uet ibm

1. Electrical Power
Systems – An Overview
Dr. Muhammad Asghar Saqib

2. 2
• The main components of a power system:
Generators, transformers, transmission lines, load, and protection
and control equipment.
•Interconnected power system: generation is generally far away for
load centres.
•Electrical power can be very efficiently transmitted over very large
distances.
•Delivery of good-quality electrical energy at competitive prices is the
objective.
•The measure of the Quality of Electrical Supply in terms of:
Constant voltage magnitude, e.g. no voltage sags (lower than
the rated voltage) or swells (higher voltages)
Constant frequency
Higher power factor (will be explained later)
Balanced phases
Sinusoidal waveforms, e.g. no harmonic content
Lack of interruptions
Ability of withstand faults and recover quickly.
1.1 INTRODUCTION

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1.2 General Composition of a Power Network
• For most practical purposes a power network maybe divided
into Generation, transmission, distribution and utilization (see
Figure).
500 kV Transmission Line
Power Plant
Generation
Residential
Customer
Commercial/
Industrial
Customer
Residential
Customer
Distribution
Pole
Urban
Customers
Primary Distribution Line
132 kV
SubTransmission Line
Distribution Transformer
(11/0.415 kV)
Secondary Grid
(132/11 kV)
Primary Grid
(500/132 kV)
Secondary Distribution
Underground Cable
To Other
132 kV
Substations
Switch Yard/Substation
(11-30 kV/500 kV)

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1.2 General Composition of a Power Network Continued

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1.2.1 Power Generation
•Generation is generally far away from load centres.
•Different primary energy resources to stabilize pricing mechanism.
•Most of the electricity consumed worldwide is generated by three-
phase synchronous generators.
•In future with increased wind power generation asynchronous
induction generators will also be abundant.
•Fuel cells and photovoltaics will also contribute significantly in future.
•A synchronous generator has two parts; stator and rotor.
•Rotor, under steady-state conditions, rotates at synchronous speed.
•Synchronous machines are of two kinds:
 Salient-pole machines
 Round-rotor machines
•Steam turbine driven generators work at high speeds and have round
rotors.
•The rotor carries DC field winding.
•Hydro units work at low speed at have salient-pole rotors.
•Salient-pole rotors have damper windings besides field windings.
•Damper windings consist of bars, put in slots, on pole faces and
connected together at both ends.

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1.2.1 Power Generation Continued
•In general steam turbines contain no damper windings, but solid steel
of rotor offers a path for eddy currents having similar damping effects.
•For simulation purpose currents circulating either in damper windings
or solid steel can be treated as currents circulating in two closed
circuits.
•A synchronous machine can thus be assumed as having three stator
and three rotor windings; magnetically coupled together.
Coupled windings of a synchronous machine
•Relative position of rotor w.r.t. stator phase a is given by angle in the
above diagram.
•In rotor direct (d-) axis is magnetically centred in the north pole.

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1.2.1 Power Generation Continued
•Quadrature (q-) axis is located 90 degrees behind d-axis.
•In general, three main control systems affect the turbine-generator
set:
 The boiler’s firing angle control
 The governor control
 The excitation system control.
Main controls of a generating unit
•The excitation system consists of an exciter and the AVR, automatic
voltage regulator.

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1.2.1 Power Generation Continued
•AVR controls the generator’s terminal voltage by controlling the
current in the field winding supplied by the exciter.
•Measured terminal voltage and desired voltage are compared to
produce error, which is used to change the exciter’s output.
•Exciters are either rotating or static.
•Static ones are preferred these days due to their fast dynamic
response and smaller response.
•Static exciters use thyristor rectifiers to adjust the field current.

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1.2.2 Power Transmission
•Transmission of large blocks of energy at high voltages is more
efficient.
•Transmission voltages in Pakistan are 500 kV and 220 kV.
•132 kV, in Pakistan, is generally called sub-transmission voltage.
 Lines operating at 132 kV are known as sub-transmission lines.
•Step-up transformers in generating substations are used to increase
the voltage to transmission voltages.
•Step-down transformers, in distribution substations, are used to
decrease the voltages to manageable levels such as 11 kV or 66 kV.
•High voltage transmission is mostly AC overhead lines.
•DC overhead transmission lines are increasing being used to
increase the stability of power systems or for inter-connections
between different countries.
•Transmission lines are usually meshed to increase the reliability of
power (see Figure).
•Overhead transmission lines maybe double circuit lines (see Figure).
•Transmission lines used bundled ACSR (aluminium conductor steel
reinforced) conductors.

10. 10
1.2.2 Power Transmission Continued
 Bundled conductors, with increased diameter, reduces inductance
of a transmission line.
 Corona power loss also decreases due to lower potential gradients.
•One or two sky wires are used to provide protection against lightning.
•Underground cables are used in areas where overhead transmission
lines are impractical.

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1.2.2 Power Transmission Continued
Three-core and single-core cables.
Double-circuit transmission line.

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1.2.3 Power Distribution
•Distribution may either be radial or mesh/loop.
•Radial system are simple, less costly but also less reliable.
•Primary distribution in Pakistan is 11 kV: distribution transformers are
11 kV delta/415 V wye (240 V; line to grounded neutral).

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1.2.4 Power Utilization
•Customers of electricity venders maybe classified as industrial,
commercial and domestic.
•In industrialized societies the first group may account for 2/5th of the
total demand.
•Some loads draw constant current from a power system and their
operation is affected by the variation of voltage and frequency.
•Such loads are induction motors, synchronous motors, DC motors.
•Other types of loads are less susceptible to variations in voltage and
frequency and exhibit constant-resistance characteristics.
•Some end-user loads can generate significant harmonic currents into
the system. The examples of such loads are: colour TV sets,
microwave ovens, energy-saving lamps, computer equipment,
industrial variable speed motor drives, battery recharging stations.
• For industrial applications, except for pumped hydro storage there is
little storage in the system.
•The generation must meet the demand and system’s losses.
•Load shedding may be used as last resort.