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.

Standardization of Transmission System Voltage

There is much variation in transmission voltages in different countries. A country adopts a system of voltage levels to suit its own requirements. Earlier individual attempts were made to fix voltage levels for higher power transmission but such an attempt had resulted in wastage of time and higher cost because of designs of varied nature. Thus, the transmission voltages had to be standardized. The various advantages of standardization of transmission voltage are:

1.      Standardization provides better facilities for research and development.

2.      The equipments can be manufactured with greater economy and reliability.

3.      Systems are easily interconnected.

Hence standardization enables to carry out joint efforts to tackle Extra High Voltage (EHV) or Ultra High Voltage (UHV) problems. By standardizing, the voltage level can be adopted for a reasonable period of time before next change. The choice of the highest system voltage for a country is a matter of great significance. It is not merely the economic factors that influence the next higher voltage but the site of power station, location and density of load, and the technological developments are also kept in mind. The next higher voltage level should also be selected on the basis of future load enhancements. The interval between the existing and the proposed voltage level should be judiciously spaced, as too small interval between the voltages will result in a short life of the proposed voltage level. At the same time too large interval would lead to heavy expenditure. It is therefore desirable that the next voltage selected should be at least two steps higher than the existing one.  

The various AC voltages adopted by different countries above 220 kV are 275, 345, 380, 400, 500, 735, 765, 1000, 1100, 1200 kV etc. The AC transmission voltages adopted in India are 220 kV, 400 kV and 765 kV. The next higher AC transmission voltage selected is 1200 kV.
Figure below shows a 765 kV Indian transmission line 

A number of High Voltage Direct Current (HVDC) schemes have been in operation throughout the world since 1954. Transmission voltages of HVDC line have increased from 100 kV to ±800 kV. The various voltages adopted by different countries for overhead HVDC lines are ±100 kV, ±250 kV, ±300 kV, ±400 kV, ±500 kV, ±600 kV and ±800 kV. The existing HVDC lines in India are of ±500 kV and a ±800 kV line between Biswanath and Agra has been under construction.

Modern Trends in Indian AC Transmission System

Long distance bulk power transfer:

The need to economize investments in generation reserves, sharing of benefits in utilizing variability in generation mixes and load pattern have given rise to interconnection of neighboring power systems and development of large power grids. 

Rapid increase of load (which is the case in developing countries like India), remote generation and system interconnections have made it necessary to transmit more power over longer distances efficiently and easily. Long distance bulk power transfer is possible only with Extra High Voltage (EHV) and Ultra High Voltage (UHV) transmission lines.  

Research and Development activities:

Priority is given to research and development activities for the optimization of cost of power transmission, efficient utilization of existing Right of Way (ROW) and increased power transfer capability. The recent technological developments and application of power semi-conductor devices, digital electronics, control equipments and satellite communication have increased the capabilities of EHV and UHV AC transmission lines.

Modern Trends in AC Transmission:

The modern trends in AC transmission are:

1.      To utilize Flexible AC Transmission Systems (FACTS) employing power electronic based and other static controllers to enhance the controllability and capability of AC transmission lines.

2.    To opt for higher transmission voltages exceeding the EHV range. Transmission voltage of 765 kV AC has been introduced in India in the year 2007. Research and development is going on for the next higher voltage of 1200 kV UHV AC transmission system. In this regard a 1200 kV test station and test line at Bina (Madhya Pradesh) is in advanced stages of research and development.

3.      To deploy satellite imagery technique (which is supposed to enhance the transmission line survey). Survey techniques are improved through GIS and Airborne Laser Terrain Mapping (ALTM) or Light Detection and Ranging (LiDAR). Laser mapping produces the detailed elevation measurements at a faster rate and accuracy. It can be used at locations where the approach is limited or restricted. With this technique, long transmission corridor can be mapped with speed to determine the exact location of towers. The same data can be used for monitoring the transmission lines, for it's repair and modification activities.  

4.     Tall and multi-circuit towers are increasingly being used to avoid deforestation, protection of wild life and effective utilization of existing ROW.

5.      Recent trend is to use multi-conductor bundled conductors having four, six, and eight sub-conductors.

6.   To use high temperature endurance conductors for increased loading and higher power transfer capability.

7.      More and more use of compact gas insulated substations of 765 kV and 1200 kV class.

8.      Use of high strength polymer insulators.        

MATLAB coding for Y-bus

In recent years, the analysis and design of power system have been influenced greatly by the high end performance of personal computers. These computers can be used to perform the steady-state and transient analysis of large interconnected power systems.   

MATLAB which stands for MATrix LABoratory, is a powerful software package developed by MathWorks Inc.  This software having analysis capability, flexibility, reliability and powerful graphics is currently the main software package used by power system engineers. MATLAB provides matrix as one of the basic elements and does the basic operation as addition, subtraction, multiplication using simple mathematical operators. With hundreds of reliable and built in functions, MATLAB helps in solving a variety of mathematical problems including differential equations, linear systems, non-linear systems, optimization and many other type of engineering computations.The most appreciable feature of MATLAB is its programming capability and the several optional toolboxes for simulating specialized problems of different areas.  

In power system, nodal admittance matrix or bus admittance matrix or Y matrix or Y bus is an n x n matrix describing a power system with n buses. It represents the nodal admittance of the buses in a power system. In a real power system, each bus is usually connected to only a few other buses, hence the Y bus matrix is sparse. The Y bus is one of the data requirements needed to formulate a power flow study.

Power flow studies, commonly known as load flow, are necessary for planning, operation, economic scheduling and exchange of power between utilities. Power flow analysis is also required for transient stability and contingency studies.

Y bus is a tool that provides a method of systematically reducing a complex power system to a matrix that can be solved by a computer program. The equation used to formulate Y bus is based on Kirchhoff’s Current Law (KCL) and Kirchhoff’s Voltage Law (KVL), applied to a circuit with steady state sinusoidal operation. These laws are applied to all the nodes of a power system and elements of the admittance matrix are determined, which then represents the admittance relationship between nodes to further find the voltages, currents and power flows in the system.

The below given MATLAB program is for the formulation of bus admittance matrix or the Y bus.

The input data required for Y bus formulation is “linedata” which contains 4 columns. The 1^st column gives the branch number. The 2^nd column is the “from bus” number whereas the 3^rd column is the “to bus” number. The 4^th column is the admittance of the corresponding branch.

% Declaring function [Y] that takes the “linedata” as input and returns Y bus matrix as output.

 function [Y]=ybus(linedata)

% extracting the maximum numerical value of column 1 of the “linedata” which gives the maximum number of %branches in the network.

elements=max(linedata(:,1));

%   defining the total number of buses in the network.

buses=max(max(linedata(:,2)),max(linedata(:,3)));

Y=zeros(buses,buses);

% defining a loop for the diagonal and off-diagonal elements of Y -bus

 for row=1:elements,

i1=linedata(row,2);

j1=linedata(row,3);

Y(i1,i1) =Y(i1,i1) + linedata(row,4);

Y(i1,j1) =Y(i1,j1) - linedata(row,4);

Y(j1,i1) =Y(i1,j1);

Y(j1,j1) =Y(j1,j1) + linedata(row,4);

end

Y

Live Transmission Line monitoring Robot

Transmission line monitoring is to have a complete and continuous real-time picture of conductor clearances, temperature, current and vibrations. Today robotic devices are able to inspect these parameters of live High and Extra High Voltage transmission lines in order to improve the reliability and safety. These remote controlled robots are equipped with cameras and several other sensors so that the grid operator can have access to strategic data on which maintenance and certain investment decisions are based. 

With the help of these robots, the utilities can inspect the irregularities in a transmission line and the smart navigation system of the device provides the ability to pinpoint the exact location which needs attention and to carry out maintenance tasks such as strengthening of broken strands etc. These robots are able to manoeuvre the splices, hardware components and other similar obstacles.

Several prototype line inspection robots were developed over the years around the globe. LineScout, is one such robotic device developed by Hydro-Quebec Research Institute of North America, is the widely used robot designed and developed for live transmission lines. These robots are in operation in various countries since 2006. The institute has recently developed a prototype robot for the 735 kV system that performs visual inspection and can operate certain equipments such as an isolator.   

Indian transmission grid is one of the largest in the world and hence has to be more reliable and efficient. The need for an intelligent transmission network has been stressed by the Chairman cum Managing Director (CMD) and other senior officials of the Power Grid Corporation of India. It is good news that world renowned Hydro-Quebec is actively seeking partnership with major Indian companies for the commercial production of LineScout technology. 

Another transmission line monitoring robot is the Lindsey TLM conductor robot. These robots are self-communicating and can be easily installed for live transmission line installation and monitoring for voltage levels up to 765 kV. These monitors or robots, as stated earlier, consists of different sensors and are self-powered from the line current itself. The on-board LiDAR sensor provides the accurate line to ground clearance. Similarly other sensors are there to track the conductor temperature and thus to prevent conductor annealing. Three-axis accelerometer detects the conductor galloping and vibrations. 

These robots come with a secure communication gateway for the safe transmission of data.    

India in pursuit of Energy Efficiency in Agricultural Sector?

Agricultural sector uses energy mainly in the form of electricity and diesel for its various activities such as water pumping, threshing, processing etc. Currently there are about 20 million agricultural pump sets in India which accounts for about 20% of the total electricity consumption. Nearly 0.25 to .5 million pump sets are added annually. Significant energy losses are associated with the distribution of electricity to the agricultural pump sets. Poor selection, installation, and operation of electrical pump also add to these losses. The poor management of load demand by the local supply authorities compounds the problem of poor power quality. To compensate for this, farmers have resorted to the usage of over-sized and inefficient local made pump motors which can operate under these conditions of power supply. Free or subsidized electricity to this sector is also one of the reasons that these consumers do not care for energy saving.

The average operational efficiency of these pump sets has been found to be only 20 to 30%. As per the Planning Commission Annual Report on State Power Utilities and Electricity Department, low or free electricity coupled with inefficiencies in the sector resulted in a revenue loss of 45,000 Crore INR in the year 2011-12. This huge amount can be well utilized in some social development activities like setting up schools, hospitals, and other basic amenities which is badly needed in our country. Do you agree?    

Proper and efficient use of equipments and resources can help in energy conservation. The energy efficiency of agricultural pump sets and hence the sector can be increased by:

1.      Using star rated Energy Efficient Pump Sets (EEPS). These pump sets have an efficiency of 40 to 45%.

2.      Selecting a pump of the right capacity as per the irrigation requirement. Improper selection of pump can lead to large wastage of energy. 

3.      Matching the motor with appropriately sized pumps.

4.      Proper installation of the pump system, including shaft alignment and coupling of motor-pump.

5.      Using low friction rigid PVC pipes, foot valves and non-return valves.

6.      Avoiding the use of unnecessary bends and too long pipes.

7.      Periodically carrying out the corrective measures such as lubrication, alignment, tuning of engines (for diesel pumps) and replacement of worn out parts. Efficiency of worn out pumps can drop by 10%.

8.      Using drip irrigation for specific crops like vegetable, fruits, flowers etc. Drip irrigation systems can conserve up to 80% water and reduce the pumping requirement.  

According to some estimates, the potential for energy saving is highest in agricultural sector although there is contradiction to this. The sector has a potential to save nearly 28 billion units (billion kWh) which is more than 36% of the total estimated energy saving potentials. Study also suggests that this sector offers the minimum payback period and hence should be on the top of the priority list.

To induce energy savings in agricultural sector, the Ministry of Power, Government of India, has initiated Agriculture Demand Side Management (Ag DSM) program on Public Private Partnership (PPP).  The scheme was initiated in 8 agriculture intensive states, viz. Maharashtra, Haryana, Punjab, Rajasthan, Gujarat, Andhra Pradesh, Madhya Pradesh and Karnataka. In these states the agriculture sector accounts for 70% of the total electricity consumption.

In one of the pilot projects carried in Maharashtra, nearly 2200 old and inefficient pump sets were replaced by star rated EEPS on PPP mode. The outcome of the project reflects that the average efficiency has improved from 22.19% to 39.6%. Now estimate how much times the average efficiency has increased?

Thus creating awareness among agricultural consumers, carrying out different studies related to improvement of energy efficiency, initiating and implementing the DSM program in this sector, one can say that India is in pursuit of Energy Efficiency in Agricultural sector.   

Methods to reduce Distribution Losses

What is Transmission & Distribution (T&D) Losses?

The losses occurring in the transfer of power from the generator to the end consumer is known as Transmission and Distribution (T&D) losses. Apart from T & D losses there are commercial losses too and therefore, the concept of Aggregate Technical and Commercial (AT & C) loss was introduced which truly reflects the technical as well as commercial losses in the whole electrical network.

Value of AT & C Losses at various levels:

The AT & C losses vary in various Indian states from 20 to 45% because of the difference in load density, type of network (urban or rural lines), pre-dominant sub-transmission voltage, voltage of supply, geographical size of the area etc. Other factors such as inadequate design, monetary investments and bad maintenance of the system over the years contribute to higher losses. 
  

Nearly 8% of the total AT & C losses take place above 33 kV level and the rest do happen below 33 kV.  

Methods to reduce AT & C Losses:

Aggregate Technical and Commercial (AT & C) loss of 20 to 45% for any state is very much higher from any international standard and has to be curtailed down. 

The various methods that can reduce these losses are:

1.      Installation of static VAr controller in the distribution system. Switchable shunt capacitors at the sub-station or on lines can be very useful in providing the required reactive power.

2.     Use of low loss transformers, particularly the distribution transformers. Distribution transformers with amorphous core have very low iron losses.

        

                   Fig: Standard Iron & Copper losses of some of the Distribution Transformers

3.      Re-conductoring the overhead line with conductors of appropriate size.

4.      Keeping the loads on power and distribution transformers within limit.

5.      Shifting the distribution transformers to load centres.  

6.      Balancing the load on the three phases of the lines.

7.      Distributed generation or local generation can also reduce the line losses.

8.  Adopting High Voltage Distribution System (HVDS), employing small capacity distribution transformers, for agricultural pumps and theft prone residential areas.

9.     Installation of microprocessor based static (electronic) energy meters incorporating special features like tamper data, Time-of- Day (TOD) metering, load pattern analysis, remote meter reading.

10. Adopting for Distribution Automation System (DAS).   

Capacitor Bank installed at a sub-station

Transmission and Distribution losses in India

Nearly all the modern generating power plants are located far from the actual electricity load centres. Transmission of generated power to major load centres and their further distribution is carried out through transmission and distribution lines. The losses occurring in the transfer of power from the generator to the end consumer is known as Transmission and Distribution (T&D) losses.

Transmission and distribution network comprises of overhead lines, cables, transformers, switchgears and other equipments to facilitate the transfer of electricity. The efficiency of these networks is improved by reducing the T & D losses. 

In India electricity is usually generated at a voltage ranging from 11 kV to 25 kV and is stepped up to a voltage of 400 kV or 765 kV (EHV range) for their transmission. Of course bipolar HVDC lines of voltages 500 kV and 600 kV are in use. The voltage is stepped up mainly to reduce the I²R losses. At the distribution end it is again brought down to a very low value. Most of the residential, commercial and agricultural consumers are supplied at a low voltage of 230/415 V. Larger commercial and industrial consumers are typically supplied at voltages of 11 kV, 33 kV or greater values.

In the process of supplying electricity to consumers, some power is dissipated in the transformers while stepping up or down the voltage levels. Some power, in the form of I²R losses, is lost in the lines and cables that carry the power. Losses occurring at various stages of power transformation and loading of transmission system at 132 kV and above are known as transmission losses, whereas losses at 33 kV and below this voltage are distribution losses. Normally the transformer losses are 0.5% of the total losses, for each voltage transformation. The line losses are nearly 2% for transmission voltage and are 4% and 5% respectively for primary and secondary distribution system.

Apart from T & D losses there are commercial losses too. Therefore the concept of Aggregate Technical and Commercial (AT & C) loss was introduced. The AT & C loss reflects the technical as well as commercial losses in the whole network and hence is the true indicator of the system efficiency.

Causes of losses

The technical losses depend largely on: 

• system configuration, 
• pattern of loading of transmission and distribution lines, 
• magnitude and types of loads, 
• characteristics of equipments. 

The losses are higher due to weak and inadequate sub-transmission and distribution lines, inappropriate sizing of conductors, lengthy transmission and distribution lines and inadequate reactive compensation in the system. The non-technical or commercial losses are component of distribution system losses and are because of unidentified and uncollected revenue arising from metering errors, shortfalls in billing and revenue collection and consumer malpractices such as meter tampering, illegal connections etc.

Comparison between networks of different countries is not straight forward and it is difficult to define an optimum level of losses for a network. The AT & C losses vary in various Indian states from 20 to 45% because of the difference in load density, type of network (urban or rural lines), voltage of supply, geographical size of the area etc. Other factors such as inadequate design, monetary investments and bad maintenance of the system over the years contribute to higher losses.   

Bundled Conductors for Extra and Ultra High Voltage transmission lines

Last updated: January 20, 2017

Transmission of large amount of power over long distances can be accomplished most economically using Extra and Ultra High Voltage lines. An increase in transmission voltage results in reduction of electrical losses, increase in transmission efficiency, improvement in voltage regulation and reduction in conductor material requirements.

Effect of Voltage Gradient on Atmospheric Air: 

In an overhead transmission line, the atmospheric air in between the conductors behaves perfectly like an insulator when the potential difference between the conductors is small. With the increase in system voltage, there is a corresponding increase in the electric field intensity. When the electric field intensity or voltage gradient reaches a critical value of 30 kV/cm, the air in the immediate vicinity of conductors no more remains a dielectric but it ionizes and becomes conducting.

Corona phenomenon at High Voltage:

Corona phenomenon is the ionization of air surrounding the power conductors. Free electrons are normally present in the atmosphere. The free electrons will move with certain velocity depending upon the field strength.  These electrons on their movement collide with the molecules of air and liberate more electrons. The process of ionization is cumulative and ultimately forms and electron avalanche.

The electrical breakdown of surrounding air around the conductor is accompanied by-

1. a faint glow around the conductor, 
2. a hissing sound, 
3. vibration in conductors, 
4. formation of ozone and oxides of nitrogen, 
5. loss of power, and 
6. radio interference. 

Factors affecting Corona loss in an overhead Transmission line:

The important factors that affect the corona loss in an overhead transmission line are -

1. frequency and waveform of supply, 
2. spacing between conductors, 
3. condition of conductor surface, 
4. atmospheric conditions, 
5. conductor diameter and 
6. number of conductors per phase.   

Bundled Conductors

At voltages above 300 kV, corona causes a significant power loss and radio interference if a single conductor per phase is used. Instead of using a single conductor, it is preferable to use two or more conductors per phase, in close proximity, which is called bundled conductors. 

Thus, a bundled conductor is a conductor made up of two or more sub-conductors and is used as one phase conductor. The high voltage gradient is reduced considerably by the use of bundled conductors.

Number of sub-conductors:

The number of sub-conductors used per phase in an Indian transmission system are two (for 400 kV), and four (for 765 kV). The bundled conductors for the proposed 1200 kV Ultra High Voltage (UHV) transmission system in India will have 8 sub-conductors. Figure 1 shows Bundled conductors with twin, triple and quadruple conductor. Figure 2 shows a 765 kV Transmission line in India with quadruplex sub-conductors.

Fig.1: Bundled conductors with twin, triple and quadruple sub-conductors.

Fig.2: 765 kV transmission line in India with 4 sub-conductors per phase (Quadruplex bundle).

Bundle spacing:

The spacing between adjacent sub-conductors is called bundle spacing and is almost 30 cm or more. In figure 1, bundle spacing is denoted by 'B'. In almost all cases, the sub-conductors are uniformly distributed on a circle. The radius of the pitch circle on which the sub-conductors are located is called bundled radius.

The various advantages of using bundled conductors are reduced reactance, voltage gradient, corona loss, radio interference, and surge impedance.  

Commercial floriculture a rising industry with great export potentials

Flowers play an essential role in people’s life. They are so indispensable that weddings, parties, inaugural functions, political activities; celebration of certain days such as teacher’s day, mother’s day, valentine's day, festivals such as Diwali, Christmas etc. cannot be accomplished without flowers. So the demand for flowers and variety of flowers are increasing day by day and has to be fulfilled.

Floriculture is the growing of cut flowers, potted flowers, foliage plants etc. in green houses or/and fields. The cut flowers are usually sold in bunches or as bouquets with cut foliage. Floriculture has been identified as a rising industry and Government of India has accorded it 100 % export oriented status. Commercial floriculture has emerged as hi-tech activity producing flowers of different variety inside a well developed greenhouse with the ability of giving controlled climatic conditions. The per unit yield of commercial floriculture is more than most of the field crops and the government is promoting this industry very hardly by providing significant subsidies (upto 50% depending upon the case) and other functional helps.

Figure shows a green house establishment near Bhopal with Gerbera plantation. 

With the opening of world market, there is a free movement of floriculture products worldwide. More than 140 countries are involved in floriculture. The main floriculture products in the international market are rose, carnation, chrysanthemum, gladiolus, gypsophila, orchids, marigold, tulip, lilies, gerbera etc. 

USA continues to be the largest consumer of floriculture in the world followed by Japan. More liberalized industrial and trade policies paved the way for import of seeds and plants of international varieties and the export of cut flowers produced in India. The country has exported 22485 MT of floriculture products worth 455 Crore INR to countries like USA, Netherlands, Germany, United Kingdom, United Arab Emirates, Japan and Canada in 2013-14.

The major producers of floriculture in India are Maharashtra, Karnataka, Andhra Pradesh, Haryana, Tamil Nadu, Rajasthan, West Bengal etc. States like Madhya Pradesh is also gearing up to play its role in this field. Countries like Netherlands, which dominates floriculture with 90-95% of world trade, is planning to ramp up its agri-business activities in Madhya Pradesh.

There are certain crucial training aspects of floriculture viz. pruning of plants for optimal flower output, post harvest treatments, storage and packaging etc. which needs to be well addressed. Some of the challenges in this industry are capital cost, labour cost, seasonal demand, environmental issues, global concern over pesticides use etc.

Design parameters and Standard layout of a 33/11 kV substation

An electrical substation:

An electrical substation is a combination of a number of major electrical equipments like power transformers, circuit breakers, instruments transformers, bus bars, lightning arresters, control panels, interconnecting cables, capacitor banks, battery set, and other related equipments designed as per the requirements, size, capacity and class of the substation. 
Control room is provided to house the various control panels and battery backup and to station the employ/operator.    
Power transmission and distribution in densely populated urban areas is a great challenge because of land availability and low noise and electromagnetic limitations. With the development in technology and enough improvement in substation design, Gas Insulated Substations are finding increasing use because of their safety, reliability, less space requirement and enhanced life.

Factors affecting the Layout and Design of a substation:

The physical layout and the design of the substation are governed by:

1.      The number of incoming and outgoing feeders (existing and proposed)

2.      Existing and expected load demand on the substation,

3.      Soil resistivity,

4.      Expected fault levels at 33 kV and 11 kV level ,

5.      Climatic conditions, such as temperature, altitude, rainfall etc.

Governing Rules & Regulations:

All electrical and civil works related to the installation and commissioning of an electric substation shall be carried out in accordance to the provisions given in Indian Electricity Act 2003 and the Indian Electricity Rule 1956 amended upto date. 
The standard instruction for construction of 33/11 kV substation is given in REC standards which specifies the layout along with the switchgear. The standard layout guides for the general arrangement of the equipments, structures, bus-bars etc. of outdoor substations. The general specifications of a substation cover the details of the major equipments and other items to be supplied. 

The tender for a particular substation clearly indicate these specifications. The tender specification also has the reference drawings, which indicate the schematic of major connections and form the part of the specification. Suitable provisions can be made in the switchyard depending upon the local condition and locations so as to facilitate transportation of heavy equipments, especially power transformers. While carrying out the substation construction all the labour regulations, safety codes and measures must be followed.

Number of Transformers and Feeders in a substation:

The REC standard mentions that an outdoor type 33/11 kV substation should have 2 power transformers and 3 outgoing feeders. There must be provision for 2 additional 11 kV outgoing feeder for load enhancements in future. The number of 33 kV incoming feeder may be one or two depending upon the requirement. The various arrangements for the 33 kV incoming and outgoing lines may be according to the standard practice adopted by the electricity board. 

Fig.1: The power transformer, circuit breakers, gantry of an under construction 33/11 kV substation in rural area. 

Control Panels: 

All the control and equipments needed to monitor and remote control the vital equipments such as transformers, circuit breakers, feeders etc. are housed in control panels. In these control panels, relays, meters, and instruments are mounted as per the drawing arrangements. Separately earthed and effectively segregated compartments/ panels (earthed panels) should be provided for circuit breaker, bus-bars, relays, CTs ,PTs etc so that faults in any one panel do not cause damage to other equipments or panels.

Relays:

Relays are the sensing device which sense the abnormal condition in any power system and accordingly give alarm and trip signal to circuit breaker. The different relays used normally in a 33/11 kV sub-station are earth fault relay, and over-current relay. Earlier electro-mechanical relays were used which are now being replaced. 

Fig.2: Control Panel with old Electro-mechanical relays.

  

The relays used in modern substation are of microprocessor based numerical relays with overload, earth fault and short circuit protection. 24/30 V DC supply with battery pack and battery charger is used for the protection as well as indication circuits. 

Current Transformers (CTs) and Potential Transformers (PTs):

Current Transformers (CTs) and Potential Transformers (PTs) used for metering and protection purpose. They should be of the burden more than the minimum burden mentioned and of accuracy class 0.5.

Earthing of a Sub-station:

The earthing of a substation is of prime importance for the satisfactory operation of protective devices and for the safety of equipments and personnel. All equipments must have separate earthing and in the numbers given in the manual. These earths should be well connected to each other and should form a grid. The resistance of the earthing should be maintained at the required minimum value.

There should be satisfactory drainage facility in the yard so that accumulation of rain water should not take place. Yard should be covered with granite metal to prevent the growth of grass and weeds, formation of mud etc. the substation may also have capacitor banks of appropriate capacity to improve the power factor and to control the harmonics.

Inspection is necessary before dispatch, delivery at site, installation, commissioning, putting into operation and handing over the equipments.