ALL INDIA RADIO, REWA

20092009

PRESENTED BY:

A L L I N D I A R A D I O , 6 , C I V I L L I N E S R E W A ( M . P . )

This is to certify that student of Thakral Collage of Technology, Bhopal, 4thyear (VIIthSem.) in Electronics & Communication engineering has successfully completed the industrial training at ALL INDIA RADIO, REWA for 4 weeks (from 01-07-2009 to 30-07-2009).

This training report is hereby approved for submission towards partial fulfillment for the degree in bachelor of engineering from Thakral Collage of Technology, BHOPAL.

Under the Guidance of: - Under the Supervision of:-

1) S.C. Dubey (S.E.A) ( V.P. Yadav)

2) Ajay Shrivastava (S.E.A) (Station Engineer)

3) O.P. Vishwakarma(E.A.) AIR REWA (M.P.)

4) V.K.Mishra(Tech.)

“Knowledge is power”

For an engineering student, this power of knowledge is unattainable unless an element of practical observation and practical performance is not added. This basic stream of engineering – Electronics & Communication Engineering calls not only for an exhaustive knowledge of the basic theory of the subject, but an equally important practical exposure of the same.

First & Foremost we would like to express our gratitude to Shri V.P.Yadav (Station Engineer, All India Radio, Rewa) for their valuable and precious support.

We pay regards from bottom of our heart to Shri S.C. Dubey Sir (S.E.A), Shri Ajay Shrivastava Sir (S.E.A), Shri O.P.Vishwakarma Sir (E.A) and Shri V.K. Mishra Sir (Tech) for their constant encouragement and unlimited acumen.

Last but not the least we want to thank all the co- trainees of the different colleges for their valuable guidance and help.

CONTENTS

1. Overview of AIR

2. AIR Rewa

3. Control Room

4. Receiving Unit

5. Stereo Console

6. Studios

7. Air Conditioning Plant

8. Transmitter Section

9. Transmitter

10. Power Supply System

OVERVIEW OF ALL INDIA RADIO

AIR headquarters, Akashvani Bhavan, in Delhi, India

All India Radio (abbreviated as AIR), officially known as Akashvani is the radio broadcaster of India and a division of Prasar Bharati (Broadcasting Corporation of India), an autonomous corporation of the Ministry of Information and Broadcasting, Government of India. Established in 1936, today, it is the sister service of Prasar Bharati's Doordarshan, the national television broadcaster.

The word Akashavani was coined by Professor Dr. M.V. Gopalaswamy for his radio station in Mysore during 1936.

All India Radio is one of the largest radio networks in the world. The headquarters is at the Akashwani Bhavan, New Delhi. Akashwani Bhavan houses the drama section, the FM section and the National service. The Doordarshan Kendra (Delhi) is also located on the 6th floor of Akashvani Bhavan.

During his regular broadcasts from the Azad Hind Radio, Subhas Chandra Bose used to refer to the pre-independence AIR as Anti Indian Radio.

AIR is a national service planned, developed and operated by the Ministry of Information

& Broadcasting under the Government of India (Now Prasar Bharti, a Corporate). The Radio

Club of Bombay broadcast the first radio programmer in India in June 1923. The setting up of a

Broadcasting Service that began broadcasting in India in July 1927 on an experimental basis at

Bombay and Calcutta simultaneously under an agreement between Government of India and a

private company called the Indian Broadcasting Company Ltd followed it. The operations of All

India Radio began formally in 1936, as a government organization, with clear objectives to

inform, educate and entertain the masses.

When India become independent, the AIR network had only six Stations located at Delhi,

Bombay, Calcutta, Madras, Lucknow and Trichurapalli with a total compliment of 18

transmitters – six on the medium wave and the remaining on short wave. Radio listening on

medium wave was confirmed to urban limits of these cities. As against a mere 2,75,000 receiving

sets at the time of Independence, now there are about 111 million estimated radio sets in about

105 million household in the country.

AIR today has a network of 214 broadcasting centers with 143 medium frequency (MW),

54 high frequency (SW), and 139 FM transmitters. The coverage is 91.37% of the area, serving

99.13% of the people in the largest democracy of the world. AIR covers 24 Languages and 146

dialects in home services. In External services, it covers 27 languages; 17 national and 10 foreign

languages.

Services

AIR has many different services each catering to different regions/languages across India. One of the most famous services of the AIR is the Vividh Bharati Seva (roughly translating to "Multi-Indian service"). Vividh Bharati celebrated its Golden Jubilee on 3 October 2007. Vividh Bharati has the only comprehensive database of songs from the so termed "Golden Era" of Hindi film music (roughly from 1940s to 1980s). This service is the most commercial of all and is popular in Mumbai and other cities of India. This service offers a wide range of programmes including news, film music, comedy shows, etc. The Vividh Bharti service operates on different MW band frequencies for each city as shown below.

Some programs broadcast on the Vividh Bharti:

Hawa-mahal - Skit (Radio Play) based on some novels/plays. Santogen ki mehfil - Jokes & humour.

BROADCASTING NETWORK:

All India Radio’s network comprised six radio stations in 1947. All India Radio has at

present (as of March 1996) 185 radio stations which includes 177 full – fledged stations, four

relay centers, one auxiliary center and three exclusive Vividh Bharti commercial centers. During

1995-96, eight radio stations at Mussoorie, Rourkela, Puri, Joranada, Jowai, Daman,

Mokokchung and Diphu have been added to the entire network. There are 146 MW transmitters,

50 SW transmitters and 89 FM transmitters. AIR, at present, provides radio coverage to a

population of 97.30 % spread over 90 % area of the country. The concept of local radio stations was

introduced during the Sixth Plan when six stations were planned as pilot projects. The number of local

radio stations in India today is 72.

The FM service of AIR, Delhi was Launched 24 hours daily, on 14 February 1995, followed by

AIR Mumbai. Calcutta, and Chennai.

.

AIR REWA

AIR REWA is having a glorious 29 years history. The foundation stone of the 4 acres campus

was laid on 10th March 1973 by the then Home Minister of India Shri Uma Shankar Dikshit under the

efficient effort of Chief Minister of M.P. Shri Prakash Chandra Sethi.

Moreover, AIR REWA was Formally Inaugurated on 17th October 1977 by the then Information

& Broadcasting State Minister Shri Jagbir Singh under the benign presence of Chief minister of M.P.

Shri Kailash Joshi.

AIR REWA is centrally located in REWA city in the posh area of Civil Lines and it is easily

accessible from all parts of of the city. The Transmitting and Broadcasting station of AIR REWA is

situated at Mehsaw which is 16 kms away from the city on NH- 75. Station is having 37 acres campus.

CONTROL ROOM

BLOCK DIAGRAM OF STUDIO & CONTROL ROOM

Control Room as the name suggests is the centrally located room from where all the operation regarding

transmission are performed. The different modules of stereo console of control room are as follows:-

1. REM-1,2

2. TALK

3. MUSIC

4. DRAMA

5. PLAYBACK

6. MASTER 1,2

7. MONITERIG

8. POWER SUPPLY

RECEIEVEING UNIT

It consists three sections

1. S-band receiver (analog only) 2. C-Band receiver(analog/digital)

3. Studio transmitter link (S.T.L.)(transmitter)

S-Band Receiver:-

C- Band Receiver :-

Audio

Demodulator

Frequency Translator

(active)

Synthesized

Translator

Front End Converter

Frequency Translator (pasive)

AudioO/P

ANALOG RECEVEIER

DIVIDER

LNBC

C- BAND DIGITAL RECEIEVER-

1DIGITAL RECEIEVER-

2

CH # 2

CH # 1 o

STEREO CONSOLE

Multifunction stereo console is a feature enriched mixing console ideally suited for broadcasting

applications of All India radio.The specifications, highly attractive for Sound Engineers, are built into a

modular state of art mechanical design and reliable assembly which make it a best bargain for different

areas of Audio Recording and Processing.

The stereo console has solid state switching throughout the system on the analog domain and uses

programmable logic array (PAL) in digital domain for low component count and high reliability. Use of

conductive plastic faders and potentiometers along with high quality PCB mount rotary switches enable

noise free operation and long operating life. Remote source selector with input channel fader extension

and bi-colors LEDs for indication of selected channel are other features of the design. The state-of-the-art

mechanical design enables easy interconnection, servicing and maintenance.

The multifunction stereo console designed by the ER&DC, Thiruvananthapuram & manufactured by

Webel Mediatronics limited meets all stringent technical performance requirements of AIR. The

user manual for Multifunction Stereo Console contains information for installation, operation and

maintenance of the unit. The guidelines given in this manual should be followed strictly during

installation and maintenance. The servicing/maintenance should be carried out by technically competent

persons having through knowledge of the circuit.

DUMB TERMINAL

MONITORING SWITCH SELECTOR

MONITOR

AMPLIFIER

STUDIO

MUSIC STUDIO

This is the studio in which recording of programmes (musical) is done. The studio has an announcer room

and separate studio. This is designed in a manner that the wall are covered with asbestos sheet to cut

down the reflected sounds. Moreover the outlet of air is given on the bottom corner of room instead of top

as in other studios. There are 04 Nos. of microphone connection which can be extended to 06 in

emergency situations. The output of any type of microphone is –70db. This output is given at the input of

console.

RECORDING CONSOLE

The console of music studios is ER & DC stereo recording console RS 12 A manufactured by WML

(Webel Mediatronics Ltd.). These are different modules on the console consisting 12 channels. These

are:-

1. Two re-source Selector.

2. One module for announcer dealing with artist.

3. 5 Artist modules (Mono, mic/ line).

4. 4 Stereo H/L I/P module (2 for CD, & 2 for CTR).

5. Two Master modules (stereo o/p).

6. One monitor module.

7. One Talk Back module.

8. One Module for Power Supply. (+5V, -18V, +18V, +48V).

Other then this console has connection with CTR(console tape recorder), Amplifier,

distribution amplifier, Resource selector etc.

DUBBING STUDIO

This is the studio where the editing, mixing and formation of final programe is done. This

consists of a control console (Dub/edit),three CTRs and also have one more CTR for message

relay from other stations. Among the three CTRs one is main that is also known as mixer CTR

and the two other are used to play different programmes which are to be mixed.

The console tape recorder for message relay can also be used to record the talks which are

recorded in a tape farther from the station.

Dubbing studio also consists of a tape-block eraser which is used to erase the content of cassette

within few seconds. This is done by removing the granuals formed in different tracks and sectors

of the tape by creating movable contact with tape-block eraser.

CTR (CONSOLE TAPE RECORDER)

Console tape recorder is a recording/Play back unit which have three heads:-

1. Erase Head2. Recording Head3. Playback Head

In addition to this there is a capstan motor which starts as soon as power supply is given if any speed selection is “ON”. This is synchronized in operation with other two motors one of which is Supply Motor and other is Take-Up motor. Both these motors have forward/reverse direction rotation.

In addition to above there is no. of electronic P.C.B., which is as under:

1. Re-Produce Amplifier2. Record amplifier3. Oscillator4. Stabilizer5. T.T.C. (Tape Transport Control P.C.B.)6. Counter PCB7. Monitoring amplifier8. Bias Stabilizer9. Sensor board (L&R)10.Spooling Motor Control (Take-up & Supply)11.Capstan Speed Control12.Counter PCB

AIR CONDITIONING UNIT

AIR Rewa consists of an ac plant known as central unit plant. This is so called because the

cooling, filtering and the inlet and outlet are combined in a single unit.

PRINCIPAL OF WORKING

This works on the principal that hot air rises up and cold air settles down. The outlet

through which cold air comes are placed in every room (may be two or three) and there is also an

inlet in every room goes back for being compressed and condensed into the cold air and

circulation of air constitutes a cycle form. One more advantage is that the humming sound of air

pressure which is there in window ac is completely eliminated since a small sound can also be

amplified. The capacity of one ac plant installed is 27 tones and there are two in number.

DESCRIPTION

The cooling through central plant unit is performed in a cycle process. The outlet through

which cold air spreads out and settles down being denser and the hot air rises up and goes into an

inlet from where it is blowed upon with pressure to filters to remove dust particals other than air.

The gas used in this is FREON (F-22) which is very expensive.

The gas is compressed heavily in a compressor in which the air continuously comes from suction

valve and going out from exhaust valve simultaneously keeping the volume constant inside the

compressor and also pressure is maintained to a safe value. A HP/LP cutout is also provided to

meet unnecessary high/low pressure conditions.

From the compressor the gas is passed through a long zigzag path in the presence of cold

water inside a condenser. The condenser also have a inlet for cold water and outlet for hot water.

Warm water extracts the heat of the gas and through motor is sent back to the overhead tank. It is

connected through pipe to the inlet of condenser. During the path it goes pass through fins which

holds up water for some time and also a exhaust fan is there which sucks the heat of water and it

becomes colder henceforth. From the condenser the gas goes into the expansion valve from

where it is sprayed where it meets the warm air through filters and thus the mixing results into

cold air. The most important property of FREON is that it is heated and cooled very easily.

FUNCTIONAL DIAGRAM OF AC PLANT:

STUDIO TRANSMITTER LINK (S.T.L.)(TRANSMITTING END)

Studio Transmitter link (Transmitting end).

Studio Transmitter link (Receiving end).

TRANSMITTER SECTION

This wing is situated 15 km away from Rewa in the outskirts of mahasaon village. Here the transmitted signal from control room is receieved via STL. The receieved signal is then transmitted through HARISS DX-20 MW transmitter through 128.25m high MAST antenna

Here for receiving the centre has three devices which are used for transmitting the control room signal to the transmitter section. These are as follows

1. STL2. DR 10003. RT 43

In all of these STL is mainly used whose block diag is as follows

HARISS DX-20 TRANSMITTER

This transmitter can be viewed as below:-

FRONT VIEW OF HARISS DX-20 TRANSMITTER

BLOCK DIAGRAM

BUFFER PRE RF

AMPLIFIER DRIVER AMPLIFIER

DUMMY LOAD

In radio this device is also known as a dummy antenna or a radio frequency termination. It is a device used in place of an antenna to aid in testing a radio transmitter. It is substituted for the antenna while adjusting the transmitter, so that the transmitter does not interfere with other radio transmitters during the adjustments. If a transmitter is tested without a load, such as an antenna or a dummy load, the transmitter could be damaged. Also, if a transmitter is adjusted without a load, it will operate differently than with the load, and the adjustments may be incorrect.

The dummy load ordinarily should be a pure resistance; the amount of resistance should be the same as the impedance of the antenna or transmission line that is used with the transmitter (usually 50 Ω or 75 Ω). The radio energy that is absorbed by the dummy load is converted to heat. A dummy load must be chosen or designed to tolerate the amount of power that can be delivered by the transmitter.

The ideal dummy load provides a standing wave ratio (SWR) of 1:1 at the given impedance.

Pictorial view of dummy load

OSSCILATOR

AUDIO INPUT

A/D CONVERTER

MODULATIONENCODER

DC REGULATOR

BAND PASS FILTER

OUTPUT N/W

OUTPUT MONITOR

AUTOMATIC VOLTAGE REGULATOR (AVR)

This is an older type of regulator used in the 1920s that uses the principle of a fixed-position field coil and a second field coil that can be rotated on an axis in parallel with the fixed coil.

When the movable coil is positioned perpendicular to the fixed coil, the

magnetic forces acting on the movable coil balance each other out and voltage output is unchanged. Rotating the coil in one direction or the other away from the center position will increase or decrease voltage in the secondary movable coil.

This type of regulator can be automated via a servo control mechanism to advance the movable coil position in order to provide voltage increase or decrease. A braking mechanism or high ratio gearing is used to hold the rotating coil in place against the powerful magnetic forces acting on the moving coil.

Electronic symbol for Voltage regulator

Moving coil Voltage Regulator

AC VOLTAGE STABILIZERS

A voltage stabilizer is a type of household mains regulator which uses a continuously variable autotransformer to maintain an AC output that is as close to the standard or normal mains voltage as possible, under conditions of fluctuation. It uses a servomechanism (or negative feedback) to control the position of the tap (or wiper) of the autotransformer, usually with a motor. An increase in the mains voltage causes the output to increase, which in turn causes the tap (or wiper) to move in the direction that reduces the output towards the nominal voltage.

An alternative method is the use of a type of saturating transformer called a Ferro resonant transformer or constant-voltage transformer. The Ferro resonant approach is attractive due to its lack of active components, relying on the square loop saturation characteristics of the tank circuit to absorb variations in average input voltage. Older designs of ferroresonant transformers had an output with high harmonic content, leading to a distorted output waveform. Modern devices are used to construct a perfect sine wave. The Ferro resonant action is a flux limiter rather than a voltage regulator, but with a fixed supply frequency it can maintain an almost constant average output voltage even as the input voltage varies widely.

It accepts 100% single-phase switch-mode power supply loading without any requirement for de rating, including all neutral components. Input current distortion remains less than 8% THD even when supplying nonlinear loads with more than 100% current THD.

FEEDER LINE

The Feeder line in a radio transmission, reception or transceiver system is the physical cabling that carries the RF signal to and/or from the antenna. It is also called a transmission line. When operating properly, and under ideal conditions, the feed line should successfully carry all of the RF energy without any signal loss, and without radiating any energy or absorbing any energy. There are three common types of feed lines in use in modern wireless systems: the coaxial type, the twin-lead, and, at frequencies above 1 GHz, a waveguide is used.

The feed line is a very important part of the antenna system especially in certain conditions such as high frequency, weak signals or both, as they often go hand in hand. Feed line cabling has a

specific impedance that must be matched with the transmitter/receiver/transceiver and the antenna to prevent signal loss. This matching is accomplished with an antenna tuning unit.

Picture how feed lines are extended

ANTENNA TUNER

An antenna tuner, trans match, or antenna tuning unit (ATU) matches a transceiver with a

fixed impedance (typically 50 ohms for modern transceivers) to a load (feed line and antenna)

impedance which is unknown, complex or otherwise does not match. This mismatch is usually

caused when using a non-resonant antenna (one that is not the correct electrical length as

compared to the wavelength of the signal). An ATU allows the use of one antenna for a broad

range of frequencies. A tuned antenna is never as good as a naturally resonant antenna due to

additional induced losses on the feed line due to the SWR (multiple reflections), and losses in the

ATU itself.

Strictly speaking the 'ATU' is only an antenna matching unit, as it is unable to

change the resonance frequency of the aerial.

MAST RADIATOR

As in any costly project, the planning of a high power transmitter site requires great care. This begins with the location. A minimum distance, which depends on the transmitter frequency, transmitter power, and the design of the transmitting antennas, is required to protect people from the radio frequency energy. Antenna towers are often very tall and therefore flight paths must be evaluated. Sufficient electric power must be available for high power

transmitters. Transmitters for long and medium wave require good grounding and soil of high electrical conductivity. Locations at the sea or in river valleys are ideal, but the flood danger must be considered. Transmitters for UHF are best on high mountains to improve the range (see radio propagation). The antenna pattern must be considered because it is costly to change the pattern of a long-wave or medium-wave antenna.

Antenna tower

Transmitting antennas for long and medium wave are usually implemented as a mast radiator. Similar antennas with smaller dimensions are used also for short wave transmitters, if these send in the round spray enterprise. For arranging radiation at free standing steel towers fastened planar arrays are used. Radio towers for UHF and TV transmitter can be implemented in principle as grounded constructions. Towers may be steel lattice masts or reinforced concrete towers with antennas mounted at the top. Some transmitting towers for UHF have high-altitude operating rooms and/or facilities such as restaurants and observation platforms, which are accessible by elevator. Such towers are usually called TV tower. For microwaves one uses frequently parabolic antennas. These can be set up for applications of radio relay links on transmitting towers for FM to special platforms. For the program passing on of television satellites and the funkkontakt to space vehicles large parabolic antennas with diameters of 3 to 100 meters are necessary. These plants, which can be used if necessary also as radio telescope, are established on free standing constructions, whereby there are also numerous special designs, like the radio telescope in Arecibo.

Just as important as the planning of the construction and location of the transmitter is how its output fits in with existing transmissions. Two transmitters cannot broadcast on the same frequency in the same area as this would cause co-channel interference. For a good example of how the channel planners have dovetailed different transmitters' outputs see Crystal Palace UHF TV channel allocations. This reference also provides a good example of a grouped transmitter, in this case an A group. That is, all of its output is within the bottom third of the UK UHF television broadcast band. The other two groups (B and C/D) utilise the middle and top third of the band, see graph. By replicating this grouping across the country (using different groups for adjacent transmitters), co-channel interference can be minimised, and in addition, those in marginal reception areas can use more efficient grouped receiving antennas. Unfortunately, in the UK, this carefully planned system has had to be compromised with the advent of digital broadcasting which (during the changeover period at least) requires yet more channel space, and consequently the additional digital broadcast channels cannot always be fitted within the transmitter's existing group. Thus many UK transmitters have become "wideband" with the consequent need for replacement of receiving antennas (see external links). Once the Digital Switch Over (DSO) occurs the plan is that most transmitters will revert to their original groups, source Ofcom July 2007 .

Further complication arises when adjacent transmitters have to transmit on the same frequency and under these circumstances the broadcast radiation patterns are attenuated in the relevant direction(s). A good example of this is in the United Kingdom, where the Waltham transmitting station broadcasts at high power on the same frequencies as the Sandy Heath transmitting station's high power transmissions, with the two being only 50 miles apart. Thus Waltham's

antenna array does not broadcast these two channels in the direction of Sandy Heath and vice versa.

Where a particular service needs to have wide coverage, this is usually achieved by using multiple transmitters at different locations. Usually, these transmitters will operate at different frequencies to avoid interference where coverage overlaps. Examples include national broadcasting networks and cellular networks. In the latter, frequency switching is automatically done by the receiver as necessary, in the former, manual retuning is more common (though the Radio Data System is an example of automatic frequency switching in broadcast networks). Another system for extending coverage using multiple transmitters is quasi-synchronous transmission, but this is rarely used nowadays.

Main and relay (repeater) transmitters

Transmitting stations are usually either classified as main stations or relay stations (also known as repeaters or translators).

Main stations are defined as those that generate their own modulated output signal from a baseband (un-modulated) input. Usually main stations operate at high power and cover large areas

Relay stations (translators) take an already modulated input signal, usually by direct reception of a parent station off the air, and simply rebroadcast it on another frequency. Usually relay stations operate at medium or low power, and are used to fill in pockets of poor reception within, or at the fringe of, the service area of a parent main station.

Note that a main station may also take its input signal directly off-air from another station, however this signal would be fully demodulated to baseband first, processed, and then demodulated for transmission.

AUSTIN TRANSFORMER

An Austin transformer is a special type of an Isolation transformer used for feeding the air-traffic obstacle lamps and other devices on a mast radiator antenna insulated from ground. As such antennas have high voltage to ground, (up to 300 kV), feeding the lamps directly is impossible. One method of feeding them, is an isolation transformer, the so called Austin transformer. It consists of two ring-like windings. The secondary circuit has a large air space between the winding and the magnetic core, to provide isolation from high voltage and low inter-winding coupling capacitance.

WBZ AUSTIN TRANSFORMER

WLS AUSTIN TRANSFORMER

POWER SUPPLY SYSTEM

A systematic planning of power supply requirement of any particular broadcasting station of

AIR/DD is carried out in the beginning. Loads to be added in future, as per extension program

of station are also taken into consideration. Major stations of AIR/DD has the following types of

loads:

1) Equipment loads

2) Air conditioning and associated loads

3) Lighting and fans load

4) Workshop load

5) Loads relating to mast light and aerial hut

6) Future provision

The total demand of the station is worked out and then decision is taken whether H/T or L/T

supply would meet the requirement. H.T supply is required in case the maximum demand of

station exceeds 30 kVA.

AC POWER SUPPLY

The electric power at 11 kV is received in the H.T. substation through underground feeders from

the power supply of the P/S authority. The H.T substation is located nearby the

studio/transmitter and generally houses metering facilities for reading maximum demand, power

factor and the energy consumed. It also houses the H.T OCB. Separate feeder is provided

from the P/S station to our centres whose total demand exceeds 50 kV. Standby feeder from

the same or other P/S station is invariably provided. A typical P/S schematic is shown below.

STUDIO SUB STATION

TRANSMITTER

33KV SUB STATION TRANSMITTER

The incoming power through metering system and H.T OCB is fed to the step-down transformer where voltage is reduced from 11 kV to 400 V, 3 phase 4-wire system. This supply is now called L.T supply. The step down transformer is housed normally in a separate enclosure at a suitable distance from the bus bar and switchgear room. In some stations two nos. of L.T transformers are provided (fig.2). One is used with normal and the other one with the standby feeder. In such cases Isolators, ACB, mechanical and electrical interlocks are provided to prevent earthing and back feed. Power from the secondary of the transformer is fed to the bus bar (sometimes through L.T OCB) for distribution through switchgears. This bus bar is called the main bus bar. The main bus bar feeds power to the auxiliary bus bar, located in the vicinity of the studio/transmitters.

Arrangement for Preventing back feed in case of feeders

Normally one auxiliary bus bar feeds supply to loads like A/C plants and lighting. The other one is used to meet the requirement of the main studio/transmitting equipments. A tie line if provided between two auxiliary bus bars proves beneficial in case of trouble with one of the feeders.

From the bus bars, the power is distributed to the loads or sub-distributors (for redistribution) through medium voltage switchers. The main switchgear may comprise full circuit breaker units; oils or air breaks type. Basically a hand operated (Iron clad) switch gear is required for each separate circuit system and it should be capable of interrupting load current at some degree of over current which is normally three times the normal rating of the switch at a power factor of 0.8.

The switchgears can be mounted above or below the bus bar. This type of switch is of double break type and with the switch, open fuse links are completely isolated so that the same can be removed and replaced in safety. The fixed contacts are completely enclosed within the insulating shrouds. A protective gear (interlock) ensures that switch must be opened before the cover can be opened and the switch cannot be closed with the cover opened.

Load distribution - Essential and Non-essential loads

Load should be distributed in such a manner that each phase shares equal load as far as possible. Total load should be divided into two groups (I) the essential load and (ii) Non-essential load. The essential loads (equipment load) are those minimum loads, which are necessary to maintain the continuity of the service. Continuous P/S to these loads should be ensured by connecting them to the standby generator in case of power failure. Therefore, P/S to these loads are required to be arranged accordingly to meet this requirement.

Earthing

Earthing is very important in power supply system. The purpose of earthing is to ensure safety of the equipment from damage due to earth fault and prevent operating personnel from getting electric shock. Earthing means making a connection to the general mass of the earth. Earthing associated with the current carrying equipment (switchgears, transformer body etc) is normally essential for the safety of the system and is generally known as system earthing. All L.T/H.T. switchgears are provided with double earth. H.T/L.T transformer neutral is normally connected to earth for safety of transformer. Earthing of non-current carrying metal work and conductor is essential to the safety of human life and is known as equipment earth. System and equipment earth’s are provided separately. Water may be required to be put into earth pits at regular intervals in case the land is very dry.

AVR

Automatic voltage regulator is a very vital part of the P/S system. Power to most of the loads in studio/transmitter equipment is supplied through AVR, as they require constant voltage all the time. Notable exceptions are the loads relating to A/C plants and lighting etc. Input to the AVR is provided through ACB/MCB, depending upon the load rating of AVR from the switchgear incorporating h.r.c. fuse. The output of AVR is brought to a distribution panel from where the power is distributed to different loads through respective ACBs.The principle and working of an AVR is described below:

Electrical equipments are designed to operate at certain supply voltage. If the supply voltage is fluctuating from the rated voltage, the performance of the equipment is bound to suffer. Both under and over voltages are determintal to the load and effectiveness of the equipment. Present state of art to control the fluctuation is with the help of servo stabilizers. A servo stabilizer has four basic components.

1) Stepless toriodally wound autotransformer.2) Instantaneous start-stop reversible sync motor.3) Solid state sensing circuit.4) Series transformer.

Servo Stablizer

The secondary of the series transformer is connected in series to the input and the primary is fed through the stepless toriodally wound auto transformer. The tapping of this transformer, which is coupled with the shaft of the servomotor, is automatically adjusted by its movement. The movement of the motor is controlled by the solid state comparator, which continuously compares the output voltage with a fixed reference voltage. In case of error, motor moves in such a direction so as to correct the error. With the movement of the motor, the voltage supplied to the primary of the series transformer changes. Thus the voltage of the series transformer is added or subtracted to the input voltage. The circuit is arranged in such a way that if the output is more than the set voltage the motor tends to reduce the voltage applied to the primary of the series transformer and vice versa.

The single-phase stabilizer employs only one variable autotransformer driven by a motor and one sensing circuit. This type of AVR is suitable for single-phase loads. A balanced type 3-phase servo stabilizer employs a single control circuit. These variable autotransformers are mechanically coupled and driven by single-phase motor. The sensing is done in one of the three phases and the other two are corrected correspondingly. This type is suitable for use with balanced loads. However, these can be used with some unbalanced loads if less accuracy in output voltage is acceptable.

H.R.C. fuse

In low and medium voltage range H.R.C (high rupturing capacity) fuse is the most commonly used interrupting device. It is relatively cheap and occupies less space. It is used as a complementary to the other circuit breaker. The fuse has a disadvantage that it is required to be replaced after each operation.

Basically, the H.R.C. fuse has a ceramic body that contains specially designed fuse element. Fuse element is connected to metal caps. The caps also serve the purpose of sealing the body after it has been filled up with pure granulated quartz. Vaporization of the metalic elements occurs on melting and then there is a fusion which leads to rapid are extinction. The chemical reaction produces a substance of high resistance, which becomes insulator and therefore the current is interrupted.

Circuit Breakers

The devices used for making and breaking an electrical circuit under some pre-determined condition are called circuit breakers. The functions of a circuit breaker are as follows:

It must close on and carry full load currents for long period.

It must open automatically to disconnect the load, on over load under pre-determined condition.

It must rapidly interrupt the heavy current, which may flow under a short circuit condition in any part of the system.

The circuit breaker must be capable of withstanding the effect of arcing at its contact and the thermal conditions, which arise due to flow of current. Breaking current capacity is more important than making current capacity.

All circuit breakers consist essentially of pairs of matting contacts, each pair comprising fixed and moving elements. Under normal conditions, these elements are in contact and carrying full load current; but on receipt of a tripping signal initiated by hand or protective gear, the circuit will be interrupted.

At the start of the separation, an arc will be established which is required to be extinguished as early as possible. Generally, we come across two types of circuit breakers at medium and high voltage, for indoor application. They are called Oil Circuit Breaker (OCB) and Air Circuit Breaker (ACB).

In oil circuit breaker, oil insulates the live contact from the earthen metal tank and provides insulation between the open contacts when the arc is extinguished. Hydrogen is produced during arcing period, which in turn interrupts the arc. In air circuit breakers, the arc exists in the mixture of nitrogen, oxygen and metalic vapor and the successful arc interruption takes place due to cooling by diffusion. The OCB and ACB’s require yearly maintenance. Contacts are required to be cleaned and oil is replaced if needed, after carrying out oil test for its electric strength and acidity. Before test, oil is subjected to filtering process for removal of sludge and other foreign material.

DC POWER SUPPLY

Every electronic equipment and operating system need regulated power supply specially at lower voltages. This constant voltages were earlier provided by the Linear regulated power supplies and recently by switch mode power supplies(SMPS). Without doubt the SMPS has become very popular with the equipment manufacturers and users because of its various advantages. It is also being said that at last the power supply is also catching up with the technical advancement of the other devices. We shall analyse merits and demerits of both the Power Supply Units.

The Linear Power Supply System

The block diagram of a Linear regulated Power Supply is shown in the fig. 4. 230 V AC supply is fed to an isolation transformer which steps down the voltage to the required low level. Here the rating of the transformer depends on the current requirement of the load. Therefore, the transformer is normally bulky.

The Linear Power Supply System

The stepped down voltage is rectified and filtered by full wave or bridge rectifiers and electrolytic capacitors.

The series pass element, normally a Power transistor or FET, gets input from the filter. The output voltage is fed back to a comparator, which compares it with a standard reference and gives an error to the base of the transistor such that its conduction is increased or decreased. In short we can say that the series pass element works like a variable resistor giving constant voltage output.

The merits and demerits of this system are as follows

Merits

1. Very low output noise.

2. Very low ripple.

Demerits

1. Because of the bulky transformer the power supply unit is usually bulky.

2. Relatively narrow input voltage range. Normally + 10%.

3. Very low output hold up time about 1 milli sec.

4. Low efficiency about 40 to 50%.

5. Heat dissipation is more.

The Switch Mode Power Supply System (SMPS)

The block diagram of a SMPS is given below.

The Switch Mode Power Supply