By Dietmar Retzmann, Siemens Energy, Germany
Electric power supply is essential for the survival of society, like the blood in the body. Lack of power brings about devastating consequences for daily life and for business. However, deregulation and privatization are posing new challenges to transmission systems. System elements will be loaded up to their thermal limits, and wide-area power trading with fast-varying load patterns will contribute to increasing congestion.
In addition to this, global climate developments call for changes in the way electricity is supplied. Environmental constraints, such as loss minimization and CO2 reduction, will play an increasingly important role. Consequently, we have to deal with conflicts between reliability of supply, environmental sustainability as well as economic efficiency. The power grid of the future must be secure, cost-effective and environmentally compatible. The combination of these three tasks can be tackled with the help of intelligent solutions and innovative technologies.
Innovative solutions with HVDC (high voltage direct current) and FACTS (flexible AC transmission systems) have the potential to cope with the new challenges. By means of power electronics, they provide the necessary features to avoid technical problems in the power systems, they increase transmission capacity and system stability very efficiently, and they help prevent cascading disturbances.
HVDC and FACTS Technologies
In the second half of the last century, high-power DC transmission technology was introduced, offering new dimensions for long-distance transmission of power. This development started with the transmission of power within a range of a few hundred MW and was continuously increased. Transmission ratings of 3 GW over large distances with only one bipolar DC line have become state-of-the-art in many grids today. The world’s first 800-kV DC project in China has a transmission rating of 5 GW, and future projects at 6 GW and higher are at the construction or planning stage. In general, for transmission distances above 600 km, DC transmission is more economical than AC transmission (greater than or equal to 1,000 MW).
In the course of HVDC’s development, different kinds of applications were introduced. The first commercial applications were HVDC sea cable transmissions. Then, long-distance HVDC transmissions with overhead lines were built. To interconnect systems operating at different frequencies, back-to-back (B2B) schemes were applied. B2B converters can also be connected to long AC lines. A further application of HVDC transmission, which is very important for the future, is its integration into a complex interconnected AC system. Hybrid solutions such as these can lower transmission costs and potentially bypass heavily loaded AC systems.
Since the 1960s, flexible AC transmission systems (FACTS) have evolved to a mature technology with high power ratings. FACTS, based on power electronics, have been developed to improve the performance of weak AC systems and to make long-distance AC transmission feasible. FACTS can also help solve technical problems in the interconnected power systems. FACTS are applicable in a parallel connection (SVC, Static VAR Compensator - STATCOM, Static Synchronous Compensator), in a series connection (FSC, Fixed Series Compensation - TCSC/TPSC, Thyristor Controlled/Protected Series Compensation - S³C, Solid-State Series Compensator), or as a combination of both (UPFC, Unified Power Flow Controller - CSC, Convertible Static Compensator) to control load flow and to improve dynamic conditions.
HVDC Promotes Green Power in Australia
The title-pages of this article show a view of the Basslink project in Australia (pages 20-21), which transmits electric power from wind farms and hydro plants from George Town in Tasmania to Loy Yang in Victoria and the same way back very cost-efficiently. This HVDC system consists of a combination of submarine cable (with 295 km the longest submarine cable in the world up to now), subterranean cables (8 km for reasons of landscape protection) and overhead lines over a total transmission distance of 370 km. The nominal power is 500 MW at a DC voltage of 400 kV and a current of 1,250 A. The overload capacity of the transmission system is 600 MW during 10 hours per day.
Both Victoria and Tasmania benefit from the interconnection of their networks: During peak load, Tasmania delivers “green energy” from its hydro power stations and wind farms to Victoria, while Tasmania can cover its base-load demands out of Victoria’s grid during dry seasons when the hydro-reservoirs are not sufficiently filled. Furthermore, the island of Tasmania receives access to the Australian continent power market.
HVDC and FACTS for Bulk Power Transmission in India
Since 2003, the HVDC East-South interconnection in India has been transporting electrical energy with low losses from the Talcher power plant complex in the state of Orissa in the east of India over 1,450 km to the industrial region around Bangalore in the south. This system was originally designed for a 2,000 MW transmission. In April 2006, Siemens was awarded an order by Power Grid Corporation of India to increase the transmission capacity of the DC transmission from 2,000 MW to 2,500 MW. As the upgrade is now completed, it is possible to make maximum use of the system’s overload capacity, and thereby enable the power supply company to keep pace with the growing demand for energy in the region around Bangalore. Photo 2 (labeled, pg 20) shows a view of the HVDC station at Talcher.
Furthermore, in March 2007, Siemens and consortium partner Bharat Heavy Electricals were awarded an order by the Power Grid Corporation of India, New Delhi, to construct a new HVDC transmission system. The purpose of the new system is to strengthen the power supply to the growing region around New Delhi. The system is scheduled to go into service in November 2009.
This project is the fourth long-distance HVDC transmission link in India and will have the highest power rating of all. In the future, the bipolar 500-kV DC transmission system at a power rating of 2,500 MW will transport electric power with low losses from Ballia in the east of Uttar Pradesh province to Bhiwadi, 780 km (485 miles) away in the province of Rajasthan near New Delhi. In comparison with a conventional 400-kV AC transmission line, this HVDC transmission link improves energy efficiency so that 688,000 tons of CO2 per year will be saved.
Series compensation is an economical solution which helps improve stability in bulk-power long-distance AC transmission systems. The basic effect of a series capacitor is the reduction of line impedance between two substations: the line becomes virtually shorter. As a consequence, the transmission angle is reduced, and the system stability is increased. This provides a higher transmission capacity of the existing system, and helps avoids the necessity to install new lines.
The world’s biggest FACTS project with series compensation (TCSC/FSC) is installed at Purnea and Gorakhpur in India at a total rating of 2x 1.7 GVAr. Photo 3 (labeled, pg. 20) depicts a view of the installation at Purnea.
DC Transmission Prospects in China
At present, Siemens (together with Chinese partners) is building an UHV DC transmission system between the province of Yunnan in the south west of China and the southern province of Guangdong, for China Southern Power Grid Company, Guangzhou. This system will be the first in the world to transmit 5,000 MW of electric power at a DC transmission voltage of +/- 800 kV. The system is scheduled to go into operation by 2010. The additional electric power from Yunnan is intended to supply the rapidly growing industrial region of the Pearl River delta in the province of Guangdong and the megacities of Guangzhou and Shenzhen. In the future, the electricity generated by several hydro power plants will be transported from Yunnan 1,400 km to Guangzhou over this long-distance HVDC link. Hydro power generation is economical, environmentally friendly and does not emit any CO2. This HVDC link will save more than 30 million tons of CO2 emissions a year, which equals the amount of harmful gases produced otherwise, for example if additional fossil-fired power plants will be constructed in the province of Guangdong to serve the regional grid.
In tangent with those Chinese partners, Siemens designed a complex HVDC system and supplied HVDC main components, including ten 800-kV and ten 600-kV converter transformers as well as DC filters and 800-kV DC components. Furthermore, Siemens supports its Chinese partners in designing and manufacturing thyristor valves, converter transformers and control systems. The converter stations operate in bipolar mode with two series-connected, 12-pulse valve groups per pole, in which light-triggered thyristors are used.
This 800-kV long-distance link is the fifth Siemens HVDC transmission project in China. The first one was the “Ge-Sha” HVDC transmission link between Gezhouba and Shanghai (1,200 MW, +/-500 kV over 1,040 km or 645 miles). This system has been in operation since 1989. The second project, “Tian-Guang,” was installed between Tianshengqiao in the south-west of the country and Guangzhou (1,800 MW, +/- 500 kV over 960 km or 600 miles) and has been in operation since June 2001. The third system, “Guizhou-Guangdong I” (Photo 4, labeled, pg. 20), between Anshun in the province of Guizhou and Zhaoqing in Guangdong province (3,000 MW, +/- 500 kV over 940 km or 585 miles) has been transporting electric power since the end of September 2004. Recently, number four, Guizhou-Guangdong II (Photo 5, labeled, pg. 20), has gone into service and is already playing its part in providing China’s power supply.
An overview of these DC projects is given in Photo 6 (labeled, pg. 21). China is the fastest growing market in the field of power transmission and distribution and will become the world’s largest single market for HVDC transmission technology in the next 20 to 30 years.
Advanced DC Converter Technologies for EHV and UHV
After the 2003 blackout in the United States, new projects are gradually coming up to enhance the system security.
One example is the Neptune HVDC project. Siemens PTD was awarded a contract by Neptune Regional Transmission System in Fairfield, Conn., to construct an HVDC transmission link between Sayreville, N.J., and Long Island, N.Y. As new overhead lines cannot be built in this densely populated area, power should be brought directly to Long Island by HVDC cable transmission, bypassing the AC sub-transmission network. For various reasons, environmental protection in particular, it was decided not to build a new power plant on Long Island near the city to cover the power demand of Long Island with its districts Queens and Brooklyn, which is particularly high in summer. The Neptune HVDC interconnection is an environmentally compatible, cost-effective solution which will help meet these future needs. The project was carried out by a consortium of Siemens and Prysmian Energy Cables and Systems. The interconnection is carried out via a combination of submarine and subterranean cable directly to the network of Nassau County, which borders on the city area of New York. Rating is 660 MW at a DC voltage of 500 kVwith the world’s first 500-kV cable in service.
During trial operation, two weeks ahead of schedule, Neptune HVDC proved its blackout prevention capability in a very impressive way. On June 27, 2007, a blackout occurred in New York City. Over 380,000 people were without electricity in Manhattan and the Bronx for up to one hour, subways came to standstill and traffic lights were out of operation. In this situation, Neptune HVDC successfully supported the power supply of Long Island and 600,000 households could be saved there. In Photo 7 (labeled, pg. 22), an inside view of the Duffy Avenue HVDC EHV converter station is given.
For comparison, in Photo 8 (labeled, pg. 22) , the UHV DC converter is depicted which will be used for the above mentioned 5,000 MW project Yunnan-Guangdong in China. The figure shows the impressive dimensions of the UHV DC valve tower arrangement.
Dietmar Retzmann has been at Siemens since 1982. He is technical director in the HVDC FACTS sales department of Siemens Energy, Power Transmission Solutions Business Unit. Dr. Retzmann is author and co-author of more than 170 technical publications in international journals and conferences.