1
Overview

This chapter introduces the basic concept of non-service interruption working and the development of working techniques, describes and analyzes indexes of power supply reliability, and explains the effect of non-service interruption working techniques, thereby giving readers a basic understanding of the significance of non-service interruption working.

1.1 Basic Concept of Non-Service Interruption Working

Generally, there are two ways for the construction or maintenance of electric power lines and equipment.

1. Service interruption working. It is a traditional way in which lines or equipment to be maintained is isolated for service interruption before being constructed and maintained and power supply is restored after working.
2. Non-service interruption working. It is a way for testing, maintaining and constructing electric power lines or equipment without interrupted supply to customers. It includes two kinds of ways as follows:
   1. Work directly on live lines or equipment, namely live line working.
   2. Continue to supply power to customers through bypass or mobile power, and then work on de-energized lines or equipment.

1.1.1 Basic Method of Live Line Working

Live line working refers to an activity in which a worker makes direct contact with live lines (or equipment) or works on live lines (or equipment) with special working tools, equipment (or device), to realize maintenance and testing on live lines (or equipment). It is an effective measure to avoid service interruption for maintenance and ensure normal power supply.

According to the relationship between a worker and a live part, that is, whether or not the worker makes direct contact with the live part with parts of his or her body, live line working method can be divided into two basic methods, namely, contact working and at a distance working; according to the body potential of the worker, live line working can be classified into earth potential working, middle potential working and equipotential working.

1. Earth Potential Working

   Earth potential working refers to an activity in which the worker makes contact with the live part with insulating tools while is at the same potential as that of the earth (tower). At this time, the relationship between the body and the live part is "earth - body - insulating tools - live part." See Figure 1.1 for the schematic diagram of earth potential working. Earth potential working is also called zero potential working, known as distance working abroad. Earth potential working is performed using four basic methods of "supporting, dragging, tightening and hanging", and the coordination of these methods serves as a major means for at a distance working. The key of this method is to maintain a sufficient safety distance between the body and the live part, and that the insulating tools should have useful insulation distance.

2. Middle Potential Working

   Middle potential working refers to an activity in which the worker makes contact with the live part with insulating tools while is always at the middle potential between the earthed part and the live part. At this time, the relationship between the body and the live part is "live part - insulator - body - insulator - earth." See Figure 1.2 for the schematic diagram of middle potential working. The body is isolated respectively from the earthed part and the live part by two sections of isolators which restrict electric current from flowing through the body, while the air gap from the body to the earthed part and to the live part prevent the live part from discharging to the earthed part through the body. The sum of the two sections of air gaps is generally known as the complex gap, expressed by Sz. To adopt the middle potential working, it is important to meet the requirement for the complex gap (Sz?=?S1?+?S2). The concept of the complex gap will be specifically introduced in subsequent chapters.

3. Equipotential Working

   Equipotential working refers to an activity in which the worker makes direct contact with the live part while the body potential is equal to the live part's potential. Equipotential working is also called contact working, known as bare hand working abroad. At this time, the relationship between the body and the live part is "live part - body - insulator - earth (tower)." See Figure 1.3 for the schematic diagram of equipotential working.

Figure 1.1 Schematic diagram of earth potential working. 1-Live part; 2-Insulator; 3-Body; 4-Earthed part.

Figure 1.2 Schematic diagram of middle potential working. 1-Live part; 2-Insulator; 3-Body; 4-Earthed part.

Figure 1.3 Schematic diagram of equipotential working. 1-Live part; 2-Body; 3-Insulator; 4-Earthed part.

1.1.2 Bypass Working and Mobile Power Working

1. Bypass working

   Bypass working refers to an activity in which temporary current-carrying bypass lines or equipment, such as bypass cables (lines) or bypass switches, replace operating lines or equipment (such as circuits, breakers, or transformers) to be de-energized for maintenance and replacement, and is then removed after the normal power supply is restored upon the working is finished. This method ensures uninterrupted power supply to customers during the working.

   Bypass working brings a new concept to the conventional live line working and realizes "non-service interruption working" by effectively combining several conventional live line working operations. This shows that the flexible combination of bypass working and conventional live line working can fundamentally change the situation where the existing operations on power grids mainly rely on service interruption working supplemented by live line working.

2. Mobile power working

   Many operations on power grids, such as replacement of distribution transformers (for capacity increase), relocation of pole lines, or change of conductors, cannot be directly realized through live line working. However, it is possible to realize less (the interruption time is the switching operation time) or no interrupted supply to customers by separating the lines or equipment to be maintained from the power grids and continue to supply power to customers through the mobile power in an independent network, which is then removed after the normal power supply is restored upon the working is finished. This is the basic idea of mobile power working, and mobile power includes mobile generator vehicles, emergency power vehicles or mobile box-type transformers.

1.2 Development of Working Techniques

1.2.1 Development History of Live Line Working

1. Domestic development history

   Live line working in China started in the 1950s, which witnessed the recovery and development of national economy with so much to do. At that time, due to the rapidly increased electric energy production, inadequate power supply facilities, and higher demands of large industrial customers for continuous power supply, normal service-interruption maintenances were limited. To solve the conflict between the service-interruption maintenance of lines or equipment and the uninterruptible power supply to customers, live line working emerged. In 1953, Anshan Electric Power Bureau successfully developed simple tools for live cleaning, replacement, and disassembly and assembly of distribution lines or equipment and leads. In 1954, the live replacement of cross arms, wooden poles, and insulators on a 3.3?kV distribution line achieved success. In 1956, the live replacement of wooden suspension poles, cross arms, and insulators on 44-66?kV distribution lines was realized. At the end of 1957, a full set of tools for live replacement of insulators on 154-220?kV transmission lines were successfully developed, and a full set of tools for live line working on 3.6-66?kV lines were further improved. In 1958, Shenyang Central Laboratory began the study on the direct contact of human body with live lines or equipment again and successfully completed the equipotential test on the direct body contact with a 220?kV live line in the test site for the first time. All these attempts laid a solid substantial and technical basis for the popularization and development of live line working in China.

   In 1959-1966, live line working was popularized as large- and medium-sized power supply enterprises in different local areas successively developed live line working operations and did research on tools for live line working. Maintenance methods were developed from at a distance working and equipotential working to hot washing. Maintenance tools were developed from original hard tools such as supporting, dragging, and hanging sticks to rope and portable tools, which can be in any combination, whereas working operations were expanded to fields like live replacement of conductors and lightning conductors.

   In 1968, Anshan Electric Power Bureau successfully completed the test on the access to potential along two strings of tension insulators on a 220?kV transmission line.

   In 1977, the former Ministry of...