The Famous Shanghai Yangshan Deepwater Harbor is located in the small Yangshan Island sea area east of Hangzhou Bay and southeast-east Shanghai. The harbor is connected to the mainland of Shanghai by the Donghai Bridge, which is 32.5 km (20 miles) long.

To satisfy the harbor's maximum demand of 130 MW (120 MVA), 110-kV cables were on the bridge to connect the harbor to the mainland. Shanghai Electric Power Co. began the design work in 2003, and completed the associated construction work on the bridge for the cable installation in 2005.

The project included two circuits, each having a 38-km (24-mile) route of which 25 km (15.6 miles) is on the bridge, making it one of the longest cable systems installed on a bridge in the world. Table 1 shows the key parameters of the project.

PROJECT FEATURES

The Donghai Bridge is an important highway with four lanes plus an additional two lanes for backup. The various sections of the bridge structure that had to accommodate the 110-kV cables are described in Table 2. The Shenyang Furukawa Cable Corp. Ltd. was awarded the cable-supply contract. Cross-linked polyethylene (XLPE)-insulated cable was selected for its superior electrical performance and ease of installation.

To avoid mechanical damage that may occur due to changes in cable temperature, snaking of the cable was used on the bridge. Because the cable is installed in the box girder of the bridge and the duct platform, the cable installation was affected by the complex structure of the bridge. As a result, different wavelengths for the snaking were used to satisfy the condition. The wavelengths varied from 5 m to 9 m (16 ft to 30 ft).

DESIGNING FOR LONGITUDINAL DISPLACEMENT AND ANGULAR DEFLECTION

Unlike conventional buried cable, longitudinal displacements (due to temperature changes), the vehicular load and wind will affect the cable installed on a bridge. The value of this displacement is significant and varies according to the type of bridge structure as shown in Table 2. Special measures have been adopted at each end of the bridge.

When the bridge is in service, it is affected by the traffic loads and weather conditions. To absorb longitudinal displacements, cable offset devices are installed at each end of the bridge. These devices overcome the problems of bridge displacement by varying the contour of cables.

When designing cable offset devices, the design of the bridge must be considered to ensure the shape and design of the cable offset devices can be installed. It is also important to consider the fatigue characteristics of the cable's metallic sheath.

The specified cable bending radius should be satisfied. But in order to allow for the continuous distortion of the cable device, the specified cable bending radius needs to be increased. To allow for the maximum bridge expansion, the bending radius on the Donghai Bridge is some 30 × D, where D is the average diameter of the metallic sheath.

It would have been difficult to make the cable in the offset devices move smoothly if the cables were not fixed. Therefore, the cables are fixed at the middle of each device with the movable structure. Also, the sliding system and uniform action device for the cable expansion was set in the stayed-cable sections of the bridge, where the expansion and cable displacement are greatest.

BENDING AND VIBRATION

The end of the bridge is designed to allow for vehicular loading on the stayed-cable sections of the bridge, which results in a slight angle where the bridge is anchored. Normally this displacement is not a problem with the angle being less than 2 degrees. The cable can withstand this bending, but the steel used for the offset devices is distorted. As a result, the special bending device is an integral part of the cable offset device on the stayed-cable bridge. Two hinge joints are set at the end of the offset device so that the steel bracket of the device can rotate in any direction.

The frequency and magnitude of the vibration in bridges differs according to the structure, shape and loading conditions. The vibration has two components. First is a uniform low-frequency vibration, less than 1 Hz, which is a function of the bridge length and rigidity. The second component is the vibration generated by vehicle movement. To minimize the effect of these vibrations on the Donghai project, an aluminum-sheath cable was used because it is superior to a lead-sheath cable in conditions subject to vibration. Additionally, correct spacing of the cable support brackets prevent resonance between the cable and bridge.

The formula used to determine the optimum spacing is:

L≤[???·(EI·g/W)^1/2/2f]^1/2,

where f = bridge frequency, EI = bending rigidity, g = acceleration of gravity and W = cable weight.

The distance between cable brackets based on this formula was less than 2 m (6.5 ft) so the spacing on the Donghai Bridge varies between 1.5 m to1.6 m (4.9 ft to 5.2 ft). Rubber pillows are placed under the brackets to absorb most of the vibration energy; rubber layers used in the cable clamps have the same effect.

The position of cable joints is critical to prevent the adverse effect of bridge vibration; therefore, the positions of joints were near the bridge piers, the points of least vibration.

CABLE GROUNDING ON THE BRIDGE

The bridge piers were chosen as the grounding pole for the cable to minimize the earth resistance, reduce the grounding material required and balance the electric potential. On the Donghai Bridge, the measured value of the earth resistance is about 0.2 Ω, less than 1 Ω required by the designer. The values of touch voltage and dangerous voltage are within the specified limits.

To ground the metal sheath of the cable, the length of the cable was divided into segments to reduce the number of cable joints; the maximum length between segments was 930 m (3051 ft) with all segments more than 800 m (2625 ft) long. Under the maximum load conditions, the induced voltage on the metal sheath is about 92 V, so the sheath is cross-bonded, surge-voltage limiters are installed and the neutral point is directly earthed.

By employing cross-bonding, the induced voltage, touch voltage and dangerous voltage can be limited effectively, and as the value of induced voltage on adjacent cables is low, there is no need to install a special return path adjacent to the cable. The calculation of transient single-phase short-circuit conditions indicates that the overvoltage can be reduced by 20% due to the effect of the reinforcing steel components on the bridge.

SUCCESSFUL INSTALLATION

Construction of the bridge started in August 2004 and was completed in 15 months. The cable installation work, undertaken by the Shanghai Cable Transmission & Distribution Co., was completed concurrently with the bridge construction. On completion to guarantee the quality of the cable system, a 24-hour no-load test was conducted and partial-discharge measurements were taken on all joints after one month's operation of the circuits.

The Shanghai Electric Power Co. has succeeded in designing and installing one of the world's longest high-voltage cable circuits on a bridge in a project that cost some US$55 million. The utility solved the problems that could cause damage to the cables, namely expansion and vibration, and developed and installed an effective means of grounding the circuits.

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Meng Yu graduated from Shanghai Jiao Tong University in August 1996 and joined the Shanghai Electrical Power Design Institute in September 1996. Currently, Yu is chief engineer of the Power Transmission Department, where he is engaged in the design and research of transmission lines.
mengy@sepd.com.cn

Gong Zun graduated from Shanghai Jiao Tong University in August 1983, and joined Shanghai Cable Transmission & Distribution Co. in September 1983. Currently, he is board chairman of the Shanghai Electrical Power Design Institute, where his main responsibilities are the research and design of power cable transmission.

Table 1. Key Cable System Parameters

Type of cable	110-kV single-core XLPE-insulated cable
Conductors	Copper
Cross-sectional area	630 mm2 (1 inch2)
Cable length	25 km (15.6 miles) installed on the bridge 13 km (8.13 miles) installed at the land shore
Accessories	Prefabricated insulating joints — 312 sets GIS joints — 12 sets
Circuit transfer capacity	103 MW

Table 2. Type of Cable Installation on the Different Bridge Sections

Type of bridge section	Length	Maximum value of expansion	Type of cable installation
Concrete box girder of the bridge Cable route section A	22 km (13.8 miles)	+200 mm, -200 mm (+7.87 inches, -7.87 inches)	Installed inside the box girder at the horizontal snake shape
PVC /PE duct platform (spanned between two box girders)	1.54 km (0.96 miles)	+150 mm, -150 mm (+5.91 inches, -5.91 inches)	Installed on the special PVC/PE duct platform (with the water pipes and communication lines) at the horizontal snake shape
Donghai stayed-cable bridge	0.83 km (0.52 miles)	+380 to 440 mm (+15 to 17.3 inches)	Installed inside the steel structure box girder in the horizontal snake shape
Kezhushan stayed-cable bridge Cable route section D	0.63 km (0.39 miles)	+480 mm, -480 mm (+18.9 inches, -18.9 inches)	Installed on the special platform for cable at the trefoil snake shape

Table 3. Cable Cross-Section Dimensions

Figure number	Item	Dimension mm (inches)
Diameter of conductor	30.3±0.3 (1.19±0.011)
Thickness of inner screen	1.0 (0.04)
Insulation thickness	17.0 (0.67)
Thickness of outer screen	1.0 (0.034)
Diameter of insulation	67.9 +1.5, -1.0 (2.67 + 0.06. -0.04)
Diameter of metal sheath	90.3±2.0 (3.56±0.08) (aluminum)	82.1±2.0 (3.23±0.08) (lead)
Thickness of outer screen	5.0 (PVC) (0.196)	5.0 (PE) (0.196)
Diameter of cable	100.3±2.0 (3.95±0.08)	93.3±2.0 (3.67±0.08)
Weight of cable (kg/m)	13.96	22.35