CTs Randhir Singh

Approximate Population of Substation Equipments Transformers 300 Nos. Reactors 325 Nos. Circuit Breakers 2100 Nos. Current Transformers 5200 Nos. Capacitive Voltage Transformers 3200 Nos. Lightning Arresters 3000 Nos. Isolators 5000 Nos.

Current Transformers

Population of CTs in POWERGRID Make Population (Approx.) ABB 1100 BHEL 1200 CGL 1000 AREVA/ WSI TELK 200 Others/ Imported 300 Total 5200

Types of CTs Dead Tank Design Live tank Design Hair Pin Design Eye Bolt Design Live tank Design

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

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

Active Part Manufacturing IT range

Live Tank CTs

Dead tank CTs

CT Standards IEC 60044 – 1 IS 2705

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

CT accuracies As per IEC-60044(1) Metering Core – ±0.2 or 0.5% at rated Currents Protection Cores – ± 1% at rated current

Accuracies as per IEC-60044-1 Class 5% of rated I 20% of rated I 100% of rated I 120% of rated I 0.2 0.75 0.35 0.5 1.5

Protection Cores Class Current Error at rated Primary Current Composite Error at rated accuracy limit Primary Current 5P ±1% ±5% 10P ±3% ±10%

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.,

Ratio Error Ratio Error = (KnIs- Ip)*100/ Ip Kn = Rated transformation ratio Ip = Actual primary current Is = Actual secondary current

Instrument Transformer Error Secondary Primary Ip Is K Current transformer : :Ratio error Ip : Phase error K.Is TC : K= Ip Is

Phase Angle Error The difference in Phase between the Primary and Secondary current vectors

Knee Point Voltage 10% increase in Voltage will lead to 30% or more increase in Current.

Insulation Levels For 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 voltages

Insulation Levels System Voltage 1 min Power Freq. Voltage Switching Impulse withstand Voltage Lightning Impulse withstand Voltage 220kV 395kV 460kV - 950kV 1050kV 400kV 630kV 1425kV

Creepage Distances Pollution Levels Creepage distance Light 16mm/kV Medium 20mm/kV Heavy 25mm/kV Very Heavy 31mm/kV

Pre Commissioning Tests Polarity Test Magnetization Curve Test Ratio Test Primary Current Injection Test Secondary Current Injection Test

Reasons of CT Failures About 30 nos. CTs have failed due to poor impregnation and paper wrapping at works. About 90 nos. CTs have failed due to pre-mature ageing of almost all makes 2 no. CT failed after repair at site. Moisture entry due to N2 gas leakage

Primary Insulation Failure due to moisture entry

Raipur CT failure (AREVA)

Insulation puncture

Failure of Primary Insulation

Condition Monitoring Checking of Bellow expansion - M Visual Inspection for leakages - M Tan Delta Measurement – 2Y Thermovision Scanning - Y Nitrogen Pressure Checking – 2Y DGA testing of Oil - SOS

CT TESTING –TAN DELTA

CT Insulation

Capacitance and Tan Delta Measurement CTs with Test Tap- Ungrounded Specimen Test mode (UST) CTs without Test Taps – Grounded Specimen Test (GST) mode with jumpers disconnected Values to be monitored w.r.t. factory/ pre- commissioning values Sudden 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

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.

CVTs

Population of CVTs in POWERGRID Make CVTs ABB 800 BHEL 315 CGL AREVA/WSI 1200 Others 85 Total 3200

CVT Construction Details

Capacitor stack Inductive VT

CVT Construction Details There are 280 – 300 elements in C1 & C2 C1 will be about 260 to 280 elements C2 will be 15 to 20 elements Ratio of C1/ C2 is 20 to 22 400/ 20 = 20kV (Tap Voltage)

Compensating Reactor Compensating Reactor is provided to compensate for the phase displacement in Capacitor elements wL = 1/w (c1+c2) L = 1/ w2 (c1+c2)

Ferro Resonance Ferro resonance in CVTs is due to the Capacitance in Voltage Divider in series with the inductance of the Transformer and series reactor. This circuit is brought to resonance by various disturbances in the network that may saturate the iron core of the transformer, over heat electro magnetic unit and lead to insulation breakdown.

Ferro Resonance Circuit Ferro resonance circuit is provided in CVT Secondary to suppress Ferro resonance oscillations There can be active or passive Ferro resonance circuits It can be RLC circuit (ABB) or RL circuit (CGL) or Resistance (BHEL, WSI, AREVA)

CVT Secondary Voltage CVT Secondary Voltage v = k * V * C1/ (C1+C2) V – Primary Voltage k – Secondary Transformer Transformation ratio Note: Puncturing of C1 – Secondary Voltage will increase Puncturing of C2 – Secondary Voltage will decrease

CVT VA ratings As per POWERGRID specifications, VA ratings for core-1, core-2 and core-3 are 50VA, 50VA and 50VA respectively. Earlier CVTs it was 200/ 200/ 50VA CVT accuracies are guaranteed if connected burdens are within 25% to 100% of the rated burdens In POWERGRID, with static meters and static/ numerical relays, connected burdens are 10 to 20 VA in each core which are very low as compared to earlier rated burdens.

PD Measurement CT PT CVT Test Voltage Before Revision IEC-185 IEC-186 Pre-stress Voltage-436kV Test Voltage- 266kV(10pC) After revision IEC- 60044(1) IEC- 60044(2) Yet to be revised Pre-stress Voltage–504kV Test Voltage- 420kV(10pC)

REASONS OF FAILURES OF CVTs WRINKLES ON ALUMINUM FOIL POOR SOLDERING QUALITY POOR QUALITY OF PAPER(LOCAL SOURCE) PINHOLES IN BELLOWS SNAPPING OF BELLOW CONNECTION OVERHEATING OF DAMPING RESISTOR SHORTING OF TRANSFORMER CORES FAILURES OF FR CIRCUIT COMPONENTS RUSTING OF COUPLING BOLTS (BETWEEN FLANGE AND EMU TANK) RUSTING OF FLANGE

Reasons of CVT Failures LOOSENESS OF CORE BOLTS SNAPPING OF CONNECTION BETWEEN PRIMARY WINDING AND COMPENSATING REACTOR FAILURE OF VARISTORS PROVIDED IN SECONDARY ENTRY OF MOISTURE IN CAPACITOR STACKS POOR GASKET QUALITY ALMOST ALL COMPONENTS OF CVTs HAVE SHOWN FAILURE TREND

Rusting of EMU Tank

EMU Tank Transformer Winding shorted

Top Bellow with uneven surface and soldered material

Failure of Capacitor Element in

Rusting and shearing off of Coupling Bolts and also entry of Moisture in the stack

Mechanical shearing of the Bolt/ Stud

Matter of Concern In POWERGRID, there have been many pre- mature failures of CVTs. CVTs have given unreliable performance so far. Against life of about 30-35 years, failures have occurred after 8-10 years of service. About 900 CVTs of almost all makes CVTs needs to be refurbished/ repaired at manufacturers works. This will need huge expenditure to be met from O&M budget affecting POWERGRID profitability.

Condition Monitoring of CVTs Secondary Voltage Measurement – variation in voltages indicates shorting/ puncturing of capacitor elements. Capacitance and Tan Delta Measurement Precautions: Porcelain surfaces need to be thoroughly cleaned before measurements.

Secondary Voltage measurement Periodic measurement to be carried out. In case of doubt, simultaneous measurement to be carried out with another feeder/ Bus CVT. For 400kV CVTs puncturing of one Capacitor element in C1 side is likely to increase Secondary Voltage by about 0.35 – 0.45% (0.22 – 0.28V) Failure of one Capacitor element in C2 side is likely to decrease Secondary Voltage by 5 – 6% (3.2 – 3.8V)

Capacitance and Tan delta measurement of stacks Change in Capacitance value above 6%, CVT need to be replaced Tan delta values more than 0.003 from pre-commissioning value needs replacement

Low/ Negative values of Tan Delta Lower values of Capacitance and Tan Delta in case of CVTs are due to parallel tapping at Intermediate point (20-22kV). At this point, there will be two parallel paths. Hence measured current flowing through C2, will be less than C1 and lower value of measured capacitance. Due to losses in Voltage Transformer Winding, Tan Delta value may also be less or even negative. Hence, values should be compared w.r.t.pre- commissioning values only and not with factory values

Thank you Randhir Singh