WORKSHOP ON POWER TRANSFORMERS PRINCPLES,APPLICATIONS,TESTING AND COMMISSIONINGHELD AT PROFESSIONAL SKILLS TRAINING CENTRE, KAINJIFROM 10 -14TH MARCH, 2008 BYPROTECTION,CONTROL AND METERINGDEPARTMENT ( P C& M )

Topics To Be CoveredTransformer PrinciplesTransformersRight-Hand RuleMagnetic FluxMagnetic Induction Left-Hand Rule Turns Ratio Transformer Losses Transformer Types

Topics To Be Covered (Contd)Tap Changers Off-Load Tap Changer On-Load Tap Changer Phase Angle Control Transformer Connections Single-Phase Circuits Two-Phase Circuits Three-Phase Circuits

Topics To Be Covered (Contd)Transformer MaintenanceInsulation (AC Resistance) TestingHigh Potential TestingTurns Ratio TestingPolarity TestingPower Factor (Loss Factor) TestingExcitation CurrentDC Winding ResistancePolarization Index testInsulating Fluid DielectricDissolved Gas Analysis

Topics To Be Covered (Contd)Transformer and RelayingTransformer FaultsDifferential Relaying (Normal Load, External Faults, Internal Faults

Introduction The transformer is a static electric device, consisting of a winding, or two or more coupled windings, with or without a magnetic core.

It transfers power by electromagnetic induction between the circuits at the same frequency, but usually at changed values of voltage and current (either serving as step-up or step-down function).

Introduction (Contd)The generation of electricity or voltage requires motion between a magnetic field and conductors.

In power plants, this motion is supplied by moving the wires, by means of turbines, through a fixed (or DC) magnetic field.

Introduction (Contd)A transformer, however, depends on a Constantly changing magnetic field to transmit power.The wires are fixed, and the magnetic field moves. This continual movement of oscillating magnetism is set up by the 50 Hz AC power supply.

First Law of Magnetics Shortly after, it was discovered that a looped wire produces a more intense magnetic field or flux. Both of the foregoing progressive steps illustrate the First Law of Magnetic-a current-carrying conductor produces a magnetic field.

Right-Hand Thumb RuleLater, the direction of the magnetic field produced by a current-carrying was determined by the so-called Right-Hand Thumb Rule.If the wire is gripped by the right hand such that the thumb of the right hand points in the direction of the current, then the fingers will point to the direction of the magnetic field.

Origin of Simple ElectromagnetWhen a current-carrying coil is wrapped around a bar of iron, the magnetic field becomes still more intense.Thus a simple electomagnet is a current-carrying coil of wire with an iron core.

Second Law of MagneticsThe electromagnet illustrates the Second Law of Magnetics A current-carrying conductor surrounding a stationary magnetic field produces an alternating voltage

Magnetic Flux DensityMagnet density flux may be thought of as an amount of magnetic field passing through an area.

Principle of ElectromagneticBetween 1831 and 1832, Sir Michael Faraday at the Royal Institution in London and Joseph Henry at the Albany Academy in the USA, independently discovered the principle of electromagnetic induction by combining the two basic laws of magnetics.

Principle of Electromagnetic Induction (Contd)Whenever a conductor moves relative to a magnetic field, electric potential or voltage, and therefore current, is induced in the conductor.In other words, electric potential is induced in a conductor whenever flux through the conductor changes.

Lenzs LawThe average induced electric potential, regardless of how it is produced, is proportional to the rate of change of flux linkage, and is given by Lenz as: = - N d dt

LENZS LAW (Contd)The instantaneous induced electromotive force (emf) acts in a way to oppose the flux inducing it.This is indicated by the minus sign in the equation. = - N d dt

Transformer major Parts The magnetic circuitThe windings (primary and secondary, and tertiary)Solid insulation materials of the windings and coreTank enclosure

Major Parts of Transformer (Contd)Bushings and leadsCoolants (Insulants) and cooling arrangementTaps and tap-changing arrangementProtective gear, circuit breakers, protective relaysOther auxiliary equipment

Magnetic CircuitThe core, a magnetic circuit with a clamping structure, is the part of the transformer in which a magnetic field oscillatesThe metallic composition of the core is a special high grade silicon sheet steel.A typical sheet of steel is 0.014 inches (0.3mm) thick.

Magnetic Circuit (Contd)These sheets are laminated into sections that are several inches wide.These core laminations help reduce eddy currents or currents induced in the iron parts of the unit.Each lamination in turn is coated with insulating material. This coating helps prevent magnetic losses and reduces heating losses

WindingsThe primary and secondary windings include clamping arrangements.There are four types of coils or windings used on core-type transformers, these having the following designations:Spiral typeCrossover typeHelical typeContinuous disc type

Tank Enclosure and Cooling surfacesThe tank contains the transformer together with cooling surface and coolant.It serves as a surface to radiate heat to the surrounding air.For self-cooled transformers, the main types of tanks in modern use are.

Tank TypesPlain sheet steelBoiler plate with external cooling tubesRadiator tanksTanks with separate coolers

BushingsBushings are usually made of porcelainThe inside of the bushing may be oil, paper, epoxy, or fiber glass.Bushings serve to insulate the primary and secondary windings from the tank (ground).

Insulants or CoolantsAir, gas, oil, or synthetic liquid may be used.

Tap-Changing ArrangementTappings can be used for the following purposes.Primary tappings to vary the primary voltage Secondary tappings to vary the secondary voltagePrimary tappings to compensate for variations in the primary voltage

Insulation SystemInsulation system is made up of Liquid coolantSolid insulating materialsThe life of the transformer depends primarily on the life of this insulation

Principle of OperationFor the purposes of understanding the main principle of operation of transformers, a two-winding transformer would be considered.In its simplest form, the two-winding transformer consists of two coils of wire primary P and secondary S. Insulated from each other and wound on a common iron coreThe core may take one of many forms, such as a toroid or a rectangular frame.

Diagram of TransformerLaminated coreLaminated coreMutual or useful FluxV2E2E1V1SP

Principle of Operation (Contd)Each of the coils P and S is connected to an external circuitThe coil P, referred to as the primary windings, is connected to an electrical source, an AC supply whose voltage or current varies with time.

Principle of Operation (Contd)In line with Faradays observation, the Alternating primary current which flows in the primary windings P generates an alternating magnetic flux, known as useful or mutual fluxThe mutual flux links or cuts not only the primary windings P but also the secondary windings S.

Principle of Operation (Contd)Consequently, the generated flux linkage (product of flux and number of turns, = N) induces an alternating emf in the secondary windings.Because the secondary coil is in circuit, it will also carry a time-varying current, the secondary current.

Principle of Operation (Colntd)This secondary current will also contribute to the magnetic field, so that the total magnetic field is created by contributions from both currents.The two coils are thus said to be magnetically or inductively coupled

Basic Transformer RelationshipsIn considering the basic theory, we would be making a few assumptions.Firstly, if the applied or source voltage to the primary windings is sinusoidal, it is assumed that the emf induced in the secondary windings is also sinusoidal.Secondly, we will also assume that all the flux generated by each current links both primary and secondary coils that is, the flux linking each coil is the same, so that there is no flux leakage.

Basic Transformer Relationship (Contd)Since the flux varies sinusoidally, we have for the flux at any instant.= max sin t

Basic Transformer Relationships (Contd)Hence the instantaneous emf e2 induced in the secondary coil of N2 turns linked by this flux is given by: 2m = -rate of change of flux linkage = - d [N2] dt = - max N2 cost

Basic Transformer Relationships (Contd)The maximum value of the induced emf e2 is thus:E2max = max N2 = 2 max N2

Basic Transformer Relationships (Contd)The r.m.s. value of the secondary voltage can be obtained from the maximum value as:E2 = E2max = 2 max N2 2 2 = 4.44 max N2 = 4.44 Bmax A N2

Basic Transformer Relationships (Contd)Now due to the effect of mutual inductance, the same flux links with the primary winding also, and so an instantaneous e.m.f is also induced in the primary.Its value is give by the corresponding equation:e1 = - m N1 cos t

Basic Transformer Relationships (Contd)

v1 = - e1 =- m N1 cos t = m N1 sin (t + 900)

Basic Transformer Relationships (Contd)It can be seen that the supply voltage leads the induced magnetic flux by 90 degrees.The induced voltages e1 and e2 in the primary and secondary respectively are represented by the equations:e1 = - E1max cost = E1max sin (t 90o)e2 = - E2max cost = E2max sin (t 90o)

Basic Transformer Relationships (Contd)The above equations show that the two induced voltages are in phase opposition to the supply voltage of equation.To obtain the r.m.s. value of the supply voltage, we haveV1 = E1max = max N1 = 4.44 max N1 2 2

Basic Transformer Relationships (Contd)

Since there are no voltage loss across the windings, the primary induced emf is equal to the supply voltage in magnitude, thus E1 = V1.HenceE1 = 4.44 max N1 = 4.44 Bmax A N1

Transformation or Turns RatioCombining the equations and, we obtain the transformation or turns ration:E2 = N2 = n or E2 = nE1E1 N1

Operation On No-LoadA transformer is said to be operated on no-load when the secondary winding is open-circuited.The secondary current is consequently zero and it is clear that the secondary winding can have no effect whatsoever on the magnetic flux in the core or the current in the primary.

Phasor Diagram of Transformer On No-Load

ACE1E2Open endV1= - E110101010

Components Of No-Load CurrentThe no-load current has two components, The reactive or magnetizing componentThe active or power component,The No-load current services two functions:The magnetizing component is needed to produce the m.m.f. (mmf = N.I) necessary to generate the flux mutual or useful

Components Of No-Load Current (Contd)The active or power component is needed to convey the power necessary to supply the core losses resulting from the alternating magnetization.The core losses are of two types, namely, hysteresis and eddy-current losses.

Components Of No-Load Current (Contd)The active and magnetizing components and of the no-load current can be calculated from the no-load phasor diagram.I w = I 0 cos 0I = I 0 sin 0

Operation Under Load

E1E2Z2 = R + X2V11010 I Io Ro V1 V1 I1 I1 I2 E2I2 I1 I1 2 2

Operation Under Lod (Contd)When a load is connected to the secondary side, a current flows due to the induced voltage, and sets up a secondary mmfThis mmf produces a secondary flux 2 that has the same magnetic path as and in the opposite direction to the useful flux originally set up in the primary due to the no-load current, and links with the primary winding.This secondary flux 2 tends to have a demagnetizing effect to reduce the mutual.

Operation Under Load (Contd)But the slightest decrease in the mutual flux due to the secondary current would cause a corresponding decrease in the primary induced voltage.If the applied voltage, however, is constant, then the primary induced emf and thus mutual flux in the core must also remain constant.

Current Transformation RatioThis can happen, only when the primary draws more current from the source to neutralize the demagnetizing effect of the mmf.Thus the load current causes the primary to take more current in addition to the no-load current such that N1I1= N2I2 or I1 = N2 I2 N1

Operation Under Load (Contd)This component of the primary current which neutralizes the demagnetizing effect is termed the load component of the primary current and is drawn opposite to the load current in the phasor diagram.The total primary current is thus the phasor sum of the no-load current and the load componentI 1 = I 0 + I 1

Voltage RegulationBy voltage regulation, we are referring to measures to ensure that voltage variation at the secondary terminals is reduced to practically acceptable limitsThe voltage regulation V.R. is defined as the change in secondary terminal voltage expressed as a percentage (or p.u.) of the secondary rated voltage under no-load conditions:

Voltage Regulation (Contd)

V.R. = No-load sec.Vol- Full-load sec.VolNo-load sec.Vol

=E2 V2 E2

Voltage Regulation in Terms of Primary Winding ParametersConsider the equivalent circuit referred to the primary and the phasor diagram for a lagging (inductive) load.V2I1R1N1E1E2V1

Corresponding Vector Diagram V1E111R1B-1 22o11Z111X112E2A/AE,B = /V,E,B = 2/V,E,A = (1 2)

Calculation of VRFrom the phasor diagram,V12 = (E1 + E1A)2 + (V1A)2But V1A is negligible compare to (E1+E1A)V 1 = E 1 + E 1 AV 1 - E 1 = E 1 AV 1 - E 1 = E 1 A V 1 V 1

= V . R.

Calculation of VR (Contd)V.R. = E1A F1= I1Ze1cos ( e1 - 2) F1= I1Ze1 V1= 11 V1V.R. = 11 V1

[cos e1cos 2 + sin e1sin 2][cos e1cos 2 + Z e1 sin e1sin 2][R e1cos 2 + X e1 sin 2]

General Equation of VRThe V.R. could also be expressed in terms of the equivalent secondary values.And so more generally, the voltage regulation could be expressed asV.R.= [Recos+Xesin ]IV

Transformer LossesThe losses in the transformer could be classified into two, namely,Core or iron losses andCopper losses though these amount to only a small portion of the total input power (1% to 15%)Since the losses in a transformer are very low compared with the output, the efficiency is very high, varying from 85% to 99% (the larger the transformer the higher the

Iron LossesThe iron losses are due to hysteresis and eddy current losses in the core of the transformer.For a better understanding of the iron losses, consider briefly how these losses occur in the core.

Hysteresis LossThe alternating induced flux in the core causes an alternating magnetizations of the core at a frequency of the time-changing magnetic flux.These alternate cycles of magnetization can be represented by the so-called hysteresis loop on a B-H curve. The iron subsequently experiences hysteresis losses. The B-H curve is shown below

Hysteresis Loop

SaturationNormalMagnetizationcurveRC B H = N1 I

Steinmetz Empirical Formula for Hysteresis LossThe magnitude of the hysteresis loss is dependent on the grade of the ferromagnetic material used for the core.It is also proportional to the frequency. If Cc is the volume of the core, the hysteresis loss is obtained by the Steinmetz empirical formularPhys = KhVcf Bm1.6

Eddy-Current LossSince the iron is electrically conducting, a solid core in the presence of an induced emf would constitute electrical paths of very low resistance and consequently currents would circulate within the core.These circulating currents inside the solid conducting core, arising out of the induced emf, are referred to as eddy-currents.

Eddy-Current LossThese eddy-currents result in so-called eddy-current loss, and produces heat which lowers the efficiency and raises the temperature of the windings, lowering its output capacity.In practice, however, the effects of the eddy-currents are greatly reduced through a process of lamination of the core.

Formula For Eddy-Current LossThe eddy-current power loss is proportional To the square of the cross-sectional area normal to the direction of the field,The square of the maximum flux density,The square of the frequency andInversely proportional to the resistively of the ferromagnetic material from which the core is made P eddy=K eddy 2 f 2 Bm2 P

Tests Performed on TransformerTwo main tests known as the Open-Circuit (No-Load) and Short-Circuit TestsThese tests are performed to determine the Voltage regulation V.R.,The efficiency nAnd other equivalent circuit parameters of the transformer without actually loading it.

Open-Circuit Test WattmeterAVC.C

V.CVARIACA2

Open-Circuit Test (Contd)The secondary is left openAnd a rated or full-load voltage is applied through a VARIAC to the primary of the transformer.The following measurements are made:

Open-Circuit Test-MeasurementsThe ammeter measures the no-load current in the primary, which is about 2.5% the rated or full-load current.The voltmeter measures the rated or full-load voltage.The wattmeter measures the open-circuit power. Since the secondary current is zero, the only copper losses are due to the no-load current in the primary.

Open-Circuit Test - CalculationsNo-Load p.f. = cos 0 = P x Vx I0I w = Io cos o:I = Io sin o

Re = Voc = Voc : X = Voc = Voc Ic Iccos o Ic Iccos o

Short-Circuit Test WattmeterA2A1VC.C

V.CVARIAC

Short-Circuit Test (Contd)The secondary of the transformer is shorted through an ammeterAnd a reduced voltage is applied through a VARIAC to the primary of the transformer until the rated or full-load current flows in the primary

Short-Circuit Test - MeasurementsThe following measurements are made:The ammeters A1 and A2 measure the primary and secondary currents respectively.The voltmeter measures the short-circuit voltage.The wattmeter measures the short-circuit power. This gives the full-load copper losses, if the currents flowing in the primary and secondary are rated or full-load currents. These are obtained by adjusting the variac.

Short-Circuit Test - CalculationsShort-circuit p.f. = cossc =Psc V sc I sc

Psc Vsc I2sc IscRel = ; Zel = Xel = Z2el-R2el

Short-Circuit Test-CalculationThe V.R. of the transformer can be calculated from the short-circuit measurements.V.R. = I1 Z e1 cos (sc-2) V1

Transformer Design The outstanding features of transformer design which should be carefully considered may be summarized as follows: ReliabilityMaximum allowable losses (Iron and Copper Losses)Impedance.

Transformer Design (contd)General arrangement of coils with regard to core Type of component material chosen Insulation system (Liquid insulant and solid insulation material clearances and thickness) Short-circuit, impulse and switching surge withstand.

Transformer Design ReliabilitySound mechanical construction Liberal oil ducts and electrical clearances Current and flux densities such as to avoid local heating Liberal radiating surfaces Good insulating oil.

Transformer Design LossesLosses must be within the allowable limits, so as not to injure the windings

Transformer Design Windings Transformer windings are designed to get the required number of turns into a minimum space. At the same time, the cross section of the conductor must be large enough to carry the current without overheating and sufficent space must be provided for the winding insulator and for cooling ducts, if any.

Transformer Design Windings (Contd)In design, the windings and their arrangements must be done to achieve:Sufficient dielectric strength against various voltage stresses, such as lighting or switching surges Adequate winding ventilation Adequate mechanical strength (for instance, to withstand short-circuit forces)Minimum cost Specified loss maximums.

Insulation SystemInsulation system consists of The liquid insulants together with The solid insulating materials around the windings The insulation system isolates the transformer windings from each other and from the ground.

Insulation System (Contd) The most widely used transformer insulation systems continue to use two basic items:A liquid insulation (transformer mineral oil askarel or sillcone, of which more than 90% are oil-filled) and Solid insulation (Kraft paper, cellulose products, PVC, PE and XLPE.

Liquid InsulantsThe insulating fluid serves three primary purposes:Provides dielectric strength Provides sufficient cooling Protects the insulation system.

Solid Insulating Materials The solid material insulates the winding because it possesses two distinct properties.It has the ability to withstand both electrical and mechanical stresses due to the voltages used. It is such a poor conductor that a negligibly small current can flow through it and leak away. In other words, a good insulator will neither allow current to break through it nor to steal through it.

Solid Insulting materials (Contd)Consequently, a practical insulation system must contain material that performs the four major function listed below: The ability to withstand the relatively high voltages encountered in normal service (dielectric strength). This voltage includes both impulses and transient surges The ability to withstand the mechanical and thermal (heat) stresses which accompany a short circuit.

Solid Insulting Materials (Contd)The ability to prevent excessive heat accumulation (heat transfer). The ability to maintain desired characteristics for an acceptable service life period, given proper maintenance.

Cooling in Transformers No-load losses and load losses are the two significant sources of heating considered in thermal modeling of power transformers. Load losses are the more significant source of transformer heating, consisting of copper loss due to the winding resistance and stray load loss due to eddy currents in other structural parts of the transformer.

Cooling In Transformers (Contd)The basic method for cooling transformers is transferring heat from the core and windings to the insulating coolant such as oil. The wasted energy in the form of heat generated in the transformers due to the foregoing iron and copper losses must be carried away to prevent excessive temperature rise and injury to the insulation about the conductors.

Cooling In Transformers (Contd) Cooling may be by Natural circulation Forced cooling using of fans, pumps, etc.

Cooling System

Cooling System

Cooling Arrangements and Designations Both the IEEE and the IEC established standard designations for the various cooling modes of transformers The designation completely describes the cooling method for the transformer, and the cooling method impacts the response of the transformer insulting oil to overload conditions.

Cooling Arrangements and Designation (Contd).

DRY-TYPE TRANSFORMERSNatural Cooling: type AN Forced Cooling: type AF

OIL-IMMERSED TRANSFORMERSOil CirculationCooling MethodIEC AbbreviationNatural Thermal Head only Air naturalONANAir blastONAFForced Oil Circulation by Pumps Air natural OFANAir blastOFAFWater forcedOFWF

Transformer Loading And Temperature Limits Design standards express temperature limits for transformers in rise above ambient temperature. The use of ambient temperature as a base ensure a transformer has adequate thermal capacity, independent of daily environmental conditions.

Transformer Loading And Temperature Limits (Contd)The useful life of a transformer is dependent on the life of its insulation, which ages rapidly at elevated temperatures To ensure a reasonable expectancy of life, it is essential that the transformer is loaded according to the prevailing ambient temperature and also to the temperature of the windings before loading.

Transformer Loading And Temperature Limits (Contd)

Type Insulation ClassTemperature Rise (oC)ANA60AFB80ANC150ONANA65ONAFA65OFANA65OFAFA65OFWFA65

Tap Changing FacilityOne of the simplest and most inexpensive methods of providing for adjustments in supply voltages is to arrange tapings on transformer windings Tapings are usually provided for the following purposes:

Purposes of Tap ChangersFor maintaining the secondary voltage constant with a varying primary voltage.For varying the secondary voltage. Consumers terminal voltages are reduced on account of impedance drops, and this necessitates tap-changing facilities to effect a slight change in the turn ratio. Seasonal (5-10%), daily (3-5%) and short-period (1-2%) adjustments are needed in accordance with the corresponding variations of load.

Purposes of Tap Changers (Contd)For providing an auxiliary secondary voltage for a special purpose, such as lightingFor providing a reduced voltage for starting rotating machinery Control of active and reactive power flow in the power system networkFor providing a neutral either for earthing, or for dealing with out-of-unbalance current in single-phase, three-wire circuits, in three-phase four-wire circuits, etc.

Tap Changing ModesOff Load Tap Changing On-Load Tap Changing.

Three-Phase Connections The main connections to be considered are:Primary SecondaryY-Y, Y-Delta, Delta-Y, Delta- DeltaY connections provide the opportunity for multiple voltages, while Delta connections enjoy a higher level of reliability (if one winding fails open, the other two can still maintain full line voltages to the load).

Transformer Connections (Contd).Probably the most important aspect of connecting three sets of primary and secondary windings together to form a three-phase transformer bank is proper winding phasing The dots are used to denote polarity of windings.

Transformer Connections (Contd)With these phase angles, the center point of the Y must the other all - or all + winding ends togetherWith these phase angles, the winding polarities must stack together in a complementary manner (+ to -)

Transformer Connections (Contd)Getting this phasing correct when the windings arent shown in regular Y or Delta configuration can be tricky. An illustration is shown below:

Y-Y Configurations

Y- Delta ConfigurationY - Delta

Delta Y Configurations

Delta Delta Connections

Tertiary WindingsTertiary windings, in the form of delta connection, have been used on star-star connected three-phase transformers and groups.As its name implies, a tertiary winding is simply a third winding of a transformer unit or group, and the general form is the closed delta for three-phase working.

Tertiary Windings (Contd)The star-star connection has been regarded with some disfavour on account of Its third-harmonic phenomena and Its behaviour when transforming very seriously unbalanced loads.So that the desire to eliminate these two effects is the principal reason for incorporating tertiary delta windings.

Tertiary Windings (Contd) The uses or purposed of the tertiary delta windings for star-star connected transformers and groups are to:Reduce third-harmonic voltage components Permit the transformation of unbalanced three-phase loads Supply an auxiliary load in addition to the main load.

Transformer Service ClassificationsPower TransformersGenerator Unit Transformer Transmission TransformersSub-Transmission Subtraction TransformersDistribution TransformerInstruments Transformers Current TransformersVoltage Transformers

Transformer MaintenanceThree kinds of maintenance are normally recognized:Unscheduled maintenance- leads to inevitable breakdown.Ordinary maintenance repairs, adjustment and replacement of parts shown to be necessary by visual inspections made at irregular intervals:Scheduled maintenance regularly scheduled inspections and periodic dismantling or testing of equipment to check every detail likely to cause trouble.

Transformer Maintenance (Contd)The possibility of a fault occurring in a transformer is NOT something remote. In the light of this, the need to undertake protective maintenance is obvious.There are three types of protective maintenance.

Protective MaintenancePredictive maintenance Preventive maintenance Corrective maintenance

Predictive Maintenance Predictive maintenance involves more frequent monitoring (inspection and testing) of critical equipment by location, function and by operating environment.

Preventive Maintenance Preventive maintenance of transformers is an integral part of an annual maintenance program.It may involve monthly inspection, annual energized testing of equipment (oil testing, gas-in-oil analysis, infrared inspection), and de-energized biennial or triennial dismantling or testing of equipment To check every detail likely to cause trouble (electrical insulation test, switchgear, and so forth)

Corrective MaintenanceCorrective maintenance is concerned with units which have shown some definite warning signals (such as cloudy oil or unusual odour).When equipment performance begins to tail off and the deterioration so recognized, corrective maintenance is carried out to pinpoint the causes.

Important Insulation Oil TestsDielectric Breakdown Test Neutralization Number/Acidity Test Moisture Content Test Interfacial Tension Test Oil Power Factor.

Insulation Oil Tests (contd)Oil Colour Test Visual Examination Specific Gravity Test Sediment in Transformer Oil Test Dissolved Gas Analysis.

Dielectric Breakdown TestDielectric strength is a measure of the electrical strength of a material as an insulator.Dielectric strength is defined as the maximum voltage required to produce a dielectric breakdown through the material and is expressed as volts per unit thickness.The higher the dielectric strength of a material the better its quality as an insulator. Instrumentation available now for dielectric breakdown test is quite compact and efficient.

Dielectric Breakdown Test (Contd)The primary tests that measures insulation quality are ASTM D-877 and D-1816, D-924 The dielectric breakdown potential tests use two electrodes of fixed geometry and a specified separation. D-877D-1816Cap 10Cap 10

Dielectric Breakdown Test (Contd)Voltage is applied and uniformly increased at a rate of approximately 3 kV/s (rms value) until breakdown occurs. When the current arcs across the gap, the voltage recorded at that instant is the dielectric breakdown strength of the insulating liquid.For new oils, this is considered to be 35 kV or above. Used oils would not be acceptable below 25kV.

Effect of Moisture on Dielectric BreakdownIt is commonly acceptable that water that is dissolved in the oil has a minimum effect on the dielectric breakdown voltage.Free water, on the other hand significantly lowers the dielectric strength of the oil. The variation of dielectric breakdown potential with moisture is shown in Fig. Below.

Effect of Moisture on dielectric Breakdown Diagram.

Acceptable test Values For Dielectric Strength

Liquid TypeTestSatisfactoryNeed Reconditioning Mineral Oil Dielectric Strength23kV< 23kV

Neutralization/Acidity Test Mineral oil will be oxidized when dissolved oxygen is presentThe extent of oxidation can be determined by measurement of the acid concentration in the oil (due to oil oxidation) according to ASTM D-974.The ASTM D-974 and D-664 are laboratory tests whereas D-1534 is a field test that determines the approximate total acid value of the oil.

Neutralization/Acidity Test (Contd0The acid number or the neutralization number is the milligrams (mg) of potassium hydroxide (KOH) required to neutralize the acid contained in 1 g of transformer oil. Test data indicate that the acidity is proportional to the amount of oxygen absorbed by the liquid.Therefore different transformers would take different periods of time before sludge would begin to appear.

Acidity Test LimitsIEEE Guideline, C57.106-1991, gives recommended acid levels at which the oil should be reclaimed, reprocessed or replaced. Their recommendations are summarized in the Table below:

Acidity Test Limits (Contd)

Test and Method Group IIGroup IIIAcid number (mg KOH/g, maximum)0.20.5Interfacial Tension (dynes/cm, minimum)2416

IEEE Guide for Acceptance and Maintenance of Insulating Oil in Equipment Group I: This group contains oils that are in satisfactory condition for continued use. Group II: This group contains oils that require only minor reconditioning for further use. (Mechanical removal of moisture and insoluble contaminants).Group III: This group contains oils in poor condition. They should be reclaimed or disposed of, depending upon economic considerations. (Requires mechanical and chemical clean up procedures.)Group IV: This group contains oils that are in such poor condition that it is technically advisable to dispose of them.

Moisture content TestMineral oil is hydrophobic and the amount of water that will dissolve in oil is very small.The solubility of water in oil is dependent on temperature as shown in Fig. Below.

Moisture Content Test Effect of Temperature.

Moisture Content Test The amount of moisture that can be dissolved in oil increases rapidly as the oil temperature increases as shown in Fig above Therefore insulating oil purified at too high a temperature may lose a large percentage of its dielectric strength on cooling, because the dissolved moisture is then changed to an emulsion unless vacuum dehydration is used as the purification process.

Moisture Content Test (Contd)It is commonly accepted that water that is dissolved in the oil has a minimum effect on the dielectric breakdown voltage. Free water, on the other hand significantly lowers the dielectric strength of the oil. The variation with moisture is shown in Fig below.

Moisture content Test.The maximum allowable moisture in oil for different voltage levels are as shown in table below:

Voltage level (kV)Maximum moisture530153035256920> 13815

Oil power Factor TestThe power factor indicates the dielectric loss of the liquid and thus its dielectric heating.The power factor test is used as an acceptance and preventive maintenance test for insulting liquid.Liquid power factor testing in the field is usually done with portable, direct-reading power factor measuring test.

Oil Power Factor Test (Contd)Power factor tests on oil are commonly made with an ASTM D-924 test cell.The power factor test has been a traditional field test. However, this test is considered a negative screening test. It measures the leakage current through an oil, which is a measure of the contamination or deterioration.

Oil Power Factor Test (Contd)Unfortunately, the power factor test is not specific in what it detects.It does tell of the presence of polar materials, but other tests must be made to determine what polar compounds are present.Good new oil has power factor of 0.05 percent or less at 20 degC

Oil Power Factor Test (Contd)Higher loss indicators represent deterioration and/or contamination with moisture, carbon or other conducting matter, varnish, sodium soaps, asphalt compounds or deterioration products. The power factor can gradually increase in service to a value as high as 0.5 percent at 20 degC, without, in most cases, indicating deterioration.

Oil Power Factor Test (contd) When the power factor exceeds 0.5 percent, an investigation is needed to actually ascertain what is causing the high loss factor.Dielectric strength tests should be made to determine the presence of moisture.For maintenance purposes, loss factor test values are as shown in the table below:

Oil Power Factor Test Limits

Liquid TypeTestSatisfactory yNeed ReconditioningMineral OilLoss factor 0.5%> 0.5%

Oil Colour Comparison Test/ Visual Examination of Oil SampleColour determination according to ASTM D-1524 is a quick method to measure the extent of oxidation.This test consists of transmitting light through oil samples and comparing the colour observed with a standard colour chart.The colour chart ranges from 0.5-8, with the colour number 1 used for new oil.

Oil Colour Comparison Test/Visual Examination (Contd)The range of colour test values for transformer oil is listed in the table below:

Liquid TypeTest SatisfactoryNeeds ReconditioningMineral Oil Colour3.5> 3.5

Specific Gravity Test This ASTM D-1298 test is conducted by floating a hydrometer in oil and taking the reading at the meniscus.For oil free of contaminants, such as water, askeral or silicone, the reading should be less than 0.84

It is the most important diagnostic tool available today for transformer maintenance.It is the only test that can determine the operating status of oil-filled electrical equipment. Insulating materials within transformers and related equipment break down to liberate gases within the unit.Gas-In-Oil Analysis

Gas-In-Oil Analysis (Contd)The distribution of these gases can be related to the type of electrical fault and the rate of gas generation can indicate the severity of the fault.The identity of the gases being generated by a particular unit can be very useful information in any preventive maintenance program.

Methods of Gas DetectionTotal Combustible Gases (TCG) Method Gas Blanket Analysis Dissolved Gas Analysis.

Total Combustible Gases (TCG) MethodThe TCG method determines the total combustible gases present in the gas above the oil.The major advantage of the TCG method is that it is fast and applicable to use in the field and it can be used to continuously monitor a unit.

Total Combustible Gases (TCG) Method - DisadvantagesHowever, there are a number of disadvantages to the TCG method Although it detects the combustible fault gases (hydrogen, carbon monoxide, methane, ethane, ethylene, and acetylene), it does not detect the non-combustible ones (carbon dioxide, nitrogen, and oxygen).This method is only applicable to those units that have a gas blanket and not to the completely oil-filled units of the conservator type.

Total Combustible Gases (TCG) Method Disadvantages contd.Another disadvantage of the TCG method is that it gives only a single value for the percentage of combustible gases but does not identify which gases are actually present. The gases present give an indication of the type of fault that has occurred

Gas Blanket AnalysisIn the gas blanket analysis, a sample of the gas in the space above the oil is analysed for its composition.This method detects all of the individual components:However, it is not applicable to the oil-filled conservator type units.

Gas Blanket Analysis - DisadvantagesIt suffers from the disadvantage that the gases must first diffuse into the gas blanket.In addition, this method is not at present best done in the field.A properly equipped laboratory is needed for the required separation, identification and quantitative determination of these gases at the part per million level.

Dissolved Gas Analysis The third and most informative method for the detection of fault gases is the dissolved gas analysis (DGA) techniqueIn this method a sample of the oil is taken from the unit and the dissolved gases are extracted. Then the extracted are separated, identified, and quantitatively determined. This entire technique is best done in the laboratory since it requires precision operations.

Dissolved Gas Analysis-AdvantagesSince this method uses an oil sample it is applicable to all type unitsAnd like the gas blanket method it detects all the individual gas 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 advantages alone outweighs any disadvantages of this technique.

Dissolved Gas Analysis-Advantages (Contd)Advance warning of developing faults Determining the improper use of units Status checks on new and repaired unitsConvenient scheduling of repairs Monitoring of units under overload.

Fault Gases The causes of fault gases can be divided into three categories:Corona or partial discharge Pyrolysis or thermal heating, and Arcing.The most severe intensity of energy dissipation occurs with arching, less with heating, and least with corona.

Fault Gases (Contd)A partial list of fault gases that can be found within a unit are shown in the following three groups:Hydrocarbons and hydrogen

MethaneCH4EthaneC2H6EthyleneC2H4AcetyleneC2H2HydrogenH2

Fault Gases (Contd)Carbon Oxides

Carbon monoxideCOCarbon dioxideCO2

Non-Fault Gases

Nitrogen N2Oxygen O2

Other Transformer TestsTurns and Polarity TestsInsulation power Factor Core Excitation Current Polarization Index

Other Transformer Tests (Contd)Insulation (AC) resistance D.C. Winding Resistance D.C. Over-potential Core Ground Ground Resistance.

Turns Ratio And Polarity TestsRatio and polarity tests are carried out on every transformer to ensure that the turns ratio of the windings is correct and also the tapping on any of windings have been made at the correct position.The turns ratio test is primarily used as an acceptance test. It is also useful as a tool for investigating problems, as well as an integral part of a routine maintenance program.

Turns Ratio And Polarity Tests (Contd)The turns ration does not tell how many turns of wire are on the primary or secondary coil, but only gives the ratio of the primary to secondary turns.During the manufacture of new transformers, the turn ratio test is performed on all tap positions to verify that the internal connections are correct and that there are no short circuited turns

Turn Ratio And Polarity Tests (Contd)During routine maintenance tests, the turns ratio test could be performed to identify short circuited turns, incorrect tap settings, errors in turn count, mislabeled terminals, and failure in tap changers.

Power Factor Tests Insulation power factor should not be confused with system power factor in an AC network.Insulation power factor provides an indication of the quality of the insulation Any winding in a transformer is separated from all other winding and ground potential by solid insulation

Power Factor Tests (Contd)Cellulosic insulation forms an effective capacitance networkIn each capacitance are dielectric losses which can be conveniently represented by a resistor in series with a capacitor.The insulation power factor is commonly defined as the ratio of the resistance R to the impedance Z of this combination and can be measured by applying a voltage across this capacitance and measuring the amperes and watt loss and then calculating the power factor.

Power Factor Tests (Contd) This measurable dielectric loss will develop heat in the insulation during transformer operation (in the equivalent resistor) and his heat, along with moisture and other factors can cause deterioration of the insulation.

Core Excitation Current TestExcitation current measurement is used for Field detection of shorted turns and Heavy core damage such as shorted laminations or core bolt insulation breakdown.

Polarization Index (PI)The polarization index is obtained from the dielectric absorption data.Polarization index is the ratio of the 10 minute resistance to the 1 minute resistance valueIt is dimensionless and often used in dielectric evaluation.

Polarization Index (PI) Guide

Polarization index Guide for Evaluation of Transformer Condition.Polarization IndexConditionLess than 1.0Dangerous1.0-1.1Poor1.1 1.25Questionable1.25 2.0FairAbove 2.0Good.

DC Winding Resistance Test The DC winding resistance test, indicates a change in DC winding resistance when there are short circuited turns, poor joints or bad contacts.This reading should be compared to factory test information. High readings could indicate loose or dirty connections. Prior readings, are however, needed for comparison.

DC Over potential TestingInsulation failure even in properly designed equipment may result from such causes as:Ageing and embrittlement due to evaporization of plasticizers, stabilizers or antioxidants or electrolytic deterioration, chemical deterioration, Lowered strength due to the absorption of moisture,

DC Over potential Testing (Contd)Physical damage Insulation cold or hot (polymers) flow or gradual tracking due to corona in internal voids or across the surface with eventual breakdown due to transients or abnormally high voltage stresses

DC Overpotential Testing (Contd)The DC test voltage should be applied to the winding in approximately 10 steps, recording the leakage current in each step.The leakage current is plotted against the test voltage as the test progresses.In this way, the condition of the equipment is under constant surveillance and the test can be stopped if the current is rising too rapidly.

DC Overpotential Testing (Contd) As long as the leakage versus voltage curve is relatively flat (Point A to Point B), the item under test is considered to be in good condition. At some point the current will start rising at a more rapid rate (point C).

Instrument Transformers For currents greater than 100A and voltages higher than 500V, it is difficult to construct ammeters and current coils of wattmeters, energy meters and relays carrying alternating currents greater than 100A. Specially designed transformers known as instrument transformers are used for this purpose.

Instrument Transformers They are used for extending the range of AC instruments in preference to shunts and series resistors. Furthermore, it is dangerous to connect such instrument to voltages exceeding 500V.Instrument transformers therefore insulate the instruments from high voltage.

Instrument Transformers- (Do)As transformers, they are electromagnetic device By using instrument transformers, electrical instruments have been standardized to operate on 220V and 5A or 1A. They are essential parts of many electrical systems, and are used for Measuring (metering) and Monitoring (relaying) devices.

Instrument Transformers (DO)The quality of instrument transformers will affect directly the overall accuracy and performance of these metering and monitoring systems.Instrument transformer performance is critical in protective replaying, since the relays can only be as good as the instrument transformers.

Basic Function of Instrument TransformersTo change the magnitude (but not the nature) of the quality (voltage or current) being measured to a suitable level for use with standard instruments (protective relays, metering equipment, etc).To provide insulation between primary and secondary circuit for equipment and personnel safety

Types of Instrument TransformersInstrument transformers are of two types,depending upon whether it is used to excite the current or voltage coil of the measuring instrument Current Transformers- CTsVoltage Transformers VTs (also referred to as Potential Transformers, PTs).

Types of Instrument Transformers (Contd) Both of these types act as insulators between high-voltage primary and low-voltage secondary.The primary of the VT is connected either line-to-line-to-neutral, and the current that flows through its winding produces a flux in the core. The ratio of primary to secondary voltage is in proportion to the turns of ratio and will usually produce 230-240V at the secondary terminals with rated primary voltage applied.

Accuracy of Instrument Transformers To be a useful part of a measurement system, instrument transformers must change the magnitude of the quantity being measured without introducing any excessive unknown errors.The accuracy of an instrument transformer must either be of a known value, so that errors may be allowed for, or the accuracy must be sufficiently high that errors introduced by the instrument transformer may be ignored.

Factors Affecting Accuracy of Instrument Transformers Design of the instrument transformer Circuit conditions such as voltage, current and frequency Burden connected to the secondary circuit of the transformer

Burden of Instrument TransformersIn instrument transformer operations, the primary quantities are reduced by the turns ratio to provide a secondary current or voltage to energize protective relays and other equipment. The totality of the impedances of the loads connected to current or voltage transformers are referred to as burden.

Burden of Instrument Transformers (Do)The burden consists of the impedances of the following:Secondary winding of the instrument transformerInterconnecting leads Relay and/or other connected devices.

Burden of Instrument Transformer (Do).For the devices, the burdens are usually expressed in volt-amperes at a specified current or voltage.Thus for CTs or VTs, if Zb is the total connected burden impedance and is the volt-ampere burden, then the following burdens are obtained for CTs and VTs.

Rated Characteristics of CTs Rated primary current. Rated short time current (primary) Rated secondary current. Rated exciting current. Rated burden.Insulation level (primary).

CTS Characteristics ( Do) Current error or ratio error. Phase angle error. Composite error. Accuracy class. Over current factor.

Calculation of Burden of CTs VA= IZB x I2ZB ZB=VAI2

Example 1: Sample Calculation of CT Burden A current transformer rated 30VA has a secondary rated current of 5A and impedance of 0.211 ohms. If this CT supplies a relay through as lead of resistance 0.4 ohm, calculate the relay burden.

Solution 1: Calculation of CT Burden The Permissible burden of the CT is ZB=VA = 30 = 1.2 1252ZB= relay burden+ lead resistance + CT secondary impedanceRelay burden = ZB lead resistance-CT secondary impedances = 1.2-0.4-0.-0.211 = 0.589

CTs For Relay ApplicationsCurrent transformers, which step down primary currents to lower, safer, measurable values, are required for Indicating and graphic ammetersEnergy meters and wattmeter (kWh and kW meters)Telemetering Protective relays.

CTs For Relay Application- (Do) A CT has a high-current primary winding which is connected in series with the line or load whose current is to be metered, Whilst its secondary winding is connected in series with the current coil of the meter (e.g. ammeter, wattmeter, energy meter, relay, etc).

CTs For Relay Applications- (Do)The primary current rating should be selected from standardized values. The value of the rated secondary current shall be either 1A or 5A.

Caution When Operating CTs It is VERY DANGEROUS to operate the secondary of the CT on OPEN-CIRCUIT while on loadREASONS if the secondary is open-circuited while the primary is carrying a heavy load, The secondary current suddenly drop to zero, and The demagnetizing or balancing effect of the secondary mmf will no longer exist.

Reasons for Not Open Circuiting The CT When On-Load (Contd) But at that instant of the secondary being open-circuited, the primary current continues to flow, so that there is now a large unbalanced primary mmf, which will drive the core heavily into saturation on each half-cycle of the AC supply.The excessive core flux will induce a VERY HIGH VOLTAGE in the multi-turn secondary circuit, which may puncture the insulation or produce a dangerous shock to personnel who come into contact with it.

Reasons For Not Open-Circuiting The CT When On-Load (Contd)In a high-quality transformer, the induced voltage can be several kilovolts Furthermore, the unopposed primary ampere-turns will produce abnormally high flux far in excess of the useful (or mutual) flux produced by the no-load currentThis excess flux in the core causes large eddy-current and hysteresis losses and excessive heating in the core and windings, and the resulting temperature rise can damage the insulation.

Some Standard CT Radios

CT RatioCT RatioCT Ratio50:5300:5800:5100:5400:5900:5150:5450:51000:5200:5500:51200:5250:5600:5

Selection Criterion For CT RatioThe major criterion for the selection is almost invariably the maximum load current In other words, the CT ratio should be selected such that the CT secondary current at maximum load should NOT exceed the continuous current rating or the thermal limits of the connected relay and equipment. This is particularly applicable to phase-type relays where the load current flows through the relays.

Example 2: Selection of CT RatioConsider a circuit to be protected by an overcurrent relay maximum load current of 90A.Select the appropriate CT ratio.

Solution 2: Selections of CT RatioThe CT should be selected to provide just under 5 A secondary current for the maximum load current. And so select, say, a CT ratio of 100:5, that isCTR = 100 =20 5

Solution 2: Selection of CT Ratio (Contd)For the maximum load of 90A, this gives a maximum continuous secondary current of, Is = Imax load = 90 = 4.5ACTR 20Which is slightly below the rated 5A

IEC Specification of CT Accuracy 15 VA Class to 10 P 20Where 15 represents the continuous VA burden 10represents the accuracy class Pstands for protection 20 represents the accuracy limit factor Thus for such a 15 VA burden CT rated at 5A, the VA/load is 15/5 =3V, and will have no more than 10% error up to 20x3 = 60 V secondary.

Standard Values For Relaying CTsContinuous VA burden 2.5, 5, 10, 15 and 30.Accuracy classes- 5% and10%Accuracy limit factor- 5, 10, 15, 20 and 30Rated secondary amperes 1 A, 5 A(5A preferred)

VTs For Relay ApplicationsVoltage transformers, which step down system voltages to sufficiently low, safer, measurable values, are required for Indication of the voltage conditions. Energy meters and watt meters (kWh and kW meters) Protective relays Synchronizing

Caution When Operating VTsAs a precaution, voltage transformer should NEVER be operated on SHORT-CIRCUIT.REASON- The full-load voltage at the primary will produce a very large current at the secondary that may damage the windings.

Types of VTs for Protective Relaying.Voltage transformers have wound primaries that are Either connected directly to the power systems (VTs)Or across a selection of capacitor string connected between phase and ground, that is, coupling-capacitor voltage transformers (CCVTs)

Normal CTs and CCVTs

Points To Note About VTs VTs are used at all power system voltages,and are usually connected to the bus. Usually the CCTVs are connected to the line, rather than to the bus, because the coupling capacitor devices may also be used to couple radio frequencies to the line for use in pilot relaying At about 115kV, the CCVT types becomes applicable and generally more economical than VTs at the higher voltages.

Points to Note About VTs (Do)Either type of voltage transformer (VT or CCVT) provides excellent reproduction of primary voltage, both transient and steady-state, for protection functions.Saturation is not a problem because power systems should not be operated above normal voltage, and faults result in a collapse or reduction in voltage.VTs are normally installed with primary fuses, which are not necessary with CCTVs. Fuses are also used in the secondary.

Transformer Protection The degree of protection provided for a transformer depends to some extend upon its size, rating and importance of the unit. This protection will comprise a number of systems each designed to provide the requisite protection for the different fault conditions identified.

Transformer Protection (Contd)A further important factor is the economic aspects.The cost of protection of transformers tends to be proportionally higher than the cost of protection for other plants items or equipments.

Transformer Protection Types Differential protection Overcurrent protection Restricted earth-fault protection Tank earth-fault protection Gas generation and oil surge protection (Buchholz relay) Winding temperature protection.Over fluxing protection.Thermal overload protection.

Differential Protection In Transformers Fault protection for transformers is obtained principally by differential type relays. Differential protection provides the best protection for both phase and ground faults, expect in ungrounded systems or where the fault current is limited by high impedance grounding.

Differential Protection In Transformers (Contd)This types of protection is usually given to large power transformers that are in excess of 500kVA.However, differential relays cannot be as sensitive as the differential relays for generator protection because of the following factors not ordinarily present for generators

Problems of Differential Relaying In Transformers

Transformer taps for voltage control Different voltage levels, including taps. Hence different primary current in the connecting circuits.Different CT types, ratios and performance characteristics, with possible mismatch of ratios.

Problems of Differential Relaying In Transformers (Contd)Phase shifts in star-delta connected banks, affecting accurate or faithful reproduction of current, both in wave shape and magnitude.Magnetizing inrush current which appears to the differential relay as an internal fault (current into, but not out of the transformer)

Magnetizing Inrush Currents In Transformers When a transformer is first energised, a current transient known as magnetizing inrush current flows, and appears as an internal fault to the differential-connected relays.Peak inrush currents of 8 to 30 times full-load peaks are common.

Magnetizing Inrush Currents In Transformers (Contd)If the transformer had been energised previously, there is a high possibility that on de-energization, some residual flux r was left in the iron core Upon re-energization, the residual flux may either add or subtract from the total flux , depending on the time point of re-energization, thereby increasing or decreasing the magnetizing inrush current.

Factors Affecting Magnitude And Duration of Inrush CurrentSize of the transformer bank Time point of energization with relation to the flux requirements Size and nature of the power system source Type of iron used in the manufacture Prior history of the transformer bank (residual flux) L/R ratio of the transformer and system.

Magnetizing Inrush Currents in Transformers (Contd)Power transformers are operated normally near the knee of the saturation curve And so the additional flux from the magnetizing inrush current plus any residual flux will definitely saturate the iron and thereby increase the magnetizing current components.

Magnetizing Inrush Current In Transformers (Contd)The inrush current decays rapidly for the first few cycles, and then very slowly, sometimes taking 4 to 5 seconds to subside, where the resistance is low. The time constant (L/R) of the circuit is not a constant because L is variable because of the transformer saturation.Time constants for magnetizing inrush currents vary from 10 cycles for small units to 1 minutes for large units.

Magnetizing Inrush Current In Transformers (Contd) The resistance from the source to the transformer bank determines the damping of the Inrush current wave. Hence transformer banks adjacent to near a generator will have a long inrush current.

Solutions to Differential Relaying Problems In Transformers Since the inrush current appears as an internal fault to differential relays some means of desensitizing them during an inrush is necessary. Several methods exist, and all are in general use. They include the following:

Solutions to Differential Relaying Problems In Tranformers (Contd) A differential relay with reduced sensitivity to the inrush current wave (a higher pickup, plus time delay to override the high initial peaks)A voltage-operated automatic tripping suppressor unit in conjunction with the differential relay.

Solution to Differential Relaying Problems In Transformers (Contd)Harmonically desensitizing the differential relay during transformer bank enegization. Use of the harmonic content of the inrush current to restrain, and thus desensitize the relay unit. The magnetizing inrush current contains a predominance of even harmonics, particularly the second.

Solutions to Differential Relaying Problems in Transformers (Contd)In the harmonic desensitization:It is necessary to provide sufficient restraints for the inrushesYet provide some degree of sensitivity for internal faults which may also contain a large amount of harmonics because of the nature of fault or because of a combination of light fault with an inrush.

Overcurrent Protection In Transformers.The degree of protection that is provided by an overcurrent relay is often limited when applied to transformers. Since the relay must not operate under emergency loading conditions, it requires a high current setting (often about 200% rating).

Overcurrent Protection In Transformers (Contd) On large transformers, therefore overcurrent protection is often employed as a back-up protection for terminal faults or uncleared low voltage (LV) system faults.In such cases overcurrent relays are installed on both sides of the transformer according to that requirements, and may trip the side of the transformer with which they are associated, or they may trip both.

Restricted Earth-Fault Protection In Transformers The difficulties inherent in the provision of adequate earth-fault sensitivity in the overalll differential protection system often requires that the restricted earth-fault protection should be added to both windings of the transformer Separate CTs are mainly used for this purposed or CTs associated with the overall differential protection can be used for the restricted earth-faulty protection.

Short-Circuit Protection With Overcurrent Relays in Transformer Overcurrent relaying is used for fault protection of transformers having circuit breakers only when the cost of differential relaying cannot be justified.Overcurrent relaying cannot compare with differential relaying in terms of sensitivity.

Short-Circuit Protection in Transformer (Contd)Three CTs, one in each phase, and at least two overcurrent phase relays and one overcurrent ground relay should be provided on each side of the transformer bank that is connected through a circuit breaker to a source of short-circuit current.

Short-Circuit Protection In Transformers (Contd).The overcurrent relays should have An Inverse-time element whose pickup can be adjusted to somewhat above maximum rated load current, say about 150% of maximum, and Sufficient time-delay so as to be selective with the relaying equipment of adjacent system elements during external faults.

Tank Earth-Fault Protection In TransformersThe transformer tank is lightly insulated from earth and all earthed cables sheaths, and then bonded to earth via a single copper strap, over which is mounted a CT connected to a relay.Any earth faults within the transformer tank will produce a current in the earthing strap which operates the relay.

Gas Generation and Oil Surge Protection In TransformersAll faults within the transformer give rise to the generation of gasGas production may be slow for minor (or incipient) faults or violent in the case of heavy faults.The generation of gas is used as a means of fault detection in the gas-and oil-operated relay (Buchholtz relay).

Gas Generation and Oil Surge protection In Transformer (Contd)The rising bubbles produced by the slow generation of gas, due to a minor fault passes upwards towards the consevator but are trapped in the relay chamber causing a fall in the level inside it. This disturbs the equilibrium of the gas float, thereby closing its contacts which would normally be connected to give an alarm.

Gas Generation and Oil Surge Protection In Transformer (Contd)A heavy fault will produce a rapid generation of gas, causing violent displacement of the oil which moves the surge float system of the relay in passing the conservator.This will result in the closure of the surge float contacts which are arranged to trip the transformer.

Winding Temperature Protection In Transformers Large transformer with forced cooling are usually fitted with temperature devices to Detect overloading of the transformer or Failure of the cooling equipmentTwo winding temperature instruments are generally fitted on each transformer; each instrument is fitted with two mercury switch contacts.

Winding Temperature Protection In Transformers (Contd)Operation of one instrument is arranged to start the cooling fans and pumps, and this gives an alarm.The other instrument is arranged to give the same alarm and to trip the low voltage circuit breaker.

Grounding Protection In TransformersOn grounded-neutral systems, protection can be provided by insulating a transformer tank from ground And connecting it to ground through a CT whose secondary energizes an overcurrent ground relay.

Remote Tripping In Transformers When a transmission line terminates in a single transformer bank, the practice is frequently to omit the high-voltage circuit breaker and thereby avoid considerable expense.Such practice is made possible by what is called transferred tripping or, preferably, remote tripping.

Remote Tripping in Transformers (Contd) Remote tripping is the tripping of the circuit breaker at the other end of the transmission line for faults in the power transformer. The protective relays at that other end of the line are not sensitive enough to detect inter-turn faults inside the transformer bank.

Remote Tripping In Transformers (Contd)Consequently, the transformer banks own differential-relaying equipment trips the banks low-voltage breaker and initiates tripping of the breather at the other end of the line.

System grounding Ungrounded (or Isolated) SystemsArching Ground Faults Solidity-Grounded Systems Impedance-Grounded Systems Arc-Suppression (or Resonant) Grounding SystemGround-Fault Detection Methods

Introduction The subject of earthing may be divided into two. General Equipment earthing System neutral earthing The main objects of earthing are to: Reduce the voltage stresses due to switching, lighting, faults, etc Control fault currents to satisfactory values.

General Equipment Earthing It is the practice of earthing the metallic frames of electrical equipment Purpose Improve safety to Operational staff The general public Property in general and System electrical equipment

System Neutral Earthing It is the practice of earthing the star-point or neutral of the electrical power system.The method of earthing employed affects the system behaviour, the levels of currents and voltages in the even of a fault In general, a low grounding impedance leads to higher earth-fault current but lower overvoltages

Factors Affecting Earth Fault Current Phase voltage Neutral earthing arrangement of the system and Local earthing resistance between the metallic frames and earth.

Concept of Hazard Voltage, Step Voltage and Touch VoltageHazard voltage- refers to the type of potential or voltage distribution in the immediate vicinity of a tower or mast, following a ground faultStep voltage the voltage that exists between the two feet of a person standing on the grounding system.

Hazard voltages Types Touch Voltage the voltage that exists between the hand and both feet of the person, upon touching the tower or mast having earth fault current through it (occasioned by, say a flashover).

Hazard Voltage Distribution Around Steel Pole

Hazard Voltage Distribution (Continued)Fig shows the variation in hazard voltage (voltage gradient) around a steel pole The further one is from the pole, the lower the potential to which he is subjected.

System Neutral Earthing ArrangementsIsolated or Ungrounded Neutral Solidly or Direct Earthing Impedance Earthing Arc-suppression or peterson-coil Earthing

Isolated or Ungrounded Neutral SystemIn an isolated neutral system, there is no physical or direct connection (I.e., infinite impedance) between the systems neutral and earthThis results in the zero-sequence impedances of the generators and transformers having infinite value, and the network zero-sequence impedance being determined by the earth capacitances of the lines

Isolated or Ungrounded Neutral System (Continued)This results in the zero-sequence impedance of the generators and transformers having infinite values.And so the network zero-sequence impedance is determined by the earth capacitances of the lines Ungrounded neutral systems are thus in effect capacitively grounded neutral systems, the capacitance being the conductor capacitance to earth.

Isolated System Voltage and Current Phasor Representations

Isolated System-Voltage and Current Phasor Representations (continued) The line conductors have capacitances between one another and to ground. The former as represented by the delta set of capacitances have little influence on the grounding characteristics of the system, and will be discarded from the considerations.

Isolated System Voltage and Current Phasor Representations (Continued)In normal balanced operation, the capacitive currents in each of the earth capacitances will be equal and displaced 1200 from each other.The voltages across each branch are therefore equal and also displaced by 120 degrees to each other The capacitive currents of all three-phase lines are leading the respective line to neutral voltages by 90 degrees, and the vector sum all three currents is zero.

Isolated System Voltage and Current Phasor Representations (continued)But during a phase-earth fault on say, phase B, the charging/capacitive current of the faulted phase goes to zero because its voltage to earth is zero The phasor relationships after the line ground fault on the isolated system are shown in fig below:

Isolated System Phase Ground Fault

Isolated System-Phase Ground Fault (Contd)When phase B is grounded, the voltages V-an and V-cn of the healthy phases across the other two earth capacitance branches will increase to line-to-line values V-ag and V-cg respectively with respect to ground

Isolated System Phase-Ground Fault (Contd)As can be seen, these voltages V-ag and V-cg are no longer 120deg out of phase, but 60 deg. Hence the sum of currents I-ag and I-cg is three times the original capacitance current to neutral That is, IBG = 3IAG = 3ICGThis fault current being capacitive, leads the original phase to neutral voltage by 90 deg, and appears in the neutral, returning to the system through the fault.

Isolated System Phase Ground Fault Arching GroundsIf the fault can be interrupted, it is most likely to be done at current zero. However, since the current leads by 90 deg in the capacitive circuits, current zero occurs at the instant of a voltage maximum.Thus, if the fault momentarily clears, a high voltage immediately appears across the fault and restrike of the fault will probably occur. This is the so-called phenomenon of arcing grounds.

Isolated System Phase-Ground Fault Arching Grounds (Contd)In the momentary interval of time that the fault has been cleared, the excessive voltage charge of the capacitors on the healthy lines has been trapped as a DC charge When the arc restrikes again, the capacitors are again recharged by a line-to ground voltage added to the trapped chargeThus a restrikes after current zero clearing is more inevitable, adding another charge.

Isolated System Phase-Ground Fault Arching Grounds (cont)The phenomenon thus probably becomes an oscillating and self-perpetuating build-up in voltage, which eventually will lead to an insulation failure on another phase and a major two-phase fault.

Isolated System Phase-Ground Fault-Arching Grounds (Contd)While the first failure may have been a tree branch in the line, the second failure (as a result of arc restrikes and build-up of overvoltages) may occur at some other location, entirely, perhaps involving expensive equipment insulation, such as a transformer.Thus arching grounds leading to arc restrike and voltage build-ups in ungrounded system actually caused troubles that resulted in its abandonment.

Isolated System AdvantagesOperating a system with the neutral isolated results in low values of earth-fault current equal to the system capacitance current.The hazard voltage between faulted equipment and earth is consequently small, which improves safety. Thus for the same hazard voltage, relatively higher protective earthing resistances are acceptable, compared with most other neutral earthing systems.The voltages of the healthy phases are unaffected by a ground fault, thus avoiding outrages of healthy phases.

Isolated System Disadvantages There exists the high probability of arc restrike when interrupting the fault current, and this can lead to the phenomenon of unsafe buildup of transient overvoltages in the system, dangerous to both personnel and equipment. This trouble coupled with other factors led to the adoption or grounded neutral systems in some form.

Adoption of Grounded SystemsBecause of the greater danger to personnel, code authorities frowned on ungrounded systems. Equipment costs were generally lower for equipment rated for grounded neutral systems because of the reduction in insulation permissible. At high voltages being used today (69kv and above), material savings in transformer costs can be realized by employing reduced basic insulation level (BIL). The requirement for safety reducing insulation level demand that system neutral be earthed

Solid or Direct Earthing Due to the solid grounding of the system, the neutral point of the transformer is always at ground potential Therefore the healthy phases, in general, remain at their normal phase value, almost unaffected by the ground fault.

Solid or Direct Earthing Phase-Ground Fault

Solid or Direct Earthing Phase- Ground Fault (Contd)When a ground fault occurs on phase B, the voltage to earth V-bn of phase B becomes zero And capacitive current I-bg flows from faulty phase B to earth, and is then divided into two components I-ag and I-cg.

Solid or Direct Earthing Phase Ground Fault (contd)In addition, the power source provides a fault current component I-fbg which flows through the faulty phase conductor to the fault location and returns to the power source by way of the earth path and the neutral connection

Solid or Direct Earthing Phase Ground Fault (contd)The fault current I-fbg is predominantly inductive and lags behind the original voltage V-bn of the faulty phase, by approximately 90 deg. The resultant flow of the current, by superimposition of the leading capacity current I-b g and the lagging predominantly inductive fault current I-fb g

Solid or Direct Earthing Phase-Ground (contd)The flow of the heavy lagging current through the fault will almost completely nullify the effect of the capacitive current. The possibility of arching ground phenomenon or its resultant over voltage conditions occurring is greatly reduced.

Solid or Direct Earthing AdvantagesIt is simple and inexpensive in that it requires no extra equipment. The expense of the earth-current limiting device such as resistors, reactors, etc, is eliminated. This is an important consideration on HV systems when multiple earthing is used.The neutral point is held at earth potential under all operating conditions. Consequently, the voltage of any conductor to earth under earth-fault conditions will Not exceed the normal phase voltage of the system.

Solid or Direct Earthing Advantages (Continued)Hazard voltages are reduced to acceptable levels. Power frequency phase-earth overvoltages are lowest, typically below 1.4 p.u., and this explains why HV systems are solidly earthed.On HV systems 132 kV and above, additional savings are available because transformer windings with graded insulation can be used.

Solid or Direct Earthing Advantages (continued)The protection of the system is simplified by virtue of the fact that the ground fault current compares in magnitude with inter-phase fault currents, making detection relatively easier.

Solid or Direct Earthing-DisadvantagesA solidly grounded system produces the greatest magnitude of fault current when a ground fault occurs The increased ground fault current result in greater influence (interference) on neighbouring communication circuits.

Solid or Direct Earthing Disadvantages (contd)The increased ground fault current produced more conductor burning.Any third harmonic currents that may circulate between neutrals tends to be excessive. This applies when earthed neutrals are those of star-connected generators or transformers without a delta winding.

Impedance EarthingMV system often use different types of impedance earthing.When it becomes necessary to limit the earth fault current, a current-limiting device is introduced in the neutral and earth.Impedance earthing involves connecting a resistor or reactor between the system neutral point and earth.

Impedance Earthing-Phase-Ground Fault.

Impedance Earthing-Phase-Ground Fault (Contd)The principle of current flows in the impedance earthing system is similar to that of solid grounding system,However, in the event of a ground fault, the phase-earth voltages of the healthy phases will increase to line values.Furthermore, the magnitude and phase relationship of the inductive fault current I-fbg depends on the relative values of the zero sequence reactance of the power source circuit and the ohmic value of the impedance.

Impedance Earthing- Phase-Ground Fault (Contd)The fault current can be resolved into two components, one I-fbgr being in phase with the voltage to neutral of the fault phase and the other I-fgbx lagging it by 90deg. The lagging component of the fault current will be in direct phase opposition to the resultant capacitive current at the fault location.

Impedance Earthing Phase-Ground Fault (Contd)By suitable choice of the ohmic value of the impedance, the lagging component of the fault current can be made equal to or more than the capacity current, so that no transient oscillation due to arcing grounds can occur.However, if the ohmic value of the impedance is sufficiently high so that the lagging current is less than the capacity current, then the system condition approaches that of the ungrounded neutral system with the risk of transient overvoltages occurring.Another important but conflicting consideration in the choice of the ohmic value is the power loss during line to ground faults.

Impedance Earthing AdvantagesIt permits the use of discriminative protective gear.It minimizes the hazard of arching grounds (only in case of low resistance value) The ground fault currents are reduced, thus obviating the harmful effects of the heavy currents associated with solid earthing such as interference with communication circuits and burning of conductors, switchgear, motors, cables.

Impedance Earthing Advantages (continued)It improves system stability under ground fault conditions.It reduces momentary line-voltage dip by clearing of ground fault It minimizes stray ground fault currents for personnel safety.

Impedance Earthing- Disadvantages With an impedance-earthed MV system, the phase-earth voltage of a healthy phase can reach 1.732 times the normal value under earth-fault conditions, and occasionally some 5% higher.This should, however, not pose problems with system equipment, since the insulation level in MV systems is based on much higher lightning overvoltages.

Impedance Earthing-Disadvantages (continued)The system neutral will almost invariably be displaced during ground fault The provision of the earth-current limiting device (resistor or reactor) means extra investment cost in the system.The inductive nature of the impedance-earthing is of particular disadvantage with overhead lines exposed to lightning, since travelling waves or impulses are subject to positive wave reflection. This higher reflected wave voltage may unduly stress the insulation of the equipment and cause breakdown.

Arc-Suppression Earthing (or Resonant Grounding)Arc-suppression-coil, also called Peterson coil after the inventor, is an attempt to eliminate the fault current that could cause the arching ground condition.Arc-suppression coil earthing is can be seen as special reactance earthing, whose inductance can be adjusted to closely match the network phase-earth capacitances, depending on the system configurations.

Arc-Suppression Grounding (Continued)The inductance of the arc-suppression-coil is adjusted such that the inductive current due to the coil approximately neutralizes capacitive current through the total network capacitance 3c, at the fault The resultant earth-fault current is theoretically suppressed and in any case inadequate to maintain the the arc. Hence the name arc suppression coil.

Arc-Suppression Grounding (Continued)

Arc-Suppression Grounding (continued)Voltage to earth of the fault phase at the point of fault becomes zero Voltage on the healthy phases is increased to 1.732 times the normal value.A resultant capacity current I-bg equal to three times the normal line to neutral charging current flows through the fault. This leads the voltage of the fault phase by 90 deg.

Arc-Suppression Grounding (Continued)Voltage of the faulty phase, I.e., phase voltage, is impressed across the arc suppression coil, and a fault current I-fbg restricted in magnitude by the impedance of the coil, flows through the faulted conductor, lagging the voltage of the faulty phase by 90deg. The capacity current I-bg and the fault current I-fbg are in direct phase opposition. The tuned inductance is given by L = 1 32 C

Ground Fault Detection MethodIn a 3-phase circuit, a combination of overload and earth-leakage relays supplied from a CT is often used. A simple earth-leakage protection can be implemented in a star-connected circuit by placing a relay between the neutral point and earth. The relay is operated by a CT as shown in the Fig below.

Ground Fault Detection Method (Contd)

Ground Fault Detection Method (Contd)If an earth occurs in any phase, for example at point F, then the fault current will flow through the earth path to the neutral.The current in the CT will operate the relay, which will trip the CB.

Thank you. Nagode. Eseun. Imeela. Masabuke.