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. CONTENTS .
I. AcknowledgementII. About NDPLIII. GRIDS
1) Power Transformers2) Circuit Breakers3) Current Transformers4) Potential Transformers5) Isolators6) Lightening Arresters7) Auto Reclosures8) Capacitor Bank9) Busbars10) Insulators11) Battery Bank And Charging System12) Control Panel13) Scada System14) Switch Gear
IV. TRANSFORMER-Installation, Testing & Maintenancea) Installation
Unloading & Handling Inspection on Receipt Storage Fittings & Accessories Installation
b) Pre Commissioning Checksc) Routine Tests
Insulation Resistance Test Winding Resistance Test Magnetic balance Test Magnetizing current test Voltage Ratio Test Load balance test Vector Diagram test Tan Delta testing
d) Periodic Inspectionf) Oil Parameterse) DGA
Rogers Ratio Method IEC 599 Method
g) Maintenance Preventive Maintenance Corrective Maintenance
f) Thermography
V. Analysis of working of a power Transformer a) Name plate Detail
b) Functional checksc) Routine Tests d) DGA e) Thermographyf) Conclusion & Recommendation
I take the auspicious opportunity to thank Mr. Pillai, under the aegis of whom, I undertook my summer training. I owe my most sincere gratitude to Mr. Dharmadhikari whose valuable and consistent guidance enabled me to complete this report. My warm thanks to Ms.Sameeksha Raina, Mr.Vinay K. and Mr. Anoop Kumar Roy for their encouragement and efforts throughout. Their kind support and patient guidance have been of great value in this study.
I am deeply grateful to NDPL which gave me a chance to explore and comprehend the practical way of working, apart from acquainting me with the importance and scope of electrical engineering.
With all the knowledge and skills I have acquired during this period, I hope that I will apply them in a positive way wherever and whenever possible.
I express my thanks to the whole NDPL family for their kind cooperation and congenial demeanor.
NDPL (North Delhi Power Limited) distributes electricity in the North of Delhi.The company is a joint venture of Tata Power and Govt. of NCT and came into existence as a result of the privatization of electricity in Delhi. Tata Power Company acquired 51% stake in NDPL and took control of the management effective from July 1st, 2002.
The NDPL area of distribution in Delhi has been organized into the following districts of Delhi:
NORTH AREA
City Circle encompassing Moti Nagar (MTN), KeshavPuram (KPM) and Pitampura (PRR) districts.Town Circle encompassing Civil Lines (CVL), Shakti Nagar (SKN) and Model Town (MDT) districts.
NORTH WEST AREA
Urban Circle encompassing Shalimarbagh (SMB) and Badli (BDL) districts.Metro Circle encompassing Rohini (RHN) and Mangolpuri (MGP) districts.Sub-Urban Circle encompassing Bawana (BWN) and Narela (NRL) districts.
At NDPL, the peak demand (17:00/23:00) is around 1055MW with a daily energy requirement of 22.4Mu.
NDPL has been the frontrunner in implementing power distribution reforms in the capital city and is acknowledged for its consumer friendly practices. Since privatization, the AT&C losses in NDPL areas have shown a record decline. On the power supply front too, NDPL areas have shown remarkable improvement. The company has embarked upon an ambitious plan to implement high-tech automated systems for its entire distribution network. Systems such as SCADA& GIS are the cornerstone of the company's distribution automation project. To fight the menace of power theft, modern techniques like High Voltage Distribution (HVDS) System and LT Arial Bunch Conductor have been adopted.
NDPL has to its credit several firsts in Delhi: SCADA controlled Grid Stations, Automatic Meter Reading, GSM based Street Lighting system, SMS based Fault Management System. To ensure complete transparency, the company has provided online information on billing and payment to all its 1 million consumers. This happened in the first year of operations itself. NDPL believes in providing more value than just electricity and is even rewarding its consumers for timely payment.
NDPL is the youngest company and the first power utility in India to receive the prestigious CII EXIM Award for 'Strong Commitment to Excel'. It is also the only distribution utility to receive the ISO 9001, ISO 14001 and OHSAS 18001 certification.
Grids Under NDPL
North
1 CIVIL LINES NEW 33
2 G.T.K. 33
3 GULABI BAGH 33
4 HUDSON LINE 33
5 INDER PURI 33
6 INDRA VIHAR 33
7 PUSA 33
8 PANDAV NAGAR 33
9 ROHTAK ROAD 33
10 SARASWATI GARDEN 33
11 SHAHZADA BAGH 33
12 SHAKTI NAGAR 33
13 TRPOLIA 33
14 SUDARSHAN PARK 33
15 RAMA ROAD 33
16 RIWARI LINE 66
17 WAZIRABAD 33
18 DIFR 33
19 PAYAL 33
North Central
1 ASHOK VIHAR 33
2 AZAD PUR 33
3 RAM PURA 33
4 RANI BAGH 33
5 TRI NAGAR 33
6 WAZIR PUR - I 33
7 WAZIR PUR - II 33
8 HAIDER PUR 33
9 SHALIMAR BAGH 33
10 MANGOL PURI - II 66
11 PITAM PURA - I 66
12 PITAM PURA - II 66
13 PITAM PURA - III 66
14 MANGOL PURI - I 66
15 ROHINI - I 66
16 ROHINI - II 66
17 ROHINI - III 66
18 ROHINI - IV 66
19 SAHLIMAR BAGH 33
20 FACILITY CENTRE 33
North West
1 A.I.R. KHAM PUR 33
2 S.G.T. NAGAR 33
3 BADLI 66
4 DSIDC NARELA 66
5 NARELA A-7 66
6 JAHANGIR PURI 66
7 POOTH KHURD 66
8 ROHINI - 23 66
9 ROHINI - V 66
10 ROHINI - VI 66
11 RG-22 66
12 Bawnan-6 66
13 Bawana Clear Water 66
14 DSIDC-II Narela 66
15 TIGGIPUR 66
16 Bawana 7 66
STUDY OF 33KV GRID SUBSTATION
What is a grid substation?
A grid is essentially an interconnected power system that connects neighboring power stations into a ‘power pool’. The assembly of apparatus used to change some characteristics of the electric supply (frequency, voltage etc.) is called substation.
Introduction:
In a 33 KV grid station, the incoming 33 KV lines are connected to the incoming buses through a line isolator, a current transformer and a bus isolator. The 33KV supply reaches the HV side of the power transformer through another bus, isolator, a circuit breaker, current transformer and a potential transformer.The power transformer steps down the voltage level to 11 KV for further distribution purpose. The LV side of the transformer is connected to the 11KV bus through a CT and a circuit breaker which are located indoor in a panel. The LV side of the transformer is connected to the panels using underground cables.Outgoing 11KV feeders are out of the 11KV buses each one connected to the bus through circuit breakers, CTs and isolators.The various incoming and outgoing voltages are measured using PTs.
At G.T. Karnal Road Grid sub station, the incoming feeders from Azadpur and Tripolia supply in power (33KV) via underground cables. The phase lines are taken out of each cable through a P.G.clamp and are fed to the HV side of power transformer, rated 20/25 MVA, through isolators, CTs, circuit breaker and busbar. The PTs and lightening arrestors are connected across the lines.
The stepped down voltage (11KV) from the LV side of the power transformers is connected to the switchgear room through underground cables. From the switchgear room, the power is fed into the various outgoing feeders-C.C Colony, Ashok Vihar, Model Town, Arya Bhatt Poly Tech., Satyawati College, FCI and Gurjwala town.
The d.c. power back up to the grid is supplied through a battery bank room. The control panel room enables monitoring and remote control of its operation. The grid is SCADA controlled substation.
GRID SUBSTATION
The following are the important components at a grid substation:
1. POWER TRANSFORMERS2. CIRCUIT BREAKERS3. CURRENT TRANSFORMERS4. POTENTIAL TRANSFORMERS5. ISOLATORS6. LIGHTENING ARRESTERS7. CAPACITOR BANK8. BUSBARS9. INSULATORS10. BATTERY BANK AND CHARGING SYSTEM11. CONTROL PANEL12. SCADA SYSTEM13. SWITCH GEAR
POWER TRANSFORMER
A power transformer in a grid substation steps down the incoming voltage (66KV or33KV) to 11(KV) for further distribution. It transfers electric energy via electromagnetic induction, from one circuit to another, at the same frequency, usually with changed values of current and voltage.Usually, the HV winding is delta connected while LV winding is star connected. Typical power ratings for power transformer are 12.5,16,20,25 or 50 MVA.
COMPONENTS OF POWER TRANSFORMER:
1. Main Tank It encloses the core and the windings. Structural steel is used for fabrication of main tank. Size of tank is dependent on its MVA rating.
2. Core The core is made from silicon steel or cold rolled grain oriented (C.R.G.O steel) in the form of laminations of 0.28-0.30mm in thickness with insulated coating. It may be shell type or core type. In core type, there is only one iron path and windings are wound on opposite limb. In shell type there are two parallel magnetic paths into which flux from the central limb can divide.
3. Windings Hard drawn Copper of high conductivity in the form of round conductors with paper/ enamel insulation is used.
4. Solid Insulation Paper, cotton, fiber based board (laminated board and wood/wood laminations) are used for inter turn and winding to earth insulation.
5. Insulating Oil Oil forms a significant part of the transformer. It acts as a coolant to dissipate the heat losses in the transformer and, also as an insulating medium. It is obtained by fractional distillation and subsequent distillation of crude petroleum.
6. Radiators These limit the temperature of oil and winding, by dissipating heat which is generated due to losses in the Transformer. The different types of cooling may be:
ONAN (oil natural air natural)ONAF (oil natural air forced)OFAF (oil forced air forced)OFWF (oil forced water forced)
7. Conservator This is a cylindrical vessel made of structured steel. It if used for accommodating the volumetric expansion of oil in the transformer due to increase in pressure. Due to its cylindrical shape, the area of interface of air and oil is reduced and hence reducing the oxidation of oil. Its capacity is approximately 5% of the tank capacity and is filled up to one-third of its volume.
8. OLTC On Load Tap Changer is used to regulate the incoming system voltage while the transformer is delivering the load. Tap changing is completed within 40-70 milliseconds. This high speed transfer is obtained by using a bank of energy storing springs. When energy is released, the tap change is complete. The tap changing is done on the HV side for the two reasons. Firstly, much lesser current flows on the HV side and secondly, because it is easier to draw out connections from the HV windings which are wound on the outer portion of the core.
9. Silica Gel Breathers In oil filled transformers, space is required for volumetric expansion of oil due to change in surrounding temperature and load. Hence oil always comes in contact with air. Silica gel breather is a dehydrator which prevents entry of moisture from air to insulating oil. Cobalt Chloride is used as Silica gel in the form of crystals.
10. Bushings It is an insulating structure which provides a central passage for a conductor. These are meant for bringing out the LV & HV side leads out of transformer tank and enable connection between transformer windings and phase lines.
Two types of bushings are mainly employed:
I. Porcelain Bushing II. Condenser Bushing
Generally, there are three bushings on the delta connected HV side while on the star connected LV side, three bushings are for the phase lines and one for neutral. Each bushing is provided with arcing horns for protection.
PROTECTIVE ELEMENTS:
11. Buchholz relay It is used in oil immersed transformers to detect the faults such as arcing, partial discharge, or local overheating, which normally results in generation of gas. The relay is mounted in the pipe connected between the tank and the conservator tank at an inclination of 3-7 to the horizontal to ensure that all the gases are directed to this relay housing and actuates an alarm circuit. In case of occurrence of incipient fault, bubbles if gases are evolved by the heat generated which rise up and the oil level drops, whereupon, the hollow float tilts, short circuiting the mercury switch and an alarm device operates.If however a serious fault has occurred, the gas generated is considerable which causes the gas surge flap valve to be deflected, thereby closing the mercury contact switch and energizing the trip coils of the circuit breaker.
12. Oil Surge Relay It gives trip command in case OLTC electric fault. Its functioning is same as of Buchholz relay. It is mounted in OLTC to OLTC conservator pipelines.
13. Marshalling Box It consists of WTI, OTI and in some transformers cooler fan control system.
14. OTI Oil Temperature Indicator works on the principle of liquid expansion with change in temperature and consequent change in volume of the liquid, which causes the expansion/contraction of the bellows and is transmitted through linkage mechanism to the indicating pointer and switching disc.
15. WTI Winding Temperature Indicator works by image process technique. An operating bellow is provided with an additional heating element. A CT mounted on the LV side of the tank feeds this heating current. The operating bellow thus gets an additional movement and indicates the temperature of the winding.
16. Oil Level Indicator Plain or prismatic glass is generally used as oil level indicator. In large transformers, magnetic oil gauge (MOG) is used. MOG works on bevel gear system. Oil in conservator lifts up float and float arm. This rotates the bevel gear system, which through proper magnetic linkage mechanism operates a mercury switch, wired to an alarm.
17. Pressure relief Valve PRV protects the transformer from excessive pressure which may occur due to internal fault or other reasons. If a short circuit occurs inside a transformer, the arc vaporizes the transformer oil and a heavy pressure is built. It is mounted on the top of the tank. It has stainless steel diaphragm which gets lifted under high pressure conditions, to reduce the pressure.
CURRENT TRANSFORMER
A current transformer is a device used to step down the current from high values to low values. It is essentially a step-up transformer, which steps down the current to a known ratio.
Its functions may be enumerated as:
Measure / monitor current. Differential Current Protection. Over Current Protection. Earth fault Protection.
PRINCIPLE OF OPERATION
As the primary no of turn is less then secondary so the voltage we get in the secondary is more than primary. Typical secondary current is 5A or 1A rms.In case of fault, current of considerable value flows through its secondary and energizes the relay. The plunger of the relay moves to close the contact of the trip circuit and the the circuit breaker operates.
TYPES OF CURRENT TRANSFORMER
Wound type: It has primary winding of more than 1 full turn on the core Bar type: Primary winding consists of a bar of suitable size.Dry type: It does not require any liquid or semi-liquid medium for cooling or insulating.Oil type: It requires oil for cooling and insulation.Live tank design: It has its secondary at the top and its housing at live potential.Dead tank design: It has its secondary at bottom and its housing at earth potential.
NAME PLATE OF A CURRENT TRANSFORMER
MAKE SKIPPERSERIAL NO 1637A/70RATIO 800-400/1-1-1RATED VOLTAGE 36 KVFREQUENCY 50 HZRATED SHORT TIME CURRENT
25 KA,3 SEC
INSULATION LEVEL 70KV/170KV
CLASS 0.5 - 5P20 - PSBURDEN 30VA - 30VA - NA
POTENTIAL TRANSFORMER
A Potential Transformer is a special type of transformer that allows meters to take readings from electrical service connections with higher voltage (potential) than the meter is normally capable of handling without at potential transformer. It is essentially a step down transformer.The rating of the PT used at the grid studied is 20/25MVA, 33KV.
ISOLATORS
Isolators are provided for isolating equipment from buses or live apparatus or for sectionalizing buses or circuits or for transfer of loads. These devices break the circuit under off-load conditions. The main purpose of isolator is to ensure protection of the maintenance person under any circumstance. The isolator is operated only when circuit Breaker connected to its line breaks.
TYPES OF ISOLATORS:
BASED ON THEIR MOUNTING1. Vertically mount2. Horizontally mount
BASED ON THE TYPE OF BREAKING1. Centre break
2. Two-way break
LIGHTENING ARRESTER
These are protective devices used to safely ground any surge in line e.g. due to lightening. These conduct the HV surges on the power system to the ground. Typically, they are connected in parallel with the equipment to be protected; between the phase and ground for three phase installation. It essentially consists of a spark gap in series with a non-linear arrester. Its one end is connected to the equipment to be protected and the other to the ground. The length of gap is so set that normal voltages not enough to cause an arc across the gap but a dangerously high voltage will break down the insulation and form an arc. The property of non-linear resistor is that its resistance decreases as the voltage or current increases and vice-versa.
LIGHTENING ARRESTORS
Operation: Under normal operation, the LA is off i.e. , the line does not conducts On occurrence of over voltage, air insulation across the gap breaks down and arc is formed,
providing low resistance path to the surge to ground. Thus excess charge on the line is conducted through LA to the ground.
CIRCUIT BREAKER
A Circuit Breaker is required to isolate the faulty section of the power system in case of abnormal conditions. A CB has two contacts: a fixed contact and a moving contact.
Basic Operation: Under normal conditions, these two contacts remain in closed position. When the CB is required to isolate the faulty section, the moving contact moves to interrupt the
circuit. On separation of the contacts, the flow of the current is interrupted resulting in formation of arc
between the contacts. The contacts are placed in close chamber containing some medium which extinguishes the arc.
Classification:1) Air Blast Circuit Breaker2) Minimum Oil Circuit Breaker3) Bulk oil Circuit Breaker4) Vacuum Circuit Breaker5) SF6 Circuit Breaker
TERMINOLOGIES IN CIRCUIT BREAKERS
Recover Voltage: Voltage that appears across the terminals of each pole of the CB immediately after breaking of the circuit is known as recover Voltage.Rate of Rise of Recovery Voltage: It is the rate at which recover voltage rises. It is expressed in Volts per microseconds.Breaking Current:
Symmetrical breaking current: It is the rms value of a.c. component of current at the instant of contact separationAsymmetrical breaking current: It is expressed as the rms value of the total current (ac + dc) present at pole at the instant of contact separation.
Breaking Capacity:Symmetrical breaking Capacity: The value of the Symmetrical breaking current that the CB is capable of breaking at the stated recovery voltage frequency and under prescribed conditions.Asymmetrical breaking Capacity: The value of the Symmetrical breaking current that the CB is capable of breaking at the stated recovery voltage frequency and under prescribed conditions.
Making Capacity:It is the peak value of the maximum current in the first cycle of current after the circuit is closed by the circuit breaker.
VACUUM CIRCUIT BREAKER
A vacuum circuit breaker utilizes a vacuum to extinguish arcing when the circuit breaker is opened and to act as a dielectric to insulate the contacts after the arc is interrupted.High vacuum has two outstanding properties:
The highest insulating strength known. When an AC circuit is opened by the separation of contacts in a vacuum, interruption
occurs at the first current zero, with a dielectric strength across the contacts building up at a rate thousands times higher than that obtained with conventional CBs. The contacts are of butt joints.
A high vacuum gap recovers most of its dielectric strength within 10μs..
These properties make the VCBs more efficient, less bulky and cheaper.
Vacuum Circuit Breaker
SF6 BREAKERS
Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as sulfur hexafluoride (SF6), having excellent dielectric and arc quenching properties. After contact separation, current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity.
SF6 CIRCUIT BREAKER
The SF6 Gas circuit Breakers are extremely simple and compact in their design owing to the fact that no gas blast valves are required and the force of contact separation simultaneously drives the puffer piston which builds up gas pressure according to the contact stroke. It consists of male-female contact joint comprising two coaxially aligned electrode sections.The gas breaker assures a high level of performance required for reliable operation of electrical system. Its reliability increases further by the use of SF6 gas insulating system (GIS) and a single pressure dual flow gas puffer interrupter.
NAME PLATE SPECIFICATION OF A SF 6 USED IN 33KV GRID SUBSTATION
Circuit breaker type ABBNo. EDF SK1-1Order 312331 Rated Voltage 72.5KV Power Freq. withstand voltage 140KVLightening imp. with voltage 325KVSwitching imp. with. Voltage - Rated frequency 50Hz Rated normal current 1600AGas Pressure SF6 abs (+20ºC)Maximum working pressure 9barFilling 7barSignal 6.2barBlocking 6barVolume/pole 18Operating Device Type FSA-1(T)Rated breaking current 25KADC component 40%Rated making current 62.5KAFirst pole to clear factor 1.5Rated short time current 25KADuty cycle O-0.3 sec-CO-3 min- CO/CO-15 sec-COLine charging breaking factor 10AMass total 1053KgMass of gas 25KgTemperature Class -30CYr. of Manufacture 2006Inst. Manual no. 1HYB 80001-25
CAPACITOR BANK
A capacitor bank is used to improve the power factor at which the power is supplied. The power factor of a system is given as:
P.F. = Useful power/apparent power=KW/KVAThey improve the voltage regulation and/or restore it to an acceptable level for a given load.
PRINCIPLE OF WORKING
In case of induction motors, magnetizing of the core first occurs in each half cycle of the ac supply, and then the power is imparted. The magnetizing power is then after returned to the generator. This magnetizing Reversal Energy is not actually doing any job in the motor, but is still absolutely essential in the working of the motor. This is called “Reactive Power”.
Capacitor Bank
If suitable capacitors are connected at the terminals of the motor, the Magnetic reversal energy, which would otherwise been pushed back into the system, could e stored and released i.e. supplied to the load for magnetizing purpose when needed in the other parts of the electrical cycle. The advantage here being that he capacitor eliminates the need for feeding reactive energy along power lines and reduce the burden. In other words, the capacitor behaves as ‘local generator of the reactive power’.
BUSBARS
An ideal busbar is characterized by zero impedance .The materials normally used for bus bars and connectors are copper and aluminum. Maximum current density in copper bus bars and connections in direct contact with air should be such that as not to exceed the following operational temperature:
Maximum permissible temperature: 70ºCMaximum hot spot temperature: 75ºCCurrent Density generally adopted:Indoor or enclosed bus bars:-750 A/sq. inch (116 A/sq. mm)Outdoor bus bars:-1200-1400 A/sq. inch (186-216 A/sq. mm)
For Aluminum bus bars the current density of 61% of that of copper are usually permissible. Maximum permissible current density under short circuit conditions should not normally exceed 18600 A/sq. cm.
BUS BAR CONFIGURATIONS:
SINGLE BUSBAR:Suitable for smaller installations, a bus sectionalizer allows the station to be split into two separate parts and the parts to be disconnected for the maintenance purpose.
DOUBLE BUSBAR:Provides for possibility of cleaning and maintenance without interrupting supply. Possibility of separate operation of station sections exists. Busbar sectionalizing increases operation flexibility.
MAIN & TRANSFER BUS:Duplicate busbar with one spare breaker allows considerable flexibility for maintenance.
BUS COUPLER
This is a section comprising of circuit breaker, CTs and isolators made to connect the two buses or bus sections in case of failure in any incomer. This ensures continuity in supply from both the buses.
BATTERY ROOM
The battery room consists of 24 batteries, each of 2V, connected in series, giving the supply of 48V.It is meant for power back-up in case of supply failure. The following points should be kept in mind for safety reasons:
1. Room should be well ventilated i.e. provided with an exhaust fan.2. The flooring should be resistant to acidic corrosion.3. The room should be free from birds and rodents.4. The specific gravity of the lead acid should have an optimum value of 1290 m/s2.
The bank is supplied with two chargers. The boost charger is for quick charging at constant current. The float charger is for steady charging at constant voltage.
BATTERY ROOM BATTERY CHARGER
SCADA (Supervisory Control And Data Acquisition)
SCADA is used to monitor, control and alarm plant or regional operating system from a central location. It essentially has three main elements:
1. RTUs (Remote Telemetry Units)2. Communication3. HMI (Human Machine Interface)
It significantly reduces the operating labor cost, while at the same time, it actually improves plant or regional system performance and reliability.
SCADA ROOM
CONTROL PANEL ROOM
It is used for remote control and monitoring of the various equipments in the grid yard. The L/R switch enables selection of local or remote control operation. TNC switch may be set in either Tripped, Neutral or Closed condition of the circuit breaker. Each relay has a graphic display screen, a trip circuit supervisor and a dc fail supervisor. For each transformer, the CT brings in current that may be monitored and is fed into a numeric relay 50/51/50n/51n that works as instantaneous, inverse over-current, earth fault instantaneous and earth fault overcurrent relay. A differential protection relay is meant to detect any unbalance in the current in the HV and LV windings. In case of operation of any of the relays, the Master relay, 86 is operational. The BCU switch enables the interconnection of the grid with the SCADA control room. All the equipments are operated at 50V dc.
CONTROL PANEL
SWITCH GEAR ROOM
The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream.
The 11KV voltage supply through the underground cables is headed to the switchgear room and is fed to the various outgoing feeders through circuit breakers, bus coupler, bus riser etc.
SWITCH GEAR ROOM
Based on the size & input voltage rating, transformers may be classified into two categories:
1. Power Transformers
2. Distribution Transformers
Distribution Transformer Power Transformer
Both power and distribution transformers are used for T&D applications (transmission & distribution). The difference between power and distribution transformers refers to:
Distribution transformers vary between 25 kVA and 10 MVA, with input voltage between 1 and 36 kV.
Power transformers are typically units from 5 to 500 MVA, with input voltage above 36 kV.
NAMEPLATE DETAILS OF A POWER TRANSFORMER
Make Crompton Greaves LimitedMVA 16/20 KV (No load) 66-HV 11-LVAmperes 140/175-HV 839.7/1049.7-LVPhases 3Frequency 50 HzMakers W.O. No. T 8251Makers serial No. 8251/2Diagram DRG No. T 62B1511HYear of manufacture 1990Type of cooling ONAN/ONAFRating MVA 16 20Guaranteed temp. rise Oil 50ºC Winding 55CConnection symbol Dyn11 Untanking Mass 21000 KgTotal oil 9800/11200 Kg/litreTotal mass 42000 kgHeaviest package Without oil- 26180kg With oil-3330kg
OLTC position
No.
HV volts HV amperes
In each phase OLTC connection
Impedance voltage at 75ºC on 20 MVA base
1 69300 166.6 20-19 14.44%5 66000 175 20-15 14.56%
The transformers are dispatched and full assembled, oil filled condition and ready for erection, testing and commissioning.
UNLOADING &
HANDLING
Transformer should be unloaded by means of crane, or lifting device of sufficient capacity.
All lifting lugs should be used to avoid unbalanced and uneven stress.
While lifting, slings should not rest/touch any other parts of the transformer especially for bushings.
INSPECTION ON RECEIPT
AT SITE
Immediately after received at site, transformers must be physically verified with a packing slip and drawings and closely inspected for any transit damage and welding, gasket leakages.
Transformers which are dispatched with oil filled in them; oil level must be checked for any pilferage or leakage during transportation. Oil level is indicated by the “oil level gauge” provided on the oil filling pipe.
If any damages, shortages and leakages are observed, then open delivery certificate from the carrier is to be obtained, for others enclosed format to be filled and reported to the firm within 15 days of receipt to enable the firm to lodge insurance claim if applicable and to attend the complaint.
STORAGE
On receipt of the transformer at site, it is desirable to install and commission the transformer with minimum delay.
Transformers should be kept over wooden support to avoid rusting.
It is desirable to keep the transformer in a place where temperature variation is minimum or energized even at a low voltage so that the oil temperature is about 10º-15ºC higher than the surrounding ambient temperature.
STEPS POINTS TO BE KEPT IN MIND
FITTINGS & ACCESSORIES
Rating & Terminal marking plate : The transformer is supplied with rating and terminal marking of plate made out of noon corrosive metal. The plate contains information concerning the rating, voltage ratio, weights, oil quantity, vector group, etc. the plate also includes unit Sr. No. and Year of Manufacturing. Earthing Terminals: The core laminations assembly is connected to core clamping frame which is in turn connected to the tank. Two earthing terminals are provided on the transformer tank. The earthing terminals should be connected to the earth.
Lifting lugsTwo lifting lugs of adequate capacity arc provided on top cover to lift fully assembled transformer filled with oil.
ValvesEvery transformer is provided with drain cum filter valve at bottom of the tank, and valves are fitted with blanking plates with locking and sealing to stop oil coming out.
BUSHINGS: Oil Communicating Type: Transformer windings are connected to the external circuit through bushings. The bushings are installed on the cover of the transformer tank and hence protrude into the tank. These ends are provided with suitable fasteners to connect the line leads inside the transformer and external conductors outside it. Tank with Corrugated walls
1. The corrugated walls panels are manufactured from CRCA sheet in thickness ranging between 1.2mm and 1.5mm.
2. The role of the corrugated panels is to increase the surface area of the tank which is in contact with the cooling air.
3. As the panels are welded on transformer tank, keen attention is required during unloading, handling and erection to protect from physical damages.
Pressure Relief ValvePRV is provided on the transformer tank. In case of heavy fault in transformer, this valve will operate and release pressure.
INSTALLATION
Location:The transformer should be installed in a well ventilated place, free from excessive dust, corrosive fuels etc. Adequate ventilation is necessary for tank and corrugation so that they can dissipate heat,. There should be clear place of about 1.25m on all sides of the transformer if it is enclosed in a room.
Foundation:Foundation should be firm, horizontal and dry.
PARTS OF TRANSFORMERS BEING INSTALLEDAT NARELA 66/11kV GRID
Bushings Transformer Testing
Radiators Marshalling Box
Cooling Fans
OLTC
Prior to energizing the transformer, several pre-commissioning checks are made to ensure that transformer has not suffered damage during transit, for maintenance purpose and, in future comparison if the transformer gets mal-functioning.
Check the oil level in the oil filling pipe gauge.Check for oil leakage in all gasketed joints and any crack in bushings.Check the drain value for closing.Check the transformer for proper and neutral earthing.Check the external electrical connections.Check the HRC fuse rating.Check for any tools lying above the transformer.Check the protection circuits.If any painting peel off, touching up can be done after erection.
Pre-Commissioning Check Points Records
1. Oil level in oil filling pipe.2. There is no oil leakage from anywhere.3. Tank earthing is done.4. Neutral earthing is duly done.5. External connection is duly tightened.
The routine tests are performed as a part of: Pre-commissioning test, during installation.Commissioning test, at regular interval.
Insulation Resistance Test
It is done to measure the reliability of the transformer insulation. The test reveals the condition if insulation (i.e. degree of dryness of insulation paper), presence of any foreign containments/moisture in oil and also any gross defect in the transformer.It is measured as a function of leakage current, which passes through the volume of insulation or external surface under a constant DC voltage.
Procedure:
The circuit arrangement is done as shown in the figure. A Megger is used (either hand driven or motored) of suitable voltage rating. IR measurements are done between H.V winding to L.V winding, H.V winding to earth,
L.V. winding to earth.
Acceptance:
IR values are checked with the values in the manufacture’s test certificate and these values. With every 10ºC drop in temperature, IR values increase by approximately 1.5 times.
Rated Voltage Insulation resistance at 20ºC
Up to 1.1KV 200 MEG.OHMS (MIN).1.1 to 3.3KV 250 MEG.OHMS(MIN).3.3 to 6.6KV 400 MEG.OHMS(MIN).6.6 to 11KV 450 MEG.OHMS(MIN).
Precautions:
It is ensured that the test specimen is discharged by short-circuiting for a period at least four times as long as the test voltage was applied.
In case of hand driven megger, it is rotated at constant speed for 60 seconds. Transformer tank is properly grounded.
Winding Resistance Test
These are measured at site in order to check for any abnormalities due to loose connections, broken strands and high contact resistance in tap changers.
The resistance is usually measured either with the bridge techniques, (Kelvin Bridge or Wheat Stone Bridge), Voltmeter Ammeter Method or a micro Ohmmeter. The resistance is below 1 ohm.
Procedure:
For star connected winding with neutral brought out, the resistance is measured between the line and neutral terminal and average of these shall be the tested value.
For delta connected windings, such as tertiary winding of auto transformer, measurements is done between pairs of line terminals and resistance per winding shall be winding is calculated as:
Resistance per winding=1.5 × Measured Value. The winding temperature is noted while recording the resistance. The resistance at 75ºC is measured as: R 75=R1 (235+75)/ (235+t) for copper. R1 is the resistance measured at temperature t.
AcceptanceThe Measured values are compared with the factory value a deviation of 5% of the factory value is acceptable.
Precautions: Care is taken to minimize the self inducting effects. To reduce the high inductive effect, a sufficiently high current is used to saturate the core, but
should not exceed 15% if the rated current as it would cause heating and thereby change the resistance.
Care is taken for accurate measurement of the winding temperature.
Magnetic Balance Test
This test is carried only in three phase transformers to check the imbalance in the magnetic circuit.
Procedure:
The OLTC is kept at its nominal tap position. The transformer neutral is disconnected from ground. Single phase 230V supply from a 50Hz regulating ac power source is applied across one
phase of HV winding terminals and neutral (v1). The voltage is measured in other two HV terminals across neutral (v2 & v3) respectively. The test is repeated for each of the three phases.
Acceptance:
The acceptance criteria are usually when the supply is fed to outer (extreme) phase of a transformer, the voltage induced in the centre phase shall be 50-90% of the applied voltage.However, when the centre phase is excited, the voltage induced in the outer phases shall be 30-70% of the applied voltage.
Precautions:
None of the winding terminals is grounded. The test is performed before being subjected to direct voltage/current tests.
Magnetizing Current Test
The test is performed to locate defect in magnetic core structure, shifting of windings, failures in turn insulation or problems in tap changers.
Procedure:
The tap position is kept in the lowest position and LV terminals are opened. Single phase 415V supply is applied on HV terminals. The voltage and current value is measured for each phase on HV terminals. The above procedure is repeated for normal and highest tap position.
Acceptance:
Magnetizing current measured on HV side shall confirm to increasing trend between highest voltage tap positions to lowest voltage tap position for all phases. The set of reading for current measurement for each of the tap position should be equal. Unequal current shall indicate possible short circuits in the winding.
Precautions:
This test is done before DC measurement of winding resistance to reduce the effect of residual magnetism.
On HV, low range AC ammeter is used.
Voltage Ratio Test
The test is performed to determine the correctness of the turn ratio between the different windings on each tapping of the transformer.It is required to confirm constant output voltage on LV windings at all tap positions.
Procedure:
The OLTC is kept at nominal tap position and LV is kept open. 415 V supply is applied on HV terminals. The phase to phase voltage between the LV terminals is measured by means of switch
selectable voltmeters and current on each phase of LV terminals is also measured. The above steps are repeated for each tap position separately.
Acceptance:
The tolerance for turn ratio should be within 0.5% of the declared ratio in the name plate specification.
The measured voltage on LV phase shall confirm to increasing trend between highest voltage tap positions to lowest voltage tap position for all phases .
Precautions:
Suitable fuses or tripping arrangement to be provided on HV side before starting test. The voltage is applied only in the high voltage winding in order to avoid unsafe voltage.
Load Balance Test
The test is performed to confirm the balancing of currents in all three phases when transformer is fully loaded.
Procedure:
The circuit is arranged as shown in figure below. Single phase 415 V voltage is applied on HV winding. The values of voltage, current of HV, and current of LV windings are recorded
instantaneously. The test is performed for each phase of the transformer.
Acceptance:
Considering applied voltage constant on HV, the ammeter reading of all three phases should be within 2-5% variation.
Precautions:
The secondary terminals of all the CT’s are shorted before starting the test. Suitable value of cross section of copper cables is selected such that the current density
should not exceed 2.5 A/sq. mm.
VECTOR DIAGRAM TEST
The test is performed to verify the polarity and phase relationship of HV and LV connections.
Procedure: The terminals 1U, 2U are shorted. Three phase supply is applied on 1U, 1V and 1W. The voltage between the following terminals is measured and recorded:
1U-1V1V-1W1U-1w1V-2v1V-2w1W-2w1W-2u
Acceptance:
Condition to be satisfied is given by the vector diagram
Precautions:
Neutral terminal is maintained in floating condition. The three phase supply is connected to the HV windings only. 1U-2U terminals are shorted with insulated wire.
Tan Delta testing:
Dielectric Dissipation factor
The dielectric dissipation factor of oil is the ratio of the power dissipated in the oil in watts, to the product of the effective voltage and current in VA, when tested with a sinusoidal field under prescribed conditions. This is numerically equivalent to the cosine of the phase angle or sine of the loss angle. It is a dimensionless quantity.This angle varies according to the quality of insulation, moisture content, contamination, heterogeneity, ageing of the material etc. The ratio Ir/Ic is a measure of the dielectric loss in the insulation and is known as the dielectric loss angle or dissipation factor.
The watt loss =VI CosΦ=VI cos (90-δ)=VI sin δ
=VIc sin δ cos δ
= VIc tan δ=VωVCtan δ= V²ωC tan δ
Dielectric loss in insulating equipment is thus proportional to tan δ and to the square of voltage. Since the amount of resistance loss of the insulating material is very small, the angle δ is also very small and hence tan δ is approximately equal to cosΦ (power factor). TAN DELTA AND CAPACITANCE MEASUREMENT OF TRANSFORMER BUSHING
It is one of the most powerful tests to monitor the health of the transformer. It is an indication of the quality and soundness of the insulation in the bushing.
Procedure:
The capacitance multiplier dial is set to the short position and the capacitance measuring dials to their respective “O” position.
The interference suppressor switches is kept in OFF position. The ground terminal of the test is set to low impedance. The control unit is connected to the high voltage unit using shielded cables. The HV cable is connected to the top terminal of the bushing and the LV cable is connected to the
test tap (strip/central stud) to the C and tan-delta kit through a screen cable and measurements are recorded.
Acceptance:
Maximum values of tan- delta of class A insulation should be 0.007.For bushings, tan-delta value should not exceed 0.7% and the main capacitance value should be within 10% of the factory test values.
Precautions:
The operator must use all practical safety precautions to prevent contact with energized parts of the test equipment and related circuits.
Measurement should be made at low voltage usually below 10 KV. Porcelain of the bushings should be cleaned and dried and removal of any dirt /oil with clean dry
cloth should be ensured.
TAN DELTA AND CAPACITANCE MEASUREMENT OF WINDING INSULATION OF TRANSFORMER
This test is carried out to ascertain the general condition of the ground and inter winding insulation of transformer and reactors.
Procedure: Measurement is made between all windings connected together and the grounded tank. The test set is positioned at least 6 feet away from the test specimen to be tested. The capacitance multiplier dial is set to the short position and the capacitance measuring dials to
the respective “O” positions. The interference suppressor switches is kept in OFF position. The ground terminal of the test is set to low impedance. The control unit is connected to the high voltage unit using shielded cables. The HV cable is connected to the top terminal of the bushing and the LV cable is connected to the
test tap (strip/central stud) to the C and tan-delta kit through a screen cable and measurements are recorded
Acceptance:
Maximum values of dissipation factor of class “A” insulation should be 0.007Rate of change of tanδ and capacitance value should be within 10% of the factory test value.
Precautions:
The ground cables must be connected first and removed last. The test set should never be connected to the energized equipment. The high voltage flux should be kept free from moisture, dust during installation and operation. It was ensured that the test specimen is denergized and grounded before making any further
connections.
INSPECTION FREQUENCY
ITEMS TO BE INSPECTED
INSPECTION NOTES
ACTION, IF REQUIRED
Monthly Winding temperature Checking for reasonable temp. rise & no abnormal increase w.r.t earlier records.
Check for:.Correct indication by instruments Winding temperature under
no loads conditions. Dust & foreign particles on
the finned area of radiators
Monthly Oil temperature Checking for reasonable temp. rise & no abnormal increase w.r.t earlier records.
Check for: Correct indication by
instruments Winding temperature under
no loads conditions. Dust & foreign particles on
the finned area of radiators
Monthly Load & Type of cooling
Checking against rated values
Reduction of load in case oil/winding temperature reading is excessive.
Monthly Voltage & noise of main unit
Checking the tap position matches with voltage
Ensuring proper tap position to avoid core loss/noise due to over fluxing.
Monthly Oil level in Conservator
Checking against transformer oil temp.
If low, it is planned to top with filtered oil, and transformer is examined for leakages.
Monthly Dehydrating Breather Checking the colour of the active agent & oil level in oil cup.
If silica gel is pink, it is replaced/ reactivated.
Monthly Main unit radiators/Cooler bank/pipe connection
Checking leaks in pipes, main tank, etc
For oil leakages from gaskets, bolts are tightened.
Monthly Fans Checking for abnormal noise.
Ensure proper mounting.
Monthly All external connections
Checking visually that all connection is normal without any discoloration.
In case of any sign of heat, bolts are cleaned and tightened.
Quarterly Bushings Examination of cracks & dirt deposits.
Cleaning for dirt/ deposits. Replacement in case of cracks.
Quarterly Oil in transformer Reference to the maintenance table
Reference to the preventive maintenance.
Quarterly Dissolved gases in oil Reference to the maintenance table
Reference to the preventive maintenance
Yearly Insulation Resistance Comparison with value at time of commissioning.
Checking for the cleanliness and soundness of bushing. If very low, process needs to be planned.
Yearly Gasketed joints Checking for any leaks.
Bolts are tightened evenly to avoid uneven pressure.
Yearly Relays, alarm, their circuit etc.
Examination of the relays n alarm contact and their operation fuses.
Components are cleaned. Contacts & fuses replaced if necessary.
Yearly Control boxes, terminal box, cables
Checking the water tightness of terminal boxes and wiring connections.
If required, gaskets are replaced.
Yearly Temperature Indicator Pockets holding thermometer is checked
Oil to be replenished, if required.
Yearly Dial type oil gauge Checking pointer for freedom.
Adjust, if required.
PARAMETER DESCRIPTION LIMITSElectric Strength It is the voltage at which a breakdown
occurs between two electrodes when oil is subjected to an electric field under prescribed conditions.
The transformer oil should have high dielectric strength in order to minimize clearance between coils and from windings to tank.
The minimum electric strength of new oils should be 30kV (RMS) and after filteration60KV (RMS).
Water Content Water content in oil is to be measured periodically to access the quality of oil.
It is measured in ppm (parts per million)
The moisture content in new unfiltered oil should be less than 50ppm and after filter it should be less than 20ppm.
Specific Resistance It should be as high as possible. An increase in temperature reduces the specific resistance.
At 90ºC, 30 X 1012ohms cmAt 27ºC, 500 X10 12 ohms cm
Dielectric Distribution factor
Known as tan-delta, is a measure of imperfection of dielectric nature of oil.
It is the ratio of power dissipated in the oil to the product of the effective voltage and current in volt amperes when tested under prescribed conditions in a sinusoidal field.
For a perfect dielectric, when applied with a sinusoidal ac voltage, the current should lead the voltage by 90ºC
Acidity It is due to the acid products which are
formed by the oxidation of oil. This encourages deterioration of insulating paper and press board.
It is measured in mg of KOH required to neutralize the acids present in a gram of oil. Hence it is also known as neutralization number.
Total acidity, max-0.03mg KOH/g
PARAMETER DESCRIPTION LIMITSInterfacial Tension It is a force necessary to detach a
planar ring of platinum wire from the surface of the liquid of higher surface
The interfacial tension of the mineral insulating oil should be
tension i.e. upward from water oil surface. It is expressed in dynes/cm or milliN/m and is measured using a Pt tensiometer.
0.04.
Flash Point It is the temperature at which oil vapor ignites spontaneously. It characterizes the tendency of oil to evaporate. Lower the flash point, the greater is the vaporization of oil.
The minimum value of the flash point of the transformer oil should be 140ºC
Sludge It is a slow formation of solid hydrocarbons due to heating and oxidation. It is a poor conductor of heat and hence produces temperature gradient across winding insulation causing overheating of conductors. Its deposition in the oil duct blocks the free circulation of oil, thereby impairing cooling, the process of sludge formation continues till the transformer becomes unserviceable.
The lowest temperature of the mineral insulating oil is -10ºC
When a fault occurs in a very small area, or if the severity of the fault is less, the gases evolved due to decomposition of oil, get dissolved in oil. As the composition and quantity of the gases generated is dependent on type and severity of fault, regular monitoring of these dissolved gases reveals useful information about healthiness of a transformer.
The advantages of fault gas analysis are as follows:
Advance warning of developing faults. Determining the improper use of units. Status checks on new and repaired units. Convenient scheduling of repairs. Monitoring of units under overload.
DECOMPOSITION OF OIL
Small amounts of H2, CH4and CO are produced by normal aging.Thermal decomposition of oil impregnated cellulose produces CO, CO2, H2, CH4 and O2.Decomposition of cellulose insulation begins at only about 100ºC or less.Faults will produce internal “hot spots” of far higher temperatures than these, and the resultant gas shows up in DGA.Hydrogen and methane begin to from in small amounts around 150ºC.At about 250ºC, production of ethane (C2H6) starts.At about 350ºC, production of ethylene (C2H4) begins.At about 450ºC, hydrogen production exceeds all others until about 750ºC to 800ºC.Acetylene (C2H2) production starts between 500ºC and 700ºC.
FAULT GASES
The cause of fault gases can be divided into three categories; corona or partial discharge, pyrolysis or thermal heating, and arcing. The major fault gases can be categorized as follows by the type of material that is involved and the type of fault present:
FAULT TYPE GASES EVOLVED
1. CORONA a. Oil H2
b. Cellulose H2, CO, CO2
2. PYROLYSISa. Oil Low Temperature CH4 C2 H 6
High Temperature C2H4, H2 (CH4, C2 H 6)b. Cellulose Low Temperature CO2 (CO) High Temperature CO (CO2)3. ARCING H2, C2 H2 (CH4 C2 H 6,C2H4)
The solubility of the fault gases in mineral oil as well as their temperature dependence are also important factors for consideration in fault gas analysis.
SOLUBILTY OF GASES IN TRANSFORMER OIL.STATIC EQUILIBRIUM AT 760mm Hg & 25ºC.Hydrogen 7% by volumeNitrogen 8.6%Carbon monoxide 9%Oxygen 16%Methane 30%Carbon dioxide 120%Ethane 280%Ethylene 280%Acetylene 400%
FAULT GAS DETECTION
One of the methods of fault gas detection is dissolved gas analysis (DGA) technique.In this method a sample of the oil is taken from the unit and the dissolved gases are extracted. Then the extracted gases are separated, identified, and quantitatively determined. Since this method uses an oil sample it is applicable to all type units and like the gas blanket method it detects all the individual components.
The main advantage of the DGA technique is that it detects the gases in the oil phase giving the earliest possible detection of an incipient fault. This advantage alone outweighs any disadvantages of this technique.
If a transformer has been operating normally for some time and a DGA shows a sudden increase in the amount of gas, the first thing to do is take a second sample to verify there is a problem.. If the next DGA shows gases to be more in line with prior DGA’s, the earlier oil sample was contaminated, and there is no further cause for concern. If the second sample also shows increases in gases, the problem is real.
Two of the frequently used interpretation techniques of DGA are:
Rogers Ratio Method IEC 599 Method
ROGERS METHOD OF DGA
Rogers ratio method compares quantities of different key gases by dividing one into the other. This gives a ratio of the amount of one key gas to another. By looking at the Gas Generation Chart we can see that at certain temperatures, one gas will be generated more than another gas. Rogers’s ratio method uses four key gas ratios.
CH4/H2
C2H6/CH4
C2H4/ C2 H 6
C2H2/C2H4
These ratios and the resultant fault indications are based on large numbers of DGA’s and transformer failures and what was discovered after the failures.
Code Definition of Rogers Refined Ratio Method
Gas Ratio Range Code R1: CH4/H2 Not greater than 0.1
Between 0.1 and 1.0Between 1.0 and 3.0Not less than 3.0
5012
R4: C2H6/CH4 Less than 1.0Not less than 1.0
01
R5: C2H4/C2H6 Less than 1.0Between 1.0 and 3.0Not less than 3.0
012
R2: C2H2/C2H4 Less than 0.5Between 0.5 and 3.0Not less than 3.0
012
DIAGNOSIS BY RATIO
CODE0 0 0 0 Normal5 0 0 0 Partial discharge1,2 0 0 0 Slight over heating<150º C1,2 1 0 0 Slight over heating 150-200ºC 0 1 0 0 Slight over heating 200-300ºC 0 0 1 0 General conductor overheating 1 0 1 0 Winding circulating currents 1 0 2 0 Core & Tank circulating current 0 0 0 1 Flash over, no power flow through 0 0 1,2 1,2 Arc, with power flow through 0 0 2 2 Continuous sparking to floating potential 5 0 0 1,2 Partial discharge with tracking
CO2/CO>11 Higher than normal temp. in insulation
GUIDELINE BY ABSOLUTE VALUEGAS FORMULA NORMAL< <ABNORMAL> INTERPRETATION
Hydrogen H2 150 ppm 1000 ppm Corona, ArcingMethane CH4 25 80 SparkingEthane C2 H 6 10 35 Local over
heatingEthylene C2H4 20 150 Severe
overheatingAcetylene C2 H2 15 70 Arcing
Carbon Monoxide
CO 500 1000 Severe over heating
Carbon Dioxide
CO2 10000 15000 Severe over heating
Nitrogen N2 1 to 10% NA NAOxygen O2 0.2 to 3.5% NA NA
Total Combustibles
0.03% 0.5 % NA
Applicability:
Ratio methods are only valid if a significant amount of the gases used in the ratio is present. A good rule is: Never make a decision based only on a ratio if either of the two gases used in a ratio is less than 10 times the amount the gas chromatograph can detect.
DISSOLVED GAS ANALYSIS DETECTION LIMITS
GAS LIMITS Hydrogen (H2) 5 ppm Methane (CH4) 1 ppm Acetylene (C2 H2) 1 to 2 ppm Ethylene (C2H4) 1 ppm Ethane (C2 H 6) 1 ppm
Carbon monoxide (CO) & Carbon dioxide(CO2)
25 ppm
Oxygen(O2) & Nitrogen (N2) 50 ppm
To apply Ratio Methods, it helps to subtract gases that were present prior to sudden gas increases. This takes out gases that have been generated up to this point due to normal aging and from prior problems. This is especially true for ratios using H2 and the cellulose insulation gases CO & CO2. These are generated by normal ageing.
IEC599 METHOD
The following table shows the IEC-599 method of gas interpretation where in various limits of the ratio corresponding to normal ageing and to various types of fault from which diagnosis of the nature of a fault may be determined.
Code range of ratios
C2H2
C2H4
CH4
H2
C2H4 C2 H 6
Detection limits and 10x detection limits are shown belowC2H2 1 ppm 10 ppmC2H4 1 ppm 10 ppmCH4 1 ppm 10 ppmH2 5 ppm 50 ppm C2H6 1 ppm 10ppm
<0.1 0.1-1 1-3 >3
0 1 1 2
1 0 2 2
0 0 1 2
Case
Fault Type Problems Found
0 No Fault 0 0 0 Normal Aging 1
Low energy partial discharge
1 1 0
Electric discharge in bubbles caused by insulation voids or super gas saturation in oil or cavitations or high moisture in oil.
2
High energy partial discharge
1 1 0Same as above but leading to tracking or perforation of solid cellulose insulation by sparking, or arcing; this generally produces CO & C O2.
3
Low energy discharges, sparking, arcing
1-2 0 1-2Continuous sparking in oil between bad connections of different potential breakdown of oil dielectric between solid insulation materials.
4
High energy discharges, arcing
1 0 2
Discharges with power follow through; arcing breakdown of oil between windings or coils, or between coils and ground. Or load tap changer arcing across the contacts during switching with the oil leaking into the main tank.
5
Thermal fault temp. range 150ºC
0 0 1
Insulated conductor overheating; this generally produces CO and C O2 because this type of fault generally involves cellulose insulation.
6
Thermal fault temp. range 150-300ºC
0 2 0Spot overheating in the core due to flux concentrations. Items below are in order of increasing temperatures of hot spots. Small hot spots in core. Shorted laminations in core. Overheating of copper conductor from eddy currents. Bad connection on winding to incoming lead, or bad contacts on-load or no-load tap changer. Circulating currents in core; this could be an extra core ground, this could also mean stray flux in the tank
These problems may involve cellulose insulation which will produce CO & CO2
7
Thermal fault temp. range 300-700ºC
0 2 1
8
Thermal fault temp. range over 700ºC
0 2 2
Transformer maintenance is needed for:1. To ensure highest availability2. To ensure serviceability at all times3. To ensure safety of personnel and 4. To extend useful life.
Presently, the emphasis is given more towards predictive maintenance as the downtime of the power equipment is unaffordable.The term ‘preventive maintenance’ describes measures aimed at discovering potential faults or preventing the faults from developing for smooth operation of the equipment.
The maintenance activity consists of:
Regular Inspection Testing Reconditioning wherever necessary.
Recommendations for maintenance:
Transformer tank, cover and other parts should be inspected periodically for oil leakages, peeling of paint or rust formation.
The rusted portion should be properly cleaned and painted. Oil leakages should be immediately attended to. Clamping bolts on gasketed joints should be tightened properly and if necessary, gaskets to
be replaced.
The maintenance of transformer is categorized into following types:
1. Preventive Maintenance2. Corrective Maintenance
TRANSFORMER PARTS
MAINTENANCE REQUIRED
Conservator & Oil level indicator
Conservator: The inside of the conservator should be cleaned
periodically. Detachable end plate is provided to facilitate
cleaning on all power transformers.Oil level indicator: Oil indicator glass should be kept clean so that oil
level is clearly visible. Broken indictor glass should be replaced
immediately. Float should be checked to see that there is no oil in
the float.
Silica Gel breather
Whenever temperature and humidity changes are considerable and transformer is subjected to fluctuating loads, breather should be inspected frequently.
When the colour of the breather changes from dark blue to pale blue/pink, the breather of the silica gel should be reactivated.
Buchholz Relay It should be ensured that isolating valves of buchholz relay are kept fully open for unhindered oil flow.
Explosion Vent
If the diaphragm of the vent is broken, because of fault in the transformer, an inspection should be carried out to determine the nature and cause of the fault.
The level of the oil in the pockets holding the thermometer bulbs should be checked and the oil replenished, if required.
Dial glasses should be kept clean and if broken should be replaced immediately.
Temperature Indicator
Temperature Indicator found reading incorrectly should be calibrated with standard thermometer immersed in hot water bath.
Bushings
Porcelain insulators and connectors should be cleaned at convenient intervals and minutely examined for any cracks or defects.
Oil inside the oil communicating type bushings should be checked by unscrewing air release screws provided on bushing top.
External Connections
including Earthing
All connections should be tight If the connections appear corroded, unbolt the
connection and clean down to the bright metal with emery paper.
Gaskets
Check the transformer for leakages periodically The bolts should be tightened evenly around the joints
to avoid uneven pressure.
Rollers
They should be greased and rotated to see that they turn freely.
OLTC The temperature of the OLTC should not exceed the temperature of the main tank. Any increase in the temperature indicates internal problem.
FAULT PREPARATION REQUIRED
DISMANTLING PROCEDURE
SAFETY NOTES
REPAIR REAASEMBLY PROCESS
Abnormal indication of temperature at OTI/WTI
Replace with spare instrument & inform supplier for repair of the instrument.
Disconnect sensor from thermometer pockets, disconnect alarm/trip contacts & CT connection. Remove the instrument.
Short WTI CT terminals before disconnection.De-energize transformer.
Replace Fix the new instrument, insert the sensor in the pockets, and reconnect the trip contacts & CT terminals
Abnormal drop in oil level of main conservator
De-energize transformer. Connect oil filter machine to top filter valve of transformer
-
Test & ensure oil BDV of 60KV & water of 15ppm before filling.
Admit oil to normal level.
Air release from conservator process valves.
Low oil level in condenser bushing
De-energize transformer & open the top filling plug of the bushing .Prepare 1 adaptor between the filling plug & filter hose
-
Oil BDV should be of 60 KV moisture of 5ppm before filling.The plug should not be left open when not filling.
Admit oil to normal level
Close the filling plug.
De-hydrating breather active agent changes to pink colour.
Oven for heating the active agent to 60 ºC.
Dismantle the breather & take out the active agent
_Re-activate till blue colour appears.,
Refill in the breather
FAULT PREPARATION DISMANTLING SAFETY REPAIR REAASEMBLY
REQUIRED PROCEDURE NOTES PROCESS
Low oil in the de-hydrating breather
Transformer oil for filling
Unscrew the oil cup & take it out.
_Adjust oil level. Refix the oil. Cap.
Gaskets leak, not stopped by tightening.
Spare gasket.Unfasten, and dismantle using crane. Open inspection covers for access to disconnect electrical connections.
Foreign particles should not fall in the active part. Ensure minimum exposure.
Replace the gasket.
Fit the component back.
Low insulation resistance in the wiring of central panel.
Cables Dismantle the old cables.
_Replace the cable
_
Non –functional MOG, B Relay, OSR
Spare instruments. Oil filter machine and oil storage tank.
Drain oil to required level.
_ Replace
After refilling oil, air release.
Abnormal tan delta valve of bushings.
Spare Bushings. Disconnect the electrical connection referring the bushing manual.
Excess pull on internal connection should be avoided.
Replace the bushing
Refer the manual.
Low insulation resistance
High vacuum pump and filter machine.
Drain oil from main tank and evacuation to one Torr for 24 hrs.
Refer the manual
Fill fresh oil or right quality.
Complete oil filling & air release.
Thermography is the use of an infrared imaging and measurement camera to see and measure thermal energy emitted from an object.
Thermal, or infrared energy, is light that is not visible because its wavelength is too long to be detected by human eye. Infrared thermography cameras produce images of invisible infrared or heat radiation and provide precise non contact temperature measurement capabilities. Nearly everything gets hot before it fails, making infrared cameras extremely cost effective, valuable diagnostic tools in many diverse applications.
ELECTROMAGNETIC SPECTRUM
As seen from the electromagnetic spectrum, the short wavelength end of the infrared radiation lies at the limit of visual perception, in the deep red. At the long wavelength end, it merges with the microwave radio wavelengths in the millimeter range.
The infrared band is further subdivided into four smaller bands. They include:
The near infrared (0.75 to 3 μm).The middle infrared (3 to 6 μm).The far infrared (6 to 15 μm).The extreme infrared (15 to 100 μm).
THERMOGRAPHIC MEASUREMENT TECHNIQUES
An infrared camera measures and images the emitted infrared radiation from an object. The fact that radiation is a function of object surface temperature makes it possible for the camera to calculate and display the temperature.
However, the radiation measured by the camera does not only depend on the temperature of the object but is also a function of the emissivity. Radiation also originates from the surroundings and is reflected in the object. The radiation from the object and the reflected radiation will also be influenced by the absorption of the atmosphere.
To measure temperature accurately, it is therefore necessary to compensate for the effects of a no. of different radiation sources. This is done online automatically by the camera. The following object parameters must however be supplied by the camera:
The emissivity of the object The reflected temperature The distance between the object & the camera The relative humidity
EMISSIVITY
It is a measure of how much radiation is emitted from the object, compared to that from a perfect blackbody.
Normally, object materials and subject treatments exhibit emissivity ranging from approximately0.1 to 0.95. A highly polished (mirror) surface falls below 0.1, while an oxidized or painted surface has much higher emissivity. The emissivity of metals is low, while for non metals, with high emissivity, it tends to decrease with temperature.
Finding the emissivity of an object: Using a thermocouple
Select a reference point and measure its temperature using a thermocouple. Alter the emissivity until the temperature measured by the camera agrees with the thermocouple reading. This is the emissivity value of the reference object. However, the temperature of the reference object must not be too close to the ambient temperature.
Using a reference Emissivity. A tape or paint of a known emissivity should be put onto the object. Measure the temperature of the tape using the camera, setting emissivity to the correct value. Note the temperature. Alter emissivity, until the area with the unknown emissivity adjacent to the tape/paint has a same temperature reading. The emissivity value can now be read.
REFLECTED TEMPERTUREThis parameter is used to compensate for the radiation reflected in the object and the radiation emitted from the atmosphere between the camera and the object.
If the emissivity is low, the distance very long, and the object temperature relatively close to that of the ambient, it will be important to set and compensate for the ambient temperature correctly.
DISTANCEIt is the distance between the object and the front lens and the camera.This parameter is used to compensate for the fact that radiation is being absorbed between the object and the camera and the fact that transmittance drops with distance.
RELATIVE HUMIDITYThe camera can also compensate for the fact that transmittance is somewhat dependent on the relative humidity of the atmosphere. To do this, set the relative humidity to the correct value. For short distances and normal humidity, the relative humidity can normally be left at a default value of 50%.
THEORY OF THERMOGRAPHY
BLACKBODY RADIATION
A blackbody is defined as an object which absorbs all radiations that impinges on it at any wavelength.The construction of a blackbody source is, in principle, very simple. The radiation characteristics of an aperture in an isotherm cavity made of an opaque absorbing material represents almost exactly the properties of a blackbody. A practical application of the principle to the construction of a perfect absorber of radiation consists of a box that is light tight except for an aperture in one of the sides. Any radiation which then enters the hole is scattered and absorbed by repeated reflections so only an infinitesimal fraction can possibly escape. The blackness which is obtained at the aperture is nearly equal to a blackbody and almost perfect for all wavelengths.
If the temperature of blackbody radiation increases to more than 525ºC the source becomes to be visible so that it appears to the eye no longer black. This is the incipient red heat temperature of the radiator, which then becomes orange or yellow as the temperature increases further.
Planck’s Law
The spectral distribution of the radiation from a blackbody is given by the given formula, called as Planck’s formula
Wλb= 2πhc 3 x 10-6[Watt/m2μm] λ5(ehc/λkT-1)
where Wλb is blackbody spectral radiant emittance at λ wavelengthc is velocity of lighth is Planck’s constantk is Boltzmann’s constantT is absolute temperatureλ is wavelength
Wien’s Displacement Law
By differentiating Planck’s formula wrt to λ, and finding the maximum, we have:
λ max=2898[μm] TThis is Wien’s formula.This implies that colour variance from red to orange or yellow as the temperature of the thermal radiator increases.
Stephan-Boltzmann’s Law
The integration of Planck’s formula from λ=0 to λ =00 , we obtain the total radiant emittance (Wb) of a blackbody.
Wb=σT4 [Watt/m2]
This is Stephan Boltzmann formula which states that the total emissive power of a blackbody is proportional to the fourth power of its absolute temperature.
NON-BLACKBODY EMITTERS
There are three processes which can occur that prevent a real object from acting like a black body: a fraction of the incident radiation α may be absorbed, a fraction ρ may be reflected and fraction τ may be transmitted. All these factors are more or less wavelength dependent.The sum of these three factors adds up to the whole at any wavelength.αλ+ρλ+τλ=1 ………..(1)
For opaque material τλ =0.hence eqn (1) reduces toαλ+ρλ =1
Another factor, called the emissivity, is required the fraction ε of the radiant emittance of a black body produced by an object at a specific temperature.The spectral emissivity ελ is equal to the ratio of the spectral radiant power from an object to that from a black body at the same temperature and radiant.Mathematically, it is expressed as the ratio of spectral emittance of the object to that of a black body.ελ = Wλo
Wλb
There are three types of radiation source distinguished by the ways in which the spectral emittance of each varies with wavelength.
A black body, for which ελ = ε=1. A gray body, for which ελ = ε<1. A selective radiator, for which ε varies with wavelength..
TRANSFORMERRewari Line
20 MVA TR-3
Name Plate Details
Make: Bharat Bijlee LimitedRating (MVA): 20Sl No.: 3195/3Amp HV/LV: 175/1050Cooling: ONAFRatio (kV): 66/11% Imp: 13.19%Vector Group: Dyn11M.Year: 2005
FUNCTIONAL CHECKS
Alarm TripBuchholz OK OK
OSR NA OKPRV NA OKMOG OK NAOTI OK OKWTI OK OK
Insulation Resistance & PI measurement test:
Connection 15secs 60secs PIHV-Earth 526 M ohms 582 M ohms 1.11LV-Earth 749 M ohms 918 M ohms 1.23HV-LV 732 M ohms 985 M ohms 1.35
Turn ratio Test
TAP POSITION
RATIO
R Y B1 6.297 6.298 6.2972 6.223 6.221 6.2213 6.151 6.151 6.1514 6.077 6.076 6.0755 6.003 6.002 6.0016 5.925 5.926 6.9277 5.850 5.853 5.8518 5.775 5.775 5.5779 5.702 5.703 5.70310 5.627 5.626 5.62611 5.552 5.553 5.55212 5.478 5.477 5.47813 5.401 5.402 5.40314 5.375 5.328 5.32615 5.252 5.252 5.25316 5.117 5.178 5.17817 5.102 5.103 5.102
Winding resistance measurement Test
Tap Position
HV winding Resistance in M ohms LV winding Resistance in M ohmsRY YB BR r n y n b n
1 933 934 931 11.36 11.26 11.342 916 907 9123 911 897 904
4 897 887 8925 889 877 8816 880 865 8697 873 854 8588 862 845 8519 864 831 83710 841 821 82511 832 812 81412 817 800 80513 810 798 79314 807 788 78015 811 769 77116 792 764 76117 773 671 767
Magnetic Balance Test
Tap Position Voltage applied(Volts) Magnetizing
current (mA)HV UV VW WU Normal 240.2 210.7 28.8 10 Tap 123.5 239.9 116.2 7.98
35.2 205.7 242.8 10.52LV Un Vn Wn
241.5 212.7 28.25 135.6123.3 240.4 116.7 10330.52 210.3 240.1 136.7
Capacitance and Tanδ Measurement test
Switch position
Tanδ (%) Capacitance (F)
HV/LV + Gr 2.88 0.003163LV/HV + Gr 2.62 0.0068HV/LV(UST) 3.1 0.005708
CONCLUSION & RECOMMENDATIONS
The Power transformer Rewari Lines TR-3 has been satisfactory examined for all the functional checks.
The routine tests have been satisfactory performed and the results are acceptable. It has been observed that OLTC electric operation is getting stuck between taps due to over
travel and operation of limit switch is to be attended.
