Download Instrumentation and Measurement Techniques

General power system
objective
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Review general energy and power system
• Application of fundamental knowledge of principal machines ,
transformer and other power device to large electrical
systems.
• To present general picture of typical electrical for High
voltage generation , transformation and distribution
• Difference between shipboard and land based HV power
generation
RECAP
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Principle of electrical machines
Power generation, transformation and distribution
Electromagnetism
Induction and transformer
Losses
Single and 3 phase circuit
Active and reactive power
Electric machine and power
Ancillaries
General Power Systems
• Majority of merchant ships have a 3-phase 3 wire, 440 V insulated neutral
earth power systems
• This power system falls in the category of LV and meets the power
demands of medium capacity motors up to 200 kW
• When large loads are connected to the LV system the magnitude of current
flow becomes too large resulting in overheating due to high iron and copper
losses
• P = VI Cos 
• Copper loss = I2 R [kW]
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Losses in Electrical Systems
Copper Losses in electrical cables and machine windings
Hysteresis Loss in magnetic cores
Eddy current loss in conductors and cores
All these losses are current dependent and rise to very high levels in LV
machines for large power ratings
Introduction to Machinery Principles
An electric machine:
is a device that can convert either mechanical energy to
electric energy or vice versa.
A generator :
converts mechanical energy to electrical energy.
A motor:
converts electrical energy to mechanical energy.
Almost all practical motors and generators convert energy from
one form to another through the action of a magnetic field.
Another closely related device is the transformer.
Introduction to Machinery Principles . . . . .
A transformer:
is a device that converts ac electrical energy at one
voltage level to ac electrical energy at another voltage level, but
with the same frequency.
In general, transformers operate on the same principles as
generators and motors, and are usually studied together with
generators and motors.
These 3 types of devices are inevitable in modern daily life.
*Motors find applications in several home appliances.
*In the work place, motors provide the motive power for
almost all tools.
*Generators are essential to supply the power used by all these
motors.
INTRODUCTION
The electromagnetic system is a necessary element of all rotating
and static electric machinery and electromechanical devices.
The role of electromagnetic system is to establish and control
electromagnetic fields for carrying out conversion of energy, and
transfer.
Practically all motors and generators, depend upon the magnetic
field as the coupling medium allowing interchange of energy in
either direction between electrical and mechanical systems.
A transformer though not an electromechanical conversion
device, provides a means of transferring electrical energy
between two electrical ports via the medium of a magnetic field.
MAGNETIC FIELD . . . .
2.
A time-changing magnetic field induces a voltage in
a coil of wire if it passes through that coil. This is the
basis of transformer action.
3.
A current-carrying wire in the presence of a magnetic
field has a force induced on it. This is the basis
of
motor action.
4.
A moving wire in the presence of a magnetic field has a
voltage induced in it. This is the basis of the generator
action.
THREE – PHASE CIRCUITS
Introduction
Three-phase systems differ from single-phase systems in
that they use a set of three voltages instead of one.
In general, a three-phase system is merely a combination
of three single phase systems of which the three voltages
differ in phase by 120 electrical degrees from each other
in a particular sequence.
There are two basic three-phase connections, the wye
(star) and the delta connections.
Analysis of three-phase balanced circuits . . . .
Wye connection
Delta connection
Voltage magnitudes
VLL  3VPh
VLL  VPh
ABC phase
sequence
L  IV
Ph by
VAB Ileads
A

30
3I by
L  IAB
Ph
IAIlags
30
ACB phase
sequence
VAB lags VA by
30
IA leads IAB by
30
Current
magnitudes
Active power and reactive power calculations
The total average power absorbed by a three phase balanced load delivered
by a three-phase generator, is equal to the sum of the powers in each phase.
The voltage and current in each phase are equal.
The total 3-phase real power is, PT  3PPh  3VPh  I Ph  cos  3  VLL  I L  cos
where,  is the phase
angle between the voltage and the current or,
2
V
2
PT  3I Ph
 RPh  3 Ph
RPh
Similarly, the total reactive power is, QT  3QPh  3VPh  I Ph  sin   3  VLL  I L  sin 
or QT  3I  X Ph
2
Ph
VPh2
3
X Ph
P1
P1
P1
P1
Total apparent power is,
*
ST  PT  jQT  3VPh  I Ph
 3  VLL  I L*
P2
P2
2
 3  I Ph
 Z Ph
V Ph2
or, S T  3
Z Ph
P3
P3
PT = P1 + P2 + P3
P2 P2
PT = P1 + P2