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Abstract- Energy consumption in the world is increasing as well as the demand for efficient and quality electric power supply that ensures a greener environment. Consequently, deployment of renewable energy sources has become imperative. The use of renewable resources like hydropower, solar and wind cannot be possible without the use of electric transmission lines, using High Voltage Alternating Current, though an established technology of more than 100 years has its pros and cons. This paper presents an investigation into the use of HVDC technology as an alternate option for power transmission, the benefits, economic and reliability issues and most importantly, the environmental impact. In view of the crucial role a fault-tolerant reliable Electric Power Transmission Grid plays in the economic development and improvement of the living standard of people, integration of HVDC technology is recommended for developing countries, for neighboring countries who want to share spinning reserves and for developed countries who can tap the enormous solar and wind energies in remote deserts/offshore islands and transfer it to consumers via HVDC lines.
Keywords-HVDC; Power Transmission; HVAC; Power Grid; Renewable Energy.
When the choice was made between AC and DC technology about one hundred years ago, the reasons for choosing AC were convincing. There were two main arguments that made AC the best alternative: The simplicity and efficiency of AC machines compared to DC machines, and the possibility of using transformers to make long distance AC transmission simpler and more flexible.
Since then a lot has happened in the development of HVDC technology including its load and generation characteristics. Converter valve technology has developed from mercury-arc valves via thyristors to high frequency switched IGBTs (Insulated Gate Bipolar Transistors). Researchers and manufacturers have also improved HVDC cable technology with minimal losses compared to equivalent HVAC. . Today, with the increasing need for electricity and cleaner environment, it has become imperative for countries of the world to shift from fossil fuel to renewable energy sources (hydro, wind and solar), which require transmission of power over long distances. The traditional way of transmission of power over long distances with HVAC has several issues.
Thus, the objective of this work is to investigate HVDC technology, the economic, environmental and technical consequences of using HVDC technology in the existing AC power system network and for new installations.
HISTORICAL BACKGROUND OF HIGH VOLTAGE DIRECT CURRENT (HVDC) SYSTEMS
It has been widely documented in the history of the electricity industry, that the first commercial electricity generated by Thomas Alva Edison was DC power. The first transmission systems for electrical power were also DC systems. The problem was that low voltage DC power could not be transmitted over long distances, the DC machines were not efficient, there were no semiconductor valves and therefore the success for AC systems was inevitable. This gave rise to High Voltage Alternating Current (HVAC) electrical systems which in the very words of Edison "is as unnecessary as it is dangerous, and has no element of permanency, and every element of danger to life and property."
Nevertheless, with the development of high voltage valves, it was possible to transmit DC power once again at high voltages and over long distances, giving rise to HVDC transmission systems.
The first HVDC valve was Hewitt's mercury-vapor valve, presented in 1901, but the first commercial HVDC transmission was not until about 1954, when the 20MW Â± 100kV link between the Swedish mainland and the island of Gothland was commissioned. This facility was mercury-arc based and it brought HVDC technology as an alternate transmission facility to limelight.
Thereafter, with the emergence of semiconductor device revolution, the first large HVDC transmission system designed with thyristor technology was built in Canada in 1972. This was a back-to-back asynchronous interconnection with a rating of 320MW at Â± 80kV. This followed the testing of the thyristor technology on a smaller scale, for the upgrade of the Gothland link in 1967 .
In 1976, an HVDC transmission project, the Skagerrak link (500MW), between Norway and Denmark was commissioned. Since then, a large number of thyristor based HVDC converters have been installed all over the world. The first multi-terminal HVDC transmission, Quebec- New England (2000MW Â± 500kV) was commissioned in 1992.
The numerous advances in thyristor and microprocessor technologies have greatly improved HVDC installations and applications. Consequently, by the late 90's, a new high switching frequency component, Insulated Gate Bipolar Transistors (IGBT), which uses pulse width modulation for Voltage Source Controlled (VSC) Converter stations had become matured. This new concept makes it possible to control active and reactive power flow separately. So far, this VSC technology has only been used for small or medium power applications of the order of 600MW. however research is still on going to increase its capability.
HVDC INSTALLATIONS AROUND THE WORLD
Identify applicable sponsor/s here. If no sponsors, delete this text box. (sponsors)Since the first commercial installation in 1954, a huge amount of HVDC transmission systems have been installed around the world. A pictorial view of major HVDC links in the world is shown in . The rationale for choosing HVDC some of the projects are listed as follows:
In Itaipu, Brazil, HVDC was chosen to supply 50Hz power into a 60Hz system, and to economically transmit large amount of Hydro-power (6300MW) over large distances (800km).
In Leyton-Luzon project in Philippines, HVDC to enable supply of bulk geothermal power across an island interconnection, and to improve stability to the Manilla AC network.
In Rihand-Delhi project in India, HVDC to transmit bulk (thermal) power-1500MW to Delhi, to ensure minimum losses, least amount right-of-way and better stability and control.
In Garabi, an independent transmission project (ITP) transferring power from Argentina to Brazil, HVDC back-to-back system was chosen to ensure supply of 50Hz bulk (1000MW) power to a 60Hz system under a 20-year supply contract.
In Gothland, Sweden, HVDC was chosen to connect a newly developed wind-power site to the main city of Visby, in consideration of the environmental sensitivity of the project area (an archaeological and tourist area) and improve power quality.
In Queensland, Australia, HVDC to interconnect two independent grids (of New South Wales and Queensland) to enable electricity trading between the two systems (including change of power direction flow), ensure very low environmental impact and reduce construction time 
All these successful projects have become major application areas for HVDC. The prospects are high and more research is ongoing, the list of confirmed application areas for HVDC is itemized in the next section.
Presently, the major players in HVDC technology can do installations with losses of about 2 percent per 1000 kilometers and an additional 1.5 percent at the transmitting and receiving ends. Research and development is continuing for improved application areas. At the time of writing, confirmed HVDC applications include:
Power transmission of bulk energy through sea cable
Power transmission of bulk energy through long distance overhead line.
Linking of systems with different frequencies using back-to-back interconnection.
Using HVDC transmission system to link renewable energy resources, such as wind power, when it is located far away from the consumer.
The other application areas where HVDC nicely complements the AC transmission system include:
Reduction of fault currents, bypass of network congestion, sharing utility rights-of-way without degradation of reliability and mitigate environmental concerns .
In an HVDC transmission system, electric power is taken from a three-phase AC network, converted to DC in a converter station, transmitted to the receiving point by a cable or overhead line and then converted back to AC in another converter station and injected into the receiving AC network. HVDC has been in the market for 50 years. Fig 3.1 shows a simplified schema of an HVDC system with the basic principles of transferring electric energy from one AC system or node, to another in any direction. It consists of three blocks: the two converter stations and the DC line. Within each station block, there are several components involved in the conversion of AC to DC and vice versa.
Components of A Converter Station
Every converter station has AC and DC sections. The HVAC lines come into AC switchyard of the converter station. The major components in the AC switchyard include: AC filters, circuit breakers, disconnectors, busbars and shunt capacitor banks. The AC feeds into the Converter transformers and the output from the transformer is converter to DC by Thyristors or IGBT. A schematic diagram of an HVDC is shown in Fig 3.2 and three dimensional illustration is shown in Fig 3.3[ ].
Fig 3.3: Three dimensional illustration of a Converter Station.
The function of Converter station components are briefly described as follows.
They are used to absorb harmonic currents generated by the HVDC converter, thus reducing the impact of the harmonics on the connected AC system. They equally supply reactive power to the converter station.
These are used to provide reactive power to the valves in the converters. They consist of a series of capacitors connected in parallel to the transformer.
The circuit breakers on the AC side of the converter transformer are used to take the HVDC link out of service and for clearing transformer faults. Converter control is used for clearing DC side faults.
The converter transformer is the second block in the schematic diagram, and it serves as a galvanic isolator between the AC and the DC side. It transforms the voltage to an appropriate and optimum level for the converter valves. Usually, they are of the single-phase, three-winding type, but depending on the transportation requirements and rated power, they can be arranged in other ways.
These are the most important components of the converter station, since they make the conversion from AC to DC and vice versa. The thyristor valves can be built up in different ways, depending on its application and the manufacturer.
These are large inductances connected in series with each pole. Its main functions are to limit the DC fault current and reduce harmonic current caused by interruption from overhead lines.
HVDC converters create harmonics in all operational modes. Such harmonics can create disturbances in telecommunication systems. Hence DC filters are used to reduce disturbances.
These include the transformer cooling systems, control and communication systems, and the internal station power supply with the battery backup. The valve cooling system is especially critical, since an outage may result in serious damage of the valves
The fundamental process that occurs in a 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. The two basic converter technologies used in modern HVDC transmission systems are Line-commutated Current source converters (CSC) and Self-commutated Voltage Source converters (VSC).
The line-commutated CSC requires a synchronous voltage source in order to operate. The line CSCs can only operate with the AC current lagging the voltage, thus requiring lagging power for the conversion process. The VSC-based systems are self-commutating with Insulated Gate Bipolar Transistor (IGBT) valves and solid dielectric extruded HVDC cables. The control capability of the VSC gives it total flexibility to be placed anywhere in the AC network since it has no restriction on minimum short-circuit capacity.
HVDC vs HVAC: BENEFITS/DRAWBACKS
Benefits of HVDC over HVAC
The emergence of semiconductor power devices such as thyristors, IGBT and microprocessors has increased the benefits and opened up new application areas for HVDC. The key benefits and limitations of HVDC over HVAC are briefly discussed in the following section:
a) Long Distance Bulk Power Transmission
Although converter stations are more expensive than AC stations, when large amounts of power are to be delivered over long distances (> 600 km), HVDC transmission is a preferred option. This is the so called break-even distance (about 50km for submarine cables, and perhaps 600 - 800 km for overhead cables), above this break even distance the lower cost of the HVDC electrical conductors outweighs the cost of the electronics. In addition, HVDC cable losses are as low as .0.3 -0.4% per 1000km
Another key benefit for HVDC for long distance power transmission is that the DC system requires fewer conductors (two), one only for submarine with earth return, smaller right-of-way and a less obtrusive tower than AC lines.
b) Interconnection OF Asynchronous HVAC Grids
Interconnections allow the sharing of spinning reserves, however, connecting independent systems running at different frequencies is an issue for AC systems. This issue is easily resolved by HVDC Back-to-Back stations because DC removes any constraints concerning stability and control problems.
c) Limitation of Faults
With AC, interconnections provide doorways for the propagation of disturbances. Faults causing depression of voltage on power swings do not transmit across a DC barrier hence, HVDC technology provides a firewall against disturbances in high-voltage grids [ ].
d) Voltage Control
HVDC links are also useful for voltage control. The converter absorbs reactive power depending on its control angle, which normally will be compensated for by filters and/or capacitor banks. By extending the control angle, operating range (to a lower voltage) and additional capacitor banks (to raise voltage) together with a fast acting transformer tap-changer, the reactive power demand can be used for independent voltage control at both connection points.
e) Ease of controllability
Today's advanced semi-conductor technology, utilized in both power thyristors and microprocessors for the control system, has created a lot of flexibility for the control of the HVDC transmission systems more than is available for HVAC systems, some installed about 80 years ago or more. This ease of controllability of power flow, enables efficient power trading between regions.
f) AC Support System
AC load flow depends on the difference in angle between voltage rectors in different parts of the network. This angle cannot be influenced directly but depends on the power balance. Also, a change in power generation or in the load demand will cause a change in system frequency that has to be restored by altering generation. This task is fulfilled by the generator speed controllers, thus frequency restoration is very slow. HVDC systems can fulfill this task at a faster rate by drawing the energy from the remote network. Due to its ability to change the operating point virtually instantaneously, HVDC can feed (or reduce) active power into the disturbed system to control the frequency much faster than a normally controlled generator.
Limitations of HVDC Transismission System
HVDC converter stations are expensive and not beneficial for power transmission for distances shorter than 600km for overhead transmission. The other limitations of DC transmission links include
Transient stability margin of the system may be reduced unless special control measures are adopted.
Earth return can lead to undesirable corrosion of earthed structures.
Operating a HVDC scheme requires many spare parts to be kept, often exclusively for one system as HVDC systems are less standardized than AC systems and technology changes faster
Research is on going to tackle some of these identified limitations.
environmental impact of hvdc system
The purpose of power transmission is to carry energy from generation stations to urban or industrial places. To satisfy the growing need of energy, transmission line capacities have been increased in recent years. The typical high voltage transmission line carry 400 - 1000kV, this voltage has to cross all kinds of terrain - urban areas, villages, oceans, desert and mountains. The effects of these lines on the environment and human beings have become a controversial issue in recent years. Increasing environmental awareness throughout the world is impacting both on the implementation and the cost of transmission projects.
In many countries, the public resistance to overhead lines has grown steadily during the last couple of decades. The objections are caused by fear of detrimental health effects from magnetic fields. Furthermore, objections are raised on environmental grounds, including visual impact, audible noise, impact on birds and other wildlife. In some countries, the public seems prepared to pay the extra cost of mitigation of the environmental issues . Some of the ways HVDC system impacts the environmental positively are as follows:
a) Visual Impact: HVDC overhead lines require less space per MW than the traditional AC solution and thereby reduce the visual impact of the towers.
b) Right-of-Way (ROW) Width: The ROW width of a DC line compared to an AC line is considerably reduced. This facilitates suitable routes in densely populated areas and in regions with difficult terrain.
c) Magnetic Field: The magnetic field produced by a DC line is stationary while that of AC is alternating, which can cause induction of body currents.
d) Electric Field: Electric field is produced by the potential difference between the overhead conductor and the earth .Directly under the conductor has the highest electric field and is approximately 20kV/M for a Â±450kV transmission line. DC has less electric field problem than that of AC because of the lack of steady-state displacement current.
e) Radio and Telephone Interference: The radio-interference level of an HVDC overhead line is lower than that of HVAC overhead lines. For the HVDC, it is 40dB (MV/m); while for the HVAC 380 KV overhead transmission line, the value is 50dB (MV/m).
In order to compare the cost of using either HVDC or HVDC for bulk power transmission, all main system elements must be taken into consideration. For instance, for the DC alternative, the capital cost for the converter terminals, AC input/output equipment, filters, the interconnecting transmission line must be all be accounted for. Equally, for the AC system, the capital cost for the step-up/step-down transformer, the overhead line, light load compensation (if required), reactive power compensation, circuit breakers, buildings, should be evaluated. From the investigation carried out, HVDC technology has proven a viable alternative for power transmission for certain break - even distances. A more detailed comparative cost analysis of HVDC and its HVAC Transmission alternative is tabulated in[ABB].
For many countries, with already established HVAC networks, HVDC transmission technology provides opportunities for expansion and improvement. For emerging economies, yet to deregulate their power sector, deployment of HVDC at appropriate network points will provide a smarter grid and level playing field for operators. For the oil and gas industries, new developments in IGBT and HVDC submarine cables will open new doors of opportunities for offshore energy systems.
CONCLUSION AND RECOMMENDATIONS
The primary advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. Depending on voltage level and construction details, losses are quoted as about 3% per 1,000 km.[wiki.]. This HVDC technology is now mature, having been in the market for 50 years . With the ever increasing demand for energy and cleaner environment, HVDC technology has become imperative because of its numerous advantages. Another driver for HVDC is the advances in high power semiconductor devices that can further improve thyristors and IGBTs so that they not only carry out the AC/DC conversion but perform more smart functions to make the power grid smarter.
From the study of successful HVDC projects all over the world and in view of the crucial role a fault-tolerant reliable Electric Power Transmission Grid plays in the economic development and improvement of the living standard of people, integration of HVDC technology is recommended for developing countries, for neighboring countries who want to share spinning reserves and for developed countries who can tap the enormous solar and wind energies in remote deserts/offshore islands and transfer it to consumers via HVDC lines.
It is also recommended that Power Grid System Planners should consider HVDC when planning for upgrade of ageing transmission systems and that that countries that own several thousands of kilometers of HVAC transmission lines should consider splitting into regional grids with HVDC technology to avoid propagation of faults.