Power system operational reliability evaluation based on real-time operating state

Abstract—Traditional reliability evaluation theory, which reflects long-term average reliability level of power systems in the typical operating mode, ignores the impact of real-time operating conditions on system’s reliability, so that the system’s reliability level in fault states or special operating modes can not be expressed. In order to evaluate power system’s reliability in real-time operating states, the concept and algorithm of operational reliability are proposed for the first time. Analyzing the impact of such operating conditions as transfer capacity of lines, bus voltage and system frequency on failure probability of components, component’s reliability models based on real-time operating conditions are established in the algorithm of operational reliability. Fault consequences are analyzed with the influence of operating conditions, such as power generation dispatch, load level and network configuration. The evaluation results of a simple system show that real-time operating conditions have a strong impact on system’s operational reliability which can be reflected by the presented algorithm.

Index Terms—Operational reliability; Real-time operating condition; Reliability model; Fault analysis

I. I NTRODUCTION

ith the rapid development in power demands, the power

grid is becoming even larger and more complicated. It will be more difficult to ensure the reliability during power system’s operation. The challenging task encountered by power system managers is how to evaluate the reliability of power system in the course of operation, especially in the fault states and special operating modes.

Based on probability and statistics, traditional power system reliability evaluation theory is applied to off-line evaluation and planning of power system. A variety of achievements have been accomplished on the concept, model, algorithm, software and application of power system reliability [1]~[3]. Nevertheless, the models and algorithms, which are based on the constant operating conditions and component’s reliability model[4]~[6], ignore two aspects. One is the impact of transfer capacity of lines, bus voltage and system frequency on the component’s failure probability, the other is the impact of power generation dispatch, load level and network configuration on the fault consequence. Due to the two system-wide blackouts in WSCC system in North American happened in 1996, CERTS was organized in 1999 to research reliability of power grid. Adopting the advanced real-time This work was supported in part by National Basic Research Program of China under Grant 2004CB217901.

The authors are with State Key Laboratory of Power Systems, Electrical Engineering Department, Tsinghua University, Beijing 100084, P. R. China (email: yzsun@) measurement technique, CERTS tried to introduce the theory and method of risk evaluation into power dispatch/control center to evaluate the reliability of large-scale system during operation[7],[8].

In order to evaluate power system’s reliability in the current operating states, the concept and algorithm of operational reliability are proposed in the paper. The impact of real-time operating conditions on the component’s reliability models and fault consequences is considered.

II. C OMPONENT’S R ELIABILITY M ODEL B ASED O N THE R EAL-

TIME O PERATING C ONDITIONS

Fig.1 Reliability models’ parameters (dashed line denotes the mean

value of solid line)

Traditional planning reliability evaluation theory reflects long-term average reliability level of power system, so failure probability and failure rate of components are chosen as the mean value of statistic, as shown in the dashed line in Fig.1; however, the fault in special operating modes can not be explained and evaluated by traditional planning reliability evaluation theory.

A. Failure probability of transmission lines based on the transfer capacity

The increase in transfer capacity will result in the increase of the failure probability of transmission lines. The reasons are as follows.

1). With the increase in transfer capacity, the heat in lines will increase, which has two undesirable effects: a). Annealing and gradual loss of mechanical strength of the conductor caused by continued exposure to temperature extremes, b). Increased sag and decreased clearance to ground due to conductor expansion at higher temperature. Moreover, lines will fuse when transfer capacity exceed the thermal limit of lines for a long time.

2). When transfer capacity exceeds the thermal limit of lines, over-load break and over-current break will trip. The relation between trip setting and transfer capacity is as shown in Fig.2. The bigger the transfer capacity L is, the smaller the

Power System Operational Reliability Evaluation Based On Real-time Operating State

Yuanzhang Sun, Senior Member, IEEE，Lin Cheng, Haitao Liu, Shan He

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