Chapter 1.
Introduction to Electrical Power Systems
(Giới Thiệu Chung Về Hệ Thống Điện Năng)
Electrical energy is produced through an energy conversion process.
The electric power
system is a network of interconnected components which generate electricity by
converting different forms of energy, (potential energy, kinetic energy, or chemical
energy are the most common forms of energy converted) to electrical energy; and
transmit the electrical energy to load centers to be used by the consumer. The production
and transmission of electricity is relatively efficient and inexpensive, although unlike
other forms of energy, electricity is not easily stored and thus must generally be used as it
is being produced.
The electric power system consists of three main subsystems: the generation subsystem,
the transmission subsystem, and the distribution subsystem. Electricity is generated at
the generating station by converting a primary source of energy to electrical energy. The
voltage output of the generators is then stepped-up to appropriate transmission levels
using a step-up transformer. The transmission subsystem then transmits the power close
to the load centers. The voltage is then stepped-down to appropriate levels. The
distribution subsystem then transmits the power close to the customer where the voltage
is stepped-down to appropriate levels for use by a residential, industrial, or commercial
customer. In this chapter, a brief description of the common methods of converting
energy to electric power, and each power subsystem will be discussed.
1.1
Sources of Energy
Electricity is produced by converting energy from one form to electricity.
The process
used may be a direct conversion process, where the energy source is converted directly to
electricity. An example of this is solar photovoltaic cells, which converts the energy
found in solar radiation directly to electricity. An indirect conversion process consists of
converting energy from one form, to an intermediate form, to electricity. Coal-fired
generating plants are an example of this process, as the chemical energy released as heat
by burning the coal is changed to rotating kinetic energy by the steam turbine, and then
the rotating kinetic energy is converted to electricity by the generator. The majority of
the electricity produced today is produced through an indirect energy conversion process.
Major sources of energy for the production of electricity are fossil fuels, hydro energy,
solar radiation, and nuclear energy.
Fossil fuels are coal, petroleum, and natural gas. Fossil fuels are a finite, non-renewable
resource, and are the primary source for the production of electricity. These fuels are
burned to release their chemical energy, which produces heat to power steam turbines.
The steam turbines power rotating electric generators, which turn kinetic energy into
electricity. No energy conversion process converts all the energy present in one form
completely into the new form. Since the production of electricity from fossil fuels
involves several energy conversion steps, the overall efficiency of a fossil fuel power
plant is quite low, somewhere in the range of 40%.
The environmental effects of electricity generation from fossil fuels are also a concern.
Fly-ash is the physical matter left after coal is burned. Since fly-ash is harmful to
humans, it must be disposed in an environmentally safe matter. Combustion of fossil
fuels also produces carbon monoxide, carbon dioxide, sulfur dioxide, and nitrous oxides
gases. These are the "greenhouse gases" that contribute to acid rain and global warming.
Nuclear energy, just like fossil fuels, is a finite, non-renewable, energy source that uses
an indirect conversion process to produce electricity. In the nuclear power plants, the
fission reaction is used to produce electricity. The fission reaction involves the splitting
of the nuclei of a heavy element. The heat output from this reaction is used to power a
steam turbine, which is used to drive a rotating electric generator, just as in a fossil fuel
power plant. The advantage of nuclear fission compared to fossil fuels is the energy
content of fissionable materials. The energy content of uranium is approximately 1010
Btu/kg, which is about one million times the energy content of fossil fuels. The
disadvantage of nuclear fission is the environmental cost. After fission, the nuclear fuel,
the reactor vessel where fission occurs, and the steam pipes are highly radioactive. Also,
plant failures can lead to the release of radioactive steam into the atmosphere.
Solar radiation includes energy used directly as intercepted solar radiation, or indirectly
as wind and hydropower. Solar radiation is a renewable energy source. The average
incident power at the earth's surface is 182 W/m2, which corresponds to a daily average
energy of 4.4 kWh/m2. Direct use of solar power includes active types involving
photovoltaic cells, and passive types using radiation to heat solar collectors. Photovoltaic
cells directly convert sunlight into electricity. Solar collectors are normally incorporated
into a solar thermal system, converting sunlight into heat for various uses, including
space heating, water heating, industrial process steam, and electricity production. At
present, large-scale utilization of solar energy is limited by several factors, including the
cost of solar cells and solar collector-heat exchanger systems, and by the requirement of
an adequate energy storage system to smooth out the daily variation. Since solar cells
generate direct current, inverting equipment is needed to obtain the desired alternating
current for most large-scale operations. Since sunlight is available everywhere, use of
solar radiation for energy production is not site specific. The best photovoltaic cells have
efficiency in the 14% to 17% range. Photovoltaics are attractive from an environmental
aspect, since there is no gases or wastes produced from the energy conversion process,
and the fuel source does not have to be extracted from the ground. However, the most
efficient solar cells use gallium arsenide, which is a toxic material. Solar cells are too
new for an understanding of the disposal requirements and costs involved for worn out
cells.
Wind energy is a form of indirect use of solar radiation. Solar radiation produces wind
by heating the air. During the day, the air over land is heated much faster than air over
water bodies because the land absorbs much less sunlight, and the evaporation is less.
The heated air over land expands, becomes lighter, and rises. The cooler, heavier air over
large water bodies moves in to replace the lighter and warmer air, creating a horizontal
motion of air. During the night, since the land cools faster than water, the cool air moves
seaward to replace the warm air that rises from the surface of the water. Wind energy to
electricity is an indirect energy conversion process, because turbine type wind generators
transform the kinetic energy of the wind into rotary-shaft motion and, in turn, to electrical
energy. Since most wind turbine generators require a sustained wind speed of 20
km/hour, the location of the wind turbine generator is important, as a consistent adequate
wind velocity must be present. This means that bulk wind generators are limited to
specific sites, generally in coastal areas or mountain passes. Wind turbines have a
maximum possible efficiency of 59.3%, with a more common efficiency of around 40%.
The significant environmental problems associated with wind turbines are noise,
aesthetics, and interaction with birds.
Hydropower is also an indirect means of using solar power to produce electricity, since
hydropower uses the stream-flow part of the hydrological cycle. In a hydropower plant,
the potential energy of a mass of water in a reservoir a distance above the stream bed is
converted to kinetic energy by flowing through a hydraulic turbine. The resulting kinetic
energy of the turbine drives an electric generator. Hydropower is available where ever a
suitable site exists having enough stream flow, potential drop, and area. Industrialized
nations contain about 30% of all hydropower potential, and are responsible for about 80%
of all electricity produced from hydro. Asia accounts for 30% of hydropower potential,
and produces only 7% of electricity so produced. Africa accounts for 20% of
hydropower potential, yet produces only 2% of electricity so produced. Hydropower is
attractive because it is a non-polluting renewable resource, but it can be disruptive
environmentally. The dam and reservoir affect the normal ecology of the stream and the
surrounding habitat by altering water use, changing natural water flow cycles of the
stream, and taking up land area for the reservoir. New dams normally requires the
relocation of people and buildings.
Tidal energy uses the tidal flow of oceans to run a hydropower plant to produce
electricity. Basically, a dam encloses a tidal pool. The tidal pool fills during periods of
high tide, then empties during periods of low tide. The water flow into and out of the
pool drives a reversible hydraulic turbine. Since the turbine is reversible, the flow of
water into and out of the bay may be used to produce electricity. Therefore, tidal power
is available twice during each 12h 25min tidal period. The ideal sites for tidal energy
have a large difference in tides.
1.2
Electromechanical Energy Conversion
The most common method for bulk power generation is by rotary generators located in
electric power stations. These generators are electromechanical energy converters, also
known as electric machines. In practice, a mechanical prime mover coupled to the
generator rotates the magnetic field created by the rotor winding, inducing a voltage in
the generator stator windings. These windings supply electrical load on the generator.
Conversely, if a current carrying conductor is placed in a magnetic field, the conductor
experiences a force according to Ampere's law. In general, electric machines are
reversible, and capable of operating both as generators and motors. There are three major
types of rotating electric machines: dc commutator, induction, and synchronous
machines.
Energy
(fossil, hydro,
wind, nuclear,
etc.)
Prime
Mover
Mechanical
Energy
Electric
Generator
Electric
Energy
Figure 1-1 Prime Mover Driving an Electric Generator
The source of mechanical energy for a rotating electric generator is known as the prime
mover. The prime mover is directly coupled to the generator. Energy sources for prime
movers are thermal, hydro, and wind. The prime movers normally are turbines, but some
thermal units use internal-combustion engines. A turbine is mechanical device that is
forced to rotate by the pressure of a gas (such as steam for thermal units or air for wind
units) or fluid (such as water for hydro units).
An electromechanical energy converter converts mechanical energy into electrical
energy, and vice versa. A generator converts energy from mechanical to electrical form,
and modulates in response to an electric signal. A motor converts energy from electrical
to mechanical form, and modulates in response to an electrical signal derived from
mechanical speed. Rotating machines, if lossless, operate on the principle of
electromechanical power equivalence as given by
Pmech=Mωm = vi=Pelectrical
Where: M is mechanical torque (N-m), ωm is mechanical angular velocity (rad/s), v is
instantaneous electrical volts (volts), and i is instantaneous electrical current (amperes).
Electric generators are governed by Faraday's law of electromagnetic induction: an
electromotive force (emf) is induced in a conductor "cutting" magnetic lines of flux.
Specifically if a conductor of length l (m) moves with a velocity u (m/sec) in a uniform
magnetic field B (tesla), such that l, u, and B are mutually perpendicular, then the induced
emf in the conductor is given by E = Blu.
Example Problem
1.1. An ideal energy converter develops 500 N-m of torque while running at 3000 rpm. If
the input voltage is 1000 volts, determine the input current for this generator.
M m  vi
 3000 x 2 
500

M m
60

  157.08 amps

i
1000
v
1.2. Calculate the power output of the ideal energy converter.
M m  Power
 3000 x 2 
Power  500  
  157.08 kW
60


1.3
Power System Load
An electric load (or demand) is the power requirement of any device or equipment that
converts electric energy into light, heat, or mechanical energy. The power system load is
the total of all such loads connected to the system. As such, the load is never constant,
varying daily, weekly, monthly, and yearly as loads are added or subtracted from the
power system. The minimum system load for a given period is called the base load. The
maximum system load for a given period is known as the peak load or peak demand. The
peak demand is usually quite short in duration. The operation of generation plants must
be closely coordinated with the load demands to ensure that enough generation capacity
is on line. On weekdays, the base load generally begins increasing at about 5:00 a.m.,
and hits peak load around 7:00 p.m. Maximum yearly peak loads generally occur during
the summer.
As more people and businesses connect to an electric system, the amount of load on the
system will increase. Load forecasting is performed to ensure that power system
generating capacity will be adequate to meet these future load demands. Power stations
take years to build, so it is necessary to plan well in advance. One important part of load
forecasting is the idea of the load growth rate. This is the estimated rate at which load on
the power system will increase. It is generally based on historical data. The growth rate
of the system load L is mathematically represented by
dL
 aL
dt
where a is the constant of proportionality, also known as the per-unit growth rate. The
solution to this equation is written as
L  L0 e at
where L0 is the value of L at t = 0. At any two values of time, t1 and t2, the ratio of the
corresponding L1 and L2 is
L2
 e a ( t2 t1 )
L1
This equation may be used to determine the time tk such that L2 = kL1 and t2-t1 = tk, given
by
ln k
tk 
a
When talking about the growth rate of a quantity, the term "doubling time" is often used.
The doubling time is the period necessary to double the initial value of load L, given a
constant value of a.
ln 2 0.693
t2 

a
a
Doubling time is used to describe how long it will take, at a constant growth rate, to use
twice what is currently used. For example, assuming a present peak energy demand of
10000 MW, and a 10 percent growth rate in the peak demand, the doubling time for
energy demand is 6.93 years. This means that peak energy demand in about 7 years will
equal 20 GW, or twice the current peak demand. Obviously, setting a steady growth rate
for the use of any quantity is unrealistic, as the growth rate depends on many factors.
1.4
Environmental Impact of Electricity Generation and
Transmission
All energy conversion methods used to produce electricity have some environmental
impact. The impact may have an active effect like the emission of airborne pollutants, or
may have a passive effect like aesthetics or habitat modification. Even methods
considered environmentally friendly, like wind, solar, and hydro, have some impact on
the environment. Not only does the production of electricity have an environmental
impact, but the transmission of electricity, with concerns over electromagnetic fields,
aesthetics, and land use, has an impact as well.
The whole cycle of electricity generation must be considered when looking at the
environmental impact. This includes the production and transportation of fuel for the
conversion process. This is especially true of fossil fuel and nuclear power plants, which
use large quantities of fuel taken from the earth.
Fossil fuel power plants generally have the most widespread effect on the environment,
as the combustion process produces airborne pollutants that spread over a wide area.
Nuclear power plants have the most potentially dangerous effect. An operating accident
at a nuclear station could allow a large release of radioactive particles to occur. Solar,
hydro, and wind power plants generally have a small effect on the environment.
Fossil Fuel Power Plants
Fossil fuel power plants produce environmental problems including land and water use,
air emissions, thermal releases, climatic and visual impacts from cooling towers, solid
waste disposal, ash disposal (for coal), and noise. Due to the need for large amounts of
steam, plants can have a great effect on water use. For example, a typical 500 MW coal
fired power plant uses 25 x 109 litres/GW-year of water, which must be taken from a
water source, and then cooled to return to the water source with as little environmental
effect as possible. The biggest effect fossil fuel plants have overall is the emission of air
pollutants, particularly SOX, NOX, CO, CO2, and hydrocarbons. CO, CO2 and
hydrocarbons are the “greenhouse gases,” possibly responsible for global warming. SOX
and NOX can produce acid when released into the atmosphere, leading to the production
of acid rain. Table 1.1 list approximate amounts of airborne pollutants produced.
Generally, air emissions are controlled by the use of scrubbers and precipitators located at
the plant.
Plant Type
CO
NOX
SO2
CO2
Coal
0.11
3.54
9.26
1090
Oil
0.19
2.02
5.08
781
Gas
0.20
2.32
0.004
490
Table 1.1 Power Plant Emissions (g/kWh)
Hydro
The use of hydropower to produce electricity can have both positive and negative effects
on the environment. At some sites, a dam may help with flood control, flow regulation,
or the reservoir may provide recreational opportunities. At other sites, the dam may have
adverse effects on the hydrological cycle, water quality of the stream, stream ecology,
fish migration, and cause the destruction of landscapes and ecosystems. Building new
high-head dams requires the displacement and compensation of populations. Low-head
dams generally have a benign effect on the environment. Dam failures can lead to
catastrophic floods.
Nuclear Power Plants
Nuclear power plants have one environmental issue no other form of electrical power
plant does. An accident at a nuclear power plant may release large amounts of
radioactive particles, possibly resulting in a direct loss of life, and rendering a large land
area immediately around the plant unlivable. The largest regular environmental impact is
the disposal of the high level nuclear waste contained in spent fuel rods, as this waste
must be stored safely for thousands of years. A long term issue is the decommissioning
of nuclear power plants. Decommissioning is shutting down a nuclear plant after its
operational life is over. At this point the entire reactor vessel becomes a high level
radioactive waste that must be disposed. The current methods of decommissioning a
plant are to completely remove and dispose of all radioactive components, to entomb the
reactor in concrete, or simply to shut the plant down and restrict access until the
radioactivity dies out.
Transmission of bulk electricity from the generating station to the load uses conductors
suspended on large towers, known as transmission lines. Traditionally these lines have
been viewed only as an aesthetic nuisance that could cause communications interference
and be a hazard to low flying aircraft. Today, there are other concerns considered about
the effect of transmission lines on the environment. Greater concern is placed on the
effect of the lines on the natural habitat. The major new issue is the effect of
electromagnetic fields (EMFs) on human health.
1.5
The Generation Subsystem
The electric power system consists of three main subsystems: the generation subsystem,
the transmission subsystem, and the distribution subsystem. Electricity is generated at
the generating station by converting a primary source of energy to electrical energy.
Alternating current synchronous generators are the main source of electrical energy.
These generators convert energy from a primary form to the electrical form. Energy is
input to the generator in the form of mechanical torque from a prime mover or turbine.
The turbine in turn is powered by a moving fluid either steam or water. The current
sources of nearly all the electrical energy distributed come from the conversion of
chemical energy of fossil fuels, kinetic energy of water, and nuclear fission energy.
Some renewable sources of energy like wind, solar, and geothermal are also used. The
synchronous generators typically range in size from 50 kW to over 1300 MW, and
operate at voltage levels from 480 V to 25 kV. The output delivered by the generators is
a balanced set of three phase ac voltages.
Extra-High-Voltage
Transmission Substation
500 kV
Step-up
Transformer
Turbine
T
110 kV

Generator
220 kV
Transmission
Lines
0.4 kV
Transmission
Substation
Distribution
Transformer
Distribution Subsystem (35, 22,10, 6 kV)
Sub-Transmission
Substation
Figure 1.2 Example of a Power System in Vietnam
In fossil fuel plants, the source of heat energy is the combustion of the fossil fuel. When
coal is used, crushed coal is conveyed to a pulverizer where the fuel is ground to a
consistency of face powder. The pulverized coal is then fed to burners where it
undergoes combustion. The thermal energy produced is then used to heat water in a
boiler to produce steam, and stored in the form of internal energy of the steam. The
steam flows into the turbine where this internal energy is released as mechanical energy.
Alternatively, natural gas or petroleum can also be combusted to produce the source of
heat energy.
The turbine is a device used to convert the stored energy of high-pressure and hightemperature steam into rotational energy. The steam fed to the turbine is passed through
a series of stages, each of which consists of stationary blades, and moving blades attached
to a rotor. The stationary blades accelerate the steam to high velocity, and this kinetic
energy is converted into shaft rotation by the moving blades. The turbine then provides
the rotational energy to the synchronous generator.
Combined cycle plants using gas turbines, heat recovery steam generators, and a steam
turbine have the advantage of superior heat rates. Gas turbines are an important part of
the utility system generation mix. The gas turbine has characteristics that make it
attractive both for generation to meet peak loads and for base-load operation.
In a simple cycle combustion turbine, air at atmospheric pressure and temperature is
compressed to raise its temperature and pressure. The pressurized air is partly combusted
with fuel and passed to the turbine. The fuel used could be gaseous, or liquid fuels, or
combinations of different fuels.
The gases exhausting from the combustion turbine are quite hot. In a combined cycle
plant, these gases are used to generate steam for a steam turbine. This system
incorporates some of the features of a conventional fossil-fired unit.
With the advent of energy consciousness, and the drive to improve energy efficiency,
cogeneration has become an important source of energy production. Electricity is
produced coincidetally with thermal energy. Two different schemes are normally used in
co-generation: “topping” and “bottoming” cycles. In the topping system electricity is
first produced ("on top"). Waste heat is used in some industrial process or to provide
heating. In the bottoming system, the heat energy is first used in the process. Waste heat
from the process is used to produce electricity ("on bottom").
In a hydroelectric plant, the electrical energy is derived from falling water. Water is
typically stored by a dam. It is then delivered using pen-stocks or pipes to the hydraulic
turbine, which in turn supplies mechanical energy to the generator. Hydraulic turbines
are of two basic types, impulse and reaction. The impulse turbine is also known as the
Pelton wheel. It is used in high-head plants, where the head (fall of water) is 305 meters
or more. The impulse turbine usually has a horizontal shaft. In the Pelton wheel, kinetic
energy is derived by converting the fall of water into one or more high-velocity jets
located around the periphery of the wheel and directed into spoon-shaped buckets.
Reaction type turbines are of two general types, Francis and propeller. In both types the
water passages are completely filled with water. The energy which drives the wheel is in
both kinetic and "pressure head" form.
The output of the generators is fed to a unit transformer which steps-up the voltage to the
appropriate transmission level.
1.6
The Transmission Subsystem
The electrical power produced at generating stations is transported or moved to areas
close to load locations by the transmission subsystem. Transmission lines are a major
component of this subsystem.
Transmission lines could be underground or overhead. The latter kind are more common.
Transmission line voltages range from 110 kV to 765 kV. Overhead transmission lines
span long distances and use bare conductors. As a result, they must be connected by
insulated mechanical connections to towers or poles which support them. A transmission
line consists of conductors, insulators, mechanical supports, and usually shield wires.
Aluminum conductor, steel reinforced (ACSR), conductors are most commonly used for
high-voltage transmission lines. Compared to other conductor material, they have low
cost and high strength-to-weight ratio. Suspension type insulators are used. The material
most commonly used for insulators is porcelain. New polymer materials are also being
used to make insulators. Shield wires are used to protect the energized conductors from
lightning.
Underground transmission is mainly used in urban areas where right-of-way costs are
high. Underground cables cost on an average, about 8-15 times more than overhead lines
on a $/MVA-km basis. Underground cables also have the disadvantage of poor
accessibility.
Transformers are an important part of the transmission subsystem. The transformer is a
static device which transfers electrical energy from one circuit magnetically coupled with
another. In doing so, they can transform voltage levels in the magnetically coupled
circuits. Different types of transformer are used in the transmission subsystem. Power
transformers are used to step-up, or step-down voltages. On load tap changing
transformers are used to regulate voltages. Auto-transformers are used for most ties
between moderate-voltage and high voltage transmission, high-voltage and extra-high
voltage circuits. Phase shifting transformers are used to regulate the flow of real power.
Switchgear constitutes an essential component of the transmission subsystem. Under
normal operating conditions, it provides the means to perform routine switching
operations, e.g., disconnecting and isolating various equipment for maintenance,
inspection, or replacement, transferring load, isolating regulators, etc. Under abnormal
conditions, switchgear provides the means for automatically isolating parts of the system
in trouble to prevent damage and to localize the problem. The main components of the
switchgear include, circuit breakers, disconnecting switches, fuse, instrument
transformers, buses and connections, supporting insulators, protective and control relays,
and control switches.
A substation houses all equipment involved in the switching or regulating of electricity.
Substations can be large or small. Control can be automatic or manual.
1.7
The Distribution Subsystem
The sub-transmission system designates the circuits, which deliver energy from the
transmission subsystem to the distribution subsystem. Usually the transmission
substations supply the sub-transmission system, but it is still referred to as the subtransmission. Many sub-transmission systems were previously transmission lines. Load
growth and demand for more power resulted in the transmission voltage being too low.
As a result, in Vietnam, voltages from110 kV down to 35, 22, 10, 6 kV are found in subtransmission systems.
The distribution subsystem includes the primary circuits and the distribution substations
that supply them, the distribution transformers, the secondary circuits, including the
components all the way up to the entrance of the customer's premises, and the protective
and control devices. The primary circuit is three-phase and is operated in the 6-35 kV
range. The secondary circuits serve most of the customers at levels of 220 volts singlephase, or 380 volts three-phase.
The various other components used in the distribution subsystem include: distribution
transformers, automatic circuit reclosers, cut-outs (a combination fuse and knife switch
used on power poles), surge arresters, separable connectors, capacitors and voltage
regulators, and metering.
1.8
Power Industry Structure
The traditional structure of the electric power system was a single company that owned
and operated all of the components of the system. This included the generation,
transmission and distribution subsystems along with all communications and control
systems. This structure is changing dramatically since the deregulation of the industry
was implemented in some industrialised countries in the early 1990s. Since this time,
each company has been reorganized to fit the expected structure of the industry within the
very near future. This structure assumes that the companies are segmented into
generation (GENCO), transmission (TRANSCO), distribution (DISTCOS) and
independent system operator (ISO) companies or independent contract administration
(ICA) business units. The GENCOs are responsible for providing (selling) electric
generation according to contracts along with any auxiliary services as required for the
customer or the transmission grid. The TRANSCOs are responsible for the maintenance
of the transmission equipment to enable the transmission grid to transport electricity
according to schedules with auxiliary services as required. The DISTCOs are responsible
for providing (purchasing) electricity along with any auxiliary services to the ultimate
customer(s). The ICA is responsible for matching the schedules (transmission grid
support, schedule control and accounting, control and communication facilities and
personnel) such that the transmission grid can transport the electricity and such that
auxiliary services are available as required. The ISO is responsible for coordination of
the maintenance schedules subject to the contracts and operating restrictions of the
transmission system.