Structure of Electrical Power Systems

An electric power system or electric grid is known as a large network of power generating
plants, which connected to the consumer loads. The lines network between Generating Station
(Power Station) and consumer of electric power can be divided into two parts. (i)Transmission
System (ii)Distribution System

Electrical energy, after being produced at generating stations (TPS, HPS, NPS, etc.) is
transmitted to the consumers for utilization. This is due to the fact that generating stations are
usually situated away from the load centers.

The main objective of an electric power system is to obtain electrical energy and make it
reachable safely to the load point where it is being used in usable form. This is done in five
stages namely

 Generating Station
 Primary Transmission
 Secondary Transmission or Subtransmission
 Primary Distribution
 Secondary Distribution

Generation means the conversion of a form of energy into electrical energy. Transmission
implies the transport of this energy to very long distance with very high amount of voltage
magnitude. Moreover, distribution is fulfilling the demand of the consumers at certified voltage
level and it is done in terms of feeders.

Generation:

Generation is the part of power system where we convert some form of energy into electrical
energy. The ordinary power plant capacity and generating voltage may be from 2.5 kV to 30kV.
But economically, it is good to step up the produced voltage from 11kV to 132kV, 220kV or
400kV or more by using Step up transformer (power Transformer). The various generating
stations are Thermal power plant, Hydel power plant (Hydro-electric), Nuclear power plant,
Wind power plant and so on.
Primary transmission:

The electric supply (in 132kV, 220 kV, 400kV or greater) is transmitted to load center by three
phase three wire (3 Phase – 3 Wires also known as Delta connection) overhead transmission
system.

Secondary or sub transmission

Area far from the city (outskirts) which have connected with receiving stations by lines is called
secondary transmission. At receiving station, the level of voltage reduced by step-down
transformers up to 66 kV, and electric power is transferred by three phase three wire (3 Phase
– 3 Wires) overhead system to different sub stations.

Primary Distribution

At a substation, the level of secondary transmission voltage of 66 kV reduced to 11kV by step

down transforms.
Generally, electric supply is provided to consumers where the demands is 11 kV and they make
a separate sub station to control and utilize the power in industries and factories. These
substations deliver power to smaller units called ‘Feeders’. This is done by either ‘Overhead
lines’ or ‘Underground cables’.

Secondary distribution:

Electric power is transferred by (from primary distribution line i.e.11kV) to distribution sub
station is known as secondary distribution. This sub station is located near domestic &
consumers areas where the level of voltage reduced to 440V by step down transformers. These
transformers called Distribution transformers, three phase four wire system (3 Phase – 4 Wires
also known as Star connection). So there is 400 Volts (Three Phase Supply System) between
any two phases and 230 Volts (Single Phase Supply) between a neutral and phase (live) wires.
Residential load (i.e. Fans, Lights, and TV etc) may be connected between any one phase and
neutral wires, while three phase load may be connected directly to the three phase lines.

The various components in the transmission and distribution system includes Conductors,
Transformers, Insulators, towers and protective devices.

• Conductors: A conductor is a substance or material that allows electricity to flow

through it. All Aluminium Conductor (AAC), All Aluminium Alloy Conductor
(AAAC) and Aluminium Conductor Steel Reinforced (ACSR) are used as power
transmission and distribution lines.

• Transformers: Step-up transformers are used for stepping up the voltage level and
step-down transformers are used for stepping it down.

• Line insulators: Insulators are the elements of transmission system, which provide
necessary insulation between line conductors and supports and hence, prevent any
leakage current from the conductors to the earth. An insulator gives support to the
overhead line conductors. In the transmission lines, it plays an essential role in its
operation. The designing of an insulator can be done using different materials like
rubber, wood, plastic, mica, etc.

• Support towers: The transmission towers carry high-voltage transmission line to

transport power from the generating station to electrical substations. The electrical
substations transport power to the end users through distribution lines. The distribution
 line uses utility poles to carry the low-voltage conductor. It helps to support the line
conductors suspending in the air overhead.

• Protective devices: to protect the transmission system and to ensure reliable operation.
These include ground wires, lightening arrestors, circuit breakers, relays etc.
OVERHEAD VERSUS UNDERGROUND SYSTEM

 The distribution system can be overhead or underground.

 Overhead lines are generally mounted on wooden, concrete or steel poles which are
arranged to carry distribution transformers in addition to the conductors.
 The underground system uses conduits, cables and manholes under the surface of streets
and sidewalks.
 The choice between overhead and underground system depends upon a number of
widely differing factors. Therefore, it is desirable to make a comparison between the
overhead and underground system.

Public safety.
 The underground system is more safe than overhead system because all distribution
wiring is placed underground and there are little chances of any hazard.
Initial cost.
 The underground system is more expensive due to the high cost of trenching, conduits,
cables, manholes and other special equipment.
 The initial cost of an underground system may be five to ten times than that of an
overhead system.
Flexibility.
 The overhead system is much more flexible than the underground system.
 In the underground system, manholes, duct lines etc., are permanently placed once
installed and the load expansion can only be met by laying new lines.
 In the overhead system, poles, wires, transformers etc., can be easily shifted to meet the
changes in load conditions.
Faults, Fault location and repairs.
 There is a frequent occurrence of fault in the overhead system due to accidents.
 On an overhead system, the conductors are visible and easily accessible so that fault
locations and repairs can be easily made.
 The chances of faults in underground system are very rare as the cables are laid
underground and are generally provided with better insulation.
 If a fault occurs in the underground system, it is difficult to locate and repair on this
system.
Appearance.
The general appearance of an underground system is better as all the distribution lines are
invisible. This factor is exerting considerable public pressure on electric supply companies to
switch over to underground system.
Current carrying capacity and voltage drop.
An overhead distribution conductor has a considerably higher current carrying capacity than
an underground cable conductor of the same material and cross-section. On the other hand,
underground cable conductor has much lower inductive reactance than that of an overhead
conductor because of closer spacing of conductors.
Useful life.

The useful life of underground system is much longer than that of an overhead system. An
overhead system may have a useful life of 25 years, whereas an underground system may have
a useful life of more than 50 years.
Maintenance cost.
The maintenance cost of underground system is very low as compared with that of overhead
system because of less chance of faults and service interruptions from wind, lightning as well
as from traffic hazards.
Interference with communication circuits.
An overhead system causes electromagnetic interference with the telephone lines. The power
line currents are superimposed on speech currents, resulting in the potential of the
communication channel being raised to an undesirable level. However, there is no such
interference with the underground system.

It is clear from the above comparison that each system has its own advantages and
disadvantages. However, comparative economics ( i.e., annual cost of operation) is the most
powerful factor influencing the choice between underground and overhead system. The greater
capital cost of underground system prohibits its use for distribution. But sometimes non-
economic factors ( e.g., general appearance, public safety etc.) exert considerable influence on
choosing underground system. In general, overhead system is adopted for distribution and the
use of underground system is made only where overhead construction is impracticable or
prohibited by local laws.
 Balanced and Unbalanced system
 Three phase balanced system or load and Three phase unbalanced system or load are
the two most commonly used concepts in power system.
 It is not the source that decides if a system is balanced or unbalanced. It is the load
which decides if a system is balanced or unbalanced.
Three phase Balanced System
 Balance load is the load that draws equal current from the supply source. In other words,
when a three-phase load draws an equal current in every phase, it is called a balanced
load, and the circuit is said to be a balanced circuit.
 In 3 phase system, phase voltages or currents are displaced from each other by 120 deg.
This is because, the windings of a generator are placed 120 deg apart from each other.
 Consider a star connected winding as shown in the below figure.

 In an AC circuit, the magnitude of electrical current depends on the applied voltage and
the impedance of the circuit.
 The load that draws the balance phase or line current from the constant voltage source
is called the balanced load.
 As the load on the system is identical, current flowing through each phase is same.
 Also, if your load is perfectly identical on all the three phases, current flowing through
neutral conductor is also zero.
Properties of Balanced System
 Waveform is perfectly sinusoidal i.e. in-terms of magnitude and phase shift of 120 deg
 Current flowing through each phase is identical.
 No current flows through the neutral.
 Power loss is very low or not present.
Balanced system is ideal and existence of which is doubtful. Most of the systems are
unbalanced like our distribution system.
Three phase Unbalanced system
 When there is variation in voltage or impedance or both, then the load draws unbalance
current. In three phase system, if one or more phase current is not equal in magnitude
and phase difference, then the circuit is said to be an unbalance circuit.
 As the load on one phase is increased, it will draw more current than the other two
phases. And this will create imbalance in the system.
 The waveform is of the unbalanced system is disturbed in terms of magnitude and
phase shift.
 The current flowing through each phase is not identical. This will further cause the
power loss in the system.
Properties of Balanced System
 Waveforms are disturbed in terms of magnitude & phase angle.
 Current flowing through phases is not same.
 Neutral in needed.
 Power losses are more.
Effects of Balanced System
 Heating of the 3 phase machines
 Heating will decrease overall life of machine
 This imbalance, will also increase the I2 R losses,
 Also, unbalanced system may cause tripping of variable frequency drives used for
induction motor.
 Types of Tariff
 The rate at which electrical energy is sold/supplied to the consumers is termed as
‘tariff.’
 The main purpose of the tariff is to recover the capital investment and maintaining the
service without any interruption.
 There are 10 different types of tariff is in practice for collecting electricity bill from the
consumer. They are
1. Flat demand tariff
2. Simple tariff
3. Flat tariff
4. Step rate tariff
5. Block rate tariff
6. Two-Part Tariff
7. Maximum demand Tariff
8. Power Factor Tariff
9. Three-Part tariff
10. Off-peak Tariff.

1. Flat demand tariff

 When the consumer is charged based on the connected load, it is called flat demand
tariff.
 Flat demand tariff is initiated for helping poor people (Below Poverty Line – BPL).
 In India, consumers having load up to 0.2kW with the average consumption of 30 units
per month consumer shall come into these types of tariff.
 There is no need to install metering equipment.
 Electricity Bill for Flat demand tariff= connected load in kW x Rate

2. Simple Tariff

 In this type of tariff, a fixed rate is applied for each unit of the energy consumed. It is
also known as a uniform tariff.
 The price per unit (1 kWh) of energy is constant.
 This energy consumed by the consumer is recorded by the energy meters.
 Electricity Bill for Simple tariff= kWh of energy consumed x Rate of kWh
3. Flat rate Tariff

 In this tariff, different types of consumers are charged at different rates of cost per unit
(1kWh) of electrical energy consumed.
 Different consumers are grouped under different categories. Then, each category is
charged money at a fixed rate similar to Simple Tariff.
 The different rates are decided according to the consumers, their loads and load factors.
 For Example: domestic, commercial, industrial are charged in different rates.

4. Step rate Tariff

 Step rate tariff is the most used type of tariff around the world. In India almost 29 states
use step rate tariff. It is also called as slap tariff.
 In this type of tariff, the consumer shall be charged with different prices for a different
level of consumption.
5. Block Tariff

 In this tariff, the first block of the energy consumed (consisting of a fixed number of
units) is charged at a given rate and the succeeding blocks of energy (each with a
predetermined number of units) are charged at progressively reduced rates. The rate per
unit in each block is fixed.
 For example, the first 50 units (1st block) may be charged at 3 rupees per unit; the next
30 units (2nd block) at 2.50 rupees per unit and the next 30 units (3rd block) at 2 rupees
per unit.

 It is also called as a sessional tariff. The power sector offers such tariff when they have
excess power.

6. Two Part Tariff

 In this tariff scheme, the total costs charged to the consumers consist of two
components: fixed charges and running charges. It can be expressed as:

The total electricity charges = electricity bill + Fixed charges (connected load * rate per kW)

7. Maximum demand Tariff

 It is similar to two-part tariff. The electricity bill be will be the sum of the energy bill
and maximum demand charges.
 The consumer will be charged electricity bill along with the maximum demand.
 The total maximum demand will be metered by the maximum demand meter.
 Maximum demand tariff is in practice at the commercial and industrial level consumers.
  This type of tariff is in practice in Karnataka state.

8. Power Factor Tariff

 In this tariff scheme, the power factor of the consumer’s load is also considered.
 The power sector will give the upper and lower limit of the power factor. Upon
exceeding or lowering the said threshold, the consumer will be charged.
 In this, reactive power will be accounted through the reactive power meters. It is also
called as penalty tariff.
 Hence, the total electricity bill amount is equal to the summation of the electricity bill
(kWh) and the kVARh.
 This type of tariff is in practice in Tamil Nadu and Karnataka state.

9. Three Part Tariff

 A three-part tariff is nothing but a charging a consumer based on their consumption,

maximum demand and fixed cost. Hence your electricity bill can be calculated as below
formula,
Electricity Bill = kWh + Fixed charges + Maximum demand

10. Off-Peak Tariff

 The rate of per unit cost varies on each interval.

 This types of the tariff are only in practice at the industry level.
 Example: The rate per unit cost at peak hours 10 to 12 will be very high. Sometimes it
varied from ₹ 6 to 16 INR. At the same time, night hour from 1 AM to 4 AM, the price
will be ₹ 2 to 5. Hence the consumer shall be encouraged to consume power during
night time.
 STAR AND DELTA CONNECTED SYSTEM

A star connection, also known as a wye connection, is a type of electrical connection in which
all of the terminals of a three-phase system are connected to a common point, forming a star
shape. This common point is sometimes referred to as the "star point" or the "neutral point."

In a delta connection, also known as a delta connection, the three terminals of a three-phase
system are connected together in a closed loop, forming a triangle shape. There is no common
or neutral point in a delta connection, and all three phases are connected in series.

Phase voltage: In a star connection, the phase voltage is equal to the voltage between each
winding end and the neutral point. In a delta connection, the phase voltage is equal to the
voltage between any two winding ends.

Line voltage: In a star connection, the line voltage is equal to the phase voltage. In a delta
connection, the line voltage is equal to the phase voltage multiplied by the square root of 3
(√3).

Current: In a star connection, the current in each winding is equal to the phase current. In a
delta connection, the current in each winding is equal to the phase current divided by the square
root of 3 (1/√3).
 TUTORIAL
(i)Three loads, each of resistance 30, are connected in star to a 415 V, 3-phase supply.
Determine(a) the system phase voltage, (b) the phase current and (c) the line current.

A ‘415 V, 3-phase supply’ means that 415 V is the line voltage, VL

(a) For a star connection, VL =√3Vp and IL=IP. Hence phase voltage, Vp = VL/√3
= 415 /√3
= 239.6 V or 240 V
(b) Phase current, Ip = Vp/Rp
= 240/30
=8A
(c) For a star connection, Ip = IL Hence the line current, IL = 8 A

(ii) Three loads, each of resistance 30, are connected in delta to a 415 V, 3-phase supply.
Determine(a) the system phase voltage, (b) the phase current and (c) the line current.

A ‘415 V, 3-phase supply’ means that 415 V is the line voltage, VL

(a) For a delta connection, VL =Vp and IL=√3 IP Hence phase voltage,
Vp = 415 V
(b) Phase current, Ip = Vp/Rp
= 415/30
= 13.8 A
(c) For a delta connection, Ip = √3IL Hence the line current, IL =√3 (13.8) A = 24A
 POWER QUALITY
Power quality is the study of the quality of the voltage or current(quality of sine waves).
As per IEEE power quality is defined as “The concept of powering and grounding sensitive
equipment in a manner that is suitable to the operation of that equipment”. “
1.1 Power Quality Problems
Any power problem manifested in voltage, current or frequency deviations that result in
failure or mal-operation of customer equipment” is called power quality problem. The most
common source of power quality problems are Power electronic devices, arching devices, Load
switching and other natural/ environmental accidents(trees).
The various power quality problems are categorized as
1. Transients
2. Long duration variations
3. Short duration variations
4. Voltage unbalance
5. Waveform distortions
6. Voltage fluctuations
7. Power frequency variations
1.1.1 Transients
Transients are defined as the power quality disturbances that involve destructive high magnitudes
of current and voltage or even both which is undesirable and momentary in nature. It may reach
thousands of volts and amperes even in low voltage systems. However, such phenomena only exist
in a very short duration from less than 50 nanoseconds to as long as 50 milliseconds.
Sources
• Lightning Strikes
• Switching activities
– Opening and closing of disconnects on energized lines
– Capacitor bank switching
– Reclosing operations
– Tap changing on transformers
• Loose connections in the distribution system that results to arcing
• Accidents, human error, animals and bad weather conditions
Effects
• Electronic Equipment
– Equipment will malfunction and produces corrupted results
– Efficiency of electronic devices will be reduced
• Motors
– Transients will make motors run at higher temperatures
– Degrades the insulation of the motor winding resulting to equipment failure.
– Increases the motor’s losses (hysteresis) and its operating temperature
• Lights
– Fluorescent bulb and ballast failure
– Appearance of black rings at the fluorescent tube ends (indicator of transients)
– Premature filament damage leading to failure of the incandescent light.
Classification
• They are classified as
– Impulsive transient
– Oscillatory transient
• An impulsive transient (IEEE 1159) is defined as a sudden change in the steady-state
condition of voltage, current, or both that is unidirectional in polarity (primarily either
positive or negative).
• It is normally a single, very high impulse like lightning. Impulsive transients are
generally described by their rise and decay times.
• The impulsive transient can be further classified into
– Nanosecond impulsive transient
– Microsecond impulsive transient
– Millisecond impulsive transient
• An oscillatory transient is a sudden change in the steady-state condition of voltage,
current, or both, that includes both positive and negative polarity values.
• The Oscillatory transient can be further classified into
– Low frequency oscillatory transient
– Medium frequency oscillatory transient
– High frequency oscillatory transient
1.1.2 Long duration voltage variation
• A Voltage variation is considered to be long duration when the voltage deviations are
exceeded for greater than 1 minute.
• The Long duration voltage variation can be classified into three types
• Overvoltage
• Undervoltage
• Sustained interruption
• Overvoltage: An increase in the RMS AC voltage greater than 110% at power frequency
for duration more than 1 minute
Causes: Switching off a large load or energizing a large capacitor bank.
• Undervoltage: A decrease in rms ac voltage to less than 90% at power frequency for
duration more than 1 minute
Causes: Switching on a large load or Switching off a large capacitor bank.
• Sustained interruption: When voltage is 0 for duration more than 1 minute.
• The overvoltage and undervoltage is not the result of system faults but they are caused by
the load variations on the system.
• Incorrect tap setting on transformers can also cause undervoltage and over voltage
• Effects: Since they are greater than one minute, they stress computers, controllers and
motors and shorten the life of power system equipment's and motors.
1.1.3 Short duration voltage variation
• Short Duration Voltage Variations are defined as the variations in the supply voltage for
durations not exceeding one minute.
• “The variation of RMS voltage for a time greater than 0.5 cycle to one minute/60
seconds” is called short duration variation.
Causes: fault conditions, intermittent loose connections in wiring.
• The Short duration voltage variation can be classified into three types
 – Sag (dip)
– Swell
– Interruption
• Sag/Dip: A sag is a decrease to between 0.1 and 0.9 p.u in rms voltage or current at the
power frequency for durations from 0.5 cycle to 1 min.
Causes: voltage drop due to fault current or starting of large motors.
• Swell: An increase in rms voltage in the range of 1.1 to 1.8 p.u. for duration from 0.5
cycles to 1 minute.
• Interruption: a reduction in the supply voltage, or load current, to a level less than 0.1
p.u for a time not exceeding 1 minute.
• They are further classified into Instantaneous, momentary and Temporary
1.Sag
(a) Instantaneous 0.5 – 30 cycles
(b) Momentary 30 cycles - 3s
(c) Temporary 3s – 1min

2.Swell
(a) Instantaneous 0.5 – 30 cycles
(b) Momentary 30 cycles - 3s
(c) Temporary 3s – 1min

3.Interruption
(a) Momentary 0.5 – 30 cycles
(b) Temporary 30 cycles - 3s

1.1.4 Voltage unbalance

• It is defined as the variation in the amplitudes of three phase voltages relative to one
another.
• It is also defined as the deviation of each phase from the average voltage of all three
phases.
– Most equipment can tolerate voltage unbalance of 2%.
 – Causes: unequal distribution of loads in the distribution system, Large single-
phase loads (induction furnace, traction loads)
– Can cause network problems such as mal-operation of protection relays and
voltage regulation equipment, and also overheat of motor and transformer. It
affects three phase loads (three phase induction machine)
• Voltage unbalance can be estimated as the maximum deviation from the average of the
three-phase voltages divided by the average of the three-phase voltages, expressed in
percent.
Max deviation from Average Voltage
Voltage unbalance = Average Voltage

Example:
• Assume the following phase-to-phase voltage readings of 226, 232, and 235.
• Average Voltage = (226 + 232 + 235) / 3 = 231
• Maximum Deviation from Average Voltage = 231 - 226 = 5 V
• Voltage Unbalance = 5 / 231
• Voltage Unbalance = 0.0216 or 2.16%

1.1.5 Waveform Distortion

• Waveform distortion is defined as a steady-state deviation from an ideal sine wave of
power frequency.
• There are 5 types of waveform distortion
– DC offset
– Harmonics
– Inter-Harmonics
– Notching
– Noise
• DC offset: It is the defined as the presence of DC voltage/current in the AC power
system.
Causes: Asymmetry of power converters
Effects: heating and reduce the life span
• Harmonics: It is the sinusoidal voltages or currents having frequencies that are integer
multiples of the fundamental frequency.
 Sources: Nonlinear devices like computers, VFD, UPS

Harmonic Indices
• The Total Harmonic Distortion (THD) and Total Demand Distortion (TDD) are the
commonly used harmonic indices.
• The Total Harmonic Distortion (THD) describes the deviation of nonlinear waveform
from ideal sine wave.
• The THD is defined as the ratio between the RMS value of the harmonics to the RMS
value of the fundamental.
• The amount of harmonic distortion is measured by a factor called total harmonic
distortion (THD%).
• The Total Demand Distortion (TDD) is defined as the square root of the sum of the
squares of the RMS value of the currents from 2nd harmonic current to the highest
harmonic divided by the peak demand load current and is expressed as a percent.
• The TDD index most often describe the current harmonic distortion level
• The formulas to calculate the voltage THD, Current THD and TDD are as follows
 • Inter-Harmonics: It is the sinusoidal voltages or currents having frequencies that are not
the integer multiples of the fundamental frequency.
Sources: Static frequency converters, cyclo-converters, power line carrier signals.
• Notching: It is a periodic voltage disturbance caused by normal operation of power
electronic devices when current is commutated from one phase to another (two phases of
supply are effectively short-circuited for a short time).
• Noise: It is defined as unwanted electrical signals with broadband spectral content lower
than 200 kHz superimposed upon the power system voltage or current in phase
conductors, or found on neutral conductors or signal lines.
• Electrical noise adds “hash” or mess onto the fundamental sine wave
Causes:
– Power electronic devices
– Arcing equipment
Effects:
– Affects Microcomputer
– Affects Programmable controllers
1.1.6 Voltage fluctuation/flicker
• Rapid changes in voltage within the allowable limits of the nominal voltage, e.g., 0.9 to
1.1 p.u.
Causes: loads that exhibit continuous rapid variations in current magnitude.
• If the impact of the voltage fluctuation on lamps perceived by the human eye then it is
called flicker.
• Flicker: undesirable result of fluctuation
• These two terms are linked together in standards and hence the common term voltage
flicker is used.
1.1.7 Power frequency variation
• It is defined as the “Deviation of power system fundamental frequency from its specified
nominal value” (50Hz to 60Hz).
• Frequency variations that go outside of accepted limits for normal steady-state operation
of the power system can be caused by faults on the bulk power transmission system, a
large block of load being disconnected, or a large source of generation going off-line.
• On modern interconnected power systems, significant frequency variations are rare.
SINGLE PHASE VS THREE PHASE
 AC and DC current

 Alternating current (AC) is an electric current, which periodically reverses direction

and changes its magnitude continuously with time.
 Direct current (DC) flows only in one direction. Pure DC has constant magnitude with
time.

 The voltages in AC system can be raised or lowered with the help of a device called
transformer. In DC system, raising and lowering of voltages is complex.
 As the voltages can be raised, electrical transmission at high voltages is possible.
Now, higher the voltage, lesser is the current flowing through transmission line.
Less the current, lesser are the copper losses and lesser is the conducting material
required. This makes AC transmission always economical and efficient.
 It is possible to build up high AC voltage; high speed AC generators of large
capacities. The construction and cost of such generators are very low. High AC
voltages of about 11 kV can be generated and can be raised upto 220 kV for
transmission purpose at sending end, while can be lowered down at 400 V at
receiving end. This is not possible in case of DC
 A.C. electrical motors are simple in construction, are cheaper and require less
attention from maintenance point of view.
 Whenever it is necessary, AC supply can be easily converted to obtain DC supply.
This is required as DC is very much essential for the applications like cranes,
printing process, battery charging, telephone system, etc. But, such requirement of
DC is very small compared to AC.
 Due to these advantages, AC is used extensively in practice.
 Victims who have experienced the electric shock with DC current say that they are
unable to pull their hand back because DC current flows continuously. Hence, it is
believed that the DC current shock is more dangerous.
 Whereas, in the case of AC current, the person experiencing the electric shock can pull
their hand back as the current goes to zero. Hence, it is believed that the AC current
shock is least dangerous than the DC current.
 An electric shock with an alternating current of 15 to 20 milliamperes can be extremely
painful. However, an electric shock with 100 milliamperes may cause death.