UNIT-I THE POWER SYSTEM-AN OVERVIEW

EE 1352 POWER SYSTEM ANALYSIS


UNIT-I THE POWER SYSTEM-AN OVERVIEW

What is Power system?
A Power system consists of Generation, Transmission and Distribution.
What is power system analysis?
The evaluation of power system is called as power system analysis
What are the functions of power system analysis?
To monitor the voltage at various buses, real and reactive power flow between buses.
To design the circuit breakers.
To plan future expansion of the existing system
To analyze the system under different fault conditions
To study the ability of the system for small and large disturbances (Stability studies)
What are the components of power system?
The components of power system are Generators, Power transformers, Transmission lines, Distribution lines, Loads and compensating devices like shunt, series, and static VAR compensator.
What is modern power system?
A modern power system can be subdivided into four major parts: Generation, Transmission and Sub transmission, Distribution and Loads.
Define per phase analysis.
A balanced three phase system is always analyses on per phase basis by considering one of the three phase lines and neutral.
Draw the per phase model or equivalent circuit model or representation all components of power system?
- Refer Table-1(Modeling of components) -
What is an infinite bus bar?
A large system whose voltage and frequency remain constant, independent of the power exchange between synchronous machine and bus, and independent of the excitation of the synchronous machine.
What is single line diagram?
A single line diagram is diagrammatic representation of power system in which the components are represented by their symbols and interconnection between them are shown by a straight line9eventhough the system is three phase system0.The ratings and the impedances of the components are also marked on the single line diagram.
What is the purpose of using single line diagram?
The purpose of the single line diagram is to supply in concise form of the significant information about the system.


What is impedance diagram? What are the approximations made in impedance diagram?
The impedance diagram is the equivalent circuit of power system in which the various components of power system are represented by their approximate or simplified equivalent circuits. The impedance diagram is used for load flow studies.
Approximation:
(i) The neutral reactances are neglected.
(ii) The shunt branches in equivalent circuit of transformers are neglected.
What is reactance diagram? What are the approximations made in reactance diagram?
The reactance diagram is the simplified equivalent circuit of power system in which the various components of power system are represented by their reactances. The reactance diagram can be obtained from impedance diagram if all the resistive components are neglected. The reactance diagram is used for fault calculations.
Approximation:
(i) The neutral reactances are neglected.
(ii) The shunt branches in equivalent circuit of transformers are neglected.
(iii) The resistances are neglected.
(iv) All static loads are neglected.
(v) The capacitance of transmission lines are neglected.
Define per unit value.
The per unit value of any quantity is defined as the ratio of the actual value of the any quantity to the base value of the same quantity as a decimal.
Per unit value=(Actual value)/(Base value)
What are the advantages of per unit system?
Per unit data representation yields valuable relative magnitude information.
Circuit analysis of systems containing transformers of various transformation ratios is greatly simplified.
The p.u systems are ideal for the computerized analysis and simulation of complex power system problems.
Manufacturers usually specify the impedance values of equivalent in per unit of the equipments rating. If the any data is not available, it is easier to assume its per unit value than its numerical value.
The ohmic values of impedances are refereed to secondary is different from the value as referee to primary. However, if base values are selected properly, the p.u impedance is the same on the two sides of the transformer.
The circuit laws are valid in p.u systems, and the power and voltages equations are simplified since the factors of √3 and 3 are eliminated.
What is the need for base values?
The components or various sections of power system may operate at different voltage and power levels. It will be convenient for analysis of power system if the voltage, power, current and impedance rating of components of power system are expressed with reference to a common value called base value.


Write the equation for per unit impedance if change of base occurs.
Z_(p.u,new)=Z_(p.u,old)×[〖Base KV〗_old/〖Base KV〗_new ]^2×[〖Base MVA〗_new/〖Base MVA〗_old ]

A generator rated at 30MVA, 11KV has a reactance of 20%.Calculate its per unit reactance for a base of 50 MVA and 10KV.
MVA new = 50 ; KV new = 10 ; MVA old = 30 ; KV old = 11
X p.u = 20% = 20/100 = 0.2 p.u
X_(p.u,new)=X_(p.u,old)×[〖Base KV〗_old/〖Base KV〗_new ]^2×[〖Base MVA〗_new/〖Base MVA〗_old ]
X_(p.u,new)=j0.2×[11/10]^2×[50/30]=j0.4033 p.u
What is the new p.u impedance if the new base MVA is twice the old base MVA?
〖 MVA〗_new=2〖 MVA〗_old
Z_(p.u,new)=Z_(p.u,old)×[〖Base KV〗_old/〖Base KV〗_new ]^2×[〖Base MVA〗_new/〖Base MVA〗_old ]
Z_(p.u,new)=Z_(p.u,old)×[〖Base KV〗_old/〖Base KV〗_new ]^2×[(2〖 Base MVA〗_old)/〖Base MVA〗_old ]
Z_(p.u,new)=2 Z_(p.u,old)×[〖Base KV〗_old/〖Base KV〗_new ]^2