Power Generation Introduction

Structure of Electrical Power System

The flow of electrical power from the generating station to the consumer is called an electrical power system or electrical supply system. It consists of the following important components :

1. Generation station

2. Transmission network

3. Distribution network

     
All these important networks are connected with the help of conductors and various step up and step down transformers. A typical transmission and distribution scheme is shown in the Fig. 1.

Fig. 1  Schematic representation of a typical transmission distribution scheme

       A scheme shows a generating station which is located too far away from cities and towns. It is generating an electrical power at 11 KV. It is required to increase this level for the transmission purpose. Hence a step up transformer is used which steps up the voltage level to 220 KV. This level may be 132 KV, 220 KV or more as per the requirement.

       Then with the help of transmission lines and the towers, the power is transmitted at very long distances. Design of the transmission lines is based on the factors like transmission voltage levels, constants like resistance, reactance of the lines, line performance, interference with the neighbouring circuits etc. Its mechanical features are strength of the supports, sag calculations, tension etc. Transmission of power by the overhead lines is very much cheaper. Similarly the repairs also can be carried out comparatively more easily. The transmission is generally along with additional lines in parallel. These lines are called duplicate lines. Thus two sets of three phase lines work in parallel. This ensures the continuity during maintenance and also can be used to satisfy future demand. The power is then transmitted to the receiving station via step down transformer. This transformer is 220/33 KV or 220/22 KV transformer.

       The power is then transmitted to the substations. A substation consists of a step down transformer of rating 33 KV to 6.6. KV or 3.3 KV. The transfer of power from receiving station to the substation is with the help of conductors called feeders. This is called secondary transmission.

       From the substations, power is distributed to the local distribution centres with the help of distributors. Sometimes for bulk loads like factories and industries, the distributors transfer power directly. For the light loads, there are distribution centres consisting of distribution transformers which step down the voltage level to 230 V or 400 V. This is called primary distribution. In the crowded area like cities, overhead system of bar conductor is not practicable. In such cases insulated conductors are used in the form of underground cables, to give supply to the consumers. These cables are called service mains. This is called secondary distribution.

Fig. 2  Line diagram of a typical transmission distribution scheme

       This is the complete flow of an electrical power from the generation station to the consumers premises.

       Let us study the line diagram of such a typical scheme of transmission and distribution and discuss the various components and voltage levels at the various stages in details. The Fig. 2 shows the line diagram of a typical transmission and distribution scheme.

       At the generating station, an electrical power is generated with the help of three phase alternators running in parallel. In the scheme shown, the voltage level is 11 KV but the voltage level may be 6.6 KV, 22 KV or 33 KV depending upon the capacity of the generating station. After the generating station, actual transmission and distribution starts. The overall scheme can be divided into four sections which are,

1. Primary transmission : It is basically with the help of overhead transmission lines. For the economic aspects, the voltage level is increased to 132 KV, 22 KV or more, with the help of step up transformer. Hence this transmission is also called high voltage transmission. The primary transmission. The primary transmission uses 3 phase 3 wire system.

2. Secondary transmission : The primary transmission line continues via transmission towers till the receiving stations. At the receiving stations, the voltage level is reduced to 22 KV or 33 KV using the step down transformer. There can be more than one receiving stations. Then at reduced voltage level of 22 KV or 33 KV, the power is then transmitted to various substations using overhead 3 phase 3 wire system. This is secondary transmission. The conductors used for the secondary transmission are called feeders.

3. Primary distribution : At the substation the voltage level is reduced to 6.6 KV, 3.3 KV or 11 KV with the help of step down transformers. It uses three phase three wire underground system. And the power is further transmitted to the local distribution. For the large consumers like factories and industries, the power is directly transmitted to such loads from a substation. Such big loads have their own substations.

4. Secondary distribution : At the local distribution centres, there are step down distribution transformers. The voltage level of 6.6 KV, 11 KV is further reduced to 400 V using distribution transformers. Sometimes it may be reduced to 230 V. The power is then transmitted using distributors and service mains to the consumers. This is secondary distribution, also called low voltage distribution. This uses 3 phase 3 wire system. The voltage between any two lines is 400 V while the voltage between any of the three lines and a neutral is 230 V. The single phase lighting loads are supplied using a line and neutral while loads like motors are supplied using three phase lines.

1.1 Components of Distribution

       The distribution scheme consists of following important components.

1. Substation : Transmission lines bring the power upto the substations at a voltage level of 22 KV or 33 KV. At the substation the level is reduced to 3.3 KV or 6.6 KV. Then using feeders, the power is given to local distribution centres.

2. Local distribution station : It consists of distribution transformer which steps down the voltage level from 3.3 KV, 6.6 KV to 400 V or 230 V. Then it is distributed further using distribution substation.

3. Feeders : These are the conductors which are of large current carrying capacitor. The feeders connect the substation top the area where power is to be finally distributed to the consumers. No tappings are taken from the feeders. The feeder current always remains constant. The voltage drop along the feeder is compensated by compounding the generators.

4. Distributors : These are the conductors used to transfer power from distribution centre to the consumers. From the distributors, the tappings are taken for the supply to the consumers. The voltage drop along the distributors is the main criterion to design the distributors.

5. Service mains : These are the small cables between the distributors and the actual consumers premises.

       The interconnection of feeders, distributors and service mains is shown in the Fig. 3.

Fig. 3

       There is no tapping on feeders. PQ, QR, RS and PS are the distributors which are supplied by the feeders. No consumer is directly connected to the feeder. The service mains are used to supply the consumers from the distributors. Tappings are taken from the distributors.

Single Line Diagram of Power System

The different elements of power systems are always required to be connected with each other so that the complete power system can be modelled. The supply system is normally three phase balanced system so while representing the system on of the three lines is shown with return through neutral. The representation of all three lines with neutral return is rarely required in practice. The representation of all power system elements along with their interconnection is done with the help of single line diagram of the system.

       The single line diagram can still be simplified by excluding the completed circuit through the neutral and the components are represented by their standard symbols rather than by their equivalent circuits. The circuit parameters are not shown while the transmission line is indicated by a single line between its two ends.

Note : Thus the single line diagram is nothing but the simplified representation of power system components with each other, with each component represented by its symbol.

       The standard symbols used for drawing single line diagram of a power system are shown below.

       The significant information about the power system can be obtained in concise from the single line diagram. The various features of the system are different based on the problem that is considered. The information to be induced in the single line diagram is dependent on the purpose for which the diagram is drawn.

       The location of circuit breakers and relays is not important when one undergoes load flow study of the system. In such case they are not shown on the single line diagram and the intension of single line diagram in such case to study the load flow.

       But if it is required to determine the stability of power system under transient conditions during fault then the role of relays and circuit breakers is vital. This is because the stability under transient conditions due to fault is dependent on speed with which relays and circuit breakers operate to isolate the faulty part. Under such case, information related to circuit breakers must be included in the single line diagram as it is important.

       The single line diagram may also include also the information about current and potential transformers which are either connected to the relays or are used for metering purpose. Hence it can be seen that the information obtained from single line diagram varies with its purpose and problem. It is also dependent upon the standard practice of the company which is preparing these single line diagrams.

       The various organisations such as ANSI (American National Standards Institute). IEEE (Institute of Electrical and Electronics Engineers) have published standard symbols for drawing electrical diagrams. It is expressed that these symbols must be used in all the diagrams everywhere in order to have simplicity, clarity and uniformity. If someone is not consistent with these diagrams and symbols then it is better to represent the machine with its basic symbol followed by information on its type and rating.

       The point at which the system is connected to ground is important as it helps in determining the amount of current flowing when an unsymmetrical fault occurs on the system involving ground. The current to the ground is limited by connecting either a resistance or reactors between the neutral of  Y and ground. The correct symbol for resistance or inductance should be added in the diagram when Y grounded connected transformer is shown. Most transformer neutrals in transmission system are solidly grounded. The generators neutrals are normally grounded through comparatively higher resistances or sometimes through inductance coils.

       The Fig. 1 shows a single line diagram of a simple power system.

Fig. 2   Single line diagram

       One generator grounded through reactor is connected to bus and through a transformer to the transmission line while two generators, one grounded through resistor and one grounded through reactor is connected to bus through a transformer to a transmission line at the other end. A load is connected on each of the bus. The ratings of the generators, transformers loads and reactances of different components of the circuit can also be represented on this diagram.

       The only limitation of single line diagram is it can not represent the conditions during unbalanced operation of a power system. Under the unbalanced operation of a power system, all three phases are to be shown for currents and voltages and single line diagram proves to be insufficient.

Elements of Power System

The power system is comprised of various elements such as generator, transformer, transmission lines, bus bars circuit breakers, isolators etc. Now we will discuss in brief about these elements.

1.1 Generators

       The generator or alternator is the important element of power system. It is of a synchronous type and is riven by turbine thus converting mechanical energy into electrical energy. The two main parts of generator are stator and rotor. The stationary part is called stator or armature consisting of conductors embedded in the slots. The conductors carry current when load is supplied on the generator. The rotating part or rotor is mounted on the shaft and rotates inside the stator. The winding on rotor is called field winding. The field winding is excited by d.c. current. This current produces high m.m.f. The armature conductors react with the m.m.f. produced by the field winding and e.m.f. gets induced in the armature winding. The armature conductors carry current when the load is connected to an alternator. This current produces its own m.m.f. This m.m.f. interacts with the m.m.f. produced by the field winding to generate an electromagnetic torque between stator and rotor.

      The d.c. current required for field winding is supplied through exciter which is nothing but a generator mounted on the same shaft on which alternator is mount. The separate d.c. source may also be used sometimes to excite the field windings through brushes bearing on slip rings.

       The generators are driven by prime mover which is normally a steam or hydraulic turbine. The electromagnetic torque developed in the generator while delivering power opposes the torque provided by the prime mover.

       With properly designed rotor and proper distribution of stator windings around the armature, it is possible to get pure sinusoidal voltage from the generator. This voltage is called no load generated voltage or generated voltage. The representation of generator is shown in the Fig.1.

Fig. 1  Representation of alternator

1.2 Transformers

       For stepping up or down the system voltage, power transformer are used in the substations. At generating end, the voltage is only stepped up for transmission of power while at all the subsequent substations the voltage is gradually stepped down to reach finally to working voltage level.

       Instead of using a bank of 3 single phase transformers, a single three phase transformer is used nowadays. The advantage of using this transformer is the easiness in its installation and only one three phase load tap changing mechanism can be used.

       Generally naturally cooled, oil immersed, two winding, three phase transformers upto the rating of 10 MVA are installed upon lengths of rails fixed on concrete slabs having foundations 1 to 1.5 m deep. For more than 10 MVA ratings, forced oil, water cooling and air blast cooling type may be used. The tap changers are used for regulating the voltage of transformers.

1.3 Transmission Line

       The Transmission line forms the connecting link between the generation stations and the distribution systems. It carries the power generated by generating stations and makes it available for distribution through distribution network.

       Any electrical transmission line has four major parameters which are important from the point of view of its proper operation. These parameters are namely resistance, inductance, capacitance and conductance.

       The resistance and inductance is uniformly distributed along the line. It forms series impedance. The resistance of a line is responsible for power loss. It is expected that the resistance of a line should be as low as possible so that the transmission system will be more efficient. Due to flux linkage, the conductor is associated with inductance which is distributed along the length of the line. For analysis, both resistance and inductance are assumed to be lumped.

       The capacitance also exists between the conductors and is the change on the conductors per unit of potential difference between them. The conductance between conductors or between conductors and the ground due to leakage current at the insulators of overhead lines and through the insulation of cables. The leakage at conductors is negligible so the conductance between conductors of an overhead line is taken as zero. The conductance and capacitance between conductors of a single phase line or from conductor to neutral of a three phase line from the shunt admittance.

        Depending upon the length of the transmission line it is classified as short transmission line, medium transmission line and long transmission line. For short line, its length is small so capacitance effects are small and are neglected.

1.4 Bus Bars

       Bus bars are the common electrical component that connect electrically number of lines which are operating at the same voltage directly. These bars are of either copper or aluminium generally of rectangular cross-section. The can be of other shapes such as round tubes, round solid bars or square tubes.

       The outdoor bus bars are of two types viz the rigid type or strain type.

       In the rigid type of the bus bars, pipes are used. The pipes are also used for making connecting among different components. The pedestal insulators support the bus bars and the connections. The equipments and bus bars are spread out and requires large space. The clearance remain constant as the bus bars are rigid.

       It has following advantages.

1) The maintenance is easy as bus bars and connections are not very high from ground.

2) As pipe diameter is large, the corona loss is less.

3) Reliability is more than strain type.

      Following are its limitations.

1) Large area is required.

2) It requires comparatively high cost.

       In strain type, bus bars are an overhead system of wires between two supporting structure and supported by strain type insulators. As per the size of the conductors, the stringing tension can be limited (500 - 900 kg).

       The advantage of this type is its economy and it is recommended presently due to general shortage of aluminium pipes.

       The material used in case of rigid type bus bars is aluminium pipes. The general sizes of pipes commonly used for voltages are as given below.

                              33 KV                            40 mm

                              66 KV                            65 mm

                              132 KV                          80 mm

                              220 KV                          80 mm

                              400 KV                         100 mm

       Due to rapid oxidization of aluminium, proper care must be taken while doing connections. In order to avoid strain of supporting insulators due to thermal expansion or contraction of pipe, joints should be provided.

        In case of strain type arrangement, material used is ACSR (Aluminium Conductors with Steel Reinforcement) and all aluminium conductors. For high rating of bus bars bundled conductors are used. The commonly used sizes are as below.

                      66 KV            37/2.79 mm                  ACSR

                      132 KV          37/4.27 mm                  ACSR

                       220 KV         61/3.99 mm                  ACSR

                       400 KV         61/4.27 mm                  ACSR in duplex

1.5 Circuit Breaker

       The circuit breakers are used to open or close a circuit under normal and faulty conditions. It can be designed in such a way that it can be manually operated or by remote control under normal conditions and automatically operated during fault. For automatic operation, relay circuit is used.

       The circuit breakers are essential as isolators can not be used to open a circuit under normal conditions as it has no provision to quench arc that is produced after opening the line. It has to perform following functions.

i) Full load current is to be carried continuously.

ii) Opening and closing the circuit on no load.

iii) Making and breaking the normal operating current.

iv) Making and breaking the fault currents of magnitude upto which it is designed for.

       Upto 66 KV voltages, bulk oil circuit breakers are used. Voltages greater than 66 KV, low oil circuit breakers are used. For still high voltages, air blast, vacuum or SF6 circuit breakers are used.

1.6 Isolators 

      In order to disconnect a part of the power system for maintenance and repair purposes, isolating switches are used. These are operated after switching off the load by means of a circuit breaker. The isolators are connected on both sides of circuit breakers. Thus to open isolators, circuit breakers are to be opened first.

       An isolator is essentially a knife switch and is designed to open a circuit under no load that is lines in which they are connected should not be carrying any current.

       Use of isolators in a substation is shown in the Fig. 1.

Fig.1 Line diagram of substation with use of isolating switches

       As shown in the Fig.1, there are 5 sections. With the help of isolators, each section can be disconnected for repair and maintenance. If it is required to do maintenance in section 4, then the circuit breaker in that section is to be opened first and then open the isolators 3 and 4. Thus section 4 is open for maintenance. After maintenance, the isolators 3 and 4 are to be closed first and then circuit breaker is closed.

       In some cases, isolators are used as circuit breaking devices. But is limited by particular conditions such as power rating of given circuit. The isolators are of two types viz single pole and three pole isolators.

Source of Electric Energy

The various energy sources are classified into two main groups.

a) Non-conventional or renewable energy sources.

b) Conventional or non-renewable energy sources.

1.1 Non-Conventional or Renewable Energy Sources

       These energy sources are available abundantly in nature and can be reused again.

        The various non-conventional energy sources are as follows.

 i) Solar energy                             ii) Wind energy

 iii) Hydraulic energy                     iv) Tidal energy

 v) Wave energy                           vi) Geothermal energy

 vii) Ocean thermal energy             viii) Biogas energy

 ix) Biomass energy                        x) Fuel cells

1.1.1 Advantage of Non-conventional energy sources 

       The leading advantage of non-conventional energy sources are

1) They are abundantly available in nature.

2) They do not pollute the atmosphere.

3) They are available in large quantities.

4) They are well suited for decentralized use.

5) The plants using these sources have very less (theoretically no) maintenance cost.

1.1.2 Disadvantages of Non-conventional energy sources

       The Disadvantage of non-conventional energy sources are

1) They are available at very low intensities are

2) These sources are available in nature during particular periods which is uncertain.

3) Less efficiency of the power plants.

4) High initial cost.

1.2 Conventional or Non-renewable Energy Sources

       These are the energy sources which once used can not be recovered any more. They are depleting in nature.

       The various non-renewable energy sources are

i) Thermal energy from a) Coal coke    b) Petroleum products like petrol, Diesel, Kerosene etc. c) Natural gas.

ii) Nuclear Energy.

1.2.1 Advantages of Conventional Energy Sources

Following are the advantages of conventional energy sources.

1) Their efficiency is more.

2) Their initial cost is comparatively less.

3) Their intensities are high.

1.2.2 Disadvantages of Conventional Energy Sources

       The disadvantages of Conventional energy sources are

1) Their running and maintenance cost is high.

2) They are depleting in nature.

3) They cause pollution to atmosphere by different means.

Comparison between renewable and non-renewable energy sources