Growth of Transmission System in India

At the time of Independence, the power system in India consisted of small isolated power generating plants catering the electrical needs of major cities and towns. The total installed capacity at that time was merely 1300 MW and the highest transmission voltage was 132 kV AC. The post independence era witnessed an appreciable growth in the power sector.

With the goal to rapidly develop India at the power sector front, the country was divided into 5 power regions viz. Northern, Western, Southern, Eastern and the North-Eastern power region. Also by mid 60s, Regional Electricity Boards came into existence in the above mentioned five power regions. The move has facilitated interconnected operation of the power system within the regions. Interconnected power system reduces the investments in generation reserves, and helps to utilize the benefits of generation mixes and load pattern to a greater extent. The transmission voltage had increased to 400 kV AC by the 70s. Significant transmission networks were developed by Uttar Pradesh, Maharashtra, Madhya Pradesh, Gujarat, Orissa, Andhra Pradesh and Karnataka as these states were having the bulk of the electrical load.

In the year 1975, to enhance the generation capacity, which till the time was carried out at the state level, Central sector generation utilities i.e. National Hydroelectric Power Corporation (NHPC) and National Thermal Power Corporation (NTPC) were formed. These corporations established generating power plants of large capacity, say of 1000 MW capacity, and developed the much needed transmission systems. To speed up the transmission infrastructure development, Power Grid Corporation of India (POWERGRID) was created in 1989.

Till the time all the 5 power regions were operated independently and that too at different operating frequency. Therefore in 1990 asynchronous interconnection between regional power grids were made with the help of back-to-back HVDC links. This was the introduction of HVDC system in the country. In the year 2007, India touched the 765 kV AC transmission voltage and by 2009 we had a couple of ± 500 kV bipolar HVDC lines. 

Fig: Under construction 765 kV lines in Madhya Pradesh

Shortly (by 2015) we are going to have the first multi-terminal UHV DC system. The ±800 kV, 1728 km long Biswanath- Agra UHV DC transmission system with a  8 GW converter capacity, including a 2 GW redundancy, will transmit hydroelectric power from the country’s northeast region to Agra in Uttar Pradesh.

In the next decade 1200 kV UHV AC system is expected to emerge as the main transmission level in India along with the 800 kV UHV DC system. The power transfer capacity of 1200 kV UHV AC transmission system is expected to be between 6000 to 8000 MW. To develop 1200 kV AC transmission system in India, a joint venture for a test sub station and test line by Power Grid Corp. of India and CPRI is under progress at Bina in Madhya Pradesh.

Watch out for development of national grid in the coming blog.

Optical Ground Wire for Transmission System Protection

An Optical Ground Wire (OPGW) or an Optical Fiber Composite Overhead Ground Wire, as known in the IEEE standards, is a type of cable mainly used by electric utilities to facilitate the function of grounding and communications in a transmission system. These cables are run at the top of a transmission line parallel to the power conductors. The conducting part of the cable shields the power conductors from direct lightning strokes whereas the inner fiber optics are used for high speed data transmission for the purpose of protection and control of the transmission system, communication etc. These cables look the same as an ACSR conductor normally used as ground wire and have the same dimension and weight. The OPGW are suitable for high loads and longer spans. From 1985 onwards OPGW has been extensively used as ground wires in transmission lines particularly in China.  

The OPGW contains one or more optical fibers surrounded by a layer of steel and aluminum wire. A typical cable may have 8 and 48 optical fibers placed in a plastic tube which is again inserted in a tube made up of stainless steel or aluminum. Buffer tubes filled with jelly are provided to protect the fibers and steel tubes from water and corrosion respectively. The number of fibers may go up to 144 fibers in a cable. Several other types of cable construction are also available in the market which may employ aluminum rods with spiral grooves for fibers. These cables are also custom made to suit the requirement of customers. figure shows a optical fiber ground wire.

Optical Fiber Ground Wire

Optical fiber ground wire is seen as an alternative to power line carrier system used for data transfer and communications. Optical fiber cables when used as ground wire on transmission lines has lesser installation cost as compared to buried optical fiber cables. As they are provided on towers hence are unlikely to get damaged by excavation and other repair works. Since the optical fiber is an insulator it prevents the induction effect of power line and the lightning stroke, external noise and cross talks. Vibration dampers are also provided on OPGW cables to reduce Aeolian vibrations.

Optical fiber ground wires have been so successful that power utilities are replacing the existing steel or ACSR ground wires of their transmission system. In India, Sterlite Industries and several other manufacturers have the technical expertise to manufacture optical fiber composite ground wire and its related hardware. These OPGW technologies have been implemented in many Indian transmission projects commissioned by Adani Power, UPPTCL, Areva & Vedanta Aluminum, etc.

Carrier-current protection schemes for Long Transmission lines

Different protective schemes and the choice

The overhead transmission and distribution lines are more prone to faults because of their length and exposure to atmospheric conditions. There are several protective schemes for the protection of these lines and feeders viz. over-current protection, distance protection and pilot protection. The choice of a particular protection scheme depends upon the cost of the scheme, type of the feeder, length of the feeder, method of operations etc.

Pilot relaying protection

Pilot relaying protection is a form of unit protection used for the protection of transmission line sections. In these protection schemes some electrical quantities such as the phase angle of current, direction of power flow etc. at the two ends of the transmission lines are compared. Some form of interconnecting channel, called pilot, is required to transmit information from one end to the other. The three different types of interconnecting channel or pilots used are wire pilot, carrier-current pilot, and microwave pilot.

Carrier-current protection

For long overhead lines the power line itself may be used as the interconnecting channel between the terminal equipments. Carrier-current protection is the most widely used scheme for the protection of Extra High Voltage (EHV) and Ultra High Voltage (UHV) power lines. The carrier signal is directly coupled to the power line itself which is to be protected. Carrier-current protection is faster and superior to distance protection schemes and is more reliable when used for long transmission lines, although the terminal equipments are more expensive and complicated. In addition to protection the carrier signals can also be used for communication, supervisory control and telemetering.

In carrier-current protection or any other unit protection, the circuit breakers at both the ends of the line trip simultaneously when a fault occurs at one of the ends of the protected line sections. This helps in improving the stability. The carrier signals can be used either to initiate or to prevent the tripping of a protective relay according to which they are classified. When a carrier signal is used to initiate tripping of relay, the scheme is known as carrier inter-tripping, or transfer tripping or permissive tripping scheme. The scheme is known as carrier-blocking scheme when the carrier signals are used to prevent the operation of a relay.

Different operating techniques used in carrier-current protection

The two operating techniques mainly used in carrier-current protection are:

1.      Phase comparison technique, and

2.      Directional comparison technique.

In phase comparison technique, the phase angle of the current entering at one end is compared with the phase angle of the current leaving the other end of the protected section. During normal operating conditions or in case of an external fault, the currents at both the ends of the protected line are in phase. In case of an internal fault i.e. fault in the protected section, the currents at the two ends will be 180^o out of phase.

The direction of power flow at the two ends of the protected sections is compared in the directional comparison technique. During normal conditions or external faults, the power flows into the protected section at one end and leaves at the other end. During internal faults, the direction of power flow is inwards at both the ends.

The signals generated in a carrier-current protection scheme are at a frequency between 50 and 500 kHz. Below 50 kHz the size and cost of the coupling equipments would be too high and above 500 kHz the line losses and therefore the signal attenuation would be too high on long lines. Carrier-current protection can be used only on overhead lines and cannot be used for underground cables as the capacitance of a cable would attenuate the carrier signals appreciably.   

Shielding method Protection of Transmission Lines against Lightning

Last updated: January 20, 2017

Surges due to lightning are mostly injected into the power system through the long transmission lines. Substation apparatus is always well protected against direct lightning strokes. The commonly adopted and effective method of protecting transmission lines against direct strokes is by the use of overhead ground wires. This method of protection of transmission lines is known as shielding method which does not allow an arc path to form between the line conductor and the ground.

Ground wire is a conductor run parallel to the main conductors of the line. It is supported on the same towers, is placed higher than the main conductors and is adequately grounded at every tower. For horizontal arrangement of conductors, there are two ground wires to provide effective shielding to power conductors from direct lightning strokes whereas in vertical configuration of conductors there is only one ground wire.

Fig 1: A 765 kV transmission line with ground wire

The ground wire is made up of galvanized steel or ACSR conductors. Modern Extra High Voltage (EHV) transmission lines have ground wires of ACSR conductors of the same size as the power conductors. In case of a direct lightning stroke, the ground wire intercepts the stroke and by providing multiple paths for conducting the stroke to the ground they reduce the induced voltage. It also helps to increase the effective capacitance between the conductor and the ground which in turn reduces the voltage induced on the conductors from nearby strokes.

The ratio of the induced voltage on a conductor of a line provided with ground wire protection to the induced voltage which would exist on the conductor in the absence of ground wire is known as protective ratio. Each ground wire has a protective angle which is defined as the-

" Angle between the vertical line passing through the ground wire and the line passing through the outermost power conductor is called the protective angle."

 The protective angle is in the region of 20^o to 45^o.

The voltage to which a transmission line tower is raised when a lightning strikes the tower is independent of the operating voltage of the system. 

Basic design requirements for protection against direct lightning strokes:

The basic requirements for the design of a line to safeguard it against direct lightning strokes are-

1.      The ground wire used should be mechanically strong and should be so located that they provide sufficient shielding.

2.      There should be sufficient clearance between the power conductors and the tower structure.

3.      There should be an adequate clearance between the line conductors and the ground wires, particularly at the mid-span, so as to avoid flashover to the power conductor upto the protective voltage level used for the line design.

4.      The tower footing resistance should be as low as permissible.  

Right of Way Requirement for Transmission System

Last Updated: January 20, 2017

A Right of Way (ROW) is an integral part of a transmission system (transmission lines, towers, sub-station etc.) that carry electricity. In other words, 

" ROW is the strip of land immediately below and adjacent to a transmission line or tower. This is the strip of land used by Electrical Utilities to construct, operate and maintain the transmission line facilities."

 The obstacles like tall trees etc. have to be removed in the width of the ROW so as to prevent electric power outages.

As per the sub section 10 (b) of The Indian Telegraph Act, 1885, ROW is not purchased. The power utility in India only acquire the users right in the property under, over, along, across, in or upon which any transmission line or tower is placed. The ROW is used by the utility to construct, operate and maintain the transmission system. The owner of the land retains the ownership and use of land. The Act also states that the construction agency and the power utility will have to prevent un-necessary damage to the property during the construction as well as operation of the transmission system. They have to pay compensation for any damage to standing crops or fruit bearing trees as approved by the competent local revenue officers. In fact this is way leaves; means the right to use the property of another without possessing it.

The width of the ROW depends on the voltage of the transmission system and the height of the tower used. As per the rule, the ROW includes the area extending for a distance of 26 meter on each side from the centre of the tower for 400 kV transmission system and hence the ROW is 52 meter. The ROW requirements for transmission facilities of other voltages are:   

Transmission Voltage	Recommended Right of Way (ROW)
132 kV	27 m
220 kV	35 m
400 kV	52 m
800 kV	85 m

      

For technical and safety reasons, the following vertical clearances shall be maintained within the transmission line ROW.

Transmission Voltage	Minimum clearance between conductor and trees
132 kV	4.0 m
220 kV	4.6 m
400 kV	5.5 m

Stability of an Electric Power System

Electric power is generated by synchronous generators, also called alternators. These generators are synchronized with the rest of the system and the voltage, frequency, and the phase sequence of the system as a whole is the same. 

"Stability of an electrical power system is the ability of the system to return back to normal state after being subjected to a disturbance."

Or in other words stability is the tendency of a power system to develop restoring forces equal to or greater than the disturbing forces to maintain the state of equilibrium. If the forces tending to hold these synchronous machines in synchronism with one another are sufficient to overcome the disturbing forces, the system remains stable. 

Thus, we can say that the problem of stability is concerned with the behavior of a synchronous machine after a disturbance.  The disturbance can be a gradual change in power, occurrence of a fault in a line, sudden removal of loads etc.

Types of Stability:

Stability is generally divided into two major classes:

1.      Steady state stability, and

2.      Transient state stability.

Steady state stability refers to the ability of the power system to regain synchronism after a small and slow change in the system operating conditions. This slow disturbance can be a gradual power change. The study of steady state stability is mainly concerned with the determination of the upper limit of the loading on the machine before losing synchronism. 

An extension of steady state stability is dynamic stability which is concerned with small disturbances but lasting for a long time with the inclusion of automatic control devices. Small disturbances such as variation in loads, change in turbine speed etc. are continually occurring in a power system. These disturbances are quite small to knock the system out of synchronism but do excite the system into the state of natural oscillations. If the amplitude of these oscillations is below a certain value they die out quickly and the system is dynamically stable. Dynamic system study has to be carried out for 5 to 10 seconds and sometimes up to 30 seconds. 
  

Transient stability deals with the effect of large, sudden disturbances such as the occurrence of a fault, the sudden removal of a line or loads. Transient studies are needed to ensure that the system can withstand the conditions following a major disturbance. The angle between the rotor axis and the resultant magnetic field axis is known as power angle or torque angle. Under normal conditions, the relative position of these two axes is fixed. During any disturbance, rotor will decelerate or accelerate with respect to the synchronously rotating air gap mmf, and a relative motion begins. If after this oscillatory phase, the rotor locks back into synchronous speed, the generator will maintain its stability. If the disturbance is created by a change in generation, load, or in network conditions, the rotor comes to a new operating power angle relative to the synchronously revolving field. If there is no net change in power, the rotor returns to its original position.   

Often such studies are conducted when a new generating plant or transmission system is planned. These studies are needed to determine the nature of the required relaying system, critical clearing time of circuit breakers, voltage level of systems, and the available transfer capacity between the various power systems.

Gerbera Plantation has the potential of foreign exchange

The aesthetic value of flowers, their significant use in social events and the high income generating ability are attracting modern entrepreneurs to invest money in the floriculture industry. Commercial floriculture has been recognized as an economic activity with the potential of generating employment and earning valuable foreign exchange due to its demand potential in overseas market.

The global consumption of cut flowers and plants is increasing steadily at an annual rate of 10 to 20% in all floriculture importing countries viz. Netherlands, USA, Japan, Germany, Italy, Denmark, etc. Many flower producing countries have extreme winters with sub-zero temperatures and low sun light. This results in higher production cost and hence seasonal variation in flower production. Thus, they have to depend largely on imports as most of the festivals fall during this period. On the other hand, India has varied agro-climatic and soil conditions, which are conducive for the production of cut flowers.

Gerbera, named in honour of German naturalist Traugott Gerber, is a very attractive commercial cut flower crop with huge international demand. In modern hi-tech method Gerberas are grown in poly-houses. The quality and quantity of the flowers produced are far better because of the controllable temperature, humidity, light, ventilation etc. The height of poly-house is normally 3.5 to 4 m and sufficient ventilation is provided on the top and sides. The light intensity required for the plants are maintained using shade nets. The normal life of a poly-house in nearly 20 years.    

    Gerbera plantation in poly-house near Bhopal

Normally Gerbera plants are grown on soil bed of height 1.5 feet and width of 2 feet. The soil used should be highly porous, airy and well drained to have better root growth. The optimum pH value of soil should be between 5.5 to 6.5; so as to have efficient nutrients absorption. Before plantation the soil should be disinfected by methyl bromide or formalin to get rid of fungus. Generally two rows with a distance of 37.5 cm are planted on each bed. A separation of nearly 30 cm is kept between the plants in the same row. Pathway of approximately 1 foot is left after every bed to facilitate movement.

Planting can be done round the year but months of September and October are preferred. After the plantation, the plants are irrigated with overhead micro-sprinklers for 4 weeks. The plants start flowering in 7 to 8 weeks after the plantation. Organic manure is recommended for soil texture and nutrition. Super phosphate and MgSO₄ are also used for better root establishment. The optimum temperature for flower initiation is 23 to 25 ^oC and the humidity should be between 80 to 85%. Plants are irrigated by micro sprinklers until the flowers are produced, thereafter drippers are used. The water requirement is approximately 700 ml/plant/day. In summer season foggers may be used to get the needed humidity, but care should be taken that the humidity should not exceed 90 to 92% as it will lead to flower deformation. Leaf servicing and loosening of soil are done to maintain the plants. Pesticides or fungicides are also sprayed as per the need. The annual yield is 30 to 32 flowers/plant.  

After harvesting flowers are sorted into different grades according to stem length, size of bud, etc. Each flower is covered with plastic leaf to prevent damage to stamens. Flower are kept in bunches and are tightly packed in CFB boxes to avoid damage during transportation. Great care is needed while packing, handling, storage and transportation.  

Thus, floriculture is basically a labour intensive industry. Major constraints faced by hi-tech floriculturist in India are:

1.      Huge investment,

2.      Irregular supply of electricity,

3.      Scarcity of labour,

4.      Non-availability of good quality indigenous plants,

5.      Poor harvest during rainy season,

6.      Pest and disease attack,

7.      Demand variation according to season,

8.      Inadequate cold storage facility,

9.      Price fluctuations,

10.  Absence of organized retail market, etc.