TRANSFORMERS

What is a transformer? An electrical equipment used to transform ac voltages from one to another value Device which works on the principle of electro-magnetism

Transformer Widely used in power systems Possible to transmit power at an economical transmission voltage and Utilize power at an economic effective voltage

Why do we need transformers in an electrical system? Bulk of generating voltage limited to 15 ~ 25 kV, though power generated is in hundreds of MW Impractical to distribute power at generated voltage: High power loss in transmission/distribution Greater cross section of conductors Higher voltage drop in distribution system

Why do we need transformers in an electrical system? High transmission voltage required - achieved by using transformers Convert AC voltage of any value to any desired value - Use suitable turns ratio for transformer windings Applicable only for AC circuits, cannot be used for DC circuits

How can transformers regulate voltage in electrical power system? Maintaining constant voltage for equipment vital for many power consumers when supply is availed from utility grid - especially whose processes are critical Simple transformer law - Primary to secondary voltage ratio equal to its primary to secondary turns ratio and vice versa Change of turns-ratio accomplished by adding/ subtracting required number of turns to either primary or secondary winding

Transformers for Oil & Gas Applications Transformers for oil & gas applications may range between 250kVA and 40MVA May be air-cooled or water cooled with indoor and outdoor enclosure Generally fall under two categories of construction – Dry type and liquid immersed type Air-insulated and solid insulated constructions are included under the dry type

Transformers for Oil & Gas Applications

Transformers for Oil & Gas Applications Epoxy Cast dry type transformer This type does not use insulation papers in the windings Instead, pure epoxy resin reinforced with glass fiber rovings are wound directly with the wire Winding processes controlled by advanced electronics also ensure even distribution and high levels of precision

Transformers for Oil & Gas Applications Vacuum cast coil dry type transformer

Transformers for Oil & Gas Applications Maxi cast-resin transformer Combines advantages of liquid-filled & dry type transformers Safe, powerful, and environmentally safe alternative to liquid-filled transformers Siemens cast resin power dry-type transformer

Transformers for Oil & Gas Applications Liquid immersed transformers

Transformers for Oil & Gas Applications Small, medium and large power Transformers Small power transformers are distribution transformers with a range of 5 to 30 MVA, and a maximum service voltage of 145 kV. Medium power transformers with a power range between 40 and 250 MVA and a voltage over 72.5 kV are used as network and generator step-up transformers. Depending on onsite requirements, transformers in the power range above 200 MVA can be designed as multi-winding transformers or autotransformers, in 3-phase or 1-phase versions. Small Medium Large

Types of transformers Generator transformer Earthing transformer Station transformer Unit transformer

Typical scheme for Generator transformer

Generator Transformer Important link between generator in power generating station and transmission lines Normally step-up transformers Most present day power generators generate voltages between 10 kV to 30 kV Stator voltage kept as high as possible to keep current within manageable values Generators of around 150 MW generate power at 11 ~ 13.8 kV - Line currents around 9000 amperes at 0.85 power factor

Earthing Transformer

Scheme for Earthing Transformer 400 kV Generator Transformer Earthing Transformer 23.5 kV With a generator shut down, station supplies can be obtained from the generator transformer and unit transformers connected to the grid, therefore the 23.5 kV busbars must have a return path for any earth fault which may occur. 23.5 kV busbars on the transformer side of the generator load switch connected to an earthing transformer. Unit Transformer Generator 11 kV 11 kV

Role of transformers in transmission, distribution networks Reduce transmission, distribution losses Reduced size of transmission, distribution line conductors Reduced voltage drops in transmission, distribution

Transformers - Typical Scheme in a Plant 20

Transformer Theory

What is magnetism? Magnetism is a phenomena by which materials exert attractive or repulsive forces on other materials A property governed by the atomic characteristics of the substance

Electricity and magnetism Inter-related Relationship is often called electromagnetism Physics of electromagnetic field: a field which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles

Electricity & Magnetism Current flowing in an electric conductor produces magnetic field in its vicinity Magnetic field linked to the medium in the vicinity of current varies depending on material of medium Further classified as Magnetic and Non magnetic depending on ability to retain / allow higher magnitude of this magnetic field

Electro Magnetic Induction Similar to production of magnetic field by flow of external current, the same magnetic field also produces an electrical force when it is in vicinity of a good conductor which is not connected to any external source Called EMF (Electro motive force) induced in the conductor due to external magnetic field

Electro-magnetic induction Induced emf - Use magnetic field to produce electrical force when magnetic flux is in vicinity of good conductor Induced emf classified as: Dynamically induced emf Statically induced emf

Basic theory of transformer Statically induced emf - Emf induced in a stationary conductor due to a change in flux linkage arising from a change in the magnetic field around the conductor Dynamically induced emf - Emf induced in a moving conductor when the conductor is moved in a stationary magnetic field Statically induced emf is the basic principle used in a transformer

Transformer principle AC voltage applied between terminals A1 and A2 Voltage induced between terminals a1 and a2

Transformer principle Magnetic flux links primary and secondary windings

Transformer principle Applied voltage causes current to flow in first winding Current variation in coil creates varying magnetic flux in core material Varying magnetic flux in core induces voltage in second winding Load connected across terminals a1 and a2 causes current flow in load Voltage induced in secondary winding depends on its number of turns in relation to number of turns in primary winding T1 V1 I2 ------------- = ------------ = ------------ T2 V2 I1

Transformer Principle Primary current is small under open circuit conditions so that applied voltage is almost equal and opposite to the emf induced in P. Hence, Secondary output power is almost equal to the primary input power and can be expressed as V1 I1 Cos 1 = V2 I2 Cos 2

Transformer principle Secondary voltage can be reduced/ increased by changing turns ratio between primary and secondary winding Primary current increases/ decreases in accordance with secondary load current This balance of ampere-turns in primary and secondary windings of transformer holds the key for transformer action and design

Important points about transformers Used to transfer energy from one AC circuit to another Frequency remains same in both circuits No ideal transformer exists Also used in metering, protection applications - Current transformer (CT), potential transformers (PT) Used for isolation of two different circuits (isolation transformers) Transformer capacity expressed in VA (Volt amperes) Transformer polarity indicated by dots

Types of transformers Core Type transformers are the most common type employed in majority of transformers in the world Shell Types are mostly employed for furnace transformers Windings are normally made of electrolytic grade Copper and the metallic parts are basically made of steel

Types of transformers By type of construction: Core type: Windings surround a considerable part of the core Shell type: Core surrounds a considerable portion of the windings

Jeumont-Schneider Shell type Transformer

Types of transformers (By method of cooling) Oil filled self-cooled: Small and medium sized distribution transformers Oil filled water-cooled: High voltage transmission line outdoor transformers Air-Cooled type: Used for low ratings and can be Natural air circulation (AN) type Forced circulation (AF) type Dry type transformer

Types of transformers (By method of application) Power transformer: Large transformers used to change voltage levels and current levels as per requirement. Power transformers. Usually used in distribution or a transmission line Potential transformer: Precision voltage step-down transformers used along with low range voltmeters to measure high voltages Current transformer: Used for measurement of current where current carrying conductor is treated as primary transformer. Isolates instruments from high voltage line, as well as step down current in a known ratio Isolation transformer: Used to isolate two different circuits without changing voltage level or current level

Dry Type Transformers No oil in the transformer Air cooled and built for various insulation classes up to class H Insulation between High and Low voltage windings by FRP (Fire Retardant Paper) cylinders Epoxy, Cast Resin & VPI are the most common

Dry Type Transformers Normally indoor Type in suitable enclosures Limited to capacities up to 10 MVA and up to 36 KV class Lesser thermal efficiency and lesser over load capacities compared to oil filled types

Transformer construction and main parts

Transformer Parts Main parts of a transformer may be grouped as below: Transformer Core Transformer windings Winding Insulation Cooling medium Transformer Tank/Enclosure Transformer accessories for measurements and protection

Core Laminations Transformer cores are not made of solid steel but are normally built up in the form of laminations to reduce the eddy current losses Made in the form of E’s and I’s (Also T’s) to get the closed magnetic circuit of the core

Transformer Core Cold Rolled Grain Oriented (CRGO) steel Low reluctance, low loss flux path Low hysteresis loss Phosphate coated, lacquered laminations - Reduced eddy current loss Silicon steel stamping usually shaped like "E" and "I" Typical Transformer Stampings

Typical Lamination Sections Cruciform type

Core Laminations Circular cross section formed by combining rectangular laminations of different cross sections to form into an overall circular shape A maximum of 95% fill can be possible by planning the number of steps and choosing proper thickness of core laminations

Typical Windows Construction

Mitred Joints of Transformer Core Boltless construction normally employs mitred joints with overlaps of 450.

Comparison of Flux Orientations Flux direction follows orientation of grains more smoothly with least resistance in magnetic core having mitred joints. Reduces power required to pass magnetic flux in direction of grain orientation

Hysteresis Loop Magnetic Materials have characteristic called Hysteresis Ability to retain certain amount of flux (B) even if magnetizing force (H) is removed or made Zero Hysteresis loop is formed when H completes a full cycle

Three phase transformers Large scale power generation - Generally 3 phase Requires use of 3 phase step-up and step-down transformers 3 phase transformer - Combination of 3, single-phase transformers (three primary and three secondary windings mounted on a core having three legs) Commonly used configurations: 3 phase three wire (Delta) 3 phase four wire (Star)

Delta connection 3-phase windings connected end-to-end 120 degrees apart electrically Generally, delta 3-wire system used for unbalanced load systems 3-phase voltages remain constant regardless of load imbalance

Possible combinations of Star and Delta Primary in delta – secondary in delta Primary in delta – secondary in star Primary in star – secondary in star Primary in star – secondary in delta

Physical connection of Delta (D) - Star (Y) configuration

3 Phase, 4-wire star connections Allows minimum number of turns/ phase (phase voltage 1/3 of line voltage) Higher conductor cross section One end of each winding connected to common end (neutral point) Better to use a star-connected 4-wire source when feeding to star connected unbalanced load

Transformer Windings Winding materials Electrolytic grade Copper and Aluminium conductors are suitable in transformer windings but Copper is still the most preferred winding material in the transformer construction Brief comparison gives an indication for copper to be a preferred conductor in transformer windings Property COPPER ALUMINIUM Electrical conductivity at 20 deg C 100% 62% Weight at 20 deg C 33% Melting point 10830 C 6600 C Mechanical strength 2250 Kgf/cm2 915 Kgf/cm2 Thermal conductivity 0.941 cal/cm2 0.57 cal/cm2 Specific heat 0.003 cal/gm 0C 0.21 cal/gm 0C

Concentric windings

Design Requirements for Windings Design criteria for transformer windings Minimum resistance To withstand the normal currents and forces associated with short circuit conditions Good mechanical strength without becoming brittle under operating temperatures Good heat transfer capability to the cooling medium, whether oil or air Cost also plays crucial role

Transformer Insulation Major materials used as insulation in transformers: Mineral oil and Kraft Paper / Press Board / Wood basically called cellulose products Transformers subjected to high temperatures because of loads, high electrical stresses due to nature of power source, loads, etc. Life of transformer relies mainly on design, condition of insulation, its ability to withstand above conditions Oil filled Transformers use both solid (cellulose / Paper) and liquid (oil) as insulation

Winding insulation Insulating materials (cellulose) used in a transformer Kraft paper Cotton cellulose Press board Characteristics of insulation materials High dielectric strength Dielectric constant close to transformer oil Low power factor Freedom from conducting particles

Transformer paper insulation Kraft Paper Made by sulphate process from Unbleached soft wood pulp basically removing carbohydrates, waxes, etc to leave only cellulose fibres Cotton Cellulose Mixing cotton fibre with wood pulp Press Board Number of paper layers laid to produce thicker press boards either by laying papers together at wet stage or by using bonding adhesives between individual boards

Winding insulation Degradation of solid insulation induced by: Heat Moisture - Free water, suspended water (trapped in oil), dissolved water and chemically bound water Oxygen Acids

Transformer paper aging Degradation depends upon temperature, moisture content, oxygen and acids in the system Heat and moisture are the major issues which affect the life of paper insulation Moisture consists of free water, suspended water (Trapped in oil), dissolved water and chemically bound water ( used during manufacture), etc. and hence complete removal of moisture is impossible

Transformer paper aging As transformer ages, cellulose molecular chains get shorter and produce chemical products such as furanic derivatives, CO and CO2 which get dissolved in the oil Furanic derivatives ((5 hydroxymethyl-2-furfural, 2 furfuryl alcohol, 2 furfural, 2 acetylfuran, 5-methyl-2-furfural) dissolved in oil are main cause for cellulose degradation 2-Furaldehyde concentration - major contributing item in paper degradation

Winding insulation Transformer oil: Can be regularly filtered to keep characteristics intact Can be replaced with new oil, if required However, in transformer paper insulation: Degradation of cellulose irreversible Solid insulation cellulose products cannot be replaced End of insulation life = End of transformer life Maintenance very important to ensure long transformer life

Tap changer - Main functions To take care of: Variations in primary voltages - Mainly large transformers/ transformers used for critical applications Inherent regulation of transformer - Maintain constant O/P voltage irrespective of load conditions Unknown system conditions at time of planning Electrical system Effective control of VAR, mainly in generator applications

Transformer Tap Changer Secondary voltage of transformer varies in line with input voltage and on load current Other factor which decides secondary voltage is ratio of primary and secondary turns Proper operation of many electrical drives depend on keeping applied voltage close to its rated voltage It is difficult to have control on primary voltage and hence turns ratio is varied to keep secondary voltage close to the desired value

Tap changer - Basic principle

Tap changer High Voltage Low Voltage 7 6 Tap Positions 5 4 3 2 1

Tap changer types Based on operation mode: On load tap changer - Continuity of supply Off load tap changer - Supply interruption during change Based on location: In tank External mounted

Tap changer (On Load and Off circuit) Voltage control (or) turns ratio control achieved by using either Off circuit Tap Changer or On load Tap Changer (OLTC) Switch can be manual operated or motor operated Broadly classified as: Off Circuit Tap Changer (To be operated in de-energized condition only) On load Tap changer (Can be operated in energized condition with load)

Off Load Tap Changer Universally it is standard practice to have off load tap switch positioned at +5%, +2.5%, 0%, -2.5% and -5% so that the LV voltage can be kept close to its rated value for variation of +/- 5% of High voltage magnitude Off load tap changer draw back - Makes it non operable with transformer in energized condition, which is unacceptable for utility substation transformers and transformers used in continuous process industries

Off Circuit Tap Changers Generally all oil filled transformers are provided with OCTC where brief interruption of service is not an issue Established practice is to have taps at +5%, +2.5%, 0, -2.5%, -5% Users can specify different ranges also but normally limited to +/- 10% and normally not below 2.5% steps

On Load Tap Changer Continuously monitors secondary voltage, changes tap position on primary side by using a motor so that voltage can be kept very close to desired values continuously either by manual or auto operation Range of OLTC can be anything depending on system requirements and values like +7.5% to -12.5% in steps of 1.25% OR + 15% to -15% in steps of 1.25% are common

Need for On load tap Changer OCTC can not be operated with transformer in energized condition Unacceptable for utilities feeding many customers and for continuous process industries Includes features to take of arcing currents during switching operations and normally operated by a motor which is controlled based on HV magnitude

Basic connections of tap changers Typical arrangement in 3 phase transformer with star connection

Typical motor drive of tap changer

In-Tank Tap-changer

Fittings and Accessories Bushings Main connection between transformer windings and external source and load. Made of porcelain with conducting material passing through centre Solid type bushings Used for applications up to around 30 kV Condenser type bushing Consists of alternate layers of paper and metal foil used as condenser surrounding the conductor tube at centre of porcelain. Paper used can be synthetic bonded or oil impregnated one

HV Bushings Fitted to each phase of HV transformer Condenser type Oil impregnated paper insulation Anti fog sheds Shall be required to have impulse voltage withstand characteristics Need to protect against corona discharges and lightning surges

Solid or Non-Condenser type bushings Used for applications up to around 30 kV Voltage not evenly distributed through the material or along length of bushing Bushing dimension increases with increased rated voltage – Very large dimensions not a practical proposition Partial discharges due to concentration of stress in insulation, and its surface - Limits use not beyond 30 kV

Condenser type bushing Employed for EHV applications above 33 KV Conducting cylinders inserted into insulation to divide wall thickness into number of capacitors More uniform voltage distribution in material and along surface Alternate layers of paper and metal foil surrounding conductor tube at center of porcelain part Paper - Synthetic resin bonded paper (SRBP) or oil impregnated paper (OIP)

Transformer cooling Thermal considerations Transformer losses: Eddy current losses Copper losses Necessary to cool windings to keep temperatures below max. allowable limits Transformer insulation also affected by operating temperature Life of paper insulation a function of temperature Insulation life halves for every 6°C rise in temperature

Types of transformer cooling ONAN – Oil natural Air natural ONAF – Oil natural Air forced OFAF – Oil forced Air forced OFWF – Oil forced Water forced ODAF – Oil directed Air forced ODWF – Oil directed Water forced

Cooling medium Coolant medium ensures that temperature is within limits of allowable insulation temperature. (rate of aging of transformer doubles for every 6 deg C increase in winding temperature) Maximum hot spot temperature permissible is around 98 deg C Depending upon coolant medium the transformers are widely classified as below Oil cooled transformers Air Cooled / Dry Type transformers

Cooling medium Temperature of hottest part of winding is arrived by summing up: Ambient temperature Temperature rise of winding by resistance. Average of temperature readings at top and bottom of oil Temperature difference between the maximum and average gradient of the windings To consider a maximum temperature rise of 50/55 deg C for guaranteed oil temperature rise and 60/65 deg C by winding resistance method for oil cooled transformers for an ambient temperature of 45/40 deg C

Transformer oil requirements Chemically stable to ensure minimum oxidation at higher operating temperatures. Low water Content to keep its dielectric strength High specific heat High thermal conductivity Low Density Non Toxic / Non PCB to avoid pollution problems. Good Arc quenching properties Simple to produce and cost is reasonable

Transformer cooling system (radiators and fans)

Transformer cooling system (Pump and heat exchanger system)

Role of transformer oil Dual role of oil: Electrical insulation medium and Coolant for removing heat from core, windings For large transformers (like generator transformer) Oil cooled by oil/ water heat exchanger (OFWF – Oil forced water forced) Forced oil circulation by pumps Gravity fed water cooling for oil Standby oil cooler and pump provided – Automatic operation of standby pump in case of pump failure

Conservator Reservoir for transformer oil, located at a height above main transformer tank Takes up volumetric changes in oil induced by load changes Sufficient volume to allow for expansion/ contraction of oil under load conditions Designed to withstand full vacuum Oil level indicated by: Prismatic type oil gauge Magnetic, float type gauge

Conservator Conservators are so arranged that the lower part act as a sump in which any impurities entering the conservator can collect A valve/plug is fitted at lowest point of the conservator for draining these impurities along with oil

Oil level gauge and its importance Proper oil level vital for transformer safety Indicates, monitors oil level in conservator tank Magnetically coupled to float arm in oil (no glands used) Enables Detection of oil leakages Preparation for replenishment of oil Protection of transformer against low oil level Low level oil switch: Float actuated, operates if oil falls below predetermined level

Oil level gauges Float type Oil level gauge Prismatic Oil level gauge

Moisture in Transformer Main cause - Oil subjected to temperature cycles depending on load and ambient temperature Transformers absorb moisture from air when oil leaks are not arrested properly Water solubility increases as oil temperature increases

Moisture in Transformers Allowable moisture for transformers in service per ANSI C57 Aver. Oil <69KV 69-230 KV >230KV temperature 500 C 27ppm 12ppm 10ppm 600 C 35ppm 20ppm 12ppm 700 C 55ppm 30ppm 15ppm Water saturation 15% 8% 5% Paper moisture 3% 2% 1.25%

Effects of Moisture on insulation Life of insulation reduces by half for each doubling of water content in oil Electrical discharges in high voltage region due to imbalance in moisture equilibrium - Leads to incipient faults Possibilities of bubble formation with gases Rate of thermal deterioration of paper directly proportional to water content

Moisture affects Dielectric Strength

Breather (Silica gel type) Breather connected to main conservator tank Silica gel/ Cobalt chloride mixture used as moisture absorbing agent Oil seal at base prevents moisture ingress when transformer is not aspirating Replace silica gel when colour changes from blue to pink across mid level of container

Breather (Silica gel type)

Protection devices Lightning arrestor Pressure release valve/ diaphragm Buchholz relay Oil level gauge Winding temperature monitoring Oil and water flow indicators

Lightning arrestor Over voltage protection of large transformers Protection against steep surge voltages (ex. Lightning) coming from overhead lines Installed as close to transformer as possible (preferably near bushings) Connected to station earthing by shortest way

Pressure release valve/ diaphragm Transformer faults can: Cause breakdown in cooling oil Quickly generate large amounts of gas Resulting pressure can rupture transformer tank if not relieved quickly Gas and oil actuated relay does not operate quickly enough to relieve pressure Pressure relief device must be fitted

Pressure release valve/ diaphragm Fitted on top of each phase oil tank Protection against dangerous internal pressure build up - Operates within 2 milliseconds Pressure released before equipment damage can occur Trip initiated Mechanical indicator pin must be reset by hand Investigate cause in the event of valve operation

Oil and water flow indicators Counter balanced vane actuated by flow rate of transformer oil or cooling water Dial chamber completely sealed off from main body by rigid diaphragm Transfer of vane movement to dial chamber by magnet coupling (No glands employed) Second magnet in dial chamber follows vane movement – Actuates indicator pointer and mercury switches

Winding temperature indicator Generates signals for indication, alarm and trip and cooling control Consists of: Compensated bellows system Transmission system Indicating pointer Switches Comprises of two detectors: Liquid filled thermometer bulb – Reacts to slow response time of change in oil temperature CT – Reacts quickly to change in current flow in phase winding

Winding temperature indicators Compensated bellows system comprises: Operating bellows, Thermometer bulb, Interconnecting capillary tubing, CT, Heating coil, adjustable shunt resistance Compensating bellows, Interconnecting capillary tube Compensating system enables sensing of winding temperature alone Movement of operating bellows transmitted to indicating pointer by linkages Mercury switches provide alarm, trip signals

Marshalling kiosk Houses transformer instrumentation and all control equipment All steel weather proof construction Multi compartment with provision for padlocking Compartments for incoming supplies, fan and pump control, winding temperature controllers, ammeters Anti condensation heaters Power supply sockets, telephone socket for maintenance purposes

Transformer Tanks / Enclosures Transformer tanks constructed with welded boiler plates of sufficient thicknesses depending on internal pressure requirements Tanks shall have removable inspection cover basically to inspect internal core, facilitate removal and reinstalling core and windings in case of any repair jobs

Tank and Painting Coating of 6 to 10 mils on tank surface is recommended to protect against deterioration due to atmospheric conditions Epoxy paints preferred in corrosive areas

Transformer Operation and Maintenance

Transformers In Service Transformer breakdowns can be avoided by adopting Standard operating procedures and recommended maintenance practices. Factors causing Failure of a transformer: Overload Incorrect installation or use Faulty design or construction Neglect Wear and tear and other deterioration Accidents

Transformer Inspection A rigorous system of inspection and preventive maintenance ensures long life, trouble-free service and low maintenance cost Maintenance consists of regular inspection, testing and reconditioning where necessary

Inspection Intervals Following table gives preferred schedule of inspection for a typical transformer Device Inspection interval Operational interval Temperature indicators 1 month N.A. Oil level 3 months Annual samples Pressure relief device Silica Gel breather 3~ 6 months Reactivation, as required Gas actuated relay 1 year Tap changer Fans/pumps 6 months

Tap-changer Maintenance Lift out diverter, clean vessel, change oil Inspect contacts, change them if required Inspect transition resistors Check flexible connections for brittleness Change main spring, check spring relaxed Check drive, molybdenum grease Check drive and bevel gears

Electrical tests and oil quality tests

Objectives of transformer testing Verifying configuration (new or repaired transformers) Winding health Oil quality verification Functional checks of mounted instruments/ relays/tap changer Residual life assessment

Winding Resistance Test Resistance decides copper losses Use Wheatstone bridge or Kelvin bridge Apply DC voltage and wait till core saturation Ensure windings not in very hot condition during measurement Unequal or infinity values may indicate possibility of open winding or loose connections Winding Resistance values shall be uniform to ensure healthiness of windings internally

Turns Ratio Test Apply around 400 volts AC on primary terminals (take open circuit voltage readings on corresponding secondary terminals – ratio indicates approximate turns ratio) Take readings at all tap positions Values should not differ by more than 0.5% (of expected design voltage ratio) Portable instruments available to take measurements

Turns Ratio Test Turns ratio measurements shall ensure that transformer meets application needs Results shall be within 0.5% of calculated voltage ratio

Vector Group Test Transformers for paralleling must have same polarity and phase relation to avoid partial or complete short circuits Transformer polarity and phase relation tests important when two or more transformer are to be paralleled

Dielectric Tests Tests applicable Applied Potential Test at rated power frequency for a duration of one minute Induced Over voltage test at higher frequency for reduced duration

Partial Discharge Test For transformers rated 220 kV and above To check discharges along cavities, cracks, etc Caused by improper drying of insulation and presence of sharp edges Requires special equipment. 1.3 times rated voltage applied for 5 minutes, 1.5 times for 5 seconds and 1.3 times the rated voltage maintained for 30 minutes Should be within 300 pC at 1.3 times and within 500pC at 1.5 times the rated voltage

Bushing testing Power factor test or Tan  test basically carried out to check deterioration and contamination of bushings RIV test done basically to determine corona discharges (lowers performance and life) in bushings at rated operating voltage Moisture content checked for oil type bushings

Oil Testing The condition and safe operation depends on testing of oil to check the following parameters and taking corrective steps Dielectric Test Acid Neutralization Number Interfacial Tension Test Colour Relative Density Dielectric Dissipation Factor Dissolved Gas Analysis

Moisture in Transformers Typical moisture contents of new transformers Aver. Oil <69KV >69 KV temperature 500 C 7ppm 2ppm 600 C 12ppm 4ppm 700 C 20ppm 7ppm Water saturation 6% 2% Paper moisture 1% 0.5%

Measurement of Moisture Content Power factor test or tan  measurement Power factor value of 0.5% considered unacceptable for bigger transformers at high voltages Value of around 1% still accepted for smaller transformers Constant high power factor at various voltages indicate presence of moisture in transformers

Measurement of Moisture Content IR Values - Log-log graph with IR values against time Good insulation gives almost straight-line curve increasing with time Moist/ contaminated insulation gives curve that raises slowly, flattens out shortly Polarization index Ratio of 10 minutes IR to 1 minute IR Value of 1 totally unacceptable For dry type transformers, minimum value of 2 required

Dissolved Gas Analysis Content of gases indicate the internal fault conditions of a transformer Following are some established methods. Permissible Gas Concentration Limits Regression Method Combustible Gas Method Key Gas Method Ratio Method- Rogers & IEC Duval’s Triangle method

Main Gases analyzed in DGA Hydrogen H2 Methane CH4 Ethane C2H6 Ethylene C2H4 Acetylene C2H2 Carbon monoxide CO Carbon dioxide CO2 Oxygen O2 Nitrogen N2

Duval Triangle Method Accurate and trustworthy method using DGA for deduction of transformer problems Based on data base of thousands of transformers About one million DGA analyses performed every year by more than 400 laboratories worldwide Cause determined based on percentages of combustible gases evolved

Duval Triangle PD - Partial Discharge T1 - Thermal Fault less than 300°C T2 - Thermal Fault between 300°C and 700°C T3 - Thermal Fault greater than 700°C D1 - Low Energy Discharge (Sparking) D2 - High Energy Discharge (Arcing) DT - Mix of Thermal and Electrical faults

Symbols and Problem Causes Fault Examples PD Partial Discharge Corona discharge in voids, gas bubbles with possible formation of X-wax in paper D1 Discharges of low energy Partial discharges of the sparking type, inducing pinholes, carbonized punctures in paper Low energy arcing inducing carbonized perforation or surface tracking of paper, or the formation of carbon particles in oil D2 Discharges of high energy Discharges in paper or oil, with power follow-through, resulting in extensive damage to paper or large formation of carbon particles in oil, metal fusion, tripping of equipment and gas alarms T1 Thermal Fault T<300ºC Evidenced by paper turning brownish (>200ºC) or carbonized (>300ºC) T2 Thermal Fault, 300<T<700 ºC Carbonization of paper, formation of carbon particles in oil T3 Thermal Fault, T>700ºC Extensive formation of carbon particles in oil, metal coloration (800ºC) or metal fusion (> 1000ºC) DT Electrical Fault and Thermal Fault Development of one type of fault into another type of fault

Oil Dielectric Test Collect sample oil and immerse electrodes with 2.5 mm gap Apply high voltage and increase till flashover - called Break Down Voltage (BDV) Standard value 30 kV but new oil may have up to 80 kV Take sample of five or six readings

Life Expectancy of Transformer Estimation by deterioration of paper insulation Determination of paper insulation condition Furan Analysis Testing of Kraft paper

Furan Analysis Compounds of cellulose decomposition Reliable method: Estimation of paper insulation life Furans > 250 ppb Paper insulation deterioration Range: 100 ppb ~ 70,000 ppb (ppb - parts per billion) Important Furans 5H2F (5-hydroxymethyl 1-2-furaldehyde) - Oxidation (aging, heating of paper insulation) 2FOL (2-furfurol) - High moisture content in paper 2FAL (2-furaldehyde) - Overheating of paper 2ACF (2-acetylfuran) - (rarely observed) 5M2F (5-methyl-2-furaldehyde) - Local overheating (hot spots)

DP (Degree of Polymerization) Test Reliable assessment of Paper deterioration Cellulose - Long chains of glucose rings DP - Average number of rings in molecule DP value of new insulation - 1000 ~ 1400 DP value ≤ 200 - End of insulation life

DP and % of Remaining Life of paper New insulation material 1000 DP ~ 1400 DP 60% to 66% remaining life 500 DP 30% remaining life 300 DP Zero remaining life 200 DP

Operation of transformers

Conditions for parallel operation of multiple transformers Transformers shall have Same transformation ratio, rated voltages Identical indices – ex. Dy11, Yd11 Terminals with same polarity (HV and LV side) connected in parallel Tap changers (if provided) in same tap positions Rated outputs not deviating more than 1:3 Same short circuit impedance (within +/- 10%) Transformer with lowest short circuit impedance most heavily loaded

Importance of regular inspection and maintenance Long life of equipment Trouble free service – Improved reliability Low maintenance expenditure Minimum down time of power system Improved safety Improved morale

Visual external inspection What it can reveal Oil temperature indicator Winding temperature indicator High oil/ winding temperatures – Probable causes Blocked radiator, heat exchanger Cooling fan, circulation pump failure Transformer over loads Internal faults

Visual external inspection What it can reveal Conservator oil level gauge – Low oil level Oil stains around transformer Probable causes Leaks from valves, radiators, welded joints etc Very low load with very low ambient temperatures Insufficient oil - Need for topping up

Visual external inspection What it can reveal Abnormal noise, smell, vibration Probable causes Termination problems – Partial discharge, arcing, sparking Bushing problems – Corona, insulator failure Internal fault Loosening of internals

Sonic and vibration analysis Valuable tools for locating internal faults of transformers Sonic analysis: Low energy arcing/ sparking and partial discharges emit energy in ultrasonic range Ultrasonic emissions detected by sensors placed outside the tank – Converted to audio signals or oscilloscope traces Fault location can be ascertained before opening up transformer

Sonic and vibration analysis Vibrations due to loose components, loose core and coil segments, defective bearings of fans and pumps Vibrations detected and measured by Vibration analysers Expertise needed to pin point cause of vibration

Life Expectancy of Transformer Estimation by deterioration of paper insulation Determination of paper insulation condition Furan Analysis Testing of Kraft paper

Visual inspection What it can reveal Silica gel Breather Silica gel turned to pink beyond mid level Probable causes Spent silica gel. Need for replacement of silica gel Inadequate oil level at bottom of breather

Visual inspection What it can reveal Buchholz Relay Gas accumulation in relay Discolouration of oil Probable causes Air entrapment - Leakages in joints Increased moisture level in oil Oil deterioration Internal fault

Visual inspection What it can reveal Pressure Relief Valve - Activation of valve Cause Internal fault Thoroughly investigate cause for valve operation

Visual inspection What it can reveal Rusting, corrosion of tank, transformer parts, pipe lines Check cause for rusting – Water leakage, ageing, deterioration of protective paint Clean the surface and provide corrosion protection

Internal (borescope) examination Specially designed instrument for internal inspection without opening up oil filled transformer Core, windings, connections can be inspected, photographed Can ascertain exact problem to be attended before opening up transformer Proper use can save: Lot of time Repair expenses

Online gas in oil monitoring Comprises of Hydran 201Ti intelligent transmitter and Hydran controller connected to plant data network Hydran system: An intelligent fault monitor Reads composite value of gases in ppm generated by faults Warn personnel when diagnostic or remedial actions required Gas in oil information – Can be monitored locally or remotely

Safety aspects while working near energized transformers HAZARD 1: Hazardous high voltages – Extreme care must be taken whilst in vicinity of energized transformers With transformer energized, no work may be performed at transformer tank and cooling system other than following: Replenishing oil within conservator Collecting gas samples from Buchholz relay Not applicable to jobs effected from marshalling kiosk or motor drive compartment of tap changer Isolate fire fighting system if access to compound exceeds a period longer than few minutes

Transformer compound access HAZARD 2: EXPOSED HV CONDUCTORS Precautions: Generator Transformer Acoustic Compound is an enclosed area Isolate fire system for Work in this area Star point conductor is exposed & may become live under fault conditions Star point is hard earthed. Need not be regarded as High voltage conductor Generator Transformer Compound, a HV Zone No climbing or work above ground level except from installed platforms is permissible Long objects e.g. ladders, scaffold poles that would reduce clearances, should not be taken into compound

Transformer compound access HAZARD 3: FIRE PROTECTION DELUGE SYSTEM OPERATION Precautions: i) For external work at ground level the fire system may remain available provided that: a) Work at heights and hot work is excluded b) Work in any enclosed area is excluded c) Control room is informed of compound entry and exit d) For external work at heights, fire system should be isolated

Protection of transformers

Why do transformers need protection? Abnormal operating conditions Increased operating temperatures Over loading External faults in system Internal faults System voltage surges (ex. Lightning, switching surges)

Transformer Short Circuit Currents Transformer windings are subjected to mechanical and thermal stresses during internal short circuits and also during system short circuits outside terminals Transformer short circuit current during faults depend on system parameters as well as on its own impedance Transformer windings and construction shall take care of such short circuits which need to be considered at design stage itself

Transformer Short Circuit Currents Normally duration of 2 seconds of short circuit current considered for transformer design unless client specifically asks to consider higher/ lower duration depending on his backup protection Short circuit withstand capacity is normally based on the maximum allowable temperature rise of the windings under short circuits

Transformer Short Circuit Currents Main reason for failure of transformer due to short circuits is more because of mechanical forces produced on windings under short circuits rather than thermal damage on the insulation Electromagnetic force varies directly in line with square of current, which means 20 times normal current during short circuit will produce 400 times the normal stress and the design to take care of such high forces

Magnetic Stray Field & Forces

Short circuit Effect – Radial collapse

Types of Stresses Stresses are to be faced by transformer winding insulations during its life: Dielectric stress Short circuit stress Switching surges Lightning impulses Through faults

Internal problems and external influence High winding temperatures: Overloading of transformer High ambient temperatures System fault conditions Decreased cooling efficiency

Internal problems and external influence Deterioration of transformer oil: Moisture ingress High operating temperatures due to overload Inadequate maintenance practices: Breather maintenance Oil inspection and maintenance Deteriorated gaskets, seals

Internal problems and external influence Deterioration of winding insulation: External system faults Over load Moisture ingress Inadequate maintenance practices: Breather maintenance Oil inspection and maintenance Deteriorated gaskets, seals

Temperature based protection Oil temperature monitoring and protection Winding temperature monitoring and protection Forced cooling based on temperature rise

Winding Temperature Indicator Generates signals for indication, alarm and trip and cooling control Consists of: Compensated bellows system Transmission system Indicating pointer Switches Comprises of two detectors: Liquid filled thermometer bulb – Reacts to slow response time of change in oil temperature CT – Reacts quickly to change in current flow in phase winding

Winding Temperature Indicators Compensated bellows system comprises: Operating bellows, Thermometer bulb, Interconnecting capillary tubing, CT, Heating coil, adjustable shunt resistance Compensating bellows, Interconnecting capillary tube Compensating system enables sensing of winding temperature alone Movement of operating bellows transmitted to indicating pointer by linkages Mercury switches provide alarm, trip signals

Gas/oil Surge Protection Buchholz relay Oil high pressure protection: Pressure relief device (explosion vent) Sudden pressure relay

Buchholz Relay

Buchholz Relay

Buchholz relay Located between conservator and main tank Detects low oil level, faults within transformer Float with mercury switch mechanism Two stage device operated by: Gas buildup – Initiates Alarm Gas created by arcing within transformer tank Air ingress Oil surge – Initiates Trip Due to internal arcing fault Buchholz relay settings not adjustable Windows permit observation of oil level inside relay In the event of faults developing within the transformer a gas/oil surge operated (Buchholz) relay is fitted in the pipework between the conservator and main tank. An alarm element operates after a specified volume of gas has collected, in the event of oil leakage or air ingress to the oil system. The Buchholz relay also contains a trip element which is operated by an oil surge.

Buchholz Relay Buchholz relay can detect both gas and oil surges as it is mounted in pipe to conservator Monitors any such kind of pressure buildup to avoid explosions Buchholz relay is connected between main tank and conservator in all breathing type transformers with isolating valves on either side

Explosion Vent

Sudden Pressure Relay Transformer faults can: Cause breakdown in cooling oil Quickly generate large amounts of gas Resulting pressure can rupture transformer tank if not relieved quickly Gas and oil actuated relay does not operate quickly enough to relieve pressure Sudden pressure relay very sensitive to variations in internal pressure, operates quickly

Over voltage protection (by lightning arrestors) Over voltage protection of large transformers Protection against steep surge voltages (ex. Lightning) coming from overhead lines Installed as close to transformer as possible (preferably near bushings) Connected to station earthing by shortest way

Differential protection Compares currents entering and leaving protected zone Operates when differential current exceeds pre-determined level Types: Current balance Circulating current scheme

Differential protection Relay operation under internal fault conditions Detection of unbalance by relay within its protective zone Called UNIT protection - Operates only for faults on unit it is protecting

Differential protection Current balance scheme (External fault conditions)

Differential protection Current balance scheme (Internal fault conditions)

Restricted Earth Fault Protection Simple over-current and earth fault relay cannot provide adequate protection for winding earth faults Even biased differential relay ineffective for certain earth faults within winding Separate earth fault protection necessary

Restricted Earth Fault Protection Relay Type: Instantaneous high impedance type

Over Fluxing Relay Transformer overfluxing due to: Over voltage Low system frequency Transformer cores designed to operated below certain magnetic flux density Excess flux density causes over heating and damage Over excitation occurs during Startup/ shutdown of generator connected transformers Over voltages during load rejection Over flux protection does not need high speed tripping – Instantaneous tripping may cause damage

Electrical protection Over Current Relay Most common protection in a transformer Protects against excess withdrawal of current Uses IDMTL (Inverse Definite minimum Time) over-current and earth fault relay on transformer HV side Operating time varies inversely with respect to current value

Neutral E/F Relay Reactance decreases towards the neutral Fault current is controlled mainly by leakage reactance, which varies in a complex manner Earth fault current does not vary in a linear fashion Use sustained or sensitive earth fault protection with CT on star point of winding, or Use restricted earth fault protection

Any questions ?