6.1 CTs by HKRajput

CURRENT TRANSFORMERS PRESENTATIONBYHITESH KUMAR MANAGER (T) PROTECTION DELHI TRANSCO LIMITED

CURRENT TRANSFORMERS DefinitionDesign and Construction Working PrincipleRelated Terminology: KPV, VA Burden, ALF, Accuracy Class, Error etc.TestingCommissioningMaintenance

CURRENT TRANSFORMERS Instrument TransformerA transformer intended to supply measuring instruments, meters, relays, and other similar apparatus. Current TransformerAn instrument transformer in which the secondary current, in normal condition of use, is substantially proportional to the primary current and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections. Primary Winding The winding through which flows the current to be transformed.Secondary WindingThe winding which supplies the current circuits of measuring instruments, meters, relays or similar apparatus.

CURRENT TRANSFORMERS Secondary CircuitThe external circuit supplied by the secondary winding of a transformer.Rated Primary CurrentThe value of the primary current on which the performance of the transformer is based.Rated secondary CurrentThe value of the secondary current on which the performance of the transformer is based.Actual Transformation RatioThe ratio of the actual primary current to the actual secondary current.Rated Transformation RatioThe ratio of the rated primary current to the rated secondary current.

CURRENT TRANSFORMERS Current Error (Ratio Error)The error which a transformer introduces in to the measurement of a current and which arises from the fact that the actual transformation ratio is not equal to the rated transformation ratio.Current Error (%) = (Kn x Is - Ip) X 100 IpKn is the rated transformation ratioIpis the actual primary currentIsis the actual secondary current when Ip is flowing under the conditions of measurement

CURRENT TRANSFORMERS Phase displacementThe difference in phase between the primary and secondary current vectors, the direction of the vectors being so chosen that the angle is zero for a perfect transformer.

The phase displacement is said to be positive when the secondary current vector leads the primary current vector. Composite ErrorIn its stationary state, the composite error Ec based on the rms value of the primary current is the difference betweena) the instantaneous values of the primary currentb) the instantaneous values of the secondary current intensities multiplied by the rated transformation.

CURRENT TRANSFORMERS Composite ErrorComposite Error (%) =

Kn is the rated transformation ratioIpis the actual primary currentip is the instantaneous value of the primary currentisis the instantaneous value of the secondary currentTis the duration of one cycle

CURRENT TRANSFORMERS Accuracy ClassA designation assigned to a current transformer the errors of which remain within specified limits under prescribed conditions of use.BurdenThe impedance of the secondary circuit in ohms and power-factor.

CURRENT TRANSFORMERS Rated BurdenThe value of the burden on which the accuracy requirements of this specification.

CURRENT TRANSFORMERS Instrument Security Factor (FS)The ratio of rated instrument limit primary current to the rated primary current.Rated Instrument Limit Primary Current (IPL)The value of the minimum primary current at which the composite error of the measuring current transformer is equal to or greater than 10 %, the secondary burden being equal to the rated burden.Secondary Limiting EMFThe product of the instrument security factor FS, the rated secondary current and the vectorial sum of the rated burden and the impedance of the secondary winding.The product of the accuracy limit factor , the rated secondary current and the vectorial sum of the rated burden and the impedance of the secondary winding.

CURRENT TRANSFORMERS Rated Accuracy Limit Primary CurrentThe value of primary current up to which the transformer will comply with the requirements for composite error.Accuracy Limit FactorThe ratio of the rated accuracy limit primary currentto the rated primary current.Rated Knee Point E.M.F.That minimum sinusoidal e.m.f. (r.m.s.) at rated power frequency when applied to the secondary terminals of the transformer, all other terminals being open circuited, which when increased by 10 % causes the rms exciting current to increase by no more than 50 %.

CURRENT TRANSFORMERS Dimensioning Factor (K)A factor assigned by the purchaser to indicate the multiple of rated secondary current (Isn) occurring under power system fault conditions, inclusive of safety factors, up to which the transformer is required to meet performance requirements.

CURRENT TRANSFORMERS Earth Fault FactorRatio at a selected location of a three phase system (generally the point of installation of an equipment) and for a given system configuration, of the highest r.m.s. phase to earth power-frequency voltage on a sound phase during a fault to earth (affecting one or more phases at any point) to the r.m.s. phase to earth power frequency voltage which would be obtained at the selected location without the fault.

Types of CTsHair Pin DesignEye Bolt DesignLive tank Design

CURRENT TRANSFORMERS

CURRENT TRANSFORMERS Hair Pin Design

Dead tank CTsHairpin design and Eye Bolt designCURRENT TRANSFORMERS

CT StandardsIEC 60044 1IS 2705CURRENT TRANSFORMERS

CT DesignCore Material The main aim is to give high accuracy with low saturation factor.

Core Material is made of CRGO Silicon steel

For very low loss characteristics, material (Alloy of Ni-Fe) is usedCURRENT TRANSFORMERS

Accuracies as per IEC-60044-1


Class5% of rated I20% of rated I100% of rated I120% of rated I0.20.750.350.20.20.51.50.750.50.5

Protection CoresCURRENT TRANSFORMERS

ClassCurrent Error at rated Primary CurrentComposite Error at rated accuracy limit Primary Current5P1%5%10P3%10%

Insulation LevelsFor Windings having Um greater than 300kV, the rated insulation level is determined by rated switching and lightning impulse withstand voltage

For voltages < 300kV, insulation levels are decided by lightning impulse and power-frequency withstand voltagesCURRENT TRANSFORMERS

Insulation LevelsCURRENT TRANSFORMERS

System Voltage1 min Power Freq. VoltageSwitching Impulse withstand VoltageLightning Impulse withstand Voltage220kV395kV460kV-950kV1050kV400kV630kV1050kV1425kV

Partial Discharge TestCURRENT TRANSFORMERS

System VoltagePre-stress Voltage for 1 minTest VoltageUm80% of 1 min P.F. VoltageUm- 10pC1.2Um/3 - 5pC

Creepage DistancesCURRENT TRANSFORMERS

Pollution LevelsCreepage distanceLight16mm/kVMedium20mm/kVHeavy25mm/kVVery Heavy31mm/kV

Routine Tests at FactoryVerification of Terminal MarkingsPower Frequency withstand test on Primary windingPartial Discharge measurementPower Frequency withstand test on Secondary windingPower Frequency withstand tests between sectionsInter-Turn Over voltage TestDetermination of ErrorsCURRENT TRANSFORMERS

Testing of CTsTan Delta Measurement

Thermo vision Scanning

DGA testing of Oil Primary Injection TestKnee Point Voltage measurementCURRENT TRANSFORMERS

CT TESTING TAN DELTACURRENT TRANSFORMERS

CT InsulationCURRENT TRANSFORMERS

Tan delta measurementCTs with Test Tap- Ungrounded Specimen Test mode (UST)CTs without Test Taps Grounded Specimen Test (GST) mode with jumpers disconnectedValues to be monitored w.r.t. factory/ pre-commissioning valuesSudden change in measured values indicate faster deterioration of insulation.Precautions: P1/P2 to be shorted. Porcelain surface to be thoroughly cleaned. Test Tap to be reconnected to Earth after the Test


Tan delta measurementConnection of Test Tap to be ensured otherwise it may lead slow arcing in the soldering area and insulation may fail in due course of time.Measurement of Tan Delta of C2 (insulation between last foil on which test tap wire is soldered to the ground) to be carried out.Measurement in GSTg mode with P1/P2 terminal guarded.CURRENT TRANSFORMERS

CT Equivalent CircuitCURRENT TRANSFORMERS

UST Mode of MeasurementEarth will be connected to N PointHence only CHL will be measuredCURRENT TRANSFORMERS

GST Mode of MeasurementHere measurement is made for the Parallel combination of CH and CHLCURRENT TRANSFORMERS

GSTg Mode of MeasurementHere measurement is done for CHE as L is guardedCURRENT TRANSFORMERS

C1 and C2 Tan delta measurementFor C1: Apply 10kV between HV and test tap on UST mode.For C2: Apply 500 V between Test tap and Ground with HV Guarded on GSTg modeCURRENT TRANSFORMERS

Service Conditions: Pollution Levels-- Normal, HeavyRated Primary CurrentRated Short Time CurrentB.I.LNumber of RatiosNumber of CoresOutput ,Accuracy, Accuracy limit factor etc of Individual Cores Specification RequirementsCURRENT TRANSFORMERS

Requirement of Metering And Protective Current TransformersMetering Coresshould maintain accuracy within limits over-Entire Current range of 20 to 120 % of rated current and burden range of 25 to 100 % andPreferably saturate beyond the Rated currentThe accuracy is designated by the max ratio error at 100% rated burden and current viz0.2 ,0.5, 1.0 etcProtection coresShould maintain accuracy from beyond the rated current to the highest possible fault current in any locationConsequently should not saturate up to highest fault currentThe accuracy is designated by the max current error at accuracy limit current viz 5P, 10P,15P


Accuracy of Metering C.TsAccuracy % R.E at Phase Error at Class % of Rated Current % of Rated Current 20 100 120 20 100 1200.2 0.35 0.2 0.2 15 10 100.5 0.75 0.5 0.5 45 30 301.0 1.5 1.0 1.0 90 60 60CURRENT TRANSFORMERS

INTERDEPENDANCE OF VA , ACCURACY, A. L. FVA= OUTPUT OF CT IN VOLTAMPERESA= CROSS SECTIONAL AREA OF CORE OF CTB normal=FLUX DENSITY IN CORE AT NORMAL OUTPUTB sat =FLUX DENSITY IN CORE AT A.L.FA.L.F- ACCURACY LIMIT FACTOR

TAKING A SECONDARY CURRENT OF 1 AMP , VOLAGE ACROSS SECONDARY WINDING VA V=--------=VA 1THEREFORE, VA= 4.44*FREQUENCY(f)*B normal* A*No OF TURNS IN SECONDARY(N)

AS FLUX DENSITY AT A.L.F I S RESTRICTED BY SATURATION IN THE COREHIGHER A.L.F MEANS LOWER VA AND VICE-VERSA


Requirements for Special Purpose Current TransformersThe performance is specified in terms of Minimum Knee Point Voltage Maximum Exciting Current at the Rated Knee Point Voltage(K.P.V ) Maximum Resistance of Secondary Winding at No Turns CorrectionCURRENT TRANSFORMERS

1CURRENT TRANSFORMERS

Current TransformersCURRENT TRANSFORMERS

1. Dome2.Nitrogen filling valve 3. Primary terminal4. Collar5. Porcelain insulator6. Primary conductor with insulation7. Adaptor cylinder8. Secondary cores9. Base10. Oil drain plugHair Pin DesignHair-Pin designIT 400 Cross sectionCURRENT TRANSFORMERS

1. Oil filling plug2.Dome 3. Nitrogen filling valve4. Collar5. Primary terminal6. Porcelain insulator7. Insulated primary8. Cover plate for tank9. Tank10. Secondary coresInternal detailsEye bolt designCURRENT TRANSFORMERS

Active Part ManufacturingIT rangeCURRENT TRANSFORMERS

CT DesignCore Material The main aim is to give high accuracy with low saturation factor.Core Material is made of CRGO Silicon steel

For very low loss characteristics, material (Alloy of Ni-Fe) is used


Current transformer :Instrument Transformer ErrorCURRENT TRANSFORMERS

Current Transformers Saturation curve10203040InductionB [T]Primary currentIp/InCURRENT TRANSFORMERS

Pre Commissioning TestsPolarity TestMagnetization Curve TestRatio TestPrimary Current Injection TestSecondary Current Injection TestCURRENT TRANSFORMERS

Capacitance and Tan Delta MeasurementCTs with Test Tap- Ungrounded Specimen Test mode (UST)CTs without Test Taps Grounded Specimen Test (GST) mode with jumpers disconnectedValues to be monitored w.r.t. factory/ pre-commissioning valuesSudden change in measured values indicate faster deterioration of insulation.Precautions: P1/P2 to be shorted. Porcelain surface to be thoroughly cleaned. Test Tap to be reconnected to Earth after the TestCURRENT TRANSFORMERS

Capacitance and Tan Delta Measurement Contd.Connection of Test Tap to be ensured otherwise it may lead to slow arcing in the soldering area and insulation may fail in due course of time.Measurement of Tan Delta of C2 (insulation between last foil on which test tap wire is soldered to the ground) to be carried out.Measurement in GSTg mode with P1/P2 terminal guarded.CURRENT TRANSFORMERS

Equivalent circuit of transformerCURRENT TRANSFORMERS

Electromagnetic Voltage TransformerCURRENT TRANSFORMERS

Vector Diagram For CT (referred to secondary)CURRENT TRANSFORMERS

Current Transformer type IMBThe the most sold current transformer in the world, more than 140 000 units are installed all over the world, from desert to arctic climates. Improved in several generations during more than 60 years.

A reliable product (failure rate 4 times lower than average) with no need for regular maintenance.ReferencesCURRENT TRANSFORMERS

IMB ManufacturingAutomated insulation technique Improves quality and minimises variationsReduced dimensions and increased withstand TechnologyCURRENT TRANSFORMERS

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Metering core: Class: Normal class for revenue metering: class 0.2 (IEC) , 0.3 (IEEE) Burden of today metering equipment is very low < 5 VA. (See earlier page) FS (Instrument security factor) figure tell us when the metering core should be saturated in order to limit the current to protect the instruments and meters. FS 5 means that the secondary current not should be higher than 5 times rated secondary current. A too high rated burden compared to actual burden can mean, that the metering equipment is destroyed as the FS value is valid at rated burden. Relay cores: I will not at this time in detail discuss the different demands for different relays but some important values to be considered are:Burden: Note that the burdens today are pure resistive and much lower than several years ago when electromagnetic relays were used.ALF (Accuracy limit factor) figure tell us when the relay core not should be saturated. The ALF is always related to rated burden. Accuracy: Ex 5P or 10P ( The figure 5 or 10 tells the total % error at rated ALF value. P means protection)Class X: When class X cores are asked for there is three values that should be specified (Ekn=knee point voltage, Rct = secondary winding resistance and I0 = magnetisation current at knee point voltage.

TPY, TPZ: When there is a risk for DC-component that will saturate the core and course unwanted tripping of circuit breaker, airgaps in the cores can be used. They are most used for line protection in systems > 245 kV and with very high short circuit currents.*****

6.1 CTs by HKRajput

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