paper describes a new technique for reducing the computational effort in Monte-Carlo based composite reliability evaluation.     

多能源数据驱动的电力信息物理系统综合态势感知模型

针对高比例可再生能源以及多类型能源设备接入的电网,以电力信息物理融合为基础,研究面向可靠性评估的电力信息物理系统运行状态综合分析方法,提出多能源数据驱动的电力信息物理系统综合态势感知模型。首先,考虑电力信息物理系统中影响系统运行状态的关键因素,研究面向可靠性评估的电力信息物理系统运行状态变化过程;其次,针对电力信息物理系统不同运行状态下关键设备扰动问题,研究基于改进鲁棒估计的电力信息物理系统正常状态、临界状态、紧急状态和恢复状态辨识方法,在此基础上,建立多能源数据驱动的电力信息物理系统综合态势感知模型;最后,以东北某地区电网运行数据为例进行验证。仿真结果表明,文章所提出的方法能够实现以可靠性辨识为目标的电力信息物理系统综合态势感知,并能保证较高的准确性和实时性。     

A heuristic-based approach for reliability importance assessment of energy producers

Reliability of energy supply is one of the most important issues of service quality. On one hand, customers usually have different expectations for service reliability and price. On the other hand, providing different level of reliability at load points is a challenge for system operators. In order to take reasonable decisions and obviate reliability implementation difficulties, market players need to know impacts of their assets on system and load-point reliabilities. One tool to specify reliability impacts of assets is the criticality or reliability importance measure by which system components can be ranked based on their effect on reliability. Conventional methods for determination of reliability importance are essentially on the basis of risk sensitivity analysis and hence, impose prohibitive calculation burden in large power systems. An approach is proposed in this paper to determine reliability importance of energy producers from perspective of consumers or distribution companies in a composite generation and transmission system. In the presented method, while avoiding immense computational burden, the energy producers are ranked based on their rating, unavailability and impact on power flows in the lines connecting to the considered load points. Study results on the IEEE reliability test system show successful application of the proposed method.     

Generating capacity adequacy evaluation of large-scale,grid-connected photovoltaic systems

Large-scale, grid-connected photovoltaic systems have become an essential part of modern electric power distribution systems. In this paper, a novel approach based on the Markov method has been proposed to investigate the effects of large-scale, grid-connected photovoltaic systems on the reliability of bulk power systems. The proposed method serves as an applicable tool to estimate performance (e.g., energy yield and capacity) as well as reliability indices. The Markov method framework has been incorporated with the multi-state models to develop energy states of the photovoltaic systems in order to quantify the effects of the photovoltaic systems on the power system adequacy. Such analysis assists planners to make adequate decisions based on the economical expectations as well as to ensure the recovery of the investment costs over time. The failure states of the components of photovoltaic systems have been considered to evaluate the sensitivity analysis and the adequacy indices including loss of load expectation, and expected energy not supplied. Moreover, the impacts of transitions between failures on the reliability calculations as well as on the long- term operation of the photovoltaic systems have been illustrated. Simulation results on the Roy Billinton test system has been shown to illustrate the procedure of the proposed frame work and evaluate the reliability benefits of using large-scale, grid-connected photovoltaic system on the bulk electric power systems. The proposed method can be easily extended to estimate the operating and maintenance costs for the financial planning of the photovoltaic system projects.     

New concept and procedure for reliability assessment of an IEC 61850 based substation and distribution automation considering secondary device faults

Smart grid is a power grid consists of extensive monitoring systems which deal with the monitoring of attributes such as current, voltage, power, and energy at distribution transformers, substations transformers, distribution switching devices and smart meters. Smart grid with advanced communication technologies can be used for several purposes such as efficiency and reliability improvement. IEC 61850 is the core standard in the smart grid domain for distribution and substation automation. This paper introduces a vision of modern substation and distribution systems using the IEC 61850. Network operators mainly assume that the modern substation and distribution systems based on the IEC 61850 are reliable for a long-time of operation. However, similar to any other systems, the implemented IEC 61850 might fail because of the operational failures or aging failures. This paper proposes a novel method for reliability evaluation of modern substation and distribution systems. A typical IEC 61850 based distribution and substation system is developed and analyzed using the proposed method. The fault tree analysis (FTA) is used to quantify the reliability of the system. The technique is implemented and demonstrated on the Roy Billinton test system (RBTS). The analysis is further extended on a 400/63 kV substation with a breaker- and-a-half configuration. In addition, the technique proves to be robust under different operations. The results verify the feasibility and applicability of the proposed method.     

Reliability evaluation of power system incorporating wind farm for generation expansion planning based on ANLSA approach

Reliability evaluation of power‐generating systems gives a mechanism to guarantee proper system operations in the face of different uncertainties including equipment failures. It is regularly not attainable to identify all possible failure states to figure the reliability indices because of the large number of system states engaged with system operations. Therefore, a hybrid optimization technique is required to analyse the reliability of the power system. This paper proposes a hybrid optimization technique to evaluate the reliability of a power system for the generation expansion planning incorporating wind energy source. The proposed hybrid methodology is the joined execution of both ant lion optimization algorithm (ALO) and lightning search algorithm (LSA), and it is named as ANLSA. ALO searching behavior is enhanced by LSA. Through the inherent convergence mechanisms, ANLSA search the meaningful system states. The most probable failure states contribute reliability indices of power generating system including mean down time (MDT), loss of load probability (LOLP), loss of load expectation (LOLE), loss of load frequency (LOLF), and expected demand not supplied (EDNS). Furthermore, ANLSA is utilized to assess the reliability of system under normal condition, integration of wind farm with capacity of 150 MW, and considering spinning reserve requirement (SRR). By then, the proposed work is actualized in MATLAB/Simulink platform and it is demonstrated on IEEE reliability test system (IEEE RTS‐79). Furthermore, the statistical analysis of proposed and existing techniques such as Monte Carlo simulation (MCS) and discrete convolution (DC) is considered. The comparison results demonstrate that proposed approach confirms its ability for evaluating the power system reliability.     

A Coordinated Control Method for Leveling PV Output Power Fluctuations of PV–Diesel Hybrid Systems Connected to Isolated Power Utility

A Photovoltaic (PV) system's power output is not constant and fluctuates depending on weather conditions. Fluctuating power causes frequency deviations and reduction in reliability of the isolated power utility or microgrid when large output power from several PV systems is penetrated in the utility. In this paper, to overcome these problems, a simple coordinated control method for leveling the fluctuations of combined power output from multiple PV systems is proposed. The conflicting objective of output power leveling and acquisition power increase is achieved by means of the proposed method. Here, output power command is generated in two steps: central and local. Fuzzy reasoning is used to generate the central leveling output power command considering insolation, variance of insolation, and absolute average of frequency deviation. In local step, a simple coordination is maintained between central power command and local power commands by producing a common tuning factor. Power converters are used to achieve the same output power as local command power employing PI control law for each of the PV generation systems. The proposed method is compared with the method where a modified maximum power point tracking control is used for smoothing the short-term change in each of the PV system's output. Simulation results show that the proposed method is effective for leveling output power fluctuations and feasible to reduce the frequency deviations of the isolated power utility to maintain reliability.      

Full electromagnetic transient simulation for large power systems

For building Global Energy Interconnection (GEI), it is necessary to implement new breakthroughs on large- power system simulation. Key routes for implementing full electromagnetic transient simulation of large-power systems are described in this paper, and a top framework is designed. A combination of the new large time step algorithm and the traditional small-time step algorithm is proposed where both parts A and B are calculated independently. The method for integrating the Norton equivalence of the power electronic system to the entire power grid is proposed. A two-level gird division structure is proposed, which executes a multi-rate parallel calculation among subsystems and element parallel calculation in each subsystem. The initialization method of combining load flow derivation and automatic trial-and-error launching is introduced. The feasibility of the method is demonstrated through a practical power grid example, which lays a foundation for further research.     

汽轮机密封油系统故障树分析

建立了汽轮机密封油系统的故障树，求出了用最小割集法表示的汽轮机密封油系统的失效模式。通过对失效模式的分析，提出了减少密封油系统故障的有效措施，指出了提高密封油系统可靠性的研究方向。     

储能设备对孤立风力发电系统的影响

采用序贯蒙特卡罗法对含有储能设备的风/柴孤立发电系统进行充裕度评估。针对样例系统,在发电系统强迫停运率、储能设备容量以及峰值负荷取值不同的情况下,计算发电系统的充裕度指标;研究储能设备对孤立发电系统充裕度的影响,并对产生影响的原因进行分析。结果表明,加入储能设备可改善发电系统的充裕度,提高系统的供电可靠性水平,减少风力发电机组输出功率波动对系统的影响。分析方法和结果可为储能设备在风力发电系统中的应用和储能设备容量的选择等方面提供参考。     

风光水互补发电系统的优化配置

为提高风光水互补发电系统的供电可靠性,降低经济成本,研究了电源容量的优化配置。首先提出独立和并网运行调度策略,然后考虑供电可靠性和蓄水库水量平衡等约束,建立了以全寿命周期成本最小为目标的优化配置模型,采用改进遗传算法进行求解。算例探讨了供电可靠性和联络线传输容量对优化配置的影响,验证了所提方法的有效性。由仿真结果可知,独立运行时,利用风光水互补发电即可保证较高的供电可靠性;并网运行时,采用联络线的双向交互电能能力,可进一步提高系统供电可靠性,有效降低经济成本。     

A multi-stage stochastic transmission expansion planning method

This paper presents a multi-stage stochastic model for short-term transmission expansion planning considering the available transfer capability (ATC). The ATC can have a huge impact on the power market outcomes and the power system reliability. The transmission expansion planning (TEP) studies deal with many uncertainties, such as system load uncertainties that are considered in this paper. The Monte Carlo simulation method has been applied for generating different scenarios. A scenario reduction technique is used for reducing the number of scenarios. The objective is to minimize the sum of investment costs (IC) and the expected operation costs (OC). The solution technique is based on the benders decomposition algorithm. The N-1 contingency analysis is also done for the TEP problem.The proposed model is applied to the IEEE 24 bus reliability test system and the results are efficient and promising.     

Operational reliability and economy evaluation of reusing retired batteries in composite power systems

To support a long-term sustainable electric future, there is ongoing development of electric vehicles, and thus, a serious problem is rising regarding recycling and the echelon application of batteries retired from electric vehicles. Herein, we propose to reuse such retired vehicle batteries as large-scale storage, based on which the operational reliability and the economy of composite generation and transmission systems are analyzed. In details, Gaussian functions were fitted according to data obtained using a holistic aging model. The proposed aging method simulated the capacity degradation of the retired batteries, with a low state of health, under various operating conditions. A universal generating function was adopted for probabilistic modeling of the capacity for battery modules consisting of hundreds of cells. The feasibility and effectiveness of using retired batteries in composite power systems were verified. The test results show that a significant investment can be saved, and system reliability improved, compared with systems without retired batteries. Finally, a method for scheduling the second retirement of retired batteries was established from the perspective of asset management.     

A new method for estimating the longevity and degradation of photovoltaic systems considering weather states

The power output of solar photovoltaic (PV) systems is affected by solar radiation and ambient temperature. The commonly used evaluation techniques usually overlook the four weather states which are clear, cloudy, foggy, and rainy. In this paper, an ovel analytical model of the four weather conditions based on the Markov chain is proposed. The Markov method is well suited to estimate the reliability and availability of systems based on a continuous stochastic process. The proposed method is generic enough to be applied to reliability evaluation of PV systems and even other applications. Further aspects investigated include the new degradation model for reliability predication of PV modules. The results indicate that the PV module degradation over years, failures, and solar radiation must be considered in choosing an efficient PV system with an optimal design to achieve the maximum benefit of the PV system. For each aspect, a method is proposed, and the complete focusing methodology is expounded and validated using simulated point targets. The results also demonstrate the feasibility and applicability of the proposed method for effective modeling of the chronological aspects and stochastic characteristics of solar cells as well as the optimal configuration and sizing of large PV plants in terms of cost and reliability.     

Reliability/cost implications of PV and wind energy utilization insmall isolated power systems

The application of renewable energy in electric power systems is growing rapidly due to enhanced public concerns for adverse environmental impacts and escalation in energy costs associated with the use of conventional energy sources. Photovoltaics and wind energy sources are being increasingly recognized as cost effective generation sources in small isolated power systems primarily supplied by costly diesel fuel. The utilization of these energy sources can significantly reduce the system fuel costs but can also have considerable impact on the system reliability. A realistic cost/reliability analysis requires evaluation models that can recognize the highly erratic nature of these energy sources while maintaining the chronology and interdependence of the random variables inherent in them. This paper presents a simulation method that provides objective indicators to help system planners decide on appropriate installation sites, operating policies, and selection of energy types, sizes and mixes in capacity expansion when utilizing PV and wind energy in small isolated systems     

A new synchronous generator based wind energy conversion system feeding an isolated load through variable frequency transformer

This paper aims to explore the possibility of synchronous generator (SG) based wind energy generation system feeding an isolated load using a latest power transmission technology i.e. variable frequency transformer (VFT). The proposed configuration does not employ any power electronics based interface as in conventional SG based stand-alone wind energy conversion systems (SWECS). For analysis, the simulation models of proposed configuration as well as conventional configuration have been developed under MATLAB-Simulink environment. A series of studies on power fed from the SG to the different loads at various SG input speeds has been carried out with the developed models. Further to analyze the effectiveness of the proposed method; the efficiency, total harmonic distortion (THD) of output voltage and THD of output current of the proposed method have been compared with those of the conventional method. From obtained results, it is observed that the proposed method is simple and does not produce harmonics. Moreover to validate the proposed scheme, an experimental analysis has been carried out. Further, the cost analysis of both systems has also been carried out. From the cost analysis, it is observed that the proposed system is cheaper than the conventional system.