During the last few decades, the electrical power needs in most of the developing countries have been increasing continuously at a tremendous rate following the growth of industrialization in major cities and towns. In order to meet such an increasing demand for power economically, and with good efficiency, electrical utilities have increased their output voltages and also sizes of their generating plants.

HVDC power transmission is an emerging technology in India. It is used for bulk transmission of electric power from one system to another and is particularly advantageous for the distribution of due shares of electric power to the states in case centralized generation near the coal pit head. The charging of Back to Back HVDC system in Rihand and Vindhyachal . HVDC lines are possible milestones for development of HVDC system in our country. The HVDC transmission lines can be through air or through Sulphur Hexafluoride (SF6) gas as the primary insulating media (gas insulated transmission lines.)

HVDC is now being considered in several developed countries as an economic alternative to AC high voltage transmission.

Supply of reliable and economic electrical energy is a basic requirement for industrial growth. To improve the reliability of power supply, adequate generation resources and reliable transmission and distribution networks are also necessary. In addition, the power system has to be operated by minimizing production costs maintaining, at the same time,the required degree security. System security implies that credible contingencies (such as tripping of a single transmission line or a generator) do not result in cascading outages leading to partial or complete blackouts. Factors that threaten system security include

The requirement of synchronous operation of all generators in a system connected by ac lines

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Inability to store energy in significant quantities close to load centers

Uncontrolled loop flows in a mesh type network

Slow and inadequate reactive power control and

Instabilities caused by low frequency power swings and voltage collapse.

These problems are typically present when operating under stressed system conditions. To overcome these problems, it is essential to carry out security assessment and implement preventive control whenever feasible. However, in Indian power systems, which are perennially deficit in generation reserves, it is more realistic to plan emergency control, which is deployed whenever the system enters an emergency state.

Environmental and resource constraints are retarding the growth of transmission networks all over the world. Fast acting and energy efficient controllers that enable flexible system operation will contribute to better operation of existing transmission networks with reduced margins.

HVDC PROJECTS IN INDIA

The first HVDC project was started in Sweden in 1952 and slowly it found its base in India . The various HVDC projects in India are;-

1. Vindhyachal back-to-Back HVDC transmission between U.P & M.P
2. Rihand Delhi HVDC Project between Rihand(U.P) & Delhi
3. National experimental HVDC project between Barsur in M.P and Sileru in A.P

The increasing popularity of HVDC transmission can be attributed to the following advantages that it offers over HVAC transmission system:

• Economic Consideration

  The cost of transmission line includes the investment and operational costs. The investment costs include cost of Right of Way, transmission cost, conductors , insulators and terminal equipments. Operational costs mainly include the costs of losses. for the lines designed with same insulation levels , a dc line can carry as much power with 2 conductors as the AC with 3 conductors. Thus for a given power level DC requires less RoW, simpler & cheaper towers and reduced conductors and insulator costs. Power and corona losses are also reduced for DC transmission.
  
  The total capital cost of a transmission system is equal to the sum of the capital cost of substations plus capital cost of lines. DC lines and HVDC lines are cheaper than AC lines and HVAC cables. Figure shows the variation of costs of transmission with line length for AC and DC transmission. The abscissa of point of intersection of the two curves is called the breakeven distance . When the transmission distance is greater than the breakeven distance HVDC is more economical than equivalent HVAC. The breakeven distance is different for different projects . The breakeven distance varies from 500-900 km in overhead lines. For submarines cables the breakeven distance varies from 25-50km and for the under ground cables the range is between 50-100 km.
  
  

• Technical performance

  The DC transmission has some positive features that are lacking in AC transmission. These are mainly due to the fast controllability of power in DC lines
  Full control over power transmitted
  The ability to enhance transient and dynamic stability in associated AC networks
  Fast control to limit fault current in DC lines

• Reliability

  The reliability of DC transmission systems is quite good and comparable to that of AC system. There are 2 measures of overall system reliability:
  Energy Availability = 100*(1-equivalent outage time/total time)%
  Transient Reliability =100*No. of times HVDC system performed/ No.of recordable AC faults
  

An HVDC Transmission has the following main components:-

1. AC substation and HVDC substation at each terminal


2. Interconnecting HVDC Line(s)


3. Electrode lines and earth(ground) electrodes.

AC substation is of conventional type having busbars, AC switch gears, CTs , VTs , surge divertors etc. The HVDC substation is called converter station.

At sufficiently high voltage levels (such as those used in HVDC Systems,), the electrostatic and electromagnetic field radiation's associated with transmission are of significant intensities.
Physical phenomena like ozone discharge, sparking, corona , radio interference and audible noise at sub station are some of the typical associated effects. In most of these cases, these phenomena do not call for undue concern, since they are merely manifestations of leakage losses in the transmission system, and, as such do not result in great environmental pollution. On the other hand, possible biological and physiological harm to human and plant life cannot be treated lightly. Realizing the fear, expressed anger, and possible legal implications involved of real or imaginary consequences of these fields, many utilities have now begun to give serious attention to studies relating to the effect of high voltage fields on the biological and physiological characteristics of human beings and animals in habitat close to these high voltage lines.

HV Fields and their area of influenece
There are two basic fields associated with high voltage transmission systems namely the electrostatic and electromagnetic fields.

• Electrostatic Fields
  They are related directly to operating high voltage and inversely to the distance. E=kV/d
• Electromagnetic Fields
  These are directly proportional to current magnitude and inversely to the distance. B=KI/d

Effects of Conventional HVDC Systems
Coherent time varying fields, which coincide with natural ion magnetic resonance, can have biological effects at very low levels. They affect the pineal gland whose main function is secretion of hormones (melatonin and serotonin) playing a major part in controlling human circadian rhythms. The production of melatonin is suppressed by extremely bright lights and low ELF (Extremely Low Frequency) magnetic fields. Level of serotonin results in mood changing and illness. Changed spin states in haemoglobin molecules can produce a significant net magnetic moment. This attracts and traps the lymphotic cells, greatly impairing the immune system response.
Four major areas of health effects due to conventional HVDC Systems are

• Effects on the immune system with a reduction in the ability of circulating white blood cells to kill tumor cells.
• Effects on foetal development
• Control and regulation of cell growth including tumor formation
• Effects on very powerful hormonal mechanisms in the central nervous system and brain, which have connection to cancer. Power line magnetic fields are also associated with headache, depression and suicide.

Effecs of SF₆ Gas Insulated Systems
Gaseous insulation other than air presents greater hazard than any other media. SF6 is a colourless and odourless, physiologically inert gas. The production of SF6 involves the use of elemental Flourine in the presence of an electric spark, sparck or corona, SF6 decomposes to a wide variety of chemically active products which posses completely different properties from it. The breakdown products are themselves gases usually of acid nature or possessing oxidizing characteristics. The accumulation of these decomposition products in the equipment or GIS has caused concerns regarding the issue of health, safety and material compatibility problems. Although minute in quantity, the traces of SF6 have some environmental effects such as :-
*Global warming and green house effect
*Ozone layer depletion effect

With the increased concern in personnel health and safety, there is growing interest in the toxicity of SF6 and its decomposition products in gas insulated systems. They fall under simple asphyxiation effect, which attacks the lungs of living beings. The greatest risk of personnel exposure to the hazardous product is during repair of maintenance of the compartments. For this prior knowledge of decomposition products , their rates and toxicities should be known. The Threshold Limit Value(TLV), Time Weighted Average(TWA) are the most common guidelines used in determining safe levels. Table 2 is a summary of animal toxicity to SF6 and its decomposition products. These levels are compared with existing allowable exposure level to decide the procedure and protection to be adopted.

The hazardous by products produced during the action of an electric arc , spark or partial discharge (corona) may be absorbed anthem totally enclosed or bypassing the arced gases through an absorbent such as activated alumina or soda lime. In conventional HVDC power lines, by adopting suitable clearance level, one can check the adverse health effects of electrostatic and electromagnetic fields.

Protective Measures
The protective methods and safety practices may be observed to secure protection against electric field effects as below:

• Designing

The electric field strength and potentials may be decreased by selecting the appropriate parameters (conductor's height, spacing and configuration). But a reduction in phase spacing which reduces the field at ground, might require larger or more) conductors to avoid an increase in radio interference. The average ground clearance may be maintained between 10m to 22m.

• Shielding

Proper shielding (or screening) employed when working at energized power transmission lines and sub-stations promote conditions, which do not produce any ill-aftereffects on human organism.


• Shielding by trees
  

Tress offer an attractive and pleasing solution to reduce the electric field close to ground level in those conditions which do not produce any ill after effects on human organism.


• Shielding by Lower Voltage Lines

This type of shielding is described as active because shield consists of energized wires. If the magnitude and phase of voltage are properly selected, a high degree of field reduction can be achieved.


• Shielding by Ground wires

The shield wire has maximum efficiency in controlling the maximum voltage gradient at ground level, when it is suspended beneath or the slightly outward the outer phase conductor having maximum efficiency. The earthed wire rope may be 2.5m above the ground


• Shielding by Make shift Device

They may be divided into two groups
1. Protective equipment employed for operations performed directly in sub station area. This includes protective mesh screen attached directly to workplace. The screen may be used a make shift operation or as a permanent protection incorporated design
2. The second group covers hot line inspection and maintenance. Protective equipment relating to this group includes screened hoist and insulating ladder with platform


• Grounding

The presence of vehicles near operating high voltage power system could present two safety problems, electrical shock and ignition of fuel vapour. Grounding straps are effective in reducing these hazards for large vehicles

1. Long distance bulk power transmission by overhead lines
2. Underground or underwater cables.
3. Interconnections of AC systems operating at different frequencies.
4. Back -to-Back HVDC coupling stations
5. MTDC asynchronous interconnections between three or more AC Networks.
6. Control and stabilization of power flows in AC Interconnections of large interconnected systems

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With the successful development of HVDC transmission, schemes are now being preferred for long transmission lines, underground/submarine cables and system interconnection and also for multi terminal HVDC interconnecting systems However, above 400 kV, discharge pulses may lead to painful sensation, momentary nervous shock or even an involuntary contraction of muscles, but prevention of the ill effects with technical and organizational means is feasible and thus mere transfer of knowledge to the public about the nature of electric field will solve many of the problems.