UNIVERSITY OF NAIROBI

FACULTY OF ENGINEERING

DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING

FINAL YEAR PROJECT

TIITLE: AN ELECTRICAL POWER SUPPLY QUALITY MONITOR

PROJECT NO. 48

AUTHOR: ODHIAMBO K. O

REG.NO. F17/2141/2004

SUPERVISOR: PROF. ELIJAH MWANGI

EXAMINER: DR. MBUTHIA

DATE: MAY 2009

Submitted to the department of electrical and electronic engineering in partial fulfillment for the award of a degree in Bachelor of Science in Electrical and Electronic Engineering.

PROJECT NO: 48

TITTLE: ELECTRICAL POWER SUPPLY QUALITY

MONITOR.

BY: ODHIAMBO K.O

F17/2141/2004

SUPERVISOR: PROF.ELIJAH MWANGI

This project has been submitted in partial fulfillment of the

requirements for the Bachelor of Science degree in Electrical and

Electronics Engineering of the University of Nairobi.

May, 2009

Dedication

To my parents, brothers and sisters and Keren for the shoulders to always lean on.

ACKNOWLEDGEMENT

I would like to take this opportunity to thank all who contributed to this project through technical

guidance and moral support.

I would like to particularly thank my supervisor Professor Elijah Mwangi for his guidance on the

entire project approach.

Finally I would like to appreciate all members of the class of 2009 for their support.

ABSTRACT

The project is an electrical power supply quality monitor which examines the quality of the

mains supply voltage and frequency.

The project uses analogue devices which includes diodes, op-amps and mos-fet to monitor

frequency and voltage being supplied from the available mains. This translates to three sections;

the frequency monitoring section, the voltage monitoring section and the spikes and noise

detection section.

The objective being to design a cheap power supply quality monitor, the project was designed

and simulated according to the specifications with results achieved.

TABLE OF CONTENTS

1. CHAPTER ONE : INTRODUCTION----------------------------------------------1

2. CHAPTER TWO: LITERATURE REVIEW------------------------------------2

2.1 Power quality ------------------------------------------------------------------------2

2.1.1 Voltage dips---------------------------------------------------------------------2

2.1.2 Voltage Surges/Spikes---------------------------------------------------------5

2.1.3 Overvoltages--------------------------------------------------------------------6

2.1.4 Harmonics-----------------------------------------------------------------------6

2.1.5 Frequency Variations----------------------------------------------------------7

2.1.6 Voltage Fluctuations-----------------------------------------------------------7

2.1.7 Voltage Unbalance-------------------------------------------------------------7

2.1.8 Supply Interruptions-----------------------------------------------------------8

2.1.9 Undervoltage--------------------------------------------------------------------8

2.1.10 Transients----------------------------------------------------------------------8

2.1.11 Rapid voltage change---------------------------------------------------------9

2.2 Power Quality monitoring---------------------------------------------------------9

2.3 Types of Installation----------------------------------------------------------------9

2.4 Improving power quality----------------------------------------------------------9

2.5 Voltage Monitor-------------------------------------------------------------------10

3.0 CHAPTER THREE: DESIGN------------------------------------------------------12

3.1 Noise and Spikes Detector-------------------------------------------------------13

3.2 Frequency Detector---------------------------------------------------------------14

3.3 Overvoltage circuit----------------------------------------------------------------15

3.3.1 Rectification---------------------------------------------------------------------16

3.3.2 The transformer--------------------------------------------------------------19

3.3.3 Construction features--------------------------------------------------------20

3.3.4 Principle of operation--------------------------------------------------------20

3.3.5 Transformer connection-----------------------------------------------------20

3.3.6 E.M.F Equation---------------------------------------------------------------21

3.3.7 The 741 Operational Amplifier---------------------------------------------22

3.3.8 metal-oxide-semiconductor FET-------------------------------------------23

3.3.8.1 Circuit symbols-------------------------------------------------------------24

3.3.8.2 MOSFET operation--------------------------------------------------------25

3.3.8.3 Modes of operation---------------------------------------------------------26

4.0 Principle of operation-------------------------------------------------------------28

5.0 Design simulation and results---------------------------------------------------29

6.0 Conclusion and Recommendation----------------------------------------------37

REFERENCE

1. INTRODUCTION

An electrical power supply quality monitor is a device which examines the status of the mains

electrical parameters that is the frequency and the voltage.

This project objectively entails the design and simulation of a mains power supply quality

monitor. The distinctive objective of the design being to lower the cost of a power quality

monitor by using cheaply available analogue devices to design one.

The design task is to simulate a real time quality monitor that can be directly connected to the

mains or interfaced with digital devices for improved monitoring. The measured parameters are

to be obtained from the various parts of the design which includes: spikes and noise detection

circuit, voltage detection circuit and the frequency monitoring section.

For visual indications the design made use of cheap LEDs and for audio indications the use of a

speaker and a buzzer was employed.

Finally, to aid observation oscilloscope and digital multi-meter were used to ascertain the

simulated results.

2.0 LITERATURE REVIEW

2.1 Power quality

`Power quality’ is the absence of various kinds of disturbances on single-phase low voltage ac

supply mains. Such disturbances include transient overvoltages from lighting and switching

reactive loads, large departures from nominal root mean square voltage, severe harmonic

distortion, and outages. Disturbances on the ac supply mains can cause electronic circuits and

systems to malfunction or even be damaged. A disturbance that affects a process control

computer in a large industrial complex could easily result in shutdown of the process. The lost

production and product loss or recycling during start-up represents a large cost to the business.

Similarly, a protection relay affected by a disturbance though conduction or radiation from

nearby conductors could trip a feeder or substation, causing loss of supply to a large number of

consumers. At the other end of the scale, a domestic user of a PC has to re-boot the PC due to a

transient voltage dip, causing annoyance to that and other similarly affected users. The

disturbances may come in the following forms:

• Large magnitude like transient overvoltage and surge

• Sudden decrease in magnitude as in the case of a notch

• Severe harmonic distortion

• Sudden loss of useful power caused by severe sag or beginning of outage

• High frequency noise

• Changes in frequency of mains voltage

• Unacceptable value of root mean square voltage like sag or swell.

2.1.1 Voltage Dips

The major cause of voltage dips on a supply system is a fault on the system, that is sufficiently

remote electrically that a voltage interruption does not occur. Other sources are the starting of

large loads (especially common in industrial systems), and, occasionally, the supply of large

inductive loads. Voltage dips due to the latter are usually due to poor design of the network

feeding the consumer. A voltage dip is the most common supply disturbance causing interruption

of production in an industrial plant. Faults on a supply network will always occur, and in

industrial systems, it is often practice to specify equipment to ride-through voltage dips of up to

0.2s. The most common exception is contactors, which may well drop out if the voltage dips

below 80% of rated voltage for more than 50-100ms. Motor protection relays that have an under

voltage element setting that is too sensitive is another cause. Since contactors are commonly

used in circuits supplying motors, the impact of voltage dips on motor drives, and hence the

process concerned, requires consideration. Other network-related fault causes are weather–

related (such as snow, ice, wind, salt spray, dust) causing insulator flashover, collisions due to

birds, and excavations damaging cables. Multiple voltage dips, as illustrated in Figure 23.3,

cause more problems for equipment than a single isolated dip. The impact on consumers may

range from the annoying (non-periodic light flicker) to the serious (tripping of sensitive loads

and stalling of motors). Where repeated dips occur over a period of several hours, the repeated

shutdowns of equipment can give rise to serious production problems. Figure 23.4 shows an

actual voltage dip, as captured by a Power Quality recorder. Typical data for under voltage

disturbances on power systems during evolving faults are shown in Figure23.5.

Disturbances that lie in the front right-hand portion of the histogram are the ones that cause most

problems, but fortunately these are quite rare.

Fig 2.1 Voltage dip profile

Vrms Nominal high

Nominal low

Interruption

Time

Dip duration

Percentage below nominal Retained voltage

Fig 2.2 Recording of a voltage dip

Fig 2.3 Voltage surge profile

V

Time

Vrms

Time

Surge duration

Above nominal

Nominal high

Nominal low

Interruption

Fig 2.4 Supply waveform distorted due to presence of harmonics

2.1.2 Voltage Surges/Spikes

Voltage surges/spikes are the opposite of dips – a rise that may be nearly instantaneous (spike) or

takes place over a longer duration (surge). These are most often caused by lightning strikes and

arcing during switching operations on circuit breakers/contactors (fault clearance, circuit

switching, especially switch-off of inductive loads). Figure 2.3 shows the profile of a voltage

surge. Equipment may suffer serious damage from these causes, ranging from insulation damage

to destruction of sensitive electronic devices. The damage may be immediate and obvious by the

fact that equipment stops working, through to failure at a much later date from deterioration

initiated from a surge or spike of voltage. These latter failures are very difficult to distinguish

from random failures due to age, minor manufacturing defects, etc.

Time

V

2.1.3 Overvoltages

Sustained overvoltages are not common. The most likely causes are maladjusted voltage

regulators on generators or on-load tap changers, or incorrectly set taps on fixedtap transformers.

Equipment failures may immediately result in the case of severe overvoltages, but more likely is

accelerated degradation leading to premature failure without obvious cause. Some equipment

that is particularly sensitive to overvoltages may have to be shut down by protective devices.

2.1.4 Harmonics

This is a very common problem in the field of Power Quality. The main causes are Power

Electronic Devices, such as rectifiers, inverters, UPS systems, static var compensators, etc. Other

sources are electric discharge lamps, arc furnaces and arc welders. In fact, any nonlinear load

will be a source of harmonics. Figure 23.7 illustrates a supply waveform that is distorted due to

the presence of harmonics. Harmonics usually lead to heating in rotating equipment (generators

and motors), and transformers, leading to possible shutdown. Capacitors may be similarly

affected. If harmonic levels are sufficiently high enough, protective devices may shut the

equipment down to avoid damage. Some equipment, such as certain protection devices, may mal

operate and cause unnecessary shutdowns. Special provision may have to be made to filter

harmonics from the measured signals in these circumstances. Interference may be caused to

communication systems. Overloading of neutral conductors in LV systems has also occurred (the

harmonics in each phase summing in the neutral conductor, not cancelling) leading to failure due

to overheating. This is a particular risk in buildings that have a large number of PC’s, etc., and in

such cases a neutral conductor rated at up to 150% of the phase conductors has been known to be

required. Bus bar risers in buildings are also at risk, due to harmonic-induced vibration causing

joint securing bolts, etc. to work loose.

2.1.5 Frequency Variations

Frequency variations that are large enough to cause problems are most often encountered in

small isolated networks, due to faulty or maladjusted governors. Other causes are serious

overloads on a network, or governor failures, though on an interconnected network, a single

governor failure will not cause widespread disturbances of this nature. Network overloads are

most common in areas with a developing electrical infrastructure, where a reduction in frequency

may be a deliberate policy to alleviate overloading. Serious network faults leading to islanding of

part of an interconnected network can also lead to frequency problems. Few problems are

normally caused by this problem. Processes where product quality depends on motor speed

control may be at risk but such processes will normally have closed-loop speed controllers.

Motor drives will suffer output changes, but process control mechanisms will normally take care

of this. Extreme under- or over frequency may require the tripping of generators, leading to the

possibility of progressive network collapse through network overloading/under frequency causes.

2.1.6 Voltage Fluctuations

These are mainly caused by load variations, especially large rapid ones such as are likely to

occur in arc and induction heating furnaces, rolling mills, mine winders, and resistance welders.

Flicker in incandescent lamps is the most usual effect of voltage fluctuations. It is a serious

problem, with the human eye being particularly sensitive to light flicker in the frequency range

of 5-15Hz. Because of the wide use of such lamps, the effects are widespread and inevitably give

rise to a large number of complaints. Fluorescent lamps are also affected, though to a lesser

extent.

2.1.7 Voltage Unbalance

Unbalanced loading of the network normally causes voltage unbalance. However, parts of the

supply network with unbalanced impedances (such as untransposed overhead transmission lines)

will also cause voltage unbalance, though the effect of this is normally small. Overheating of

rotating equipment results from voltage unbalance. In serious cases, tripping of the equipment

occurs to protect it from damage, leading to generation/load imbalance or loss of production.

2.1.8 Supply Interruptions

Faults on the power system are the most common cause, irrespective of duration. Other causes

are failures in equipment, and control and protection malfunctions. Electrical equipment ceases

to function under such conditions, with undervoltage protection devices leading to tripping of

some loads. Short interruptions may be no more than an inconvenience to some consumers (e.g.

domestic consumers), but for commercial and industrial consumers (e.g. semiconductor

manufacture) may lead to lengthy serious production losses with large financial impact. Longer

interruptions will cause production loss in most industries, as induction and synchronous motors

cannot tolerate more than 1-2 seconds interruption without having to be tripped, if only to

prevent excessive current surges and resulting large voltage dips on supply restoration. On the

other hand, vital computer systems are often fed via a UPS supply that may be capable of

supplying power from batteries for several hours in the event of a mains supply failure. More

modern devices such as Dynamic Voltage Restorers can also be used to provide continuity of

supply due to a supply interruption. For interruptions lasting some time, a standby generator can

be provide a limited supply to essential loads, but cannot be started in time to prevent an

interruption occurring.

2.1.9 Undervoltage

Excessive network loading, loss of generation, incorrectly set transformer taps and voltage

regulator malfunctions, cause undervoltage. Loads with a poor power factor (see Chapter 18 for

Power Factor Correction) or a general lack of reactive power support on a network also

contribute. The location of power factor correction devices is often important, incorrect location

resulting in little or no improvement. The symptoms of undervoltage problems are tripping of

equipment through undervoltage trips. Lighting will run at reduced output. Undervoltage can

also indirectly lead to overloading problems as equipment takes an increased current to maintain

power output (e.g. motor loads). Such loads may then trip on overcurrent or thermal protection.

2.1.10 Transients

Transients on the supply network are due to faults, control and protection malfunctions, lightning

strikes, etc. Voltage-sensitive devices and insulation of electrical equipment may be damaged, as

noted above for voltage surges/spikes. Control systems may reset. Semiconductor manufacture

can be seriously affected unless the supplies to critical process plant are suitably protected.

2.1.11 Rapid voltage change

Rapid voltage change is caused by motor starting and transformer tap changing. In this case

undesired effects may be felt in the form of overheating in motors and generators and

interruption of three phase operation.

2.2 POWER QUALITY MONITORING

If an installation or network is thought to be suffering from problems related to Power Quality,

suitable measurements should to be taken to confirm the initial diagnosis. These measurements

will also help quantify the extent of the problem(s) and provide assistance in determining the

most suitable solutions. Finally, follow up measurements after installation will confirm the

effectiveness of the remedial measures taken.

2.3 Type of Installation

Monitoring equipment for Power Quality may be suitable for either temporary or permanent

installation on a supply network. Permanent installation is most likely to be used by Utilities for

routine monitoring of parts of their networks to ensure that regulatory limits are being complied

with and to monitor general trends in respect of power quality issues. Consumers with sensitive

loads may also install permanent monitoring devices in order to monitor Power Quality and

provide supporting evidence in the event of a claim for compensation being made against the

supplier if loss occurs due to a power quality problem whose source is in the Utility network.

2.4 Improving power quality

There are many ways to reduce the effects of poor power quality, and it is important to select an

appropriate solution with an adequate rating.

• Constant voltage transformers (CVTs) use saturation of transformer core to provide

output voltage regulation. The regulation is automatic, with no electronic circuitry and

moving parts.

• Automatic voltage stabilizers (AVS) use a transformer to add or subtract a small

percentage to the incoming voltage. Microprocessor control monitors the incoming mains

supply, and electronically selects a transformer tap so that output voltage stays close to

nominal.

• Regulators are electro-mechanical devices, consisting of a variable transformer (variac)

driven by a motor to adjust the output voltage close to the required level.

• Conditioner is a term usually applied to the device that regulate the voltage against input

voltage variations and absorbs surges and transients. They are effectively a stabilizer or

regulator, with added surge suppression and filtering functions.

• Passive harmonic filters consist of an array of passive components (capacitors, inductors

and resistors), designed to absorb harmonic currents. They are generally applied at larger

installations and require careful design, depending on measurements of voltage distortion.

• Power electronic compensators perform a similar function to passive filters, but they can

self-adapt to changing conditions and are much more controllable. In case of an overload

they will provide all the compensation they can within their rating, where a passive filter

may trip or fail.

• Surge suppressors are devices that conduct little until a threshold voltage is exceeded,

when they conduct and absorb energy to limit the voltage. They are available in sizes

from those to protect a single appliance, to those for high voltage electrical distribution.

Devices are available that combine suppresors with passive filtering for a single device or

group. Self-monitoring model that indicates continuing protection is preferable as

repeated operation or transients of too high a level can cause failure.

2.5 Voltage Monitor

Voltage monitoring circuits report when a voltage is beyond one or more pre-defined limits.

These circuits are used for many purposes, ranging from determining when a power supply has

achieved stable operation to determining when a sensed variable in a manufacturing process has

drifted out of tolerance. Although it is possible to monitor a voltage with an analog-to-digital

converter and make an in-bounds/out-of-bounds determination digitally with a microprocessor, a

completely analog solution is often simpler to implement or offers performance advantages such

as fast reaction time.

The design is also done by use of readily available and easy to use components such as diodes,

capacitors, op amps and resistors. Any complexity may be seen in the choice of transformer

which is relatively simpler in comparison to the use of a microprocessor.

3.0 DESIGN

The design approach employed voltage and frequency monitoring. The design consists of an

overvoltage detector, noise and spikes sensor and frequency detector.

Fig 3.1 Block diagram of the system design

This consists of:

• Noise and spike section

• Frequency monitor section

• Rectification and smoothening

• Overvoltage and undervoltage circuit

The noise and spikes section is responsible for detecting the presence of noise the mains.

Frequency monitor section is in charge of evaluating over and under frequency against the set

frequencies. This has a step down transformer which is connected to the overvoltage circuit. It

supplies the voltage to this circuit. The construction of the circuit is described in the section that

follows.

NOISE AND SPIKES SECTION

FREQUENCY DISTURBANCE DETECTOR

RECTIFICATION AND SMOOTHENING

OVER VOLTAGE AND UNDERVOLTAGE CIRCUIT

AC

3.1 Noise and spikes detector

Fig 3.2 Noise and spikes detector circuit

The noise section consists of a 50 hertz filter, a speaker and a bicolor light emitting diode.

The filter works to eliminate the spikes and allows for the detection of noise. The filter has three

400V 470 nano farads capacitors, two in parallel which in turn are in series with the other

capacitor. It also has a 2 watts 47 kilo ohms resistor in series with the two capacitors in parallel.

The bicolor light emitting diode gives the visual indication of noise or asymmetry in the wave.

The noise overimposed in the pure sinusoidal wave is visibly indicated by this set-up. The 5 kilo

ohms potentiometer is adjusted to allow for the least light from the light emitting diodes. In this

case it is possible to vary the sensitivity of the noise detection.

The 8 ohms speaker is connected through an audio transformer. This gives an audio indication of

noise.

The spike detector circuit has a constituent of a silicon controlled rectifier, a potentiometer and a

buzzer. By controlling the gate current through the SCR the buzzer is able to give a spike

presence indication.

C1470nF

R13.3kΩ

C2

470nF

C3470nF

R2

47kΩ

R3

5kΩKey=A 50%

LED1

LED2

R4270Ω

D1

1N4948GPD2

1N4948GP

T1

TS_AUDIO_VIRTUAL

XLV1

Input

R5

1kΩKey=A 50%

D31N4007

D5BT145_500R

R65.6kΩ

6

45

120

10

9

8731

D4

1N4007

U1

BUZZER200 Hz

R7

10kΩ C4470nF

1411 13

2

0

3.2 Frequency detector

The main part of this circuit makes use of the rectifying characteristics of the diode.

Fig 3.3 Frequency detector circuit

By setting an optimal range of frequency the behavior of the diode D1 or D3 is monitored. In this

case the full-wave rectification characteristic of the diode is the principle pointer to the frequency

variation from the set value. The values of capacitances and resistors are selected to match this

calibration. When the frequency deviates either below or above the predetermined value the

waveform observed will not be that of fully rectified diode.

The fluctuations in the mains frequency causes motors and generators to run at high or low

synchronous speed. The speed of the synchronous machines is dependent on the frequency of the

supply and the size of the poles of the machine. The speed increases with increase in supply

frequency.

However, in a production industry like the soft drinks industries, vehicle manufacturing

industries, precision is highly valued. In this case any slight variation in the speed of the motors

used to tighten the bottles, move them or even tighten bolts in the case of vehicles will results to

an undesired result. Thus these companies employs a frequency monitoring device to determine

whether it is necessary to put in place frequency stabilizers.

C1

470nF

C2470nF

C3570nF

C4

100uFC5

400nF

C6470nFC7

470nF

R13.9kΩ

R24.02kΩ

R347kΩ

R4

25kΩ

R547kΩ

R62.2kΩ

R73.777kΩ

R83.9kΩ

D1

1N4007

D21N4007

D3

1N4007

D41N4007

25

4

6

7 8 9

10

11

1

0

3.3 Overvoltage circuit

Fig 3.4 Overvoltage circuit

For overvoltage and undervoltage, the configuration in Figure 1b. In this case, it is assumed

|VWIN–|>|VWIN+|, where |VWIN–| is the magnitude of the negative window voltage and |VWIN+| is

the magnitude of the positive window voltage. Q1 is an NMOS enhancement-mode MOSFET

that has a threshold voltage of approximately 1V. The source terminal of Q1 connects to the

negative input of the op amp; thus, it remains at a virtual-ground potential. The gate terminal

connects to the op amp's output, which turns Q1 on whenever the output voltage exceeds Q1's

threshold voltage.

For input voltages greater than 0V, the op amp produces a negative voltage and Q1 turns off. The

ratio of R2 and R1 sets the op-amp gain, and the output clamps at the on-state voltage of the green

LED, approximately –2.1V. For input voltages lower than 0V, Q1 turns on once the op amp's

output exceeds the threshold voltage of Q1. In this case, the ratio of R1 and the parallel

combination of R2 and R3 sets the op-amp gain, and the maximum output voltage is the on-state

U1

741

3

2

4

7

6

51

Q1ZVN4424G

LED1LED2

R1

5kΩ

R2

5kΩ

R3

4.3kΩ

R4

5kΩ

2

3

VCC15V

VEE-15V

VDD-5V

VDD

VCC

VEE

7T20

1

2

3

D4

1N4004

D5

1N4004

D1

1N4004

D2

1N4004

R5

1kΩ

139

0

51

4

0

voltage of the red LED, 2V. Resistor R4 again serves as a current limiter for the LEDs. The

relationship between the resistor values and the positive and negative window voltages is given

by the following equations. For simplicity, the positive magnitude of the voltages was used, and

the difference between the forward voltages of the red and green LEDs was also neglected.

The value of R1 is chosen such that the feedback current through R4 is small in comparison

with the on-state LED current. Choosing R3 is such that its value is much greater than the on-

resistance of Q1.

3.3.1 Rectification

The voltage monitor circuit accepts a dc voltage. This implies that the mains supply voltage must

be rectified before being fed in this system. The rectifier unit converts the a.c voltage from the

step down transformer secondary winding into pulses of unidirectional current. Three types of

rectifier circuit are used for single phase: the half-wave, the full-wave and the bridge rectifier.

The half-wave rectifier although being a simple circuit has the main disadvantage of low

efficiency.

Fig 3.5 Diagram of half wave rectifier

Fig 3.6 Rectifier input and output voltage waveforms.

Vo1

Vd

T1

The diode conducts only during one half of the cycle so the efficiency cannot be greater than

50%

The full wave rectifier uses two diodes each conducting on alternate half cycles to give much

higher efficiency. However to achieve this, a transformer with a centre tapped secondary

winding is necessary. This means that twice the number of turns is required on the secondary

winding. This circuit was common when valve rectifiers were in use since it was cheaper to wind

extra turn on the transformer than to use more valves.

Fig 3.7 Full-wave rectifier circuit, input and output voltage waveforms

U1

2

R1

4.7kΩ

D1

1N4148

D2

1N4148

1

2

3

45

6

The bridge rectifier, now the circuit of choice, uses four diodes to achieve rectification over the

whole cycle and no centre tap of the transformer is required. The four diodes can now be

supplied in one encapsulated unit which is more convenient and somewhat cheaper than wiring

in four separate diodes. However should one part of the encapsulated bridge circuit fail, the

whole unit has to be replaced.

Fig 3.8 Bridge rectifier circuit

3.3.2 The transformer

This is a significant component of the voltage monitoring circuit. It takes in the ac power output

and sends it out at a different level of voltage and current depending on whether it is a step up or

step down transformer. The step up condition is utilized in transmission of electrical power since

high voltage links carry low currents and hence low power loss.

D1

3N258

1

2

4

3

U1

2

1

2

3

4

3.3.3 Construction features

Transformer consists of two coils which are magnetically coupled (through mutual inductance).

The coils can be wound on a laminated steel core or just air cored. The most commonly used

transformers are sheet-steel laminated and then assembled to provide a continuous magnetic path

with minimum air gaps. The steel used is of high silicon content sometimes heat treated to

produce a high permeability and low hysteresis loss at the usual operating flux densities. The thin

sheets of steel are separated from one another by a thin film of insulation. Such a film can be a

coat of insulating varnish, a sheet of paper pasted on the surface and sometimes even the film of

oxide remaining on the surface. This minimizes the eddy currents loss whose magnitude is

directly proportional to the square of the frequency and the square of the thickness of the

material.

3.3.4 Principle of operation

3.3.5 Transformer connection

Fig 3.9 Transformer connection.

One of the coils is connected to the supply and is therefore termed the primary coil and the other

is connected to the load (secondary coil). An alternating voltage applied to the primary circulates

an alternating current through the coil and this current produces an alternating flux in the steel

core; the mean path of this flux linking the primary and the secondary winding. The e.m.f

induced in each turn is the same for both winding. Thus if the primary winding and the

secondary winding have Np and Ns turns respectively the ratio of the secondary induced voltage

to primary is Ns/Np. For step up transformer Ns>Np while for the step down transformer

Ns<Np.

U1

2

V1

230 Vrms 50 Hz 0°

3

4

3.3.6 E.M.F EQUATION

Let the flux produced due to the current in the primary be given by

φ=φmsinωt

φm is te peak value of magnetic flux, while ω=2∏f. where f is the frequency of the supply.

Flux change with time

From the figure above the flux changes from + φm to – φm in T/2(s) or 1/2f (s).

Hence the average rate of change of flux

∆ф/∆t = 4фmf webers per second

Average e.m.f induced per turn= 4фmf

For sinusoidal waves the root mean square or effective value of the wave is 1.11 times the

average value. The RMS value of induced e.m.f per turn is

= 1.11 х 4фmf

= 4.44фmf

Therefore,

Ep = 4.44fфm Np

Es= 4.44фm Ns

Where Ep and Es are the rms values of induced voltage in the primary and secondary winding

respectively.

The above equations are employed in the design and final construction of a transformer to meet

the load requirement specification.

3.3.7 THE 741 OPERATIONAL AMPLIFIERS

The Operational Amplifier is probably the most versatile Integrated Circuit available. It is very

cheap especially keeping in mind the fact that it contains several hundred components. The most

common Op-Amp is the 741 and it is used in many circuits.

The OP AMP is a ‘Linear Amplifier’ with an amazing variety of uses. Its main purpose is to

amplify (increase) a weak signal - a little like a Darlington Pair.

The OP-AMP has two inputs, INVERTING ( - ) and NON-INVERTING (+), and one output at

pin 6.

The chip can be used in a circuit in two ways. If the voltage goes into pin two then it is known as

anINVERTINGAMPLIFIER.

If the voltage goes into pin three then the circuit becomes a NON-INVERTING AMPLIFIER.

Fig 3.10 The 741 Op-amp IC.

The 741 integrated circuit looks like any other ‘chip’. However, it is a general purpose OP-AMP.

You need only to know basic information about its operation and use. The diagram opposite

shows the pins of the 741 OP-AMP. The important pins are 2, 3 and 6 because these represent

inverting, non-inverting and voltage out. Notice the triangular diagram that represents an Op-

Amp integrated circuit.

In this case it is used to amplify the voltage fed to the two LEDs.

3.3.8 metal–oxide–semiconductor field-effect transistor

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS

FET) is a device used to amplify or switch electronic signals. The MOSFET includes a channel

of n-type or p-type semiconductor material, and is accordingly called an NMOSFET or a

PMOSFET (also commonly nMOS, pMOS). It is by far the most common transistor in both

digital and analog circuits, though the bipolar junction transistor was at one time much more

common.

Usually the semiconductor of choice is silicon, but some chip manufacturers, most notably IBM,

have begun to use a mixture of silicon and germanium (SiGe) in MOSFET channels.

Unfortunately, many semiconductors with better electrical properties than silicon, such as

gallium arsenide, do not form good semiconductor-to-insulator interfaces and thus are not

suitable for MOSFETs.

The gate is separated from the channel by a thin insulating layer of what was traditionally silicon

dioxide, but more advanced technologies uses silicon oxynitride

When a voltage is applied between the gate and source terminals, the electric field generated

penetrates through the oxide and creates a so-called "inversion layer" or channel at the

semiconductor-insulator interface. The inversion channel is of the same type – P-type or N-type

– as the source and drain, so it provides a channel through which current can pass. Varying the

voltage between the gate and body modulates the conductivity of this layer and makes it possible

to control the current flow between drain and source.

3.3.8.1 Circuit symbols

A variety of symbols are used for the MOSFET. The basic design is generally a line for the

channel with the source and drain leaving it at right angles and then bending back at right angles

into the same direction as the channel. Sometimes three line segments are used for enhancement

mode and a solid line for depletion mode. Another line is drawn parallel to the channel for the

gate.

The bulk connection, if shown, is shown connected to the back of the channel with an arrow

indicating PMOS or NMOS. Arrows always point from P to N, so an NMOS (N-channel in P-

well or P-substrate) has the arrow pointing in (from the bulk to the channel). If the bulk is

connected to the source (as is generally the case with discrete devices) it is sometimes angled to

meet up with the source leaving the transistor. If the bulk is not shown (as is often the case in IC

design as they are generally common bulk) an inversion symbol is sometimes used to indicate

PMOS, alternatively an arrow on the source (not depicted below) may be used in the same way

as for bipolar transistors (out for NMOS, in for PMOS).

Compari

symbols

voltages

JFET

For the

connecte

configura

of the M

all the tra

3.3.8.2 M

Fig 3.11

lights up

son of enha

(Note, the 2

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3.3.8.3 Modes of operation

The operation of a MOSFET can be separated into three different modes, depending on the

voltages at the terminals. In the following discussion, a simplified algebraic model is used that is

accurate only for old technology. Modern MOSFET characteristics require computer models that

have rather more complex behavior. For example, see Liu [4] and the device modeling list in [1].

For an enhancement-mode, n-channel MOSFET the three operational modes are:

Cut-off or Sub-threshold or Weak Inversion Mode

When VGS < Vth:

where Vth is the threshold voltage of the device.

According to the basic threshold model, the transistor is turned off, and there is no

conduction between drain and source. In reality, the Boltzmann distribution of electron

energies allows some of the more energetic electrons at the source to enter the channel

and flow to the drain, resulting in a sub threshold current that is an exponential function

of gate–source voltage. While the current between drain and source should ideally be

zero when the transistor is being used as a turned-off switch, there is a weak-inversion

current, sometimes called sub threshold leakage.

In weak inversion the current varies exponentially with gate-to-source bias VGS as given

approximately by:[5][6]

,

where ID0 = current at VGS = Vth and the slope factor n is given by

n = 1 + CD / COX,

with CD = capacitance of the depletion layer and COX = capacitance of the oxide layer. In

a long-channel device, there is no drain voltage dependence of the current once VDS > >

V

v

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4.0 Principle of operation

The following circuit was therefore designed.

Fig 4.1 Power quality monitor circuit

This design can detect spikes, noise overimposed in the mains, undervoltage overvoltage and

frequency instability.

5.0 Design simulation and results

C1470nF

R13.3kΩ

C2

470nF

C3470nF

R2

47kΩ

R3

5kΩKey=A 50%

LED1

LED2

R4270Ω

D1

1N4948GPD2

1N4948GP

T1

TS_AUDIO_VIRTUAL

XLV1

Input

R5

1kΩKey=A 50%

D31N4007

D5BT145_500R

R65.6kΩ

C4

470nF

C5470nF

C6570nF

C7

100uFC8

400nF

C9470nFC10

470nF

R73.9kΩ

R84.02kΩ

R947kΩ

R10

25kΩ

R1147kΩ

R122.2kΩ

R133.777kΩ

R143.9kΩ

D4

1N4007

D61N4007

D7

1N4007

D81N4007

U1

741

3

2

4

7

6

51

Q1ZVN4424G

LED3LED4

R16

5kΩ

R17

4.3kΩ

R18

5kΩ

VCC15V

VEE-15V

VDD-5V

T20

1

2

3

D9

1N4004

D10

1N4004

D11

1N4004

D12

1N4004

R19

1kΩ

V1

300 Vrms 50 Hz 0°

2

00

28

2726

25

VEE

VCC

VDD

24

23

20191816

17

15

1413

6

45

120

11

10

9

8731

XSC2

A B

Ext Trig+

+

_

_ + _

21

0

R20

10kΩKey=A 65%

29

22

The designed circuit was simulated using Multism pro-edition simulator. The result were as

shown below.

For the frequency monitor circuit the waveforms at various frequencies were as displayed in the

figures thereof.

At frequency of 0Hz there was no reading on the oscilloscope as shown.

At the frequency of 0Hz

At frequency of 15Hz

At frequency of 30Hz

At frequency of 45Hz

At frequency of 49.5Hz

At frequency of 50Hz

At frequency 50.5Hz

At frequency of 51Hz

At frequency of 55Hz

Voltage monitor showing an over voltage

Showing normal operation at 230V

Input and output waveforms to overvoltage circuit

6.0 CONCLUSION AND RECOMMENDATION

The technique described in this report works well to detect transient disturbances on low-voltage

ac supply mains. These techniques can be applied to mains with any nominal frequency. While

this circuit was designed for use on single-phase mains, it could still be connected between one

three-phase line and neutral. The disturbance detector circuit can be used to trigger a digital

waveform recorder, as in our application, or operate a counter circuit to simply record the

occurrence of a disturbance. While the event counter is much less expensive, it does not provide

as much information. A low-cost commercial disturbance counter that transfers its data via a

telephone line to a remote computer is envisioned. Such counters could be located at electric

utility substations and in buildings with critical electrical equipment. The computer would

routinely interrogate each counter to determine at each site both (1) the average number of

disturbances per hour and (2) the duration and time of every outage. In addition, when there is an

abnormally large number of disturbances or a long-duration outage, the counters could call a

central computer and report the problem. Changes in the power quality at a site might possibly

be detected, and corrected, before the problem adversely affected operation of critical equipment.

Moreover, susceptible equipment could be interfaced to a local disturbance counter to improve

reliability of operation. When disturbances were occurring at unusually high rate, a computer, for

example, could make more frequent backups of its data to a nonvolatile medium.

REFERENCES

1.www.delab.com

2. COUGHLN R.F.``Operational Amplifiers and Linear Integrated circuits’’

3. Dr. Ogaba’s class notes on Power Electronics.

4. RASHID M., 1993`` Power Electronics’’

5. F.F. MAZDA,``Electronic Instruments and Measuring techniques’’

6. www.wikipedia.org