Maquette11 Current Transformers Protection accuracy classes CEI Accuracy classes (Protection) Maximum current error in % of I P Accuracy limit primary current = f L.I N Burden :S N
Maquette12 Factors for Protection Parameters 1. ALF ( accuracy limiting factor) 2. Composite error Over Current and Earth Fault Protection
Maquette13 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.
Maquette14 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 CEI Current Transformers Protection accuracy classes K n is the rated transformation ratio; I p is the r.m.s. value of the primary current; i p is the instantaneous value or the primary current; i s is the instantaneous value of the secondary current; T is the duration of one cycle.
Maquette15 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.,
Maquette16 Factors for Protection Parameters 1. Knee Point Voltage requirement(V k ) 2. Exciting current ( I o ) 3. Resistance of CT (R ct ) Transformer and Busbar Differential Protection
Maquette17 Current Transformers Saturation curve Induction B [T] Primary current Ip/In Protection CT gauss Metering CT 8000 gauss
Maquette18 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 I n (R c t +2R L )- for Transformer Differential Where I n : Relay rated current R L = Total lead Resistance R c t = CT secondary resistance
Maquette19 Current Transformers Type Tests IEC 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
Maquette20 Current Transformer Routine tests CEI CEI 60-1 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.
Maquette21 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).
Maquette22 Partial discharge test
Maquette23 Multiple Chopped Impulse test 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 As per IEC ( 2002)
Maquette24 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.
Maquette25 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.
Maquette26 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 (I k >40 kA)
Maquette27 Capacitor Voltage Transformer Definitions Element Pack (or pack) Element Pack (or pack) Pile of elements : ± 10 to 25 kV
Maquette28 Capacitor Voltage Transformer Definitions Assembly of elements in an insulating container: ± 245 kV. Can be connected to a HV line Capacitor Unit (or unit) Capacitor Unit (or unit) HV Power line Ground
Maquette29 Capacitor Voltage Transformer Definitions Capacitor STACK (or stack) Capacitor STACK (or stack) Assembly of elements to reach higher voltage levels : ± 800 kV HV Power line Ground In general, the term CAPACITOR stands for a capacitor element as well as a capacitor stack.
Maquette30 Capacitor Voltage Divider ( or CVT ) Capacitor Voltage Divider ( or CVT ) Connecting an MV inductive voltage transformer to a tap. HV Power line Ground
Maquette31 Intermediate Voltage : 10 to 20 kV/ 3 S1S1 S2S2 P1P1 P2P2 C1C1 C2C2
Maquette32 L µ = Inductance equivalent to magnetic losses of the magnetic circuit. R w = Resistance equivalent to the watt losses of the magnetic circuit. L fs = Secondary leakage inductance of the magnetic VT. Rs= Resistance of the secondary winding CeCe LaLa L fs RaRa RsRs LμLμ RwRw ZcZc UsUs U P = V p. C 1 k. (C 1 +C 2) Equivalent Diagram
Maquette35 Capacitor Voltage Transformer Capacitor Elements All Paper Dielectric designMixed dielectric design PaperPPR film + paper Copper tabs for connection Aluminum foil folded for contact with next element
Maquette36 Excessive Frequency Variations Nominal burden OVER-ESTIMATED Real load < 25% VA N Nominal burden UNDER-ESTIMATED Real load > 100% VA N Capacitor Voltage Transformer Typical Error Curve Typical graph of variation of errors with VA & frequency Phase error Ratio error f 25% VA f+ f f - f 100% VA
Maquette37 Voltage transformers Protection accuracy classes IEC Accuracy classes (Protection) Maximum error in % of V P - Voltage between 5 % and f T x V NP - Burden between 25% and 100% of S N - Maximum error doubled for V NP =2%