Electrical Power System Analysis--Sector Outlook in India (part 2)

(cont. from part 1)

10. CONSTITUENTS OF A PRESENT-DAY POWER SYSTEM

An electrical power system is a complex network of several subsystems, which convert some form of latent energy into electrical energy and transform, transmit, and distribute it for consumption at the customers' terminals. FIG. 4 is a single- line representation of a three-phase ac power system.

The various subsystems of an electrical power system may be classified as follows:

(a) Generation

(b) Transmission and distribution

(c) Loads

(d) Protection and control

It may be noted that transformers are used in all the subsystems. A transformer transfers power with very high efficiency from one voltage level to another voltage level. The insulation requirements limit the generated voltage to low values up to 30 kV. Thus step-up transformers are used for transmission of power at the sending end of the transmission lines. At the receiving end of the transmission lines, step-down transformers are used to reduce the voltage to suitable values for distribution to consumers of electric energy. Furthermore, depending on power handling capacity, two types of transformers, namely power transformers and distribution transformers, are shown in FIG. 4. Power transformers are usually rated from 250 kVA up to 1000 MVA, and distribution transformers are rated between 20 kVA and 250 kVA. In FIG. 4, voltage levels at various subsystems are indicated. EHV designates extra-high voltage, usually above 220 kV and up to 800 kV. HV denotes high voltage, usually from above 66 kV to no more than 220 kV. MV means medium voltage, usually from above 1 kV but less than 66 kV. LV stands for low voltages, which are 1 kV or less.

FIG. 4 Single line representation of an ac network

10.1 Generation Subsystem

Generation of electric energy commenced with the setting up of individual power stations at the pitheads to supply electric power to individual consumers. As the demand for electric energy increased, power systems came into existence. Thus, the generation subsystem is constituted of groups of generating stations, which convert some form of primary energy into electrical energy.

The simplest form of a generating station is constituted of a prime mover coupled to an electric generator. A primary source of energy is employed as the input to the prime mover, which in turn rotates the generator to produce electric energy.

Primary Sources of Energy

The important primary sources of energy employed for generation of electric energy can be broadly classified into three categories:

(i) fossil fuels, for example, coal (including lignite and peat), oil, and natural gas

(ii) renewable energy from hydro, wind, and solar

(iii) nuclear energy from uranium or plutonium

In modern-day electric power systems, majority of the generating stations employ these three types of primary energy sources. The amount of electric power contributed by each type of generating station is governed primarily by the market costs of primary energy sources. For example, water stored in dams and wind as primary input sources of energy have zero cost compared to the cost of fuel such as coal, oil, gas, and uranium used in thermal and nuclear generating stations. In nuclear generating stations, energy costs are low compared to thermal generating stations. The economics of generating stations employing fossil fuels as source of energy are dependent on market prices of the fuels.

Types and Characteristics of Generating Stations

Generating stations, based on the type of primary source of energy employed, can be classified into the following four categories:

(i) thermal,

(ii) hydro,

(iii) nuclear, and

(iv) non-conventional.

In thermal generating stations, coal, oil, natural gas, etc. are employed as a source of primary energy, while the head and volume of water is employed as the primary source of energy in hydro generating stations. Controlled nuclear fission is the source of energy in a nuclear power station. In non-conventional generating stations, wind, geothermal (heat deep inside the earth) energy, tidal energy, etc. are used as sources of energy to generate electric power.

Thermal Generating Stations

Coal fired--A simple schematic diagram of a thermal generating station is shown in FIG. 5. The chemical energy in coal is utilized to generate electrical energy.

Pulverized coal is burnt to produce steam, at high temperature and pressure, in a boiler. The steam so produced is passed through an axial flow steam turbine, where the internal heat energy of the steam is partially converted into mechanical energy.

The steam turbine, which is the prime mover, is coupled to an electric generator. Thus, mechanical energy produced by the rotating turbine is converted into electric energy.

The efficiency of the process of conversion of chemical energy into thermal energy and then into mechanical energy is poor. Due to heat losses in the combustion process, rejection of large quantity of heat in the condenser and rotational losses, the maximum efficiency of the conversion process is limited to about 40%. In order to increase the thermal efficiency of conversion of heat into mechanical energy, steam is generated at the highest possible temperature and pressure. To further increase the thermal efficiency, steam is reheated after it has been partially expanded by an external heater. This reheated steam is returned to the turbine where it is expanded in the final stages of bleeding.

FIG. 5 Schematic diagram of a thermal generating station

Modern practice is to design and build generating units having large megawatt generating capacity, since their capital cost per kilowatt decreases as the megawatt capacity is increased. Increasing the unit capacity from 100 MW to 250 MW results in a saving of about 15% in the capital cost per kilowatt. It is also established that units of this magnitude result in fuel saving of the order of 8% per kilowatt-hour.

Additionally the cost of installation per kilowatt is considerably lower for large units. Currently, the maximum capacity of turbo-generator sets being produced is nearly 1200 MW. In India, super thermal units of capacity 500 MW are being commissioned by BHEL. Thermal generating stations also employ cogeneration in order to utilize the large amount of waste heat. In cogeneration, electricity and steam or hot water are simultaneously made available for industrial use or space heating. It is claimed that cogeneration results in an overall increase in efficiency of up to 65%. Cogeneration has been found to be particularly advantageous for industries such as paper, chemicals, textiles, fertilizers, food, and petroleum refining.

The waste gases produced by coal fired generating stations contain particles and gases such as oxides of sulphur and NOx. These gases are released to the atmosphere resulting in pollution of air. Thermal pollution also results due to the large amount of heat released via the condenser to the cooling water.

Oil fired--In the oil fired steam station, oil is employed to produce steam, which is used to run the steam turbine. In the oil fired stations, the oil used is of two types: crude oil, which is the oil pumped from oil-wells, and residual oil, which is the oil left behind after the more valuable fractions have been extracted from the crude oil.

Cost of transporting oil through pipelines is less than shipping coal by rail. However, residual oil fired stations have to be located close to the oil refinery because it is uneconomical to transport residual oil by pipelines because of its high viscosity.

Gas fired--The primary source of energy in such types of generating stations is natural gas. A gas turbine engine, which is similar to a turbo-prop engine used in an aircraft, is employed as a prime mover to run the generator. In order to achieve higher thermal efficiency, combined-cycle method is used to generate electricity.

In the first stage, gas turbine engines coupled to electric generators produce power.

In the second stage, the hot gases exhausted from the gas turbines are passed through a heat exchanger to generate steam, which is used to run a conventional steam generator to produce electric energy. Alternatively, the hot gases exhausted from the gas turbine can be used for producing steam for an industrial process.

FIG. 6 shows a schematic layout of a combined-cycle gas fired power station.

FIG. 6 Schematic layout of a combined-cycle gas fired power station: To the grid Heat exchanger. * Gas turbine Hot gases Natural Chimney Cooling tower

For the same amount of power generated, combined-cycle gas fired stations are more environment friendly. The flue gases emitted by these stations contain almost zero sulphur dioxide, 50% carbon dioxide, and 25% NO, as compared to those produced in a coal fired power station. Compared with coal fired steam power stations, the installation cost of gas fired stations is lower and they can be quickly started. The operational cost of gas fired stations is high due to the high fuel cost when employed to supply power on their own. As such, they are used to supply peak load demand and for short periods.

The world over, gas turbines in conjunction with 100 MW generators are being used to generate electrical power. In India, a gas power station with an installed capacity of 180 MW (6 x 30 MW) is operational in Delhi.

Diesel oil fired--Diesel oil is used to run large internal combustion engines of the type employed in ships. The diesel oil fired stations exhibit characteristics similar to those of a gas fired station. However, the speed of a diesel oil fired station is considerably low and its fuel efficiency is higher than that of a steam power station. Since diesel is more expensive than oil, in an oil fired steam station, the use of diesel oil fired stations is limited to supplying stand-by power.

Hydro Generating Stations--In a hydroelectric generating station, the potential energy and quantum of water are utilized to generate electrical power. In other words, hydroelectric schemes function on flow of water and difference in level of water known as head. Due to the difference in head, considerable velocity is imparted to the water, which is used to drive a hydro turbine. This hydro turbine acts as a prime mover and is coupled to an electric generator to produce electrical energy.

Hydroelectric stations depend on the availability of a head of water. As such they are often sited in mountainous terrain and require long transmission lines to deliver power to the load centers. Hydroelectric schemes are classified on the basis of the head utilized to generate power: high head storage type, medium head pondage type, and run-of-river. In low head type of hydro generators, both the velocity of water and difference in levels are used to rotate the turbine. In high head generators, the difference in levels is used to impart high velocity to the water to run the turbine. As the name suggests, in the run-of-river hydro generators, the natural flow of river water is used to drive the turbines. FIG. 7 shows a schematic diagram of the high head storage type hydroelectric scheme.

FIG. 7 Schematic diagram of a high head storage type of a hydroelectric scheme

The power P generated in a hydroelectric station is given as P = 9.81pQhq x kW (eq.1)

where Q is the discharge of water in m3/s through the turbine, p is the specific weight of water in 1000 kg/m3, h is the head of water in meters, and q is the generation efficiency.

The merits of a hydroelectric station are as follows:

• Minimal operational costs (since there is no fuel cost involved)
• No air pollution
• No waste products
• Minimal maintenance
• Quick start-up time (within five minutes)
• Long life (minimum fifty years)

The demerits of a hydroelectric station are as follows:

• High capital costs
• Long gestation period
• Ecological damage to the region
• Nuclear Power Stations

The fuel in a nuclear power station is uranium. Of the two isotopes of uranium, uranium-235 and uranium-23 8, found in natural uranium, only uranium-235 is capable of undergoing fission. Fission in uranium-235 is brought about by bombarding it with neutrons. Due to the fission reaction, heat energy and neutrons are released. The released neutrons further react with fresh uranium-235 atoms to generate more heat and produce more neutrons. Thus the fission reaction is a chain reaction and is required to be conducted under controlled conditions in a nuclear reactor.

In a nuclear power station, the nuclear reactor constitutes the heart of the station and replaces the boiler in coal or oil fired stations. FIG. 8 shows a schematic layout of a nuclear power station.

FIG. 8 Schematic layout of a nuclear power station

In the reactor pressure vessel, nuclear fuel rods are embedded in neutron speed reducing agents such as heavy water and graphite called moderators. These moderators reduce the speed of neutrons to a critical value. The nuclear reaction is controlled by inserting boron steel rods, which have the property to absorb neutrons.

Thus, the rate of nuclear fission is controlled by controlling the neutron flux.

A primary coolant such as heavy water or carbon dioxide is used to transfer the heat generated due to the fission reaction to the heat exchanger. Steam is produced in the heat exchanger, which is used to run a conventional steam turbine.

The fuel requirements of a nuclear generating station are minimal compared to a coal fired generating station. In addition, the cost of transporting nuclear fuel is negligible. Another advantage of nuclear power stations is that they do not produce any air pollution. Nuclear stations, therefore, can be sited close to load centers.

However, since radioactive fuel waste is produced in the nuclear reactor, safety considerations demand that nuclear stations be sited away from the populated areas. Nuclear stations require a high capital investment. The operational cost of such stations, however, is low.

Non-conventional / Alternative Generating Stations

Wind power stations--Wind as a source of energy has been used for centuries to grind grain and pump water. It is particularly attractive since it is non-polluting.

However, it is unpredictable and unsteady. The expression for theoretical power generated, in watts, by wind of average velocity V meters per second is given by where p is the air density (1201 g/m^2 at normal temperature and pressure) and A is the swept area in square meters.

The success of wind power generating stations is governed by the initial capital cost, maintenance cost, useful life, and power output. Wind power generating stations are useful for meeting low power requirements in small isolated areas.

In India, the gross potential of wind power has been assessed at approximately 45,000 MW, and the technical potential is estimated at 13,000 MW. Wind power stations have been set up in the states of Gujarat, Maharashtra, Orissa, Andhra Pradesh, and Tamil Nadu. The largest installation of wind turbines in the country so far is in the Muppandal-Perungudi area near Kanyakumari in Tamil Nadu with an aggregate installed capacity of about 500 MW. State-of-the-art technology is now available in India for manufacturing wind turbines of capacity up to 750 kW. Geothermal power stations Geothermal power generation involves conversion of the heat energy contained in hot rocks inside the core of the earth into electricity through steam. Water is used to absorb heat from the rock and transport it to the earth's surface, where it is converted to electric energy through conventional steam-turbine generator. Geothermal energy has been employed to generate steam in a limited way in Italy, New Zealand, Mexico, USA, Japan, etc. In India, the use of geothermal energy is still at the developmental stage with feasibility studies for a 1-MW station in Ladakh being undertaken. Though the efficiency of a geothermal station is less than that of a conventional fossil fuel plant, geothermal stations have become attractive due to their low capital cost and zero fuel cost.

The total available geothermal power globally has been estimated at 2000 MW of which only about 500 MW has been tapped. In India, despite a number of hot springs, the availability of exploitable geothermal energy potential appears to be unattractive.

Tidal power stations The gravitational effects of the sun and the moon and the centrifugal forces of the earth's rotation on its axis cause sea tides. In about 25 hours there are two high tides and two low tides. The minimum head required for generation is about 5 m. Tidal power stations use periods of high tides to fill reservoirs, through open sluice gates, behind embankments along the seashores.

P = 0.5~ AV3 W (eq.2)

During low-tide periods, when the tide is falling on the seaward side of the embankments, the sluice gates are closed and stored water is made to flow through turbines coupled to generators. This is known as ebb generation.

A tidal power station is usually sited at the mouth of an estuary or a bay. A barrage or an embankment is constructed at the site to store water. A two-way generation can be achieved in a tidal power station. As the tidal waves come in, water flows through the reversible turbines to generate power and fill the estuary/ bay. As the tide falls, water flows out of the estuary hay and the turbines. Since the turbines are reversible, power is again generated.

The disadvantage of tidal power stations is that, due to variation in high and low tide timings, they may be generating at peak demand on some days and idle for other days. Another disadvantage is the high cost of civil engineering works required.

With hundreds of kilometers of coastline, a vast potential source of tidal energy is available in India. It has been planned to set up a 600-MW tidal power station in India by constructing a dam at Kandala on the Gujarat coast. Other sites under exploration are at Bhavnagar, Navalakhi (Kutch), Diamond Harbour, and Ganga Sagar.

Solar power The earth receives radiation continually from the sun to the equivalent of 1.17 x 1017 W. This energy from the sun is utilized to generate electricity. A solar cell is a thin silicon wafer of thickness 0.25 mm and can have a round or square form. It has the property of converting light energy of the sun into cur- rent. FIG. 9 shows the one- dimensional geometric view of a PN junction solar cell.

Light photons from the sun penetrate into the PN junction diode (cell) and impart enough energy to the valence electrons to make them jump into the conduction band. Due to the unaccounted number of photons penetrating the cell, an extremely large number of electrons enter the conduction band and are pushed out of the cell by the internal electric field which has already been produced during the manufacture of the PN junction diode. This flow of electrons leads to the flow of current. The process of direct conversion of solar light energy into electric current is called 'photovoltaic' (PV) effect.

The electrons will continue to flow out of the cell as long as light photons from the sun continue to penetrate the cell. As such, a cell never loses power, like a battery, since cells do not 'consume' electrons. Therefore, a cell may be viewed as a converter since it changes (sun) light energy into electric energy.

FIG. 9 Direct conversion of solar energy to electricity in a photovoltaic PN junction diode

FIG. 10 shows a view of a basic circular PV device. The metal contacts placed in front and at the back draw and deliver the electrons to the II cell. In this manner, the same electrons continue to travel the same path and in the process deliver light energy to the load.

A typical silicon PV cell produces only 0.5 V DC. Therefore, a PV cell forms the basic device to form modules or panels to obtain higher voltages. Fig. 10 Basic PV device Since a minimum of 12 V is required to charge a storage battery, a typical 65 x 140 cm panel will be made of 36 PV solar cells connected in series to produce 18 V. When loaded, the output voltage of the panel drops to 14 V which is the minimum voltage required to charge a storage battery.

Thus, 36 solar cells panel has become the standard or basic module for the solar battery charger industry. Solar panels can be connected in series to obtain higher voltages of 24 V, 48 V, and more. Higher current capacity and therefore more power output can be obtained by connecting the basic modules of 36 cells each in parallel. FIG. 11 provides a pictorial view of a basic module of 36 cells.

FIG. 11 Pictorial view of a basic solar PV module

As on 31 March 2012, out of an installed capacity of about 200 GW in India, the share of renewable energy was 24,915 MW, which constitutes 12% of the total capacity. The share of solar energy in the installed renewable energy component stood at 905 MW (4%). Under the National Action Plan on climate change, one of the eight missions that the GOI has set up in January 2010 is the Jawaharlal Nehru National Solar Mission (JNNSM). The aim of the Mission is to develop and promote the use of solar energy technology. The Mission aims to achieve, in three phases, a cumulative target of 20,000 MW and 2,000 MW, respectively, in grid and off-grid solar power generation by 2022.

MHD generation--The magnetohydrodynamic power generation technology (MHD) is the production of electrical power utilizing a high-temperature conducting fluid (plasma) moving through an intense magnetic field. The con- version process in MHD was initially described by Michael Faraday in 1893.

However, the actual utilization of this concept remained unthinkable. The first known attempt to develop an MHD generator was made at Westinghouse Research Laboratory (USA) in around 1936. Since the 1960s, many different types of MHD power generators have been classified according to the type of cycle (open loop or closed loop) and the type of fluid used. Open loop MHD generators were first realized in 1965 in the USA. It was a 32 MW facility, fuelled with alcohol, which had a start-up time of three minutes. In 1971, an MHD pilot plant using natural gas fuel was commissioned at the Institute of High Temperatures, USSR. This pilot plant had 75 MW of power (25 MW of MHD and 50 MW from steam). In 1984, a coal-fired MHD pilot plant was constructed in USA. Closed loop MHD generators are usually associated with nuclear reactors as heat source, where the working fluid can be a noble gas or liquid metal. In India, BHEL Tiruchirappalli started work in MHD technology in 1978 in close cooperation with BARC and High Temperature Institute, Moscow, which was a pioneer in large-scale MHD activities, and a 5 MW pilot plant was commissioned in Timchirappalli in 1985.

Later in 1993 there was a proposal for installing a 200 MW retrofit in an existing thermal station, but it was later shelved.

Working principle

The MHD generator can be considered to be a fluid dynamo.

This is similar to a mechanical dynamo in which the motion of a metal conductor through a magnetic field creates a current in the conductor, except that in the MHD generator the metal conductor is replaced by conducting gas plasma.

When a conductor moves through a magnetic field, it creates an electrical field perpendicular to the magnetic field and the direction of movement of the conductor.

This is the principle, discovered by Michael Faraday, behind the conventional rotary electricity generator. Dutch physicist Hendrik Antoon Lorentz provided the mathematical theory to quantify its effects.

The flow (motion) of the conducting plasma through a magnetic field causes a voltage to be generated (and an associated current to flow) across the plasma, perpendicular to both the plasma flow and the magnetic field according to Fleming's Right Hand Rule. This is illustrated in FIG. 12.

FIG. 12 Magnetohydrodynamic power generation

The MHD system The MHD generator needs a high-temperature gas source, which could be the coolant from a nuclear reactor or more likely high-temperature combustion gases generated by burning fossil fuels, including coal, in a combustion chamber. FIG. 13 shows the possible system components.

The expansion nozzle reduces the gas pressure and consequently increases the plasma speed through the generator duct to increase the power output.

Unfortunately, at the same time, the pressure drop causes the plasma temperature to fall which also increases the plasma resistance.

FIG. 13 Magnetohydrodynamic electricity generation

The following are some of the merits of MHD power generation:

• Simple structure
• Works at high temperatures
• High Carnot-cycle efficiency
• Easy to realize combined cycle with other systems

The following are the disadvantages of MHD power generation:

• Simultaneous presence of high temperature and a highly corrosive and abrasive environment
• MHD channel operation under extreme conditions of electric and magnetic fields
• Expensive initial installments

10.2 Transmission and Distribution Subsystem

The transformer and transmission line subsystems are designed to transmit bulk electric power for consumption at the load centers. In the generating stations, power is generated at voltage levels, which vary between 11 to 30 kV. The transformers at the generating station end step up the voltage to the level of transmission voltage suitable for transmission of bulk power. Since these transformers step up the voltage, they are also known as step-up transformers.

The power transmitted over a transmission line is proportional to the square of the transmission voltage. Therefore, ideally it is desirable to have the highest transmission voltages. As such, continuous efforts are undertaken to increase the transmission voltages. In the western countries, transmission of power is undertaken at transmission voltages of 765 kV. In India, the transmission voltage levels vary between 66 to 400 kV. High voltage direct current (HVDC) transmission of bulk power over long distances is more economical than high voltage alternating current (HVAC) transmission when bulk power is to be transmitted over distances greater than 600 km. The dc voltages at which transmission takes place are 400 kV and above.

At the generator end the ac voltage generated is stepped up to the transmission voltage level by a step-up transformer, which is converted to high voltage dc by a converter circuit. A converter is a three-phase full wave bridge circuit consisting of silicon-controlled rectifiers that can operate as a rectifier converting ac voltage to DC voltage and can also operate as an inverter converting dc voltage to AC voltage.

At the receiving end or the load end of HVDC transmission, a converter operating as an inverter is used to change high voltage dc to high voltage ac, and then the ac voltage is stepped down by a step-down transformer to lower voltage level for distribution to consumers of electric energy.

The level of voltage at which distribution of power is undertaken depends on the type of industry in the region. The first step down in voltage may be from the transmission or grid level to the subtransmission level and may range between 132 kV to 33 kV.

For the purpose of supplying power to small industries and commercial and domestic consumers, the voltage is again stepped down at the distribution substation. The distribution of power is undertaken at two voltage levels; the primary or feeder voltage at 11 kV and the secondary or consumer voltage at 415V for three-phase supply and 230V for single-phase supply.

Subtransmission System

The portion of the transmission system that connects the high-voltage substations through step-down transformers to the distribution substations is called the subtransmission network. There is no clear demarcation between the transmission and subtransmission voltage levels. The voltage level of a subtransmission system ranges from 66 kV to 132 kV. Some heavy industrial consumers are connected to the subtransmission system.

A distribution subsystem constitutes the part of the electric power system between the step-down distribution substation and the consumers' service switches.

A distributed system is designed to supply continuous and reliable power at the consumers' terminals at minimum cost. A typical distribution system is shown in FIG. 14.

FIG. 14 Layout of a typical distribution system

At the distribution substation, the voltage level is brought down from 66kV at the subtransmission level to 11kV at the distribution level. Each distribution substation normally serves its own area, which is a subdivision of the area served by the distribution system. Distribution transformers are ordinarily connected to each primary feeder and its sub-feeders and laterals. Each transformer or banks of transformers serve to step down the voltage to utilize voltage of three-phase 415 V or single-phase 230 V and supply a consumer or a group of consumers over its secondary circuit. Each consumer is connected to the secondary circuit through service leads and a meter.

The subtransmission and distribution systems remained neglected for a long time and it was only in the mid 1990s when the development of their infrastructure was recognized as a core issue in the power sector. The reason for lack of initiative to update the subtransmission and distribution infrastructure was attributed to a generation-centric focus by both the central and state governments. The bias towards generation is obvious when it is observed that till 1993 the ratio of plan outlay for the development of the generation subsystem to the transmission and distribution subsystem was 3:1 as against the desired 1:1. However, during the period 1997-2002 (ninth plan period) this ratio improved to 1.3:1, which is mainly due to a reduction in investment by the government in the generation subsystem.

The shortcomings in the distribution infrastructure, on the other hand, have been identified as follows:

(a) Insufficient transformation capabilities

(b) High technical losses

(c) High non-technical losses, such as pilferage and commercial losses

(d) Inadequacy in addressing consumer concerns including poor service

(e) Absence of redundancies

Another very pressing issue related to the subsystem is that of unbearably high T&D losses. The collective T&D losses in the power industry in India increased from 7.5% to an ... are estimated at 40%, which is very high when compared to the international average of approximately 6-7%. TABLE 7 provides the year-wise T&D losses in the power industry in India.

Due to concerted efforts, the transmission and distribution (T&D) losses have come down but have stagnated at 23.97%. The aggregate technical and commercial losses (AT&C) are of the order of 50% power generation. Billing for generated power is approximately 55% of the total power generated while the realization is only about 41%.

TABLE 7 Year-wise T&D losses in the Indian power industry.

National Grid

Hitherto transmission networks were developed with a focus on self-sufficiency on regional basis. As such the period from the mid 1970s to the early 1990s saw the building up of strong state grids and the emergence of regional grids. Presently, the regional grid networks are adequately strong to meet the inter-state transmission requirements while the state grids can focus on meeting the intra-state needs of their respective states.

The spotlight is now on building a national grid for better utilization of hydro resources, saving the transportation cost of coal (since it is economical to transmit electrical energy), sharing of reserves, etc.

10.3 Load Subsystem

From the perspective of a power supplier, an item (component) consuming electrical energy is a load. Therefore, loads on a power system can be broadly categorized as follows: (a) industrial, (b) commercial, and (c) domestic.

Industrial loads, which are voltage and frequency dependent, are a combination of motor loads, lighting loads, etc. Induction motors comprise a high percentage of the industrial load and consume considerable reactive power. Both commercial and domestic loads are voltage dependent and are mainly constituted of lighting, heating, and cooling. A few terms related to load subsystems are described here.

Load curve of a utility is a plot of variation of composite load against time. If the variation of load is on a 24-hour basis it is called a daily load curve.

Peak or maximum demand is defined as the maximum load occurring in a 24- hour cycle.

Loadfactor (LF) is defined as the ratio of average load during a period to maximum load during the same period. Thus,

Average load in kW/MW Maximum load in kW/MW

LF = for a specified period

If the specified period is a 24-hour cycle, the LF is called a daily LF. Multiplying Eq. (eq.3) by 24 yields

Average load in kW/MW x 24 Maximum load in kW/MW x 24 Daily LF = (1 *4)

- Energy consumed in 24 h in kWh/MWh

If the specified period is one year (24 x 365 = 8760 h), the LF is called an annual LF. The annual LF is used to assess the performance of a generating station. Thus,

- Maximum load x 24 in kWh/MWh (1.5)

Energy consumed in 8760 h in kWh Maximum load x 8760 in kWh/MWh Annual LF =

Higher the annual load factor, more economical is the plant operation. The desirable range of annual load factor of a system is between 55% to 70%. Diversity factor is defined as the ratio of the sum of maximum demands of individual category of consumers, such as industrial, commercial, and domestic, to the maximum load on the system. Thus, Max. demand of individual category of consumers

Diversity factor = Max. demand on system (1 4 It is a parameter which provides the diversification in load and is used to decide the installed capacity of a generating station. It is greater than unity, and therefore the installed capacity will be less than the sum of the maximum demands of individual category of consumers.

Utilization factor is defined as the ratio of maximum demand to installed capacity, that is, Max. demand in MW Installed capacity in MW

Utilization factor = Plant factor is defined as the ratio of annual energy generated to the possible annual energy that can be generated based on installed capacity. Thus, (eq.8)

Annual energy generated in MWh Installed capacity in MW x 24 x 365 Plant factor =

Example 1: load factor, if the daily variation of load on a power company is as follows: Compute the (i) average load, (ii) maximum load, and (iii) daily average

Solution

to the program consists of the following: ni ntrvl 1 oad The MATLAB function 1 odata is used to draw the daily load curve. The input The number of intervals which is 8 in this case It is a matrix whose order is equal to (number of intervals) x 3. The first two columns of the matrix represent the duration in clock hours and the third column represents the load in MW.

The output variables are energy, Pavg (average power), LF (load factor), and Pmax (maximum power demand).

The daily load curve is shown in FIG. 15.

FIG. 15 Load curve of Example 1

10.4 Protection and Control Subsystem

Protection and control subsystem is constituted of relays, switchgear, and other control devices, which protect the various subsystems against faults and overloads, and ensure efficient, reliable, and economic operation of the electric power system.

11. ENERGY CONSERVATION

A unit of energy saved is a unit of energy generated at no extra cost. One of the objectives of the GOI under its Mission 2012: 'Power for All' is to evolve a conservation strategy to optimize the utilization of electrical energy. The focus will be on (i) demand management, (ii) load management, and (iii) technology up-gradation to provide energy-efficient equipment and gadgets. It is possible to bring about an energy saving of the order of 20% in various sectors without sacrificing any of the end-use benefits of energy.

Keeping in mind the need and importance of energy conservation, the GOI has enacted the Energy Conservation Act (2001) under which a Bureau of Energy Efficiency has been established for the promotion of conservation and efficient use of energy.

12. COMPUTERS IN POWER SYSTEM ANALYSIS

Historically, digital computers were first employed for analyzing power system problems, in a restricted manner, in the late 40s of the last century. With the advent of computers having capabilities to handle large volumes of data with adequately fast processing speeds, in the mid 1950s, their usage in analyzing varied and more intricate problems related to larger and complex power system networks was a natural outcome.

The operation and control of present-day interconnected power networks, each constituting of substations, transmission lines, and transformers, has become so complex that from the perspective of economy and reliability of supply it is essential that these be monitored through a central point called an Energy Control Center (ECC). An ECC is an online computer which undertakes signal processing based on remote data acquisition system and performs in both normal and emergency situations. The constituents of an ECC are as follows.

(a) An operator who acts as a human-machine interface (b) A visual display unit (VDU) which enables the selection of presentation of the desired portion of the network, along with the data summaries and performance indices, through paging buttons (c) Editing and special function keyboards to change operating conditions, system parameters, transformer taps, switch-in-out line capacitors, etc.

(d) Light pen cursor for operating circuit breakers, switches, etc. and for changing displays directly on the VDU

SUMMARY

After Independence, the GOI, amongst other development plans, took upon itself to develop the power sector.

Vision 2012 for the power sector, in addition to providing 'Power for all', also envisioned reliable and quality power at optimum cost along with the development of a competitive power industry in addition to providing sufficient power to achieve a GDP growth of 8%.

Vision 2020 envisages efficient and environment-friendly energy resources which would become the growth engines to provide speedy and sustainable future economic development.

In order to achieve the objectives of Vision 2012 for the power sector, Electricity Act 2003 was amended and enacted to bring about a market-oriented management, through restructuring and deregulation of the power sector, so as to introduce a spirit of competition. Electricity Act 2007 further amended Electricity Act 2003 to allow setting up of captive power units without obtaining licenses and making power theft a criminal offence.

A power sector is a complex network constituted of (a) generation, (b) transmission and distribution, (c) loads, and (d) protection and control subsystems.

Primary sources of energy are (a) fossil fuels such as coal, oil, and natural gas, (b) renewable energy sources like hydro, wind, solar, and (c) nuclear energy.

Based on the type of primary source employed, generation stations are categorized into (a) thermal, (b) hydro, (c) nuclear, and (d) non-conventional. In 2009-10, the total all-India generation was 771,173 GWh. Generation voltages range between 11 kV and 30kV. DC transmission voltage is 500 kV while the AC voltages range between 66 and 400 kV. By 2017, it is planned to add transmission lines operating at 765 kV, along with HVDC Bipole lines. Distribution of power is undertaken at AC voltages up to 500V. The load subsystem consists of (a) industrial, (b) commercial, and (c) domestic loads.

The important terms used to define a load subsystem are: (a) load curve, (b) daily and annual load factor, (c) diversity factor, (d) utilization factor, and (e) plant factor.

EXERCISES

Review Questions

Trace the history of the growth of the power sector after Independence.

Write a short essay on 'Vision 2020' for the power sector.

Describe the models of regulation and deregulation associated with the power sector. Discuss the features of a regulated power sector and explain the structure of a regulated authority.

With the help of block diagrams, illustrate and explain the structure of a deregulated power sector.

Explain the mechanism of competition.

Write short notes on: (i) wholesale power market, (ii) instruments of sale transaction, and (iii) responsibilities of the SSMSO. Draw a neatly labeled diagram of a power network and indicate the various subsystems along with their operation voltages.

Enumerate the various sources of energy and categorize generating stations based on primary source of energy employed.

Draw a block diagram of a thermal station and describe its main features.

Explain cogeneration.

Write short notes on: (i) oil-fired, (ii) gas-fired, and (iii) diesel oil-fired generating stations.

Describe with the help of a diagram the salient features of a hydro generating station and itemize its merits and demerits.

Describe the working of a nuclear generating station.

List the various types of non-conventional generating stations and describe any two of them.

(a) Briefly describe the transmission and distribution subsystems.

(b) Highlight the concerns of the T&D subsystem in the Indian power sector.

(a) Describe the components of the load subsystem.

(b) Discuss the utility of load curve and define peak demand.

Define and write notes on: (i) load factor, (ii) diversity factor, (iii) utilization factor, and (iv) plant factor.

Write a note on the importance of computers in power system analysis.

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

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Numerical Problems

A power utility with an installed capacity of 100 MW is supplying a composite load whose details are as follows:

Calculate the diversity factor.

A generating station has a plant factor of 50% and a maximum demand of 450 MW. If the annual load factor is 60%, determine the additional load the station can supply. Assume that the generating station can be fully loaded.

The demand for power on a 100-MW generating station is as follows: 80 MW for 4 h, 50 MW for 8 h and 20 MW for 6 h. For the remaining part of the day it is switched off. Determine the annual load factor of the station. Assume that the station is under maintenance and repair for 45 days in a year.

The demand for energy on a utility is growing exponentially and can be expressed as P = POeut where a and t are the growth rate and time respectively.

Determine the growth rate if the energy consumption is expected to increase 1.5 times in 10 years.

Mathematically the demand on a utility is estimated to be P = Poeu('- 'o), where P is the demand in the year t, Po is the demand in the base year to, and a is the per annum growth rate. If the maximum power demand in the base year was 250 GW, write a MATLAB function to plot the growth of demand for the next 15 years. What is the demand after 10 years?

The month-wise load on a generating station is as follows:

Multiple Choice -- Objective Questions

1. Who introduced the first electric supply system? (a) Edison (b) Faraday (c) Tesla (d) Marconi (a) NHPC (b) NTPC (c) NPCIL

2. Which of the following was set up for the development of nuclear power? (d) None of these

3. Which of the following is not an objective of 'Vision 2012' for the power sector? (a) Reliability of power (b) Quality power (c) Sufficient power to achieve 8% GDP growth rate (d) None of these The process of reforming the power sector was started in (a) 1981 (b) 1991 (c) 2001 (d) 2011

5. Electricity Act 2003 was enacted in the month of (a) April (b) May (c) June (4 July

6. Loss reduction and theft control was which part of the strategy to achieve 'Power for All'? (a) Power generation (b) Transmission (c) Distribution (d) Conservation

7. As per 'Electricity Act 2003' which of the following type of generating stations requires a license? (a) Oil fired (b) Hydro (c) Gas fired (d) Solar (a) Fertilizer (b) Petroleum refining (c) Paper Which of the following is not true of a hydroelectric station? (a) No ecological imbalance (c) No air pollution (a) Oil-fired (b) Coal-fired (c) Gas fired Which of the following is a reason to locate nuclear stations away from populated areas?

8. Which of the following industries employs cogeneration? (d) All of these (b) Long life (d) None of these

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10. Which of the following contains nearly zero sulphur dioxide? (d) Diesel fired

11. (a) Primary coolant is heavy water.

(b) High capital investment is required.

(c) Radioactive waste is generated.

(d) All of these.

The success of wind power stations is based on (a) initial capital cost (c) useful life (a) Orissa (b) Punjab (c) Tamil Nadu (d) Maharashtra 1.12 (b) power output (d) all of these

13. Which of the following states has not set up wind power stations?

14. The voltage produced by a typical silicon PV solar cell is (a) 0.25 V DC (c) 0.5 V DC

Which of the following is not required for the flow of electrons in a PV cell? (a) External field (b) Internal field (c) Penetration of a photon of light (d) None of these The minimum voltage required to charge a 12 V storage battery is (b) 0.25 VAC (d) 0.5 VAC

16. (a) 18V (b) 16V (c) 14V (d) 12V (a) 765 kV (c) 400 kV (a) 17.5% (b) 21.7% (c) 25% (d) 40% A higher annual load factor indicates (a) economical plant operation (b) uneconomical plant operation (c) no effect on economics of plant operation (d) none of these Which of the following is employed to determine the installed capacity of a generating station?

17. Which of the following will become a part of the transmission system by 2017? (b) HVDC Bipole (d) All of these

18. Currently the level of T&D losses stands at (a) Maximum demand (c) Diversity factor (b) Annual load factor (d) Plant factor