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SILVER

CERTIFICATION COURSETELECOM SUPPORT INFRASTRUCTURE

OVERVIEW OF TELECOMINFRASTRUCTURE & POWER PLANT

Version 2 June 2014


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Telecom Support I nf rastructure (TSI) Overview of Telecom Infrastructure & Power Plant

EETP/ BSNL Silver Certification Course/Ver.02/June2014 Page 1 of 23

For Restricted Circulation

1 OVERVIEW OF TELECOM INFRASTRUCTURE &

POWER PLANT

INDEX

1.1 Introduction ................................................................................ 2

1.2 Objective ..................................................................................... 3

1.3 Telecom Infrastructure ............................................................. 4

1.4 Components Of Telecom Support Infrastructure .................. 5

1.5 Sources Of Power ....................................................................... 8

1.6 Commercial Ac Power Supplies................................................ 9

1.7 A.C To D.C Conversions ......................................................... 10

1.8 Earthing Of One Pole Of D.C ................................................. 10

1.9 Major Subsystems Of Power Plants ....................................... 11

1.10 SMPS Power Plants .................................................................. 15

1.11 Summary ................................................................................... 20

1.12 References And Suggested Further Readings ....................... 21

1.13 Self Assesment Questions ........................................................ 21


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Telecom Support Inf rastructure (TSI ) Overview of Telecom Infrastructure & Power Plant



1 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

1.13 SELF ASSESMENT QUESTIONS

1.1 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 operators business.

Telecommunication systems require electrical energy for transmission of signalsenergization of subscribers 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 subscribers 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.


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The power system is intended primarily to provide uninterrupted DC power to

Telecom equipments and current for charging the batteries in the presence of AC Mains.

The system works from commercial AC mains which is rectified and regulated to 50V

DC and is fed to the equipment (exchange).

1.2 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


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1.3 TELECOM INFRASTRUCTURE

A typical telecom network infrastructure can be categorized into three distinct categories

namely Passive Infra, Active Infra and Backhaul.

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:

1.3.1

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.

Telecom NetworkInfrastructure

PassiveInfrastructure

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 partof the networkconsists of the

intermediate linksbetween the core ofthe network and the

various sub-networks

Active

Infrastructure

Key Components

-Spectrum

- Base tower station

- Microwave radio

- Switches

- Antennas

- Transceivers


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1.4COMPONENTS OF TELECOM SUPPORT INFRASTRUCTURE

Figure 1.Telecom support infrastructure

1.4.1SMPS (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.

Life of Power Plant:


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Static P/P : 15 years

SMPS P/P: 15 years

1.4.2BATTERY

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

1.4.3INVERTER

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.

1.4.4

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.

1.4.5EARTHING:

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.


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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.

1.4.6

AIR- CONDITIONING SYSTEM: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.

1.4.7FIRE 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 combustiblematerials, 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 measuresin 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


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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:

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:

1.6 COMMERCIAL AC 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.


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EETP/ BSNL Silver Certification Course/Ver.02/June2014 Page 10 of

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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

1.7 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.

AC DC to LOAD

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 eachcell 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)

1.8 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.


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1.9 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.

1.9.1FLOAT RECTIFIER

Function of Float Rectifier

The function of the Float Rectifier is to receive three phases 440 V AC and to give aconstant 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.


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If FR output is 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.

2) By varying the secondary of the main transformer automatically depending on output

voltage.

3) By SCR method.

4) 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.


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-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 PointA 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.


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Figure 4.Silicon Controlled Rectifier Type Float Rectifier

1.9.2 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.

1.9.3 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

Current

Transformer

AC INPUT

10 V 50 c/s


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except for the battery on charge, other batteries remain connected across the exchange

during or after any switching operation.

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.

1.10SMPS 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.

1.10.1 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)


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The output voltage = Input voltage x D

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.


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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.

1.10.2

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


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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 diodes.

Proper operation of the power converters is managed by individual controller

circuits and supervised by the housekeeping electronics. Remote commanding andmonitoring of the modules are possible through a power system controller housed in the

system.

1.10.3 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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1.10.4 TROUBLE SHOOTING IN POWER PLANT


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1.11

SUMMARY

This unit 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.


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1.12REFERENCES AND SUGGESTED FURTHER READINGS

www.tec.gov.in

www.tnd.bsnl.co.in

intranet.bsnl.co.in/digital library

www.wikipedia.org/

1.13 SELF ASSESMENT QUESTIONS

1. The function of the Float Rectifier is________________

2. _______________of A.C may affect the speech signals.

3. Transistors and I.Cs etc are ____________ devices

4. In Float rectifier _____________are used for rectification

5. In electrolysis __________electrode will be normally corroded

6. By earthing one pole, ___________ can be avoided

7. Power plants are required to feed currents for _________

8. In Filtering______________ are used

9. The out put voltage of switching regulator in SMPS is I/P voltage+ D (T/F)

10. The mode of operation of rectifier modules depends on the coded signal from the

controller ( T /F )

11. SMPS works in the principle of _____ regulation ( I/P voltage/Switching/O/P

voltage)

12. For maximum efficiency the duty cycle should be less than_______


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Answers

1. To receive three phases 440 V AC and to give a constant 51.5 Volts D.C without AC

ripples.2 Harmonics

3. Uni directional

4. Diodes & SCRs

5. Positive electrode

6 Cross talk

7. The subscribers transmitters, signaling and control and operation of exchanges and

switches

8 Multi-stage L.C. Filters

9 False

10.True

11 Switching

12. 0.5

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