Unit 1

Mount Zion College of Engineering and
Technology
BE8254 Basics of Electrical and
Instrumentation Engineering
P. Maria Sheeba
AP/ECE

OOBJECTIVE
 To analyze the operation of Three phase electrical circuits
and power measurement.
 To deal with the working principles of Electrical machines.
 To understand the principle of various measuring
instruments.
MZCET- ECE

OUTCOMES
 Concept of three phase power circuits and measurement.
 Comprehend the concepts in electrical generators, motors
and transformers.
 Choose appropriate measuring instruments for given
application.

• 1.1 Three phase power supply
• 1.2 Inter Connection of windings
• 1.3 Balanced and Unbalanced loads
• 1.4 Power Equation
• 1.5 Star Delta Conversion
• 1.6 Three phase power measurement
• 1.7 Transmission and Distribution of electrical energy
• 1.8 Over head Vs Underground system
• 1.9 Protection of power system
• 1.10 Types of Tariff
• 1.11 Power factor improvement
MZCET-ECE
Unit-I AC Circuits and Power Systems

IMPORTANCE OF THREE PHASE SYSTEM
• Uniform power transmission and less vibration of
three phase machines.
– The instantaneous power in a 3φ system can be
constant (not pulsating).
– High power motors prefer a steady torque especially
one created by a rotating magnetic field.

IMPORTANCE OF THREE PHASE SYSTEM
• Three phase system is more economical than the
single phase.
– The amount of wire required for a three phase system
is less than required for an equivalent single phase
system.
– Conductor: Copper, Aluminum, etc

Differences
Single Phase Power Supply
 230 V, 50 Hz
 Not sufficient for producing large amount
of power.
 With single-phase current, the voltage
rises to a peak in one direction of flow,
subsides to zero, reverses, rises to a peak
in the opposite direction, subsides to zero,
and so on.
 Single-phase current requires the use of
one transformer.
Three Phase Power
Supply
 440 V, 50 Hz
 Sufficient for producing large amount of
power.
 There are three separate and distinct
single-phase currents, which are combined
so they can be transmitted over three or
four wires and these rise to a peak in one
direction, subside, reverse, and so on;
however they do not peak at the same
time.
 Three-phase current requires two or three
transformers.

1.1.1 Introduction:
• The generator , motor , transformer or rectifier have only one winding is
called a single phase system
• If the current or voltage follows a phase difference 900
in a two windings,
called two phase systems
• If the phase difference is 1200
between voltages or currents in a three winding,
called as Three phase systems
• In poly-phase systems , there are more than three windings
Advantages of three phase system:
• More efficient than single phase system
• Cost is less
• Size is small . Compared to single phase system
10

Advantages of Three Phase Circuits
• The amount of conductor material is required less for transmitting same power,
over the same distance , under same power loss
• Three phase motors produce uniform torque , where as torque produced by
single motor is pulsating
• Three phase generators not produce the harmonics when they are connected in
parallel
• Three phase motors are self starting whereas single phase motors are not self
starting
MZCET 11

FARADAYS LAW
• Three things must be present in order to
produce electrical current:
a) Magnetic field
b) Conductor
c) Relative motion
• Conductor cuts lines of magnetic flux, a voltage is
induced in the conductor
• Direction and Speed are important

GENERATING A SINGLE PHASE
Motion is parallel to the flux.
No voltage is induced.
N
S

x
N
S
Motion is 45° to flux.
Induced voltage is 0.707 of maximum.

x
N
S
Motion is perpendicular to flux.
Induced voltage is maximum.

x
N
S

N
S
Motion is parallel to flux.

x
N
S
Notice current in the
conductor has reversed.
Induced voltage is
0.707 of maximum.

N
S
x
Motion is perpendicular to flux.
Induced voltage is maximum.

N
S
x

Motion is parallel to flux.
N
S
Ready to produce another cycle.

GENERATOR WORK
• The generator consists of a rotating magnet
(rotor) surrounded by a stationary winding
(stator).
• Three separate windings or coils with terminals a-
a’, b-b’, and c-c’ are physically placed 120° apart
around the stator.

• As the rotor rotates, its magnetic field cuts the
flux from the three coils and induces voltages in
the coils.
• The induced voltage have equal magnitude but
out of phase by 120°.

GENERATION OF THREE-PHASE AC
N
xx
S

THREE-PHASE WAVEFORM
Phase 2 lags phase 1 by 120°. Phase 2 leads phase 3 by 120°.
Phase 3 lags phase 1 by 240°. Phase 1 leads phase 3 by 240°.
Phase 1 Phase 2 Phase 3
120° 120° 120°
240°
120° 120° 120°
240°

Phase 1 Phase 2 Phase 3
GENERATION OF 3φ VOLTAGES
Phase 1 is ready to go positive.
Phase 2 is going more negative.
Phase 3 is going less positive.
N
xx
S

1.1.2 Generation of Three Phase Voltages
MZCET 29

BALANCED 3φ VOLTAGES
• Balanced three phase voltages:
– same magnitude (VM)
– 120° phase shift
( )
( )
( ) ( )°+=°−=
°−=
=
120cos240cos)(
120cos)(
cos)(
tVtVtv
tVtv
tVtv
MMcn
Mbn
Man
ωω
ω
ω

• Vectorially r.m.s values of voltages
induced in three windings are represented
in the diagram
• ER = E ∟0o
v,
• EY = E ∟-120o
v
• EB= E ∟+120o
v MZCET 31

 Three voltages are not in phase, since there is a phase difference
of
 The order of voltage waveform sequences in a polyphase system
is called phase rotation or phase sequence.
 This sequence of phase shifts has a definite order. For clockwise
rotation, the order is 1-2-3 (i.e) RYB (winding 1 peaks first, them
winding 2, then winding 3), which is known as a positive phase
sequence.
MZCET
1.1.3 PHASE SEQUENCE

120

 For anti-clockwise rotation, the
order is 3-2-1 (i.e) RBY (winding 3
peaks first, them winding 2, then
winding 1), which is known as a
negative phase sequence.
 If we’re using a polyphase voltage
source to power resistive loads,
phase rotation will make no
difference at all.
 Whether 1-2-3 or 3-2-1, the
voltage and current magnitudes
will all be the same.

PHASE SEQUENCE
( )
( )°+=
°−=
=
120cos)(
120cos)(
cos)(
tVtv
tVtv
tVtv
Mcn
Mbn
Man
ω
ω
ω
°+∠=
°−∠=
°∠=
120
120
0
Mcn
Mbn
Man
VV
VV
VV
POSITIVE
SEQUENCE
NEGATIVE
SEQUENCE
°−∠=
°+∠=
°∠=
120
120
0
Mcn
Mbn
Man
VV
VV
VV

1.2 INTERCONNECTION OF
WINDINGS
36

THREE PHASE QUANTITIES
QUANTITY SYMBOL
Phase current Iφ
Line current IL
Phase voltage Vφ
Line voltage VL

1.2.1 PHASE VOLTAGES and LINE
VOLTAGES
• Phase voltage is measured between the neutral
and any line: line to neutral voltage
• Line voltage is measured between any two of the
three lines: line to line voltage.

1.2.2 PHASE CURRENTS and LINE
CURRENTS
• Line current (IL) is the current in each line of the
source or load.
• Phase current (Iφ) is the current in each phase
of the source or load.

Interconnection of windings
Three phase connections:
• There are two types of three phase connections
• Star connection (Y)
• Delta connection (Δ)
1.2.3 Star connection (Y):
• In this method of inter-connection, the similar ends, say, “start”
ends of three coils (it could be “finishing” ends also) are joined
together at point ‘N’
40

• The point ‘N’ is known as star point or
neutral point
• If this three-phase voltage is applied across
a balanced symmetrical load, the neutral wire
will be carrying three currents which are exactly
equal in magnitude but are 120o
out of phase
with each other. Hence, their vector sum is zero
IR + IY + IB = 0
Voltages and Currents in Y-Connection:
• The voltage induced in each winding is called the
‘phase’ voltage and current in each winding is
known as ‘phase’ current.
MZCET 41

• The vector diagram for phase voltages and currents in a star connection shows
that
ER = EY = EB = Eph (phase e.m.f)
• Line voltage VRY between line 1 and line 2 is the vector difference of ER and EY.
• Line voltage VYB between line 2 and line 3 is the vector difference of EY and EB.
• Line voltage VBR between line 3 and line 1 is the vector difference of EB and ER.

MZCET 42

• The p.d. between lines 1 and 2 is
VRY = ER - EY (Vector difference)
• VRY is found by compounding ER and EY
reversed and its value is given by the
diagonal of the paral1elogram in figure.
• The angle between ER and EY reversed is 60°.
If ER = EY = EB = Ephthe Phase e.m.f then,
•
MZCET 43
C
O
ph
RY
o
E
V
230cos =
o
phRY EV 30cos2 ××=
phph EE 3
2
3
2 =××=

• Hence, in star connection
It will be noted from figure that
• (a) Line voltages are 120° apart.
• (b) Line voltages are 30° ahead of their respective phase voltages.
• (c) The angle between the line currents and the corresponding line voltages is (30 + )ɸ
with current lagging.
44
ph
BYYB
E
DifferenceVectorEEV
similarly
3
)(
=
−=
ph
RBBR
E
DifferenceVectorEEVand
3
)(
=
−=
L
BRYBRY
Vvoltageline
VVV
,=
==
phL EV 3=

Line Currents and Phase Currents:
• Current in line 1 = IR
• Current in line 2 = IY
• Current in line 3 = IB
Since IR = IY = IB = say,
Iph - the phase current
Line current IL = Iph
Power:
• The total power in the circuit is the sum of the three phase powers. Hence
• Total Power =3 x phase power=
•
45

1.2.4 Delta (Δ) or Mesh Connection:
• Phase sequence is R, Y, B
• R leads Y by 120° and Y leads B by 120°.
• The voltage between lines 1 and 2 as VRY
• The voltage between lines 2 and 3 as VYB
VRY leads VYB by 120
VYB leads VBR by 120°.
• VRY =VYB = VBR = line voltage VL
• Then, it VL = Vph
MZCET 46

Line Currents and Phase Currents:
• Current in line 1 is I1 = IR – IB
• Current in line 2 is I2= IY – IR
• Current in line 3 is I3= IB - IY
• Current in line 1 is found by
compounding IR with IB reversed
and its value is given by the diagonal
of the parallelogram
• The angle between IR and IB reversed (-IB) is 60°.
• If IB = IR = IY = Iph phase current, then current in line 1 is
48

Contd..
Since all line currents are equal in magnitude i.e., I1= I2 = I3= IL
From Vector diagram, it should be noted that
• (a) Line currents are 120o
apart.
• (b) Line currents are 30o
behind the respective phase currents.
• (c) The angle between the line current and the corresponding line voltage is 49

Power:
MZCET 50
φcos3 phph IEP ×=
Lph VEbut =
3
L
ph
II =
φcos
3
3 ×××= L
L
I
VP
φcos3 LL IVP =

1.3 Balanced and unbalanced loads
51

1.3.1 Balanced three phase supply:
• A three phase supply is said to be balanced, when all the
three voltages have the same magnitude but differ in
phase by 120° with respect to one another.
• The three phase supply is said to be unbalanced, even if
one of the above conditions is not satisfied.
MZCET 52
Balanced Supply

MZCET 53
1.3.2 Balanced Load:
• A three phase load is said to be balanced, when the impedances
of all the three phases are exactly the same. Even if one of them
is different from the other, then the three phase load is said to be
unbalanced

• In a three phase balanced load, whether star connected or delta connected, the
magnitudes of the phase currents are the same but differ in phase by 120o
with
respected to one another
1.3.3 Unbalanced Load
• But in an unbalanced load, when a three phase balanced supply is given, the
magnitudes and phases of all the three phase currents will be different.
MZCET 54

1.4 Power Equation
1.4.1 Star Connection
The total power in the circuit is the sum of the three phase powers.
Hence ,
Total Power =3 x phase power=
56

1.4.2 Power for delta connection
MZCET 57
φcos3 phph IEP ×=
Lph VEbut =
3
L
ph
II =
φcos
3
3 ×××= L
L
I
VP
φcos3 LL IVP =

Resistive Circuits (Basics)
 Resistors in parallel:
 Resistors in series:
R1
R2
R3

Current Division in Parallel Circuits
R1
R2
V
I I 2I 1

Voltage Division in Series Circuits
R 1
R 2
V
+ V 1 -
- V 2 +
I

1.5.1 Star -> Delta Conversion
A
B
C
Rab
Rbc
Rca
B
C
Ra
RbRc
A
Star Connection Delta Connection

Star- Delta conversion
Advantages
1. The primary side is star connected. Hence fewer number of
turns are required. This makes the connection economical
2. The neutral available on the primary can be earthed to avoid
distortion.
3. Large unbalanced loads can be handled satisfactory.
63

Star- Delta conversion
• Disadvantages
• The secondary voltage is not in phase with the primary. (30 ⁰
phase difference )
• Hence it is not possible to operate this connection in parallel
with star-star or delta-delta connected transformer.
64

Wye(star) to Delta Transformation:
Consider the following:
••
• • • •
a
bc
a
bc
R a
R bR c
R 1 R 2
R 3
( a ) w y e c o n f i g u r a t i o n ( b ) d e l t a c o n f i g u r a t i o n
a
accbba
c
accbba
b
accbba
R
RRRRRR
R
R
RRRRRR
R
R
RRRRRR
R
++
=
++
=
++
=
3
2
1
321
31
321
32
321
21
RRR
RR
R
RRR
RR
R
RRR
RR
R
c
b
a
++
=
++
=
++
=

Using the following circuit. Find Req.
9 Ω
1 0 Ω 5 Ω
8 Ω 4 Ω
V
+
_
R e q 1 0 Ω
I
a
bc
Convert the delta around a – b – c to a wye.

Continued….
2 Ω
2 Ω4 Ω
4 Ω8 Ω
9 Ω
R e q
It is easy to see that Req = 15 Ω

1.5.2 Delta-star Transformation
BC
Ra
RbRc
AA
BC
Rab
Rbc
Rca
 Delta to Star Transformation:

Delta - Star Transformation
Features
 secondary Phase voltage is 1/√3 times of line voltage
 neutral in secondary can be grounded for 3 phase 4 wire system
 Neutral shifting and 3rd
harmonics are there
 Phase shift of 30⁰ between secondary and primary currents and
voltages
69

1.6 THREE PHASE POWER MEASUREMENT

 Three-phase power is a common method of alternating-
current electric
power generation, transmission, and distribution.
 It is a type of polyphase system and is the most common
method used by electric grids worldwide to transfer power.
 A polyphase system is a means
of distributing alternating-current electrical power.
 An electrical grid is an interconnected network for
delivering electricity from suppliers to consumers.

1.6.1 Measure of power in Three Phase Circuits:
• Wattmeter is the instrument which
is used to measure power in an electrical circuit.
• It consists of (i) a current coil ML’
through which the line current flows
• (ii) a potential coil PV, which is connected
across the circuit.
• The full voltage is applied across the potential coil and it carries a very small
current proportional to the applied voltage.
• Three single phase watt-meters may be connected in each phase
• The algebraic sum of their readings gives the total power consumed by the three
phase circuit.
• It can be proved that only two watt-meters are sufficient to measure power in a
three phase circuit.
MZCET 73

 Various methods are used measurement of three
phase power in three phase circuits on the basis of
number of wattmeter used.
 We have three methods:-
1) Three wattmeter method
2) Two wattmeter method
3) Single wattmeter method

1.6.2 MEASUREMENT OF THREE
PHASE POWER BY THREE
WATTMETER METHOD

 Three Wattmeter method is used to measure
power in a 3 phase, 4 wire system.
 However, this method can also be used in a 3
phase, 3 wire delta connected load, where
power consumed by each load is required to
be determined separately.
 The Three-wattmeter method can be used for
star and delta connected unbalnced loads.

• The connections for Star/Delta
connected loads for measuring
power by Three wattmeter
method is shown below:-
 The pressure coil of all
• the Three wattmeter namely W1,
W2and W3are connected to a
• common terminal known as the
neutral point. The product of the
phase current and line voltage
represents as phase power and is
recorded by individual
wattmeter.
77

 The total power in a Three wattmeter method of
power measurement is given by the algebraic sum
of the readings of Three wattmeter. i.e.
 Where, W1 = V1I1 W2 = V2I2 W3 = V3I3
 Except for 3 phase, 4 wire unbalanced load, 3
phase power can be measured by using only Two
Wattmeter Method.

1.6.3 MEASUREMENT OF THREE
PHASE POWER BY TWO
WATTMETER METHOD

 Two Wattmeter Method can be used to measure
the power in a 3 phase, 3 wire star or delta
connected balanced or unbalanced load.
 In Two wattmeter method the current coils of the
wattmeter are connected with any two lines, say R
and Y and the potential coil of each wattmeter is
joined across the same line, the third line i.e. B.
 The two wattmeter method is used for the power
measurement in the 3-phase system, irrespective of
whether the load is balanced or unbalanced.

MEASUREMENT OF POWER
BY TWO WATTMETER
METHOD IN STAR
CONNECTION

Let W1 and W2 Be the two
wattmeter.
Let andthe
phase voltages across the
three loads
be the phase currents
respectively.

 The instantaneous current through the current coil
of Wattmeter, W1 is given by the equation shown
below.
 Instantaneous potential difference across the
potential coil of Wattmeter, W1 is given as
 Instantaneous power measured by the Wattmeter,
W1 is

of Wattmeter, W2 is given by the equation
 Instantaneous potential difference across the
potential coil of Wattmeter, W2 is given as
 Instantaneous power measured by the Wattmeter,
W2 is

 Therefore, the Total Power Measured by the Two
Wattmeter W1 and W2 will be obtained by adding the
equation (1) and (2).
 Here P is the total power absorbed in the three loads at
any instant.

MEASUREMENT OF POWER BY
TWO WATTMETER METHOD IN
DELTA CONNECTION

There are similar notations for delta also.

 The instantaneous current through the coil of the
Wattmeter, W1 is given by the equation
 Instantaneous Power measured by the Wattmeter,
W1 will be
 Therefore, the instantaneous power measured by
the Wattmeter, W1 will be given as

of the Wattmeter, W2 is given as
 The instantaneous potential difference across the
potential coil of Wattmeter, W2 is
 Therefore, the instantaneous power measured by
Wattmeter, W2 will be

 Hence, to obtain the total power measured by the Two
Wattmeter the two equations, i.e. equation (3) and (4)
has to be added.
 Here P is the total power absorbed in the three loads at
any instant.

 The power measured by the Two Wattmeter at any
instant is the instantaneous power absorbed by the
three loads connected in three phases.
 In fact, this power is the average power drawn by the
load since the Wattmeter reads the average power
because of the inertia of their moving system.

1.6.4 MEASUREMENT OF POWER BY SINGLE
PHASE WATTMETER METHOD

 Power is measured in the electric
circuit using a wattmeter.
 A single phase wattmeter
consists of two coils; namely the
current coil and the pressure coil.
 The Current coil is connected in
series with the line and thus
carries the line current.
 The Pressure coil is connected in
parallel with the line.
 The Wattmeter gives the power
per phase.

25
• The wattmeter gives the value of power per phase.
• Therefore, Total power = 3 X Power per phase
= 3 X wattmeter reading
•The one wattmeter method is used for power
measurement in the 3-phase star connected balanced
load.

1.7 Transmission and Distribution of
Electrical Energy
95

9/20/20151
BY 01,02,03,04,05,06,12604

2 9/20/2015
1.7.1 Introduction

9/20/20154
1.7.2 Structure of Electrical Power System

1.7.3 Distribution System
“The part of power system which distributes
electrical power for local use is known as
DISTRIBUTION SYSTEM.”
This system is the electrical system between the
substation fed by the transmission system and
consumer meter.
Distribution line generally consist of
Feeders
Distributers
Service mains
9/20/2015

FEEDERS DISTRIBUTORS SERVICE MAINS
Distribution

Feeder
 A Feeder is conductor which connects the substation to the
area where power is to be distributed
 Feeder are used to feed the electrical power from the
generating station to the substation
 No tapings are taken from the feeder
 So the current in it remains the same throughout
 Main consideration in the design of feeder is
the Current carrying capacity.
9/20/20158

Distributer
 A distributer is a conductor from which tapings
are taken from pole mounted transformer to the
consumer
 The current through a distributer is not constant
because tapings are taken at various places along
its length
 Voltage drop is main consideration
 Limit of variation is 6% of rated at
consumer
9/20/20159

Service mains
 A service mains is a generally a small cable which
connects the distributer to the consumer ‘s meter.
 The connecting links between the distributor and
the consumer terminals.
9/20/201510

1.7.4 classification of dc
distribution
 Distribution system is a part of power system,
existing between distribution substations and
consumers.
 It is further classified on the basis of voltage
 Primary distribution system- 11 KV or 6.6 KV or 3.3 KV
 Secondary distribution system- 415 V or 230 V
9/20/201516

Classification Of Distribution
System:
It can be classified under different considerations as;
1. Type Of Current:
a)AC Distribution System
b)DC Distribution System
2. Type Of Construction:
a)Overhead System
b)Underground System
9/20/201517

3. Type Of Service:
a)General Lighting & Power
b)Industrial Power
c)Railway
d)Streetlight etc
c) Interconnected Distribution System
4. Number Of Wires:
a)Two Wire
b)Three Wire
c)Four Wire
5. Scheme Of Connection:
a)Radial Distribution System
b)Ring or Loop Distribution System
9/20/201518

Ac distribution
 A.c. distribution system is the electrical system
between the step-down substation fed by the
transmission system and the consumers’ meters. The
a.c. distribution
system is classified into
 ( i) primary distribution system and
 ( ii) secondary distribution system.
9/20/201519

1.7.5 Primary distribution system
 voltages somewhat higher than general utilisation and
handles large blocks of electrical energy than the average
low-voltage consumer uses.
 Commonly used primary distribution voltage 11KV, 6.6
KV,3.3 KV.
 Electric power from the generating station is transmitted at
high voltage to the substation located in or near the city.
 At this substation, voltage is stepped down to 11 kV with the
help of step-down transformer.
 Power is supplied to various substations for distribution or
to big consumers at this voltage.
 This forms the high voltage distribution or primary
distribution.
9/20/201520

1.7.6 Secondary distribution system
 It is that part of a.c. distribution system which includes
the range of voltages at which the ultimate consumer
utilizes the electrical energy delivered to him.
 The secondary distribution employs 400/230 V, 3-
phase, 4-wire system.
22

D.C. Distribution
 D.c. supply is required for the operation of variable speed
machinery ( i.e., d.c. motors), for electro-chemical work and
for congested areas where storage battery reserves are
necessary.
 For this purpose, a.c. power is converted into
d.c. power at the substation by using converting machinery
e.g., mercury arc rectifiers, and motor-generator sets.
24

Type of DC distributor
 The dc supply from the substation may be
obtained in form of
 ( i) 2-wire or
 ( ii) 3-wire for distribution.
25

27 Two outer and a middle or neutral wire is earthed at
the s/s.
 Voltage between the outer is twice the voltage
between either outer and neutral wire.
 Advantage – available two voltage at the consumer
terminal.
 Loads requiring high voltage connected across the
outers.
 Lamps and heating circuits requiring less voltage are
connected between either outer and neutral.

Method of obtaining 3 wire D.c
system
 Two generator method.
 3-wire D.c. generator.
 Balancer set
9/20/201528

Two generator method
 D.c generator G1 and G2 are connected in series and
the neutral is obtained from the common point btwn
generator
 G1 supplies a I1, G2 supplies a I2
 Difference of load current on both side (I1-I2) flow
through the neutral wire.
 Disad.:two separate generator are required.
9/20/201530

3-wire D.c. generator.
9/20/201531

3-wire D.c. generator.
 Consist of a standard 2 wire machine with one
or two coils of high reactance and low
resistance that connected to opposite points of
the armature winding.
 Neutral wire is obtained from common
point.
9/20/201532

Connection schemes of
distribution system
 Radial system
 Ring main system
 Interconnected system
9/20/201533

Radial Distribution System:
9/20/2015
 separate feeders radiate from a single substation and feed the
distributors at one end only.
 Only one path is connected between each customer and
substation.
 Electrical power flows along a single path.
 If interrupted, results in complete loss of power to the customer.
.
34

• Advantages:
 Low cost .
 Simple planning.
• Disadvantages :
 The radial system is employed only when power is generated at low
voltage and the substation is located at the centre of the load.
 Distributor nearer to feeding end is heavily loaded.
 Consumers at far end of feeder would be subjected to serious
• voltage fluctuations
128

Ring or Loop Distribution System:
 It consists of two or more paths between
• power sources and the customer.
 The loop circuit starts from the substation bus-bars, makes a loop
through the area to be served, and returns to the substation
• Advantages:
 Less conductor material is required.
 Less voltage fluctuations.
 More reliable.
• Disadvantages:
 It is difficult to design as compared to the
design of radial system.
37 9/20/2015

Interconnected Distribution System:
 It is supplied by a number of feeders.
 Radial primary feeders can be tapped off from the
interconnecting tie feeders.
 They can also serve directly from the substation.
Advantages:
 Increases the reliability of supply
 Losses are less
 Quality of service is improved.
Disadvantages:
 Its initial cost is more.
 Difficult in planning, design and operation.
9/20/201539

1.7.7 D.C. Distribution
 D.c. supply is required for the operation of variable
speed machinery ( i.e., d.c. motors),
 for electro-chemical work and for congested areas
where storage battery reserves are necessary.
 For this purpose, a.c. power is converted into d.c.
power at the substation by using converting machinery
e.g., mercury arc rectifiers, rotary converters and motor-
generator sets. The d.c. supply from the substation may
be obtained in the form of
9/20/201543

DC Distribution:
 Voltage drop along distributor is considered as a
main factorwhile designing a distributor.
 It depends upon the nature of load and also on
feeding, whether it is fed at one or both ends.
According to loading, a distributor can be classified
as:
i. Fed at one end.
ii. Fed at both ends. a). With equal voltages.
b). With unequal voltages.
iii. Fed at centre.
Ring mains.
9/20/2015

DC Distribution Feed at one end
9/20/2015

 a) The current in the various sections of the
distributor away from feeding point goes on
decreasing. Thus current in section AC is more than
the current in section CD and current in section CD is
more than the current in section DE.
 (b) The voltage across the loads away from the
feeding point goes on decreasing. Thus in Fig. 13.1,
the minimum voltage occurs at the load point E.
 (c) In case a fault occurs on any section of the
distributor, the whole distributor will have to be
disconnected from the supply mains. Therefore,
continuity of supply is interrupted

 In this type of feeding the distributor is connected to the supply mains at
both ends and loads are tapped off at different points along the length of the
distributor.
The voltage at the feeding points may or may not be equal. distributor A B
fed at the ends A and B and loads of I1, I2 and I3 tapped off at points C
respectively.
 Here, the load voltage goes on decreasing as we move away from one
feeding point say A , reaches minimum value and then again starts rising
and reaches maximum value when we reach the other feeding point B.
 The minimum voltage occurs at some load point and is never fixed. It is
shifted with the variation of load on different sections of the distributor.
48 9/20/2015

Advantages
 (a) If a fault occurs on any feeding point of the
distributor, the continuity of supply is main-tained from
the other feeding point.
 (b) In case of fault on any section of the distributor,
the continuity of supply is maintained from the other
feeding point.
 (C)The area of X-section required for a doubly fed
distributor is much less than that of a singly fed
distributor.
9/20/201549

9/20/201551
 In this type of feeding, the centre of the distributor
is connected to the supply mains as shown in Fig.
 It is equivalent to two singly fed distributors,
each distributor having a common feeding point
and length equal to half of the total length

1.8 Overhead Vs Underground
System
144

Transmission and
Distribution
 Transmission lines
connect power
generation plants
to substations,
other power
generating plants,
and other utilities
at high voltages.
 Distributes electricity
to each customer's
residence, business,
or industrial plant at
lower voltages.
Transmission System Distribution System

Transmission and
Distribution
Transmission Distribution

Underground and Overhead
 Undergrounding
transmission lines is
less common
 Underground cables
have different
technical
requirements than
overhead and have
different impacts
 Undergrounding
distributions lines
is more common
Transmission Distribution

Underground vs. Overhead
Transmission
 Design Issues
 Specialized engineering skills required
 Extensive study required to determine site-
specific subsurface obstructions or obstacles
 Longer timeframe for design
 Need to provide larger budget contingency
 Flood plain and wetland issues require special
consideration
 Environmental impacts

Transmission
 Construction Concerns
 Space for large vaults (8’ x 10’ X 20’)
 Longer construction time frame
 Dewatering in wet areas during construction
 Significantly more impacts to surrounding
properties
 Open trenches
 min. 5’ wide x 5’ deep
 Specialized backfill

Transmission
 Operational Concerns
 Difficult to identify outage location
 Requires specialized work force
 Long lead time for delivery of materials
 Need to warehouse specialized spare materials
 Increased maintenance
 Shorter life span
 Dewatering and cleaning of equipment in
vaults

Transmission
 Cost
 Typical underground costs are 8 to 10 times the cost of
overhead construction
 Typical life of underground is approximately one-half
the life of overhead construction
 Depending on route may have significantly more
unanticipated problems with associated costs
 4-Cable system required to increase reliability which
adds cost
 Specialized workforce increases cost
 Wetland mitigation may be substantially more
depending on route
 Warehousing of spare materials and equipment

Underground
Transmission
 Generally used:
 in densely populated and urban settings
 where sufficient right-of-way is not available
 to reduce visual impacts
 riser poles at each end of the underground cable are large
and support additional equipment that create visual impacts
 Reliability
 May have fewer outages than overhead
 When outages occur they will be more difficult to
locate and may take significantly more time to repair

1.9 Protection of Power System
157

Power-system protection
 Power-system protection is a branch of electrical power engineering that deals with the
protection of electrical power systems from faults through the isolation of faulted parts from
the rest of the electrical network.
 The objective of a protection scheme is to keep the power system stable by isolating only
the components that are under fault, whilst leaving as much of the network as possible still in
operation.
 Thus, protection schemes must apply a very pragmatic and pessimistic approach to
clearing system faults. For this reason, the technology and philosophies utilized in protection
schemes can often be old and well-established because they must be very reliable.

Components
Protection systems usually comprise five components:
•Current and voltage transformers to step down the high voltages and currents of the
electrical power system to convenient levels for the relays to deal with
•Protective relays to sense the fault and initiate a trip, or disconnection, order;
•Circuit breakers to open/close the system based on relay and autorecloser commands;
•Batteries to provide power in case of power disconnection in the system.
•Communication channels to allow analysis of current and voltage at remote terminals of a
line and to allow remote tripping of equipment.

 For parts of a distribution system, fuses are capable of both sensing and disconnecting
faults.
 Failures may occur in each part, such as insulation failure, fallen or broken transmission
lines, incorrect operation of circuit breakers, short circuits and open circuits. Protection
devices are installed with the aims of protection of assets, and ensure continued supply of
energy.
Switchgear is a combination of electrical disconnect switches, fuses or circuit breakers
used to control, protect and isolate electrical equipment. Switches are safe to open under
normal load current, while protective devices are safe to open under fault current.

Protective devices
Protective relays control the tripping of the circuit breakers surrounding the faulted part of
the network
Automatic operation, such as auto-re-closing or system restart
Monitoring equipment which collects data on the system for post event analysis
While the operating quality of these devices, and especially of protective relays, is always
critical, different strategies are considered for protecting the different parts of the system.
Very important equipment may have completely redundant and independent protective
systems, while a minor branch distribution line may have very simple low-cost protection.

There are three parts of protective devices:
•Instrument transformer: current or potential (CT or VT)
•Relay
•Circuit breaker
Advantages of protected devices with these three basic components include safety,
economy, and accuracy.
Safety:
Instrument transformers create electrical isolation from the power system, and thus
establishing a safer environment for personnel working with the relays.
Economy:
Relays are able to be simpler, smaller, and cheaper given lower-level relay inputs.
Accuracy:
Power system voltages and currents are accurately reproduced by instrument transformers
over large operating ranges.

Performance and design criteria for system-protection devices include reliability, selectivity,
speed, cost, and simplicity.
Reliability: Devices must function consistently when fault conditions occur, regardless of
possibly being idle for months or years. Without this reliability, systems may result in high
costly damages.
Selectivity: Devices must avoid unwarranted, false trips.
Speed: Devices must function quickly to reduce equipment damage and fault duration, with
only very precise intentional time delays.
Economy: Devices must provide maximum protection at minimum cost.
Simplicity: Devices must minimize protection circuitry and equipment.

Protective relays
These are compact analog or digital networks, connected to various points of an
electrical system, to detect abnormal conditions occurring within their assigned
areas.
 They initiate disconnection of the trouble area by circuit breakers. These relays
range from the simple overload unit on house circuit breakers to complex systems
used to protect extra high voltage power transmission lines.
They operate on voltage, current, current direction, power factor, power,
impedance, temperature.
In all cases there must be a measurable difference between the normal or
tolerable operation and the intolerable or unwanted condition.

 System faults for which the relays respond are generally short circuits
between the phase conductors, or between the phases and grounds.
 Some relays operate on unbalances between the phases, such as an open or
reversed phase.
 A fault in one part of the system affects all other parts. Therefore relay sand
fuses throughout the power system must be coordinated to ensure the best
quality of service to the loads and to avoid operation in the non-faulted
areas unless the trouble is not adequately cleared in a specified time.
165

The tariff is the rate at which the
electrical energy is sold. There are various types of
tariffs followed in the market. This post will give the
brief idea about different tariff types.

VARIOUS TYPES OF TARIFFS
1- Simple Tariff
2- Flat rate Tariff
3 - Block Rate Tariff
4 - Two Part Tariff/Maximum Demand Tariff
5 - Power Factor Tariff
6 – Three Part Tariff

SIMPLE TARIFF
The tariff which has a fixed rate, per unit energy consumed.
ADVANTAGES
:- This is simplest tariff.
:- Even a simple consumer can understand it.
DISADVANTAGES
:- There is no differentiation between small and big
consumers.
:- The cost is per unit is very high.

FLAT RATE TARIFF
It is a type in which different consumers have different rates. A
discrimination exists between small and large consumers.
ADVANTAGES
:- Large consumers are encouraged in this type.
:- This tariff is simple and easy to calculate.
DISADVANTAGES
:- Different energy meters are required to be installed at
consumer’s premises.
:- The supplier doesn’t get any return for the connection given
to the consumer If he doesn’t consume any energy in a
particular period.

BLOCK RATE TARIFF
When the total energy consumed is divided into
blocks for the purpose of tariff.
ADVANTAGES
:- The consumers are encouraged to consume more
energy. This increase load factor of
the system and the cost of generation is reduced.
DISADVANTAGES
:- This doesn’t take into account the maximum
demand of the consumer.

TWO PART TARIFF
This is the system in which the tariff is related to the consumer’s
maximum demand, and then, to his consumption of energy.
ADVANTAGES
:-This tariff is very suitable for industrial consumers who have higher
maximum demand.
:-It takes into considerations the maximum demand of the consumer.
DISADVANTAGES
:-A maximum demand indicator is to be installed at the premises for
assessing the maximum demand of the consumer.
:-The consumer has to pay some fixed amount per kw whether he
consumes energy or not in a particular period.

POWER FACTOR TARIFF
The tariff in which the power factor of the consumer is
taken into consideration.
In this there are following types:-
(i)KVA maximum demand tariff
(ii)Sliding scale tariff/Average P.F Tariff
(iii)KWh and KVAR Tariff

THREE PART TARIFF
The tariff which charges a consumer in 3 parts.
First Part
This represents fixed charge which includes interest
and depreciation.
Second Part
This is a semi-fixed charge which is calculated on
per kw of the maximum demand.
Third Part
This is a running charge which is calculated per kwh
of power consumed by the consumer

1.11 Power Factor Improvement
175

Active power actually performs the work and is measured in kW. This is also
what is read on a wattmeter.
Reactive power sustains the electromagnetic field and is measured in kvar.

 Power Factor is the measurement of how effectively your business uses the
electricity supplied to your site
 Ideal Power Factor is unity or 1, anything less than 1 means that extra power
is required to achieve the necessary tasks.
The higher the power factor, the more effectively electrical power is
being used and vice versa.
 Low Power Factor is expensive and inefficient, with many utility companies
charging extra, (reactive power charge), for sites with a poor power factor
 Low Power Factor can also reduce the capacity of your electrical
distribution system by increasing current flow and causing voltage drops.

Power Factor is the ratio of true power to apparent power
Power Factor = KW
KVA

 Power Factor Correction is the term given to a technology to restore Power
Factor to as close to unity as economically viable
 This can be achieved by adding Power Factor Correction capacitors to the
distribution system which provide or compensate for the Reactive Power
demand of the inductive load, and thus reduce the burden on the supply
 Capacitors work as reactive current generators “providing” needed reactive
power (KVAr) into the power supply
 By supplying their own source of reactive power, the industrial user frees the
utility from having to supply it, and therefore the total amount of apparent
power supplied by the utility will be less.
 Power Factor Correction Capacitors reduce the total current drawn from the
distribution system and subsequently increase the system’s capacity by raising
the Power Factor level.

 A reduction in electricity charges
 Elimination of utility power factor penalties, which can increase electrical bills
by up to 20%
 Reduction in I²R losses of transformers and distribution equipment
 Prolonging the life of equipment from reduced heat in cables, switchgear,
transformers and alternators
 Reduced voltage drop in cables, allowing the same cable to supply a
larger motor and improving the starting of motors at the end of the long
cable runs

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