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**Instrument Transformer**

Magnetic circuit (core) Primary Windings Secondary Windings U1 U2 Maquette

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**Instrument Transformer**

Maquette

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**Definition of an Instrument Transformer**

Voltage Transformer : Primary U1 K : Transformation ratio Secondary U2 Maquette

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**Definition of an Instrument Transformer**

Current Transformer : I1 Primary K : Transformation ratio Ampere-Turns Conservation Law Secondary I2 Maquette

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**Instrument Transformer Error**

Secondary Primary Up Us K Voltage Transformer : Up (theoretically = k.US) e:Ratio error f: Phase error K.Us Maquette

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**Instrument Transformer Error**

Secondary Primary Ip Is K Current transformer : Ip e:Ratio error K.Is f: Phase error TC : K= Ip Is Maquette

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**Internal details Eye bolt design 1. Oil filling plug 2. Dome**

3. Nitrogen filling valve 4. Collar 5. Primary terminal 6. Porcelain insulator 7. Insulated primary 8. Cover plate for tank 9. Tank 10. Secondary cores Eye bolt design Maquette

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**Manufacturing Process**

Primary steel pipe Paper insulation Seconday cores IT range Maquette

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**Active Part Manufacturing**

IT range Maquette

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**Hair Pin Design Hair-Pin design 1. Dome 2. Nitrogen filling valve**

3. Primary terminal 4. Collar 5. Porcelain insulator 6. Primary conductor with insulation 7. Adaptor cylinder 8. Secondary cores 9. Base 10. Oil drain plug IT 400 Cross section Hair-Pin design Maquette

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**Current Transformers Protection accuracy classes**

CEI Accuracy classes (Protection) Maximum current error in % of IP Accuracy limit primary current = fL.IN Burden :SN Maquette

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**Factors for Protection**

Over Current and Earth Fault Protection Parameters 1. ALF ( accuracy limiting factor) 2. Composite error Maquette

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**Factors for Protection**

1. Accuracy Limiting Factor What is Accuracy Limiting factor ? It is the factor of over current above the rated current which determines the capability of CT to maintain the error at such a condition. 2. Composite error It is the error of the CT when this over current is applied. Maquette

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**Current Transformers Protection accuracy classes**

CEI Composite error : Under steady-state conditions, the r.m.s. value of the difference between: a) the instantaneous values of the primary current, and b) the instantaneous values of the actual secondary current multiplied by the rated transformation ratio Kn is the rated transformation ratio; Ip is the r.m.s. value of the primary current; ip is the instantaneous value or the primary current; is is the instantaneous value of the secondary current; T is the duration of one cycle. Maquette

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**Factors for Protection**

1. Accuracy Limiting Factor/composite error For e.g if the class designation is 5P20 20 is the Accuracy limiting factor which signifies that when 20 times the rated primary current is applied the composite error of 5P( +/- 5%) is maintained. Typical Class designations are 5P10, 5P20, 10P10, 10P20 etc., Maquette

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**Factors for Protection**

Transformer and Busbar Differential Protection Parameters 1. Knee Point Voltage requirement(Vk) 2. Exciting current ( Io) 3. Resistance of CT (Rct) Maquette

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**Current Transformers Saturation curve**

Induction B [T] Protection CT 18500 gauss Metering CT 8000 gauss Primary current Ip/In 10 20 30 40 Maquette

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**Factors for Protection**

1. Knee Point Voltage(Vk): Knee point voltage is point beyond which an application of 10% of voltage increases the exciting current by 50%. The typical equations for Vk ( based on relay used) Vk > 24 In (Rc t+2RL)- for Transformer Differential Where In: Relay rated current RL = Total lead Resistance Rc t = CT secondary resistance Maquette

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**Current Transformers Type Tests**

a) short-time current tests b) temperature rise test c) lightning impulse test d) switching impulse test e) wet test for outdoor type transformers f) determination of errors IEC Maquette

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**Current Transformer Routine tests**

CEI CEI 60-1 Routine tests The following tests apply to each individual transformers: a) verification of terminal markings b) power-frequency withstand test on primary winding c) partial discharge measurement d) power-frequency withstand test on secondary windings e) power-frequency withstand tests, between sections f) determination of errors The order of the tests is not standardized, but determination of errors shall be performed after the other tests. Maquette

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**CT Failure and remedial action**

Remedial actions in CT at site to avoid failures Tangent delta and Capacitance measurement from the C terminal at periodic intervals once in three years or during shutdown. Dissolved gas analysis of oil taken out from CT alteast once in five years. Thermo vision scanning of CTs of rating 400kV ( or above). Maquette

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**Partial discharge test**

Maquette

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**Multiple Chopped Impulse test**

As per IEC ( 2002) Application of 100 chopped impulses of negative polarity on CTs of ratings above 300kV. These impulses will be applied at the rate of one impulse per minute. The test Voltage shall be 60% of the rated lightning impulse voltage Before the test and three days after the test the dissolved gas analysis of oil taken from CT will be carried out.Analysis procedure and fault diagnosis shall be as per IEC Maquette

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**Special test A. Thermal Stability test :**

This involves simultaneous application of rated voltage (1.1Um/Sqrt3) and rated simultaneous current (1.2,1.5 etc) by using a synthetic test circuit.Capacitance, tangent delta, secondary resistance and temperature of primary terminal are recorded until stable values are acquired. This test demonstrates the insulation capacity ( healthiness) under energised conditions. Maquette

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**Special test B. Temperature coefficient test:**

The CT is heated in a oven to approximately 90Deg C. The tan delta is measured at ambient, 80 and 90 deg C at voltages of 0.3,0.7,1.0 and 1.1Um/Sqrt3. This test demonstrates the healthiness at high extreme temperature conditions. Maquette

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**Accurate Measurement of HV**

Capacitor Voltage Transformers Coupling Capacitors CCV / CC 72.5 to 765 kV Line protection HF transmission Compliance with IEC, ANSI or equivalent standards Reduce the slope of the RRRV (Rate of Rise of Recovery Voltage (Ik >40 kA) Maquette

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**Capacitor Voltage Transformer Definitions**

Element Pack (or pack) Pile of elements : ± 10 to 25 kV Maquette

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**Capacitor Voltage Transformer Definitions**

HV Power line Ground Capacitor Unit (or unit) Assembly of elements in an insulating container: ± 245 kV. Can be connected to a HV line Maquette

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**Capacitor Voltage Transformer Definitions**

HV Power line Ground Capacitor STACK (or stack) Assembly of elements to reach higher voltage levels : ± 800 kV In general, the term CAPACITOR stands for a capacitor element as well as a capacitor stack. Maquette

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**Capacitor Voltage Divider**

HV Power line Ground Capacitor Voltage Divider ( or CVT ) Connecting an MV inductive voltage transformer to a tap. Maquette

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P1 P2 C1 C2 Intermediate Voltage : 10 to 20 kV/3 S1 S2 Maquette

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**Lµ = Inductance equivalent to magnetic losses of the magnetic circuit. **

Equivalent Diagram Ce La Lfs Ra Rs Lμ Rw Zc Us U’P = Vp . C1 k . (C1+C2) Lµ = Inductance equivalent to magnetic losses of the magnetic circuit. Rw = Resistance equivalent to the watt losses of the magnetic circuit. Lfs = Secondary leakage inductance of the magnetic VT. Rs = Resistance of the secondary winding Maquette

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Capacitor stack Inductive VT Maquette

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**Secondary terminal box**

Oil expansion device CCV 72.5 to 765 kV Capacitor elements Capacitor column Insulating oil Insulator flange Secondary terminal box Inductance MV Transformer Damping circuit Maquette

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**Capacitor Voltage Transformer Capacitor Elements**

All Paper Dielectric design Mixed dielectric design Paper PPR film + paper Copper tabs for connection Aluminum foil folded for contact with next element Maquette

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**Capacitor Voltage Transformer Typical Error Curve**

Typical graph of variation of errors with VA & frequency Nominal burden OVER-ESTIMATED Real load < 25% VAN Phase error Ratio error f 25% VA f+Df f -Df 100% VA +20’ - 20’ +0.5 - 0.5 Excessive Frequency Variations Nominal burden UNDER-ESTIMATED Real load > 100% VAN Maquette

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**Voltage transformers Protection accuracy classes**

IEC Accuracy classes (Protection) Maximum error in % of VP - Voltage between 5 % and fT x VNP - Burden between 25% and 100% of SN - Maximum error doubled for VNP=2% Maquette

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