VOCATIONAL TRAINING ON UTTAR PRADESH POWER TRANSMISSION CORPORATION LIMETEDFor

The partial fulfillment of award Of B.TECH. DegreeBy

SHAILENDRA YADAV (0705420096) Electrical Engineering (Final Year)

ACKNOWLEDGEMENTI am extremely thankful & indebted to the numerous UPPTCL Engineers, who provided vital information about the functioning of their respective departments thus helping me to gain an overall idea about the working of organization. I am highly thankful for the support & guidance of each of them. I am highly indebted to my project guide, Mr. Ramlal(A.E.), Mr. Mevalal(J.E.), Mr. P.K. Mishra (A.E.-T&C) for giving me his valuable time and helping me to grasp the various concepts of switchyard equipments and their control instruments and their testing. Last but not the least, I would like to thank my parents & all my fellow trainees who have been a constant source of encouragement & inspiration during my studies & have always provided me support in every walk of life.

SHAILENDRA YADAV B.TECH. FINAL YEAR ELECTRICAL ENGINEERING B.B.D.N.I.T.M. (LUCKNOW)

ContentsWhat is an Electrical Substation? Energy growth in UP Grid map of UP Introduction: about substation Overview of substation Single line digram of substation Brief description :Power transformer Isolators Circuit breaker Lightning arrestor Current transformer Capacitor voltage transformer Wave trap Protective relays Shunt reactor for bus voltage Capacitor bank Clearance at glance Power line communication & SCADA system Other definitions Appendix References

What is an Electrical Substation

Electric Power is generated in Power Stations and transmitted to various cities and towns. During transmissions, there are power (energy) loss and the whole subject of Transmission and Distribution...

An electrical substation is a subsidiary station of an electricity generation, transmission and distribution system where voltage is transformed from high to low or the reverse using transformers. Electric power may flow through several substations between generating plant and consumer, and may be changed in voltage in several steps. The word substation comes from the days before the distribution system became a grid. The first substations were connected to only one power station where the generator was housed, and were subsidiaries of that power station.

Elements of a substation Substations generally have switching, protection and control equipment and one or more transformers. In a large substation, circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network. Smaller distribution stations may use reclose circuit breakers or fuses for protection of distribution circuits. Substations do not usually have generators, although a power plant may have a substation nearby. Other devices such as power factor correction capacitors and voltage regulators may also be located at a substation. Substations may be on the surface in fenced enclosures, underground, or located in specialpurpose buildings. High-rise buildings may have several indoor substations. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions. Where a substation has a metallic fence, it must be properly grounded (UK: earthed) to protect people from high voltages that may occur during a fault in the network. Earth faults at a substation can cause a ground potential rise. Currents flowing in the Earth's surface during a fault can cause metal objects to have a significantly different voltage than the ground under a person's feet; this touch potential presents a hazard of electrocution.

Transmission substation: A transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains high-voltage switches that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station may have transformers to convert between two transmission voltages, voltage control devices such as capacitors, reactors or static VAr compensator and equipment such as phase shifting transformers to control power flow between two adjacent power systems. Transmission substations can range from simple to complex. A small "switching station" may be little more than a bus plus some circuit breakers. The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, many circuit breakers and a large amount of protection and control equipment (voltage and current transformers, relays and SCADA systems). Distribution substation: A distribution substation in Scarborough, Ontario, Canada disguised as a house, complete with a driveway, front walk and a mown lawn and shrubs in the front yard. A warning notice can be clearly seen on the "front door". A distribution substation transfers power from the transmission system to the distribution system of an area. It is uneconomical to directly connect electricity consumers to the high-voltage main transmission network, unless they use large amounts of power, so the distribution station reduces voltage to a value suitable for local distribution. The input for a distribution substation is typically at least two transmission or sub transmission lines. Input voltage may be, for example, 115 kV, or whatever is common in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between 2.4 and 33 kV depending on the size of the area served and the practices of the local utility.

Energy growth in UP:

Grid map of UP:

Introduction: about substation

400 kv Unnao substation is one important substation of UPGCL & UPPTCL. It is one of the largest power grids in the state of UP and the north India. It is situated at Dahi Chowki 6.64 km far from unnao railway station. The construction of this substation completed during 1994-98 by CGL(Crompten Grives Limted) .The area of this substation is about 300 acre. The whole substation is divided in four parts: 1. 132kv switchyard 2. 400/220kv switchyard 3. 765kv switchyard For 400kv &220kv switchyard a common control room is used and for 132kv switchyard A separate control room used.

Crompton Greaves Limited (CG), an Indian Multinational with manufacturing bases in 8 countries, have signed the contract on 5th March2010 with Uttar Pradesh Power Transmission Corporation Ltd for construction of 765/400 kV Substation at Unnao, in Uttar Pradesh. The value of contract is Rs 302 Corers .

A 765/400 kV substation is the highest grade system voltage for transmission in India. UPPTCL is first state utility to enter into 765 kV arena.

The scope of the project includes Design, Engineering, Manufacture, Supply, Erection, Testing and Commissioning of 8 Bays of 765 kV & 2 Bays of 400kV, along with 7 Nos. of 333 MVA (Single Phase) 765/400 kV Power Transformers and 7 Nos. of 110 MVAR (Single Phase) 765 kV Shunt Reactor & 4 Nos. 63 MVAR (Single Phase) 765 kV Reactors. The project is expected to be commissioned in July 2011. The project is of strategic importance for entry into market of 765 kV Substations globally and widens up the horizon for the entire product range of CGL.

Overview of substation

As we said earlier the whole substation is divided in three parts:132kv site ,400/220kv site and 765 kv site 765 kv sit is on under construction. The civil work is completing by L&T Company. Other part of project Design, Engineering, Manufacture, Supply, Erection, Testing and Commissioning of Bays will complete by CGL. In 400/220kv switchyard following outdoor instrument used: 1. One 400kv transfer bus control bus coupler 2. Two 100MVA 220/132kv autotransformer 3. Two 315MVA 400/220kv autotransformer 4. Five 50MVAR shunt reactor 5. Two 63MVAR bus reactor 6. 15 lighting tower 7. SF6 circuit breaker 8. Capacitor voltage transformer(CVT) 9. Current transformer(CT)

In switchyard one room for mulsi fire system and one for generator system is also present. In 400kv switchyard following lines are present for incoming and outgoing power: i) Unnao to Lucknow ii) Unnao to Bareily-1 iii) Unnao to Bareily-2 iv) Unnao to Agara v) Unnao to Panki kanpur vi) Unnao to Anpara vii) Unnao to PGCIL-1 viii) Unnao to PGCIL-2

ix) One bus bar for 400/220kv 315MVA ICT-1 & ICT-2 line.

The buses of 400kv switchyard charged by Unnao - Anpara line. This line is the Indias first line which is made for 765KV transmission. But till today it is charged by 400kv. In futureit work on 765 kv .

From 220kv switchyard two lines for Lucknow and two lines for Panki Kanpur comes out. In whole switchyard following main equipment are used:

i) One 400kv transfer bus control bus coupler. ii) Two 100MVA 220/132 KV auto transformer manufactured from BHEL. iii) Two 315 MVA 400/220 KV auto transformer manufactured from BHEL. iv) Five 50 MVAR shunt reactor manufactured from BHEL. v) Two 63 MVAR bus reactor manufactured from HITACTI. vi) Circuit breaker from CGL. vii) Isolators from S&S. viii) ix) CVT x) Wave trap xi) Lighting arrester xii) Surge capacitor Current transformer from WS and CGL.

Single line diagram of unnao substation

Brief Description Of all Outdoor Equipment

Power transformer:Various types of transformers have been provided at 220& 400 KV Substation from UPPTCL. Capacity and voltage ratio wise 100 MVA , 315MVA & 160 MVA and 220/132/11 kV. 400/220 kV, These transformers are of TELK, BHEL, GEC, NGEF, C & G, Hitachi and Bharat Bijlee make and have most of the features common except few accessories which may be different. In this

substation all transformers made by BHEL. These transformers have following main components: 1. MAIN CORE & WINDING.

2.

BUSHING :-

(a) 220 kV High voltage bushings:

Condenser type bushings with insulating body and central conducting tubebackelised with paper wound capacitor have been provided. Innermost of the capacitor layer is electrically connected to the tube and outermost to the mounting flange on insulating body. The central tube insulating body and mounting flange are oil filled assembled. High dielectric Strength oil is filled between central tube and insulating body. Oil level indicators are provided on the bushing.

(b) 132 kV Medium voltage bushing:

These bushing are also of condenser type and are of similar construction as in the case of 220 kV bushing in 200 MVA transformers.

In 40 & 20 MVA transformers 132 kV bushings are also of oil filled type in which oil is filled up when the transformer tank is topped up. Necessary air vent screws are provided on top of the bushings for release of trapped air at the top of oil fitting.

(c) 66 kV. 33 kV. & 11 kV. Bushings:

These are oil filled bushing and simpler in construction.

3. TAP CHANGER:

The transformers have been provided with on load tap changer, which consists of diverter switch installed in an oil compartment separated from transformer oil and the tap selector mounted below it. The tap changer is attached to the transformer cover by means of tap

changer head, which also serves for connecting the driving shaft and the oil conservator.

4. PROTECTIVE RELAYS:

Generally there are two protective buchholz relays, one for main transformer tank and other for tap changer. In 40MVA GEC transformers oil surge relay has also been provided in tap changer.

5. PRESSURE RELIEF VALVE:

40 MVA GEC make transformers have been provided with pressure relief valve which operates in case of sudden pressure formation in side the transformer.

6. COOLING SYSTEM :

100 MVA transformers have been provided with cooling bank installed on separate structures. These cooling banks have provided with to groups of fans and 2 nos. pumps. These fans and pumps automatically operate, depending upon the settings of winding temperature Indicator.

7. TERTIARY BUSING:

100 MVA transformers have been provided with tertiary bushing connected with 11 kv capacitor and lighting arrestor t absorb switching surges.

ELECTRICAL PROTECTION :

The following electrical protection have been provided on the transformers :-

(i) Differential Protection (ii) Restricted Earth Fault (iii) Winding temp high

(iv) Oil temp high (v) Pressure relief valve (vi) Oil surge relay (vii) Over current relay (viii) Local Breaker Back up protection (ix) Surge arrestors on HV, MV & LV sides.

The main Tank - The transformer is transported on trailor to substation site and as far as possible directly unloaded on the plinth. Transformer tanks up to 25 MVA capacity are generally oil filled, and those of higher capacity are transported with N2 gas filled in them +ve pressure of N2 is maintained in transformer tank to avoid the ingress of moisture. This pressure should be maintained during storage; if necessary by filling N2 Bushings generally transported in wooden cases in horizontal position and should be stored in that position. There being more of Fragile material, care should be taken while handling them. Rediators These should be stored with ends duly blanked with gaskets and end plates to avoid in gross of moisture, dust, and any foreign materials inside. The care should be taken to protect the fins of radiators while unloading and storage to avoid further oil leakages. The radiators should be stored on raised ground keeping the fins intact. Oil Piping. The Oil piping should also be blanked at the ends with gasket and blanking plates to avoid in gross of moisture, dust, and foreign

All other accessories like temperature meters, oil flow indicators, PRVs, buchholtz relay; oil surge relays; gasket O rings etc. should be properly packed and stored indoor in store shed. Oil is received in sealed oil barrels . The oil barrels should be stored in horizontal position with the lids on either side in horizontal position to maintain oil pressure on them from inside and subsequently avoiding moisture and water ingress into oil. The transformers are received on site with loose accessories hence the materials should be checked as per bills of materials.

The transformers that are used in Unnao substation have following specification: Specification of 100 MVA 220/132/11 KV 3- auto transformer: Types of cooling Rating of H.V. & I.V.(MVA) Rating of L.V. (MVA) Line current H.V.(Amps) ONAN 60 18 157.4 ONAF 80 24 209.9 OFAF 100 30 262.4

Line current I.V. (Amps) Line current L.V. (Amps)

262.4 944.8

349.9 1259.7

437.4 1574.6

No load voltage H.V. No load voltage I.V. No load voltage L.V. Temp. Rise winding C 55 [

220KV 132KV 11KV 55 Above ambient of 50 C 60 ]

Temp. rise oil C Phase Frequency Connection symbol

50 [ 3

Above ambient of 50 C

]

50Hz YNa0d11

Insulation level:

H. V. L. V. I. V.

-

LI950 AC395-AC38 LI170 AC70 LI550-AC230-AC38

Core & winding (Kg.) Weight of oil (Kg.) Total weight (Kg.) Oil quantity (liters) Transport weight (Kg. ) Untanking weight (Kg.)

54000 39410 127995 45300 69000 54000

Specification of 315 MVA 400/220 KV 3- auto transformer: Types of cooling Rating of H.V. & I.V.(MVA) Rating of L.V. (MVA) Line current H.V.(Amps) Line current I.V. (Amps) ONAN 189 105 272.76 495.96 ONAF 252 105 363.68 661.28 OFAF 315 105 454.6 826.6

Line current L.V. (Amps)

837.0

1857.0

1837.0

No load voltage H.V. No load voltage I.V. No load voltage L.V. Temp. Rise winding C 55 [

400KV 220KV 33KV 55 Above ambient of 50 C 60 ]

Temp. rise oil C Phase Frequency Connection symbol

50 [ 3

Above ambient of 50 C

]

50Hz YNa0d11

Insulation level:

H. V. L. V. I. V.

-

LI950 AC395-AC38 LI170 AC70 LI550-AC230-AC38

Oil quantity (liters)

:

84550 liter

Impedance volt 315 MVA Base

H.V. position 9/L.V. H.V. position 9/I.V. I.V./L.V.

71.81% 11.47% 67.92%

Vector group: 1U

N3U

2U

3V 2W 1W 1V 3W 2V

Isolators:In electrical engineering, a disconnecter or isolator switch is used to make sure that an electrical circuit can be completely de-energized for service or maintenance. Such switches are often found in electrical distribution and industrial applications where machinery must have its source of driving power removed for adjustment or repair. High-voltage isolation switches are used in electrical substations to allow isolation of apparatus such as circuit breakers and transformers, and transmission lines, for maintenance.

In the substation following type isolators are used for the protection:

Horizontal break center rotating double break isolator:

This type of construction has three insulator stacks per pole. The two one each side is fixed and one at the center is rotating type. The central insulator stack can swing about its vertical axis through about 900C. The fixed contacts are provided on the top of each of the insulator stacks on the side. The contact bar is fixed horizontally on the central insulator stack. In closed position, the contact shaft connects the two fixed contacts. While opening, the central stack rotates through 900C, and the contact shaft swings horizontally giving a double break.

The isolators are mounted on a galvanized rolled steel frame. The three poles are interlocked by means of steel shaft. A common operating mechanism is provided for all the three poles. One pole of a triple pole isolator is closed position.

Pantograph isolator:

illustrates the construction of a typical pantograph isolator. While closing, the linkages of pantograph are brought nearer by rotating the insulator column. In closed position the upper two arms of the pantograph close on the overhead station bus bar giving a grip. The current is carried by the upper bus bar to the lower bus bar through the conducting arms of the pantograph. While opening, the rotating insulator column is rotated about its axis. Thereby the pantograph blades

collapse in vertical plane and vertical isolation is obtained between the line terminal and pantograph upper terminal.

Pantograph isolators cover less floor area. Each pole can be located at a suitable point and the three poles need not be in one line, can be located in a line at desired angle with the bus axis.

Pantograph isolator

Isolator with earth switches (ES): The instrument current transformer (CT) steps down the current of a circuit to a lower value and is used in the same types of equipment as a potential transformer. This is done by constructing the secondary coil consisting of many turns of wire, around the primary coil, which contains only a few turns of wire. In this manner, measurements of high values of current can be obtained. A current transformer should always be short-circuited when not connected to an external load. Because the magnetic circuit of a current transformer is designed for low magnetizing current when under load, this large increase in magnetizing current will build up a large flux in the magnetic

circuit and cause the transformer to act as a step-up transformer, inducing an excessively high voltage in the secondary when under no load. The main use of using the earth switch (E/S) is to ground the extra voltage which may b dangerous for any of the instrument in the substation.

Circuit breaker:A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to

protect high voltage circuits feeding an entire city. Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs. The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting the circuit. Contacts are made of copper or copper alloys, silver alloys, and other materials. When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas or oil as the medium in which the arc forms. Different techniques are used to extinguish the arc including: Lengthening of the arc Intensive cooling (in jet chambers) Division into partial arcs Zero point quenching (Contacts open at the zero current time crossing of the AC waveform, effectively breaking no load current at the time of opening. The zero crossing occures at twice the line frequency i.e. 100 times per second for 50Hz ac and 120 times per second for 60Hz ac ) Connecting capacitors in parallel with contacts in DC circuits Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit.

Types of circuit breaker: Many different classifications of circuit breakers can be made, based on their features such as voltage class, construction type, interrupting type, and structural features. Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electrotechnical Commission(IEC).

High-voltage breakers are nearly always solenoid-operated, with current sensing protective relays operated through current transformers. In substations the protection relay scheme can be complex, protecting equipment and busses from various types of overload or ground/earth fault. High-voltage breakers are broadly classified by the medium used to extinguish the arc. Bulk oil Minimum oil Air blast Vacuum SF6 In unnao substation only SF6 circuit breaker is used. The breaker uses SF6 (Sulpher Hexa fluoride) gas for arc extinction purpose. This gas has excellent current interrupting and insulating properties, chemically, it is one of the most stable compound in the pure state and under normal condition it is physically inert, non-flammable, non toxic and odorless and there is no danger te personnel and fire hazard. It's density is about. 5 times that of air insulating strength is about 2-3 times that of air and exceeds that of oil at 3 Kg/Cm pressure. SF6 breaker called as maintenance free breaker, has simple construction with few moving parts: The fission products created during breaking and not fully recombined are, either precipitated as metallic fluoride or absorbed by a static filter which also absorbs the residual moisture. Since no gas is exhausted from the breaker and very little compressed air is required for operation, noise during the operation is also very Jess. Since SF6 gas is inert and stable at normal temperature, contacts do not settler from oxidization or other chemical reactions, whereas in air or oil type breakers oxidation of contacts would cause high temperature rise. SF6 gas circuit breakers, designed to conform to the same standards as air or oil breakers, but in operation it is possible to get better service even at higher fault levels. Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur in the fluorine gas, in a steel box, provided with staggered horizontal shelves, each bearing about 4 kg of sulphur. The steel box is made gas tight. The gas thus obtained contains other fluorides such as S2F10, SF4 and must be purified further SF6 gas generally supplier by chemical firms. The cost of gas is low if

manufactured

in

large

scale.

During the arcing period SF6 gas is blown axially along the arc. The gas removes the heat from the arc by axial convection and radial dissipation. As a result, the arc diameter reduces during the decreasing mode of the current wave. The diameter becomes small during the current zero and the arc is extinguished. Due to its electro negativity, and low arc time constant, the SF6 gas regains its dielectric strength rapidly after the current zero, the rate of rise of dielectric strength is very high and the time constant is very small.

Fig: SF6 circuit breaker. Gas circuit breaker: high voltage side Type 220-SFM-20B Voltage rating: 220kv Rated lightening impulse withstand voltage: 1050 kVp

Rated short circuit breaker current: 40 kV Rated operating pressure: 16.5 kg/ cm2g First pole to clear factor 1.3 Rated duration of short circuit current is 40 kA for 30 sec. Rated ling charging breaker breaking current 125 Amp Rated voltage 245 kV Rated frequency 50 Hz Rated normal current 1600 Amp Rated closing voltage: 220 V dc Rated opening voltage 220 V dc Main parts: (a) Power circuit (b) Control circuit

Gas circuit breaker: low voltage side Type 120-SFM-32A Voltage rating: 220kv Rated lightening impulse withstand voltage: 650 kVp Rated short circuit breaker current: 31.5 kV Rated operating pressure: 15.5 kg/ cm2g

Lightning arrester:High Voltage Power System experiences overvoltages that arise due to natural lightning or the inevitable switching operations. Under these overvoltage conditions, the insulation of the power system equipment are subjected to electrical stress which may lead to catastrophic failure. Broadly, three types of overvoltages occur in power systems: (i) temporary over-voltages,(ii) switching overvoltages and(iii) lightning overvoltages. The duration of these overvoltages vary in the ranges of microseconds to sec depending upon the type and nature of overvoltages. Hence, the power system calls for overvoltage protective devices to ensure the reliability.

Conventionally, the overvoltage protection is obtained by the use of lightning / surge arresters . Under normal operating voltages, the impedance of lightning arrester, placed in parallel to the equipment to be protected, is very high and allow the equipment to perform its respective function. Whenever the overvoltage appears across the terminals, the impedance of the arrester

collapses in such a way that the power system equipment would not experience the overvoltage. As soon as the overvoltage disappears, the arrester recovers its impedance back. Thus the arrester protects the equipment from overvoltages.

The technology of lightning arresters has undergone major transitions during this century. In the early part of the century, spark gaps were used to suppress these overvoltages. The silicon carbide gapped arresters replaced the spark gaps in 1930 and reigned supreme till 1970. During the mid 1970s, zinc oxide (ZnO) gapless arresters, possessing superior protection characteristics, replaced the silicon carbide gapped arresters. Usage of ZnO arresters have increased the reliability of power systems many fold.

Current transformer:Current Transformers (CTs) are instrument transformers that are used to supply a reduced value of current to meters, protective relays, and other instruments. CTs provide isolation from the high voltage primary, permit grounding of the secondary for safety, and step-down the magnitude of the measured current to a value that can be safely handled by the instruments. TECHNICAL SPECIFICATION FOR CURRENT TRANSFORMERS 1.0 GENERAL 1.1 This specification covers manufacture, test, & supply of LT Current transformers of class 0.5 accuracy. 1.2 The CTs shall be suitable for metering purpose. 2.0 TYPE: 2.1 The CTs shall be of ring type or window type (bar type or bus-bar type CTs shall not be accepted). 2.2 The secondary leads shall be terminated with Tinned Cooper rose contact terminals with arrangements for sealing purposes. 2.3 Polarity (both for primary and second leads) shall be marked. 2.4 The CTs shall be varnished, fiberglass tape insulated or cast resin, air-cooled type. Only super enameled electrolytic grade copper wires shall be used. 2.5 The CTs shall conform to IS 2705:Part-I & II/IEC:185 with latest amendments.

3.0 TECHNICAL DETAILS: 3.1 Technical details shall be as given below: 1. Class of Accuracy 2. Rated Burden 3. Power Frequency Withstand Voltage 4. Highest System Voltage 5. Nominal System Voltage 6. Frequency 7. Supply System 0.5 5.00 VA 3KV 433 V 400 V 50 Hz 3 Ph. Solidly grounded Neutral System

3.2 Transformation ratio shall be specified from the following standard ratings as per requirement : Ratio 50/5 150/5 300/5 400/5 1000/5

(Secondary with 1 A may be specified by the utility incase the same is desired.) 3.3 Bore diameter of the CT shall not be less than 40 mm. Ring type CTs shall have suitable clamp to fix the CT to panel Board, wherever required.

3.4 The limits of current error and phase angle displacement as per IS:2705 at several defined percentage of rated current are:

Accuracy % Ratio error at % of Phase displacement in minutes Class 0.5 rated current 520100 1.50.75 0.5 120 0.5 at % of rated current 520100 904530 120 30

Note : Current error and phase displacement at rated frequency is required to be as above when the secondary burden from 25% to 100% of the rated burden i.e. 50 V A. 3.5 Rated extended primary current shall be 120% of rated primary Current in accordance with IS:2705 Pt-II. 3.6 Rated ISF (Instrument Security Factor) shall be declared by the manufacturer & marked on the CT. 3.7 CTs shall be made with good engineering practices. Core winding shall evenly spread stress & avoid stress concentration at any one point. Cast resin CTs sha;; be processed by hot curing method under controlled vacuum conditions. 3.8 The base shall be of hot dip galvanized steel.

4.0 TESTS:

4.1. TYPE TESTS: 4.1.1 Copies of all type tests as per IS.2705 Part-I and II including short time current & temperature rise tests in NABL accredited laboratory shall be submitted and got approved before commencement of supply.

4.2 ROUTINE TESTS:

4.2.1 The supplier shall conduct all the routine tests such as Ratio test, phase angle error test for 0.5 accuracy class as per IS 2705 Part I & II.

4.3 Commissioning test : 4.3.1 In accordance with IS:2705, Power frequency test on primary winding shall be carried out after erection on site on sample basis.

5.0 Marking :

5.1 The CTs shall have marking and nameplate as per IS 2705 in addition to class of insulation & ISF. The markings shall be indellible. The nameplate shall be securely fixed to the body of the CT.

6.0 PACKING:

6.1 Each CT shall be securely packed so as to withstand rough handling during transit and storage.

7.0 QUALITY ASSURANCE PLAN:

7.1 The requirements of clause 29.0 of Section I of main specification for Energy Meters shall apply.

Wave trap

Current transformer

Current transformer rating table for all cores:

CTRatio Burd KneeMag. Class Sec.Purpose NaminenpointCurrentresistan gVAVoltageatce V(min.)Kpv mA(max )1CT 1N 500/1 300 40AT Vk/2 PS 5 REF

1CT 1U1 500/1 1CT 1V1 1CT 1W1 1CT 2U1 500/1 1CT 2V1 1CT 2W1 1CT 3U1 2000/1 1CT 3V1 1CT3W1

-

300

40AT Vk/2 PS

5

REF

-

300

40AT Vk/2 PS

5

REF

-

600

30AT Vk/2 PS

5

Differenti al

2CT 3U1 2000/1 2CT 3V1 2CT 3W1 3CT 3U1 2000/1 3CT 3V1 3CT 3W1 WCT 1U2

-

300

40AT Vk/2 PS

5

Spare

30VA

-

-

1.0

5

Metering

198/3.0 1.7VA -2.5

-

-

5

-

WTT+RT D

Capacitor voltage transformer:

In high and extra high voltage transmission systems, capacitor voltage transformers (CVTs) are used to provide potential outputs to metering instruments and protective relays. In addition, when equipped with carrier accessories, CVTs can be used for power line carrier (PLC) coupling.

A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down extra high voltage signals and provide low voltage signals either for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the voltage signal is split, an inductive element used to tune the device to the supply frequency and a transformer used to isolate and further step-down the voltage for the instrumentation or protective relay. The device has at least four terminals, a high-voltage terminal for connection to the high voltage signal, a ground terminal and at least one set of secondary terminals for connection to the instrumentation or protective relay. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of voltage transformers would be uneconomical. In practice the first capacitor, C1, is often replaced by a stack of capacitors connected in series. This results in a large voltage drop across the stack of capacitors that replaced the first capacitor and a comparatively small voltage drop across the second capacitor, C2, and hence the secondary terminals.

INSULATING SYSTEMS: The external insulation is provided by the porcelain housing and coordinated with the capacitor stack, consisting of virtually identical elements so that the axial voltage distribution from the line terminal to ground is essentially uniform. The capacitor elements have a mixed dielectric material consisting of alternating layers of polypropylene film and Kraft paper. The Kraft paper layers serve as a wicking agent to ensure homogenous synthetic oil impregnation. The electromagnetic unit (EMU) is housed in an oil-filled tank at the base of the capacitor stack. Mineral oil is employed as the insulating medium instead of air because of its superior insulating and heat transfer properties. The use of an oil-filled base tank removes the need for space heaters in the secondary terminal box as this area is warmed by heat transfer from the insulating oil. This results in a more reliable and cost effective design.

INSULATING OIL: We use insulating oils with excellent dielectric strength, aging, and gas absorbing properties. The synthetic oil used for the capacitor units possesses superior gas absorption properties resulting in exceptionally low partial discharge with high inception/extinction voltage ratings. The oil used for

the EMU is premium naphthenic mineral oil. The oil is filtered, vacuum dried and degassed within house processing. It contains no PCB.

CAPACITOR STACK: The capacitor stack is a voltage divider which provides a reduced voltage at the intermediate voltage bushing for a given voltage applied at the primary terminal. The capacitor stack is a multicapacitor-unit assembly. Each unit is housed in an individual insulator. A cast aluminum cover is on top of the upper capacitor assembly and is fitted with an aluminum terminal. An adapter for mounting a line trap on top of the CVT can be1 - Primary terminal 2 - Cast aluminum bellow housing 3 - Stainless steel expansion bellow 4 - Compression spring 5 - Insulated voltage connection 6 - Capacitor elements 7 - Insulator (porcelain or composite) 8 - Voltage divider tap connection 9 - Cast-epoxy bushing 10 - HF terminal connection 11 - Ferro-resonance suppression device 12 - Secondary terminals 13 - Oil level sight-glass 14 - Aluminum terminal box 15 - Intermediate transformer 16 - Oil/air block 17 - Oil sampling device 18 - Compensating reactor 19 - Aluminum cover plate

provided with an optional (and removable) HV terminal. The capacitor units are mechanically coupled together by means of stainless steel hardware passing through the corrosion resistant cast aluminum housing. The mechanical connection also establishes the electrical connection between capacitor units. This facilitates field assembly of the CVT

.

1. High Voltage terminal 2. Compensation reactor 3. Intermediate voltage transformer 4. Ground terminal 5. Ferro-resonance suppression device 6. Damping resistor 7. Carrier (HF) terminal (optional) 8. Overvoltage protective device 9. Secondary terminals 10. Link, to be opened for test purposes

PRINCIPLE CIRCUIT DIAGRAM

Wave trap:

Line trap also is known as Wave trap. What it does is trapping the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/teleportation panel in the substation control room (through coupling capacitor and LMU).

This is relevant in Power Line Carrier Communication (PLCC) systems for communication among various substations without dependence on the telecom company network. The signals are primarily teleportation signals and in addition, voice and data communication signals.

The Line trap offers high impedance to the high frequency communication signals thus obstructs the flow of these signals in to the substation busbars. If there were not to be there, then signal loss is more and communication will be ineffective/probably impossible.

Construction: 1. Main coil:

The main coil winding are encapsulated by winding continuous filament fiberglass That has been impregnated with a specially selected epoxy resin harden system. The epoxy resin fiberglass composite is then curved according to a programmed temperature Schedule. 2. Tuning pack: Tuning pack is connected in parallel with the main coil to provide a high impedance to the desired carrier frequency. 3. Lighting arresters: The line traps are protected by a lighting arrestors against high voltage surges caused by atmospheric effects or switching operations.

Protective relays:Protective relaying is one of the several features of power system design.

Every part of the power system is protected. The protective relaying is used to give an alarm or to cause prompt removal of any element of power system from service when hat element behave abnormally. The relays are compact and self contained devices which can sense abnormal conditions. Whenever abnormal condition occur , the relays contacts get closed. This in turn closes the trip circuit of a circuit breaker. For switchyard protections following type relays are used: 1. Overcurrent relay 2. Earth fault relay 3. REF relay 4. Differential relay 5. Directional relay 6. Over flux relay 7. Buchoolz relay 8. IDMT relay

Restricted earth fault protection relay:

RES E/F +O/L

The REF protection method is a type of "unit protection" applied to transformers or generators and is more sensitive than the method known as differential protection.

An REF relay works by measuring the actual current flowing to earth from the frame of the unit. If that current exceeds a certain preset maximum value of milliamps (mA) then the relay will trip to cut off the power supply to the unit.

Differential protection can also be used to protect the windings of a transformer by comparing the current in the power supply's neutral wire with the current in the phase wire. If the currents are equal then the differential protection relay will not operate. If there is a current imbalance then the differential protection relay operates. REF protection is applied on transformers in order to detect ground faults on a given winding more sensitively than differential protection.

Directional relay:Directional relays have protection zones that include all of the power system situated in only one direction from the relay location. (This is in contrast to magnitude relays which are not directional, i.e., they trip based simply on the magnitude of the relay. Consider the one-line diagram in Fig. 1.

Fig. 1 If the relays R1 and R2 in Fig. 1 are directional relays, then

-

R1 looks to the left but not to the right, and

Bus 1 y

R2 looks to the right but not to the left.

In order to understand how the directional relay works, first, consider that R2 measures the phasors V2 and I23. Now define the following parameters associated with Fig. 1: L23: length of circuit 2-3. x: distance from R2 to a fault on circuit 2-3.

x=x/L23: the fraction of the circuit length between the relay R2 and the fault at point x. I23: the current in circuit 2-3 resulting from the fault x on circuit 2-3 (a phasor). V2: the bus 2 voltage (a phasor). Z23: total series impedance of circuit 2-3. If a fault occurs on circuit 2-3, at point x, then the fraction of total circuit length is x. If the circuit has uniform impedance per unit length, then the impedance between the relay R2 and the fault point is xZ23, and with the bus 2 voltage being V2, the current flowing into circuit 2-3 from bus 2 is: I23

=

V2 Z x

23

(1)

But recall that for transmission lines, it is generally the case that R