In modern power systems network, it is essential to transmit power from one region to another region in order to meet the load demands. This can only possible by having Asynchronous power transmission between two regions operating at different frequency. This Asynchronous power transmission is called HVDC transmission.
Beginning with a brief historical perspective on the development of High Voltage Direct Current (HVDC) transmission systems, this paper presents an overview of the status of HVDC systems in the world today. It then reviews the underlying technology of HVDC systems, and HVDC systems from a design, construction, operation and maintenance points of view. The paper then discusses the recent developments in HVDC technologies. The paper also presents an economic and financial comparison of HVDC system with those of an AC system; and provides a brief review of reference installations of HVDC systems. The paper concludes with a brief set of guidelines for choosing HVDC systems in today’s electricity system development.
In today electricity industry, in view of the liberalization and increased effects to conserve the environment, HVDC solutions have more desirable for the following reasons:
v Environmental advantages
v Economical (cheapest solution)
v Asynchronous interconnections
v Power flow control
v Added benefits to the transmission (stability, power quality etc.)
3. The HVDC Technology:
The fundamental process that occurs in an HVDC system is the conversion of electrical current from AC to DC (rectifier) at the transmitting end and from DC to AC (inverter) at the receiving end. There are three ways of achieving conversion:
v Natural Commutated Converters.
v Capacitor Commutated Converters (CCC)
v Forced Commutated Converters.
The components of an HVDC transmission system:
The three main elements of an HVDC system are:
ü The converter station at the transmission and receiving ends.
ü The transmission medium.
ü The electrodes
VSC valves, Transformers, AC Filters, Capacitor Banks and DC Filters.
Transmission medium: For bulk power transmission over land, the most frequent transmission medium used is the overhead line. This overhead line is normally bipolar, i.e. two conductors with different polarity. HVDC cables are normally used for submarine transmission. The most common types of cables are the solid and the oil-filled ones. The solid type is in many cases the most economic one. Its insulation consists of paper tapes impregnated with high viscosity oil. No length limitation exists for this type and designs are today available for depths of about 1000 m. The self-contained oil-filled cable is completely filled with low viscosity oil and always works under pressure. The maximum length for this cable type seems to be around 60 km.
The development of new power cable technologies has accelerated in recent years and today a new HVDC cable is available for HVDC underground OR submarine power transmission. The new HVDC cable is made of extruded polyethylene, and is used VSC based HVDC systems.
HVDC in the new Electrical Industry:
The question is often asked to when HVDC transmission should be chosen over an AC system. In the past, conventions were that HVDC was chosen when:
ü Large amounts of power (>500MW) needed to be transmitted over long distance (>500km);
ü Transmitting power under water;
ü Interconnecting two AC networks in an asynchronous manner.
HVDC systems remain the best economical and environmentally friendly option for the above conventional applications.
v New technologies, such as the VSC based HVDC systems, and the new extruded polyethylene DC cables, have made it possible for HVDC to become economic at lower power levels (up to 200 MW) and over a transmission distance of just 60 km.
v HVDC systems enable the bi-directional power flows, which is not possible with AC systems (two parallel systems would be required).
4. Design, construction, operation, Maintenance & Cost structure considerations:
In general, the basic parameters such as power to be transmitted, distance of transmission, voltage levels, temporary and continuous overload, status of the network on the receiving end, environmental requirements etc. are required to initiate a design of an HVDC system.
In terms of construction, it can take from three years for thyristor-based large HVDC systems, to just one year for VSC based HVDC systems to go from contract date to commissioning. The following table shows the experience for the different HVDC technologies.
To the extent that the term operation denotes the continual activities tat are aimed at keeping the system availability at designed levels, modern HVDC links can be operated remotely, in view of the semiconductor and microprocessor based control systems included. There are some existing installations in operation completely unmanned. Moreover, modern HVDC systems are designed to operate unmanned. This feature is particularly important in situations or countries where skilled people are few, and these people can operate several HVDC links from one central location.
Maintenance of HVDC systems is comparable to these of those of high voltage AC systems. The high voltage equipment in converter stations is comparable to the corresponding equipment in AC substations, and maintenance can be executed in the same way. Maintenance will focus on: AC and DC filters, smoothing reactors, wall bushings, valve- cooling equipment, thyristor valves. In all the above, adequate training and support is provided by the installation, commissioning and initial operation period.
Cost structure: The cost of an HVDC system depends on many factors, such as power capacity to be transmitted, type of transmission medium, environmental conditions and other safety, regulatory requirements etc.
voltage AC and HVDC systems- one is between HVDC systems and a high voltage AC transmission system; and the other between a VSC based HVDC system; an AC system and a local generation source.
Thyristor based HVDC system versus high voltage AC system: The investment costs for HVDC converter stations are higher than for high voltage AC substations. Moreover, the operation and maintenance costs are lower in the HVDC case. Initial loss levels are higher in the HVDC system, but they do not vary with distance. In contrast, loss levels increase with distance in a high voltage AC system.
VSC based HVDC system versus an AC system: VSC based HVDC systems cater to the small power applications (up to 200MW) and relatively shorter distances (hundred of km) segment of the power transmission spectrum. The graph below shows that, the VSC based HVDC system is the better alternative economically when compared to either a high voltage AC system.
6. Historical perspective on HVDC Transmission
It has been widely documented in the history of the electricity industry, that the first commercial electricity generated was direct current (DC) electrical power. The first electricity transmission systems were also direct current systems. However, DC power at low voltage could not be transmitted over long distances, thus giving rise to high voltage alternating current (AC) electrical systems.
a) Important Milestones in the Development of HVDC technology
ü Hewitt’s mercury-vapour rectifier, which appeared in 1901.
ü Experiments with thyratrons in America and mercury valves in Europe before 1940.
ü First commercial HVDC transmission, Gotland 1 in Sweden in 1954.
ü First solid state semiconductor valves in 1970.
ü First microcomputer based control equipment for HVDC in 1979.
ü Highest DC transmission voltage (+/-600KV) in Itaipu, Brazil, 1984.
ü First active DC filters for outstanding filtering performance in 1994.
ü First Capacitor Commutated Converter(CCC) in Argentina-Brazil interconnection, 1998
ü First Voltage Source Converter for transmission in Gotland, Sweden, 1999.
7. Some successfully commissioned HVDC projects in India:
1) Rihand-Delhi HVDC Transmission:
National Thermal Power Corporation Limited built a 3000 MW coal-based thermal power station in the Sonebhadra District of Uttar Pradesh State. Part of the power from the Rihand complex is carried by the Rihand-Delhi HVDC transmission link, which has a rated capacity of 1500mw at + 500kv DC. Some of the power is transmitted via the existing parallel 400kv AC lines.
The basic aim of the HVDC link is to transmit the Rihnad power efficiently to the Northern Region,
meeting urgent needs in the area. There were several reasons why choosing HVDC instead of 400kv AC. The most important ones were better economics, halved right-of –way requirements, lower transmission losses and better stability and controllability. The Rihand-Delhi HVDC transmission is the first commercial long-distance HVDC link in India.
2) The 1500MW HVDC dipole between chandrapur & padgne has been successfully commissioned in 1999 and that supplies Mumbai.
3) Vindhyachal 500 MW (1989) back-to-back – interconnecting the northern and western regions.
4) The 2000 mw Talcher - Kolar link is the biggest so far and spans four states: Orissa, Andhra Pradesh,
Tamil Nadu and Karnataka. The 5651 towers used are as high as the Kutb Minar. In all 100, 00 metric
tonnes of steel and 80,000 tonnes of cement were used. The project cost is Rs.700 crores and was
executed by Indians.
4) The commissioning of the 200 MW, 200 KV National HVDC project has linked the 196 km. DC
Transmission line between Barsoor in Chhatisgarh and Lower Sileru in Andhra Pradesh and is manufactured by BHEL.The main purpose is to develop design and manufacturing capability of HVDC system and to gain on the operation and maintenance experience. Further to establish a facility for further experiments related to Product/system development.
6) A 500MW HVDC back to back station is successfully commissioned at Gajuwaka in Andhra Pradesh.
also a 400KV D/C Vijayawada – Gajuwaka line is installed along with bay extension.
7) India, China, Brazil and South Africa have agreed to co-operate for developing a 800 kilo volt (kV) High Voltage Direct Current (HVDC) power transmission facility for evacuation of power over long distances in the country in September 2005.
8. ADVANCED TECHNOLOGIES IN HVDC SYSTEMS:
The Electrical Power Research Institute (EPRI) continues to play a vital leadership role in the theoretical and experimental fronts in HVDC, AC/DC conversion equipment, and operation of HVDC systems. The EPRI High Voltage Laboratory in Lenox is a unique research and testing resource available to EPRI members. Lenox Laboratory has conducted pioneering research for a half a century, first under the direction of General Electrical and later a dedicated EPRI center.
HVDC work at the Lenox Laboratory was launched with the construction in 1977 of a full-scale DC test capability to +/- 1200kv and a DC source rated at +/- 1500kv. Several long –duration research projects were performed at the laboratory between 1977 and 1984 to investigate various aspects of HVDC line performance between +/- 600kv and +/- 1200kv. After 1984, emphasis shifted in response to market needs to the +/- 400kv and +/- 600kv voltage range.
The multi-year program, to begin in 2006, focuses on the following project areas:
a) Life Extension of Existing HVDC systems:
This project will develop life extension guidelines for aging HVDC lines, cables, and converter stations, enabling members to extend the life of their existing HVDC schemes.
Benefits: The project helps members:
v Design refurbishment strategies for their existing HVDC system to extend equipment life
v Evaluate reliability performance improvement strategies for their existing HVDC systems, and
v Increase existing asset utilization by extending the life of HVDC systems, and thus increase revenue by selling the extra transmission capacity.
2006 Deliverable Highlights:
Guidelines will be developed to extend the life of the existing HVDC equipment, including overhead lines, UG cables, submarine cables, and converter stations (converters, converter transformers, and ground electrode).
b) Advanced HVDC system at +/- 800kv and above:
This project will explore testing and demonstration of conductor bundles, insulators, and cables for operation of HVDC at +/- 800kv and above.
Benefits: The project helps to
v Identify the major issues in operating the HVDC systems at +/- 800kv and above
v Establish technical parameters of equipment exposed to HVDC voltages of +/- 800kv and above
v Gain experience in HVDC equipment performance at +/- 800kv and above through lab and field demonstration tests, thus solidifying confidence in building UHVDC systems.
2006 Deliverable Highlights:
Selection of technical parameters for conductor bundles, insulators, and cables will be perfomed. Issues such as power tap-off techniques, insulation coordination, building block concepts for converters, and telecommunications will be addressed
c) HVDC Reference book:
This project will develop a state-of –the art HVDC Reference Book that documents the latest technological developments in the HVDC area. It will build on prior EPRI HVDC handbooks:
HVDC Transmission Line Reference Book and HVDC Handbook
India has been a pioneer developer of HVDC since 1990 when the 1000 mw Rihand - Delhi line was commissioned in UP. Since then many 500 mw lines have come up. The 2000 mw Talcher - Kolar link is the biggest so far and spans four states: Orissa, Andhra Pradesh, Tamil Nadu and Karnataka. The project cost Rs.700 crores and was executed by Indians. The commissioning of the 200 MW, 200 KV National HVDC project has linked the 196 km. DC transmission line between Barsoor in Chhatisgarh and Lower Sileru in Andhra Pradesh.These facts should give us a measure of the little-known developmental works of very high calibre that are going on in India right now. We should be justly proud of this achievement
India is racing to a saturation point in electricity availability by 2012. 100,000 mw of power is planned to be added. HVDC technology will be waiting to ferry this power to all corners of India.