^{1}

^{2}

^{1}

^{2}

Transmission power towers play an important role in power delivery systems. In recent years, some important results on reliability of transmission towers have been obtained based on theoretical analysis, but there are very few practical application systems of real-time condition monitoring. This paper proposes a new real-time reliable condition assessment system for 500kV transmission power towers based on stress measurement. The necessity of such systems and the architecture of the online monitoring system will first be presented. Through calculating the stress distribution condition of different components of the transmission tower under typical working conditions, those positions with relatively high failure probability in the transmission tower can be identified monitored for installing the stress sensors on them. A new method is presented for calculating the reliability index of the transmission tower structure is also developed based on the mechanical structure of the tower. In particular, the tower structure is simplified to a series system, and the

Transmission power towers are important parts of power transmission and distribution systems. Their failure would lead to severe consequences. In recent years, accidents in power grid caused by adverse weather conditions and climate are increasing. For example, many transmission towers were wrecked in South China in 2008 due to a large-scale ice disaster, which caused catastrophic damages to China power grid. Preventing the occurrence of such incidents has been research focus on transmission lines safety in recent years [

Considerable works have been devoted to the system reliability assessment of structures. In [

All these aforementioned works on reliability assessment of structure are theoretical analysis or simulations which are carried out by theoretical calculation. With the development of modern measurement technologies, novel online health monitoring systems of steel structures have been developed and employed for safety inspection in the last few years [

Under different loads, the steel structures of the transmission tower will subject to certain degree of deformation. Generally, this deformation is so slight that it is difficult to be observed directly by human. However, it can be measured by stress (strain) sensors. In fact, the real-time reliable condition assessment of transmission tower has both theoretical and practical significance. It can provide online monitoring of stress at critical positions of the transmission tower and hence real-time reliability assessment of the tower.

In this paper, a real-time reliable condition assessment system for transmission power tower based on stress measurement is proposed. In particular, measured stress data from strain/stress sensors installed in the tower is used to evaluate the reliability of the structure. To this end, the positions of stress sensors need to be determined and a method based on finite element analysis is proposed in our previous work [

The rest of the paper is organized as follows: Section

The system architecture of the real-time reliable condition assessment system has been introduced in our previous work [

Block diagram of System Structure.

The method to determine the installation positions of the stress sensors on the tower is based on finite element analysis. By calculating and comparing the force of the tower the force of the tower, those positions with relatively high failure probability in the transmission tower can be identified for installing the stress sensors. The procedure has been described in our previous work in detailed [

In the reliable condition assessment of the transmission tower, the main task is to evaluate the reliability index of the tower structure. Conventionally, the reliability index of complicate system can be obtained by two steps:

For a structure component, its reliability is decided by several random factors, such as load types, material strength, and geometry. Usually, these random factors are called basic variables, expressed by

The mean and variance of

If the variables

Considering a steel rod under pure tension loading, the rod will fail if the applied stress on the rod cross-sectional area exceeds the steel yield stress. The yield stress

The system reliability is determined based on the failure probability of its components. The reliability assessment of the structural system requires analysis of the various possible failure modes. Generally, two types of systems are commonly used:

The general structure function of a series system with

For the system in series with^{th} component

For a series system, if ^{th} failure mode, according to

The

Define

Transmission power tower has complex towering structure, and its reliability is greatly influenced by wind load and ice load. Meanwhile, it consists of a large number of main bars, helical rods, diagonal braces and other components. Therefore, it is very difficult to determine its structural reliability directly. Since main bars account for a large proportion of the weight of the tower in the structural design, they are the main load-bearing structures of the tower. For the sake of simplicity, a tower can be considered as damaged when any main load-bearing bar yielding occurred. So, we can use the reliability of main bars instead of the reliability of whole transmission tower to access its reliability in practice. We now proposed a method to model the reliability of high-voltage transmission tower in form of a series system based on its mechanical structure.

The length of the each main bar in high-voltage transmission tower does not exceed 10 ~ 12 m. For example, the height of 500 kV transimission power towers are mostly 30 ~ 60 m; main bars of tower structure can be divided into 4 to 8 sections; the numbers of main load-bearing sections (called main section) are 3 to 5 [

As mentioned above, a main bar section composes of four main bars, which are controlled by the same limit state equation and each of them is considered to have same failure mode. Therefore, a section can also be considered as a series system which composes of four bars. Each bar of the same section is working under same condition, as they are made of same material and has same structure, so, they can be seen to fully correlate with each other. In [

Assume the failure probabilities of the main bars in a section is

By (

Then, using (

Since four sensors are installed symmetrically in the main bar section, the corresponding measured values

Assuming the transmission tower structure is a series system composed of

(I) Calculate the reliability index of each section according (

(II) After obtaining the above reliability indices of main sections, the reliability index of tower structure can be evaluated by (

A 500kV oxytropis tower is employed to illustrate the method suggested in the paper. This transmission tower is erecting in Yunnan Power Grid, located in southwest China. Of course, other types of transmission power towers can also be calculated accordingly.

Positions where the sensors are installed must be decided firstly. So, the finite element model of transmission tower is established and then analyzes its stress in various extreme conditions. The finite element model is shown in Figure

Finite element model of a 500kV oxytropis tower.

Through the stress analysis in the finite element model, it is possible to select the points at which the stress is relatively strong and thus determine the installation position of the stress sensor. The specific process can be seen in our preliminary work [

Positions of stress sensors installed on the transmission tower.

According to the above design procedure, 12 stress sensors have been installed in 12 main bars in this real-time assessment system of the 500kV oxytropis power structure, which belong to three sections, respectively, as shown in Figure

Measurement stress data samples and corresponding reliability indices.

_{ ti }(MPa) | _{ t1 } | _{ t2 } | _{ t3 } | _{ t4 } | _{i} | Φ(_{i}) |
---|---|---|---|---|---|---|

Section A | 127.20 | 127.20 | -158.20 | -158.20 | 2.61 | 4.53 |

Section B | 121.63 | 121.63 | -146.80 | -146.80 | 2.92 | 1.81 |

Section C | 103.32 | 103.32 | -108.76 | -132.26 | 3.32 | 0.467 |

_{K} and the coefficient of variation_{R} can be obtained from specifications [

This study proposes a real-time reliability assessment system for transmission tower structure. It aims to address and evaluate the reliability index of tower structure system based on measurement stress data samples. By finite element analysis, main bars with large compressive stress will be selected for installing stress sensors. The main bars of tower structure are divided into 3 to 5 main sections. Therefore, the tower structure system can be seen as a series system model with 3 to 5 failure modes (components). In this project, 12 stress sensors will be installed; it means there are three failure modes in our simplified series system. Afterwards,

The data used to support the findings of this study are available from the corresponding author upon request.

The authors declare that they have no conflicts of interest.

The authors would like to express their gratitude to Professor S.C Chan and Dr. H.C Wu in the Department of Electrical and Electronic Engineering, the University of Hong Kong, for their useful advice and revision. This research was supported by the State Key Laboratory of Water Resources and Hydropower Engineering Science.