The world of electricity is moving forward at an ever-increasing rate as industrialized countries face the often conflicting challenges of industry deregulation, competition and energy conservation. Conversely, developing countries must meet increasing demands for energy-the key to improving quality of life and creating economic stability. The past two decades has seen unprecedented changes in telecommunication and information technology that has revolutionized the operation, control and management of the power industry.

Major advances in technology now seem set to change the design and traditional key components of transmission and distribution networks. This article reports on five initiatives from around the globe that are incorporating new technology to ensure the economic development of power-delivery networks to serve an ever increasing appetite for electrical energy.

Sweden Installs HVDC Light Project The island of Gotland is situated in the Baltic Sea approximately 56 miles (90 km) east of the Swedish mainland. The island's annual electricity consumption is about 900 GWh, most of which is supplied from the mainland through high-voltage direct-current (HVDC) submarine link. In recent years there has been increasing interest in expanding wind power generation on Gotland, whose coastline is frequently exposed to strong winds from the Baltic Sea. Around 48 MW of generation has been added since the beginning of the 1990s. Of that, 35 MW is located in the southern tip of the island where the network is weak with low short-circuit power and low transmission capacity. A further 50 MW of wind-power capacity is due to be commissioned in the next few years. This rapid development has created a need for additional transmission capacity on Gotland's network and an improved method of maintaining acceptable power quality levels. The variable operating conditions of the wind-power system results in flicker, variations in reactive power and, in the longer term, variations in the direction of active power flow.

Initially, GEAB (Gotland's local utility) considered four options to address the island's power transmission problems. Two of these options were based on traditional high-voltage ac technology, and two were based on the new HVDC Light technology. The ac option featured an overhead transmission line or underground cable with static VAR compensators. The HVDC Light alternatives were based on overhead lines or underground dc cables.

The ac alternatives were penalized by the grid system's low short-circuit power levels, voltage control and power flow problems. Both the ac and dc alternatives featuring an overhead transmission line were discarded as a result of the stringent environmental considerations. After a careful review of the four alternatives, the HVDC Light option with underground cables was selected as the best solution based on technical viability, economics and environmental compatibility criteria. In December 1997, GEAB, Vattenfall AB, the Swedish National Energy Administration and ABB Power Systems agreed to install the world's first HVDC Light transmission system.

The principle technical requirements for this transmission system are: - System transfer capacity, 50 MW active power and at least 30 MVAR reactive power at full load. - Maximum losses at full load 10%. - DC voltage approximately 80 kV.

The general characteristics of the controllability of power and voltage specified for the HVDC Light system included voltage source converters with pulse width modulation (PWM) using insulated gate bipolar thyristors (IGBT). A PWM voltage source converter comes close to being the ideal transmission network component. By changing the PWM pattern, any phase angle or amplitude is possible. The system is able to control active and reactive power almost instantaneously, which makes the system operation characteristics more software than hardware dependent.

HVDC Light System The HVDC Light system will consist of two converter stations connected by two dc cables +/-80 kV. The converters will be connected via reactors to the 80-kV ac buses that will be fed from the 75-kV system. The transformers will be equipped with tap changers that are able to reduce the voltage on the converter side to reduce no load and low-load losses. The wind-power site seldom will operate at peak production, and sometimes will not produce at all. It is therefore important to keep the no load and low-load losses as low as possible, since they will have a much bigger influence on the overall economy than the peak load losses.

The entire terminal will be housed under a single roof. Most of the equipment is factory assembled in containers and factory tested, reducing on-site testing to a minimum. The terminal buildings on Gotland will be designed and colored to resemble a traditional Swedish farmhouse to reduce the visual impact.

The HVDC Light cables developed by ABB for the 43.5 mile (70 km) route are a new design with insulation made of an extruded polymer that is particularly resistant to dc voltage. Polymeric-insulated cables are the preferred choice for HVDC because of their excellent mechanical strength, flexibility and low weight. The 80-kV cable for this project is 340-mm2 aluminum, with a diameter of 43.4 mm that makes it comparable in size to a 20-kV ac cable.

The system performance studies for this installation considered three different aspects: - Steady state conditions evaluating losses, load flow and voltage regulation. - Flicker created by power fluctuations. - Transients for different fault situations.

Studies confirm the dc link will serve as an integrated part of a sensitive ac network that contributes to the power quality and stability of the GEAB network. The new link should reduce voltage dips caused by short-circuits by some 50% and reduce the flicker emanating from wind power installations by up to two-thirds.

Main Advantages The main advantages of the HVDC Light project are: - Low environmental impact, enabling the location of the converter stations and negotiations of rights-of-way even in sensitive environments. - Short project lead-times, as most of the components are factory assembled. Also, construction of the dc link and system studies that define operational software can be conducted in parallel. - Control of power quality and power flow in the system due to the ability to control active and reactive power fast and separately. The Gotland HVDC Light Project will be made operational during the summer of 1999.

Detroit Edison to Install Superconducting Cable The Detroit Edison Co., Detroit, Michigan, U.S., supplies more than 2 million customers in Michigan from an overhead and underground transmission and distribution system, which extends over 7600 sq miles (19,684 sq km). This Detroit-based diversified energy company is now set to create history as the first electric power utility to operate a distribution system that in the year 2000 will be reinforced by the installation of a unique power cable system-the world's first high-temperature superconductor (HTS). This project, in which Detroit Edison is investing in the research and development program, will provide a `real-life' operational installation to prove the practicality of HTS technology.

Ceramic-based HTS materials were discovered in the late1980s. To help the industry develop this technology, the U.S. Department of Energy (DOE) created a 'Superconductivity Partnership Initiative' in 1988. At the same time, the industry started to develop and commercialize the electric power applications of HTS. This latest HTS cable project, conceived by Pirelli and Electric Power Research Institute (EPRI), recognizes the potential benefits of HTS energy transmission.

This joint project has produced the world's first 115-kV HTS cable system. The project is the culmination of the entrepreneurial drive of a number of high-tech companies combined with the vast technological resources of DOE's laboratories that organized the R&D program. This combined the work of researchers on HTS wire and cable technology with those in industry where research centered on system technology and the development of prototype components capable of integration into complete power-delivery networks.

The Detroit Edison installation will consist of three single-phase 24-kV warm dielectric cables (with a circuit rating of 100 MVA) that provide the interconnection between a transformer and switchgear at Frisbie Substation in downtown Detroit. Currently, nine conventional copper-cored cables (three per phase) are used for the three-phase ducted interconnection, which has a route length of approximately 400 ft (130 m). The rating of the existing cables is 800 A per cable compared with a 2400 A rating for each of the HTS cables in this ducted installation. It would not be practical to achieve this current rating with a conventional cable. The conductor would be prohibitively large, and the cable weight and dimensions would render installation in the existing ducts impossible. The HTS cables and joints will be installed in existing 4 inch (10 cm) cable ducts.

The project is being undertaken by a consortium, lead by Pirelli Cables and Systems. Pirelli will undertake the system design, manufacture and installation. The cable will use American Superconductor Corp.'s (ASC) superconducting tapes. Lotepro, a subsidiary of Linde AG, will provide the cryogenic equipment (manufactured by Kyrotechnik in Switzerland) that cools the liquid nitrogen that is circulated through the cable ASC's conducting tape. The conducting tape used in the cable is comprised of the ceramic compound BSCCO2223 (Bismuth-Strontium-Calcium-Copper-Oxide) in a silver metal matrix. In its pure form, this compound has a current carrying capacity hundreds of times greater than copper.

The size and weight advantages of superconductors make it possible for a fully engineered, insulated cable to carry several times more current while weighing substantially less. In the case of the Detroit project, nine copper cables incorporating more than 18,000 lbs (8165 kg) of copper will be replaced by three HTS cables incorporating approximately 250 lbs (113 kg) of superconductor.

The cabling technology for HTS is different from conventional cables because the tapes are fragile, withstanding a tensile strain of about 0.3% at room temperature. Also, the cable has to incorporate a cryostat to thermally isolate the superconducting tapes (immersed in liquid nitrogen at about -200o C) from the external ambient. Special stranding techniques and equipment have been developed by Pirelli to incorporate the delicate HTS tapes into a robust cable structure whilst maintaining their performance. Figure 3 shows the key data and construction of the Detroit cable.

The project was signed in October 1998 and is based on two phases. Phase 1-the design of the HTS tapes, cable and refrigeration system-is already well advanced. Phase 2 involves installation and operation of the HTS cable system in Detroit. The project budget, which includes all design engineering, manufacture, installation, system operation and project management costs.

The benefits and 'drivers' for HTS distribution and transmission cables can be summarized in terms of the immediate and potential benefits: - Satisfy increasing power demands in urban areas via retrofit applications via the higher power transfer capacity per circuit. - Minimize the need to acquire new rights of way. - Enhance overall system efficiency by reducing network losses. - Offer potential energy 'superhighway' applications such as dc links for power trading between asynchronous ac networks. - Eliminate the need for transmission voltages in high load density areas. For example, it could eliminate 400-kV operation since HTS cable at 110 kV has the potential to transmit up to 1000 MVA.

The development of HTS technology over the last decade has paved the way for the commercialization of a technology that will transform the power-delivery systems of the world. Robert Buckler, president of DTE Energy Distribution Co., said "Detroit Edison needed to update its central city electrical system to accommodate the revitalization of the downtown area to supply two new sports stadiums and three new casinos. The superconducting cable allows us to upgrade the system using existing underground conduits, eliminating the need to disturb the dense, established urban infrastructure. Additionally, we will be able to serve more customers at the same or even lower costs."

New Substations for Southeast Queensland Power Queensland (PQ) decided to investigate and evaluate the benefits and risk of using leading-edge technology for three new substations in southeast Queensland that must be operational by 2000. The substations are: - Blackwall, a 275-kV switching station comprising 17 circuit breaker bays for 10 feeders and a static voltage compensator. Commissioning is scheduled for November 1999. - Bulli Creek, a 330-kV switching station comprising eight circuit breaker bays for four feeders and four shunt reactors. Commissioning is scheduled for November 2000. - Braemar, a 275/330-kV transformation station comprising six 330-kV bays and 11 275-kV circuit breaker bays. Commissioning is scheduled for June to November 2000.

PQ compared the life cycle cost and benefits (over 30 years) of substations equipped with air-insulated switchgear with two new types of switchgear available from two manufacturers, and the Plug & Switch System (PASS) manufactured by ABB. PQ concluded that PASS technology provided the optimum solution for its three new substations and committed US$48 million (AUS$30 million) to the project.

The PASS concept incorporates all the necessary functions of a switching bay in one common compartment. Each single-phase module brings together a gas-insulated circuit breaker, disconnect switches, earthing switch, current and voltage sensors and bushings. The circuit breaker and the integrated digital output instrument transformers are based on the well-proven gas-insulated switchgear technology.

The disconnecting and earthing switch and their integration with the circuit is a new development. The bushings are made of composite material. An epoxy resin-impregnated glass fiber ensures the mechanical characteristics, and a moulded silicone rubber housing provides the necessary electrical functions, such as creepage distance in addition to affording protection against environmental factors.

High performance current and voltage sensors, which combine both functions within one component, replace the traditional HV current and voltage instrument transformers. These sensors fulfill the different tasks of control, measurement, protection and revenue metering.

The PASS modules are available for the voltage range 220 kV to 550 kV and are completely assembled and routine tested by the manufacturer and shipped pre-assembled with the exception of the bushings. Among the many new features that make this new technology attractive are the flexibility of the PASS modules for retrofit substation extensions and new substation applications. The system requires less space and use of utility resources in erection and commissioning, which is simplified by the plug-in bushings and plug-in secondary connections.

PQ's pre-contract studies for the three 275-kV and 330-kV substations showed life cycle cost benefits ranging from 6% to 10% less than for conventional air-insulated substations. There are additional unquantified benefits arising from higher plant availability, improved protection by elimination of dead zones and future introduction of automated switching.

This new technology has already improved PQ's design practices by: - Reducing the number of drawings per substation. - Creating cost-effective site layouts. - Implementing radical changes to bay layouts and busbar arrangements. - Eliminating line disconnectors.

Time-based maintenance will be eliminated because the PASS modules are self-monitoring and have many diagnostic tools that facilitate on-line monitoring and diagnostic assessment. Maintenance, if required, will involve the removal of faulty components for repair and plugging-in replacement units. The change to digital optical fiber connected secondary systems has virtually eliminated cabling and enabled the change from a large control building to the use of lower cost prefabricated modular control building.

There is an increased risk associated with the introduction of new technology-acceptance will only occur if the decision-makers decide the risk outweighs the overall potential benefits. The decision by PQ to use the PASS technology in three substations in southeast Queensland seems set to satisfy the company's vision to produce a reliable, cost-effective transmission system-a key component to being successful in the emerging competitive environment.

Sweden Installs Thyristor-Controlled Series Compensation The Swedish power system is part of the synchronous Nordic system that connects Sweden, Finland, Norway and the eastern part of Denmark. The main sources of generation are hydro-plants (45%) in northern Sweden and nuclear plants (50%) sited in the coastal areas of the central and southern regions. As the main load centers are in the central and southern regions of the country, the transmission system operated by Svenska Kraftnet (Swedish National Grid) comprises eight 400-kV lines. To improve the power transfer capacity of these circuits that are each approximately 311 miles (500 km) long, series compensation is used on each circuit with degrees of compensation ranging up to 70%. (The degree of compensation is the reactance of the series capacitor related to the series reactance of the transmission line).

The extensive use of series compensation-without which, several additional 40-kV lines would be required-enables the north-south transfer of 8000 MW of hydropower under stable conditions. However, the use of series compensation on circuits connected to large thermal generating facilities can, under certain conditions, create increased mechanical stresses on turbo-alternator shafts that appear as torsional vibrations. This phenomenon, known as sub-synchronous resonance (SSR), traditionally has been overcome by bypassing the series capacitor or tripping the generator. With the increasing demand for improved system availability, these measures are no longer acceptable.

Forsmark, one of Sweden's main nuclear plants in mid-Sweden is connected to the transmission system via two series compensated transmission lines. After refurbishment of a series capacitor at Stode substation in 1994, the SSR current relay on a 1300-MW generator (Forsmark III) started triggering repeatedly. This resulted in the series capacitor being by-passed and a decrease in the power transmission capability-prompting the need for system stability studies. The study of SSR conditions showed that sub-synchronous oscillations could occur under certain conditions on this system.

As a result of these studies, the Swedish National Grid decided to install a thyristor-controlled series capacitor (TCSC) at Stode. The company rebuilt the existing series capacitor rated at 493 MVAr at 400 kV to create two segments. One segment, representing 70% of the original series capacitor rating, was left as a conventional fixed unit while the other segment was converted into a TCSC. This was achieved by the addition of a thyristor-controlled inductor connected in parallel with the series unit. With the system's and the appropriate control algorithm, the TCSC will display a virtual reactance, which is inductive in the sub-synchronous frequency band. This characteristic prevents any resonance between the TCSC and the power grid and the SSR hazard is eliminated.

It would have been possible to convert the existing series capacitor at Stode into a TCSC, but calculations showed that a controllable element for 30% of the rating was sufficient, as 70% of the fixed compensation did not contribute to any SSR. Hence, the rating of two elements satisfied power system operational requirements with minimum capital investment. The thyristor valve is equipped with light-triggered thyristors (LTT) in order to minimize the complexity of this outdoor equipment. The valve is water-cooled with a mixture of glycol to allow for the sub-zero outdoor temperatures prevalent throughout the winter in Sweden.

Communication between the platform and ground is performed using fiber optics. Currents, voltages and other quantities on platform level, required for monitoring and control purposes, are measured using a high bandwidth optically powered data link. A small building, housing control, protection and monitoring devices in addition to the thyristor cooling equipment, also form part of the installation.

Technical Data The main technical data of the Stode TCSC are: System Voltage 400 kV Rated Continuous Current 1500 A Rated Overall Power 493 MVAr Degree of Compensation Total 70% Fixed 49% Thyristor controlled 21%

The Stode TCSC installation includes several innovative systems that are expected to be vital in the continuing development of this technology. For example: - The VarMach control equipment for TCSC, manufactured by ABB, incorporates a control algorithm for SSR mitigation and boosts level control. - The outdoor TCSC thyristor valve uses SVC technology and experience. - The use of direct light triggered thyristors. - The high bandwidth, optically powered, data link system for measurement of quantities on the platform. - A station control and monitoring (SCM) system providing operator interface to the TCSC with remote diagnostics.

The SCM is housed in the control room that is equipped with the SCM computer and an operator workstation, from which the TCSC can be manually operated.

The Stode TCSC installation is a milestone in the application of this technology-demonstrating that series compensation continues to provide a cost-effective and environmentally friendly method of increasing transmission system capacity. Series compensation has also been used more recently with the installation of three units in a double-circuit 400-kV cross-border interconnection between Sweden and Finland, which has increased the transfer capacity from 800 MW to 1100 MW (40%).

Brazilian North-South Interconnection The Brazilian power system is composed of two high-voltage grid systems, the North-Northeast (N-NE) grid, a network of radial design and the South-Southeast (S-SE) grid that is largely an interconnected network. These two systems operate separately with an installed capacity of 60 GW supplying 95% of the country's electrical energy. Local thermal generation is used to supply a number of the isolated load centers in the north region.

The N-NE radial system consists of 230-kV and 500-kV transmission lines connecting the large hydropower plants in this region with the main load centers along the north-eastern coast of Brazil. A 1120 mile (1800 km), 500-kV transmission line interconnection between the north and northeast was constructed in 1981. This circuit includes interconnection to the Tucuri hydropower plant (4500 MW).

The S-SE meshed network includes transmission lines in the voltage range 230 kV to 750 kV and a +/- 600 kV dc link. The 560 mile (900 km), 750-kV transmission line commissioned in 1982 to link the south and southeast regions is the grid system's principal interconnector. The main characteristics of the Brazilian electrical system are: - Long distances between hydro generation facilities and load centers. - Seasonal hydrologic diversity affects the electrical interconnections between regional systems. - The two major grid networks are not electrically connected.

Due to the country's severe economic crisis in the last decade, several generation projects were discontinued in mid-construction, but a recent economic stabilization plan has stimulated load growth-identifying a shortfall in system capacity. Research was therefore conducted to identify a project that could provide a solution to the energy supply on a short-term basis. This resulted in consideration of a north-south interconnector that could take advantage of the complementary hydrologic regimes of the north and southeast river basins.

The design of interconnections is a complex process that involves evaluating, on a long-term basis, transmission system performance and the overall energy and demand characteristics. These studies are particularly difficult since the Brazilian power system is hydro-dominated by plants with large storage capacity. The magnitude and frequency of the energy flows between the two systems define the main parameters of the North-South interconnector. These were estimated and used in system operation studies for various hydrological scenarios that confirmed a 1000 MW link could increase the firm capacity of the system by 600 MW.

Both ac and dc transmission interconnection were considered, but the ac option was more economical and offered the possibility of future interconnection with new power plants on route. It was then decided to construct a 500-kV transmission line with a route length of 750 miles (1200 km) from Imperatriz substation in the north to Samambaia substation in the south.

Environmental impact studies have influenced the choice of technology, the transmission line route and new substation sites to minimize the impact on protected areas and native Indian reservations.

The interconnector, currently in the course of construction, should be commissioned in early 1999 within 12 months of construction commencing-an attractive benefit in view of the current demands being made on the country's generating facilities. With an estimated cost in excess of US$ 930 million, this project will increase the capacity of the Brazilian system by 600 MW and provide energy at US$15 per MWh compared to the US$30 per MWh cost of energy from new hydropower plants.