Chapter 1: Basics of Power Systems

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 Basic Structure of Power System
Power System is a network of high tension wires/cables by which Electrical
power transmitted and distributed throughout a region.
Power System consists of the following main components:
Generation System- Energy Conversion Methods
o Switchgear – Step-up transformer in the Generation station
Transmission System- Ultra-high, Extra-high, High and Medium Voltage
levels
o Substation-Step up or step down Transformer or switching substation
Distribution System- Low voltage levels
The Load or Energy sink- Resistive, Capacitive or inductive Electrical
devices
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 Single line diagram of a Power System Structure

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 Brief description of the main Power System Components:
1. Generation System

Types of Energy Resources:

•Oil, Natural Gas, Coal, Atomic energy (Fossil fuel resources)

•Solar, Hydro, wind, hot water/spring, Biomass (Renewable energy

Resources)

Types of Energy Conversion Methods/Generation Systems:

•Diesel Generator, Gas Turbine, Steam Turbine, Combined Cycle Gas

Turbine (CCGT), Steam Injected Gas Turbine, Nuclear power (Conventional
systems)

•PV System, Solar Thermal, Hydropower, Wind Power, Geothermal,

Biomass (Renewable conversion systems)
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 Cont....

Electrical power can be generated from several sources of energy as given

below:

By Conversion of Chemical Energy in Coal, oil, Peat, gas or other conventional

fuels into heat by burning and then into mechanical energy by hot gas or steam
rising and using it in a Gas or steam turbine respectively. The Gas turbine or
steam turbine drives a generator which inturn produce electrical energy.

By Conversion of Potential Energy in water stored in elevated reservoirs to

kinetic energy and then to mechanical and electrical energy using hydraulic
turbines and generators.

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By Conversion of Kinetic Energy in wind using wind turbines and Generators.
 Cont...
By Conversion of Solar Energy directly in to electrical energy using solar cells
and/or heat by solar radiation and then into mechanical energy by steam
turbines which derives a generator to produce electrical power.

By Conversion of Nuclear Energy via heat and steam into electrical power

By Conversion of Geothermal energy in the Earth using steam turbines to

derive generators which inturn generate Electrical power.

By the Use of Electrolytic Cells in which chemical energy can be converted

quickly into Electrical energy.
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 Cont...
Ethiopian Power System Overview

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 Cont….

Existing System in Ethiopia

Generation [Source: EEP 2017]
• Hydro Power 4,068 MW
• Diesel Power 112 MW
• Geothermal Power 7 MW
• Wind Power 324 MW
• Co-generation thermal 167 MW
Total 4678 MW

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 2. Transmission Systems

This component of the power system transmits bulk electrical energy from
generation stations where it is produced to the main load centres. The
transmission system is composed of:
• Step-up and Step-down substations
• Transmission lines
Existing Transmission Capacity in Ethiopia[EEP 2014]

• 400 kV 686.7 km
• 230 kV 4222.95 km
• 132 kV 5033 km
• 66 kV 2234 km
• 45 kV 476 km
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Total 12652.65 km
 3. Distribution Systems

The distribution system gives out the energy from the medium voltage
substations to customers’ location. It is composed of:
• Medium Voltage (MV) lines (33kV, 15 kV)
• MV/LV transformer substations (33/15/ kV to 400/380/220 V)
• Distribution lines (33/15 kV, 380 V 3-phase and 220 v single
phase)

4. Load or Energy Sink:

Load is the end equipment of the power system where the transmitted
electrical energy is converted to other forms of useful energy.

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Power Systems

• Power Production:

 Different Types:

 Traditional

 Renewable

 Capacity, Cost, Carbon Emission

 Step‐up Transformers

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Power Systems

• Power Transmission:

 High Voltage (HV) TransmissionLines

 Several Hundred Miles

 Switching Stations

 Transformers

 Circuit Breakers

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Power Systems

• The Power Transmission Grid in the United States:

Ethiopia Future tie line connection to other control
areas which then require highly secured system.

www.geni.org

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Power Systems

• Power Distribution:

 Medium Voltage (MV) Transmission Lines (< 50 kV)

 Power Deliver to LoadLocations

 Interface with Consumers /Metering

 Distribution Sub‐stations

 Step‐Down Transformers

 Distribution Transformers

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Power Systems

• Power Consumption:

 Industrial

 Commercial

 Residential

 Demand Response

 Controllable Load

 Non‐Controllable

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Power Systems

2/20/2023 Generation Transmission Distribution Load

Texas Tech University 16
Power Systems

• Power System Control:

 Data Collection: Sensors, PMUs, etc.

 Decision Making: Controllers

 Actuators: Circuit Breakers, etc.

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 AC and DC transmission
The design of the transmission addition must take into account:

 Power to be transferred
 Length of line
 Nature of Compensation required
 Protection against disturbances such as fault and over-voltage
 Line conductor and insulation design
 Insulation and conductor hardware selection
 Line tower and mechanical design
 Environmental constraints to be met
 Corona and losses
 Equipment, installation, and maintenance costs
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 Cont…

 Electric power can be transmitted either by :

AC transmission system (i.e. the voltage and current are alternating) or
 DC transmission system (i.e. the voltage and current are direct or
unidirectional)

AC Transmission System
 The AC transmission system is the one in which the alternating current is
employed for the transmission of electric power. Nowadays, electric power is
almost generated, transmitted and distributed in the form of AC supply.

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 Cont…
Advantages of AC Transmission
The AC transmission system has the following primary advantages :
 In an AC system, the electric power can be generated at high voltages (such
as 3.3 kV, 6.6 kV, 11 kV).
The AC voltage can be increased with the help of a step-up transformer or can
be decreased using the step-down transformer easily and efficiently. Therefore,
the AC transmission permits to transmit the electric power at high voltages and
to distribute it at lower voltages.
 The repair and maintenance of AC substation and transmission lines is easy
and less expensive.
The AC switchgears such as circuit breakers are cheaper than DC switchgear.
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 Cont…
Disadvantages of AC Transmission
The following are the disadvantages of AC transmission system:
 The construction of AC transmission lines is more complicated than the DC
transmission lines.
 AC transmission lines require more conductor material than the DC
transmission lines as three wire are required for AC transmission.
 The effective resistance of the AC transmission line is higher than DC
transmission line. It is because the skin effect takes place in AC transmission
line.
 An AC transmission line has line capacitance. Therefore, there is a continuous
power loss in the AC transmission line due to line charging current even when
the line is open.
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 Cont…

Advantages of DC Transmission
The high voltage DC transmission system has the following advantages:
 DC transmission requires less conductor material than AC transmission as
only two wire are required for the power transmission through DC system.
 DC transmission lines are free from the skin effect. Therefore, the entire
cross-section of the line conductor is utilized, hence the effect resistance of the
line is small.
 There is no capacitance in the DC transmission. Therefore, there is no power
loss due to the charging current.
 There is no inductance, phase displacement, and surge problems in the DC
transmission.
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 Cont…

 For the same sending end voltage and load conditions, the voltage drop in
the DC transmission line is less than the AC transmission line. It is because
of the absence of inductance in DC transmission line.
 A DC transmission line has better voltage regulation than an AC
transmission line.
 For the same voltage, A DC transmission system requires less insulation
material because the potential stress on the insulation is less in case of DC
transmission system than that in AC transmission system.
A DC transmission line has less corona loss and reduced interference with
the communication circuits.

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 Cont…

Disadvantages of DC Transmission
The main disadvantages of DC transmission are as follows :
 Electric power cannot be generated at high DC voltage because of
commutation problems.
 DC switchgears have their own limitations and they are also expensive
than the AC one.
 DC voltage cannot be directly step-up or step-down for transmitting the
power at high voltages and for distributing it at low voltages.
It requires extra equipment such rectifier and inverter, etc. which increases
the cost of transmission.

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 Cont…

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 Single-phase and three-phase transmission
 If a voltage and a current are expressed as functions of time, such as

 maximum values are obviously Vmax = 141.4 V and Imax = 7.07 A,

respectively.
 Vertical bars are not needed when the subscript max with V and I is used to
indicate maximum value. The term magnitude refers to root-mean-square (or
rms) values, which equal the maximum values divided by squere of 2.
Thus, for the above expressions for v and i

These
2/20/2023 are the values read by the ordinary types of voltmeters and ammeters.
26

Another name for the rms value is the effective value.

 POWER IN SINGLE-PHASE AC CIRCUITS
 If the voltage and current are expressed by
Van = Vm cos(ωt + θv ) and ian = Imax cos(wt + θi)

The instantaneous power is

P = Vanian = Vmax Imax cos(ωt + θv ) cos(wt + θi)

 By using trigonometric identities

1
cos A cos B  cos(A  B)  cos(A  B)
2
The expression is reduced to
1 1
p(t )  Vm I m cos( v   i )  Vm I m cos(2wt   v   i )
2 2
The average power is 1
T
P (t )   p ( t ) dt
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 Cont….
We know that the average of a sinusoid over its period is zero

average power becomes

1
P V m I m cos(  v   i )
2

 or when rms values of voltage and current are substituted

P = /V//I/cos(θv - θi)

Complex power
The phasor forms of v(t) and i(t)

1 1
= V m I m cos( v   i )  j V m I m sin( v   i )
2 2
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 Cont….

Apparent power

S  V rms I rms

Active or real power

P = V rms I rms cos(  v   i )

Reactive power

Q = V rms I rms sin(  v i)

1. Q = 0 for resistive loads (unity pf).

2. Q < 0 for capacitive loads (leading pf).
3. Q > 0 for inductive loads (lagging pf).
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 Cont….

Power triangle

S
Q


P

Power factor

In a simple series circuit where Z is equal to R + jX

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 POWER IN BALANCED THREE-PHASE CIRCUITS

 For a Y-connected load  If the load is connected in Δ

the total three-phase power is the total three-phase power is

 The total vars are  The total vars are

 The voltamperes of the load are  The voltamperes of the load are
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 Introduction to power transformers
 A transformer is one of the most common devices found in electrical system
that links the circuits which are operating at different voltages .
These are commonly used in applications where there is a need of AC
voltage conversion from one voltage level to another.

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electric transformer
Current transformer
 Also known as CTs, current transformers are devices that measure alternating
current. They are widely used to measure high magnitude currents.

A current transformer essentially lowers (steps down) a high current to a

lower, safer level that you can manage properly. It steps down the current to be
measured so that you can measure it with an average range ammeter.
 Converting large primary currents into small 1A/5A current

Potential Transformers
 On the other hand, potential transformers, also known as voltage
transformers, measure an aspect of the power supply. While a current
transformer measures current, the potential transformer measures voltage.
 Voltage transformers proportionally convert the high voltage into a standard

secondary
2/20/2023 voltage of 100V or lower for easier utilization of protective33 and
measuring instruments/devices.
Connection
 In the current transformer, the primary winding is connected in series to the
transmission line whose current is to be measured, and full line current flows via
the winding.
On the other hand, the potential transformer is connected in parallel with the
circuit, meaning full line voltage appears across the winding.

 In a current transformer, the primary winding has a smaller number of turns

and carries the current to be measured.
 In potential transformers, the primary winding has many turns and carries the
voltage to be measured.

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 Components of Power Systems.

Generators
 The device converts mechanical energy to electrical energy is called a
generator.
Synchronous machines can produce high power reliably with high
efficiency, and therefore, are widely used as generators in power systems.
A generator serves two basic functions. The first one is to produce active
power (MW), and the second function, frequently forgotten, is to produce
reactive power (Mvar).

A per phase steady-state equivalent circuit of a synchronous generator and the system
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 Cont…
The generator internal voltage can be obtained

Transmission Lines
 The equipment connecting the generated electrical energy from the
generation to the Distribution system is the transmission line.
 A transmission system is a massive interconnected network consists of
mainly AC transmission lines with various high/extra high voltage levels.

 Electrical energy is transported from generating stations to their loads through

overhead lines and cables.
 Overhead transmission lines are used for long distances in open county and
rural areas, while cables are used for underground transmission in urban areas
and for underwater crossings.
 Because the cost for cables is much more expensive than the overhead lines,
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cables are used in special situations where overhead lines can not be used.
 Cont…
The parameters for modeling of overhead transmission lines are:
1. Series (line) Resistance (R) – The resistance of the conductor.
2. Series (line) inductance (L) – The line inductance depends on the partial
flux linkages within the conductor cross-section and external flux
linkages.
3. Shunt capacitance (C) – The potential difference between the conductors
of a transmission line causes the conductors to be charged.
 Then, the series (line) impedance of the transmission line can be
expressed as Z = R + jXL = R + jωL Ω,
and the shunt admittance of the transmission line can be expressed as Y
= jBc = jωC Siemen.

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A pi network for a transmission line model

 Transformers
The main functions of transformers are stepping up voltages from the
lower generation levels to the higher transmission voltage levels and
 stepping down voltages from the higher transmission voltage levels to the
lower distribution voltage levels.

 The main advantage of having higher voltage in transmission system is to

reduce the losses in the grid. The output power of an ideal two winding
transformer equals the input power, S1 = S2 .

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 A representation of an ideal two winding transformer

An equivalent circuit of a two winding transformer.

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The transformer equivalent circuit with impedances referred to primary
 THE SINGLE-LINE OR ONE-LINE DIAGRAM

 Our present interest is in how to portray the assemblage of the

components(transformers, synchronous machines, and transmission lines)
to model a complete system.

 The purpose of the one-line diagram is to supply in concise form of

significant information about the system.

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Single-line diagram of an electrical power system

 Cont…
Apparatus symbols

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 IMPEDANCE AND REACTANCE DIAGRAMS

 In order to calculate the performance or a system under load conditions or

upon the occurrence of a fault, the one- line diagram is used to draw the
single-phase or per-phase equivalent circuit of the system.
 Combines the equivalent circuits for the various components to form the
per-phase impedance diagram of the system.

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The per-phase impedance diagram corresponding to the single -line diagram stated earlier
 Cont….

Per-phase reactance diagram adapted by omitting loads, resistances, and shunt admittances.

The capacitance of the transmission line is omitted

 Resistance is often omitted when making fault calculations, since the
inductive reactance of a system is much larger than its resistance.
Loads which do not involve rotating machinery have little effect on the
total line current during a fault and are usually omitted.
Synchronous motor loads, however, are always included in making fault
calculations
Since the shunt current of a transformer is usually insignificant compared
with the full-load current, the shunt admittance is usually omitted in the43
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equivalent circuit of the transformer.

 Per Unit Values
 In power system calculations, it is very often to normalize actual values, such
as voltages and currents, to per unit values.
 The per unit value is defined as the ratio of the actual value to the selected
base value, namely,

where X can be the power, voltage, current and/or impedance.

 For instance , if a base voltage of 120 kV is chosen, voltages of 108, 120, and
126 kV become 0.90, 1.00, and 1.05 per unit, or 90, 100, and 105 %.
respectively.
 Both the percent and per- unit methods of calculation are simpler : often more
informative than the use of actual amperes, ohms, and volt .
 The per-unit method has an advantage over the percent method because 44the
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product or two quantities expressed in per unit itself,

 Cont…

Usually the base voltage and base power (VA) are given quantities while the
base current and base impedance are to be determined accordingly.

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 Cont….

To change from per-unit impedance on a given base to per-unit impedance

on a new base, the following equation applies:

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