In Shanghai, China, the existing 35-kV substations are located indoors on large sites, and the operation and maintenance costs are relatively high. The design and development of composite switchgear technologies have resulted in wide-scale application of 35-kV and 10-kV SF₆ switchgear, which, being compact, has reduced the land area required for distribution substations.

To standardize substation equipment specifications, construction, operation and maintenance in order to improve the economic efficiency and simplify the equipment tendering process, the State Power Grid Co. (SPGC) invited several design companies to study and produce a SPGC manual for 35-kV substations. This manual now forms the specification for the design of 35-kV B5-type substations in the Shanghai power grid.

New Compact Design

A new compact design has been adopted for new substations, where available land is limited, and for substations in some central districts of Shanghai, where available sites are generally irregular in shape. For these situations, composite switchgear provides the optimum solution. Based on the premise of accepting a moderate increase in the height of the building, there are two main methods to reduce the land area occupied by a substation:

• Separate the transformer tank and the cooling radiator by vertical disposition. Applications of vertically displaced transformer cooling radiators include the Caoyang, Changqiao and Jingshajiang 35-kV substations in Shanghai.

• Compact Distribution Transformers

  Select compact 35-kV and 10-kV switchgear. For example, SF₆-insulated switchgear requires less space for operations staff.

The selection of distribution transformers available for 35-kV applications according to the type of insulation can generally be classified as dry-type, SF₆-insulated and oil-insulated transformers. Usually selection is based on the manufacturer, operational and cost differences.

Based on the production technology of most domestic transformer manufacturers at present, the maximum capacity of a dry-type transformer is 16 MVA, which can be increased to 20 MVA by fitting fans. There are examples of 20-MVA dry-type transformers in successful operation in Shanghai, such as in the Shanghai South Railway Station's 35-kV substation. However, the Shanghai power system planning standard specification for the transformer rating is 31.5 MVA for all new 35-kV substations.

There are very few SF₆-insulated transformers in Shanghai. All the units being installed today are at the 110-kV voltage level. As no SF₆-insulated transformers have been used in Shanghai, there is a lack of operational experience. In addition, in China, there are fewer manufacturers of gas-insulated (GIS) transformers than manufacturers of dry-type or oil-insulated transformers. Therefore, except for the substations integrated within commercial or civil buildings that require special fire-protection measures, SF₆-insulated transformers are not selected for 35-kV substations.

Oil-insulated transformers have the advantages of good insulation, improved cooling and a moderate price. They are widely used. The cost of SF₆-insulated transformers is three times that of oil-insulated transformers, and the cost of dry-type transformers is equally costly. Therefore, oil-insulated transformers have a significant cost advantage.

In accordance with the disposition of the transformer's tank and the cooling radiator, oil-insulated transformers can be classified as integrated, separated with horizontal or vertical disposition. Vertical disposition requires the least land occupation as the transformer's radiator is positioned above the transformer room, so the area of land required is limited to the dimensions of the transformer room.

Switchgear Selection

Based on past experience, if the integrated disposition is selected, the transformer room requires natural ventilation or a fan-induced ventilation attic for heat dissipation. This can result in low-frequency noise emission through the ventilation, making it difficult to comply with Shanghai's noise-level restrictions of 55 dB during the day and 45 dB at night. When the transformer and radiator are separated, the transformer is totally enclosed in a room that can be coated with noise-absorbent material, reducing noise emission to within acceptable environmental levels. Although the radiator room opens to the outdoors, any radiator noise emitted can be controlled to below 45 dB. Therefore, the optimized compact design scheme is based on an oil-insulated transformer with the tank and radiator vertically disposed.

Composite switchgear technology is now mature and extensively adopted for 35-kV and 10-kV indoor substations. The choice of switchgear is limited to GIS and air-insulated switchgear (AIS).

When AIS is selected, the width of the operational corridor between two facing switchboards is twice the space required to withdraw a single circuit breaker for maintenance plus an additional 900 mm (35 inches). In practice, the operational corridor used in SGPC's typical design of 35-kV Shanghai substations, equipped with two facing switchboards, should be 3000 mm (10 ft). Furthermore, the standard design scheme specifies the width of the maintenance corridor behind each switchboard, and the width of the switchroom for 35-kV and 10-kV substations is 11,000 mm (36 ft).

Transformer and Switchgear Interconnection

When SF₆ switchgear is selected for the two facing switchboards, the clearance required to perform routine operation and maintenance has to be considered, some 2000 mm (7 ft). Allowing for the clearance required behind each switchboard, the overall width of 35-kV and 10-kV substations equipped with SF₆ switchgear is reduced to 7500 mm (25 ft). Therefore, the reduction in width by selecting SF₆ switchgear results in the optimum compact design.

The interconnection between the 35-kV SF₆ switchgear and transformer is usually through SF₆-insulated bus bars or cable. Similarly, the connection between 10-kV SF₆ switchgear and the transformer is mainly through SF₆-insulated bus bars, cable or insulated bus bars. The length of each connection is almost the same and Table 1 compares the different practices.

It is apparent that the cost of connection between the transformer and switchgear is lower with the use of cable than it would be using SF₆ bus bars or insulated bus bars. Currently, the cable connection between transformers and SF₆ switchgear is the normal means of interconnection used in Shanghai's 35-kV substations. Therefore, the optimum compact design solution is based on the use of cable for the interconnection between the high-volage or low-voltage side of the transformer and 35-kV or 10-kV SF₆ switchgear.

Compact vs. Typical Design

Also for the compact design, the main transformer room and the 35-kV and 10-kV switchroom are situated on the first floor, with the vertically displaced radiator in the capacitor room together with the station service transformer room on the second floor and the control room on the third floor. The indoor substation also includes a half-underground cable floor used as the substation entrance and for incoming and outgoing underground cables. The capacitors are installed to provide reactive power compensation to improve the power factor.

To construct city center substations, it is important to make the best use of the available land. The land required for the optimum compact 35-kV substation is 46% less than that for the typical design. This significant reduction alleviates the problem of securing favorable substation sites in the metropolitan central districts, where there is a shortage of land. At present, there are no land charges applicable to substation sites.

Substations based on the optimum compact design concept require less land, but the overall cost is higher than the typical design because they use the latest technologies, which require less in-service maintenance. The capital cost of the equipment for the two design schemes reveals the equipment cost for optimum compact substations is 4.65 million yuan (RMB), or 30%, more than for the typical standard design.

Shanghai Power Grid Co. has already adopted some of the recommended compact standards by installing substations with the transformer horizontally displaced and some substations equipped with SF₆ switchgear.

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Wenliu Zhuang (zhuangwl@sepd.com.cn) graduated in 2001 from the Shanghai Electric Power Institute, where he majored in electrical engineering and automation. He then joined the Shanghai Electric Power Design Institute, where he worked as a design engineer on power transmission and substation projects. His current focus is mainly linked to substation design. In 2008, Zhuang earned the title of a national registered electrical engineer.

Table 1. Comparison of the Transformer to SF₆-Insulated Switchgear

Voltage	Means of connection	Specifications	Estimated cost (RMB)	Maintenance requirement	Application
35 kV	SF6 bus bar	1250 A	28,000	Non-maintenance	Few
	Cable	2 × (3 × 400) sq mm	2164	Few	Common
10 kV	Insulated bus bar	3150 A	18,000	Few	Few
	Cable	3×3×(1×630) sq mm	5310	Few	Common

Table 2. Comparison of Substation Designs — Typical vs. Optimum Compact Design

Item		B5-type substation design	Optimum compact substation design
Main transformer capacity		2 or 3 rated 31.5 MVA	2 or 3 rated 31.5 MVA
Outgoing circuits	35 kV	2 or 3 feeders	2 or 3 feeders
	10 kV	20 feeders	20 feeders
Means of connection	35 kV	Overhead line connected to 35-kV transformer by a circuit breaker	Overhead line connected to 35-kV transformer by a circuit breaker
	10 kV	Single bus bar with bus sections	Single bus bar with bus sections
10-kV reactive power compensator		Shunt capacitors 2 × 4800 MVAR or 3 × 4800 MVAR	Shunt capacitors 2 × 4800 MVAR or 3 × 4800 MVAR
10-kV grounding/earthing mode		Resistance	Resistance
Distribution equipment	35 kV	Indoor single switchboard	Indoor single switchboard
	10 kV	Indoor double-row switchboard	Indoor double-row switchboard
Disposition		All indoor, single building	All indoor, single building
Type of main transformer		Transformer tank and radiator in horizontal disposition	Transformer tank and radiator in vertical disposition
Land area occupied by substation		754.72 sq m (21.2 m × 35.6 m)	411.60 sq m (14.0 m × 29.4 m)
Capital cost of equipment (for phase No. 1)		15.55 million yuan (RMB) (US$2.278 million)	20.2 million yuan (RMB) (US$2.960 million)

Table 3. Comparison of Equipment Cost — Typical vs. Optimum Compact Design

Item	B5-type design scheme		Optimum compact design scheme
	Equipment	Cost (RMB) (US$)	Equipment type	Cost (RMB) (US$)
35-kV main transformer tank and radiator	In horizontal disposition	5,650,000 827,802	In vertical disposition	6,550,000 959,665
35-kV switchgear	Cabinet	930,000 136,258	GIS	1,700,000 249,073
35-kV switchgear	Cabinet	4,800,000 703,266	GIS	7,570,000 1,109,109
Connection between switchgear and transformer	Enclosed cable	140,000 20,512	Cable	350,000 51,280
Total		11,520,000 1,687,838		16,170,000 2,369,127