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Instrument Transformer

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Presentation on theme: "Instrument Transformer"— Presentation transcript:

1 Instrument Transformer
Magnetic circuit (core) Primary Windings Secondary Windings U1 U2 Maquette

2 Instrument Transformer

3 Definition of an Instrument Transformer
Voltage Transformer : Primary U1 K : Transformation ratio Secondary U2 Maquette

4 Definition of an Instrument Transformer
Current Transformer : I1 Primary K : Transformation ratio Ampere-Turns Conservation Law Secondary I2 Maquette

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

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

7 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

8 Manufacturing Process
Primary steel pipe Paper insulation Seconday cores IT range Maquette

9 Active Part Manufacturing
IT range Maquette

10 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

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

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

13 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

14 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

15 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

16 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

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

18 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

19 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

20 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

21 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

22 Partial discharge test

23 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

24 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

25 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

26 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

27 Capacitor Voltage Transformer Definitions
Element Pack (or pack) Pile of elements : ± 10 to 25 kV Maquette

28 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

29 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

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

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

32 Lµ = Inductance equivalent to magnetic losses of the magnetic circuit.
Equivalent Diagram Ce La Lfs Ra Rs 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

33 Capacitor stack Inductive VT Maquette

34 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

35 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

36 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

37 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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