POWER SYSTEM ANALYSIS

BOOK

• The Transmission and Distribution of Electrical Energy 3rd Edition by H.COTTON – H.BARBER

CHAPTER NO # 1

POWER SYSTEM ANALYSIS

THE SUPPLY SYSTEM• Energy is transmitted from generating station to consumer through the

supply system which consist of networks of conductors & associated equipment

• It may be divided into distinct parts, the Transmission Line and the Distribution system

Transmission Line

Primary Transmissi

on

Secondary Transmissi

on

Distribution

System

Primary Distributi

on

Secondary

Distribution

Tertiary Distributi

on

Elements of a transmission line

• Generally in a.c. system there is the change in the voltage where the subdivision takes place, the change being effected by transformation, and therefore there may be several working voltages in the same system

• Voltages have to be standardize• Primary transmission 400 kV and 275 kV

• Secondary transmission 132kV and 66kV

• Primary distribution 33kV

• Secondary distribution 11kV and 6.6kV

• Tertiary distribution 415V between phase, 240V to neutral

• Foe ac working the standard frequency is 50Hz. 60Hz and 50Hz are also used in some countries

• Consumer load divide themselves into secondary or tertiary distribution. Those using more than 500kVA may be supplied from secondary distribution but a bulk of consumer uses less power are supplied from tertiary distribution

• In general conductor -> 1)Feeders 2) Distributors3)Service mains

• Feeders: The conductors which connects the substations or in some cases generating station to the area served by these stations

• Distributors: They are characterized by numerous tapping which are taken from them for the supply to the various consumers

• Service mains: They are the connecting links between the distributors and the consumer terminals

The Difference b/w distributor and Feeder• The essential difference is that the current loading of the feeder is same as

the whole of the length, a distributor has a variable loading due to the large number of individual tapping taken from it

THE ANALYSIS OF POWER SYSTEM• First a picture of the system must be developed in the form of a single line

diagram on which all the relevant circuits and equipment appear.

• Secondly the relevant data for each item of equipment must then be collected and marked on the diagram

• Thirdly then the network is then solved for the various current and voltages making use of the theorems and techniques outlined below.

NETWORK THEOREM• The Reciprocity Theorem: if an emf E acting in any branch p of a network

consisting of linear bilateral impedance results in a current I in a second branch q, then the same emf acting in branch q will result in the flow of the same current I in the branch p. The ratio of the emf in branch p to the resulting current in branch q is called the transfer impedance. It states that Zpq= Zqp

• The Superposition Theorem: If several emfs of the same frequency act in a network composed of linear impedance then the current in any branch is the sum of the currents which would flow if the various emfs acted independently and all the other sources were replaced by their internal impedances

• Thevenin’s Theorem: This states that “any system of the linear impedances with two accessible terminals may be replaced by an emf acting in series with an impedance. The emf is that acting between the terminals when they are unconnected externally, and the impedance is that presented by the system when all the emfs in system is zero, the sources being represented by their internal impedance only.

• Star-delta and Delta-star Transformation:

LOOP AND NODAL EQUATIONS

AIDS TO NETWORK ANALYSIS• Models: These are small-scale replicas of the system in which the voltages and

current are reproduced to a smaller scale and impedances to a larger scale. The results of the investigations performed are converted to actual values of system currents and voltages using the appropriate scale factors.

• Analogues: These are devices whose behavior follows equations which are identical to those applicable to the power system in the particular problem under consideration

• Digital Computers: These are machines which are capable of carrying out large numbers of mathematical operation in very short time.