intellligent power sharing of transformers with auto protection

Intelligent power sharing of transformer with auto protection 1

EEE Dept.

CHAPTER 1

INTRODUCTION

1.1 General Background

The transformer is a static device, which converts energy at one voltage level to another

voltage level. The project is all about protecting the transformer under overload condition.

Due to overload on the transformer, the efficiency drops and the secondary winding gets

over heated and may burnt. So, by reducing the load on the transformer, the transformer is

protected. This will be done by arranging another transformer through a micro-controller.

The micro-controller compares the load on the first transformer with a reference value.

When the load exceeds the reference value, the second transformer will share the extra

load. Therefore, the two transformers work efficiently under overload condition and the

damage is prevented.

In this project three modules are used to control the load current. The first module is

Sensing unit, which is used to sense the current of the load; the second module is control

unit. In this module Electromagnetic relay is the main role, and its function is to change

the position with respect to the control signal and last module is microcontroller. It will

read the analogue signal and perform some calculation and finally gives control signal to

the relay. When designing low-voltage power systems to supply large load currents,

paralleled lower-current modules are often preferred over a single, large power converter

for several reasons. These include the efficiencies of designing and manufacturing

standard modular converters which can be combined in whatever number necessary to

meet a given load requirement; and the enhanced reliability gained through redundancy.


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1.2 Literature Review

At present load sharing of transformer is done by paralleling

the transformer. That is primary of the transformer is always energized. This method is

following on olden days also.

The challenge in paralleling modular supplies is to insure

predictable, uniform current sharing-regardless of load levels and the number of modules.

It provide enhanced system reliability through complete redundancy such that the failure

of one or more modules could be tolerated as long as the total remaining capacity is equal

to or greater than the demands of the load.

1.3 Aim and Scope of Project

This Project describes about, how to use power supply

when critical load. Using this module can protect the transformer form the over load. This

project will connect and disconnect the transformer automatically. This will be done by

arranging another transformer through a microcontroller. It has the advantage to maintain

a stable level of short circuit current. It reduces the voltage drop and balances the current

and it is reverse power protection. It has applications in electrical substation and industry

too.

1.4 Organization of project report

The project is all about protecting the transformer under overload condition. Due to

overload on the transformer, the efficiency drops and the secondary winding gets over

heated and may burnt. So, by reducing the load on the transformer, the transformer is

protected. By introducing this method it have advantage to maintain a stable level of short

circuit current. It reduces the voltage drop and balances the current and it is reverse power

protection.


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CHAPTER 2

SUBSTATION

2.1 Introduction

The substation may be defined as “an assembly of apparatus which transforms

the characteristics of electrical energy from one form to another say for example from

alternating current to direct current or from one voltage to another”. For economical

transmission, higher and higher voltages should be achieved. At present normal voltages

are 66kV, 110kV and 220kV; however 440kVwill be used for the national grid system in

future. The consumers do not use such high voltages and so the high voltages must be

transformed to low voltages by means of substation. Thus a substation may be called as

link between the generating stations and consumers. The distribution voltages generally

used in practice are 6.6kV, 11kV and 33kV.

Substations or switching stations are integral part of transmission system, and

function as a connection or switching point for transmission lines, substation feeders,

generating circuits and step up and step down transformers .substation of voltages 66kV

to 400kV are termed as EHV substations. Above 500kV, they come under the

terminology of UHV system.

2.2 Equipments for Substation

The substation mainly consists of the following equipments:

(a) Main bus bars

Bus bars (or bus) term is used for a main bar or conductor carrying an electric

current to which many connections may be made. Bus bars are merely convenient means

of connecting switches and other equipments into various arrangements. The usual

arrangement of connections in most of the substation permits working on almost any

piece of equipment without interruption to incoming or outgoing feeders.


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(b) Single bus arrangement

This arrangement is a simple scheme in less important substations. A breaker or bus

failure can cause total outage. By providing a bus-sectionalizing scheme, this can be

overcome to some extent. Even though the protective relay in is simple, single bus

scheme is inflexible.

(c) Main and transfer bus

A transfer bus is added to the single bus scheme. An extra bus-tie breaker is provided

to tie the main and transfer buses together. When circuit breaker is in maintenance, the

bus-tie breaker can be used for energizing the circuit. Bus-tie breaker relaying must be so

arranged to protect the transmission line or transformer, if the protective relays also are

not transferred.

(d) Double bus, Single breaker

This is superior to the single bus and main and transfers bus schemes. There are two

main buses and each circuit can be connected to either of the buses by bus isolators. A

bus-tie breaker connects the two main buses when closed allows the transfer of a circuit

from one to the other without a break in supply.

The circuit may operate all from one bus, or half of the circuit connected in each bus.

For a bus fault, only half the number of circuit will be lost. In some cases the tie breaker

is permanently closed and both the buses stand connected. A bus protection scheme will

be necessary for opening the tie breaker in the event of a bus fault. Possibility of operator

error is more as to bus isolators are involved for every circuit

(e) Insulators

They are used for supporting live conductors and bus bars. For low voltage up to

66kV, this can be used in stacks and mounted horizontally or vertically as the

circumstances permit but for voltages beyond that, they are rarely used in a horizontal

configuration. The porcelain insulators are employed in substations are of the post and

bushing type. They serve as the supports and insulators of the bus bars.


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A post insulator consists of porcelain body, cast iron cap and flanged

cast iron base. The hole in the cap is threaded so that the bus bars are either directly

bolted to the cap of fixed by means of a bus bar clamp. Past insulators are available with

round, oval and square flanged bases for fixing respectively, with aid of one, two or four

bolts. Each base in addition also has an earthing bolt. A bushing or through insulator

consist of porcelain-shell body, upper and lower locating washers used for fixing the

position of bus bars or rod in shell, and mounting flange with holes drilled for fixing bolts

and supplied with an earthing bolt. For current rating above 2000A, the bushings are

designed to allow the main bus bars to be passed directly through them.

(f) Isolators

An isolators or disconnecting switch is used to open some given part

of a power circuit after switching of the load by means of a circuit breaker. Thus isolators

serve only for preventing the some given section of the bus in a switch gear installation or

to one or another piece of apparatus in the installation from voltage applied. In some

cases isolators may be used as a circuit breaking device but their use for this purpose is

strictly limited by definite conditions, such as the power rating of the given circuit.

Isolators can be opened only after opening the circuit breaker .An isolator should be

closed before closing the circuit breaker .Opening and closing of a current carrying circuit

is performed by a circuit breaker.

Isolators (disconnecting switch) operate under no load condition. It does not have

any specified current breaking capacity or current making capacity, so it is not even used

for breaking load currents.

There are two types of isolators.

1. Single pole isolators

2. Three pole isolators

(g) Circuit breaker

Circuit breakers are switching devices, which open during fault condition and

interrupt the short circuit currents automatically within about 2.5 cycles. They are


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mounted on support structures .The circuit breakers are classified on the basis of the

medium used for arc extinction. The types of circuit breaker include the following:

1. SF6 circuit breakers

2. Air blast circuit breakers

3. Vacuum circuit breakers

4. Minimum oil circuit breakers

5. Air circuit breakers

6. Miniature circuit breakers

7. Bulk oil circuit breakers

The following circuit breakers are preferred for EHV ac substation

1. Single pressure buffer type SF6 CB .There is used for 132kV, 400kV, and 765kV.

2. Air blast circuit breakers.

3. Minimum oil CB. These were preferred for 33KV, 66kV, and 132kV.

The circuit breakers are automatically switches, which can interrupt fault

currents. The part of the circuit breaker connected in one phases is called the pole. A

circuit breaker suitable for a three phase system is called triple pole CB.

Circuit breakers are installed to perform the following duties.

a. To carry full load current continuously.

b. To open and close the circuit on no load.

c. To make and break the normal operating current.

d. To make and break the short circuit of magnitude up to which it is designed for.

2.3 Power Transformers

Power transformers are used for stepping up the voltage for transmission at

generating stations and for stepping down voltages for further distribution at main step

down transformer substation. Usually naturally cooled, oil immersed, known as on type,

two winding, 3-phase transformer, are used up to the rating of 10MVA. The transformers

of rating higher than 10MVA are usually air blast cooled. For very high rating, the forced

oil, water-cooling and air blast cooling may be used. For regulating the voltage the

transformers used are provided with on load tap changer.


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The transformers are generally installed upon length of rails fixed on concrete slabs

having foundation 1 to 1.5 meters deep.

The modern practice is to use three phase transformers as power transformers,

although 3 single phase banks of transformers can also be used. The advantages of 3

transformers are that in case of fault in one of them, it could be completely replaced by

new one. The main advantage of a 3-phase transformer is that only one 3-phase load taps

changing mechanism (on LV side) could be used. Further the installation of a single 3-

phase transformer is much simpler than three single phase transformers.

(a) Tertiary winding

Tertiary winding is an additional winding in power transformers normally delta

connected. Tertiary winding is required for the following reasons:

1. To reduce the triple harmonic contents of the output voltage, there by stabilizing

potential of the neutral point, and reducing communication interference.

2. To permit the transformation of unbalanced three phase load.

3. To reduce system zero sequence impedance for effective grounding, where solid

ground is not provided.

4. To supply additional auxiliary loads, this for some reason must be kept isolated from

that of the secondary.

5. To function as a voltage coil in a testing transformers.

6. To load large split-winding transformers.

7. To interconnect three supply systems operating at different voltages.

(b) Current Transformers

These instrument transformers are connected in ac power circuits to feed the current

coils of indicating and metering instruments (ammeters, wattmeter, and watt hour meters)

and protective relays. Thus the CT broadens the limits of measurements and maintains a

watch over the currents flowing in the circuits and over the power loads. In high voltage

installation CT’s in addition to above, also isolate the indicating and metering instruments

from high voltage.


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The current transformers are rated voltage of the installation, the rated currents of

primary and secondary windings and the accuracy class. The accuracy class indicates the

limit of the error in percentage of the rated turn ratio of given CT. CT is available in

accuracy classes .5, 1, 3, 10.

(c) Potential Transformer

The PT is employed for 380 volts to feed the potential coils of indicating and

metering instruments (voltmeter, wattmeter, and watt hour meters) and relays. These

transformers make the ordinary low voltage instruments suitable for measurement of high

voltage and isolate them from high voltage.

The primary winding of the PT is connected to the main bus bars of the switch gear

installation and to the secondary windings various indicating and metering instruments

and relays are connected. When the rated high voltage is applied to the primary of a PT,

the voltage of 110volt appear across the secondary winding. The PT is rated for primary

and secondary rated voltage, accuracy class, number of phases and system of cooling.

(d) Autotransformers

Autotransformers are well known for their reduced size and economic use of material.

In an autotransformer the primary and secondary windings have a common part. The

secondary is connected electrically to the primary at the common point. Most of the three

phase autotransformers are star connected and it is a usual practice to add an additional

delta winding called tertiary winding. The purpose of the delta tertiary is to provide an

internal path to the third harmonic current required for execution and thus eliminating

them from the AC network. The delta tertiary also helps to stabilize the neutral with

reference to the line voltage. The economy and reduced size of autotransformer as

compared with two winding transformer has resulted in development of an

autotransformer for a very wide range of applications at EHV-AC transmission level, MV

distribution levels as well as in industrial applications .A very wide choice is available for

the tapping on the autotransformer. The suitable choice of tapping must be made for

obtaining the alternatives.


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As per the rules star-star connected autotransformer is provided with delta tertiary for

the following purposes:

1. Stabilizing the phase to phase voltages during unbalanced load on secondary side.

2. Suppression of the third harmonic currents due to no load current in neutral to ground

circuit when neutral is earthed.

3. Reduction in zero sequence reactance.

4. Increase of KVAR.

(e) Earthing switch

Switch is connected between the line conductor and earth. Normally it is open. When

line is disconnected, the earthing switch is closed so as to discharge the voltage trapped

on the line capacitance to the earth. Though the line is connected, there is some voltage

on the line to which the capacitance line and earth is charged. This voltage is significant

in high voltage system. Before proceeding with the maintenance work the voltage is

discharged to earth, by closing the earthing switch.

(f) Earthing

Connecting of electrical equipment or apparatus to the earth with the help of a

connecting wire of negligible resistance is known as “earthing” or “grounding”. In an

electric installation if a metallic part of an electric appliance comes in direct contact with

a bare or live wire, the metal being a good conductor is charged and static charge on it

will accumulate. Now if any person comes in contact with charged metal parts, he will get

a severe shock. But if the metallic parts of the equipment are earthed, the charge will be

given to the earth immediately as the metallic part comes in direct contact with a bare or

live wire or breakdown occurs.

The earthing can be divided into two parts.

1. System earthing: It is possible to provide to provide low fault impedance to the

ground fault current for proper operation of the protective relays and therefore for

meeting the system requirement by effectively earthed system


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2. Safety earthing: It is required to provide protection to the operating staff working in

the yard and substation from any injury during fault conditions by keeping the voltage

gradient within the safe limits.

In electrical installation the following components must be earthed.

1. The frames, tanks and enclosures of electrical machines, transformers and apparatus,

lighting fitting and other items of equipments.

2. Operating mechanism of switch gear

3. Frame work of switch boards, control boards, individual panel boards, and cubicles.

4. Structural steel work of indoor and outdoor substations.

There are two types of earthing.

1. Pipe earthing: This is the best and cheapest method. In this method, a galvanized steel

and perforated pipe of approved length and diameter is placed up right in permanently

wet soil

2. Plate earthing: In this method either a copper of dimensions 60cmx 60cm x 3 mm or

of galvanized iron of dimensions 60cmx 60cmx6mm is buried into the ground with its

face vertical at a depth of not less than 3m from the ground level.

(g) Lightning arresters (Surge arresters)

There equipment connected between the conductor and ground, to discharge the

excessive voltage to the earth. Surge arresters divert the transient over voltages to the

earth and protect the substation equipment from lightning and switching over voltage

surges. There are two types of designs

a) Conventional gapped arresters

b) Metal Oxide (ZnO) arresters


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(h) Relays

The protective relays are connected in the secondary circuits of the CT & PT. The

relays sense the abnormal conditions and close the trip circuit of associated CB. The CB

opens its contact. An arc is drawn between the contacts as they separate. The arc is

extinguished by suitable medium and technique. The relays distinguish the normal and

abnormal condition.

Whenever an abnormal condition develops, the relay closes its

contacts. Thereby the trip circuit of the CB is closed. Current from the battery supply

flows in the trip coil of CB and the CB opens. The fault parties disconnected from the

supply.

(a) Substation Auxiliaries Supply

In small-unattended substations only a small amount of power is required for electric

lighting during regular periods of inspection, maintenance and repair is required. In

regional substations the electric power is required for the auxiliaries-the lighting circuits,

air blast fans of power transformers, battery charging sets, oil servicing facilities,

compressors units in the case of ABCB, ventilating fans of the substation buildings, water

supply, and heating system equipments. In substations incorporating synchronous

condensers the supply is also required for the operation of auxiliary equipment of the

synchronous condensers. In large substations it is wide practice to connect two

transformers to the 11kV main bus bars for supply of the auxiliaries at a voltage of

400/230volts

(b) Capacitor banks

Reactive power monitoring is desirable to maintain efficient system operation,

keeping voltage and load stability. Reactive power flow from the substation to the

transmission line varies during low loads, heavy loads and changing loads. So series

capacitors are installed in sending, receiving end station and also the intermediates

stations.

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2.4 Transformers

Transformers are used in substations for conversion of voltage from high to low

and vice versa. A transformer

circuit to another through inductively coupled conductors

varying current in the first or

transformer's core and thus a varying

varying magnetic field induces

secondary winding. This effect is called

will flow in the secondary winding and electrical energy will be transferred from the

primary circuit through the transformer to the load. In an id

voltage in the secondary winding (

given by the ratio of the number of turns in the secondary (

the primary (Np) as follows:

By appropriate selection of the ratio of turns, a transformer thus allows an

current (AC) voltage to be "stepped up" by making

by making Ns less than Np.

In the vast majority of transformers, the windings are coils wound around a

core, air-core transformers being a notable exception.

Transformers range in size from a thumbnail

stage microphone to huge units

of power grids. All operate with the same basic principles, although the range of designs

is wide. While new technologies have eliminated the need for transformers in some

electronic circuits, transformers are still found in nearly all electronic devices designed

for household ("mains") voltage

power transmission, which makes long

The transformer cannot supply load more than its rated capacity. So, by introducing

another transformer reduces the load on the transformers. The transformers share the load

as and when the load on the transformer exceeds a preset value.

Intelligent power sharing of transformer with auto protection


transformer is a static device that transfers electrical energy from one

to another through inductively coupled conductors—the transformer's coils. A

in the first or primary winding creates a varying magnetic flux

transformer's core and thus a varying magnetic field through the secondary winding. This

induces a varying electromotive force (EMF) or "voltage

secondary winding. This effect is called mutual induction.

If a load is connected to the secondary, an electric current


primary circuit through the transformer to the load. In an ideal transformer, the induced

voltage in the secondary winding (Vs) is in proportion to the primary voltage (

given by the ratio of the number of turns in the secondary (Ns) to the number of turns in

By appropriate selection of the ratio of turns, a transformer thus allows an

voltage to be "stepped up" by making Ns greater than Np, or "stepp

In the vast majority of transformers, the windings are coils wound around a ferromagnetic

transformers being a notable exception.

Transformers range in size from a thumbnail-sized coupling transformer hidden inside a

to huge units weighing hundreds of tons used to interconnect portions

. All operate with the same basic principles, although the range of designs



household ("mains") voltage. Transformers are essential for high-voltage

hich makes long-distance transmission economically practical.

transformer cannot supply load more than its rated capacity. So, by introducing


on the transformer exceeds a preset value.



is a static device that transfers electrical energy from one

the transformer's coils. A

magnetic flux in the

winding. This

voltage" in the

is connected to the secondary, an electric current


eal transformer, the induced

) is in proportion to the primary voltage (Vp), and is

) to the number of turns in

By appropriate selection of the ratio of turns, a transformer thus allows an alternating

, or "stepped down"

ferromagnetic

sized coupling transformer hidden inside a

weighing hundreds of tons used to interconnect portions

. All operate with the same basic principles, although the range of designs



voltage electric

distance transmission economically practical.

transformer cannot supply load more than its rated capacity. So, by introducing



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2.4.1 Parallel operation of Transformers

The essential conditions for successful parallel operation of transformers are given below:

I. Transformation or turn-ratios and voltage ratings are same.

II. Polarities of the transformers are same.

III. Percent impedances of the transformers are same.

IV. Ratios of resistance to reactance are same.

V. Phase displacement between primary and secondary windings of the transformers is

same.

VI. Phase sequences of the transformers are same.

a. Single Phase Transformers

For single phase transformers only the first four conditions apply as there

is no phase sequence and phase displacement due to voltage transformation.

If the turn-ratios or voltage ratings are not same a circulating current will flow even at no

load. If the percent impedance or the ratios of resistance to reactance are not same there

will be no circulating current, but the division of load between the transformers when

supplied will no longer be proportional to their KVA ratings. Hence the capacities of the

transformers cannot be utilized to a full extent.

When the polarity of one transformer is additive and that of the other is

subtractive, the transformers may be operated in parallel by reversing the connection of

primary or secondary side of either transformer. In such a case check that dielectric

strength is satisfactory when the reversed winding has a graded insulation.


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b. Three phase Transformers

The same conditions hold true for three phase transformers except in the

case the questions of phase displacement and phase sequence must be considered.

Phase sequence refers to the order in which the terminal voltages reach their maximum

values. In paralleling those terminals whose voltages reach their maximum

simultaneously are paired.

Certain transformer connections as the Wye-delta or Wye-

Zigzag produce a phase displacement of 30° between the line voltage of primary side and

those of the secondary side. Transformers of such connections cannot be run in parallel with

the transformers not having this phase displacement such as Wye-Wye, delta, - delta zigzag-

delta or zigzag-zigzag.


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CHAPTER 3

PROGRAMMING WITH PIC

6.1 PROGRAMMING IN PIC

#include<pic.h>

#include<stdio.h>

#include"delay.c"

#define LED1 RD0;

#define RELAY0 RD1;

___CONFIG(0x3f72);

unsigned char CT;

void GetADC1(void);

void main( )

{

TRISA=0xff;

ADCON1=0x00;

DelayMs(10);

While (1)

{

GetADC1 ( );

If (CT >=0.7)

{

LED1=1;


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RELAY0=1;

}

}

DelayMs (250);

DelayMs (250);

}

Void GetADC1 ( )

{

ADCON0=0x41;

DelayMs (1);

ADGO=1;

While (ADGO==1);

CT=ADRESH;

}

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4.1 Introduction

When designing low-voltage power systems to supply large load currents, paralleled

lower-current modules are often preferred over a single, large power converter for several

reasons. These include the efficiencies of designing and manufacturing standard mo

converters which can be combined in whatever number necessary to meet a given load

requirement; and the enhanced reliability gained through redundancy.

While most modem-day power supplies can be paralleled for higher currents, the load

current will not share equally between modules without some extra effort in the design

process. With unequal load sharing, the stress placed on the individual modules will be

unequal; resulting in some units operating with higher temperatures

contributor to reduced reliability .Therefore, the challenge in paralleling modular supplies

is to insure predictable, uniform current sharing

of modules. Another major goal should be to provide enhanced system reliability thr

complete redundancy such that the failure of one or more modules could be tolerated as

long as the total remaining capacity is equal to or greater than the demands of the load.

Over the years, a variety of schemes have been devised to accom

and, as one would expect, these schemes offer a wide range of performance

characteristics.

4.2 Block Diagram


CHAPTER 4

LOAD SHARING

voltage power systems to supply large load currents, paralleled

current modules are often preferred over a single, large power converter for several

reasons. These include the efficiencies of designing and manufacturing standard mo


requirement; and the enhanced reliability gained through redundancy.

day power supplies can be paralleled for higher currents, the load

not share equally between modules without some extra effort in the design


unequal; resulting in some units operating with higher temperatures--

to reduced reliability .Therefore, the challenge in paralleling modular supplies

is to insure predictable, uniform current sharing-regardless of load levels and the number

major goal should be to provide enhanced system reliability thr



Over the years, a variety of schemes have been devised to accomplish load sharing

and, as one would expect, these schemes offer a wide range of performance

Fig 4.1 Block Diagram


voltage power systems to supply large load currents, paralleled

current modules are often preferred over a single, large power converter for several

reasons. These include the efficiencies of designing and manufacturing standard modular


day power supplies can be paralleled for higher currents, the load

not share equally between modules without some extra effort in the design


--a recognized

to reduced reliability .Therefore, the challenge in paralleling modular supplies

vels and the number

major goal should be to provide enhanced system reliability through



plish load sharing


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A brief description of some of these approaches will, perhaps, be useful in comparing

their performance capabilities against the degree of difficulty in their implementation.

The connection diagram for this most basic approach is shown in Figure where it can be

seen that each unit is completely independent except for the common load. Each

individual module must have inherent current limiting because in practice, the output

voltage of all the modules will never be exactly equal. Thus, when several modules are

paralleled, the one with the highest output voltage will attempt to supply all the load

current, up to the point where its current limit is reached. As this unit goes into current

limiting, its output voltage will fall to the level of the next highest module, which then

begins to conduct and supply additional load current.

When the second module reaches its current limit, number three starts

conducting, and so on. Of course, there is no current sharing at all except for the units

which are in current limiting, and it could be expected that the dynamic load regulation,

particularly as each current limit threshold is passed, would be less than desirable.

4.3 Current Transformers

Fig. 4.2 Current transformer

The current transformer is used with its primary winding connected in series

with line carrying the current to be measured and therefore the primary current is

dependent upon the load connected to the system and is not determined by the load

connected on the secondary winding of the current transformer . The primary winding

consists of very few turns and therefore there is no appreciable voltage drop across it. The

secondary winding of current transformer has large number of turns, the exact number


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being determined by the turn’s ratio. The ammeter current coil or the over current relay

are connected directly across the secondary winding terminals. Thus a current transformer

operates it secondary windings nearly under short-circuit conditions. One of the

secondary windings is earthed so as to protect equipment and personnel in the vicinity in

the event of an insulation breakdown in the current transformer.

4.4 Current Transformer operation

• A Current Transformer (CT) is a type of instrument transformer designed to provide a

current in its secondary winding proportional to the alternating current flowing in its

primary.

• They are commonly used in metering and protective relaying in the electrical power

industry where they facilitate the safe measurement of large currents, often in the

presence of high voltages.

• The current transformer safely isolates measurement and control circuitry from the

high voltages typically present on the circuit being measured.

• The instrument current transformer (CT) steps down the current of a circuit to a lower

value and is used in the same types of equipment as a potential transformer.

• This is done by constructing the secondary coil consisting of many turns of wire,

around the primary coil, which contains only a few turns of wire.

• In this manner, measurements of high values of current can be obtained. A current

transformer should always be short-circuited when not connected to an external load.

• Because the magnetic circuit of a current transformer is designed for low magnetizing

current when under load, this large increase in magnetizing current will build up a

large flux in the magnetic circuit and cause the transformer to act as a step-up

transformer, inducing an excessively high voltage in the secondary when under no load.

4.5 Crystal Oscillator

A crystal is a solid in which the constituent atoms, molecules, or ions are packed in

a regularly ordered, repeating pattern extending in all three spatial dimensions. Almost

any object made of an elastic material could be used like a crystal, with appropriate

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transducers, since all objects have natural

crystal of quartz is properly cut and mounted, it can be made to distort in an

by applying a voltage to an

piezoelectricity.

Fig.4.3 Schematic symbol and

When the field is removed, the quartz will generate an electric field as it

returns to its previous shape, and this can generate a voltage. The resu

crystal behaves like a circuit composed of an

precise resonant frequency (See Fig.4.3).

Quartz has the further advantage that its elastic constants and its

size change in such a way that the frequency dependence on temperature can be very l

The specific characteristics will depend on the mode of vibration and the angle at which

the quartz is cut (relative to its crystallographic axes). Therefore, the resonant frequency

of the plate, which depends on its size, will not change much, either.

quartz clock, filter or oscillator will remain accurate. For critical applications the quartz

oscillator is mounted in a temperature

also be mounted on shock absorbers to prevent pertu

vibrations. A quartz crystal can be modeled as an electrical network with a low

impedance (series) and a high impedance (parallel) reso

together.


, since all objects have natural resonant frequencies of vibration

is properly cut and mounted, it can be made to distort in an electric field

to an electrode near or on the crystal. This property is known as

Schematic symbol and equivalent circuit of a crystal oscillator


returns to its previous shape, and this can generate a voltage. The result is that a quartz

crystal behaves like a circuit composed of an inductor, capacitor and resistor,

precise resonant frequency (See Fig.4.3).


size change in such a way that the frequency dependence on temperature can be very l



of the plate, which depends on its size, will not change much, either. This means that a


oscillator is mounted in a temperature-controlled container, called a crystal oven, and can

also be mounted on shock absorbers to prevent perturbation by external mechanical


(series) and a high impedance (parallel) resonance point spaced closely


vibration. When a

electric field

near or on the crystal. This property is known as

oscillator


lt is that a quartz

, with a


size change in such a way that the frequency dependence on temperature can be very low.



This means that a


controlled container, called a crystal oven, and can

rbation by external mechanical


nance point spaced closely


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4.6 Driver Circuit

Driver circuit performs amplification process. A transistor is

used for that. It is a semiconductor device used to amplify and switch electronic signals. It

is made of a solid piece of semiconductor material, with at least three terminals for

connection to an external circuit. A voltage or current applied to one pair of the

transistor's terminals changes the current flowing through another pair of terminals.

Because the controlled (output) power can be much more than the controlling (input)

power, the transistor provides amplification of signal. Today, some transistors are

packaged individually, but many more are found embedded in integrated circuits.

4.7 The Electromagnetic Relay

Diagram that a relay uses an electromagnet. This is a device consisting of a

coil of wire wrapped around an iron core. When electricity is applied to the coil of wire it

becomes magnetic, hence the term electromagnet. The A B and C terminals are an SPDT

switch controlled by the electromagnet.

Fig. 4.4 Electromagnetic relay


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When electricity is applied to V1 and V2, the electromagnet acts upon the SPDT switch

so that the B and C terminals are connected. When the electricity is disconnected, then the

A and C terminals are connected. It is important to note that the electromagnet is

magnetically linked to the switch but the two are NOT linked electrically.

Basic Design and operation

Fig. 4.5 Simple electromechanical relay

A simple electromagnetic relay consists of a coil of wire surrounding a soft

iron core, an iron yoke which provides a low reluctance path for magnetic flux, a movable

iron armature, and one or more sets of contacts (there are two in the relay pictured). The

armature is hinged to the yoke and mechanically linked to one or more sets of moving

contacts. It is held in place by a spring so that when the relay is de-energized there is an

air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the

relay pictured is closed, and the other set is open. Other relays may have more or fewer

sets of contacts depending on their function. The relay in the picture also has a wire

connecting the armature to the yoke. This ensures continuity of the circuit between the

moving contacts on the armature, and the circuit track on the printed circuit board (PCB)

via the yoke, which is soldered to the PCB.


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When an electric current is passed through the coil it generates a

magnetic field that attracts the armature and the consequent movement of the movable

contact either makes or breaks (depending upon construction) a connection with a fixed

contact. If the set of contacts was closed when the relay was de-energized, then the

movement opens the contacts and breaks the connection, and vice versa if the contacts

were open. When the current to the coil is switched off, the armature is returned by a

force, approximately half as strong as the magnetic force, to its relaxed position. Usually

this force is provided by a spring, but gravity is also used commonly in industrial motor

starters. Most relays are manufactured to operate quickly. In a low-voltage application

this reduces noise; in a high voltage or current application it reduces arcing.

When the coil is energized with direct current, a diode is often placed across the

coil to dissipate the energy from the collapsing magnetic field at deactivation, which

would otherwise generate a voltage spike dangerous to semiconductor circuit

components. Some automotive relays include a diode inside the relay case. Alternatively,

a contact protection network consisting of a capacitor and resistor in series (snubber

circuit) may absorb the surge. If the coil is designed to be energized with alternating

current (AC), a small copper "shading ring" can be crimped to the end of the solenoid,

creating a small out-of-phase current which increases the minimum pull on the armature

during the AC cycle.

4.8 Power supply

The ac voltage, typically 220V rms, is connected to a transformer, which steps

that ac voltage down to the level of the desired dc output. A diode rectifier then provides

a full-wave rectified voltage that is initially filtered by a simple capacitor filter to produce

a dc voltage. This resulting dc voltage usually has some ripple or ac voltage variation.

A regulator circuit removes the ripples and also remains the same dc value even if

the input dc voltage varies, or the load connected to the output dc voltage changes. This

voltage regulation is usually obtained using one of the popular voltage regulator IC units.


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4.9 Working principle of power supply

Fig. 4.6 Block Diagram of Power supply

(a) Transformer

The potential transformer will step down the power supply voltage (0-230V) to (0-6V)

level. Then the secondary of the potential transformer will be connected to the precision

rectifier which is constructed with the help of op–amp. The advantages of using precision

rectifier are it will give peak voltage output as DC; rest of the circuits will give only RMS

output.

(b) Bridge rectifier

When four diodes are connected as shown in figure, the circuit is called as bridge

rectifier. The input to the circuit is applied to the diagonally opposite corners of the

network, and the output is taken from the remaining two corners.

Let us assume that the transformer is working properly and there is a positive potential, at

point A and a negative potential at point B. the positive potential at point A will forward

bias D3 and reverse bias D4.

The negative potential at point B will forward bias D1 and reverse D2. At this time D3

and D1 are forward biased and will allow current flow to pass through them; D4 and D2

are reverse biased and will block current flow. The path for current flow is from point B

through D1, up through RL, through D3, through the secondary of the transformer back to

point B. this path is indicated by the solid arrows. Waveforms (1) and (2) can be observed

across D1 and D3.

LOAD IC REGULATOR FILTER RECTIFIER TRANSFORMER


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One-half cycle later the polarity across the secondary of the transformer reverse,

forward biasing D2 and D4 and reverse biasing D1 and D3. Current flow will now be

from point A through D4, up through RL, through D2, through the secondary of T1, and

back to point A. This path is indicated by the broken arrows. Waveforms (3) and (4) can

be observed across D2 and D4. The current flow through RL is always in the same

direction. In flowing through RL this current develops a voltage corresponding to that

shown waveform (5). Since current flows through the load (RL) during both half cycles

of the applied voltage, this bridge rectifier is a full-wave rectifier.

One advantage of a bridge rectifier over a conventional full-wave rectifier is that

with a given transformer the bridge rectifier produces a voltage output that is nearly twice

that of the conventional full-wave circuit.

(c) Filter

Filters are electronic circuits which perform signal processing functions,

specifically to remove unwanted frequency components from the signal, to enhance

wanted ones, or both. The most common types of electronic filters are linear filters,

regardless of other aspects of their design.

(d) IC Regulator

An IC regulator is an electrical regulator designed to automatically

maintain a constant voltage level. A voltage regulator may be a simple “feed-forward”

design or may include negative feedback control loops. It may use an electromechanical

mechanism, or electronic components. Depending on the design, it may be used to

regulate one or more AC or DC voltages.


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CHAPTER 5

PIC MICROCONTROLLER

5.1 Introduction

A micro-controller-based physiological sensing unit has been

designed, prototyped, and field-tested for recording galvanic skin response data and

relaying them to a computer for physiological analysis. Focusing on system design issues

concerning battery-driven ambulatory applications, this paper presents a special data

compression algorithm based on relative encoding to optimize memory utilization and

reduce data transfer time. Data flow coordination and timing control are enabled by a PIC

micro-controller. The embedded block is connected with the computer through serial port

and the patient reaches an abnormal condition, automatically an alert message will given

to the doctor.

5.2 PIC Microcontroller

Major Blocks in the PIC MCU,

The major parts of the PIC MCU (Microcontroller Unit) that we will be concerned with

are the program memory, data memory which is also called file registers, and the

Working Register, and finally the EEPROM memory section.

Program Memory ----- 14 bit word length

File Register Memory (Data Memory) ----- 8 bit word length

EEPROM Memory ----- 8 bit word length ---- separate address space

Working Register ----- Byte wide used in most instructions

5.3 PROGRAM MEMORY

The program memory in the PIC16F877A has a total of 8K words. The word

length for the midrange family of PIC microcontrollers is 14 bits long. Because each

word uses 14 bits this amount of memory is roughly equivalent to 14K bytes. The


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program memory can be made up of EPROM using the 16CXXX parts or it can have

FLASH memory as in the 16FXXX parts. After the program memory has been

programmed it will retain the program even if power is lost. Therefore program memory

is said to be non-volatile. The 8K words or program memory are made up of 4 sections or

pages of 2K words each. Within each page every 14 bit word has an address with the first

one starting at 000hex and ending with 7Ffhex which corresponds to 0 through 2047

decimal. The MCU can only use one of the program memory pages at a time. To read or

write to a different page requires the programmer to do page switching within his

program. How to do this will be covered in detail in a later section.

The PIC 16F877A microcontroller is at the top of the end of the range for the

midrange family. Other members have less memory than this. Members that have 4K

words of program memory will only have 2 pages. Some members have only enough

program memory for one page. For example the PIC 16F84A only has 1024 words of

program memory so consequently needs only one page to contain its program memory.

For the PIC16F84A page switching is not a consideration. For the programs used in this

course we will normally only use the first page of program memory and will not have to

worry with page switching. The methods for page switching will be covered in detail later

in the course.

Fig. 5.1 File Registers (Data Memory)


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The file registers are where data and variables are stored. File registers contain a single

byte in each address location. File registers will not retain their contents when power is

lost and therefore is said to be volatile memory.

File Registers are considered the data memory section of the MCU. File

registers can be broken down into two types of data storage. General purpose registers

(GPR) are used by the programmer for normal calculations and temporary data storage.

Special Function registers (SFR) are used to control various internal functions contained

within the microcontroller such as interrupts, serial communications, timers and counters,

A/D converters, program counter, port direction control and many others. Special

Function registers will have a name associated with them such as TMR0, STATUS,

PORTA, PORTB, etc. Each Special Function register allows control or access to data

associated with a particular hardware function within the microcontroller. General

purpose registers do not have a name since they can be used for any purpose the

programmer designates. After a power-up any special function registers that are going to

be used must be loaded with the necessary setup data before they can be used. The file

registers in data memory are divided up into Banks similar to the way program memory is

divided into pages. Each Bank can hold up to 128 bytes and uses addresses from 00h to

7Fh. Notice in the image of the register map that in each bank the special function

registers always start at address 00h and go up to a certain address at which point the

general purpose registers begin. If all four banks contained 128 bytes each there would be

512 bytes total in the file register space. However not all locations are used. The byte

locations tinted in grey are not available. Also in banks 1, 2 and 3 the last 16 bytes all

map to the same 16 bytes that are in bank 0. After these considerations we find that there

are actually 368 bytes of General Purpose Register Space. Other microcontrollers that are

at the lower end of the midrange product line may have only 2 banks that contain even

fewer bytes than the 128 bytes in each bank of the PIC 16F877A. For example the PIC

16F84A has 2 banks with each bank having addresses between 0 and 4Fh. This MCU has

a total of 68 bytes of general purpose file register space. We will be using all 4 banks of

the file register space available to the PIC 16F877A in our programs. The methods used

to switch banks will be covered in detail in a later programming section of the course.


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5.4 EEPROM Memory

The last type of memory we will deal with is EEPROM.

This memory is nonvolatile meaning that it will retain its data even if the power is lost.

EEPROM is normally used to store parameter data that needs to be saved over a long

period over many power-up and power-down cycles. Writing to EEPROM involves a

more complex series of steps than writing to the File Registers. Each write cycles is very

slow compared to writing to the File Registers. However reading is much simpler and

much faster. The procedures for using this type of memory will be covered in an

intermediate course. The PIC16F877A has 256 bytes of EEPROM memory. The

EEPROM memory is separate from program memory and data memory. It has it own

address space. To read or write to EEPROM memory the Special Function Registers are

used such as EEDATA and EEADR.

5.5 Working register

The Working register is a single byte register that is used in most of the PIC

MCU instructions. The Working register is nearly always involved in any actions

involving data in the File Registers. Whenever two values are used in arithmetic

operations or logic operations one of the values will be loaded into the Working register

at the start of the instruction. After the instruction finishes the result is often left in the

Working register. In other manufacturers microcontrollers the Working register is known

as the accumulator. However, we will stick with the term working register for our

purposes. If you look in the File Register Map you will not find the Working register. It is

not part of file register space. It exists on its own within the PIC MCU.

5.6 Special function registers

As can be seen in the File Register Map there

are many special function registers. We will cover the most important ones in this course

and leave others for more advanced course work.


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Fig.5.2 Register file


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5.7 Status register

The Status register is at address location 03 in bank 0. Notice that it is also listed

in banks 1, 2 and 3 at addresses 83h, 103h and 183h respectively. The Status register is

used so often in programming that it was designed in to all banks so that it would be easy

to access without switching banks. So there is really only one Status register but its

contents are available in all 4 File Register banks. Each of the 8 bits in the Status register

has a function. For example bit 0 is the Carry Flag. This bit is used to signal a carry when

the result of an addition is too large to fit into a single byte. It can also be used to indicate

a borrow when subtraction is performed. The carry flag can also be cleared or set directly

by an instruction. Bit 1 is DC or Digit Carry. When working with binary coded decimal

it is necessary to have a carry for each decimal digit. Since each decimal digit is

represented by 4 bits, the DC works as the carry for the low order digit and the regular

Carry bit works as the carry for the high order digit. Often we want to test to see if the

result of an arithmetic or logic operation resulted in zero or a non-zero number. The Z

flag or Zero flag will be modified after arithmetic and logic instructions. If the result was

0 the Z flag will be set to 1. However if the result was not a zero the Z flag will be cleared

to 0. The instruction set gives us the capability to check the results of C, DC and Z flags

in order to make decisions based on their contents. For right now we will not concern

ourselves with the functions of bits 3, 4 and 7. We will pick them up later when they will

make more sense. Bits 5 and 6 are the bits that control which File Register bank is active.

They are called RP0 and RP1 respectively. If both bit 5 and 6 are cleared then bank 0 is

active. When bit 6 is cleared and bit 5 is set then bank 1 is active. All that is needed to

change File Register banks is to set bit 5 and 6 as shown in the Status Register Map.

The choice as to how each pin will be used in a particular application is

controlled by programming various special function registers. The control of the various

functions available is covered later in the context of programming and initializing the

processor.


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5.8 Features of PIC 16F877A

The microcontroller has the following features:

(a) High-Performance RISC CPU:

• Only 35 single- word instructions to learn .Hence it is user friendly. Easy to use

• All single – cycle instructions except for program branches, which are two-cycle

• Operating speed: DC – 20 MHz clock input DC – 200 ns instruction cycle

• Up to 8K x 14 words of Flash Program Memory, Up to 368 x 8 bytes of Data Memory

(RAM), Up to 256 x 8 bytes of EEPROM Data Memory.

(b) Peripheral Features:

• Timer0: 8-bit timer/counter with 8 – bit prescaler. It is used for synchronization

• Timer1: 16-bit timer/counter with prescaler, can be incremented during Sleep

• Timer2:8-bit timer/counter with 8-bit period register, prescaler and postscaler

• Two Capture, Compare and some PWM modules, having following features

-Capture is 16-bit, max. resolution is 12.5 ns

-Compare is 16-bit, max . resolution is 200 ns

-PWM maximum resolution that is 10-bit

(b) Analog features:

It has an analog Comparator module with:

(1) Two analog comparators

(2) Programmable on-chip voltage reference (VREF) module (3) Programmable input

multiplexing from device inputs and internal voltage reference thus 3 parts

I CMOS Technology:

It has following features:

(1) Low-power, high-speed Flash/EEPROM technology

(2) Fully static design


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5.9 Blocks diagram of PIC 16F877A:

Fig.5.3 Block diagram of PIC 16F877A


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5.10 PIN diagram of PIC 16F877A

Fig. 5.4 Pin diagram of PIC 16F877A


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5.11 Trigeering Circuit

Fig. 5.5 Triggering circuit.

5.11.1 PIN description:

(a) OSC1/CLKI:

Oscillator crystal or external clock input. Oscillator crystal input or external clock source

input. ST buffer when configured in RC mode; otherwise CMOS. External clock source

input. Always associated with pin function OSC1 (see OSC1/CLKI, OSC2/CLKO pins).

(c) OSC2/CLKO:

Oscillator crystal or clock output. Oscillator crystal output. Connects to the crystal or

resonator in Crystal Oscillator mode. In RC mode, OSC2 pin outputs CLKO, which has

1/4 the frequency of OSC1 and denotes the instruction cycle rate.


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(d) MCLR/VPP:

Master Clear (input) or programming voltage (output). Master Clear (Reset)

input. This pin is an active low Reset to the device. Programming voltage input.

• RA0/AN0.

• RA1/AN1.

• RA2/AN2/VREF-/CVREF.

• VREFCVREF.

• RA3/AN3/VREF+.

• VREF+.

• RA4/T0CKI/C1OUT.

• T0CKI.

• C1OUT.

• RA5/AN4/SS/C2OUT/SS/C2OUT.

(e) I/O PORTS:

Some pins for these I/O ports are multiplexed with an alternate function for the

peripheral features on the device. In general, when a peripheral is enabled, that pin may

not be used as a general purpose I/O pin.

(f) PORT A AND THE TRISA PORT

PORTA is a 6-bit wide, bidirectional port. The corresponding data direction

register is TRISA. Setting a TRISA bit (= 1) will make the corresponding PORTA pin an

input (i.e., put the corresponding output driver in a High – Impedance mode). Clearing a

TRISA bit (= 0) will make the corresponding PORTA pin an output (i.e., put the contents

of the output latch on the selected pin). Reading the PORTA register reads the status of

the pins, whereas writing to it will write to the port latch. All write operations are read-

modify-write operations. Therefore, a write to a port implies that the port pins are read;

the value is modified and then written to the port data latch.


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Pin RA4 is multiplexed with the Timer0 module clock input to become the RA4/T0CKI

pin. The RA4/T0CKI pin is a Schmitt Trigger input and an open-drain output. All other

PORTA pins have TTL input levels and full CMOS output drivers. Other PORTA pins

are multiplexed with analog inputs and the analog VREF input for both the A/D

converters and the comparators. The operation of each pin is selected by clearing/setting

the appropriate control bits in the ADCON1 and/or CMCON registers. The TRISA

register controls the direction of the port pins even when they are being used as analog

inputs. The user must ensure the bits in the TRISA register are maintained set when using

them as analog inputs.

(g) PORT B and the TRISB Register

PORTB is an 8-bit wide, bidirectional port. The corresponding data direction

register is TRISB. Setting a TRISB bit (= 1) will make the corresponding PORTB pin an

input (i.e., put the corresponding output driver in a High-Impedance mode). Clearing a

TRISB bit (= 0) will make the corresponding PORTB pin an output (i.e., put the contents

of the output latch on the selected pin). Three pins of PORTB are multiplexed with the In-

Circuit Debugger and Low-Voltage Programming function: RB3/PGM, RB6/PGC and

RB7/PGD.

Four of the PORTB pins, RB7:RB4, have an interruption- change feature. Only pins

configured as inputs can cause this interrupt to occur (i.e., any RB7:RB4 pin configured

as an output is excluded from the interruption- change comparison). The input pins (of

RB7:RB4) are compared with the old value latched on the last read of PORTB. The

“mismatch” outputs of RB7:RB4 are together to generate the RB port change interrupt

with flag bit RBIF (INTCON<0>).

This interrupt can wake the device from Sleep. The user, in the Interrupt

Service Routine, can clear the interrupt in the following manner:

a) Any read or write of PORTB. This will end the mismatch condition.

b) Clear flag bit RBIF.

A mismatch condition will continue to set flag bit RBIF. Reading PORTB

will end the mismatch condition and allow flag bit RBIF to be cleared. The interrupt-on-


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change feature is recommended for wake-up on key depression operation and operations

where PORTB is only used for the interrupt-on-change feature. Polling of PORTB is not

recommended while using the interrupt-on- change feature. This interrupt-on-mismatch

feature, together with software configurable pull-ups on these four pins, allow easy

interface to a keypad and make it possible for wake-up on key depression.

(H) PORTC and the TRISC Register:

PORTC is an 8-bit wide, bidirectional port. The corresponding data direction register is

TRISC. Setting a TRISC bit (= 1) will make the corresponding PORTC pin an input (i.e.,

put the corresponding output driver in a High- Impedance mode). Clearing a TRISC bit (=

0) will make the corresponding PORTC pin an output (i.e., put the contents of the output

latch on the selected pin). PORTC is multiplexed with several peripheral functions (Table

4-5). PORTC pins have Schmitt Trigger input buffers. When the I2C module is enabled,

the PORTC<4:3> pins can be configured with normal I2C levels, or with SMBus levels,

by using the CKE bit (SSPSTAT<6>). When enabling peripheral functions, care should

be taken in defining TRIS bits for each PORTC pin. Some peripherals override the TRIS

bit to make a pin an output, while other peripherals override the TRIS bit to make a pin an

input. Since the TRIS bit override is in effect while the peripheral is enabled, read-modify

write instructions (BSF, BCF, XORWF) with TRISC as the destination, should be

avoided. The user should refer to the corresponding peripheral section for the correct

TRIS bit settings.

(i)PORT D and TRISD Register

PORTD is an 8-bit port with Schmitt Trigger input buffers. Each pin is individually

configurable as an input or output. PORTD can be configured as an 8-bit wide

microprocessor port (Parallel Slave Port) by setting control bit, PSP MODE (TRISE<4>).

In this mode, the input buffers are TTL.

(j)PORTE and TRISE Register

PORTE has three pins (RE0/RD/AN5, RE1/WR/AN6 and RE2/CS/AN7) which

are individually configurable as inputs or outputs. These pins have Schmitt Trigger input

buffers. The PORTE pins become the I/O control inputs for the microprocessor port when

bit PSPMODE (TRISE<4>) is set. In this mode, the user must make certain that the


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TRISE<2:0> bits are set and that the pins are configured as digital inputs. Also, ensure

that ADCON1 is configured for digital I/O. In this mode, the input buffers are TTL.

Register 4-1 shows the TRISE register which also controls the Parallel Slave Port

operation. PORTE pins are multiplexed with analog inputs.

When selected for analog input, these pins will read as ‘0’s. TRISE controls. The

direction of the RE pins, even when they are being used as analog inputs. The user must

make sure to keep the pins configured as inputs when using them as analog inputs.

(k) TIMER0 Interrupt:

The TMR0 interrupt is generated when the TMR0 register overflows from FFh to 00h.

This overflow sets bit TMR0IF (INTCON<2>). The interrupt can be masked by clearing

bit TMR0IE (INTCON<5>). Bit TMR0IF must be cleared in software by the Timer0

module Interrupt Service Routine before re-enabling this interrupt. The TMR0 interrupt

cannot awaken the processor from Sleep since the timer is shut-off during Sleep.

(l) TIMER1 Module:

The Timer1 module is a 16-bit timer/counter consisting of two 8-bit registers (TMR1H

and TMR1L) which are readable and writable. The TMR1 register pair

(TMR1H:TMR1L) increments from 0000h to FFFFh and rolls over to 0000h. The TMR1

interrupt, if enabled, is generated on overflow which is latched in interrupt flag bit,

TMR1IF (PIR1<0>). This interrupt can be enabled/disabled by setting or clearing TMR1

interrupt enable bit, TMR1IE (PIE1<0>). Timer1 can operate in one of two modes:

• As a Timer

• As a Counter

The operating mode is determined by the clock select bit, TMR1CS (T1CON<1>).

In Timer mode, Timer1 increments every instruction cycle. In Counter mode, it

increments on every rising edge of the external clock input. Timer1 can be

enabled/disabled by setting/clearing control bit, TMR1ON (T1CON<0>).Timer1 also has

an internal “Reset input”. This Reset can be generated by either of the two CCP modules.

Shows the Timer1 Control register. When the Timer1 oscillator is enabled (T1OSCEN is


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set), the RC1/T1OSI/CCP2 and RC0/T1OSO/T1CKI pins become inputs. That is, the

TRISC<1:0> value is ignored and these pins read as ‘0’.

(m)TIMER2 Module:

Timer2 is an 8-bit timer with a prescaler and a postscaler. It can be used as the PWM

time base for the PWM mode of the CCP module(s). The TMR2 register is readable and

writable and is cleared on any device Reset. The input clock (FOSC/4) has a prescale

option of 1:1, 1:4 or 1:16, selected by control bits T2CKPS1:T2CKPS0 (T2CON<1:0>).

The Timer2 module has an 8-bit period register, PR2. A Timer2 increment from 00h until

it matches PR2 and then resets to 00h on the next increment cycle. PR2 is a readable and

writable register. The PR2 register is initialized to FFh upon Reset. The match output of

TMR2 goes through a 4-bit postscaler (which gives a 1:1 to 1:16 scaling inclusive) to

generate a TMR2 interrupt (latched in flag bit, TMR2IF (PIR1<1>)). Timer2 can be shut-

off by clearing control bit, TMR2ON (T2CON<2>), to minimize power consumption.

(n)Analog to Digital converter:

The analog-to-digital (A/D) converter module can have up to eight analog inputs

for a device. The analog input charges a sample and hold capacitor. The output of the

sample and hold capacitor is the input into the converter. The converter then generates a

digital result of this analog level via successive approximation. This A/D conversion, of

the analog input signal, results in a corresponding 10-bit digital number. The analog

reference voltages (positive and negative supply) are software selectable to either the

device’s supply voltages (AVDD, Avss) or the voltage level on the AN3/VREF+ and

AN2/VREF.

The A/D module has four registers. These registers are:

• A/D Result High Register (ADRESH)

• A/D Result Low Register (ADRESL)

• A/D Control Register0 (ADCON0)

• A/D Control Register1 (ADCON1)


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10-bit A\D Converter:-

Fig. 5.6 10-bit A/D converter

Control register:

bit 7:6 ADCS1:ADCS0: A/D Conversion Clock Select bits

00 = FOSC/2

01 = FOSC/8

10 = FOSC/32


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11 = FRC (clock derived from the internal A/D RC oscillator)

bit 5:3 CHS2:CHS0: Analog Channel Select bits:

000 = channel 0, (AN0)








bit 2 GO/DONE: A/D Conversion Status bit

When ADON = 1

1 = A/D conversion in progress (setting this bit starts the A/D conversion which is

automatically cleared by hardware when the A/D conversion is complete)

0 = A/D conversion not in progress

bit 1 Unimplemented: Read as ‘0’

bit 0 ADON: A/D On

1 = A/D converter module is powered up

0 = A/D converter module is shut off and consumes no operating current


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(o)ADCON1 Register:

bit 7:6 Unimplemented: Read as ‘0’

bit 5 ADFM: A/D Result format select

1 = Right justified. 6 Most Significant bits of ADRESH are read as’0’.

0 = Left justified. 6 Least Significant bits of ADRESL are read as ’0’.

Bit 4 Unimplemented: Read as ‘0’

bit 3:0 PCFG3:PCFG0: A/D Port Configuration Control bits


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CHAPTER 6

HARDWARE LAYOUT

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Pic Development Board - PIC Main and PS

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Date: Sheet of

VDD

VDD

TX

MCLR

VPP

VDD

V DD

TO RA1 PIN

VDD

RX

RB5

RB1RA4

RE1

RD2

RA5

RC7

RD6

RA1

RD1RC4

RE2

RC3

RB0

RC6RC5

RA3

RD7

RA2 RB4

RD5

RD0

RB2

RB7

RB3

RB6

RD3RD4

RE0

RC0RC1RC1

T1

TRANSFORMER CT

1

5

6

4

8

JP1

230 VAC

12

- +

D1

1

4

3

2C5470 uF

R15POT1

3

2D3

5V zener

R141k

Y1

10Mhz

R5220 ohm

R131k

Q3BC547

K3

RELAY DPDT

34

5

68

712

SW3

RESET

Q3BC547

R131k

C70.1 uF

JP1

220 VAC

12

- +

D1

1

4

3

2

C6100 uF

C5470 uF

R4220 ohm

D2

LED

U2LM7805

1

2

3VIN

GN

DVOUT

R6

1 k

K3

RELAY DPDT

34

5

68

712

C927 pF

PIC16F877U3

123456

11

32

12

31

789

10

1314

15161718

1920

3334353637383940

282930

2122

242526

27

23

MCLR/VppRA0/AN0RA1/AN1RA2/AN2/Vref -RA3/AN3/Vref +RA4/T0CKI

VD

DV

DD

VS

SV

SS

RA5/AN4/SSRE0/AN5/RDRE1/AN6/WRRE2/AN7/CS

OSC1/CLKINOSC2/CLKOUT

RC0/T1OSO/T1CKIRC1/T1OSI/CCP2RC2/CCP1RC3/SCK/SCL

RD0/PSP0RD1/PSP1

RB0/INTRB1RB2

RB3/PGMRB4RB5

RB6/PGCRB7/PGD

RD5/PSP5RD6/PSP6RD7/PSP7

RD2/PSP2RD3/PSP3

RC5/SDORC6/TX/CKRC7/RX/DT

RD4/PSP4

RC4/SDI/SDA

C80.1 uF

C1027 pF

220ohms

Load 1 Load 2

220ohms


EEE Dept.

CHAPTER 7

CONCLUSION AND FUTURE SCOPE

The project describes about how to use power supply intelligently under peak

loads. The project automatically connects and disconnects the transformer thus protecting

transformer from overload. Sensing unit, ie.Current transformer plays an important role

by sensing the current through the load and sending feedback signal to the

microcontroller.PIC Microcontroller is so programmed that as soon as the load exceeds a

particular current limit it will soon generate a control signal that would be amplified by

the driver unit and finally control signal is fed to the Electromagnetic relay. For working

the relay AC supply is obtained through the inverter. The switching process occurs in the

Electromagnetic Relay which automatically connects the transformer in parallel in

accordance to the load sensed by the CT.

The future scope of our project is particularly in

Substation. In substations particularly during the peak hours there is a need for the

operation of additional transformer to supply the additional load requirement. Our project

automatically connects the transformer under critical loads. Thus there is no need to

operate both transformers under normal loads, particularly during off peak hours. Thus

power is shared intelligently with the transformers in parallel.

REFERENCES

[1] J. R. Rodriguez, J. W. Dixon, J. R. Espinoza, J. Pontt, and P. Lezana, “PWM

regenerative rectifiers: state of the art,” IEEE Transactions on Industrial Electronics,

vol. 52, no. 1, pp. 5– 22, Feb. 2005.


EEE Dept.

[2] J. Rodriguez, L. Moran, J. Pontt, J. Espinoza, R. Diaz, and E. Silva, “Operating

experience of shovel drives for mining applications,” IEEE Transactions on Industry

Applications, vol. 40, no. 2, pp. 664–671, Mar./Apr. 2004.

[3] http://www.pdf4me.net/pdf-data/load-sharing-of-transformers-using-embedded-systems.php

[4]http://www.docstoc.com/docs/10934947/IEEE-Electrical-IEEE-Project-Titles_-2009---2010-

NCCT-Final-Year-Projects

[5] www.2010ieeeprojects.com/power_electronics_projects.html

[6]www05.abb.com/1MRK500401SEN_en_Transformer_protection_monitoring_and_control.pdf

[7]http://www.chetanasprojects.com/Thread-EMBEDDED-MICROCONTROLLER-PROJECTS-

-4754

[8] ww1.microchip.com/downloads/en/devicedoc/39582b.pdf

[9]http://www.docstoc.com/docs/41554618/PIC-Microcontroller-Development-Board-%28PIC-

16F877A-40-Pin%29

[10] www.barcolair.com/PDF/BarberColman/Load%20Sharing/f22107-4.pdf

intellligent power sharing of transformers with auto protection

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