For the past 40 years the bureaus responsible for the transmission and distribution systems in Shanghai have employed live-line techniques in the maintenance of the overhead circuits. Initially, techniques were used to overcome the lack of firm capacity on the system, but later, these techniques developed along with the increase in transmission voltages. The development work has resulted in the widespread application of ground potential hot sticks and equi-potential bare hand methods on circuits with voltages ranging from 0.22 kV to 500 kV ac and +/-500 kV dc.
The electricity supply to Shanghai is under the operational control of five power bureaus or companies: -Shanghai UHV Transmission & Substation Co. (STSC) maintains the 220 kV-500 kV overhead transmission system and associated substations. - Shanghai Cable Transmission and Distribution Co. is responsible for all underground cables in the voltage range 0.22 kV to 500 kV. -Three power bureaus manage the O.22-kV to 110-kV overhead line distribution networks in Shanghai. The Shanghai Urban Power Supply Bureau (SUPSB) is responsible for the urban area. The other two bureaus are responsible for the east and south sectors of the city.
Work on Transmission Lines The STSC is responsible for 1120 miles (1800 km) of UHV transmission lines. Seven line worker teams perform all live-line operations. Figure 1 shows the live-line replacement of the four parallel insulator strings on a single-phase, 500-kV circuit on the terminal pole (see substation in background). This procedure is assisted by a specially designed, hydraulically driven line-grip that loosens the insulator string end nut. The conductor tension is transferred to the two insulated rods. Over-tensioning the conductor by approximately 1.6 inches (40 mm) enables the insulator replacement operation, which is done in pairs, to proceed. A 12-man overhead line team can replace a single-phase termination in less than four hours.
Work on Distribution Lines Continuity of supply is now a key component in customer service to industrial, commercial and domestic customers for the three large power bureaus responsible for the distribution networks. For example, the SUPSB is now responsible for supplies to 2.04 million customers in an area of 531 sq km. The main load and network characteristic features in 1996 were: -System maximum demand: 3434 MW units supplied-146.3 GWh. -Network voltages used: 220/380V, 10 kV, 35 kV, 110 kV and 220 kV. -Overhead line network: distribution 5600 miles (9000 km); transmission 250 miles (406 km). -Underground cable network: 1060 miles (1700 km). -Substation capacity 6019MVA: distribution transformer capacity 1672 MVA.
The distribution network operated by SUPSB and the two suburban bureaus is predominantly overhead, hence the need to develop and use live-line procedures for the majority of maintenance and refurbishment work on the overhead lines. Techniques have been developed to perform the following operations without disconnecting customers: -Replacing poles and installing additional poles. -Changing conductors or stringing additional conductors. -Installing pole top in-line air break line switches and drop-out fuses. -Changing and installing pole mounted distribution transformers. -Disconnecting and connecting jumpers at tee-off positions. - Replacing cross-arms and insulators. -Checking and changing drop out fuses and installing new fuses. -Checking and replacing lightning arresters (surge diverters).
These procedures use a number of methods: -Ground potential hot sticks. -Equi-potential insulated arm aerial platform. -Intermediate potential. -Split-phase grounding method.
Live Line Equipment To support its application of live-line work, SUPSB has used the technology developed by utilities in China and from across the world and has added to this technology. The bureau has designed and manufactured a new insulated arm aerial platform with a working height of 108 ft (33 m) and has developed many specially designed instruments and tools. New installation methods have been developed to fix the range of accessories at the hot stick rod tip. A new aluminum alloy is now used for casting and manufacturing accessories.
Future Development In 1996, SUPSB completed more than 3000 live-line operations on the distribution network. As a result of the continuity of supply to customers, the utility did not lose the revenue from an estimated 8 million kWh. To extend the range of work completed on the distribution network and eliminate outages for maintenance, SUPSB has extended the live-line training program to include all the line workers. This action is necessary if SUPSB is to achieve its goal that from the year 2000 all maintenance work on the distribution network will be completed without interrupting the supply to customers.
Zhou Yong-Xing is director of the Shanghai Urban Power Supply Bureau (SUPSB) and has 14 years experience in electric power distribution. During his career in SUPSB he has held engineering appointments in overhead lines and protection departments, and managerial posts as director of Regional Department and deputy director of SUPSB. He was awarded the master of management science degree at Jiaotong University of Shanghai in 1995. He is deputy chairman of the Urban Power Distribution Committee of the Chinese Society for Electrical Engineering.