Presentation on theme: "Faculty of Electrical Engineering Universiti Teknologi Malaysia"— Presentation transcript:
1 Faculty of Electrical Engineering Universiti Teknologi Malaysia Mechanical and Electrical Systems SKAA 2032Power Supply (AC and DC)Faculty of Electrical EngineeringUniversiti Teknologi Malaysia
2 Alternating voltage and current Electricity is produced by generators at power station.Electricity is then distributed by a vast network of transmission lines called National Grid System.It is easier and cheaper to generate AC than DC.It is more convenient to distribute AC than DC since the voltage can be readily altered using transformer.Whenever DC is needed, devices called rectifiers are used for conversion.
3 Alternating voltage and current RectifierPower socket
4 Generation of Single Phase An electric current can be induced in a circuit by a changing magnetic field – Faraday’s LawThe direction of the induced current is such that the induced magnetic field always opposes the change in the flux – Len’z LawDirection of current for generator – Fleming’s right hand rule.
5 Single phaseSingle phase electricity is generated by rotating a single turn coil through a magnetic field.The shape of the waveform produced by a generator (i.e. the alternator) is in the form of sine wave.Wires used:Live conductor (yellow)Neutral conductor (blue)Earth conductor (green) –connected from neutral via a protective gear to earth
7 Single phase system A general expression for the sinusoid is given by: v(t) = Vm sin (wt + q)whereVm is the amplitude or peak valueω is the angular frequency radian/s given by ω=2πftf is the frequency in hertz (Hz)t is the time in second (s)T is the period in second, given by T=1/fθ is the phase angle in degree
8 The angular frequency in radians/second Single phase systemThe angular frequency in radians/second
9 Single phase systemA sinusoid can be expressed in either sine or cosine form. When comparing two sinusoids, it is expedient to express both as either sine or cosine with positive amplitudes.We can transform a sinusoid from sine to cosine form or vice versa using this relationship:cos ωt = sin (ωt + 90o)sin ωt = cos (ωt - 90o)
10 Single phase system (a) v(t) = 12 cos (50t + 10o) Example 1.1Find the amplitude, phase angle, angular frequency, period and frequency of the sinusoidal waveform(a) v(t) = 12 cos (50t + 10o)(b) v(t) = 5 sin (4πt - 60o)(a) (12V, 10o, 50rads/sec, sec., Hz)(a) (5V, -60o, 4π rads/sec, 0.5 sec., 2 Hz)
11 Single phase systemSinusoids are easily expressed in terms of phasors.A phasor is a complex number that represents the amplitude and phase of a sinusoid.v(t) = Vm cos (ωt + θ)Time domainPhasor domainTime domainPhasor domain
12 Single phase system Instantaneous and Average Power The instantaneous power is the power at any instant of time: p(t) = v(t) i(t)Where v(t) = Vm cos (ωt + θv)i(t) = Im cos (ωt + θi)Using the trigonometric identity, gives
13 Single phase systemThe average power is the average of the instantaneous power over one period.
14 Single phase systemThe effective value is the root mean square (rms) of the periodic signal.The average power in terms of the rms values is given byWhere
15 Single phase systemExample 1.2 An ac voltage of a sinusoidal waveform has a peak value of 300 V. What is the rms value of this voltage? (212.1 V) Example 1.3 What is the peak voltage of 120 V rms? (169.7)
16 Single phase systemExample 1.4 An alternating current of sinusoidal waveform has a r.m.s value of 10A. What are the peak values of this current over one cycle? (14.14A & A)
17 Single phase systemExample 1.5 An alternating voltage can be represented by v=141.4 sin 377t. Determine: (a) r.m.s. voltage (b) frequency (c) the instantaneous voltage when t = 3 ms (100V, 60Hz, 127.8V)
18 Single phase systemApparent Power, Reactive Power and Power Factor The apparent power is the product of the rms values of voltage and current. The reactive power is a measure of the energy exchange between the source and the load reactive part.
19 Single phase system The complex power: The power factor is the cosine of the phase difference between voltage and current.The complex power:
20 Single phase system True or active power: Watts (W) Apparent power: S Qvolt·amperes (VA)θv–θiReactive power:Preactive volt·amperes (var)
22 Three phase systemA three-phase electricity is generated when three coils are placed 120° apart, and the whole rotated in a magnetic field.The result is three independent supplies of equal phase voltage - distinguished by 120° phase angle.The convention adopted to identify the phase voltages: R-red, Y-yellow, B-blue.The standard phase sequence is R, Y, B.
23 Generation of Three-phase Suppose three similar loops of wire with terminals R-R’, Y-Y’ and B-B’ are fixed to one another at angles of 120o and rotating through a magnetic field.
24 Three phase systemThree conductors (lines) to carry the three phase supply, colored red, yellow and blue.A fourth conductors called the neutral, connected through protective device to earth.The three phase system is usually connected using:star connection (sources i.e. alternators)delta connection (transformers, motors and other loads)
25 Generation of Three-phase The instantaneous e.m.f. generated in phase R, Y and B:vR = VR sin wtvY = VY sin (wt -120o)vB = VB sin (wt -240o) = VBsin (wt +120o)
26 Generation of Three-phase Phase sequences:RYB or positive sequenceVR leads VY, which in turn leads VB.This sequence is produced when the rotor rotates inthe counterclockwise direction.
27 Generation of Three-phase (b) RBY or negative sequenceVR leads VB, which in turn leads VY.This sequence is produced when the rotor rotates inthe clockwise direction.
37 Star connection - Vector diagram Phasor diagram is used tovisualize the system voltages• Star system has two type ofvoltages: Line-to-neutral, andline-to-line.• The line-to-neutral voltages areshifted with 120o• The line-to-line voltage leads theline to neutral voltage with 30o• The line-to-line voltage is timesthe line-to-neutral voltage
38 Star connection - Distribution Typical distribution voltage of 415/240V, 3 phase 4 wires system
39 Delta Connection Phase voltages are equal to the line voltages # Reference: IRY# Positive sequence.
44 TNB Supply System Voltage 3 phase, 50 Hz The main transmission and substation network are:- 275 kV- 132 kVkVThe distribution are:kVkVkVkVvolts- 240 volts (single phase) drawn from 415 volts 3 phase(phase voltage), between line (R, Y, B) and Neutral (N)
45 TNB Supply System Supply Method (two types of premises) The low voltage system (415/240 V) is 3-phase four wire.The low voltage system is a mixture of overhead lines andunder ground cables.The high voltage and extra high voltage system is 3-phase three wireConfiguration. Overhead line and under ground cable system are used.Supply Method (two types of premises)1. Single consumer such as private dwelling house, workshop, factory, etc.Single phase, two wire, 240 V, up to 12 kVA max demandThree phase, four wire, 415 V, up to 45 kVA max demandThree phase, four wire, C. T. metered 415 V, up to 1,500 kVA maxdemand
46 TNB Supply System2. Multi tenanted premises, such as high rises flats, commercial, office blocks, etcLow VoltageThree phase, four wire, C.T. metered 415 V, up to 1,500 kVA maxdemandHigh Voltage and Extra High VoltageThree phase, three wires, 6,600 and 11,000 V for load of 1, 500 kVAmax demand and above, whichever voltage is availableThree phase, three wires, 22,000 and 33,000 V for load of 5,000 kVAThree phase, three wires, 66,000 V, 132,000 V and 275,000 forexceptionally large load of above 20 MVA max demand
47 Standby SupplyStandby generator(s) may be used by the applicant at their premises, subject to compliance with the relevant laws.The generators shall remain a separate system from TNB distribution system and the applicant shall declare to TNB on the safe installation of the generator(s).This may be used in place of TNB’s supply source through a suitable, approved changeover facility.The Energy Commission and other relevant authorities govern the usage of generators and standby supply.This may be used in place of the TNB’s supply source through a suitable, approved change over facility under emergency conditions.