To supply new loads in the business center of Honolulu, the Hawaiian Electric Co. (HECO) was confronted with several problems. The space available for building and upgrading utility facilities in this area is increasingly scarce. Located within the jurisdiction of Hawaii's Kakaako Community Development District, additional requirements for utility construction work are strictly regulated. HECO met these demanding restrictions with an engineering solution that included gas-insulated substations (GIS), 138-kV underground transmission cables and an increase in the distribution voltage.

Archer — Kewalo 138-kV System

Archer Substation. Archer Substation was HECO's first totally enclosed 138-kV substation. The initial 1990 installation included 138-kV and 46-kV GIS equipment, two 138/46-kV 83.3 MVA transformers, one 46/12-kV 12.5 MVA transformer and two 138-kV high-pressure fluid-filled (HPFF) underground cable terminations. In 1993, HECO added more 46-kV and 138-kV GIS equipment for the third 138/46-kV transformer and created provisions for the Archer-Kewalo No.1 138-kV HPFF cable termination.

Kewalo Substation. HECO owned an empty lot next to an existing 46/12-kV substation. Existing buildings bordered the property line that surround the 19,252 sq ft (1789 sq m) lot located within the Kakaako Community Development District (Fig. 1). However, with land costs in this area exceeding US$250 per sq ft, purchasing additional property for the 138-kV substation was not an option.

The ultimate design for this 138-kV substation consisted of eight 138-kV circuit breakers in a ring bus configuration, three 138-kV cable terminations, four 138/25-kV 30/50 MVA transformers and 16 25-kV distribution circuits. To optimize the available land, HECO selected GIS equipment because it was compact and generally required only 10% to 25% of the footprint area and 30% to 50% of the height needed for an open-air substation. Also, the compact features of GIS make it an ideal choice for areas where it is desirable to minimize the aesthetic impact of utility facilities.

The main feature of the engineering project was the installation of two 3500-ft (1067-m), 138-kV underground cables between the Archer and Kewalo substations. Figure 2 shows a schematic of HECO's transmission system including the two underground cable circuits.

138-kV Archer — Kewalo Cables

Although HECO considered several 138-kV cable systems for this project, it chose the HPFF cable system for the 138-kV circuits. The HPFF cable system met HECO's high-ampacity requirement and had the ability to obtain higher load-transfer capacity. The ampacity requirement for the Archer-Kewalo No.1 138-kV cable was 855 A normal rating (1659 A emergency rating) with the Archer-Kewalo No. 2 138-kV cable requiring 500 A normal rating (1115 A emergency rating). A major advantage of the HPFF cable system is its ability to circulate and cool the dielectric fluid to achieve higher ampacities. This cable system is identical to the 1990 installation at Archer Substation, thus reducing the spare part inventories. With the circuit length of each cable approximately 3500 ft, cabling pulling required one manhole located about 1450 ft (442 m) from Archer Substation.

The HPFF cable system consisted of three-phase 3000-kcmil stranded-copper compact segmental conductors (Fig. 3) in an 8-inch (20-cm) diameter steel pipe with a corresponding 5-inch (13-cm) diameter fluid-return pipe to allow for increased ampacity operation. The cable pipe and fluid-return pipe are filled with dielectric fluid and are pressurized at 200 to 250 psi.

Safe operation of the cable system required a hydraulic system to circulate and cool the dielectric fluid, and to maintain fluid pressure within an acceptable range. The normal operation is slow fluid circulation where the dielectric fluid circulates at 10 to 12 gal (38 to 45 l) per minute between the cable pipe and the fluid-return pipe. This smooths out any hot spots along the cable route that can occur because of the deep burial depths or areas of high native-earth thermal resistivity.

In the event that HECO needs to increase the load-transfer capacity in the future, the system design allows for rapid dielectric fluid circulation of 150 gal (568 l) per minute with only minor modifications. This rapid circulation increases the load-transfer capacity by 10% to 20%. In addition, the system design parameters allow additional heat exchangers to increase the cable ampacity by as much as 40%.

One of HECO's major challenges was installing this new hydraulic pumping plant into the existing Archer Substation. This is the second pumping-plant installation in Archer Substation and was slightly larger than originally anticipated. The first reservoir tank had a 12,500-gal (47,316-l) capacity and the second reservoir tank had a 10,000-gal (37,854-l) capacity. The two hydraulic systems are located on opposite ends of Archer Substation with physical interconnections so that either pumping plant could back up the other and provide static pressurization for both systems.

The duct-bank design comprised:

• Two 8-inch (20-cm) diameter pipes for the 138-kV circuits

• Two 5-inch (13-cm) diameter fluid return pipes

• Two 4-inch (10-cm) diameter fiber-optic ducts

• Two 4-inch (10-cm) diameter 25-kV ducts

• Eight 6-inch (15-cm) diameter 25-kV ducts.

At some places, the dimensions of the duct bank exceeded 7 ft wide by 4 ft deep (2 m by 1 m). Low thermal resistivity fluidized thermal backfill compensated for the high thermal resistivity of the native soil.

HECO awarded the installation contract to a local contractor who used traditional open-cut trenching techniques to install the duct bank during an eight-month period. Some night work was necessary to minimize traffic congestion and reduce negative impact to businesses along the cabling route (Fig. 4).

HECO bid the 138-kV HPFF cable and hydraulic system as a purchase and install contract. Pirelli Cables and Systems North America (Clark, New Jersey, U.S.), the successful bidder, provided the materials and labor for the HPFF cable and hydraulic system. Installation of the Archer-Kewalo No.1 138-kV circuit was completed in 2000, and the No. 2 circuit was commissioned in 2001. Material cost for the two cable systems and the hydraulic pumping plant was more than $2.5 million.

The Archer-Kewalo No.1 138-kV HPFF cable was installed during a four-month period in 2000. Although the Archer Substation required no new GIS equipment, GIS work on the existing equipment was necessary to accommodate the HPFF terminators and the DC Hipot testing of the HPFF cables. This work primarily consisted of removal and re-installation of the termination chamber and conductor links that connect the HPFF terminators to the GIS bus bar. In-house HECO crews performed this work, which provided the utility staff with valuable experience.

Figure 5 shows the 138-kV cables in the basement of Archer Substation, and Fig. 6 shows the installation of the Archer-Kewalo No.1 138-kV cable.

The underground 138-kV transmission circuits enter Kewalo Substation at a level 9 ft (3 m) below ground in a large vault. This substation's location of less than 1 mile (2 km) from the ocean along with a very high water table complicated the construction of the subsurface vault.

138-kV GIS Equipment

As part of the overall project, the existing Mitsubishi 138-kV GIS equipment was increased for the Archer-Kewalo No. 2 138-kV HPFF cable termination (Fig. 7). HECO's options for the supplier of new GIS equipment for Archer Substation were limited for two reasons:

• Compatibility of the new equipment to the existing GIS bus (match height and location)

• Fit-up of the new equipment into an existing building floor plan with floor penetrations and anchoring designed and constructed for the ultimate layout of the existing GIS manufacturer's equipment.

Mitsubishi Electric Corp. (Itami, Japan), the manufacturer of the existing GIS equipment, was the only company able to meet these limitations, so HECO awarded it the contract. At an equipment cost of $1.4 million, the GIS installation was completed on schedule.

For the Kewalo Substation, the GIS equipment was bid. HECO selected the successful vendor (ABB Calor Emag Schaltanlagen AG (Hanau, Germany) based on cost and technical factors. The main requirement was the ultimate build-out of the GIS equipment needed to fit in a space approximately 50 ft long by 20 ft wide (15 m by 6 m). Initial installation included five GIS circuit breakers, three HPFF line terminations, two transformer terminations and associated GIS equipment. The cost for the GIS equipment was more than $2.5 million.

Figure 8 shows the installation of ABB's GIS equipment in Kewalo Substation in progress. The GIS installation was completed in 1999. A project to install one 138/25-kV 30/50 MVA transformer and 25-kV switchgear was also completed in 2001.

In addition, a 19-ft (6-m) high wall was required on all four sides of the substation site to minimize the visual impact of the utility facility when viewed from the surrounding areas.

HECO did the design and the installation of the GIS equipment in-house for the first time. Previously, HECO used consultants for the design work and the GIS vendor to install the equipment. This approach resulted in HECO personnel being unfamiliar with the equipment. The experience HECO personnel gained from the GIS equipment installation will help them with future GIS equipment operation and maintenance activities.

For this project, a representative from ABB was on-site to provide technical direction during the installation. HECO formed an installation crew comprised of staff with various levels of experience to undertake the complete construction of the Kewalo 138-kV GIS substation. With the timely delivery of the equipment from ABB along with the expertise of the ABB on-site representative, the HECO crew finished the GIS installation within the schedule at a cost more than 20% below the estimated cost.

The total project cost for the Archer-Kewalo No.1 circuit was nearly $24 million, which included both substation terminations and the 25-kV ducts. The total cost for the Archer-Kewalo No. 2 circuit is anticipated to be in excess of $4.5 million.

Future System Design Strategy

Gas-insulated substations provide a solution to the space problem in dense urban areas where land for new facilities is limited and only available at prohibitive cost. They also provide an acceptable substation design that minimizes the aesthetic impact of utility facilities. By designing and installing the substations in-house, HECO expanded its transmission system and increased its in-house technical expertise.

Earlynne Oshiro holds the BS degree in engineering physics from Santa Clara University and is a registered professional engineer in Hawaii. Joining HECO in 1986, Oshiro currently works in the Substation and Protection Division, and is responsible for the design and construction of substations and transmission lines. Oshiro is the project engineer for Kewalo Substation.

Lisa Ikeda holds the BS degree in electrical engineering from the University of Hawaii and is a registered professional engineer in Hawaii. Ikeda joined HECO in 1991 and is working in the Transmission and Distribution division responsible for the design of overhead and underground transmission and distribution systems. Ikeda is the project engineer for the Archer-Kewalo No.1 and No.2 underground transmission circuits.

Debbie Isler holds the BS degree in electrical engineering from the University of Hawaii and is a registered professional engineer in Hawaii. Joining HECO in 1992, Isler currently works in the Substation and Protection Division, responsible for the engineering, design and construction of substations. Isler is the project engineer for Archer Substation.