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.