LOCATED IN NORTH CHINA, TIANJIN IS A MUNICIPALITY DIRECTLY UNDER THE CHINESE CENTRAL GOVERNMENT. Fed by a 500-kV ring-connected network linking the power plant and the North China power grid, the electric network of Tianjin extends to some 11,900 sq km (4595 sq miles).

The downtown area comprises the political, financial, trading and cultural areas of the city and a population of 3.8 million. There are 13 220-kV substations and 70 35-kV substations downtown. Each 220-kV substation contains three 220/35/10-kV transformers with 150/150/70-MVA ratings. Therefore, in each 3-sq km (1.2-sq mile) area of downtown, the average available capacity is 60 MVA. The urban distribution networks are supplied by 220-kV substations equipped with transformers having secondary voltages of 35 kV and 10 kV, the voltage levels commonly used for regional networks.

In accordance with Tianjin Electric Power Co.'s plan for the next 20 years, 13 to 15 220-kV substations will be constructed and 70 to 75 35-kV substations will be constructed or increased capacity will be installed in existing substations. Hence, to supply the predicted load demand, a 35-kV substation will be required for every 1.5 sq km (0.6 sq mile). In the year 2030, the total installed capacity will be between 12,600 MVA to 14,000 MVA in the 220-kV substations in the downtown area to supply the expected demand of 7000 MW. To meet the expected future load demands, the technical decisions on network development are based on the following interrelated factors:

• Network voltage level
• Number of substations
• Number of outgoing circuits
• Distribution equipment selection.

A large number of environmental issues linked to land usage, including the provision of space for reinforcement at a later date and the control of noise, also form part of the decision-making process, which precedes the selection of the optimal economic solution.

The design of a distribution network should consider the existing load density and the long-term load density, determined by the annual growth in demand. Therefore, the ultimate load density influences the need to consider upgrading the existing network voltage, number of substations and outgoing circuits. The distribution network varies as the load density is linked to construction development, customer diversity (i.e., the percentage of industrial, commercial and domestic consumers) and several social issues.

INFLUENCE OF LOAD DENSITY TO VOLTAGE LEVEL

In high load-density areas, characterized by 7 kW/sq km to 8 kW/sq km, and considering the N-1 reliability standard, the 220-kV substations could be equipped with 140/150-MVA transformers with end users being supplied by 10-kV or 35-kV feeders. However, the exclusive use of 10-kV feeders for such high-demand users is not suitable for reliability reasons.

In areas where the load density is lower (2 kW/sq km to 3 kW/sq km), and considering the N-1 standard and system flexibility requirements, the load could be met by installing three 20-MVA, 35/10-kV transformers. This arrangement fulfills various consumer demands and economic restrictions; therefore, 10 kV and 35 kV are the preferred voltages for the end users.

The two secondary voltages used on the 220-kV main transformers are 35 kV and 10 kV. The 10-kV feeders supply nearby areas of 1 sq km to 2 sq km (0.4 sq miles to 0.8 sq miles). The 35-kV feeders supply 35/10-kV substations located in a load center equipped with three 20-MVA 35/10-kV transformers, thereby supplying the 10-kV network through low-voltage distribution transformers for consumers demands less than 3 MVA. Consumer demands greater than 3 MVA are normally supplied directly through a 220-kV substation's 35-kV busbar or indirectly through a 35-kV distribution substation.

A 35-KV DISTRIBUTION NETWORK

The average length of a 10-kV feeder, including the main line and spurs controlled by a 10-kV circuit breaker in the substation, is around 4 km (2.5 miles). Therefore, with one 35/10-kV distribution substation per square kilometer, the length of the main feeder is less than 2 km (1.25 miles) and the current carrying capacity is less than 630 A. Such an arrangement is economical and the line loss is optimal.

In Tianjin, an indoor 35/10-kV distribution substation occupies an area of 35 m by 25 m (115 ft by 82 ft) and is often placed in the basement of buildings.

The adoption of a 20-kV distribution network could replace the 35-kV and 10-kV networks, but, although this would reduce system losses, it would take many years to develop. For example, in Paris, France, the maximum capacity of a 220/20-kV main transformer is 70 MVA. EDF (Paris) unified the various voltage levels to 20 kV — a modification financed by the utility and the consumers on a 50-50 basis in a program that took 30 years to complete.

Following confirmation of the voltage level, the Tianjin Electric Power Co. considers the load density. Downtown Tianjin has an area of around 334 sq km (129 sq miles), with a planning load density of between 20 MW/sq km to 30 MW/sq km, creating a maximum demand of 6500 MW.

TARGET NETWORK STRUCTURE

To satisfy the N-1 rule used to provide system reliability, it will be necessary to install transformer capacity totaling 13,000 MVA. Assuming an average demand of some 1.5 kW per person for the predicted population in the downtown area of 4.3 million, supplying this demand will require the installation of 34 220-kV substations or, alternatively, investing in additional capacity at existing substations.

The development of the distribution network will require construction programs to commission new 220-kV substations that initially will supply a load equivalent to one-third of a substation's final capacity. Considerable planning is required to ensure the new substations have the appropriate capacity in terms of the number of outgoing feeders and their thermal ratings.

The Tianjin Electric Power Co. uses two designs for its 220-kV substations that are very compact, occupying an area of between 6000 sq m to 7000 sq m (64,583 sq ft to 75,347 sq ft). The designs are as follows:

• Key substations that are ring-connected and equipped with two to four 220-kV transformers to supply the local loads and two circuits for 220-kV transfer substations located in the suburbs

• Transfer substations that are equipped with three 220-kV, 150-MVA transformers with two transformers supplying the local load. The third transformer is supplied from an alternative 220-kV substation and connected as a line-transformer bank. The load on the three transformers is balanced by the substation's 35-kV busbar.

In the event of a 150-MVA transformer fault, at least six feeders within two 220-kV substations are available to transfer the load. Each interconnected feeder supplies two to three 35/10-kV substations, so each 35/10-kV substation has a power source from more than one 220-kV substation.

All 35-kV distribution substations are designed for the installation of three 20-MVA, 35/10-kV transformers. Often, two transformers are installed initially and the third is added later, although the 10-kV busbar for the three transformer arrangement is installed at the outset, occupying an area of between 6000 sq m to 7000 sq m. Each 35/10-kV transformer supplies two 10-kV feeders.

The standard 220-kV substation is equipped with 30 35-kV bays, of which 24 bays are used for outlet feeders. Between 15 and 18 bays are used for feeders supplying 35/10-kV substations and large power consumers, with the remainder being used for 35-kV network interconnections.

SUBSTATION EQUIPMENT

The selection of equipment is a key technical factor in the development and construction of a distribution network. The loading rate of 10-kV distribution transformers in downtown Tianjin is 40% or less during the off-peak period in order to avoid overload during the summer demand. Also, the distribution transformers used in residential areas must have a noise level less than 45 dB to 50 dB. The reduction of transformer loss and the low noise are the important problems that need a cost-effective solution. The reduction in noise for the 220-kV main transformers must first be solved to overcome any obstacles in the construction of substations in the downtown area.

PROBLEMS with THE CABLES

The selection of routes for distribution network circuits in urban areas is increasingly difficult, creating conflict with planning and infrastructure authorities. In Tianjin, there are more than 90 inlet or outlet cables for each 220-kV substation that require several road openings for cables. The 220-kV cables from the key substation to the transfer substation are now in concrete chambers, but more than 80% of the cables downtown are 35-kV and 10-kV cables. The increasing use of cables has resulted in a change to the neutral grounding mode from neutral compensating grounding to neutral low-resistance grounding in the medium-voltage network.

SUBSTATION MONITORING AND CONTROL

The centralized control of the substations is limited to the operation of circuit breakers. The operation of grounding disconnecting switches in substations still requires on-site manual switching, prolonging the total switching time. To realize improved efficiency and benefits, more investment is required in several areas: monitoring and control of transformer cooling systems; remote monitoring and control of switches and grounding switches; remote monitoring and control of dc devices; automatic tracking of arc-extinguishing coils; and automatic connecting and disconnecting of capacitors and inductors.

DISTRIBUTION AUTOMATION

Distribution automation is a system project that needs to be achieved through stages. New distribution substations are constructed according to the planned network, so the capacity/load ratio of the distribution network reaches 2.0 and substations have sufficient outgoing feeders to create the possibility for distribution network reconstruction. At this stage, the equipment used for the substation should be miniaturized, compact with minimum losses satisfying environmental standards to save space, energy and protect the environment.

Distribution networks should be redesigned and reconstructed to establish a ring-connected network for improving system reliability. Ring-connected feeders should only supply seven to eight transformers to reduce the outage range to shorten the fault-restoration time.

A fault transmission unit should be installed on each feeder. The installation of an effective communication system will transmit signals from the fault transmission unit to the dispatching center and system fault data from consumers. The system also should have a feedback signal facility from the dispatcher in control to remotely operate the equipment according to system operation and demand-side management requirements.

COMPACT, HIGH-DENSITY EQUIPMENT

The 35-kV vacuum circuit breakers are causing many overvoltage problems, so the Tianjin Electric Power Co. will replace them with SF₆ circuit breakers. The SF₆ circuit breaker is compact, saving space. Still, at the research and development stage, the open, integrated switchgear will use less SF₆ gas than the normal gas-insulated substations, to satisfy the Kyoto Protocols. Compact gas-insulated substations, integrated circuit breakers, current transformers and disconnecting switches together will reduce the space required for a substation. This will enable the Tianjin Electric Power Co. to increase the capacity of an existing substation by a factor of 1.5, when equipped with new equipment. In this way, the current capacity of the distribution network can be increased without building new distribution stations.

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Xue Jin is a professor-grade senior engineer and a former vice-general manager of the Tianjin Electric Power Co. in China. Currently, Jin is the vice-chairman of the Urban Power Distribution Committee of the Chinese Society for Electrical Engineering.