Future Of Transmission Line

There is a need for information on the attempts of EHV transmission and experiences during installation. Sharing new design concepts, tower construction ideas, operation and maintenance aspects of these EHV lines will help in avoiding duplication of efforts... - Ravi Kant Kumar, Girish A Kulkarni

Power is the basic key for growth of any country’s economy. The increased demand of electricity, need to optimise the utilisation of power generation capacity and increase in the interconnections are the major issues with which power sector is dealing with. Energy consumption per person is also rising tremendously in developing countries. However, installing a new power plant cannot be a solution every time. Dense population, availability of land, initial and installation cost can be the major issues in this case. Huge transfer of power from generating plants to load centre at long distance with bulky transmission lines is causing to upgrade voltage class to Extra High Voltage (EHV) from High Voltage (HV).

There are indications that EHV network will grow at a very fast rate worldwide as compared to previous few decades. Increase in transmission distance with maximum possible reduction in power loss with saving in the economic costs of transmission lines is the major boost worldwide for moving from HV to EHV. Efforts regarding this already initiated worldwide.

There is a need for information on the attempts of EHV transmission and experiences during installation. Sharing new design concepts, tower construction ideas, operation and maintenance aspects of these EHV lines will help in avoiding duplication of efforts. Therefore, this article is an attempt to provide a summary of the relevant efforts in terms of all these issues going on in different countries throughout the world.

Why EHV

The world will need greatly increased energy supply in the next 20 years. The demand of electricity is increasing twice as fast as overall energy use and is likely to rise by more than two-thirds till 2035. In 2012, 42% of primary energy used was converted into electricity. With the United Nations predicting world population growth from 6.7 billion in 2011 to 8.7 billion by 2035, demand for energy must increase substantially over that period. Both population growth and increasing standards of living for many people in developing countries will cause strong growth in energy demand, as outlined above. Over 70% of the increased energy demand is from developing countries, led by China and India – China overtook the USA as top CO2 emitter in 2007. Growth of Power Sector infrastructure in India since its Independence has been noteworthy making India the third largest producer of electricity in Asia. Generating capacity has grown manifold.

The distance between generating stations and load centers is increasing day by day. The amount of power to be handled is increased from 11kV to 765 kV in India. This need is the basic cause behind emergence of EHV. EHV transmission has emerged from various advantages like reduction in line drop and increase in transmission efficiency.
For HV transmission volume of conductor material is given by the formula:

So, the volume of conductor material is inversely proportional to the square of transmission voltage and power factor. So, EHV transmission reduces the volume of conductor material.

Transmission efficiency is given by formula,

Transmission efficiency is directly proportional to their line voltage where j, ρ and l is constant.

The above equation shows that the percentage line drop decreases when the transmission voltage increases. In addition, very high voltages (345 kV and above) are subject to corona losses. These losses are a result of ionization of the atmosphere, and can amount to several megawatts of wasted power. Power losses due to corona is given by formula

Where,
f = frequency
V = phase natural voltage
Vc = disruptive voltage per phase

Reduction in conductor size leads to losses. Mitigating the losses during EHV transmission is the major challenge to be faced along with other structural and geographic challenges.

Challenges

Transmission lines travels hundreds of miles in difficult terrain, under varying and extreme environmental conditions – using variety of construction equipment. The engineering challenges become much more difficult with higher voltages as structures become very tall and heavy.

Construction of tower depends on
Type of geometry: such as Peak of the transmission tower, Cross arm of the transmission tower, Boom of transmission tower, Cage of transmission tower, Leg of transmission tower.
Weight of tower: The weight of the tower varies substantially with height, duty (straight run or corner, river crossing etc.), material, number of circuits and geometry. The average weight of 670 towers for 500-kV lines included in the EPRI survey (EPRI 1982) is 28,000 lb (Pounds = kg *2.2046). The range of reported tower weights is 8,500 to 235,000 lb.
Circuit configuration: Due to unavailability of the shortest distance, straight corridor transmission line has to deviate from its straight way when obstruction comes. In total length of a long transmission line, there may be several deviation points.

According to the angle of deviation there are four types of transmission tower-

  • A – type tower – angle of deviation 0˚to 2˚.
  • B – type tower – angle of deviation 2˚ to 15˚.
  • C – type tower – angle of deviation 15˚ to 30˚.
  • D – type tower – angle of deviation 30˚ to 60˚.

Every country has individual challenges, which are unique to that country or region.

The problems posed in using such high voltages are different from those encountered at lower voltages. Major problems are:

  • Increased current density because of increase in line loading by using series capacitors.
  • Use of bundled conductors.
  • High surface voltage gradient on conductors.
  • Corona problems: audible noise, radio interference, corona energy loss, carrier interference, and TV interference.
  • High electrostatic field under the line.
  • Switching surge over voltages, which cause more havoc to air-gap insulation than lightning or power frequency voltages.
  • Increased short-circuit currents and possibility of ferro resonance conditions.
  • Use of gapless metal-oxide arresters replacing the conventional gap-type Silicon Carbide Arresters, for both lightning and switching-surge duty.
  • Shunt reactor compensation and use of series capacitors, resulting in possible sub synchronous resonance conditions and high short circuit currents.
  • Insulation coordination based upon switching impulse levels.
  • Single-pole reclosing to improve stability, but causing problems with arcing.

Efforts to mitigate the problems

The basic proof justifying the Corona Problems, Audible Noise, Radio Interference, Corona Energy Loss, Carrier Interference, and TV Interference in power transmission is explained in “Corona Effects on EHV AC Transmission lines” by Snigdha Sharma et.al. When corona is present on the conductors, EHV lines generate audible noise, which is especially high during foul weather. The noise is broadband that extends from very low frequency to about 20 kHz, Pulse type of corona gives interference to radio broadcast in the range of 0.5 MHz to 1.6 MHz, Corona on conductors also causes interference to Carrier Communication and Signaling in the frequency range 30 to 500 kHz. Extra high voltage carrying lines cause corona losses, and to reduce these losses can be reduced by use of bundle conductors. These consist of two or more conductors in bundle. Each conductor carries equally distributed among these conductors in bundle, so reduces the corona losses.

The electrostatic effects are caused by the extremely high voltage, while electromagnetic effects are due to line loading current and short circuit currents. Electrostatic field causes damage to human life, plants, animals, metallic objects – such as fences and buried pipelines.

Although, it is not a biological effect, electromagnetic interference of power frequency as low as 2 kVm-1 with certain cardiac pacemakers could have medical significance. In short, corona effects, electrostatic fields in the line, losses, audible noise, carrier interference and radio interference became recognised as steady state problems, which govern the line conductor design, line height, and phase-spacing used to keep the interfering fields within prescribed limits. Use of synchronous condensers due to high line charging currents at load end only became impractical to control voltages at the sending-end and receiving-end buses. Use of Shunt compensating reactors for voltage control at no load and switched capacitors at load conditions became necessary. All these are still categorised as steady-state problems. However, the single major problem considered with EHV.

Voltage levels is the over voltages during switching operations, which is commonly known as switching-surge over voltages. Effect of high surface voltage gradient on conductors is when in an insulation system, the voltage gradient (voltage stress) exceeds a critical voltage, the air molecules surrounding the high voltage transmission line conductors become ionized resulting in partial discharges. Corona loss occurs if the line to line voltage exceeds the corona threshold, and it can be overcome by the use of bundle conductors.

Increased short-circuit currents and possibility of ferro resonance conditions leads to the phenomenon appears after transient disturbances (transient overvoltage, lightning overvoltage or temporary fault) or switching operations (transformer energising or fault clearing). Its effects are characterised by high sustained over voltages and over currents with maintained levels of current and voltage waveform distortion, producing extremely dangerous consequences. The solution adopted is the use of switched damping resistors. Cross-tripping parallel energised circuits also can be used occasionally. Transient oscillations may stress transformer insulation or cause
circuit break.

The overall aim of insulation coordination is to reduce to an economically and operationally acceptable level of the cost – and disturbance caused by insulation failure. In insulation coordination method, the insulation of the various parts of the system must be so graded that flash over, if occurs, it must be at intended points. In order to protect electric power system equipment from lightning and switching over voltages, surge arresters are used within the system as a tool for insulation co-ordination. The purpose of using a surge arrester is to always limiting the voltage across the terminals of the equipment to be protected below its insulation withstanding voltage.

When a single phase-to-ground fault occurs on an energised transmission line the faulted phase is tripped, and automatically reclosed after a suitable dead time. Auto reclosing reduces operating cost and improves the reliability of service of the network, but further causing problems with arcing. Mitigating with above limitations to boost the performance of EHV system can be the major area of research in coming days.

World scenario

American Electric Power started transmitting power at a nominal voltage of 765kV and a maximum voltage of 800kV in 1969. Since then, 765kV transmission lines of nominal voltage have been introduced in other areas such as New York (by NYPA), Brazil, Venezuela, and South Africa. In Eastern Europe, Poland and Hungary started to operate of 750kV transmission lines of nominal voltage in the 1970s in order to receive power from the former Union. Recently, China and India also developed 765 kV and 1,000 kV transmission lines.

Indian scenario

India is presently placed at a junction of the globalised and liberalised economy, which provides a great opportunity to exploit its potential, and lead to sustained economy growth and welfare of its populace. Present transmission network in India is 765kV lines existing 8,056 ckms, 400kV lines existing 1,25,039 ckms and 220kV lines existing 144,966 ckms. The major goal for India towards self reliance is UHVAC transmission systems. Having introduced 765kV as the highest transmission voltage, the country is aspiring to shift to 1200 kV (voltage) transmission networks during the XIIth plan period. A large network comprising 1200 kV transmission superhighways is being planned as part of the National Transmission Network.

Conclusions

This article reviewed the efforts going on worldwide to meet the heavy electric power demands with minimal losses. Need, advantages and technical problems in implementing EHV technology, efforts to overcome that problem are mentioned and studied. Comparative study on EHV transmission lines used in different countries and a comparison between them and the low voltage levels have been performed. Accordingly, the advantages of EHV transmission lines are summarized. Finally, different research areas are identified.


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