design, development and fabrication of a static voltage relay using opamp 741
TRANSCRIPT
1
DESIGN, DEVELOPMENT AND
FABRICATION OF A STATIC VOLTAGE
RELAY USING OP-AMP 741
DHANSHYAM MAHAVADI: [email protected]
R NAGARAJU: [email protected]
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ABSTRACT
The primary objective of a power system is to provide a reliable and continuous
supply at the consumer end under all conditions. Due to the dynamic nature of the
loads and external conditions, the system parameters vary at every instant of time.
There are nominal voltage and frequency levels for every system. It is desirable that
the system voltage is always within the prescribed limits. However, due to various
internal and external factors, there is every possibility that the prescribed limits are
violated, which is referred to as abnormality. The abnormalities in the system
voltages may be either under-voltage or over-voltage. Both the conditions affect the
performance of the system in a negative manner. Abnormalities in the system may
lead to problems such as burning, insulation breakdown, reduced efficiency, etc. and
sometimes, even to the permanent damage of the electrical equipment. This prompts
the necessity to protect the system against the abnormal voltages. This can be
achieved using a relay.
A relay is a device, which senses the abnormalities in the system and then
initiates a signal to a circuit breaker to break the circuit, so that the equipment is
protected from being exposed to the abnormal conditions. Due to the advancements in
the field of Electronics and Integrated Circuits, the conventional electro-mechanical
relays are being replaced by static relays, which have many advantages due to the
absence of moving parts and lesser cost.
An Operational Amplifier is one of the most versatile integrated circuits which
have many general purpose applications. An op-amp in differential mode can be used
to detect the abnormalities in the system voltages by comparison with a standard
reference voltage fed at one of its input terminals. Thus, it may be applied in power
systems for relaying under abnormal voltage conditions.
The following project report on ‘Design, Development and Fabrication of a
Static Voltage Relay using an Op-Amp’ is an effort aimed at introducing the
application of an operational amplifier in the field of protective relaying and
implementing the same practically.
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CONTENTS
PAGE NO.
CHAPTER 1 INTRODUCTION 5
CHAPTER 2 SYSTEM DESCRIPTION 7
2.1 AC Supply 7
2.2 Step-Down Transformer 7
2.3 Rectifier 8
2.4 Filter 8
2.5 Reference Voltage Source 8
2.6 Comparator 8
2.7 Indicator 8
CHAPTER 3 PROTECTIVE RELAYING 9
3.1 Introduction 9
3.2 Basic principle 10
3.3 Types of Relays 11
3.4 Comparison between electromagnetic 12
and Static Relays
3.5 Static Relays 15
CHAPTER 4 OPERATIONAL AMPLIFIER 18
4.1 Introduction to Integrated Circuits and 18
Operational Amplifier
4.2 Basic Information of Op-Amp 19
4.3 Op-Amp – Modes of Operation 26
4.4 Op-Amp as Differential Amplifier 28
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CHAPTER 5 STATIC VOLTAGE RELAY USING 32
OP-AMP 741
5.1 Basic Principle 32
5.2 Practical Considerations 33
5.3 Design Considerations 37
5.4 Practical Implementation 49
CHAPTER 6 ADVANTAGES OF THE OP-AMP – 57
BASED STATIC VOLTAGE RELAY
CHAPTER 7 SCOPE FOR FUTURE ADVANCEMENTS 59
7.1 Tripping Circuit 59
7.2 Over-current Protection 62
7.3 High Power Applications 62
7.4 Inverse Time Relay 62
CHAPTER 8 CONCLUSION 63
BIBLIOGRAPHY
LIST OF FIGURES
LIST OF TABLES
APPENDIX – I SPECIAL FEATURES OF OP-AMP 741
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CHAPTER – 1: INTRODUCTION
Due to the dynamic load conditions, the system voltages are never steady at the
nominal system voltage, but undergo continuous fluctuations. This is not desirable
since it adversely affects the system performance. Hence, there should be a
mechanism to guard the system against these abnormal voltage conditions.
A static voltage relay is a device that senses the abnormalities in the system
voltages and initiates an appropriate action as per the requirement, thus protecting the
electrical equipment from damage.
The op-amp – based static voltage relay takes into consideration the minimum
and maximum voltage limits prescribed by the standards and is designed to provide a
visual indication using two LEDs – one for over-voltage indication and the other for
under-voltage indication. Personnel, watching the visual indication may switch-off or
switch-on the load as per the requirement.
In order to understand the complete concept of the static voltage relay, it is first
customary to know the phenomenon of abnormal voltages. The basic relaying system
will be explained exclusively in the following chapters.
The electrical energy is normally generated at the power stations far away from
the urban areas, where consumers are located, and delivered to the ultimate
consumers through a network of transmission and distribution. For satisfactory
operation of motors, lamps and other loads, it is desirable that the voltage at the
consumers‟ end is always maintained substantially constant. Too wide a variation of
voltages may cause erratic operation or even mal-functioning of consumers‟
appliances. The main cause responsible for voltage variation is the variation in load
on the supply system. With the increase in load on the supply system, the voltage at
the consumer premises falls due to increase in voltage drop in (i.) alternator
synchronous impedance, (ii) transmission lines, (iii) transformer impedance, (iv)
feeders, (v) distributors and (vi.) other electrical equipments. The reverse would also
happen if the load on the system falls. The voltage may also rise because of increase
in capacitive reactive power of the system. Such voltage variations are undesirable
and the voltage at the consumers‟ premises should always be within the prescribed
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limits, given by + or – 5 % of nominal supply voltage so that the consumer appliances
operate satisfactorily. If the voltage at the consumer premises exceeds the upper limit,
then the phenomenon is referred to as over-voltage. If the voltage falls below the
lower limit, it is known as under-voltage.
The most important effect of over-voltage are insulation break-down of
consumer appliances. On the other hand, the under-voltages result in reduced
intensity of lamps, reduced efficiency of induction motors, time errors in electrical
clocks, etc.
A voltage relay senses both of these phenomena and helps prevention of
damage of electrical appliances.
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CHAPTER – 2: SYSTEM DESCRIPTION
The following block diagram represents the general principle of a static voltage
relaying system:
Fig-2.1: Basic Block Diagram of the relaying system
The different elements and blocks of the system may be explained as follows:
2.1 AC Supply:
It is the system voltage of the system, whose level is to be checked for
abnormality. It is taken from the bus or the ac mains.
2.2 Step – down transformer:
The ac mains voltage is, under normal conditions, a large value of the order of
230 V. It is stepped – down to a lower and safer value that can be applied to the
comparator, with the help of a step – down transformer.
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2.3 Rectifier:
The stepped – down ac voltage is converted into a corresponding dc value, using a
rectifier. It can be done using different rectifier circuits employing different solid-state
devices like the diodes, thyristor bridges, etc.
2.4 Filter:
The output of the rectifier is a pulsating and intermittent dc signal. The ripples in
this signal are filtered out using a filter, to obtain a uni-directional dc signal. It can be
made using different combinations of passive components.
2.5 Reference Voltage Source:
It is a source of constant voltage, whose value is maintained under all
circumstances. It corresponds to a pre-decided value, representing the reference system
nominal voltage, for which the relay is rated. It may be provided using a battery or any
other method.
2.6 Comparator:
It is the heart of the entire relaying circuit. It has two inputs and one output. One
of its input is from the constant reference voltage, Vref, which is given from the constant
reference voltage source, Vref, while the other input is fed from the filter output, Vin.
The comparator compares the system voltage, Vin with the reference voltage, Vref
at every instant of time. Under normal operating conditions, the output of the comparator
is low. During abnormal conditions, i.e., either under-voltage or over-voltage, the output
magnitude is high with either sign.
2.7 Indicator:
The comparator output is fed to the indicator. It gives an indication when the
comparator output is high and gives no indication when the comparator output goes low.
Thus, as long as the system works in the normal condition, there is no indication
by the indicator, and any abnormal voltages are indicated by the indicator.
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CHAPTER – 3: PROTECTIVE RELAYING:
3.1 Introduction:
Protective relaying is that branch of the power engineering in which the design,
operation and application of the equipment required to detect the abnormal operating
condition on the system and initiate action for an automatic isolation of the faulty
section as quickly as possible so that damage to the system is minimized and the
system operation is brought back to normal operating condition, is explained.
In case of any fault, the device should be able to respond quickly in taking
suitable steps for isolating the disturbance. During this period the device should not
malfunction under any extraneous noise that may be generated by the disturbance.
Failure to protect can result in increased damage to the system equipment and
personnel apart from monetary loss.
In general, a relay is a device, which senses the abnormal conditions in the
system and gives a signal for the circuit breaker to operate.
The Institute of Electrical and Electronic Engineers (IEEE) defines a relay as
“an electric device designed to interpret input conditions in a prescribed manner and
after specific conditions are met to respond to cause contact operation or similar
abrupt change in the associated electric control circuits”. From this definition it is
clear that a relay transfers the information for a device to operate and isolate a section
of the system. Another word for transfer is of message is “relaying “. Hence, the
name “relay”. A relay whose function is to detect defective lines or apparatus or
other power system conditions of an abnormal or dangerous nature and/or to initiate
appropriate control circuit action is termed as “Protective Relay” (IEEE).
Thus, in short, a relay is a device which senses and/or causes fast disconnection
of equipment from power system during abnormal conditions and
Minimizes damage
Minimizes effects on the system operation
Maximizes power transfer
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3.2 Basic Principle:
As already stated, a relay is a relaying element. It is a device to detect the
defective lines or apparatus and initiates appropriate control action. It integrally
consists of:
Measuring device - It receives a signal from the Power System (PS) &
determines if the condition is abnormal.
Control device - For abnormal conditions, it signals Circuit Breakers (CB) to
disconnect the equipment.
It actuates a signal from the PS to the CB. Relay & CB are the parts of
protection system.
Input
Fig. 3.1: A logic representation of a relay
Any power system relay can be represented in the form of a simple block
diagram as shown in fig.3.1.
The components of each block may be electro-mechanical, solid-state or both.
While functions of the block diagram are general, they may be independently
designed or may be combined in any particular unit. In the case of static relays, for
example, a static comparator is exclusively used for comparison and the output is fed
to a slave device, which may be an electromechanical component. On the other hand,
in wholly electromagnetic relays, sensing and comparison is done by the relay itself
by balancing two fluxes provided by the input quantities.
Sensing
element
Comparing
element
Controlling
element
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3.3 Types of Relays:
Relays are classified in several different ways based on the function,
constructional features, inputs, performance characteristics or operating
principles.
The relays may be classified into different types on the basis of the actuating
quantities, which may be any one of the following:
1) Current
2) Voltage
3) Impedance
4) Power
Whenever faults occur, the voltages fall and the currents increase in
magnitude. Besides these changes, or more of the following parameters may change.
1) Phase angle
2) Frequency
3) Harmonic components
4) Rate of change
5) Direction
Relay operating principles base on these changes to distinguish between a
normal and abnormal situation.
However, the main classification is based on the type of construction. The
relays are classified and constructed based on the principles on which they are
expected to operate. The most common type of construction was based on the basics
of electromagnetic attraction/repulsion principles or electromagnetic induction
principles. These relays have become very common and stood the test of time for a
very long period almost till the mid sixties although instances do exists where they
are being used even today. They go by the name electromagnetic relays. With the
rapid expansions in the power system sizes in the recent years, it was necessary to
design relays on different philosophies to improve the times of operation of the
relays. Solid-state relays were next in the line wherein measurement is carried out by
stationary electronic circuits. Such relays, also known as static relays, have several
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merits and have been replacing the conventional electromagnetic relays for various
applications. Before the 1980‟s protective functions were independent of the control
and monitoring functions. With the advent of microprocessor operating on digital
techniques the functions of supervision, control and protection have been made
complementary rather than being independent. This has led to the combined
monitoring, control and protection functions in a single unit. These systems are
called the Combined Protection, Control and Monitoring systems (CPCMs). It is
possible, now-a-days, to develop microprocessor based units for independent or
combined functions as per requirements. Thus, a classification of relays may be stated
as follows:
1. Electromagnetic Relays
2. Static Relays
3. Microprocessor- based Relays
4. Computerized Relays
In addition, it is possible to develop relays using the non-electric quantities like
pressure, temperature etc for operation and have been developed. These go by the name
non-electric relays (thermal, pressure ...etc.).
3.4 Comparison between electro-magnetic and static relays:
The conventional electro magnetic relays are robust and quite reliable, but are
required to operate under different forces under fault conditions. This leads to delicate
setting, small contact gaps, special bearing systems, special clutch assemblies and
several measuring problems. These require instrument transformers (CTs and PTs)
with high burden and are bulky in size also.
The static relays in comparison to the corresponding electro magnetic relays
have many advantages.
3.4.1 Limitations of electro-magnetic relays:
1.) The electromagnetic relays are bulky in size. They occupy more space.
2.) Since the operation of electromagnetic relays depends on the motion of the
moving contacts, they prove to be sluggish at times, due to the inertia of the moving
parts. This may lead to unreliable response.
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3.) The response time is more due to the inertia of the moving contacts. Hence,
the time required for fault clearance is more.
4.) The efficiency of the system is low due to the frictional losses.
5.) Due to the friction, there may be wear and tear of the contacts, which
requires frequent replacement.
6.) There is a possibility of arcing at the contacts, which may damage the entire
unit.
7.) There is chance of unwanted tripping due to the ageing of springs, loss of
magnetism, decrease in magnetic strengths, etc.
8.) Their operation is much influenced by the surrounding atmosphere and
external operating conditions like gravity, presence of stray magnetic fields, etc.
9.) The life of the relays is less.
10.) A single relay seldom performs multiple functions. Thus, different units are
required for protection against different actuating quantities.
3.4.2 Advantages of static relays:
1.) The power consumption in case of static relays is usually much lower than
that in the case of their electro magnetic equivalents. Hence burden on the instrument
transformers (CTs and PTs) is reduced and their accuracy is increased, possibility of
use of air gapped CTs is there, problems arising out of CT saturation are eliminated,
and there is an overall reduction in the cost of CTs and PTs.
2.) Quick response, long life, shock proof nature, fewer problems of
maintenance, high reliability and a high degree of accuracy.
3.) Absence of moving contacts and associated problems of arcing, contact
bounce, erosion, replacement of contacts etc..
4.) Quick reset action- A high reset value and absence of overshoot can be
easily achieved because of the absence of mechanical inertia and thermal storage.
5.) There is no effect of gravity on the operation of static relays and therefore
they can be installed in vessels and aircraft, etc.
6.) Ease of providing amplification enables greater sensitivity to be obtained.
7.) Use of printed (or integrated) circuits avoids wiring errors and facilitates
rationalization of batch production.
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8.) The basic building blocks of semi conductor circuitry permit a greater degree
of sophistication in the shaping of operating characteristics, enabling the practical
realization of relays with threshold characteristics, more closely approaching the ideal
requirements.
9.) By combining various functional circuits, several conventional relays can be
substituted by a single static relay. For example, a single static relay can provide
over- current, under-voltage, and single phasing, short – circuit protection in an ac
motor by incorporating respective functional blocks.
10.) Static relays are very compact. A single static relay can perform several
functions. Single micro-processor based systems can substitute several independent
protection and control relay units.
The space required for installation of protective relays and control relays etc., is
reduced.
11.) The characteristics of static relays are accurate and superior. They can be
altered within certain range as per protection needs.
12.) Static relays can be designed for repeated operations. This is possible
because of absence of moving parts in the measuring circuits.
13.) The risk of unwanted tripping is less with static relays.
14). Static relays are quite suitable for earthquake- prone areas, ships, vehicles,
locomotives, aeroplanes, etc. This is because of high resistance to shock and
vibration.
15.) The static relays are provided with integrated features for self-monitoring,
easy testing and servicing. Defective module can be replaced easily.
16.) A Static protection, control and monitoring system can perform several
functions such as protection, monitoring, data acquisition, measurement, memory,
indication, data-communication etc..
Thus, for complex protective functions requiring accurate characteristics for
various protection systems and for protection of costly large equipment or machines
static relays are preferred technically and economically.
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These may be hard-wired or programmable. As static relays perform protective
and monitoring functions, the additional cost is justified on the basis of improved
system stability and reliability.
For integrated protection and monitoring systems programmable
microprocessor controlled static relays are preferred.
Having mentioned the advantages of the static relays, they have been
explained comprehensively in the following article.
3.5 Static Relays:
3.5.1 Introduction:
A static relay refers to a relay in which there is no armature or other moving
element and response is developed by electronic, magnetic or other components
without mechanical motion. The solid-state components used are transistors, diodes,
resistors, capacitors and so on. The function of comparison and measurement are
accomplished by static circuits.
A relay using combination of both static and electromagnetic units is also
called as static relay provided that the response is accomplished by static units
in static relays. The measurement is carried out by electronic, magnetic, optical or
other components. However, additional electro-mechanical relay units may be
employed in output stage as auxiliary relays. A protective system is formed by static
relays and electro-mechanical auxiliary relays.
3.5.2 Need for static relays:
With the rapid growth of electrical transmission and distribution systems during
last forty years and with the advent of much larger power stations and interconnected
systems, the duty imposed on protective gear become more and more severe. Thus,
relaying function became more and more complicated and many types of relays tend
to become very complex mechanically, and hence, costly and difficult to test and
maintain.
The basis of the static relaying is the use of the circuits and components to
obtain a variety of functions and operating characteristics, which for protection
purposes have traditionally been obtained using electro-mechanical devices. The need
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of fast and reliable protective schemes was realized because short-circuit levels,
circuit ratings and complexity of interconnection have increased. Shorter operating
times have become more essential for preserving dynamic stability of the system as
the characteristics and loading approach design limits. The satisfaction of the
requirements has left little scope for further improvements in further electro-magnetic
relays. Experience shows that such requirements can readily be met by static relays,
which are capable of performing electronic circuit control functions in a manner
similar to that of an electro-magnetic relay without using moving parts or elements.
The transistors have made it possible to achieve greater sensitivity and
simultaneously excellent mechanical stability, which would have never been possible
with electro-mechanical relays. The noteworthy point is that it is usually not
economical to replace the existing electro-magnetic relays with their static
counterparts just to reduce maintenance.
It is interesting to note that the static relays have been commercially
manufactured for the distance and the differential protective schemes while the much
the simpler over-current relays have not been developed. This is because the distance
and differential schemes are more amenable to mathematical analysis whereas the
over-current characteristics are more of empirical nature. With the use of static relays
it has been possible to achieve many varied and complex distance protection
characteristics, which are impossible with the conventional electro-magnetic relays.
3.5.3 Essential elements of a static relay:
The essential components of static relay are shown in figure 3.2 below:
Fig 3.2: Components of a static relay
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Here, the relaying quantity i.e., the output of a CT or PT or transducer is
rectified by a rectifier. The rectified output is supplied to a measuring unit comprising
of comparators, level detectors, filters, logic circuits. The output is actuated when the
dynamic input (i.e., the relaying quantity) attains the threshold value. This output of
the measuring unit is amplified by an amplifier and fed to the output device, which is
essentially an electro-magnetic one. The output unit energises the trip coil only when
relay operates.
In a static relay the measurement is carried out by static circuits consisting of
comparators, level detectors, filters, etc., while in a conventional electro-magnetic
relay, it is done by comparing operating torque with restraining torque. In the
individual relays, there is wide variation. The relaying quantity such as voltage or
current is rectified and measured. When the quantity under measurement attains a
certain well-defined value, the output device is triggered and thereby the circuit
breaker trip circuit is energized.
Static relay can be operated to respond electrical inputs. The other types of
inputs such as heat, light, magnetic field, travelling waves etc, can be suitably
converted in to equivalent analogue or digital signals and then supplied to the static
relay. A multi-input static relay can accept several inputs. The logic circuit in the
multi-input digital static relay can determine the conditions for relay response and
sequence of events in the response.
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CHAPTER – 4: OPERATIONAL AMPLIFIER:
4.1 Introduction to Integrated circuits and Operational Amplifier:
The present era is going through a period of micro-electronic revolution. For a
common person, the role of electronics is limited to audio-visual gadgets like radio,
T.V., DVD, but the truth today is that the growth of any industry like communication,
control or instrumentation, is dependent upon electronics to a great extent. Integrated
circuits are, now, the building blocks of modern electronics.
The integrated circuit or IC is a miniature, low cost electronic circuit consisting
of active and passive components that are irreparably joined together on a single
crystal chip of silicon. Most of the components used in the ICs are not similar to
conventional components in appearance, although they perform the similar electrical
functions.
The IC technology has got following advantages over the conventional circuits
using discrete components:
a.) Miniaturization and hence increased equipment density
b.) Cost reduction due to batch processing
c.) Increased system reliability due to elimination of soldered joints
d.) Improved functional performance
e.) Increased operating speeds
f.) Reduction in power consumption
ICs offer a wide range of applications and could be broadly classified as digital
and linear ICs. They may also be classified based on the scale of integration as Small
Scale integration (SSI), Medium Scale Integration (MSI), Large Scale Integration
(LSI) and Very Large Scale Integration (VLSI).
An Operational Amplifier, commonly referred to as an Op-Amp, is one of the
most versatile Integrated Circuits and has proven to be a revolution in the field of
electronics. It is internally a quite complex chip, which can be put into innumerable
applications.
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4.2 Basic information of OpAmp:
4.2.1 Circuit Symbol:
The µA741 is a general-purpose operational amplifier featuring offset-voltage
null capability.
The circuit schematic of an OpAmp is a triangle as shown in fig - 4.1.
Fig-4.1: Op-Amp Circuit symbol
It has two input terminals and one output terminal. The terminal with a (-) sign
is called inverting input terminal and the terminal with (+) sign is called the non-
inverting input terminal. The offsets N1 and N2 represent the biasing required for
general operation of the device.
4.2.2 Op-Amp pin configuration:
There are three popular packages in which the 741 OpAmp is available. They
are the metal can (TO) package, the dual-in-line package (DIP) and the flat package.
The most commonly used general-purpose configuration for experimental purposes is
the flat package.
The flat pack is further available in two pin configurations- the 14 pin or the 8
pin configuration. The one used widely is the 8-pin configuration.
The pin diagram of an 8-pin 741 OpAmp is as shown in the figure 4.2 below.
Fig-4.2: Op-Amp 741 Pin Configuration
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Op-Amps have five basic terminals, that is, two input terminals, one output
terminal and two power supply terminals. The significance of other terminals varies
with the type of Op-Amp and its application. Each of the pins and its significance
may be explained as follows:
Pin 1 - Offset Null: Since the op-amp is the differential type, input offset
voltage must be controlled so as to minimize offset. Offset voltage is nulled by
application of a voltage of opposite polarity to the offset. An offset null-adjustment
potentiometer may be used to compensate for offset voltage. The null-offset
potentiometer also compensates for irregularities in the operational amplifier
manufacturing process which may cause an offset. Consequently, the null
potentiometer is recommended for critical applications.
Pin 2 - Inverted Input: All input signals at this pin will be inverted at
output pin 6. Pins 2 and 3 are very important (obviously) to get the correct input
signals or the op amp cannot do its work.
Pin 3 - Non-Inverted Input: All input signals at this pin will be processed
normally without inversion. The rest is the same as pin 2.
Pin 4 - (-V): The V- pin (also referred to as -Vcc) is the negative supply
voltage terminal. Supply-voltage operating range for the 741 is -4.5 volts (minimum)
to -18 volts (max), and it is specified for operation between -5 and -15 V dc. The
device will operate essentially the same over this range of voltages without change in
timing period. Sensitivity of time interval to supply voltage change is low, typically
0.1% per volt.
Pin 5 - Offset Null: Same as pin 1.
Pin 6 - Output: The polarity of the output signal will be the opposite of
the input when this signal is applied to the op-amp's inverting input.
Pin 7 – Pos V: The V+ pin (also referred to as +Vcc) is the positive supply
voltage terminal of the 741 Op-Amp IC. Supply-voltage operating range for the 741
is +4.5 volts (minimum) to +18 volts (maximum), and it is specified for operation
between +5 and +15 Vdc. The device will operate essentially the same over this range
of voltages without change in timing period. Actually, the most significant
operational difference is the output drive capability, which increases for both current
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and voltage range as the supply voltage is increased. Sensitivity of time interval to
supply voltage change is low, typically 0.1% per volt.
Pin 8 (N/C): The 'N/C' stands for 'Not Connected'. There is no other
explanation. There is nothing connected to this pin, it is just left open to make it a
standard 8-pin package.
4.2.3 Op-Amp Ideal Characteristics:
Fig - 4.3: Circuit model of an Op-Amp
A circuit model of an operational amplifier is shown in Figure 2.3. The output
voltage of the op amp is linearly proportional to the voltage difference between the
input terminals by a factor (V1-V2) of the gain . However, the output voltage is
limited to the range [-Vcc,Vcc], where Vcc is the supply voltage specified by the
designer of the op amp. The range [-Vcc, Vcc] is often called the linear region of the
amplifier, and when the output swings to Vcc or -Vcc, the op amp is said to be
saturated.
An ideal op amp has infinite gain (A = ∞), infinite input resistance (R in = ∞),
and zero output resistance (Rout = 0).
In short, the characteristics of an ideal Op-amp may be enumerated as follows:
1) Gain is infinite.
2) Input impedance is infinite.
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3) Output impedance is zero.
4) Bandwidth is infinite.
5) Voltage out is zero (when voltages into each other are equal).
6) Current entering the amp at either terminal is extremely small.
However, in practice, no op-amp can meet these ideal characteristics. However,
the characteristics may be approximated to ideal characteristics for all practical
purposes without considerable loss of accuracy. The practical characteristics are
described in the following section.
4.2.4 Op-Amp practical Characteristics:
Unlike the ideal op-amp, the op-amp that is used in more realistic circuits today
does not have infinite gain and bandwidth. At very low frequencies, the open-loop
gain of an op-amp is constant, but starts to taper off at about 6Hz or so at a rate of -
6dB/octave or -20dB/decade (an octave is a doubling in frequency, and a decade is a
ten-fold increase in frequency).
Fig-4.4: Typical curves showing open-loop gain and frequency of 741 op-amp
This decrease continues until the gain is unity, or 0 dB. The frequency at which
the gain is unity is called the unity gain frequency. A real op-amp has a gain in the
range - (depending on the type), and hence, actually maintains a very small
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difference in input terminal voltages when operating in its linear region. For most
applications, we can get away with assuming +V = -V. We will use two operational
amplifiers in our laboratory exercises, the 741, a general purpose bipolar junction
transistor (BJT) based amplifier with a typical input resistance of 2 M . The output
resistance is 100Ω.
One of the main factors of consideration of a specific op-amp is its "gain-
bandwidth product". The product of the open-loop gain and frequency is a constant at
any point on the curve, so that: GBP = Aol.BW. Graphically, the bandwidth is the
point at which the closed-loop gain curve intersects the open-loop curve, as shown in
fig-2.4 for a family of closed-loop gains. For a more practical design situation, the
actual design of an op-amp circuit should be approximately 1/10 to 1/20 of the open-
loop gain at a given frequency. This ensures that the op-amp will function properly
without distortion.
One additional parameter is worth mentioning, the Transient Response, or rise
time is the time that it takes for the output signal to go from 10% to 90% of its final
value when a step-function pulse is used as an input signal, and is specified under
close-loop conditions. From electronic circuit theory, the rise time is related to the
bandwidth of the op-amp by the relation: BW = 0.35 / rise time.
4.2.5 Some important features and parameters of OpAmp 741:
a.) Absolute Maximum Parameters:
Maximum parameters indicate the values that the op-amp can safely tolerate,
i.e., the maximum ratings as given in the data section of such op-amp without the
possibility of destroying it. The μA741 is a high performance operational amplifier
with high open- loop gain, internal compensation, high common mode range and
exceptional temperature stability. The μA741 is short-circuit protected and allows for
nulling of the offset voltage. The μA741 is manufactured by Fairchild Semiconductor.
Some of the maximum parameters are enlisted in the table 2.1 below:
Supply Voltage (+/-Vs): It is the maximum voltage (positive and
negative) that can be safely used to feed the op-amp.
24
Dissipation (Pd): It is the maximum power the op-amp is able to dissipate
at the specified ambient temperature (500mW at 80° C).
Table 4.1: Absolute Maximum Ratings of µA741
Differential Input Voltage (Vid): This is the maximum voltage that can
be applied across the + and – input terminals of the op-amp.
Input Voltage (Vicm): The maximum input voltage that can be
simultaneously applied between both input and ground also referred to as the
common-mode voltage. In general, the maximum voltage is equal to the supply
voltage.
Operating Temperature (Ta): This is the ambient temperature range for
which the op-amp will operate within the manufacturer's specifications.
Output Short-Circuit Duration: This is the amount of time that an op-
amp's output can be short-circuited to either supply voltage.
b.) Summed-up Features:
Internal Frequency Compensation
Short-Circuit Protection
Offset voltage null capability
Excellent temperature stability
High input voltage range
NO latch-up
25
c.) Input Parameters:
Input Offset Voltage (Voi): This is the voltage that must be applied to one
of the input pins to give a zero output voltage. For an ideal op-amp, output offset
voltage is zero.
Input Bias Current (Ib): This is the average of the currents flowing into
both inputs. Ideally, the two input bias currents are equal.
Input Offset Current (Ios): This is the difference of the two input bias
currents when the output voltage is zero.
Input Voltage Range (Vcm): The range of the common-mode input voltage
(i.e. the voltage common to both inputs and ground).
Input Resistance (Zi): This is the resistance looking-in at either input with
the remaining input grounded.
d.) Output Parameters:
Output Resistance (Zoi): It is the effective resistance of the device seen
looking into the op-amp's output.
Output Short-Circuit Current (Iosc): This is the maximum output current
that the op-amp can deliver to a load.
Output Voltage Swing (Vo max): Depending on what the load resistance is,
this is the maximum 'peak' output voltage that the op-amp can supply without
saturation or clipping.
e.) Dynamic Parameters:
Open-Loop Voltage Gain (Aol): The output to input voltage ratio of the
op-amp without external feedback is termed as Open-Loop Voltage Gain.
Large-Signal Voltage Gain: This is the ratio of the maximum voltage
swing to the charge in the input voltage required to drive the output from zero to a
specified voltage (e.g. 10 volts).
Slew Rate (SR): The time rate of change of the output voltage with the op-
amp circuit having a voltage gain of unity (1.0).
f.) Other Parameters:
Supply Current: This is the current that the op-amp will draw from the
power supply.
26
Common-Mode Rejection Ratio (CMRR): This is the measure of the
ability of the op-amp to reject signals that are simultaneously present at both inputs. It
is the ratio of the common-mode input voltage to the generated output voltage,
usually expressed in decibels (dB).
Channel Separation: Whenever there is more than one op-amp in a single
package, like the 747 op-amp, a certain amount of "crosstalk" will be present. That is,
a signal applied to the input of one section of a dual op-amp will produce a finite
output signal in the remaining section, even though there is no input signal applied to
the unused section.
4.3 Op-Amp – Modes of operation:
4.3.1 Open loop Operation of an Op-Amp:
The simplest way to operate an op-amp is in the open-loop configuration. In this
mode, the signals V2 and V1 are applied at the inverting and non-inverting terminals
respectively, as shown in fig 4.5.
Fig-4.5: Open-loop operation of Op-Amp
Since the gain is infinite, the output V0 is either at its positive saturation voltage
(+Vsat) or negative saturation voltage (-Vsat) as V2>V1 or V1>V2 respectively. The
output assumes one of the two possible output states and amplifier acts as the switch
only. This has a very limited number of applications and, is hence, operated in closed-
loop configuration.
4.3.2 Closed loop operation of Op-Amp :
With positive feedback, the op-amp can be used as an oscillator. The utility of
op-amp can be greatly increased by providing negative feedback. The output in this
case is not driven into saturation, and hence, the circuit behaves in a linear manner.
The different modes of negative feedback circuits are explained below:
27
a.) The inverting Amplifier:
This is perhaps, the most widely used of all possible configurations. The circuit
is as shown in fig 4.6.
Fig-4.6: Op-Amp inverting amplifier
The output voltage Vout is fed back to the inverting terminal through the Rf-R1
network, where Rf is the feedback resistor. The input signal Vin is applied to the
inverting input terminal through R1 and non-inverting input terminal of the op-amp is
grounded.
The output of an inverting amplifier is guided by the relation:
Vout = (-Rf/R1) Vin (4.1)
The term in the brackets represents the gain of the amplifier. By suitable
selection of the values Rf and R1, the required output can be achieved.
b.) The Non-inverting Amplifier:
If the signal is applied to the non-inverting input terminal and the feedback is
given as shown in the fig 4.7, the circuit amplifies without inverting the input signal
and is referred to as the non-inverting amplifier.
As the differential voltage Vd at the input terminal of the op-amp is zero, the
voltage at the upper node is same as Vin., same as the input applied to the non-
inverting input terminal. Now, Rf and R1 form a potential divider. Hence,
Vout = (1 + Rf/R1) Vin (4.2)
28
Fig-4.7: Op-Amp Non-inverting Amplifier
The term in the brackets indicates the closed-loop gain of the non-inverting
amplifier. The gain can be adjusted to unity or more, by proper selection of resistors
Rf and R1. Compared to the inverting amplifier, the input resistance of the non-
inverting amplifier is extremely large as the op-amp draws negligible current from the
signal source.
c.) The Differential Amplifier:
A circuit that amplifies the difference between the two signals is called
difference or differential amplifier. This mode of operation is instrumental in the
design of the relay.
4.4 Operational Amplifier as a Differential Amplifier:
4.4.1 Introduction:
All op-amps are differential input devices. Designers are accustomed to working
with these inputs and connecting each to the proper potential. What happens when
there are two outputs? How does a designer connect the second output? These are a
couple of questions that need to be answered before going into the depth of the
project details. A general study of the op-amp differential amplifier is, thus, essential
for understanding the basic principle of operation of the relay.
4.4.2 History:
The idea of fully differential op-amps is not new. The first commercial op-amp,
the K2-W, utilized two dual section tubes (4 active circuit elements) to implement an
29
op-amp with differential inputs and outputs. It required a ±300 V dc power supply,
dissipating 4.5 W of power, had a corner frequency of 1 Hz, and a gain bandwidth
product of 1 MHz.
In an era of discrete tube or transistor op-amp modules, any potential advantage
to be gained from fully differential circuitry was masked by primitive op-amp module
performance. Fully differential output op-amps were abandoned in favour of single
ended op-amps. Fully differential op-amps were all but forgotten, even when IC
technology was developed. The main reason appears to be the simplicity of using
single ended op-amps. The number of passive components required to support a fully
differential circuit is approximately double that of a single-ended circuit.
Almost 50 years later, IC processing has matured to the point that fully
differential op-amps are possible that offer significant advantage over their single-
ended cousins. The advantages of differential logic have been exploited for 2 decades.
More recently, advanced high-speed A/D converters have adopted differential inputs.
Single-ended op-amps require a problematic transformer to interface to these
differential input A/D converters. This is the application that spurred the development
of fully differential op-amps. An op-amp with differential outputs, however, has far
more uses than one application.
4.4.3 Differential Amplifier Basic Circuits:
The easiest way to construct fully differential circuits is to think of the inverting
op-amp feedback topology. In fully differential op-amp circuits, there are two
inverting feedback paths:
Inverting input to non-inverting output
Non-inverting input to inverting output
Both feedback paths must be closed in order for the fully-differential op-amp to
operate properly.
4.4.4 Basic Differential Amplifier Circuit:
This type of amplifier is very useful in instrumentation circuits. A typical circuit
is shown in the fig-4.8.
Since, the differential voltage at the input terminals of the op-amp is zero, both
the input nodes are at the same potential.
30
The two inputs V1 and V2 are applied to the inverting and non-inverting
terminals respectively through the resistances R1, equal in value. The non-inverting
terminal is grounded through the resistance R2. The feedback is negative feedback
Fig-4.8: Op-Amp Differential Amplifier
and it is given through the resistance R2. It is noteworthy that usually, the grounding
and feedback resistances are equal in value here.
The output and input are related by the relation:
Vout = (R2/R1) (V1-V2) (4.3)
The transfer function gives the gain of the differential op-amp. It is given by:
A = (Vout /(V1-V2)) = R2 / R1. (4.4)
Such a circuit is very useful in detecting very small differences in signals, since
the gain (R2/R1) can be chosen to be very large. For example, if R2 = 100 R1, then a
small difference V1-V2 is amplified 100 times.
On the other hand, if the resistances are so adjusted that R1 = R2, the output is
the difference of the two inputs.
This circuit, by proper design of resistances, can be used to obtain a visual
indication in the case of over-voltages or under-voltages. This forms the basic
principle of the project.
4.4.5 Differential and Common-mode gains:
From the above explanation, it is clear that the output, which is given by Vout =
(R2/R1) ( V1-V2) reduces to zero, if V1 = V2.That is, the signal common to both inputs
31
gets cancelled and produces no output voltage. This is true for an ideal Op-Amp,
however; a practical op-amp exhibits some small response to the common mode
component of the input voltages too. The output voltage depends not only upon the
difference signal Vd as input, but is also affected by the average voltage for the input
signals, called the common-mode signal Vcm defined as,
Vcm = (V1+V2)/2
For differential amplifier, though the circuit is symmetric, because of the
mismatch, the gain at the output with respect to the positive terminal is slightly
different in magnitude to that of the negative terminal. So, even with the same voltage
applied to both inputs, the output, therefore, must be expressed as,
Vo=A1V1+A2V2
Where, A1(A2) is the voltage amplification from input 1(2) to the output with
input 2(1) grounded. since Vcm = (V1+V2)/2 and Vd = (V1-V2)
V1 = Vcm+Vd/2
And V2 = Vcm-Vd/2
Substituting the value of V1 and V2 in Vo equation, we get
Vo=AdmVd+AcVc
Where, Adm = (A1-A2)/2
And Acm = A1+A2
The voltage gain for the difference signal is Adm and that for the common-mode
signal is Acm.
4.5.6. Common Mode Rejection Ratio:
The relative sensitivity of an op-amp to a difference signal as compared to a
common-mode signal is called common-mode rejection ratio(CMRR) and gives the
figure of merit „ρ‟ for the differential amplifier. So, CMRR is given by:
ρ = | Adm / Acm|
And is usually expressed in decibels (dB). We should have large Adm and Acm
should be zero ideally. So, higher the CMRR, better is the Op-Amp.
32
CHAPTER – 5: STATIC VOLTAGE RELAY USING OP-AMP 741:
The above comprehensive explanation and description of the principles and
basics of protective relaying and the operational principles and modes of operating an
operational amplifier, has given a general idea and thought of clubbing the two to
obtain some protection using the Op-amps. This approach has been thoroughly
studied and implemented to obtain a static voltage relay for low power applications
and for different system voltages in ac distribution systems.
The ongoing chapter gives a description and explanation of the use of op-amp
741 for relaying application and the practical difficulties involved, along with the
probable solutions to some of them.
5.1 Basic Principle:
An operational amplifier operating in the differential mode can be used for
indication of over-voltage or under voltage for low power applications.
The fig- 5.1 explains the basic principle of operation of the relay.
Fig-5.1: Basic principle of operation
The above figure shows an op-amp connected in the differential mode of
operation. The terminals 2 and 3 are the inverting and non- inverting terminal
respectively. The resistances R1 and R2 are the resistances to which the input is fed.
R‟ is the grounding resistance and Rf is the negative feedback resistance.
33
The inverting terminal of the op-amp is maintained at some reference value, say
Vref as shown in the figure. The input at the non-inverting terminal corresponds to the
supply voltage, which has to be identified for normal or abnormal levels.
The output of the differential amplifier in the above fig is given by:
Vo = - (Rf/R1) Vref + (1+Rf/R1) {R‟/(R‟+R2)} Vin (5.1)
The resistance values R1 and R2 are equal in magnitude. The resistance values R‟
and Rf are simulated so that a desirable output may be obtained.
Under normal conditions, the value Vin is maintained equal to the reference value
Vref. Under this condition, the output is zero. Under abnormal conditions, the
following two cases arise:
(i.) Under voltage: Under this condition, Vin < Vref and the output goes negative.
(ii.) Over voltage: Under this condition, Vin > Vref and the output goes positive.
Thus, it may be seen that the circuit does not give any output under normal
operating conditions. However, under abnormalities, there is a substantial magnitude
of the output. This output may be used to drive any indicating device like an LED and
a tripping element or a circuit breaker.
5.2 Practical considerations:
In order to implement the above principle, certain practical considerations need
to be made. Some of them are the voltage considerations, design considerations,
output considerations, input considerations and so on.
5.2.1 Voltage considerations:
The relay under study is being designed to operate for multiple voltage levels of
the distribution systems.
In India, the nominal voltage level is 230V single phase supply (line- to-
neutral) and frequency of 50Hz at the consumer end, according to the Indian
Electricity Act. The equipments ad appliances are designed for satisfactory operation
at this voltage and frequencies. However, due to the dynamic and varying nature of
the loads and other factors, the voltage is never practically constant at the nominal
value, but varies in and around that value. Accordingly, certain limits have been
specified for the variation of frequency and general voltage levels. According to
34
Institute of Electrical and Electronics Engineers (IEEE), the maximum permissible
limits in the voltage levels for any system is (+ or -) 5% of the nominal value.
The voltage limits, thus, for a 230V system are +11.5V for over voltage and -
11.5V for under voltage. These limits should be taken into account for the operation
of the relay.
Thus, the relay should be designed as follows:
(i.) 218.5V <= Vin <= 241.5V: Normal operation. The relay should take this
condition as normal and should not respond by giving any visual indication or
tripping signal.
(ii.) Vin < 218.5V: Under-voltage: In this case, the input voltage should
become less than the reference voltage and the output should go negative. The circuit
should give a visual indication and/or give the signal for the circuit breaker to
disconnect the load and avoid it from being exposed to under voltages.
(iii.) Vin > 241.5V: Over voltage: In this case, the input voltage should become
greater than the reference voltage and the output should go positive. The circuit
should give a visual indication and/or give the signal for the circuit breaker to
disconnect the load and avoid it from being exposed to over voltages.
Apart from these considerations, the voltage drops in the various components of
the circuit should also be taken onto account.
5.2.2 Resistance values:
In the fig 5.1, according to equation 5.1, the output depends on the values of the
resistances R1, R2, R‟ and Rf..
The resistances R1 and R2 are usually equal in order to ensure a justified
distribution of voltage and equalize the voltage drops at both, the inverting as well as
the nom-inverting terminal so that the output depends solely on the difference
between the input voltages. This helps to nullify the effect of unequal voltage
distributions, leading to undesirable noise in the signal.
The other factor that needs to be taken into consideration is the number of
connections. More the number of connections, more is the number of joints, and
correspondingly, more is the noise. This leads to the false or the mal-operation of the
circuit. Thus, it is a general practice to use the resistance values that are available as
35
standard resistors as far as possible. Hence, it is advisable that during designing the
resistance values, the resistance values R1 = R2 and the grounding resistance R‟ are
fixed to standard, readily available values and vary the feedback resistance value for
different modes of operation, to obtain different resolutions and different values of
output.
5.2.3 Indicator Considerations:
The relay designed here is basically meant to give a visual indication in case of
either under voltage or over voltage considerations. Since both of these cannot occur
simultaneously, the circuit output corresponds to only one of the over voltage or
under voltage at any given instant.
Different types of indicators are available, which provide a proper and precise
visual indication under faulty conditions. The most common type is a Light Emitting
Diode (LED). The LED has many advantages over the other indicators, some of
which are low on-state voltage drop, low breakdown voltage, less turning-on time and
less power dissipation for full glow.
The On-state voltage drop, i.e., the breakdown voltage of a green LED, which is
the most readily available and commonly used LED, is about 1.79V, which may be
approximated to 1.8V. Thus, 1.8 volts is the threshold value for the LED to glow.
This means that the LED starts to glow only if the output voltage reaches 1.8V on
either side. In case of over voltage, the threshold point of operation is Vo = +1.8V and
the LED is placed at the output terminal with anode connected to the output terminal
of the OpAmp and the cathode is grounded. In case of under voltage, the threshold
point of operation is Vo = -1.8V and the LED is placed with cathode connected to the
output of the op-amp and the anode grounded.
Usually, the under voltage and the over voltage indication are obtained by two
separate op-amp circuits. Also, both modes of operations may be achieved using a
single op-amp. In such case, the two LEDs, one indicating over voltage and the other
indicating under voltage, both connected to the common output terminal of the op-
amp. In such case, the two LEDs are connected anti-parallel to each other.
The value of resistance Rf is simulated and then practically implemented such
that the resolution is same in both positive and negative LEDs.
36
5.2.4 Op-Amp Biasing Voltages:
An Op-amp has two pins –pin 7 and pin 4 which are fed with +Vcc and –Vcc
respectively. These power supplies are required for the operation of an Op-amp.
These are typical values, but in general, the supply voltage may range from about 5V
to 22V.The common terminal of the two supplies is connected to ground. The supply
source can be anything like battery, generator etc. Providing the biasing voltage for
op-amp through some source is again one of the most important challenges that need
to be overcome while implementing the relay practically.
The use of a transformer-converter circuit with some filters may serve the
purpose to much extent. Once rectified, the dc may be adjusted to be maintained at a
constant value with the help of 3-pin regulators, zener diodes, etc. Apart from these
methods, a combination of parallel RC networks can also be used, whish divides the
supply available at the input terminals into two equal values. The capacitors and
resistors should be selected judiciously.
5.2.5 Reference Voltage:
The reference voltage is fed at the non-inverting terminal through the resistance
R2 as in fig-5.1. The reference voltage should have following characteristics:
It should be a fixed value.
It should be maintained constant under all operating conditions.
The fluctuations in the voltage levels of load circuit should not affect its
operation and output.
The variation in the output, if any, should be as low as possible. It should
be such that it may be assumed to be constant practically under all
operating conditions.
It should not be affected by being loaded. That means to say that it should
maintain its voltage value at the same value at either no-load or loaded
conditions.
It should be economical and flexible enough to be used simultaneously by
more than one Op-Amp circuit.
37
The features can normally be achieved with the help of a constant battery
source, but it is prone to fall in its output gradually under continuous use. This may
prompt the user to replace the battery source frequently, which is a costly affair.
The other methods of providing the constant reference voltage Vref is to rectify
the voltage to suitable levels and then make use of 3-pin regulators or zener diodes.
However, the care should be taken that these components would not be affected by
the load on them.
5.2.6 Input Voltage, Vin:
This is the most important quantity in the circuit. It is this voltage value, which
determines the output of the circuit, once it has been designed for particular values of
resistances.
The quantity Vin is the replica of the system voltage of the ac system. It is
obtained by stepping down the supply voltage (which needs to be checked for
abnormality) to the value close to the reference dc voltage and rectifying it to the
value such that it is equal to the reference voltage, under normal conditions.
This can be done using a transformer of suitable rating with or without a centre
tap as per the requirement. In case the centre tap is not required, but is present, it is
left unconnected. The stepped-down ac voltage is rectified using a bridge rectifier
employing diodes or a rectifier IC into intermittent dc voltage. The ripple is filtered
using a C-filter across the diode bridge.
In order to achieve accurate results, the transformer must give appropriate
voltage at its output, which, under normal conditions, is very close to the reference dc
voltage. Else, it may lead to the mal-operation of the relay.
5.3 Design Considerations:
In order to implement the circuit using the basic principle discussed in the
article 5.1, the above practical considerations also need to be taken into account.
The design aspect involves theoretical calculations and simulation. It mainly
focuses on the values of resistances R1, R2, R‟ and Rf, for proper working.
38
5.3.1 Simulation:
The simulation may be either digital simulation or analog simulation.
Simulation is mainly aimed at providing a general idea to the user about the likely or
possible values of the circuit components in order that the proper output is achieved.
Digital simulation is the one done on a computer system, with the aid of many
simulating software such as PSPICE, Multisim, Simulink, MATLAB, etc. Each of
these softwares has its own advantages.
PSPICE is the most basic and versatile software available for digital simulation.
It provides many features, which is a collection of many electronic components, its
practical electrical characteristics, and gives a desired level of accuracy to the user.
The simulation using PSPICE may be done in two ways:
(i.) Using Program.
(ii.) Using circuit components.
The simulation that has been adopted here is the one using program.
While doing simulation, the following factors need to be considered:
The resistance values arte as close to standard, readily available ones as
possible.
The resistances R1 = R2 are fixed and either of Rf an R‟ is varied keeping
the other one constant at a standard value.
It is a general practice to fix the grounding resistance, R‟ constant and
vary the feedback resistance Rf to obtain the desired output.
The desired output implies that one of the LEDs glows for the abnormal
conditions and both LEDs remain in Off-state under normal operating conditions.
Each of these factors is taken into consideration while selecting suitable values
of resistances.
After having a sufficient information and approximate value of Rf through
digital simulation, the values of the circuit components is implemented using analog
simulation.
Analogue simulation is the one where all the circuit components are practically
connected together and some of the quantities are varied using variable resistances,
capacitances or inductances. Since analog simulation involves the real-time, practical
39
and actual simulation, it takes into consideration the original behaviour of circuit
components under operating conditions, which is usually neglected during the digital
counterpart.
The simulation is carried using a breadboard. A breadboard is a versatile circuit
board, which provides flexible and easy connections of different circuit elements
according to the requirement. It consists of pairs of a series of holes, which are
shorted in different directions. Any connection can be easily made, broken or varied
using the breadboard.
The values R1, R2 and R‟ are replaced by standard, pre-decided values obtained
in the digital simulation and the feedback resistance Rf is a decade resistance box.
The pairs of LEDs are connected in anti-parallel at the output terminals of the op-amp
to obtain the visual indication.
The op-amp biasing voltages (+Vcc and –Vcc), the reference voltage (Vref) and
the input voltage under test (Vin) are applied using a regulated power supply (RPS).
All other values except the Vin and Rf are fixed. Rf and Vin are variable and it is
these values that are varied to obtain he desired output.
Rf is initially fixed at the value obtained by digital simulation. The Vin is now
varied in both the directions (above and below the reference). The voltage limits are
also taken into consideration. It is desirable that both the LEDs remain off as long as
the voltage Vin is within the prescribed limits. The moment it violates the limits in
either direction, the LED should glow. This means to say that the value of Rf has to
be adjusted to such a value that the threshold of the LED (say, 1.8V) is obtained at the
output at the value of Vin that violates the limits in either direction.
Thus, it may be observed that the value for Rf under practical scenario is slightly
different from the one obtained in the digital simulation.
It would be first worthwhile focusing on the values of the standard and fixed
resistances and the reference input level, since these are the factors on which the
design is completely based upon.
5.3.2 Resistances R1, R2 and R’:
The resistance values form the heart of the circuit. These are the values which
determine the points and ranges of operation of the devices, and must be judiciously
40
selected. Some practical limitations and requirements of the op-amp must be taken
into consideration while deciding the resistance values.
Right from the beginning of the description, it has been specified that that the
resistances R1 and R2 are selected to be equal in value. This is done to ensure that the
unequal distribution of voltages at the two terminals is avoided, as already been
explained.
In any op-amp circuit, it may be observed that the output is not zero even if the
difference in the inputs is zero. This is due to the presence of some unbalance, leading
to input offset current. It is usually desirable that the value of input offset current is as
low as possible. This can be achieved by keeping the feedback resistance at a low
value.
From the fig 5.1, it can be seen that the input impedance of the op-amp is
affected by the values of R1 and R2. Ideally, we know that, the input impedance of the
op-amp is infinite. Thus, the value of input impedance should be as large as possible.
Thus, the values of input resistances R1 and R2 should be sufficiently large in
magnitude.
Now, from the explanation made in the article 4.5.2 about the basic differential
amplifier circuit, the reduced output equation is given by:
Vo = (Rf / R1) (Vin – Vref) (5.2)
Thus, The gain of the circuit depends on the value of the feedback resistance. In
order to keep the gain at a larger value, the feedback resistance is usually kept at a
value higher than the resistance R1.
Taking the above mentioned problems into consideration, the resistance values
R1 and R2 are fixed at 1KΩ(arbitrary).
The grounding resistance value is usually selected to be comparable to that of
the feedback resistance. Since the feedback resistance is varied to obtain the desired
output, and the value of gain should be considerable, the value of the grounding
resistance is fixed at an arbitrary value higher than the value of R1, say 3.3KΩ.
Thus, by proper analysis, the values of the constant resistances R1, R2 and R‟ are
fixed as:
R1 = R2 = 1KΩ, R‟= 3.3KΩ
41
The value Rf is determined by simulation.
5.3.3 Decision on the value of Reference Voltage, Vref:
The voltage inputs given to the op-amp are the dc inputs. The input at the
inverting terminal is the reference input and has to be maintained constant under all
circumstances. The value should be easily obtainable. For that, the value should
correspond to a value that can be easily stepped down using a transformer whose HV
rating corresponds to the system nominal voltage of 230V.
The centre-tapped transformers of the rating 230/(6-0-6) are readily available in
the market at low costs and various current ratings. Thus, the reference voltage is
selected as 6V constant dc source. It is maintained at constant potential using a zener
diode of the rating 6.2V, 0.5W.
5.3.4 RC potential divider circuit:
The op-amp 741 requires pair of a constant biasing voltages +Vcc and -Vcc to be
fed to the 7th
and 4th
terminals respectively. The voltage values can be anything
ranging from 5V to 22V.
There are many methods to provide the constant voltage for the operation of the
op-amp, like the use of regulator pairs of the series 7800 and 7900. However, another
method has been tried here as a matter of practical implementation.
The method adopted here employs an RC potential divider, which divides the
voltage applied across it into two equal parts. By applying a dc voltage of some 18 to
20 V using the rectifier set-up stated in the above article, such dc voltage can be
developed.
When a dc is applied across the common terminals of the output of the rectifier,
the voltage is divided into two halves, equal in magnitude, but opposite in sign. The
two RC pairs, which consist of a parallel combination of R and C, are grounded at the
common point of their connection. The upper RC pair is at the positive potential and
forms the +Vcc supply while the lower RC pair is at a negative potential with respect
to the ground and forms the –Vcc supply.
The values of the resistance and capacitance have to be selected judiciously. For
this, there are two methods- one is to design using proper analysis and the other one is
by practical trial and error method.
42
By trial and error method, it has been found that the values given by R = 1KΏ
and C = 100μF.
With this particular combination, the required biasing potential may be obtained
for satisfactory operation of the op-amp.
5.3.5 Design of Rf:
The design of the feedback resistance is the most important design aspect in the
circuit, since it is this value which determines the gain of the circuit and drives the
visual indication circuit.
The design of the resistance can be done using three different methods, given by
design calculations, digital simulation using PSPICE and analogue simulation.
Each of these designs has been described below:
(a.) Design calculations:
The output equation of the op-amp circuit with resistances R1, R2, Rf and R‟ is
given by equation 3.1 as:
Vo = - (Rf/R1) Vref + (1+Rf/R1) {R‟/(R‟+R2)} Vin (5.1)
We know that R1 = R2 and the value of grounding resistance should be
approximately equal to the feedback resistance value, i.e, R‟ and Rf are approximately
the same. Substituting this condition in the above equation, the reduced output
equation may be given as:
Vo = (Rf/R1) (Vin – Vref) (5.2)
We know that for visual indication, the output value Vo should be more than the
threshold breakdown voltage of the LED, which is equal to 1.8V.
Thus, the condition for visual indication may be specified as follows:
i. One LED should glow when Vo < -1.8V. This condition describes the
under voltage condition. It happens when Vin < Vref, taking into account
the prescribed limit.
ii. The other LED should glow when Vo > 1.8V. This condition describes the
over voltage condition. It happens when Vin > Vref, taking into account the
prescribed limit.
iii. None of the LEDs should glow for the condition -1.8V <= Vin <= 1.8V,
which indicates the normal operation.
43
Thus, it can be seen that the threshold point for the operation of the LED is Vo =
1.8V.
We have, R1 = 1KΩ, Vref = 6V and the threshold points of operation given by
Vin = 6.3 V for overvoltage
And Vin = 5.7 V for under voltage.
Substituting the above values in the equation 3.2, the value of Rf can be
determined as:
1.8 = (Rf/ 103) (0.3)
Solving, we get:
Rf = 6 x 103 Ω
Thus, Rf = 6 KΩ
(b.) Simulation using PSPICE:
The simulation is done by writing a program in PSPICE. In PSPICE, there is no
exact provision to club together both the cases. The values of known elements are,
hence, specified and the circuit is simulated separately for over voltage and under
voltage conditions. The output of the op-amp is noted down for each value of Rf. The
value of Rf at which the output is closest to the threshold of operation of LED (i.e.,
1.8V and -1.8V respectively for over voltage and under voltage) is the desired value.
The PSPICE programs and diagrams are as given below:
Fig-5.2 Modified Diagram for simulation using PSPICE
44
PSPICE program for under voltage operation:
* Simulation for Under Voltage
Vref 1 0 DC 6V
Vin 5 0 DC 5.7V
R1 1 2 1K
RL 4 0 8K
R2 5 3 1K
.PARAM VAL1 = 3K
Rf 2 4 {VAL1}
.STEP PARAM VAL1 LIST 3K 3.1K 3.2K 3.3K 3.4K 3.5K
3.6K 3.7K 3.8K 3.9K 4K
Rf2 3 0 3.3K
XA1 2 3 4 3 OPAMP
.SUBCKT OPAMP 1 2 7 4
R1 1 2 2MEG
R0 5 7 75
EA 4 5 3 4 2E+5
EB 4 3 1 2 0.1M
C1 3 4 1.5619UF
.ENDS OPAMP
.OP
.PROBE
.END
The output for this was as follows:
Rf Vo
3k -0.3V
3.1K -0.6V
3.3K -0.9V
3.5K -1.2V
3.7K -1.6V
3.8K -1.8V
45
3.9K -1.9V
4K -2.1V
Table – 5.1: Simulation Results for under-voltage conditions
From the above output, it may be seen that the output voltage, Vo = -1.8V,
which is the threshold for the LED to glow. It is obtained at Rf = 3.8K. Thus, for
under voltage operation, 3.8K is the feedback resistance value.
PSPICE program for over voltage operation:
* Simulation for Over Voltage
Vref 1 0 DC 6V
Vin 5 0 DC 6.3V
R1 1 2 1K
RL 4 0 8K
R2 5 3 1K
.PARAM VAL1 = 3K
Rf 2 4 {VAL1}
. STEP PARAM VAL1 LIST 3K 2.9K 2.8K 2.7K 2.6K 2.5K
2.4K 2.3K
Rf2 3 0 3.3K
XA1 2 3 4 3 OPAMP
.SUBCKT OPAMP 1 2 7 4
R1 1 2 2MEG
R0 5 7 75
EA 4 5 3 4 2E+5
EB 4 3 1 2 0.1M
C1 3 4 1.5619UF
.ENDS OPAMP
.OP
.PROBE
.END
The output for this was as follows:
46
Rf Vo
3k 2.4V
2.9K 2.1V
2.8K 1.9V
2.7K 1.8V
2.6K 1.6V
2.3K 0.9V
Table – 5.2: Simulation Results for Over-Voltage condition
From the above output, it may be seen that the output voltage, Vo = 1.8V, which
is the threshold for the LED to glow. It is obtained at Rf = 2.7K. Thus, for under
voltage operation, 2.7K is the feedback resistance value.
From the simulation results, it can be seen that the values of Rf are different in
case of under-voltage and over-voltage conditions. The resistance values obtained for
each case are as follows:
Rf = 3.8 KΩ, for under voltage indication
And Rf = 2.7 KΩ, for over voltage indication.
(c.) Design Verification:
The value of Rf for indication of over voltage and under voltage has been found
to be different for both the cases. This gives a general idea that for the common
operation of the two modes using a single op-amp, the value of Rf would lie between
the two values obtained earlier from PSPICE simulation.
Accordingly, the circuit is connected on the bread board with the constant
values of resistances R1, R2 and R‟. The supply and the op-amp biasing supply are
provided using a regulated power supply containing four independent outlets. A
decade resistance box is connected in place of the feedback resistance Rf.
The reference supply is adjusted to 6V and the biasing voltages are set to + and
– 9V. The input voltage, Vin is variable and is applied through the variable voltage
outlet of the RPS.
The value of Rf is varied between the two values obtained in the above PSPICE
simulation and the corresponding readings for the threshold values of Vin are noted
47
down for each resistance. The readings of analog simulation are shown in the
following table.
R‟ = 3.3KΩ, R1 = R2 = 1KΩ, Rf is varying.
Rf Under Voltage Condition
Threshold Voltage
Over Voltage Condition
Threshold Voltage
2.7KΩ 5.4V 6.3V
2.8KΩ 5.6V 6.8V
3KΩ 5.5V 6.6V
3.3KΩ 5.5V 6.5V
3.4KΩ 5.4V 6.4V
Table – 5.3: Results of Analogue Simulation
From the table, it is clear that the tolerance is different for both over-voltage and
under-voltage for different resistance values. But the tolerance is equal in both
directions when Rf = 3.3KΩ.
Thus, the tolerance is practically found to be equal to + or – 0.5V on the 6V
reference.
The tolerance of the relay is given by:
(0.5 / 6) x 100 = 8.33%
Thus, the relay has been designed for a tolerance of + or – 8% (approximately)
on both over voltage and under voltage side.
The values of the corresponding supply voltage are, thus shifted away from the
reference 230V on both sides. (Theoretically, it was + or – 5%, but practically, it is +
or – 8% of the reference voltage).
On the 230V reference, the lower limit and upper limit of operation of the relay
are obtained as 212V and 242V respectively.
5.3.6 Design Verification:
The output voltage can be verified theoretically by back – substitution using
equation 3.1:
Vo = - (Rf/R1) Vref + (1+Rf/R1) {R‟/(R‟+R2)} Vin
48
For over voltage condition, we have R1 = R2 1KΩ; R‟ = Rf = 3.3KΩ; Vref = 6V
and Vin = 6.5V.
Substituting these values, the output voltage is obtained as:
Vo = 1.65V
This value is very close to the LED threshold value of 1.8V. This shows that
the design of Rf is approximately accurate theoretically.
5.3.7 Final Design Values – Design values in a nutshell:
The basic circuit of relay is shown in the circuit diagram. It has the following
design values:
Fig – 5.3: Final Circuit with designed values
R1 = R2 = 1KΩ
R‟ = 3.3KΩ
Rf = 3.3KΩ
Voltage Levels:
Vref = 6V
Normal Condition: 5.5V ≤ Vin ≤ 6.5V
In terms of ac values, 212V ≤ Vac ≤ 248V
49
Abnormal Condition:
Under voltage: Vin < 5.5V, when Vac < 212 V. In this case, the LED2
glows.
Over voltage: Vin > 6.5V, when Vac > 248 V. In this case, the LED1 glows.
RC potential divider values:
R = 1KΩ, C=100μF.
Zener diode for Vref:
Vref = Vz, taken across the zener diode 6.2V, 0.5W
5.4 Practical Implementation:
The following article describes the practical implementation of the main relay
circuit and the accessory circuits:
5.4.1 Biasing Voltages, +Vcc and -Vcc:
The biasing voltage can be obtained by using the following equipments:
Sr. No. Component Component
Code
Specifications Qty
1. Transformer -- 230 / (6-0-6), 0.5A 1No.
2. Resistors -- 1KΩ 2No.
3. Capacitors -- 100µF, 16V 2No.
4. Capacitor -- 1000µF, 16V 1No.
5. Diodes IN4007 -- 4No.
Table – 5.4: Components Required for Biasing Voltage Circuit
This is obtained using a 230 / (6-0-6) transformer, connecting the end
terminals and removing centre tap.
The secondary voltage of the transformer is fed to a bridge rectifier. The
secondary voltage on no-load is approximately, Vac = 13.6V(ac)(rms).
The rectified voltage , Vdc is an intermittent dc signal.
The ripple is filtered using a capacitor of value C = 1000µF. This value of
capacitor is the most ideal for small power applications.
The no–load dc output, thus, obtained is Vdc=19.8V.
50
Fig – 5.4: Circuit to obtain biasing voltages
This voltage is given to an RC potential divider circuit, which halves the
voltages, giving approximately 10V at its output terminals.
5.4.2 Reference Voltage, Vref:
The circuit to obtain the constant 6V reference voltage is obtained using the
following components:
Sr.No. Component Component
Code
Specifications Qty
1. Resistor -- 1KΩ 1No.
2. Zener Diode -- 6.2V, 0.5W 1No.
Table – 5.5: Components required for Vref
Fig – 5.5: Circuit to obtain Vref
It is obtained by using a zener diode across one of the Vcc terminals
obtained above the zener diode used is a 6.2V, 0.5W zener gives gives the output
51
voltage of 6.2V under no-load and about 6.05v under loaded condition, which is
approximately equal to Vref = 6.0V.
The 1KΩ resistor in the circuit is for the protection of the zener diode.
This voltage is fed to the 1K resistor at the inverting terminal of the
input.
5.4.3 Vin Circuit:
The value Vin represents the system voltage. The system voltage is stepped
down to a value which is approximately equal to 6V dc voltage under ideal
conditions, with the help of the following circuit components:
Sr.No. Component Component
Code
Specifications Qty
1. Transformer -- 230 / (6-0-6), 1A 1No.
2. Pot (variable resistance) -- 5KΩ pot 1No
3. Capacitor -- 1000µF, 16V 1No.
4. Diodes IN4007 -- 4No.
Table – 5.6: Components required for Vin Circuit
It is obtained using a 230/(6-0-6), 1A transformer.
The secondary voltage is collected from one of the terminals and centre-
tap.
The secondary voltage under no-load is approximately 6.5V(ac)(rms).
It is rectified using a bridge rectifier, and then a 1000µF capacitor to
obtain a no- load voltage of about 8.6V.
Since the Vin should be equal to Vref under normal condition s, the voltage
should be dropped to 6V by using a 5K pot.
The 5K pot gives the flexibility to the relay and allows operations at
different nominal system voltages
52
Fig – 5.6: Circuit diagram to obtain Vin
For a 230V system, at Vac =230V, Vin =6V when the 5K pot is
approximately 650Ω (by practical measurement).
5.4.4 Relaying Circuit:
This is the main circuit of the relay. It makes the use of following
components:
Sr.No. Component Component
Code
Specifications Qty
1. Operational amplifier μA741 -- 1No.
2. Resistors -- 1KΩ 3No.
3. Resistors -- 3.3KΩ 2No.
4. LEDs -- 1.8V, green LED 2No.
5. IC Base -- -- 1No.
Table – 5.7: Components required for op-amp relaying circuit
The reference voltage, Vref is taken across the zener diode of the fig-5.5.
The varying voltage, Vin, which corresponds to the supply voltage is
obtained from the output of fig-5.6 shown in the previous section.
The two LEDs are placed anti-parallel to each other.
53
The LED1 has its anode connected to the output. It provides a visual
indication if there is an over voltage in the system voltage.
Fig – 5.7: Relaying Circuit
The LED2 has its cathode connected to the output terminal through a 1KΩ
resistor. It provides a visual indication if there is an under-voltage in the system
voltage. This resistor is required to avoid interference of LED1 on LED2. In the
absence of this LED, the under voltage indication is affected by the over-voltage
indication and the relay violates the reverse path.
The output of this circuit may be used to trip the circuit breaker or the contactor
contacts in case one of the LEDs glows, which represents the faulty condition.
However, this circuit has not been implemented owing to many practical difficulties
experienced during its implementation.
5.4.5 The Complete Circuit Diagram - the PCB Layout:
The overall circuit diagram can be obtained by combining all the circuits seen in
all the sub-topic stated earlier under article 5.4.
The circuit in fig-5.8 shows the complete circuit diagram of the static voltage
relay. It may be explained as follows:
The system voltage, which is to be tested, is taken from the mains supply
socket.
54
Fig – 5.8: Complete Circuit Diagram of Static Voltage Relay using Op-Amp
55
The ac voltage is converted into a dc voltage of low value, Vin, with the
help of the diode bridge and capacitor and is adjusted to the normal level of 6V at the
reference of 230V with the help of the 5Kpot.
This voltage, Vin is fed to the non-inverting terminal of the op-amp
through the 1K resistor R2.
The op-amp in the fig-5.8 requires a biasing voltage of +Vcc and –Vcc.
This voltage is obtained through the dc output of the other diode bridge and the
capacitor, which is halved by using an RC potential divider circuit grounded at its
common terminal.
The voltage +Vcc is fed to the 7th pin and the voltage –Vcc is fed to the 4
th
pin of the op-amp.
The reference voltage, Vref is taken across the zener diode placed parallel
to the +Vcc circuit. It gives a constant voltage reference of 6V under all conditions.
This reference voltage is given to the inverting terminal of the op-amp
through the 1K resistor R1.
The feedback resistance and the grounding resistance, which have been
simulated earlier are responsible for the gain of the operational amplifier.
The LEDs at the output of the op-amp are connected in an anti-parallel
fashion. The LED2 and LED1 provide visual indication in case of under-voltage and
over-voltage condition respectively.
If the reference nominal voltage is taken as Vs = 230 V, the pattern of
glowing of LEDs and the corresponding condition are as follows:
(i.) LED2 glows when the system voltage, Vs < 212V, i.e., under-voltage
(ii.) LED1 glows when the system voltage, Vs > 248V, i.e., over-voltage
(iii.) No LED glows when 212V <= Vs <= 248V, i.e., normal voltage.
Thus, the LED glow indicates the abnormality in the system voltage levels.
5.4.6 Complete list of components:
The total number of components required for the entire operation of the relay is as
follows given by the following table:
56
Sr.No. Component Component
Code
Specifications Qty
1. Operational amplifier μA741 ±18Vcc, 500mW 1No.
2. Resistors -- 1KΩ, 0.5W 6No.
3. Resistors -- 3.3KΩ, 0.5W 2No.
4. LEDs -- 1.8V, green LED 2No.
5. Op-amp IC Base -- 4-pin flat package 1No.
6 Capacitors -- 1000µF, 16V 2No.
7. Capacitors -- 100 µF, 16V 2No.
8. Diodes IN4007 -- 8No.
9. Zener Diode -- 6.2V, 0.5W 1No.
10. Transformer -- 230 / (6-0-6), 0.5A 1No.
11. Transformer -- 230 / 6-0-6, 1A 1No.
12. Pot (variable resistance) -- 5K pot 1No.
13. Connecting wires -- Single stranded As per
requirement
Table – 5.8: Components list for the project
57
CHAPTER – 6: ADVANTAGES OF THE OP-AMP – BASED
STATIC VOLTAGE RELAY :
The relay circuit designed above has many advantages over the other methods.
Some of them may be enumerated as follows:
a) Flexibility: This is, perhaps, the most important advantage of the circuit.
The 5K pot used in the circuit provides added flexibility to be used for different
system voltages. By varying the setting of the pot, the same circuit can be used to
indicate over voltages and under voltages for multiple system values. While that is
done, the resolution is maintained constantly at 8% irrespective of the normal
operating voltage.
b) Reduced losses: Unlike the electromagnetic counter parts, it contains no
moving parts. This ensures less loss due to the absence of friction. Thus, the
efficiency is improvised.
c) Fast response: Since the response is the indication using LEDs, which are
static devices, the response time and the delay is very less. Thus, the indication is
very fast.
d) Simplicity: The circuit is very simple to understand, analyse and simulate.
Thus, it can be easily repaired in case of any disturbances and malfunctioning. The
complexity involved is very less.
e) Robustness: It is very robust and works satisfactorily for all operating and
environmental conditions. It is not affected by the external conditions like
temperature, humidity, etc. However, the indication can be observed with great ease
in a dark room.
f) Single op-amp circuit: Bothe over voltage and under voltage can be sensed
using a single op-amp. Thus, the problems like biasing multiple ICs are eliminated.
g) Precise Indication: The indication is very precise in case of abnormal
voltage levels. The intensity of glow of the LEDs increases as the voltage levels
deviate away from the reference voltage.
h) Size: The relay, being small in size and light in weight is easily portable.
58
i) Cost: The relay is made of the components which are readily available at
a very cheaper value. The overall cost of the device is also very low. The total cost of
the device is as low as about Rs.200/-.
j) Less Noise: Since the number oif components in the circuit and the
interconnections are very less, effect of noise introduced is very less in this circuit.
k) Much scope for future advancement: The relay that has been designed is a
basic idea of implementation of op-amp circuits for power system protection. The
idea may be extended to any range by introducing suitable modifications in the
circuits. However, the basic principle remains the same.
59
CHAPTER – 7: SCOPE FOR FUTURE ADVANCEMENTS:
As has been described in the previous chapter, the relay has certain limitations.
Overcoming each of these limitations in itself signifies the scope for future
advancements of the relaying circuit that has been developed.
The advancements may include the improvement of the design or the extension
over the existing model. The advancements may be listed as follows:
7.1 Tripping Circuit:
The relay that has been designed provides a visual indication in case of over
voltages and under voltages. This is mainly a manually-controlled relay. This needs
personnel to monitor the device and disconnect the load manually under
abnormalities. This prompts the need for an automatic circuit for fast disconnection
and reclosure.
The tripping may be achieved directly by using a tripping device at the output
terminals of the op-amp. The tripping may be achieved using either solid-state
devices like SCRs, triacs, etc., or some electromagnetic devices like the contactors or
analog circuit breakers.
7.1.2 Methods of tripping:
The proper tripping may be achieved using some of the methods, stated as
follows:
Fig – 7.1: General tripping circuit
60
a.) Current amplification using Darlington pair:
A Darlington pair is two transistors that act as a single transistor but with a
much higher current gain.
Transistors have a characteristic called current gain. This is referred to as its hfe.
The amount of current that can pass through the load when connected to a transistor
that is turned on equals the input current x the gain of the transistor (hfe).
In some applications, the amount of input current available to switch on a
transistor is very low. This may mean that a single transistor may not be able to pass
sufficient current required by the load.
As stated earlier this equals the input current x the gain of the transistor (hfe).
If it is not be possible to increase the input current then we need to increase the gain
of the transistor. This can be achieved by using a Darlington Pair
Fig – 7.3: The Darlington Pair
A Darlington Pair acts as one transistor but with a current gain that equals:
Total current gain (hfe total) = current gain of transistor 1 (hfe t1) x current gain of
transistor 2 (hfe t2).
61
Thus, using a Darlington pair, the current at the op-amp is amplified to a value
which is sufficient to energise the coil of the tripping circuit. The load shown in the
above circuit may be replaced by the tripping coil contacts.
b.) Auto reset and counters:
The difficulty of closing the circuit and regaining normal operation of the load
circuit may be overcome using this method.
In this method, the breaking is achieved with the help of gate pulse provided
from the output of the op-amp during abnormality. The making may be achieved
using a combination of counters and timers set for a particular duration, say 5 sec. In
this case, once the circuit is broken, the timer starts to count for the specified time. As
soon as the preset time is reached, it releases a signal for resetting the solid switch,
which is applied to the gate pulse. The circuit is then made and if the abnormality still
exists, it is broken again. The process is repeated till the abnormality is finally
cleared.
This method is analogous to a circuit breaker with automatic reclosure.
c.) Logic Gates:
It may be seen from the general operation of the LEDs that at any given time,
either both the LEDs are in off state or one of the LEDs glows during abnormal
conditions. The circuit has to be tripped off when either of the LEDs glows and has to
be made when both the LEDs are under non-operating state.
By proper observation, it may be found that this is the basic logic of an XOR
gate. Thus, by practical implementation of logic gates, the desired tripping may be
achieved.
d.) Different breakers for both modes:
The problem discussed in the feature 5.1.1.3 can be overcome by making the
use of two separate circuit breakers, connected back-to-back for each mode. One of
the breakers responds for under voltage, while the other operates for under voltage. In
either case, the load is disconnected from the supply during abnormal condition and
vice-versa.
62
e.) Independent circuits for both modes:
Another way out for the problem discussed in 5.1.1.3 is to implement both
under voltage and over voltage using separate circuits. In this case, the circuit is
operated by applying constant reference voltage to inverting terminal for over voltage
and non-inverting terminal for under voltage. Thus, the interference between the two
operations may also be avoided.
7.2 Over-Current Protection:
This application is of much theoretical interest, since the practical implementation
might prove to be much costlier.
Since the circuit responds for changes in the voltages, by converting current to
proportional values of voltage, it may also be implemented for over-current
protection. This conversion may be done using many specialized devices like V-I
converters.
7.3 High Power Applications:
The relay designed here is only for low-power applications and is limited to the
distribution level voltages. However, this may be used by proper technology for
protection of high voltage systems as well. This may be achieved by stepping down
the high voltages and currents to lower values, low enough to be implemented using
this circuit by making use of potential transformers (PT) and current transformers
(CT).
7.4 Inverse Time relay:
The relay that has been designed is a instantaneous type of relay. Its response is
very fast and instantaneous. It responds the moment the voltage limits are violated.
However, with an introduction of a delay circuit in between the op-amp output
and the delay circuit in between the output and the LEDs, it can be made to give
inverse voltage-time characteristics. Thus, a delay can be introduced, making it an
inverse time relay, which has added applications over the existing one.
63
CHAPTER – 8: CONCLUSION:
The project ‘Design, Development and Fabrication of a Static Voltage Relay
using an Op-Amp’ has been done with an idea of applying the concept of using an
operational amplifier to protective relaying. The relaying operation has been realized
for two abnormal conditions usually experienced in the power systems either in
transmission or distribution, viz., under-voltage and over-voltage. Both the abnormal
conditions have been indicated by the LED indication, which forms the fundamental
for achieving the desirable operational criterion. This forms the basis to apply the
actual operating conditions using circuit breakers for protection of the systems
sensitive to over-voltages and under-voltage conditions.
The relay has been designed to operate for different nominal voltages with a
resolution of + or – 8% for over-voltage and under-voltage respectively.
It has been a great learning experience to know practically the difficulties
involved in protective relaying and operation of an operational amplifier in the real-
time applications. The project, though simple, is an application of the theoretical
knowledge one has achieved as a part of the engineering course.
The work has been done on an experimental basis to demonstrate the feasibility
of using an op-amp in protective relaying operation, and hence, confined to indication
(through LEDs) of abnormal voltage levels. This prompts for the future development
in the direction to make use of the circuit directly to protective relaying.
64
BIBLIOGRAPHY
REFERENCES:
A Text Book on Linear Integrated Circuits - Roy Chowdary & Shail Jain
A Course in Electrical Power - J.B. Gupta
Switchgear & Protection - Prof. C. Lakshminarayana
Electronic Devices and Circuits - Millman & Halkins
Switchgear and protection - Sunil. S. Rao.
Art and Science of Protective Relaying - C.R Mason
MANUALS:
Basic Op-Amp Circuits - TEXAS instruments (.pdf)
Op-Amp Reference Manual - TEXAS Instruments (.pdf)
Op-Amp Differential Amplifier- TEXAS Instruments (.pdf)
Darlington Pair - Google Books
Electronics For You (magazine)
WEBSITES:
Google: www.google.co.in ; www.google.com
Wikipedia: www.wikipedia.org
www.electronics4u.com
www.ask.com
65
LIST OF FIGURES
Sr. No. Fig No. Description Page No.
1. 2.1 Basic Block Diagram of the 7
Relaying System
2. 3.1 A logic representation of a Relay 10
3. 3.2 Components of a Static Relay 16
4. 4.1 Op-Amp Circuit Symbol 19
5. 4.2 Op-Amp 741 Pin Configuration 19
6. 4.3 Circuit Model of an Op-Amp 21
7. 4.4 Typical Curves showing Open-loop 22
Gain and frequency of 741 Op-Amp
8. 4.5 Open – loop operation of an Op-Amp 26
9. 4.6 Op-Amp Inverting Amplifier 27
10. 4.7 Op-Amp Non-Inverting Amplifier 28
11. 4.8 Op-Amp Differential Amplifier 30
12. 5.1 Basic Principle of Operation of Relay 32
13. 5.2 Modified Diagram for PSPICE Simulation 43
14. 5.3 Final circuit with designed values 48
15. 5.4 Circuit to obtain Biasing Voltages 50
16. 5.5 Circuit to obtain Vref 50
17. 5.6 Circuit diagram to obtain Vin 52
18. 5.7 Relaying Circuit 53
19. 5.8 The Complete Circuit Diagram of Static 54
Voltage Relay using Op-Amp
20. 7.1 General Tripping Circuit 59
21. 7.2 The Darlington Pair 60
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LIST OF TABLES:
Sr.No. Table No. Description Page No.
1. 4.1 Absolute Maximum Parameters of µA741 24
2. 5.1 Simulation result for Under-Voltage Conditions 44
3. 5.2 Simulation for Over-Voltage Conditions 46
4. 5.3 Results of Design Verification 47
5. 5.4 Components required for Biasing voltage circuit 49
6. 5.5 Components required for Vref circuit 50
7. 5.6 Components required for Vin circuit 51
8. 5.7 Components required for Op-Amp relaying circuit 52
9. 5.8 Components list for the project 56
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APPENDIX – I : SPECIAL FEATURES OF OP-AMP 741:
Electrical Characteristics of µA741:
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Table: Electrical characteristics of Op-Amp
Typical Characteristic Curves:
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Fig: Graphs showing typical characteristics of a Op-Amp
Schematic Diagram:
Fig: Schematic Diagram of Op-Amp 741