BSNL electrical engineering summer training report file

1. Summer Training Report
at
BHARAT SANCHAR NIGAM LIMITED
Submitted in partial fulfillment
of the requirement for the award of the degree of
Bachelor of Technology
in
Electrical engineering
By
AMAN SINGH
Enrollment No. 1473520007
Under the supervision of
Mr. S.K. RATHORE
Assistant Telecom Officer
BSNL
Fatehpur, Uttar Pradesh (212601)
Submitted to
Department of Electrical Engineering
Rajkiya engineering college, Bijnor
1

2. Acknowledgement
On the very outset of this report, I would like to extend my sincere & heartfelt obligation
towards all the personages who have helped me in this endeavor without their active
guidance, help, cooperation & encouragement.
I would not have made headway in the project.
I am ineffably indebted to Mr. S.K. RATHORE for conscientious guidance and
encouragement to accomplish this assignment.
I am extremely thankful and pay my gratitude to department of BSNL for their Valuable
guidance and support on completion of this project in its presently.
I also acknowledge with a deep sense of reverence, my gratitude towards my Parents and
member of my family, who has always supported me morally as well as economically. At last
but not least gratitude goes to all of my friends who directly or indirectly helped me to
complete this project report.
Any omission in this brief acknowledgement does not mean lack of gratitude.
2

3. PREFACE
Organizations are made up of people and function through people. Without people,
organizations cannot exist. The resources of men, money, material, machinery, and
mechanism are connected, coordinated and utilized through people. Engineers need to
concentrate more on mechanism and the way in which things have been made. The need of
training arises for doing things yourself, understanding its way.
Practical exposure for doing things makes a person conversant to the technicalities involved
in any job. In view of such benefits, imparting of vocational training has been made an
integral part of any academic structure.
In B.S.N.L., training is given to Engineering Aspirants to secure future in the dynamic world
of telecommunications. Today telecommunication industry is one of the very fastest growing
industries in the world.
In this order I have taken 28 days BSNL training. In my report I try to explain the entire
power plant system of BSNL.
3

4. TABLE OF CONTENTS
CHAPTER
NO.
CONTENTS PAGE NO.
Cover Page 1
Acknowledgement 2
Preface 3
Contents 4-5
List of Figures 5
Overview of telecom infrastructure 6
Chapter-1 Introduction to BSNL 7
1.1 Objective 8
1.2 Telecom infrastructure 9
Chapter-2 Components of Telecom support infrastructure 10
2.1 SMPS plant 10
2.2 Battery 11
2.2.1 Inverter 11
2.2.2 Engine alternator set 11
2.2.3 Earthing 11
2.2.4 Air conditioning system 12
2.2.5 Fire safety 12-13
2.2.6 Tower 13
2.2.7 Shelter 13
2.2.8 Sources of power 13-14
2.3 Commercial AC power supplies 14
2.4 A.C to D.C conversions 15
2.4.1 Earthing of one pole of D.C 15
Chapter-3 Major subsystems of power plants 16
3.1 Float rectifier 16-18
3.2 Battery charger 19
3.3 Switching cubicle 19-20
Chapter-4 Principle of switching regulator 20
4.1 Functional description of rectifier 21-22
4

5. LIST OF FIGURES
4.2 Functional description of power system
controller
23
4.3 Trouble shooting in plant 24-25
Conclusion 26
References 27
FIGURE NO. TITLE OF FIGURE PAGE NO.
FIG 1 Telecom support infrastructure 10
FIG 2 Float working 15
FIG 3 Scr method 17
FIG 4 Silicon controlled rectifier type Float rectifier 19
FIG 5 Switching regulator 21
FIG 6 Principle of regulation 22
FIG 7 Duty cycle pattern 23
5

6. OVERVIEW OF TELECOM
INFRASTRUCTURE & POWER PLANT
STRUCTURE
1.1 INTRODUCTION
1.2 OBJECTIVE
1.3 TELECOM INFRASTRUCTURE
1.4 COMPONENTS OF TELECOM
SUPPORT INFRASTRUCTURE
1.5 SOURCES OF POWER
1.6 COMMERCIAL AC POWER SUPPLIES
1.7 A.C TO D.C CONVERSIONS
1.8 EARTHING OF ONE POLE OF D.C
1.9 MAJOR SUBSYSTEMS OF POWER PLANTS
1.10 SMPS POWER PLANTS
1.11 SUMMARY
1.12 REFERENCES AND SUGGESTED FURTHER READINGS
6

7. CHAPTER 1
INTRODUCTION
Telecom Network consists of many elements such as switching network,
transmission network, civil infrastructure, electrical items etc.
Proper functioning of this infrastructure is necessary for delivery of quality
services to the customers which in turn leads to profitability of the operator’s business.
Telecommunication systems require electrical energy for transmission of signals
energization of subscriber’s telephone transmitters and for many miscellaneous
functions.
A telephone exchange requires a considerable large amount of energy, as the
common exchange power plants required to feed currents for the subscriber’s
transmitters, for signaling and for control and operation of exchanges switches.
It is therefore, necessary that a power source should not be only economical but
adequate to meet the needs of a particular type of the installation.
Failure of power supply system in any installation renders the communication
facilities offered by it to be instantly paralyzed.
7

8. 1.1 OBJECTIVE
The objective of this chapter is:
• To know components of telecom infrastructure
• To Know the importance of power supply
• To list the different sources available for power
• To Classify the power plants
• To understand working principle of Float rectifier, battery charger.
• To Know the Working principle of SMPS power plant
• To explain the different Features of power system
• To Understand principle of regulation
• To Explain the functions different components of the power plant
8

9. 1.2 TELECOM INFRASTRUCTURE
A typical telecom network infrastructure can be categorized into three distinct categories
namely Passive Infra, Active Infra and Backhaul.
Telecom Network
Infrastructure
Passive
Infrastructure
Key Components
- Power supply
- Battery bank
- Invertors
- Diesel enerator (DG)
- Air conditioner
- Earthing
-Fire extinguisher
- Security cabin, etc. -
Steel tower
- mounting structures -
shelter
Backhaul
The backhaul part
of the network consists of the
intermediate links between the
core of the network and the
various sub-networks
Active
Infrastructre
Key Components
-Spectrum
- Base tower station
- Microwave radio
- Switches
- Antennas
- Transceivers
Coordinated maintenance, timely up gradation of these elements is the key to success of
an operator. A brief structure of these categories is given below:
REVENUE POTENTIAL OF PASSIVE INFRA:
With boom in telecom business, many operators already exist in the market and many in
the pipeline to start business. Existing operators are under intense pressure to expand their
network and new entrants under pressure for faster roll out. This coupled with intense
price war has lead to telecom operators look for cost cutting and faster roll out
opportunities. In mobile network, a very significant cost of investment as well as time
goes towards passive infrastructure. Government of India allows sharing of passive
infrastructure.
9

10. Chapter 2
COMPONENTS OF TELECOM SUPPORT INFRASTRUCTURE
Figure 1. Telecom support infrastructure
2.1 SMPS (SWITCHED MODE POWER SUPPLY) POWER PLANT
The power plant is used to rectify the ac input supply to desired output dc (-48v). The
conventional power plants which were in use earlier were based on SCRs or Ferro-
resonant techniques. These conventional types of power plants were having following
problems:
• Very large size
• Large weight
• Lower efficiency
• No scope for modular expansion.
To get rid of all these problems now SMPS (Switched Mode Power System) power
plants are used.
10

11. 2.2 BATTERY
These days, most of the Battery used in mobile network are VRLA (Valve Regulated
Lead Acid Battery) type. Various capacities of Batteries are 120 AH, 400 AH, 600 AH,
1000AH, 1500 AH, 2000 AH, 2500 AH, 3000 AH, 4000 AH & 5000 AH.
LIFE OF A BATTERY:
• Batteries up to 200AH: 4 Years
• Batteries more than 200 AH: 6 years
2.2.1 INVERTER
In most of the telecom installations, inverters are installed to provide uninterrupted AC
supply to OMC terminals. Capacity of invertors used varies from 1KVA to 10KVA
depending on the connected AC load. The basic precautions for installation is that
inverter should be installed as close to battery room as possible so as to reduce DC
voltage loss due to cabling. The inverters may not be loaded beyond 80%of its rated
capacity and initial start up load also needs to be taken into account. Only essential
equipment may be connected to inverter output.
2.2.2 ENGINE ALTERNATOR SET
Now-a-days it is extremely difficult to get an uninterrupted power supply from the
supplier. Non availability of power supply is caused from various factors. In the present
working system, continuous supply of power is a must for telecom equipments and
computers. Hence, there is an important need of the engine alternator set. The engine
alternator is a combination of a diesel engine and an alternator. This combined unit is
called as an Alternator set.
2.2.3 EARTHING:
Earthing plays a vital role in the protection of equipments and the personnel. Apart from
protection from hazardous stray currents in electrical equipment in Telecommunication
circuits and equipments, Earthing is provided for the following purposes:
• Reduction of Crosstalk and Noise.
• Protection of costly apparatus and persons against foreign voltages and leakage
currents from power wirings touching the metallic frame of the equipment.
• Protection of buildings and equipments from lightning strikes.
• Earthing of power supply systems is used to ensure reliability of power as it helps to
provide stability of voltage conditions preventing excess fluctuations and providing a
measure of protection against lightning.
2.2.4 AIR- CONDITIONING SYSTEM:
11

12. The telecom equipments use semi-conductor based circuitry which works in a normal
way within a particular temperature band only. Beyond this band, this behaves
critically. So the exchange equipments need air-conditioning. Air conditioning means
maintaining desired conditions within a confined space. It is essentially provided to
prevent deterioration of equipments and to maintain temperature and humidity for
electrical and electronic equipments. So it is mandatory for proper functioning of
exchanges. Hence, it needs proper maintenance.
Air-conditioning system in use may be categorized in to the following types:
1) Window Type Units
2) Split Type Units.
3) Package Type unit
4) Central Air-conditioning System
Each of these has its limitations as well as advantages and the most suitable one
should be selected taking all relevant factors into account.
2.2.5 FIRE SAFETY
One of the most common types of hazard in an office is a fire hazard which can cause
personal injury to employees. The main reasons for office fire hazards are
combustible materials, poor maintenance of equipment, poor standards of
housekeeping and poor maintenance of electric circuits. Office fire hazards can cause
serious injuries to employees and may even lead to loss of life. So it is necessary to
take certain steps in order to prevent fire accidents in the office premises. Employers
should ensure there are health and safety requirements regarding workplace fire
safety.
Many fires can easily be prevented if adequate fire safety precautions are taken.
Maintaining proper fire safety standards in commercial places and buildings not only
helps in saving lives but also provides protection to buildings and the businesses
carried on within them. It is the responsibility of the employer to undertake fire safety
measures in his or her workplace
The geographical and climatic condition of the country makes many different
disciplines and attitudes in regard to design of buildings and selection of materials.
Good housekeeping, general tidiness, control on combustible materials and awareness
about surroundings may certainly minimize the fire risks. In case of fire incidence,
provisioning of efficient fire detection and alarm system helps in initiating timely
action to control the fire. Good quality and proper quantity of fire fighting apparatus
provides strength against fight with fire. Fixed and portable fire extinguishing
apparatus fully charged and in working condition should be available in sufficient
number at convenient locations to check the fires in incipient stage Smooth operation
of above elements is crucial to the successful delivery of service through active
elements in the network. In order to facilitate prompt action for any deviation in state
of health of these passive elements against set benchmarks, alarms are generated and
transmitted to OMC/NOC (Operation and maintenance Centre/ Network Operation
Centre for appropriate actions. Typical alarms extended are Mains fail, Diesel Low,
12

13. battery Low, Generator ON, DG set Cabin door open(canopy type), Fire, High
Temperature etc.
2.2.6 TOWERS
Types of Towers Telecom towers are broadly classified on the basis of their placement
as Ground-based and Roof-top.
Ground-Based Tower: Erected on the ground, ground-based towers (GBTs) are
taller (typically 200 to 400 feet) and are mostly used in rural and semi-urban areas
because of the easy availability of real-estate space there. GBTs involve a capital
expenditure in the range of Rs. 2.4 to 2.8 million, depending on the height of the
tower.
Roof-Top Tower (RTT): Roof-top towers (RTTs), which are generally placed on
the roofs of high-rise buildings, are shorter (than GBTs) and more common in urban
and highly populated areas, where there is paucity of real-estate space. Typically,
these involve a capital expenditure of Rs. 1.5 to 2 million. It is the height of a telecom
tower that determines the number of antennas that can be accommodated, which in
turn determines the capacity of the towers, apart from factors such as location and
geographical conditions (wind speeds, type of terrain, etc.). Hence, typically, while
GBTs can accommodate up to six tenants, RTTs can accommodate two to three
tenants.
2.2.7 SHELTERS
A telecom shelter is a small building that contains the major telecom equipment
located well away from the main power lines. The telecom shelters are environmental
friendly and are placed in remote areas. They hold the backup power which is
important to run the telecommunication functions. Telecom shelters are subject to
over-heating since they are located in deserted areas. Besides the environmental
pressure, the heat produced by the equipment during its operation is enormous.
Hence, the need for controlling the high temperature inside the telecom shelter is
mandatory for the better performance of the system.
2.2.8 SOURCES OF POWER
Telecommunication services are to be provided uninterruptedly round the clock and
throughout the year. For any uninterrupted power supply system, two sources are
required. One is Normal or Main source” and another is “Secondary or standby
source”. (Standby source is “Secondary cells”. Main Source is D.C derived from
commercial source). By name we can define “Normal source is one which supplies
power to the load round the clock” and “secondary source is one which supplies
power to the load only during the absence of power from normal source”. Hence it is
a must to convert AC from commercial mains to D.C.
In communication network, D.C. power is widely used. It has been found that relays
/ discrete components used in these systems could be designed to work on D.C. with
greater degree of sensitivity than an A.C. In telecom systems D.C. Power supply is
only used due to the following reasons: 13

14. • Harmonics of A.C may affect the speech signals.
• Relays/discrete components used in telecom systems are more sensitive to D.C
than A.C
• Transistors and I.Cs etc. being unidirectional devices, the use of D.C has
become necessary.
• Arranging standby source to A.C is difficult compare to D.C for which
secondary cells can be used as S/B source.
• Not hazardous to human life.
However, for certain auxiliary functions like lighting up of busy lamps, alarm
lamps etc. or for running teleprinter, motors, A.C. is also used as a measure of
economy.
Power for the communication system is derived from various sources, of which the
important ones utilized in the department are detailed below:
2.3 COMMERCIALAC POWER SUPPLIES
AC Mains of 220/230 v single phase or 440 V three phase at a frequency of 50
Hz are provided which requires conversion to DC by means of converting
equipments. It is necessary to provide a Standby power supply as an alternative
source of power plant installation feeding power to the communication system as
interruption may occur in AC power supply.Primary cells: The cells which can be
discharged only once are known as primary cells. Such cells do not have the
capability of recharging and hence they cannot be reused. Primary cells were used in
small telephone offices. Sack type Lechlanche, inert cells and dry cells are examples
of Primary cells used in the department.
Prime mover generating sets: A prime mover generating set is comprised of petrol/
kerosene/diesel-fired engine, which is coupled to an alternator. A prime mover set is
generally used as a standby source of power and also as a regular source of power in
areas where commercial power mains are not available.
Secondary cells: Secondary cells can be discharged and charged number of times.
Battery of secondary cells are used to provide reserve power for telecom systems in
the Department. Normally two sets of batteries are used for medium capacity
telephone exchanges.
Static Rectifier units : A static rectifier is an AC to DC conversion set utilizing the
AC power mains as the primary source of power and delivering DC output at the
required voltage and current for charging of secondary cells or for feeding
telecommunication equipments.
Ringers: In electronic exchanges, ringing supply and tones are derived from P.C.B.s
2.4 A.C TO D.C CONVERSIONS
Previously M.G (Motor-Generator) sets were used for A.C to D.C conversion. In this
A.C motor rotates on commercial A.C. supply. To the shaft of this AC motor, D.C.
Generator will be coupled which generates D.C. Now a days, static rectifiers using
static electronic components like metal or diode rectifiers are used.
14

15. A.C
Battery set-A Battery set-B
Figure 2. Float Working
Parallel Battery Float Scheme:
In this scheme two sets of Batteries (24 cells each set) are connected parallel to the output
of the rectifier. The output of the rectifier is 51.5v. Hence floating voltage of each cell is 51.5
divided by 24 = 2.15V. Hence always 90% of battery capacity will be available for emergency
usage. For the operation of the scheme “POWER PLANT” is designed by TRC (Telecom
research Centre)
2.4.1 EARTHING OF ONE POLE OF D.C
Reasons for earthing of one pole of D.C are as follows
• Switching can be single pole.
• Cross talk and other disturbances can be avoided.
• To make the alarm and supervisory system easy.
• Earth return signalling can be used.
Reasons for earthing positive pole of D.C
• In electrolysis positive electrode will be normally corroded. If we keep our lines and
equipment at negative potential, we can minimise troubles from the corrosive
effects.
• Partial Earth faults can be definitely identified if the conductor is negative.
Otherwise fault is liable to seal up owing to oxidation.
DC TO
LOAD
15

16. CHAPTER 3
MAJOR SUBSYSTEMS OF POWER PLANTS
Power plant comprises 3 parts
• Float Rectifier
• Battery Charger
• Switching Cubicle.
Note: Nowadays mostly 2 units p/p are used with maintenance free batteries and all
transmission power plants are 2-unit type only. The latest being P/P of SMPS with VRLA
batteries.
3.1 FLOAT RECTIFIER
Function of Float Rectifier
The function of the Float Rectifier is to receive three phases 440 V AC and to give a
constant 51.5 Volts D.C without AC ripples.
-The steps involved to achieve the function are
a) Step down
Transformer steps down the 3 phases A.C voltage from 440V to around 80 volts.
b) Rectification
Any unidirectional device rectifies the AC to DC.
Here Diodes & SCRs are used for rectification.
c) Filtering
Here multi-stage L.C. Filters are used for filtering the A.C. Ripples.
d) Regulation
i) What is Regulation?-As far as Float Rectifier is concerned,” Regulation is the mechanism
by which the output of a float rectifier is kept constant at 51.5 _+0.5V irrespective of input
voltage variations of ± 12%. Output load variations of 5% to 105% and input frequency
variations of ± 4% or 48-52 Hz”.
ii) Why Regulation is required?
Float rectifier should not only supply power to the load but also takes care of its battery
sets, which are floated across its output.
If the float rectifier output voltage is 51.5v, the cells are floated at 2.15v/cell and hence
they are continuously trickle charged and this compensates losses due to “self discharge
or local action”.
If FR output is 49.2V, the battery set is not trickle charged; hence trickle charging is to be
given once in a fortnight.
16

17. If FR output is <49.0V, the battery starts discharging. If FR output >51.5, the
floating voltage of each cell will be > 2.15V and the battery will be over charged.
Hence regulation is required.
iii) How Regulation is done
1) By “Transduction or saturable reactor or magnetic amplifier” method.
By varying the secondary of the main transformer automatically depending on
output voltage.
By SCR method.
SMPS method.
-Second method was used in olden days but not used nowadays due to mechanical
involvement in regulation. The forth method is discussed in detail chapter 2
-Any of the other three methods,. Controls the portion of the input A.C cycle to
feed to rectifier so that output voltage gets regulated.
1) Transductor Method:
-Normally this principle is used in small exchange power plants.
In this a transductor is placed in series with the rectifier and uses the principle that the
impedance of an iron cored coil can be varied by varying the degree of saturation of the
core.
-By varying the series impedance to rectifier, we can vary the portion of input
cycle that is fed to Rectifier.
2) SCR Method
Figure 3. SCR Method.
-In this the SCR is used as rectifying element.
-Let us recapitulate the working of SCR.
17

18. -SCR can be switched on by applying the positive pulse to the gate. Once if the SCR
is switched on, it will be in ‘ON’ condition as long as the current flowing through
SCR is above a threshold value called “Holding current”.
-In a Float rectifier, across each half cycle one SCR is connected. Hence for 3 phases i.e.
R, Y, B totally 6 SCRs are connected.
-Let an SCR be connected across the positive half cycle of a phase. The total time
period of a half cycle is 10 ms. Within this half cycle triggering pulses can be given at
any time. Assume that triggering pulse is given to SCR at Point’A’ after 4 ms of
starting of the half cycle, the SCR will be on. Even though the triggering pulse is
removed, the SCR will remains on. But the current flowing through SCR depends on
the amplitude applied across its terminals. At 9 ms say at point “B” let the current
flowing through the SCR is just below the holding current. The SCR will be switched
off. That means “switching on” of SCR is in our hands, but “swg off” of SCR is not in
our hands, it is automatic. In this case the portion of half cycle between the points A
and B is rectified.
-The output voltage of the FR depends on both the input AC voltage and output DC load.
-Look at the above table. Whenever the input voltage increases or output load
decreases the output DC voltage increases and vice versa. Hence if we monitor output
voltage, it is sufficient to regulate it.
If the output voltage is increased, then the triggering pulse to the SCR will be delayed
or retarded, point A moves towards B, hence the portion of AC cycle rectified will be
reduced, hence output voltage will be automatically reduced and brought to the
specified value.
-If the output voltage is decreased, then the triggering pulse to the SCR will be
advanced, hence output voltage will be automatically increased and brought to the
specified value. - This is how regulation is achieved by using SCR.
Input AC voltage O/P DC load O/P DC Vol Position of Triggering pulse
Increased Decreased Increases Retarded or delayed
Decreased Increased Decreases Advances.
18

19. Current
Transformer
AC INPUT
10 V 50 c/s
Figure 4. Silicon Controlled Rectifier Type Float Rectifier
3.2 BATTERY CHARGER
Functions of Battery charger
1) To Initial charge a battery set:- For initial charging, the battery charger capacity should
be at least 14% of AH capacity of battery set.
2) To normal charge the battery set at 10 hour rate.
3) To use as Float rectifier during emergency condition by suitable links.
4) To charge the sick cell.( provision exists in some power plants only).
Components of Battery charger
a) 3 phase step-down main transformer with links for mains variation and tap changing
points.
b) Ballast chokes.
c) 3 phase Full wave rectifier.
3.3 SWITCHING CUBICLE
The Switching Cubicle essentially provides for the termination of:
i. The paralleled output from the Float Rectifiers connected with the Exchange load.
ii. The paralleled output from the Battery Chargers.
iii. The positive and negative bus bar risers for the batteries.
iv. The positive and negative bus bar risers for the exchange.
v. Arrangement for manual operation of the knife switches for floating of either all
batteries or any one battery. The knife switches are so arranged and interlocked that
except for the battery on charge, other batteries remain connected across the exchange
during or after any switching operation.
19

20. In addition, the Switching Cubicle provides facilities for:
a) Monitoring the total exchange load current.
b) Monitoring the exchange voltage and individual battery voltages.
c) Supervision and/or alarms for abnormal operating conditions in the associated
cubicles that is the Float Rectifiers and Battery Chargers.
d) Auto-parallel working of Float Rectifiers with sequential switching on and off
of non-priority Float Rectifiers.
SMPS POWER PLANTS
SMPS means Switch Mode Power Supply. This is used for D.C-to-D.C conversion.
This works on the principle of switching regulation. The SMPS system is highly
reliable, efficient, noiseless and compact because the switching is done at very high
rate in the order of several KHz to MHz.
CHAPTER 4
PRINCIPLE OF SWITCHING REGULATOR
Figure 5. Switching Regulator
A pulse train drives the base of ‘switching or pass transistor’. When the voltage
to the Base is high, the transistor saturates, when the voltage is low, the transistor
turns off. Here the Transistor functions as a switch. When the transistor is ON, load
current is drawn through the Transistor and choke L. When the transistor is OFF the
load current is maintained by the Energy stored in the choke L. The current flows
through earth, Diode D, choke, load an Earth. Hence this diode is called ‘Retrieval
Diode’.
Duty cycle of the Transistor = On Time = D
On Time + Off Time
(One cycle time)
The output voltage = Input voltage x D
20

21. For example
If I/P voltage is 200 volts and D=0.25
O/P voltage = 200 x 0.25 = 50V.
Regulation is achieved by modifying the Duty cycle. Duty cycle depends on onetime of
transistor, which in turn depends on the width of the pulse applied to the base of the
Transistor, which is controlled by ‘Pulse width modulation’ by regulator circuit
Figure 6. Principle of Regulation
The relaxation oscillator produces a square wave. The square wave is integrated to
get a tri angular wave, which drives the non-inverting input of a triangular to pulse
converter. The Pulse train out of this circuit then drives the Pass Transistor. The output is
sampled by a Voltage divider and fed to a comparator. The feedback voltage is compared
with a reference Voltage. The output of the comparator then drives the input of the
triangular to pulse converter. If the output voltage tries to increase the comparator
produces a higher output voltage, which raises the reference voltage of the triangular- to
pulse converter.
This makes the pulse that drives the base of the switching transistor narrower. That
means duty cycle is reduced. Since the duty cycle is lower the output becomes less, which
tries to cancel almost all the original increase in output voltage. Conversely, if the
regulated output voltage tries to decrease, the output of the comparator decreases the
reference voltage of the triangular -to pulse converter. This makes the pulse wider and the
transistor conducts for larger time and more voltage comes out of the L.C. filter. This
cancels out the original decrease in output voltage.
21

22. Figure 7. Duty Cycle pattern
For maximum efficiency the duty cycle should be less than 0.5. As long as the
triangular voltage exceeds the reference voltage, the output is high. Since Vref is
adjustable, we can vary the width of the output pulse and hence the duty cycle. Switching
regulators are more efficient than conventional regulators as the power loss in the
switching element is reduced to minimum as it conducts only for a fraction of a cycle.
Now a days SMPS technology is extended to power plants also. Power plants upto 2000A
capacity has been developed using SMPS principle.
4.1 FUNCTIONAL DESCRIPTION OF RECTIFIER
The SMPS 50V-5600W rectifier is a state-of-the-art switch-mode power conversion
equipment. The unit consists of two cascaded power converters performing power factor
correction and DC/DC conversion. The power stages are synchronized and working with
constant switching frequency of 100 kHz.
The rectified AC mains voltage is processed first in the power factor corrector
circuit which is based on a boost topology. The boost converter has the inherent
advantage of continuous input current waveform which relaxes the input filter
requirements. The performance of the basic boost cell is improved by a proprietary
snubber circuit which reduces the switching losses of the power semiconductors due to
non-zero switching times. Furthermore, the snubber circuit also decreases the
electromagnetic interference (EMI) generated primarily during the turn-off process of the
boost diode. The output of the boost converter is a stabilized 400V DC voltage.
Further conversion of the stabilized high voltage output of the power factor
corrector circuit is necessary to generate the isolated low voltage output and to provide
the required protection functions for telecommunication application. These tasks are
achieved in the DC/DC converter circuit which is based on a full-bridge topology. The
full-bridge circuit is operated by phase-shift pulse with modulation with current mode
control. This control method provides zero voltage switching condition for all primary
side power semiconductors effectively reducing switching losses and electromagnetic
interference. An advanced solution reduces the stresses of the output rectifier
22

23. diodes.Proper operation of the power converters is managed by individual controller
circuits and supervised by the housekeeping electronics. Remote commanding and
monitoring of the modules are possible through a power system controller housed in the
system.
4.2 FUNCTIONAL DESCRIPTION OF POWER SYSTEM CONTROLLER
Power system controller is designed to control the modes of operation of rectifiers,
acknowledge and displays the status of rectifiers and system and controls parameters of
rectifiers.
The controller accepts signal from individual rectifiers through 8 pin telephone jack and
controls the operation of each individual rectifiers.
The mode of operation of rectifier modules depends on the coded signal M1 and M2
from the controller. Depending on the state of batteries, the ATM circuit either gives a signal
for float or charge. These signals are encoded by an encoder to obtain suitable coded signals
M1 and M2.
Depending upon the mode of operation of Rectifier modules, they acknowledge coded
signals S1 and S2. These signals are decoded to display whether the modules are in auto
float/charge or fail condition.
The total battery current can be suitably programmed to limit the current supplied from
the modules through current programming pin in modules.
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24. 4.3 TROUBLE SHOOTING IN POWER PLANT
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26. Conclusion
This report has given you the sufficient knowledge of different telecom infrastructure
components and the necessity of power supply. It also makes you aware of different power
supplies & their sources, use of DC power in telecommunication, earthling of one DC pole,
and various sources from where DC power is derived for operation of equipments. The
working of conventional and SMPS power plant and troubleshooting in case of problems is
also explained.
Engineering student will have to serve in the public and private sector industries and
workshop based training and teaching in classroom has its own limitation. The lack of
exposure to real life, material express and functioning of industrial organization is the
measure hindrance in the student employment.
In the open economy era of fast modernization and tough competition, technical industries
should procedure pass out as near to job function as possible.
Practical training is one of the major steps in this direction. I did my training from BSNL,
Fatehpur which is one of the best known communication service provider companies of
India.
The training helps me in gaining in depth knowledge of the working of telephone exchange,
various technologies of BSNL power plant. In the end, I hereby conclude that I have
successfully completed my industrial training on the above topics.
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27. 1.12 REFERENCES AND SUGGESTED FURTHER READINGS
• www.tec.gov.in
• www.tnd.bsnl.co.in
• intranet.bsnl.co.in/digital library
• www.wikipedia.org/
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