VOCATIONAL TRAINING PROJECT REPORT at 33KV/11KV/6KV OLD CALCUTTA SUBSTATION AND 33KV/11KV MOHANPUR GIS SUBSTATION under BARRACKPORE DIVISION of WEST BENGAL STATE ELECTRICITY DISTRIBUTION COMPANY LIMITED (16/07/2016 (16/07/2016 to 30/07/2016)
Presented By:RONI DUTTA
GURU NANAK INSTITUTE OF TECHNOLOGY 157/F, NILGUNJ ROAD, PANIHATI, SODEPUR, KOLKATA – 700114
DECLARATION I here certify that the project work which is being presented is done at “BARRACKPORE ELECTRICITY
DIVISION”
DISTRIBUTION
of
“WEST
COMPANY
BENGAL
LIMITED”
STATE
in partial
fulfilment of the requirement for the award of the degree of BACHELOR OF TECHNOLOGY with specialization in ELECTRICAL ENGINEERING from
“GURU
“MAULANA
NANAK
INSTITUTE
OF
TECHNOLOGY”
under
ABUL KALAM AZAD UNIVERSITY OF TECHNOLOGY ”
(formerly known as WBUT) is an authentic record of vocational training
carried out for a training period from July 16,2016 to July 30,2016.
RONI DUTTA
ELECTRICAL ENGINEERING GURU NANAK INSTITUTE OF TECHNOLOGY SODEPUR, PANIHATI
(DIVISIONAL MANAGER, BARRACKPORE DIVISION)
(ASSISTANT ENGINEER, BARRACKPORE DIVISION)
[1]
ACKNOWLEDGEMENT This report is an outcome of the contributions made by many people.Therefore it is my sole responsibility to acknowledge them. We are greatly thankful to the sincere efforts made by Mr. Bikash Kumar Dey(Asst. Engg.,Barrackpore Division),without whom this project would be abstract. I would also like to thank the staff of 33kv/11kv/6kv Old Calcutta & 33kv/11kv GIS Distribution Substation, who took out their precious time to tell us about various equipment. My special thanks is dedicated to Mr. Souvik Sarkar (D.M ,Barrackpore Division, W.B.S.E.D.C.L). W.B.S.E.D.C.L). Along with the persons mentioned above, also I would like to thank every skilled and semi-skilled technicians and others for being very supportive and hence instrumental in the successful completion co mpletion of the vocational training.
RONI DUTTA Date:-30/07/2016
[2]
CONTENTS Page No. 1. 2. 3. 4. 5. 6.
WBSEDCL at a glance...................... glance................................... ......................... ........................ ........................... ....................4 .....4 Electrical Power Distribution................ Distribution............................. ......................... ........................ ............................ ................5 5 Old Calcutta Substation................... Substation................................. ........................ ......................... ............................ ....................6 .......6 Single Line Diagram of Old Calcutta Substation..................... Substation................................. ..................7 ......7 GIS Substation..................... Substation.................................. .......................... ....................... .......................... ............................8 ............8 Different Components of a Substation a) Transformer......................... Transformer..................................... ........................ ....................... ....................... ........................10 ............10 I. Power Transformers........................ Transformers.................................... ........................ ......................... .............10 10 II. Station Service Transformer...................... Transformer.................................. ......................... ..............11 .11 III. Potential Transformers...................... Transformers.................................... ............................ .....................12 .......12 IV. Current Transformers...................... Transformers.................................... ......................... .......................12 ............12 b) Isolators...................... Isolators................................. .......................... ........................... ........................ ......................... ..................13 .....13 I. Types of Isolators...................... Isolators.................................. ........................ ............................ ..................13 ..13 II. Constructional Features of Double-Break Isolators..................... Isolators.................................. ........................... ......................14 ........14 c) Lightning Arresters......................... Arresters..................................... ....................... ......................... ........................14 ..........14 d) Circuit Breakers........................ Breakers.................................... ....................... ....................... ........................... ...................15 ....15 I. Vacuum Circuit Breakers...................... Breakers.................................. ............................ ...................15 ...15 e) Conductors...................... Conductors................................... ......................... ......................... ......................... .........................16 .............16 I. Choices Depends Upon..................... Upon................................... .......................... .....................16 .........16 II. Types of Conductors...................... Conductors...................................... ........................... ......................17 ...........17 f) Insulators...................... Insulators.................................. ........................ ........................ .......................... ........................... ................17 ...17 I. Pin Insulators................... Insulators............................... ........................ ............................ ............................ ............18 18 II. Post Insulators..................... Insulators................................. ........................ .......................... .........................18 ...........18 III. Strain Insulators.................... Insulators................................ ......................... ............................ ......................18 .......18 IV. Shackle Insulators....................... Insulators................................... ........................ ........................... .................18 ..18 g) Capacitor Bank..................... Bank................................. .......................... ........................ .......................... ......................19 ......19 h) Battery Bank....................... Bank................................... ........................ ......................... .......................... .......................20 ..........20 I. Need for the Battery bank................ bank............................ ........................... ........................20 .........20 i) Relay..................... Relay............................... ...................... ......................... .......................... ........................... ..........................21 ............21 I. Purpose of Relay...................... Relay.................................. ........................ .......................... ...................21 .....21 II. Types of Relay...................... Relay.................................. ........................... ............................ ...................21 ......21 j) Bus Coupler..................... Coupler................................. ........................ ......................... ........................... ..........................22 ............22 7. Probable Faults in a Distribution System.................. System............................... ......................... ..................23 ......23 8. Conclusion.................... Conclusion................................. ......................... ........................ ........................ ........................ ...........................25 ...............25
[3]
WBSEDCL at a glance The story of WBSEB began on the first of May, 1955. WBSEB from the very onset has set up a glorious path of service, working day and night to bring power to each and every household, locality, town and village in West Bengal. With its inception with a mere 755 employees and a consumer strength of only 1000 and a capacity of 4MW it has come a long way to the mammoth giant it is today. WBSEB has taken giant strides to progress to a strength of 22,000 people with a consumer strength exceeding 50 lakhs. The Board is proud owner of transmission lines spanning 87,500 sq. km of area with a backbone comprising 5 zones, 17 circles, 47 distribution divisions and 467 electric supply offices. The present capacity along with WBPDCL is 3165MW.
WBSEB, the first SEB in India, was constituted on 1 st May, 1955(under section 5 of Electric Supply Act 1948), celebrated its Golden Jubilee Year in 2005.
WBSEB provides power to 96% of West Bengal, while meeting 80% of the state’s peak power demand.
First SEB in India to record a commercial profit of Rs. 81.52 Crore in 2005-2006.
Achieved a cash surplus for last 3 years at an average of Rs. 293 Crore per year.
Ranked 5th in all India level in ICRA CRISIL rating(up from 13th in 2002).
Received Rs. 302.76 Crore as cash incentive under APDRP incentive scheme for reduction of loss in 2003-2004.
WBSEB was restructured as per the provision of the Electricity Act 2003 into two separate government owned companies namely West Bengal State Electricity Distribution Company Ltd.(WBSEDCL)(looking after distribution and hydro undertaking undertaking of WBSEB) and West Bengal State Electricity
Transmission
Company
Ltd.(WBSETCL)(looking
after
transmission and load dispatch undertaking of WBSEB) with effect from 01.04.2007.
[4]
ELECTRICAL POWER DISTRIBUTION An electric power distribution system is the final stage in the delivery the delivery of electric electric power; power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between 2 kV and 35 kV with the use of transformers. Primary transformers. Primary distribution lines carry this th is medium voltage power to distribution to distribution transformers located near the customer's premises. Distribution transformers again lower the voltage to the utilization the utilization voltage of household appliances and typically feed several customers through secondary distribution lines at this voltage. Commercial and residential customers are connected to the secondary distribution lines through service through service drops. Customers drops. Customers demanding a much larger amount of power power may be connected directly to the primary distribution level or the sub-transmission level.
AC became the dominant form of transmission of power. Power transformers, transformers, installed at power stations, stations, could be used to raise the voltage from the generators, and transformers at local substations local substations could reduce voltage to supply loads. Increasing the voltage reduced the current in the transmission and distribution lines and hence the size of conductors and distribution losses. This made it more economical to distribute power over long distances.
[5]
Old Calcutta SUBSTATION SUBSTATION The Old Calcutta Calcutta Substation is is located at Barrack pore in north 24 parganas. It is a 33KV/11KV/6kv substation. It receives power from two different feeders having rating of 33kv and distributes voltage voltage at the rating rating of 11kv & 6kv by stepping down the voltage from 33kv to 11kv & 6kv. It has six transformers three having capacity of 6.3MVA two having capacity of 3 MVA & One having capacity of 5MVA . Other Other than these it it has an station station service transformer of rating of 100KVA, 33KV/0.433KV to supply power to the substation premises.
Different Incoming and Outgoing Feeders at Old Calcutta Substation:
SUBSTATION FEEDER NAME
CKT-1
O L D C A L C U T T A
CKT-2
E.M.CO TALPUKUR SADAR BAZAR RAHARA-2 BRITANIA
NILGUNJ RAHARA-1 BARRACKPORE
NATURE
I N C O M I N G
O U T G O I N G
FEATURES
VOLTAGE RATING OF FEEDER
From Titagarh to Old Calcutta substation. 33KV From Titagarh to Old Calcutta Substation
11KV From Old Calcutta Substation to the respective places as in feeder name. 6KV 6KV 6KV
NILGUNJ DANGAPARA
33KV
TS L
Single line diagram of 33/11/6kv old Calcutta substation
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MOHANPUR GIS SUBSTATION The GIS Substation is also located at Barrackpore in north 24 parganas. It is a 33KV/11KV substation. It receives power from two single feeders having rating of 33kv. It distributes distributes voltage at different voltages, 11 kv by stepping down the voltage from 33kv to 11kv . It has two transformers, they are all of different power ratings. Other than these it has an station service transformer which steps down the voltage from 33kv to 0.433kv for supplying power to station premises.
Different Incoming and Outgoing Feeders at GIS Substation: SUBSTATION
G O U R I P U R
FEEDER NATURE
I N C O M I N G
FEEDER NAME
FEATURES
INCOMMER 1
From Titagarh to GIS Substation
VOLTAGE RATING OF FEEDER
33 KV
KMDA-2 O U T G O I N G
JAFFARPUR NCP
From GIS Substation to the respective places as in feeder name.
11 KV
[8]
Single line diagram of 33/11kv GIS Substation
[9] DIFFERENT COMPONENTS OF A SUBSTATION
TRANSFORMERS A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. Electromagnetic induction produces an electromotive force across a conductor which is exposed to time varying magnetic fields. Commonly, transformers are used to increase or decrease the voltages of alternating current in electric power applications.
POWER TRANSFORMERS Generation of electrical power in low voltage level is very much cost effective. Hence electrical power is generated in low voltage level. Theoretically, this low voltage level power can be transmitted to the receiving end. But if the voltage level of a power is increased, the current of the power is reduced which causes reduction in ohmic or I 2R losses in the system, reduction in cross sectional area of the conductor i.e. reduction in capital cost of the system and it also improves the voltage regulation of the system. Because of these, low level power must be stepped up for efficient electrical power transmission. This is done by step up transformer at the sending side of the power system network. As this high voltage power may not be distributed to the consumers directly, this must be stepped down to the desired level at the receiving end with the help of step down transformer. These are the uses of electrical power transformer in the electrical power system.
[10]
In Old Calcutta Substation, there are six power transformers present, two of them are 33kv/6kv transformers, three of them are 33kv/11kv p and the rest is a 33kv/6kv transformer. In Mohanpur GIS Substation, there are two power transformer present, all of them are 33/11 kv transformer. TRANSFORMER Transformer – 1,2 are parallel connection Transformer – 3,4,5 are Parallel connection Transformer – 6
RATING 3 MVA, 33KV/6KV 6.3 MVA, 33KV/11KV
FEEDERS RAHARA-1, BARRACKPORE E.M.CO, TALPUKUR,SADAR BAZAR,BRITANIA,RAHARA2 NILGUNJ
5MVA, 33KV/6KV
In GIS Substation, there are two power transformers present, all of them are 33kv/11kv transformers. TRANSFORMER RATING Transformer – 1 1 & 2 are 6.3 MVA Parallel connection 33KV/11KV
FEEDERS KMDA 2,JAFFARPUR,NCP
STATION SERVICE TRANSFORMER Station service transformers are intended to provide low voltage control power for substations, cell tower installations, and at switching stations by tapping directly from the high voltage line. They are used for running the auxiliary equipments of the substation. At Old Calcutta Substation, there is a station service transformer of 100kVA, 33KV/0.433kv present.
[11]
POTENTIAL TRANSFORMERS Potential transformer or voltage transformer gets used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays as commercially available relays and meters, used for protection and metering, are designed for low voltage. Primary of this transformer is connected across the phase and ground. Just like the transformer used for stepping down purpose, potential transformer i.e. PT has lower turns winding at its secondary. The system voltage is applied across the terminals of primary winding of that transformer, and then proportionate secondary voltage appears across the secondary terminals of the PT. The secondary voltage of the PT is generally 110 V. In an ideal potential transformer or voltage transformer, when rated burden gets connected across the secondary; the ratio of primary and secondary voltages of transformer is equal to the turns ratio and furthermore, the two terminal voltages are in precise phase opposite to each other. But in actual transformer, there must be an error in the voltage ratio as well as in the phase angle between primary and secondary voltages.
CURRENT TRANSFORMERS A current transformer i.e. CT is an instrument transformer in which the secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree. A CT functions with the same basic working principle of electrical power transformer, as we discussed earlier,
but here is some difference. In a power transformer, if load is disconnected, there will be only magnetizing current flows in the primary.
[12]
The primary of the power transformer takes current from the source proportional to the load connected with secondary. But in case of CT, the primary is connected in series with power line. So current through its primary is nothing but the current flows through that power line. The primary current of the CT, hence does not depend upon whether the load or burden is connected to the secondary or not or what is the impedance value of burden. Generally CT has very few turns in primary where as secondary turns are large in number.
ISOLATORS Isolator is a manually operated mechanical switch which separates a part of the electrical power system normally at off load condition. Circuit breaker always trip the circuit but open contacts of breaker cannot be visible physically from outside of the breaker and that is why it is recommended not to touch any electrical circuit just by switching off the circuit breaker. So for better safety there must be some arrangement so that one can see open condition of the section of the circuit before touching it. Isolator is a mechanical switch which isolates a part of circuit from system as when required. Electrical isolators separate a part of the system from rest for safe maintenance works.
TYPES OF ISOLATORS There are different types of isolators available depending upon system requirement such as 1) Double Break Isolator 2) Single Break Isolator 3) Pantograph type Isolator. Depending upon the position in power system, the isolators can be categorized as 1) Bus side isolator – the isolator is directly connected with main bus. 2) Line side isolator – the isolator is situated at line side of any feeder. 3) Transfer bus side isolator – the isolator is directly connected with transfer bus.
[13]
CONSTRUCTIONAL FEATURES OF DOUBLE BREAK ISOLATORS These have three stacks of post insulators as shown in the figure. The central post insulator carries a tubular or flat male contact which can be rotated horizontally with rotation of central post insulator. This rod type contact is also called moving contact. The female type contacts are fixed on the top of the other post insulators which fitted at both sides of the central post insulator. The female contacts are generally in the form of spring loaded figure contacts. The rotational movement of male contact causes to come itself into female contacts and isolators becomes closed. The rotation of male contact in opposite direction make to it out from female contacts and isolators becomes open. Rotation of the central post insulator is done by a driving lever mechanism at the base of the post insulator and it connected to operating handle (in case of hand operation) or motor (in case of motorized operation) of the isolator through a mechanical tie rod.
LIGHTNING ARRESTER An electrical surge can be occurred in an electrical power transmission system due to various reasons. Surge in electrical system originated mainly due to lightning impulses and switching impulses. Electrical surge produces a large transient over voltage in the electrical network and system. This over voltage wave travels through the electrical network and causes over voltage stresses on all the electrical insulators and equipment come under its travelling path. That is why all
electrical equipment and insulators of power system must be protected against electrical surges. The main equipment commonly used for this purpose is lightning arrester or surge arrester.
[14]
In electrical In electrical sub-station, arresters sub-station, arresters are mainly used at the entrance of any feeders and also they are used at both rides of electrical power transformers as transformer is also considered as inductive load and very costly equipment.
CIRCUIT BREAKERS Electrical circuit breaker is a switching device which can be operated manually as well as automatically for controlling and protection of electrical power system respectively. As the modern power system deals with huge currents, the special attention should be given during designing of circuit breaker to safe interruption of arc produced during the operation of circuit breaker. According to their arc quenching media the circuit the circuit breaker can be divided as-
1. Oil circuit breaker. 2. Air circuit breaker. 3. SF6 circuit breaker. 4. Vacuum circuit breaker. In Old Calcutta Substation all the circuit breakers are vacuum circuit breakers. and GIS Substation, all the circuit breakers are SF6 circuit breakers.
VACUUM CIRCUIT BREAKER A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. The technology is suitable for mainly medium voltage application. For higher voltage vacuum technology has been developed but not commercially viable. The operation of opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the breaker which is called vacuum interrupter. The vacuum interrupter consists of a steel arc chamber in the centre symmetrically arranged ceramic insulators. The vacuum pressure inside a vacuum interrupter is normally maintained at 10 - 6 bar.
The material used for current carrying contacts plays an important role in the performance of the vacuum circuit breaker. CuCr is the most ideal material to make VCB contacts.
[15]
OPERATION:The main aim of any circuit breaker is to quench arc during current zero crossing, by establishing high dielectric strength in between the contacts so that reestablishment of arc after current zero becomes impossible. The dielectric strength of vacuum is eight times greater than that of air and four times greater than that of SF6 gas. This gas. This high dielectric strength makes it possible to quench a vacuum arc within very small contact gap. For short contact gap, low contact mass and no compression of medium the drive energy required in vacuum circuit breaker is minimum. When two face to face contact areas are just being separated to each other, they do not be separated instantly, contact area on the contact face is being reduced and ultimately comes to a point and then they are finally de-touched. Although this happens in a fraction of micro second but it is the fact. At this instant of de-touching of contacts in a vacuum, the current through the contacts concentrated on that last contact point on the contact surface and makes a hot spot. As it is vacuum, the metal on the contact surface is easily vaporized due to that hot spot and create a conducting media for arc path. Then the arc will be initiated and continued until the next n ext current current zero. At current zero this vacuum arc is extinguished and the conducting metal vapour is re-condensed on the contact surface. At this point, the contacts are already separated hence there is no question of re-vaporization of contact surface, for next cycle of current. That means, the arc cannot be re-established again. In this way vacuum circuit breaker prevents the reestablishment of arc by producing high dielectric strength in the contact gap after current zero.
CONDUCTORS The selection of the optimum conductor type and size for a given distribution line design requires a complete understanding of the characteristics of all the available conductor types. This understanding must encompass more than just the current carrying capability or thermal performance of a conductor.
CHOICES OF OVREHEAD DEPEND UPON Power Delivery Requirements Current Carrying Capacity Electrical Losses
[16] Line Design Requirements Distances to be spanned Sag and Clearance Requirements
Environmental Considerations Ice and Wind Loading Ambient Temperatures
TYPES OF CONDUCTORS There are various types of overhead conductors used for electrical transmission and distribution. AAC – All All Aluminium Conductors AAAC – All All Aluminium Alloy Conductors ACSR – Aluminium Aluminium Conductor Steel Reinforced ACAR – Aluminium Aluminium Conductor Aluminium Alloy Reinforced AACSR – Aluminium Aluminium Alloy Conductor Steel Reinforced ACSS – Aluminium Aluminium Conductors Steel Supported ACCC – Aluminium Aluminium Conductor Composite Core The various combinations and modifications of these conductor types provide a wide variety of possible conductor designs.
INSULATORS There are various types of insulator used as overhead insulator likewise 1. Pin Insulator 2. Post Insulator 3. Strain Insulator 4. Suspension Insulator In addition to that there are other two types of electrical insulator available mainly for low voltage application - Stay Insulator and Shackle Insulator.
Here, we will discuss about pin type, post type, strain type and shackle type insulators as they are used in Old Calcutta and GIS Substation. Suspension type insulators are mainly used for high voltage transmission, that is why we will not discuss about suspension type insulators here.
[17]
PIN INSULATORS A pin insulator consists of a non conducting material such as porcelain, glass, plastic, polymer, or wood that is formed into a shape that will isolate a wire from a physical support (or "pin") on a telegraph, a telegraph, utility utility pole or other structure, provide a means to hold the insulator to the pin, and provide p rovide a means to secure the conductor to the insulator.
POST INSULATORS Post insulator is more or less similar to pin insulator but former is suitable for higher voltage application. Post insulator has higher numbers of petticoats and has greater height. This type of insulator can be mounted on supporting structure horizontally as well as vertically. The insulator is made of one piece of porcelain but has fixing clamp arrangement arrangement are in both top and bottom end.
STRAIN INSULATORS When suspension string is used to sustain extraordinary tensile load of conductor it is referred as string insulator. When there is a dead end or there is a sharp corner in transmission line, the line has to sustain a great tensile load of conductor or strain. A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties.
SHACKLE INSULATORS The shackle insulator is usually used in low voltage low voltage distribution network. It can be used both in horizontal and vertical position. The use of such insulator has decreased recently after increasing the using of underground cable for distribution purpose. The tapered hole of the shackle insulator distributes the
load more evenly and minimizes the possibility of breakage when heavily loaded. The conductor in the groove of shackle insulator is fixed with the help of soft binding wire.
[18]
CAPACITOR BANK A capacitor bank capacitor bank is a grouping of several identical capacitors interconnected in parallel or in series with one another. These groups of capacitors are typically used to correct or counteract undesirable characteristics, such as power factor lag or phase shifts inherent in alternating current (AC) electrical power electrical power supplies. The use of a capacitor bank to correct AC power supply anomalies is typically found in heavy industrial environments that feature working loads made up of electric motors and transformers. This type of working load is problematic from a power supply perspective as electric motors and transformers represent inductive loads, which cause a phenomenon known as phase shift or power factor lag in the power supply. The presence of this undesirable phenomenon can cause serious losses in terms of overall system efficiency with an associated increase in the cost of supplying the power. The use of a capacitor bank in the power supply system effectively cancels out or counteracts these phase shift issues, making the power supply far more efficient and cost effective. The installation of a capacitor bank is also one of the cheapest methods of correcting power lag problems and maintaining a power factor capacitor bank is simple and cost effective.
[19]
BATTERY BANK The heart of a substation is the battery bank. A battery bank is a primary component of a substation or switchyard DC control system. The function of the control system is to supply control power to operate critical devices such as protective relays, alarms and status indicators, supervisory and communications equipment, and switchgear operating circuits. Battery bank for switchgear and control applications are made up of many cells. These cells are typically wired in series to achieve a desired voltage and may also be wired in parallel to achieve additional ampere capacity. Sizing of these battery banks, therefore, includes selecting the number and type of cells to be used. A battery room is a room in the substation subs tation used use d to house this battery bank for backup or uninterruptible power systems.
NEED FOR THE BATTERY BANK If this were to fail, an electric utility could expose all feeders associated with the station to a condition where they could not ever trip in a fault. Not only that, but any backup devices, such as the main breaker on the low-voltage side or the high-voltage side protection of the power transformer, would all be inoperative, leaving the transmission grid protection as the only possible back up. u p. This could then cause such su ch catastrophic consequences as burning wire down across town town and eventually eventually destroying the substation transformer. This is not to mention the hazard it would cause to the public. At Old Calcutta Substation and GIS Substation, each of them has a battery room for housing battery bank. The capacity of each of the battery bank is 30 volts which is feed to the control panel components of the substation. One of the applications of battery bank here is to operate the Vacuum Control breakers(VCB).The motor of the VCB although is charged by AC current, but
the operation of the VCB requires DC current which is supplied by the battery room provided at the substation.
[20]
RELAY A relay is an electrically an electrically operated switch. operated switch. Many relays use an electromagnet an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state as solid-state relays. Relays relays. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays". relays".
PURPOSE OF RELAY A protective relay is a relay, the principal function of which is to protect service from interruption or to prevent or limit damage to apparatus. A protective relay is a device designed to trip a circuit a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as over-current, over-voltage, reverse power reverse power flow, over- and under- frequency. Microprocessor-based digital protection relays now emulate the original devices, as well as providing types of protection and supervision impractical with electromechanical relays.
TYPES OF RELAY 1. 2. 3. 4. 5.
Over current relay Earth fault relay Differential relay Time delay relay Under voltage relay
6. IDMT relay 7. Directional relay
[21]
BUS COUPLER Bus Coupler is a device which is used to couple one bus to the other without any interruption in power supply and without creating hazardous arcs. Bus coupler is a breaker used to couple two busbars in order to perform maintenance on other circuit breakers associated with that busbar. It is achieved with the help of circuit breakers and isolators.
[22]
Probable Faults in a Distribution System In an electric power system, a fault or fault current is any abnormal electric abnormal electric current. current. For example, a short a short circuit is a fault in which current bypasses the normal load. An open-circuit fault occurs if a circuit is interrupted by some failure. In three-phase In three-phase systems, a fault may involve one or more phases and ground, or may occur only between phases. In a "ground fault" or "earth fault", current flows into the earth. The prospective short circuit current of a predictable fault can be calculated for most situations. In power systems, protective devices can detect fault conditions and operate circuit operate circuit breakers and other devices to limit the loss of service due to a failure.
TRANSIENT FAULT A transient fault is a fault that is no longer present if power is disconnected for a short time and then restored; or an insulation fault which only temporarily affects a device's dielectric properties which are restored after a short time. Many faults in overhead power lines are transient in nature. When a fault occurs, equipment used for power power system protection operates to isolate the area of the fault. A transient fault will then clear and the power-line can be returned to service. Typical examples of transient faults include:-momentary tree contact, bird or other animal contact, lightning contact, lightning strike, conductor strike, conductor clashing etc.
PERSISTENT FAULT A persistent fault f ault does not n ot disappear when power is disconnected. d isconnected. Faults in underground power underground power cables are most often persistent due to mechanical damage to the cable, but are sometimes transient in nature due to lightning.
SYMMETRIC FAULT A symmetric or balanced fault affects each of the three phases equally. In transmission line faults, roughly 5% are symmetric. symmetric .[2] This is in contrast to an asymmetrical fault, where the three phases are not affected equally.
ASYMMETRIC FAULT An asymmetric or unbalanced fault does not affect each of the three phases equally. Common types of asymmetric faults, and their causes:
[23]
Line – to – Line:-A short Line:-A short circuit between circuit between lines, caused by ionization by ionization of air, or when lines come into physical contact, for example due to a broken insulator. broken insulator. Line – to – Ground:- A short circuit between between one line and ground, very often caused by physical contact, for example due to lightning to lightning or other storm storm damage Double Line – to – Ground Ground :- Two lines come into contact with the ground (and each other), also commonly due to storm damage.
BOLTED FAULT One extreme is where the fault has zero impedance, giving the maximum maximum prospective short-circuit current. current. Notionally, all the conductors are considered connected to ground as if by a metallic conductor; this is called a "bolted fault". It would be unusual in a well-designed power system to have a metallic short circuit to ground but such faults can occur by mischance. In one type of transmission line protection, a "bolted fault" is deliberately introduced to speed up operation of protective devices.
REALISTIC FAULTS Realistically, the resistance in a fault can be from close to zero to fairly high. A large amount of power may be consumed in the fault, compared with the zeroimpedance case where the power is zero. Also, arcs are highly non-linear, so a simple resistance is not a good model. All possible cases need to be considered for a good analysis.
ARCING FAULT Where the system voltage is high enough, an electric arc may form between power system conductors and ground. Such an arc can have relatively high impedance (compared to the normal operating levels of the system) and can be difficult to detect by simple over current protection. For example, an arc of several hundred amperes on a circuit normally carrying a thousand amperes may not trip over current circuit breakers but can do enormous damage to bus bars or cables before it becomes a complete short circuit. Utility, industrial, and
commercial power systems have additional protection devices to detect relatively small but undesired currents escaping to ground. In residential wiring, electrical regulations may now require Arc-fault require Arc-fault circuit interrupters on building wiring circuits, to detect small arcs before they cause damage or a fire.
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CONCLUSION The Vocational training at 33KV/11KV Old Calcutta Substation and GIS Substation under Barrackpore Barrackpore Division Division of
West Bengal State Electricity
Distribution Company Limited turned out to be a wonderful industrial experience which not only provided me with the opportunity to be aware of the electrical power distribution system process but also provided me the opportunity to interact with the experienced engineers in the power sector which has helped me increase my exposure.
The vocational training organized by WBSEDCL helps students lot more than books by giving us a practical knowledge. It helped me to correlate my theoretical conceptions with practical ones. I would like to thank everybody who has been a part of this project, without whom this project would never be completed with such ease.
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