Power system Operation and Protection
The electric power industry shapes and contributes to the welfare progress and technological advances of humanity. The growth if electrical energy consumption in the world has been nothing short of phenomenal. Complex systems supply the vast need of a country for electric power. Because of the tremendous power requirements of the present age, especially in developed countries, there is a vast concern pertaining to the efficient operation of power production and power conversion systems. Electric power systems are real-time energy delivery systems.
This means that power is generated, transported and supplied the moment one turns on a switch. That is to say electric power systems are not storage systems like water systems and gas systems. Instead, generators produce the energy as the demand calls for it (Fardo & Patrick, 2008:15). The study of electric power systems is concerned with the generation, transmission, distribution and utilization of electric power. The explanation of these aspects of the power systems along with the various protection mechanisms is the objective of the present essay.
OPERATION OF AN ELECTRIC POWER SYSTEM The figure below shows the basic building blocks of an electric power system. Fig 1 Electric Power System Overview (Blume, 2007:4) As mentioned earlier, Generation, Transmission and Distribution systems are the main components of an electric power system. As can be seen the figure above, the system starts with generation, by which energy is produced in the power plant and then transformed in the power station to high-voltage electrical energy that is more suitable for efficient long-distance transportation.
The power plants transform other sources of energy in the process of producing electrical energy. High-voltage HV power lines in the transmission portion of the electric power system efficiently transport electrical energy over long distances to the consumption locations. Finally, substations transform this HV electrical energy into low-voltage energy that is transmitted over distribution power lines that are more suitable for the distribution of electrical energy to its destination, where it is again transformed for residential, commercial, and industrial consumption (Blume, 2007:3).
A system may or may not have transmission component depending upon the distance of the generating system from the consumers of electric energy, but generation and distribution systems are integral parts of any electric power system. The intermediate phase between the generation and distribution has come into existence only because of the long distances between the energy sources i. e. generators and sinks i. e. consumers. Needless to a full-scale interconnected electric power system is much more complex than the outline given above.
In fact electrical power system is the most capital intensive and the most complex system ever developed by man. However, the basic principles and terminology is the same as given in the figure and description above (Wadhwa, 2006:260). GENERATION OF ELECTRIC POWER Electricity is produced by converting the mechanical energy on the output shaft of an engine, or more usually a turbine, into electrical energy. In the most contemporary system, the mechanical energy is either obtained from thermal energy or extracted directly from flowing water i. e. hydro-generation. The main thermal energy resources used commercially are coal.
, natural gas, nuclear fuel and oil. There is also an increasing use of non-fossil fuels such as wind tidal, geothermal and biogas in the generation of electricity. The main non-thermal source of energy is water of hydro-power. The conversion of mechanical to electrical energy is almost universally achieved by synchronous generator i. e. alternator, although some wind generation systems may use an induction generator. The synchronous generator feeds its electric power into the transmission system via a step-up transformer in order to increase the voltage from the generation level i. e.
10-20 kV to the transmission level which may be of the order of hundreds of kV (Machowski, Bialek, & Bumby, 1997:7-8). The backbone of the electric power systems is the number of generating stations, distributed over a territory and operating in parallel. In addition, electricity is not only generated locally but is also transported to and/or imported from neighboring states and even countries, if hey have the requisite trading contracts and a common power flow and frequency pool. For instance, in Europe, 25 countries such a France, the Netherlands, Germany, Austria, Switzerland, Italy etc.
cooperate in the UCTE, while NORDEL is the union of the Scandinavian grid companies (Schavemaker & Van der Sluis, 2008:46). Almost all electric power generation and most of the power transmission in the world today is in the form of three-phase ac circuits. AC power systems have a great advantage over dc systems in that their voltage levels can be changed with the help of transformers to reduce transmission losses. The three-phase ac power systems have two major advantages over single-phase ac power (Chapman, 2004:681): 1. It is possible to get more power per kilogram of metal from a three-phase machine, and
2. The power delivered to a three-phase load is constant at all times, instead of pulsing as it does in single-phase systems Three-phase systems also make the use of induction motors easier by allowing them to start without special auxiliary start windings (Chapman, 2004:681). TRANSMISSION OF ELECTRIC POWER Electrical transmission is the most efficient method of transmitting power over long distances. A transmissions system consists of structures, wires, switching and conversion stations . It forms the bone of the powers system which connects the generating stations with the load points.
Transmission systems are interconnected due to economic, security and reliability reasons (Singh, 2008, p. 146). Since the energy lost in a transmission line is proportional to the current squared, transmission lines operate at very high voltages. This can be seen from the well-known formula for electric power, which states . Therefore, the higher the voltage, the smaller the current needs to be for a certain power required, and the smaller the current, the smaller the required conductor size, hence, the lower the cost of the transmission line.
That is why voltages tend to become higher and higher, as new technologies develop to achieve higher voltages, especially over very long distances. On the other hand, higher voltages mean more expensive equipment like transformers and switchgear, thus the most economical transmission network design will depend on a multitude of factors like distance, available route generated voltage, required end voltage, power demand etc. Also overhead lines are far cheaper than underground cables for long distances mainly because air is used as the insulation medium between phase conductors, and that no excavation work is required.
The major portion of the cost is hence the support of the overhead lines. Hence aluminum is used as it is lighter than copper and hence is les expensive, though the optimal design still depends on a lot of factors considering that copper has higher current conducting capacity. Finally the long lengths of transmission lines add significant reactance to the network this worsening the power factor (cos? ), which in turn makes the power transmission inefficient due to higher reactance losses.
The installation of capacitor banks improves the situation in this case, as the added capacitance will oppose the buildup inductance (Strauss, 2003:17-18). Usually transmission network is connected in a mesh structure in order to provide many possible routes for electrical power to flow from the generators to electrical consumers, thereby improving the flexibility and reliability of the system. As the electrical energy gets closer to the load center, it is directed from the transmission network to a sub-transmission network.
When a power system expands with the addition of new, high voltage, transmission lines, some of the older, lower voltage lines may become part of the sub-transmission network. There is not strict division of the network into transmission and sub-transmission networks, and smaller power generation plants may feed directly into the sub-transmission network whilst bulk power consumers may be fed directly from the transmission or sub-transmission network (Machowski, et al. , 1997:8). The use of dc transmission is also on a rise around the world.
Its main use is to interconnect two separate areas where some power transfer is desirable but a synchronous i. e. ac connection is not. The main features of this connected are ac-dc conversion and the dc-ac inversion that allows the connection of two ac systems with a dc line (Chen, 2005:713). DISTRIBUTION OF ELECTRIC POWER Most of the electrical energy is transferred from the transmission or sub-transmission network to distribution high voltage and medium voltage networks in order to bring it directly tot the consumer.
The main difference between transmission and distribution lines is the voltage level, the latter being at comparatively lower voltages. The distribution is generally connected in a radial structure as opposed to the mesh structure of the transmission system. However, large consumers may be supplied from a weakly coupled, meshed, distribution network or alternatively they may be supplied from two radial feeders with a possibility of automatic switching between feeders in case of a power cut.
Some industrial consumers may have their own embedded or on-site generation as a reserve or as a by product of the technological processes e. g. steam generation. Ultimately power is transformed to a low voltage using step-down transformers and distributed directly to consumers (Machowski et al. , 1997:9; Grigsby, 2007:8. 10-8. 11). The power supply required by the various appliances may be ac or dc depending upon the use. However, ac distribution of supply is common. Distribution system consists of two components: feeder and distributor.
A feeder is a circuit carrying power from a main substation to a secondary substation while distributor has variable loading along its length due to its service connections tapped off at intervals by individual consumers (Wadhwa, 2006:260). Primary distribution lines are connected to distribution transformers to step-down the primary voltage for distribution over secondary mains to the consumer’s service. The lines carrying the energy at utilization voltage from the transformer to the consumer services are called secondary distribution mains and may be found overhead or underground.
Services and meters link the distribution system and the consumer’s wiring. Energy is tapped from the secondary mains at the nearest location and carried by the service wires to the consumer’s building, and as it passes on to operate the various home appliances it is measured by highly-accurate watt-hour meters (Pansini, 2004:6-7). Distribution system represents the final stage in the transfer of power to the individual consumers. The distribution infrastructure is extensive and distribution circuits are found along most secondary roads and streets.
Urban construction is usually underground while rural construction is usually overhead. Because of the extensive infrastructure, distribution systems are capital intensive businesses. Low cost, simplification, and standardization are all important design characteristics of a distribution system. Few components and/or installations are individually engineered on a distribution circuit. Standardized equipment and standardized designs are used wherever possible. Cookbook engineering methods are used for much of the distribution planning design and operations (Short, 2004:1-2). UTILIZATION OF ELECTRIC POWER
Utilization is the end result of the generation, transmission and distribution of electric power. The energy carried by the transmission and distribution system is turned into useful work, light, heat, or a combination of these items at the utilization point. Understanding and characterizing the utilization of electric power is critical for proper operation of power systems. Improper characterization of utilization can result of over or under building of power system facilities and stressing of system equipment beyond design capabilities (Andrew Hanson cited in Grigsby, 2000:7. 12).
The most common utilization voltages are 105 volts in Japan, 120 volts in United States and nations that have adopted US electrical standards, and 230 or 250 volts in Europe and many other nations. As voltages in the range of 105 to 250 volts cannot move large or even small amounts of power very efficiently over any great distance, the service level here consists of many small transformers, each providing power to only a handful of customers in its immediate vicinity, with secondary circuits operating at utilization voltage to route power no more than a few hundred feet from each transformer to property lines of each consumer.
Service drops are usually individually dedicated lines that take the power across the customer’s property to his meter (Willis, 2004:706). Load characteristics The term load refers to a device or collection of devices that draw energy from the power system. Individual loads range from small light bulbs to large induction motors to arc furnaces. The total load demand of an area depends upon its population and the living standard of people. The general nature of load is characterized by the load factor, demand factor, diversity factors, power factor and utilization factor.
In general, the types of load can be divided into the following categories: ? Domestic loads: This category mainly consists of lights, fans, refrigerators, air-conditioners, mixer & grinders, heaters, ovens, small pumping motors etc. ? Commercial loads: This category mainly consists of lighting for shops, offices, advertisements etc. , fans, heating, air-conditioners and many other electrical appliances used in commercial establishments such as market places restaurants etc.
? Industrial loads: This category consists of small-scale industries, medium-scale industries, large-scale industries, heavy industries and cottage industries. ? Agriculture loads: This category mainly consists of motor pump-sets load for irrigation purposes. Load factor for this load is very small, e. g. 0. 15-020. (Guile, Paterson & Das, 2006:3-4) Load Forecasting Typically about 8% of the electrical energy appearing at the generator terminals is lost on its way to the consumers in transmission and distribution. The demand for the electrical power is never constant and changes continuously throughout day and night.
The changes in demand of individual consumers may be fast and frequent but as one moves up the power system structure from individual consumers, through the distribution network, to the transmission level, the changes in demand become smaller and smoother as individual demands are aggregated. Consequently the total power demand at the transmission level changes in a more or less predictable way that depends of season, weather conditions, the way of life of a particular society etc. fast global power demand changes on the generation level are usually small and are referred to as load fluctuations (Machowski et al.
1997:9). Before transmission planning is started, long-term load forecasting and generation planning are completed. In long-term load forecasting, peak and off-peak loads in each area of the system under study are projected, year by year, from the present up to 25 years into the future. Such forecasts are based on present and past load trends, population growth patterns and economic indicators. The results of long-term load forecasting and generation planning are used by engineers to design the power system (Dorf, 1998:1388).
The forecasting of power systems load is an essential task and forms the basis for planning of power systems. The estimation of the load demand for the power system must be as exact as possible. Despite the availability of sophisticated mathematical procedures, the load forecast is always afflicted with some uncertainty, which increases the farther the forecast is intended to be projected into the future. Power systems, however, are to be planned in such a ay that changing load developments can be accommodated by the extension of the system.
Long-term planning is related either to principal considerations of power system development or to the extra-high voltage system, so that no irrevocable investment decisions are imposed. These investment decisions concern the short term, as they can be better verified within the short-term range, for which the load forecast can be made with much higher accuracy (Schlabbach, & Rofalski, 2008:11). Load on a distribution system varies by the hour, day and the seasons. It also consists of different types of loads on the systems such as residential, commercial, industrial etc.
Data is collected regarding loads in a system and from the load data of each type of load, the load window for each type of load is developed. The percentage of each type of load under the major head of the classification of the load as percentage of the total load is represented by what is termed as the load window (Deshpande, 1984:314). PROTECTION OF ELECTRIC POWER SYSTEM Power systems are built to allow continuous generation, transmission and consumption of energy. Protecting all equipments of the power system – generators, transformers, transmission lines, distribution feeders – against short circuits and other faults is essential.
Though rare, the electrical power system is occasionally subjected to abnormal operating conditions. They include lightning striking the transmission lines during severe weather storms, excessive loading and environmental conditions, deterioration or breakdown of the equipment insulation, and intrusions by humans and/or animals. As a result power systems may experience occasional faults, which are defined as events that have contributed to a violation of the design limits for the power systems regarding insulation, galvanic isolation, voltage and current level, power rating, and other such requirements.
Faults occur randomly and may be associated with any component of the power system. The general principle behind protection is to detect the fault and isolate the equipment (Chen, 2005:714, 788). It is fair to say that without discriminative protection, it would be impossible to operate modern power system. The protection is needed to speedily remove as speedily as possible any element of the power system in which a fault has developed. So long as the fault remains connected, the entire system remains in jeopardy from three effects of the fault: 1.
It is likely to cause individual generators in a power system, or groups of generators in different stations, to lose synchronism and fall out of step with consequent splitting of the system 2. A risk of damage to the effected plant 3. A risk of damage to healthy plant. Yet another effect, not necessarily dangerous to the system, but important from the consumers’ viewpoint is a risk of synchronous motors in large industrial premises falling out of step and tripping out, with the serious consequences that entails loss of production and interruption of vital processes.
Hence it is the function of protective equipment, in association with circuit breakers to avert these effects. In addition to avert the effect mentioned above, the role of protective equipment is ultimately to provide 100 percent continuity of supply (Electricity Training Association, 1995:2). BASIC COMPONENTS OF PROTECTION Protection if any power system us a function of many elements. Following are the main components of protection: ? Fuses: Fuse is the self-destructing one, which carries the currents in a power circuit continuously and sacrifices itself by blowing under abnormal conditions.
These are normally independent or stand-alone protective components in an electrical power system unlike a circuit breaker, which necessarily requires the support of external components ? Voltage & current Transformers: Accurate protection cannot be achieved without properly measuring the normal and abnormal conditions of a system. Voltage transformers and current transformers measure voltage and current respectively, and are capable of providing accurate measurement during fault conditions, without failure.
? Relays: The measured values are converted into analog and/or digital signals and are made to operate relays, which in turn isolate the circuits by opening the faulty circuits. In most cases relays provide the dual functions of alarm and trip, once the abnormality is noticed. With advancement in digital technology and use of microprocessors, relays monitor various parameters which give a complete history of a system during both pre-fault and post-fault conditions. ? Circuit Breakers: The opening of faulty circuit requires some time, which may be in milliseconds, which for common day life could be insignificant.
However, the circuit breakers which are used to isolate the faulty circuits are capable of carrying these fault currents until the fault currents are totally isolated. ? DC batteries: The operation of relays and breakers require power sources, which shall not be effected by faults in the main distribution system. Hence, other component, which is vital in the protection of electrical power system is batteries that are used to ensure uninterrupted power to relays and breaker coils. (Hewitson, Brown & Balakrishnan, 2005:2-3)
An electric power system is divided into protection zones for: generators, transformers, bus bars, transmission & distribution circuits, and Motors. The division is such that zones are given adequate protection while keeping service interruption to a minimum. Each zone is overlapped to avoid unprotected i. e. blind areas (El-Hawary, 1995:542). The figure below shows the zones of protection. Fig 2 Various zones of protection for a power system (Paithankar, Bhide, 2004:19) The figure below shows the required qualities of power system protection.
Fig 3 Power System protection Qualities (Hewitson et al. 2005:4) Primary & Backup protection Primary protection is the first line of defense and is responsible to protect all the power system elements from all types of faults. The backup protection comes into play only when the primary protection fails. The backup protection is provided as the primary protection can fail because many reasons such as failure of circuit breaker, failure in protective relay, failure in tripping circuit, failure in dc tripping voltage, loss of voltage or current supply to the relay.
Thus if the backup protection is absent and the main protection fails, then there is a possibility of severe damage to the system. Backup protection is also useful when the primary protection us deliberatively inoperative due to maintenance purposes. The backup protection scheme should be such that the failure in main protection should not cause the failure in backup protection as well. This is satisfied if the backup relaying and primary relaying do not have anything in common. Hence, generally backup protection is located at different stations from the primary protection.
From the economy point of view, backup protection is employed only for the protection against short circuit and not for any other abnormal conditions (Bakshi & Bakshi, 2007: 4-5). PROTECTIVE RELAYING A protective relay is the device, which gives instruction to disconnect a faulty part of the system. This action ensures that the remaining system is still fed with power and protects the system from further damage due to the fault. Hence, the use of protective apparatus is very necessary in the electrical systems, which are expected to generate, transmit and distribute power with least interruptions and restoration time.
A protections system hence continuously monitors the power system to ensure maximum continuity of electrical supply with minimum damage to life, equipment and property (Christopoulos, Wright, 1999:28). Within this context there are five basic facets of protective relay application: 1. Reliability: assurance that the protection will perform correctly 2. Selectivity: maximum continuity of service with minimum system disconnection 3. Speed of operation: minimum fault duration and consequent equipment damage 4. Simplicity: minimum protective equipment to achieve the protection objectives 5.
Economics: maximum protection at minimum total cost (Blackburn, 1997:19) LIST OF REFERENCES Bakshi MV, Bakshi UA, (2007), Switchgear and Protection, Pune: Technical Publications Blackburn JL, (1997), Protective Relaying: Principles and Applications, 2nd Edition, New York: CRC Press & Marcel Dekker Blume SW, (2007), Electric Power System Basics: For the Nonelectrical Professional, Danvers, Massachusetts: Wiley-IEEE Chapman SJ, (2004), Electric Machinery Fundamentals, 4th Edition, New York: McGraw-Hill Professional Chen WK, (2005), The Electrical Engineering Handbook, New York: Elsevier & Academic Press
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