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500kV Aluminum-Sheathed XLPE Cable in a 96m Vertical Shaft

U.D.C. [621.315.221.712.027.85: 669.71:678.742.2]: 621.315.232

He Yongquan*

Tatsuji Ishibashi**

Katsuhiko Yamamoto**

Kazutoshi Abe**

Tetsuyo Yoneda**

ABSTRACT: Three 500-kV XLPE cable circuits were supplied and installed in a 96m

vertical shaft at Tianhuangping Pumped Storage Power Station in the P. R. of China.

The cable was specially designed to prevent the cable core from slippage and to

compensate radial expansion and contraction of the thick insulation. A hauling machine

featuring regenerative braking was developed and utilized to vertically install the cable.

The cable laying work was carried out by an installation contractor appointed by the

owner under the supervision of Hitachi Cable, Ltd. The first 500-kV XLPE cable circuit

was successfully put into service in July, 1998, the second in July, 1999 and the third

circuit is under construction and will be completed by the end of 1999.

1 INTRODUCTION

To cope with increasing peak-power regulation and to

ensure stability of the power supply in the East China Network,

East China Electric Power Group Co. and the appointed

construction firm Tianhuangping Pumped Storage Power

Station Construction Co. launched the construction of a pure

pumped storage power plant in 1993. This new power plant,

Tianhuangping Pumped Storage Power Station (THP), is

situated in Anji Country Zhejiang Province, which is 260 km

southwest of Shanghai, a center of high load demand in the

network (Fig. 1). The main features of THP are (1) a total

installed capacity of 1800 MW (6300 MW) composed of six

sets of Francis-type turbine, reversible units installed in an

underground powerhouse complex: (2) a maximum 607-mgenerating head difference between the upper and lower

reservoirs: and (3) a water-conveyance system with a conduit

length to potential head ratio of 2.5. Three 500-kV XLPE cable

circuits are employed to carry the plant output from the

underground power house via a 96-m vertical shaft to

preliminary overhead transmission lines in the network (Fig. 2).

During the planning stage, both oil-filled cable and XLPE

cable were considered for use as the main feeders. The XLPE

cable was finally selected because:

(1) 500-kV XLPE cables are being used in a number of

pumped storage power plants in Japan(1).

(2) XLPE cable has low dielectric loss and superior flame-retardant characteristics due to its oil-free construction.

(3) High stati c pressure at the bottom of the shaf t and

complicated control of oil pressure during terminating work

can be avoided.

However, prior to installing applying a corrugated aluminum-

sheathed XLPE cable in the deep vertical shaft, we had to

satisfy two requirements:

(1) A mechanical design of the cable that prevents the cable

core from slipping through the corrugated aluminum sheath.

3HITACHI CABLE REVIEW No.18 (October 1999)

* Tianhuangping Pumped Storage Power Station Construction Co.** Hitaka Works, Hitachi Cable, Ltd.

Fig. 1 Location of Tianhuangping pumped storage power station

A new power station with an installed capacity of 1800 MW is

located 260 km southwest of Shanghai.

Chengdu

P.R. OF CHINA

Wuhan

Beijing

Guangzhou

Hongkong

Shanghai

TIANHUANGPINGPOWER STATION

N

(2) An appropriate cable laying technique that prevents the

cable from slipping during laying work.

The first requirement was satisfied by a specially designed

bedding layer applied to the cable core and lightly compressed

by the corrugated aluminum sheath. An experimental length of

the cable was manufactured and subjected to a mechanical

assessment which showed that no slipping occurred. Thesecond requirement was met by developing a new hauling

machine with regenerative braking.

By introducing these techniques, three circuits of 500-kV

XLPE cables, the total length of which is 2090m, were safely

and efficiently installed in the 96 m vertical shaft at the power

station. Following assembly of the sealing ends and the field

tests, the first 500-kV XLPE cable circuit was completed in

April and put into service successfully in July, 1998. The

second circuit was put into commercial operation in July, 1999.

This paper describes the main features of the cable design

for vertical installation, the verification of this design, the cable

laying equipment and the construction of the cable circuits.

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4 HITACHI CABLE REVIEW No.18 (October 1999)

2POWER CABLE SYSTEM DESIGN

2.1 Cable design for vertical installation

Table 1 lists the electrical requirements and the site

conditions to which the cable system was designed. As

previously mentioned, the salient point of the THP cable

installation was that the 500-kV aluminum-sheathed XLPE

cables were installed in a 96-m vertical shaft which lay in the

middle of the cable route. The total difference in elevation of

the entire cable route was 105 m (including the aboveground

switchgear side).In the past several decades, oil-filled cable have mainly

been utilized in vertical installations where the elevation

difference was 100m and above(2). On the other hand, as far as

the authors know, there was no precedent for an application of

Fig. 2 Power house complex

and cable route profile

Three 500-kV XLPE cable

circuits were installed in a 96-

m vertical shaft to carry plant

output to overhead trans-

mission lines.

TABLE 1 CABLE SYSTEM PARAMETERSThe 500-kV XLPE cable systems were designed according to the

parameters shown in the table.

A LIWV of 1675 kVp was required for cable and accessories.

500kVXLPEcables

DoorGIS

500kVSwitchyard

Original groundline

Transformer hall

Tailgate gallery

Machine hall

214.4

233.1238.9

228.2

350.2

285.0

216.2

246 .9I

II

III

I

II

III

IV

VI

V

500kV cable& access

shaft(8m Dia.) Lower reservoir

Dimensions in m

TRF.

Cross-section of cable &access shaft

1.6

0.6

Elevator

Platform

17.0

8.2

180.0 29.0

15.0

22.4

24.4

47.5

500kV XLPEcables(3 circuits)

a corrugated aluminum-sheathed XLPE cable to such a deep

vertical shaft. Since oil-filled cables are often used in vertical

installations, great attention has been paid to the design of the

metallic sheath and the hydraulics including accessories.

However, extensive research has not been conducted on

slippage of the cable core through the metallic sheath. This is

because the metallic sheath of the oil-filled cable can be

applied tightly over the core, and the existence of a gap

between them is not an issue.

However, this is not the case with EHV XLPE cable.

Usually, an EHV XLPE cable is designed to maintain a certainamount of gap between the metallic sheath and the underlying

component so as not to obstruct the radial expansion of the core

due to heat. This cable design, including the gap, results in a

low coefficient of friction between the core and the metallic

sheath, typically between 0.3 to 0.8 at ambient temperature(3), (4).

When we committed to the application of the 500-kV

XLPE cable at THP, we examined how to increase the

restrictive force on the core a key factor in vertical

installation. During the cable design, careful attention was paid

to both the grasping force and to the thermal behavior of the

thick XLPE insulation. Namely, the 500-kV XLPE cable had to

satisfy the following diverse conditions simultaneously: (1)restrain the cable core firmly to prevent slippage, even at low

temperatures during which the core diameter shrinks; and (2) to

not obstruct the expansion of the cable core at high

temperature. These conditions were satisfied by developing a

special bedding layer, the design of which is described in the

following section.

2.2 Construction of cable and sealing end

Table 2 lists the construction specifications and Fig. 3

shows the appearance of the 500-kV XLPE cable. The cable

core is composed of an 800 mm 2 copper conductor and 35 mm-

thick XLPE insulation. The insulation thickness was

determined according to the required lightning impulse

Category Items Particulars

Site

conditions

Electrical

ratings

Nominal system voltage (U)

Highest system voltage (Um)

Lightning impulse withstand voltage (LIWV)

Switching impulse withstand voltage (SIWV)

500 kV

550 kV

1675 kVp

1240 kVp

Three-phase symmetrical fault current

Single-phase fault current

System frequency

Nos. of circuit

Route length

Elevation difference at vertical shaft

Designed ambient temperature

50 kA for 3 s

40 kA for 2 s

50 Hz

3

219 m / 222 m / 256 m

96 m

14C to 40C

Transmission capacity for each circuit:

(a) Rated current

(b) Max. current for pumping start-up

873 A (756 MVA)

890 A (770 MVA)

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withstand voltage (LIWV) of 1675-kVp. A specially-designed

bedding layer (5.7mm thick) and a corrugated aluminum sheath

were applied over the core. The inner diameter of the

aluminum sheath was slightly smaller than the outside diameter

of the bedding in order to obtain the necessary restrictive force

on the core. The aluminum sheath was protected with a flame

retardant PVC jacket, followed by a semi-conducting coating.The bedding layer, one of the most significant components

of this cable, was carefully designed. According to the

operational conditions of the cable systems, the diameter of the

core varies by approximately 4mm in the conductor

temperature range of 14C to 90C. To overcome this core

variation, lightly-amalgamative cushion tapes, which absorb

the expansion of the core and woven copper woven semi-

conductive tape were applied over the core. The amalgamative

nature of the tape prevents slippage between the core and the

bedding layer as well as between the tapes. The thickness of

the bedding layer was selected so that the maximum

prospective indent by the aluminum sheath did not affect the

underlying cable core.

Items Particulars

Type designation XLPE/CSA/PVC

Thickness of bedding layer

Thickness of aluminum sheath

5.7 mm

3.9 mm

Diameter of cable

Mass per unit length

* L0I: Limited Oxygen Index

160 mm

28 kg/m

Conductor:

(1) Cross-section(2) Construction 800 mm

2

(copper)Segmental circular

Insulation

(1) Material

(2) Thickness

Inert gas cured XLPE

35.0 mm

Jacket:

(1) Material

(2) Thickness

Flame retardant PVC (L0I*30)

5.5 mm

TABLE 2 CONSTRUCTION OF 500-KV XLPE CABLEThe bedding layer was carefully designed to absorb any expansion

of the core and to grasp the core firmly.

Fig. 3 Appearance of 500-kV

XLPE cable

The diameter and mass per

unit length are 160-mm and28-kg/m, respectively.

At the THP power plant, the 500-kV cables were

terminated with SF 6-gas-type sealing ends on both the

transformer and the switchgear. Figure 4 illustrates the general

arrangement of the sealing end, which was composed of

synthetic-resin bell mouth, silicone-oil-impregnated

polyethylene sheets as internal insulation, and epoxy insulator

as external insulation. Expansion and contraction of thesilicone oil volume contained in the sealing end due to

temperature variation is compensated by a built-in oil reservoir

made of metallic bellow, which enables a compact and space

saving installation. The sealing ends were connected to the

transformer horizontally and to the GIS vertically. On the

transformer side, spacers were added in the sealing end to

prevent the cable core from dangling and to maintain the

specified gap between the internal and external insulations.

The electrical performance of the cable and the sealing end

was verified at the manufacturers factory. Sets of the cable and

sealing-end assemblies were subjected to special and internal

tests. In the special test, the assembly surpassed all aspects of

the electrical requirements. The internal test was performed

spontaneously in order to determine the ultimate characteristics

of the cable and verified that the characteristics of the newly

designed cable were of equally high quality to those of the

manufacturers previous 500-kV XLPE cables (Table 3).

3CONSTRUCTION DESIGN

3.1 Mechanical performance and installation design

To assess the mechanical performance of the cable, an

experimental length was manufactured. This cable was

subjected to a series of horizontal and vertical tests. Firstly, a

1m cable sample was laid horizontally, clamped and the corewas extracted from the cable using a winch through a load cell.

The required pulling forces were 10kN/m at ambient

temperature and more than 5 kN/m at 75C (an average

insulation temperature during actual service). In the latter case,

the exact force required to extract the core could not be

measured because slippage between the conductor and the

insulation occurred. Similar tests were performed on a 3m

cable sample laid horizontally and a 1m sample laid vertically.

In both tests, slippage of the conductors occurred prior to any

measurable core movements. These results imply that the

;

;

; ;

;

;

;

;

;

;

;

;

;

;

;

;

;

;

290

810

660

650

600

60

1500

1755 1620

Epoxy InsulatorInsulating oil

Bell mouthOil reservoir

PE insulating sheetsInsulating flange

62314 5

14

25

36

Fig. 4 Structure of 500-kV sealing end

A combination of bel l mouth and PE sheets pressurized by oilreservoir were employed as an internal insulation.

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designed cable has sufficient grasping force on the core and

can be considered appropriate for vertical installation.

Following these preliminary tests, the cable was subjected

to a vertical installation test involving heat cycles. Two lengths

of cables were installed in a 15-m-high scaffold construction in

a snaked formation with pitches of 6m and deflections of (a)

1.5Ds (Ds: average diameter of aluminum sheath) and (b)

2.0Ds. The cables were fixed to the scaffolding by holding

cleats at their axial centers and by sliding cleats at their apexes

(Fig. 5). Both ends of the cables were set freely. The cables

were attached with terminals and bonding wires through which

a circulating current flowed to provide daily heating cycles

(from ambient temperature to 90C, 8 h ON/16 h OFF). After

several heating cycles, the longitudinal protrusion and

contraction of the cable core followed a regular pattern with

each heating (Fig. 6). A total of 36 heating cycles were applied,

and neither significant variation of the pattern nor slippage ofthe core were observed in either cables during the entire

heating process.

The results of the heating cycles indicate that both snaked

formations were acceptable. However, the pattern of core

movement under the deflection of 2.0Ds was more stable. This

snaked formation was therefore adopted for the THP vertical

installation. On completion of this test, the cables were

dismantled and inspected. Undue deformation was not

observed in the cable components, i.e., the core, the bedding

layer, and the aluminum sheath. These tests confirmed the

propriety of the cables mechanical design.

3.2 Development of laying equipment

When cables are laid in vertical shafts, they are usually

threaded into the shaft from top to bottom. The THP cable

installation followed this traditional method due to the limited

working space in the underground power house. In previous

vertical installations, the bond pulling methods use a wire rope,

a turn pulley and winch installed at the upper mouth of a shaft

and devices to prevent the cable from rotating (5) (7). However, in

the THP cable installation, it was difficult to utilize the bond

pulling method due to there being:

No appropriate place to install a turn pulley

Anticipated undue side wall pressure on the cable at the

bending portion of the upper entry of the shaft.

TABLE 3 PERFORMANCE OF CABLE AND SEALING ENDSets of cable and sealing end assemblies withstood all the test voltages.

Category Items Test condition

Special test

Internal test

* d: Dia. of conductor; D: Overall dia. of cable.

Bending test

Impulse withstand test at 953C

AC withstand test at ambient temperature

Partial discharge test

Bending test

Heating cycle

AC withstand strength test after heating cycle

Impulse breakdown voltage test after AC test

Bending dia. 3500 mm [=18(dD)]*3 times

1675 kVp/10 shots

AC 480 kV10 h, followed by AC 625 kV/24 h

P.D. less than 10pC at AC 430 kV & AC 447 kV

Bending dia. 2450 mm [=13(dD)]*3 times

(8 h ON/16 h OFF)20 cycles

AC 1150 kV/2 h, withstood

More than 2625 kVp

3

2

1

01

2

3

4

5

6

7

1 2 3 4 5

Nos. of heating cycles

Expansion/contractionlengthof

coremm

Contra

ction

Expansion

6 7 8

Core movement at lower end

Deflection : 2DsDeflection : 1.5Ds

Fig. 5 Ex ec ut io n of 15 mhigh vertical installa-

tion test

(a) Left: deflection = 1.5 Ds,

(b) Right: deflection = 2.0 Ds.

No slippage of the cable core

was observed during 36

heating cycles.

Fig. 6 Expansion/contraction behavior of cable core in vertical

installation test

After several cycles, the core movement showed a regular pattern

until the end of the heating cycles.

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With these difficulties in mind, we selected a dispersed

hauling machine system, which has been successfully used in

laying aluminum-sheathed EHV cables in steeply-inclined

tunnels.

A conventional hauling machine, however, was not suitable

for the vertical cable laying, since it does not have a brake

function during running mode (although it does have one insuspended mode). If the conventional hauling machine is used,

gravity on the cable accelerates the speed of the machine

caterpillars during running mode, and this acceleration causes

the cable to fall. So these problems necessitated the

development of a new hauling machine equipped with a

regenerating brake. This regenerative braking is achieved by an

induction generator/motor that is used as a motor during the

sending operation and is switched to a generator function when

braking force is required. The generated energy fed back to a

power source through an inverter produces a maximum braking

force of 7.4-kN in running mode. Also, a new governing

system in the hauling machine was developed. This system

provides speed control supplemented by torque control

which enables every hauling machine to be synchronized

without slack in the cable.

4CONSTRUCTION WORK

Construction of the 500-kV XLPE cable circuits

commenced in November, 1997. Prior to cable laying work,

Hitachi Cable, Ltd. conferred with Shanghai Cable

Transmission and Distribution Co., an owner-appointed

installation contractor, on the cable laying procedure including

familiarization with the new hauling machine. During trial

operations, the hauling machines and associated main/localcontrol panels were carefully adjusted to eliminate any possible

problems. After these thorough trials, the laying operation

commenced.

The cable drum was set up outdoors in front of the

aboveground switching room. The cable was transferred from

the drum to the upper horizontal cable gallery, then lowered

into the 96-m vertical shaft. Fifteen sets of hauling machines

were utilized in the laying operation. Among them, seven sets

were installed on platforms in the vertical shaft which

supported the weight of the cable (Figs. 7 and 8). The distance

between the adjacent hauling machines in the shaft was

determined to provide continuous laying even if one of thehauling machines experienced operating difficulties.

Furthermore, four sets of the hauling machines set in tandem

near the upper entry of the shaft could hold the full weight of

the cable even if some of the hauling machines in the shaft

became disabled.

After laying the cable, it was fixed from bottom to top. The

cable was removed from the hauling machine, lowered slightly

into the shaft and given initial transverse offsets by cable

benders, then fixed to the wall of the shaft with cleats. This

procedure was repeated until the whole length of the cable was

fixed in the snaked formation. The cable laying and fixing

operation in the vertical shaft was completed in two days. Dust

and other contaminants were excluded from the sealing end due

to stringent quality control assembly procedures. A working

tent formed a contaminant-free room and an air-conditionerwas installed to maintain temperature and humidity within

acceptable ranges.

After assembling the sealing ends, an electrical test was

performed on the cable jacket. A test voltage of DC 20-kV was

applied for 15 minutes to confirm the jackets integrity. Then, a

dielectric security test was performed by an independent testing

specialist appointed by the owner. An AC test voltage of 365-

kV (85% of the factory test voltage and 1.26 Uo) generated by

a frequency-variable resonance device was applied to the cable

for 10 minutes via a test bushing equipped to the switchgear.

The voltage tests and other commissioning tests on the first

circuit were successfully completed in April 1998. The first

circuit was put into commercial operation in July, 1998. The

H15

TRF. Hall

96m

Verticalshaft

Direction of laying

Cable Gallery

E.V Floor

14m

14m

Scaffolding

HaulingMachine

to : Hauling machines

Cable Drum

GIS Room

H14

H13

H12

H11

H9

H8

H7

H6 H3

H2 H1

H1 H15

H10

14m

14m

14m

14m

25m 30m

E.V Floor

E.V Floor

E.V Floor

Cable GalleryE.V Floor

Fig. 7 Arrangement of hauling

machines

15 sets of hauling machines were

effectively dispersed to lower the

cable into the vertical shaft.

Fig. 8 Lay ing of 500-

kV XLPE cable

in vertical shaft

Newly developed hauling

machines were set on

platforms in the vertical

shaft.

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Fig. 9 Ext ernal appearance of control build ing in the power

station

The first 500-kV XLPE cable circuit in the most advanced pumped

storage power plant, THP, commenced its operation in July, 1998.

second circuit was completed and put into service in July, 1999

(Fig. 9). As for the third circuit, the cable laying work and

assembly of the sealing end on the switchgear side are

completed. It is expected that all construction work will be

completed by the end of 1999, and the THP will then become

fully operational and reinforce the stability of the power supply

in the East China Network.

He Yongquan

Deputy Chief Engineer, Tianhuangping PumpedStorage Power Station Construction Co.Received BE Degree in Electric Engineering fromZhejiang University in 1969.Chief Supervisor Engineer for EL-Mech., Equipmentinstallation of THP Project.Member of Association of China Hydropower andAssociation of Electric Engineer

Tatsuji Ishibashi

Assistant Manager, Power Cable Engineering Dept.,Hitaka Works, Hitachi Cable, Ltd.Received BE Degree in Electric Engineering fromKyoto University in 1983.Currently engaged in design of EHV cables andsystems.Member of IEE of Japan.

Katsuhiko Yamamoto

Assistant Manager, Power Cable Accessories &Distribution System Dept., Hitaka Works, HitachiCable, Ltd.Graduated in Electrical Engineering from MatsumotoTechnical High School in 1963.Currently engaged in design of EHV cableaccessories.

Kazutoshi Abe

Manager, Engineering & Development Sec., PowerCable Construction Engineering Dept., Hitaka Works,Hitachi Cable, Ltd.Received the BE Degree in Electric Engineering fromYamagata University in 1985.Currently engaged in engineering EHV cablesystems.Member of IEE of Japan.

Tetsuyo Yoneda

Assistant Manager, Power Cable Construction Dept.,Hitaka Works, Hitachi Cable, Ltd.Received BE Degree in Electric Engineering fromOsaka University in 1978.Currently engaged in construction of power cablesystems.

5CONCLUSION

A 500-kV XLPE cable designed for vertical installation

was developed and applied in a 96-m vertical shaft in theTianhungping Pumped Storage Power Plant. A hauling

machine with a regenerating brake was also developed for the

laying of the cable in deep vertical shaft. The first 500-kV

XLPE cable circuit was successfully put into service in July,

1998.

This successful vertical cable installation will accelerate the

adoption of EHV XLPE cables in hydropower stations and

deep underground transmission lines.

6ACKNOWLEDGMENT

The authors wish to express their deepest appreciation toShanghai Cable Transmission and Distribution Co. for their

generous cooperation during every stage of the construction of

the cable circuits.

REFERENCES

(1) K. Ogawa et al., The Worlds First Use of 500-kV XLPE

Insulated Aluminum Sheathed Power Cables at The

Shimogo and Imaichi Power Stations, IEEE 89 SM 643-8

PWRD (1989)

(2) N. Palmieri and G.M. Lanfranconi, Extra High-Voltage

Oil-Filled cables (From 300 to 400-kV) Laid in Vertical

Shafts, CIGRE 21-06, (1973)

(3) S. Mori et al., Investigation of Thermal Behavior of 275-

kV XLPE Cable in Vertical Installation, Proceeding of

Annual Convention 1981 of IEE of Japan, Paper No. 1191

(in Japanese)

(4) K. Kaminaga et al., Research and Development of 500-kV

XLPE Cables - Study on Thermo-mechanical Behavior andInstallation Method, Proceeding of Annual Convention

1986 of IEE of Japan, Paper No. 1350 (in Japanese)

(5) C. A. Arkell and W. E. Blake, Installation of E.H.V. Oil

Filled Cables in Deep Shaft, IEE Conference Publication

No. 44, pp. 213-217 (1968)

(6) G. Bazzi et al., 400-kV Cable Installations in Mexico,

IEEE Trans. PES F77 636-4 (1977)

(7) M. Nakanishi et al., An Installation of an EHV OF Cable

Laid in a Vertical Shaft, Sumitomo Electric Technical

Review No. 121, pp. 31-44. (1982)

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