In developing countries, the demand for electrical energy continues to rise, while in developed countries, consumers seek higher standards of system reliability and low-cost energy. For instance, the European system, which is regarded as one of the most powerful systems in the world, now supplies more than 400 million consumers who depend on a reliable supply of electrical energy.
This article presents five projects in which utilities in Northern Ireland, Mexico, China, Australia and the Baltic states have used or plan to use technology to address transmission system interconnection needs.
Moyle Interconnector Links Northern Ireland to European Network
In 1990, Northern Ireland Electricity (NIE), the electricity transmission and distribution company for Northern Ireland, signed agreements with Scottish Power for the construction of a high-voltage direct-current (HVDC) interconnector between the companies' transmission systems. It took nine years of planning applications, public inquiries and environmental statement procedures before NIE obtained the statutory consents for the transmission line, undersea crossing and converter stations. Moyle Interconnector plc (Moyle), a member of the Viridian Group, was established to construct the link.
A New Source of Electricity Energy
Figures 2 and 3 show the geographical location and schematic diagram of the Moyle Interconnector, which will transmit power between the transmissions in Northern Ireland and Scotland. The link provides Northern Ireland with an important new source of electricity supply, which will promote competition in the emerging markets in Northern Ireland and the Republic of Ireland, as well as enhance security and quality of supply. Turnkey contracts for the Moyle Interconnector, which was designed with specified power rating of 2 × 250 MW in either direction and a December 2001 completion date, were awarded to the following:
Nexans Norge AS for the supply, installation and protection of the submarine and underground cables. This included the 8.5 km (5.3 miles) 250-kV dc cable from each converter station to the coast and the two 55-km (34-mile) 250-kV dc submarine cable.
Siemens for the design, installation and commission of the two HVDC converter stations located at Ballycronan More Island Magee, County Antrim in Northern Ireland (Fig. 1), and at Auchencrosh, Ayrshire, Scotland.
Balfour Kilpatrick constructed the 275-kV single-circuit transmission line [64 km (40 miles)] from the existing Coylton Substation to the converter station at Auchencrosh for Scottish Power.
The estimated cost for the 500-MW link — which was completed on schedule and is now in service — was 234 million euros (US$214 million), a total that included a sponsored contribution of 83 million euros (US$76.1 million) from the European Regional Development Fund.
Technical Features of the Moyle Interconnector
The 250-kV 1000 mm2 (1.55 inches2) copper-conductored dc mass-impregnated paper insulated dc cable, rated at 250 MW, includes an integrated return conductor (IRC). This innovative design by Nexans has a metallic coaxial layer integrated in the HVDC cable to form the return path for the current and also to form part of the torsion-balanced armoring. A fiber-optic cable for control and communication between the converter stations is integrated into the polythene sheath of the IRC.
The submarine cables supplied in a continuous length were laid 1 km (0.62 miles) apart and then buried into the seabed by Nexan's Capjet water-jetting system or protected by concrete mattresses or rock dumping to a nominal depth of 1 m (3.3 ft).
The converter stations are comprised of two valve halls, one for each monopole constructed on either side of the control building (Fig. 1). Adjacent to each valve hall is a dc switchyard that includes measuring equipment, smoothing reactor and cable terminations.
The two converter stations are the first in the world to be completely equipped with direct-light-triggered thyristors with integrated overvoltage protection. Triggering is initiated by light pulses generated at ground potential and applied directly into the thyristor gate through a set of fiber-optic cables. The light pulses generated by laser diodes have a life expectancy of more than 40 years.
Benefits of the Moyle Interconnector
The Moyle Interconnector is designed to have low losses and low maintenance. In addition, the Moyle Interconnector provides enhanced security and improves supply quality. The converter stations are designed for a biannual scheduled maintenance with guaranteed losses of less than 1.35% and guaranteed energy availability of more than 99.6%.
Comision Federal de Electricidad Completes Interconnection in Record Time
ALSTOM's Transmission & Distribution Mexico Transmission Projects Business unit completed the first phase of a US$45 million turnkey project in the Mexican state of Veracruz, linking the new Tuxpan II power station to the country's national grid.
The Comision Federal de Electricidad (CFE) in the Central American country invited tenders for the erection of two 400-kV double-circuit transmission lines with a total route length of 111 km (69 miles), and two 400-kV switching stations sited at either end of the 400-kV lines. The most demanding feature of the tender specification was that the contractors had to commission the first phase of the new transmission line and substation project within 12 months. CFE awarded the contract to ALSTOM's Transmission & Distribution Mexico Transmission Projects business unit on Nov. 1, 2000.
Transmission Line and Substation Specifications
The 400-kV, double-circuit transmission lines will each include three ACSR conductors (1113 kcmil) per phase, one OPGW and a guard wire. The six-bay Tres Estrellas 400-kV Switching Station also will be equipped with six reactors (plus a spare unit). Figure 5 shows a schematic diagram of this two-phase project.
The first new 400-kV substation, Tres Estrellas, includes six 400-kV switch-bays and the second existing 400-kV substation — Poza Rica II — had to be extended to install two additional 400-kV switch-bays. This phase also included the erection of a 55-km (34.2 miles) 400-kV double-circuit transmission line undertaken by ALSTOM along with Spanish contractors, Abengoa and Elecnor.
A contract of this nature normally would take 15 to 18 months to complete. To meet the demanding time schedule, ALSTOM sourced products from its global manufacturing facilities. For this project, the circuit breakers were manufactured in France, isolating switches in Italy and Mexico and the current and voltage transformers in Mexico.
The site construction teams had to contend with a month-long rainy season. However, by ensuring that materials and personnel were in the right place at the right time, Phase One was commissioned in early September 2001, two months ahead of schedule.
The second phase of the project is currently under construction with the erection of an additional section of the 400-kV transmission line, the 56-km (35-mile) section that will link Tres Estellas 400-kV Substation to the town of Poza Rica. This phase of the contract has a completion date of April 2002.
The project was financed under the Pideregas scheme, under which CFE, a wholly owned Mexican state subsidiary, offers large tenders for infrastructure work in the country, which totaled US$800 million in 2000.
China's Electricity Travels East on Major HVDC Link
The Tian-Guang HVDC power transmission system delivers 1800 MW over 960 km (600 miles) from converter stations (Bipolar 2 × 900 MW) at Tianshengqiao (TSQ) in Guangxi Province to the Guangzhou (GZ) Converter Station in Guangdong Province. The TSQ Converter Station is directly connected to the substation of the hydropower stations, and the GZ Converter Station to the existing 220-kV system and — via two autotransformers — to the 500-kV system.
The HVDC transmission system forms an important power link in the Southern China electric-power network transferring bulk power to the Guangzhou area operating parallel to the existing 500-kV ac transmission lines. A reliable HVDC link among the ac-interconnected systems increases the efficiency of the power-transmission capacity between and within the two transmission networks.
Tian-Guang HVDC scheme allows bi-directional control of power interchange and improves reliability and dynamic performance of both ac-connected systems. Additionally, the converter stations also can provide the advantage to control the reactive power exchange with the connected ac grids and thus the ac system voltage. This HVDC link will play a definitive role in the new national power transmission strategy, West Electricity Goes East.
Power Transmission Capacity
The 960-km (597-mile) long-distance HVDC transmission system operates on a ±500-kV bipolar scheme rated for a continuous power of 1800 MW at the dc terminal of the rectifier converter station. Figure 7 shows the schematic single-line diagram of the Tian-Guang HVDC project.
The converter stations (constructed and equipped by Siemens Power Transmission & Distribution) can transmit full-rated power up to a maximum dry bulb temperature of 40°C (104°F) without a redundant cooling system in service. With redundant cooling, the same converter stations can achieve a continuous overload of 110% of rated load. Additionally, the dc transmission permits a three-second overload of 150% power (this overload rating being available for use in all ambient conditions and in all continuous overload previous operating conditions). Such short-time overload capabilities are important for some contingencies, such as power modulation after ac system faults.
The HVDC interconnection scheme is capable of continuous operation at a reduced dc voltage of 400 kV (80%) and 350 kV (70%) from minimum current up to the rated dc current of 1800 A with all redundant cooling equipment in service. To optimize the filter and converter transformer design, valves to operate at high firing angles combined with an extended range of the tap changer.
The energy availability and combined operational availability for the two converter stations is guaranteed to be more than 99.5%, or alternatively, a forced energy unavailability of less than 0.5%. The forced outage rate should be less than six outages per pole per year.
The utility operators specified performance requirements for audible noise, electrical noise, and radio and noise interference that were incorporated in the scheme at the design stage. Noise filter equipment provided for the ac switchyard and the dc line damps the interference between the thyristor valves and the PLC equipment.
Low loss design was of central importance for technical and economical optimizations. This resulted in converter station designs with total losses (excluding dc lines) of approximately 12 to 13 MW per station at 1800 MW of transmission power. At rated transmission capacity, the converter transformers (about 50% of the total losses) and the converter valves (about 35% of the total losses) cause the main losses.
Reactive Power Requirements
The hydropower generators partly cover the reactive power demand of the TSQ converters (up to 330 MVAr) 0.9 × 80 MVAr Q-elements balance the reactive flow to the TSQ AC system at rated power. These Q-elements in three filter banks with individual switching capability connect to the station bus bar. The reactive power supply equipment is capable of regulating the reactive power interchange with the ac system from -80 MVAr (supply) up to +80 MVAr (absorption) in whole power range and with one sub-bank out of service.
The GZ Converter Station is connected to the 220-kV substation with a design of 100% redundancy of reactive power requirement that can be met with one sub-bank out of service. Installing 11 sub-banks, rated at 100 MVAr will meet this requirement. The total reactive power supply divided into three banks is capable of regulating the reactive power interchange with the ac system from -100 MVAr (supply) up to +100 MVAr (absorption) in whole power range.
Future Transmission System Development in China
The Tian-Guang HVDC Interconnector was commissioned in June 2001. With the growing demand for power in this region, the State Power Corporation of China in Guangzhou awarded contracts for the construction of a 3000 MW, ±500-kV HVDC transmission line (940 km [585 miles] long), due for commissioning in October 2004, from the hydro and coal-fired power plants in the west of China to the rapidly developing areas in the southeast, around Guangzhou and Shenzen.
TransGrid (Australia) Provides New Supply to Sydney's CBD
Electricity demand in the Central Business District (CBD) of Sydney and the surrounding suburbs has been growing at an average of 4% per year over the last four summers. Predictions state that electricity demand will continue to grow at this strong rate because of new commercial and residential developments. Studies indicate utilities will no longer be able to meet existing reliability standards for Sydney's CBD and inner suburbs from the summer of 2003 onward.
Existing Supply System
The electricity supply to Sydney's CBD and surrounding suburbs is sourced from TransGrid's main grid of 330/132-kV substations where 132-kV capacity is available to Energy Australia (EA). EA owns and operates the 132-kV network that interconnects these substations and supplies the major sub-transmission substations in the metropolitan area. This arrangement permits the sharing of capacity between the source substations.
TransGrid and EA decided the most cost-effective and achievable solution was to install and commission a 330-kV underground cable between the Sydney South 330-kV Substation and a new CBD 330-kV Substation with associated 132-kV works prior to summer 2003 (Fig. 10).
The Sydney CBD project consists of the following major components:
A new 330/132-kV substation at Haymarket adjacent to the CBD
A single-circuit 330-kV cable with a route length of 28 km (17 miles) from Sydney South Substation to Haymarket Substation
Extensive 132-kV developments including a new inner-city substation.
Haymarket Substation will be a new 330-kV substation adjacent to Sydney's CBD. It will be predominantly underground with provision for a “high rise” commercial development on site. The site location is adjacent to public areas that link commercial, tourism and university districts.
Fire and other electrical failures were major factors in the development of this substation concept. The transformers were the critical components of the design. Because of the possibility of catastrophic failures of conventional transformers on the Haymarket site, gas-insulated transformers were selected. Toshiba will supply three 400 MVA 330/132-kV gas-insulated transformers and one 100-MVA gas-insulated reactor with cooling systems. These will be the highest-rated gas-insulated transformers in the world to date.
Siemens won the contract for the substation building and the balance of the substation plant, which includes five bays of 330-kV gas-insulated switchgear and 23 bays of 132-kV gas-insulated switchgear. Again, innovation will be a major feature of the design. The substation plant and environment will be highly monitored and will include a gas management strategy to ensure there is no significant loss of SF6 to the substation environs.
330-kV Cable Circuit
The 28-km (17-mile) single circuit that will supply Haymarket Substation will be one of the longest high-voltage ac cable installations. The first 24 km (15 miles) of the cable will be direct buried in public streets with some directional drilling. The last 4 km (2 miles) of the route to the Haymarket Substation will be in a deep tunnel.
A pre-qualification process allowed the offering of any technology to meet the needs of the project, of which 29 offers were received from 13 applicants. Sumitomo offered the best cable, a paper-polypropylene insulated cable. The high levels of cable monitoring, which include a dynamic rating system based on integrated optical-sensing fiber, is an important feature of the design. The low magnetic field design for sections of the route in residential streets is another key feature.
In parallel with these works, TransGrid and EA are pursuing investigations about using DSM or embedded generation to defer future network augmentation.
Baltic States Upgrade Interconnected Power Systems
Historically, the Soviet Union's northwestern regional planning strategy lead to concentration of the thermal power plants in Estonia while more than 70% of the installed capacity in Latvia was hydropower. The installed capacity at the Ignalina Nuclear Power Plant in Lithuania produces the majority of the state's power requirements. The structure of the generation capacity within the Baltic Interconnected Power System (IPS) is very diverse, and to optimize the power balancing on the Baltic IPS, each state's power system requires support and cooperation from the neighboring states to supply base and peak demands. Figure 13 details the annual electrical-energy interchange among the Baltic and neighboring power systems.
The three Baltic states-Estonia, Latvia and Lithuania — are located on the eastern coast of the Baltic Sea and have a combined population of about 8 million. The Baltic IPS was established in 1992 following independence from the Soviet Union. Baltic IPS operates in parallel with Russia's Unified Power System (UPS) and the Belarus IPS via an “electrical ring” of 330- to 750-kV transmission lines, which were constructed in the northwestern part of the former Soviet Union in the 1960s and 1970s. The power plants and power systems in each Baltic state operate independently. However, the operation is coordinated by the Dispatch Center of the Baltic IPS (DC Baltija), which is located in Riga, Latvia. To improve operational control, GE Harris SCADA systems have been installed in the Lithuanian and Estonian National Control Centers. Work is currently in progress to install similar equipment in DC Baltija and the Latvian National Control Center.
The Baltic IPS
The Baltic IPS has a current total installed capacity of 11,490 MW and includes a wide spectrum of generation types: nuclear plant (Ignalina NPP), combined heat and power, thermal, hydro and pumped storage power plants. The annual peak demand in 2000 on the Baltic IPS was 4551 MW (Lithuania 1910 MW, Estonia 1469 MW and Latvia 1172 MW). The transmission network of the Baltic IPS consists of 330-kV transmission lines with a total length of 4137 km (2571 miles) based upon January 2001 data.
Following independence from the Soviet Union, each power system of the Baltic states operates independently, though DC Baltija coordinates the operations. DC Baltija in Riga was founded on the basis of the former North-West Dispatch Center, a facility installed on Russia's UPS. DC Baltija's principle role and main tasks include:
The reliable operation of the Baltic IPS 330-kV network and tie lines with Russia UPS and Belarus IPS in close cooperation with dispatch centers of respective power systems
Efficient power and energy balance planning and realization for the Baltic IPS.
The regional operational control takes into account the generation structure of each country to deal efficiently with demand or generation deviations from planned values. To improve operation control, GE Harris SCADA systems were installed in Lithuanian and Estonian National Control Centers; in 2002, the SCADA systems will be fully installed in DC Baltija and Latvian National Control Center.
Currently, the Ignalina NPP generates energy from two 1300-MW RBMK-type reactors manufactured in the Soviet Union. The Lithuanian government has decided to shut down the first reactor in 2005 and the second reactor in 2010. This decision will lead to serious changes in the power balance in Lithuania and in the whole Baltic IPS as Ignalina NPP produced 35% of the total Baltic IPS electrical energy production during 2000, (8419 million kWh). Figure 14 shows the emergency protection scheme installed to safeguard the IPS in the event of the loss of the power in-feed from Ignalina NPP.
Research Activities of DC Baltija
Since DC Baltija was established, engineers, scientists and specialists from power companies and universities of the Baltic states have performed field tests to verify appropriate relay protection and to establish the power/frequency characteristics and regulating capability of Baltic IPS.
A series of field tests raised concerns about the performance of the speed governors providing unstable operation as a result of out-of-date fuel and steam supply systems of the power plants. These factors made it necessary to carry out wide-scale field tests with isolated operation of the whole Baltic IPS. This is an urgent issue. Presently, there is serious discussion about implementation of the Automatic Generation Control in Baltic IPS, and therefore, it is necessary to determine all requirements and measures for the primary control. Though there have been several attempts to complete the field tests, they continue to be deferred because of disagreements among Baltic IPS, Russia UPS and Belarus IPS on the basis of the electrical ring security and the provision of commercial contract fulfillment among these power companies.
In addition to the research activities, successful black start tests on the Ignalina NPP's two main circulation pumps (5.6 MW each) were conducted using two generators at the Pljavinas HPP supplying energy via three 330-kV transmission lines.
Future Development of Baltic IPS
The effect of the decommissioning of the Ignalina NPP on the Baltic states is the subject of the report “Baltic Regional Energy Development Program” prepared by Latvian, Estonian and Lithuanian power systems in cooperation with Electrotek Concepts Inc. USA.
This regional development analysis confirmed that to re-establish the energy balance of the Baltic states after the decommissioning of the Ignalina NPP would require the construction of the new power plants having a total generation capability of 700 MW. The location of these power plants will have to take into account economical and political considerations in addition to optimizing their location to ensure siting in close proximity to the existing 330-kV transmission network of the Baltic IPS. Furthermore, an additional factor is the possible future power system connection development with UCTE and NORDEL systems.
In 1998, the Baltic Ring project provided development scenarios of new electrical connections, such as a 400-kV ac link between Lithuania and Poland, and a 400-kV dc link connecting Estonia with Finland. Currently, the respective countries are studying the economic effectiveness of these links, taking into consideration the results of the project “Synchronous Interconnection of TESIS and UPS Networks,” prepared by experts of European Union countries within the framework of EC TACIS.
The accumulated experience of Baltic IPS' joint operation with neighboring countries and well-developed relations with power companies from the United States and Eastern, Central and Western European countries will be used by the governments of the Baltic states to create a common Baltic electricity market.