Generation, Transmission and Distribution of Electric Power an Overview

 2.1 Goals of the lesson

After going through the lesson you shall get a broad idea of the following:

 

1. Different methods of generating electrical power.

 

2. Issues involved in transporting this power to different types of consumers located generally at far off places from the generating stations.

 

3. Necessity of substations to cater power to consumers at various voltage levels.

 

2.2 Introduction

In this lesson a brief idea of a modern power system is outlined. Emphasis is given to create a clear mental picture of a power system to a beginner of the course Electrical Technology. As consumers, we use electricity for various purposes such as:

 

1. Lighting, heating, cooling and other domestic electrical appliances used in home.

 

2. Street lighting, flood lighting of sporting arena, office building lighting, powering PCs etc.

 

3. Irrigating vast agricultural lands using pumps and operating cold storages for various agricultural products.

 

4. Running motors, furnaces of various kinds, in industries.

 

5. Running locomotives (electric trains) of railways.

 

The list above is obviously not exhaustive and could be expanded and categorized in detail further. The point is, without electricity, modern day life will simply come to a stop. In fact, the advancement of a country is measured by the index per capita consumption of electricity - more it is more advanced the country is.

2.3 Basic idea of generation

Prior to the discovery of Faraday's Laws of electromagnetic discussion, electrical power was available from batteries with limited voltage and current levels. Although complicated in construction, D.C generators were developed first to generate power in bulk. However, due to limitation of the D.C machine to generate voltage beyond few hundred volts, it was not economical to transmit large amount of power over a long distance. For a given amount of power, the current magnitude (I = P/V), hence section of the copper conductor will be large. Thus generation, transmission and distribution of d.c power were restricted to area of few

kilometer radius with no interconnections between generating plants. Therefore, area specific generating stations along with its distribution networks had to be used.

2.3.1 Changeover from D.C to A.C

In later half of eighties, in nineteenth century, it was proposed to have a power system with 3-phase, 50 Hz A.C generation, transmission and distribution networks. Once a.c system was adopted, transmission of large power (MW) at higher transmission voltage become a reality by using transformers. Level of voltage could be changed virtually to any other desired level with transformers - which was hitherto impossible with D.C system. Nicola Tesla suggested that constructionally simpler electrical motors (induction motors, without the complexity of commutator segments of D.C motors) operating from 3-phase a.c supply could be manufactured. In fact, his arguments in favor of A.C supply system own the debate on switching over from D.C to A.C system.

2.3.2 A.C generator

A.C power can be generated as a single phase or as a balanced poly-phase system. However, it was found that 3-phase power generation at 50 Hz will be economical and most suitable. Present day three phase generators, used to generate 3-phase power are called alternators (synchronous generators). An alternator has a balanced three phase winding on the stator and called the armature. The three coils are so placed in space that there axes are mutually 120° apart as shown in figure 2.1. From the terminals of the armature, 3-phase power is obtained. Rotor houses a field coil and excited by D.C. The field coil produces flux and electromagnetic poles on the rotor surface. If the rotor is driven by an external agency, the flux linkages with three stator coils becomes sinusoidal function of time and sinusoidal voltage is induced in them. However, the induced voltages in the three coils (or phases) will differ in phase by 120° because the present value of flux linkage with R-phase coil will take place after 120° with Y-phase coil and further 120° after, with B-phase coil. A salient pole alternator has projected poles as shown in figure 2.1(a). It has non uniform air gap and is generally used where speed is low. On the other hand a non salient pole alternator has uniform air gap (figure 2.1(b)) and used when speed is high. Driven at n rps by prime mover (a) Salient pole generatorField coilN S YR B Driven at n rps by prime mover (b) Non salient pole generator Y RBField coilNSFigure 2.1: 3-phase generators.

Frequency, voltage & interconnected system

The frequency of the generated emf for a p polar generator is given by 2pf=n where n is speed of the generator in rps or 120pf=n when n is in rpm. Frequency of the generated voltage is standardized to 50 HZ in our country and several European countries. In USA and Canada it is 60 Hz. The following table gives the rpm at which the generators with different number of poles are to be driven in order to generate 50 Hz voltage.

Number of poles of Generator 2 4 6 8 10
rpm at which generator to be driven	3000	1500	1000	750	600