3. Instrument Transformers

High Voltage Instrument TransformerInstrument Transformer Product Training © ABB Group Septem ber 25, 2012 | Slide 1 Protection System Analogy Brain - Relay Eyes,Ears,Nose & Skin CTs, CVTs, VTs Hands & Legs Circuit Breakers © ABB Group Septem ber 25, 2012 | Slide 2 Protection System Analogy Fault in the Power System Sensed by Instrument Transformers & communicated to Relay Relay Issues Trip Command To Breaker Breaker Trips & Clears Fault © ABB Group Septem ber 25, 2012 | Slide 3 Definitions What do Instrument Transformers do? Instrument transformers measure current and voltage in high voltage transmission lines and switchgears during normal and fault conditions (IEC:60044-1 & 5 and IS:2705/3156) Insulate control circuits from the network Transform current and voltage to standardized levels for control equipment as relays and meters( e.g.1A & 110 V) A V © ABB Group Septem ber 25, 2012 | Slide 4 Interfacing Equipment / System Meters / Transducers Protective Relays Synchronizers (for CVTs) PLCC (CVTs & CCs) Energy Meters kWh kVAR Numerical Energy calculation CVT CT kWh kVAR Tomorrow Today Electronic Analog Integrator Yesterday kWh kVAR Meter Mechanical Integrator Ancient Rb A/D conversion Energy O/P Computer © ABB Group Septem ber 25, 2012 | Slide 5 Current Transformer Product Range Theory & Application Construction and Manufacturing Routine, Type and Special Tests Packing and Transportation Unloading, Unpacking and Storage Checks before Installation & Commissioning Care and Troubleshooting © ABB Group Septem ber 25, 2012 | Slide 6 Product Range Current Transformer Type IMB Voltage: 72.5, 145, 245, 420 & 800 kV Current: 50 A to 2000 A with round conductor Above 2000 A with Aluminium Oval Aluminium conductor and Population: > 30000 CTs in India abroad with Principle of Construction: Dead tank Hair pin type primary © ABB Group Septem ber 25, 2012 | Slide 7 Product Range Current Transformer Type IMB with Porcelain & Polymer Insulator Developed, type tested & supplied to various utilities. © ABB Group Septem ber 25, 2012 | Slide 8 Ie Ie Burden Ideally But Exciting impedance Ip x Np = Is x Ns . 2012 | Slide 9 CT – Theory & Application Equivalent circuit CT Is = Np X Ip Ns Is Is .Ie x Ns Introduces Error in CT © ABB Group Septem ber 25.CT – Theory & Application What is a CT used for? I1 Measurement of Current Measurement of Power Isolation between High voltage and Low Voltage I2 Inputs to Relays & Protection Systems Ns Is Np Ip Ip = Ns Is = Np Ip x Np = Is x Ns = Ampere turns (always in balance) eg.1000Apri x1 or 2 Pri turns = 1Asec x 1000 or 2000 sec turns © ABB Group Septem ber 25. 2012 | Slide 10 . ISF e. ISF < 10 V = Burden/Is =RctxIs +Meter Burden+ 2RL © ABB Group Septem ber 25. 0.CORE LOSS COMPONENT IM ..PHASE ANGLE ERROR -BURDEN ANGLE -ANGLE BETWEEN FLUX AND EXCITATION CURRENT RATIO ERROR: KT+IOSIN( )/IS PHASE ANGLE ERROR: IOCOS( )/KTIS IS ES © ABB Group Septem ber 25.VOLTAGE DEVELOPED ACROSS SECONDARY Ø-FLUX DEVELOPED .MAGNETISATION COMPONENT ES.g.SECONDARY CURRENT KT.EXCITATION CURRENT IW.5 Cl. 15 VA. 2012 | Slide 11 CT – Theory & Application Metering Types of Cores--------------------------------- Protection Metering Core: VA Burden. 2012 | Slide 12 . Accuracy.TURNS RATIO NS/NP Io.CT – Theory & Application Io ISKT IP Io Vector Diagram IW IM Ø IP.PRIMARY CURRENT IS. 2012 | Slide 14 .35 T Bs 0.5 Saturated [CRGO ] Metering Core: •CRGO – 0. same should be proportionately high on higher tapings © ABB Group Septem ber 25.44BmAfN2 Volts Working Flux Density E/B 2 Tesla Eknee 1. In such CTs.5 to 1. Multiple ratio CT with ratio selection by primary re-connection can have same ISF for all ratios as secondary turns are same on all ratios.3 to 0.4 to 0. The secondary current is restricted due to saturation and the instrument connected does not get damaged at fault current. 2012 | Slide 13 CT – Theory & Application Instrument Security factor (ISF) It is desirable that the ISF should be as low as practicable such that the metering core saturates during fault current.8 T metal – 0. if ISF is specified on lowest ratio.6 T x Bn x I0 /H © ABB Group Septem ber 25. same ISF on all ratios cannot be obtained.8 Tesla Bn Saturated [Mumetal] Protection Core: •CRGO – 1. For CTs with secondary taps.CT – Theory & Application B – H Characteristics E= [1T=10000 Gs] 4. If actual burden is too low there may be burning of instrument connected. Thus it can carry 22 Amps for 1 sec. This will require very costly Nickel Iron cores. IEC in their amendment in 2002.included 1. as confirmed by the meter manufacturers. ISF of 10 to 15 is safe for all practical purposes. © ABB Group Septem ber 25. © ABB Group Septem ber 25.2S Class CTs metering CTs These special class metering CTs require the stipulated accuracy limits to be maintained up to 1% of the operating primary current.CT – Theory & Application ISF has direct relation with VA burden connected. If ISF of 5 is assigned at 30 VA.0 Amp CT but with observation that there is uncertainty in accuracy measurement at low current like1% of operating primary current rating. 100/5 & their decimal multiples. which is below ankle point. IS 2705. However. ISF will be modified to 15.5S AND 0. 2012 | Slide 15 CT – Theory & Application Design of ‘S’ class 0. ISF of 5 on higher taps on multi-ratio CT means proportionally less ISF on lower taps. 2012 | Slide 16 . However. 50/5. it is important to note that all the meters are capable of carrying 10 Amp for 5 sec. and actual burden is 10 VA. 1992 specifies these CTs for 5 Amps secondary current and ratios of 25/5. Normal CTs require this limit to 5% of primary current. 5 1.2 0. 15 VA.75 30 1. Vs = (Burden/Is + RCT x Is) x ALF © ABB Group Septem ber 25.g. 5P or 10P. 2012 | Slide 18 .5 100% of rated burden 20 50 100 10 5 120 20 50 100 120 Rated primary current (%) Example: Plotted curves for class 0.7 50. Accuracy. 2012 | Slide 17 CT – Theory & Application PROTECTION CORE General Protection – VA Burden.5 © ABB Group Septem ber 25. ALF e. 5 0.5 Accuracy Curves:-Metering core Class 0. 0 .CT – Theory & Application Current error (%) + 1. 5 + 0. 20 For satisfactory performance of protection scheme. Class 0.5 Class 0. the CT should develop sufficient voltage to pass required current through Rp.2 25% of rated burden 0 0.0.2 Phase displacement (min) 90 60 .1.2 5 Class 0. usually equal to 2 x IF Rct = CT secondary resistance @ 75°C RL = one way lead resistance F = factor specified by relay manufacturer & may be 2 or 4. 2012 | Slide 19 CT – Theory & Application B – H Characteristics E= [1T=10000 Gs] 4. depending upon application P in mA = permissible magnetizing current.35 T Bs 0. Rct < 5 Ohms PS class is used for balance protection Information about KPV and permissible magnetizing current is required to be furnished Following equation may be used Vk (minimum) = C (Rct + 2RL) Imag (max) = P mA at Vk / F where. 2012 | Slide 20 . Rct e.8 T metal – 0.5 to 1. Vk (min) = Min KPV (Knee point Voltage) C= constant governed by system parameters and relay characteristics. Io < 50 mA at Vk/2. © ABB Group Septem ber 25.44BmAfN2 Volts Working Flux Density E/B 2 Tesla Saturated [CRGO ] Eknee 1.8 Tesla Bn Protection Core: Saturated [Mumetal] •CRGO – 1.6 T x Bn x I0 /H © ABB Group Septem ber 25.3 to 0. Io.CT – Theory & Application Special Protection – PS Class: Vk.g.4 to 0.5 Metering Core: •CRGO – 0. Vk > 400 V. 2012 | Slide 22 . which can be selected suitably with cost effectiveness. IF: the value of fault current must be selected by user in a realistic manner and not merely by using system fault current. the difference in magnetizing current drawn by cores on two sides of relay is less than the relay operating current. © ABB Group Septem ber 25. It is sufficient that at relay operating voltage. It is recommended that manufacturer shall select value of RCT.CT – Theory & Application Differential Protection © ABB Group Septem ber 25. Illustration in slide ahead may help in considering selection of fault current. 2012 | Slide 21 CT – Theory & Application Differential Protection Exact matching of cores for balance protection is neither possible nor it is required. 2012 | Slide 24 .96/35).CT – Theory & Application FAULT CURRENT CALCULATION FOR DIFFERENTIAL PROTECTION Calculation of KPV considering system fault level: KPV = 2 IF (RCT + 2 RL specified for DTH 31 relay.00 / 1000) = 52.48 Amps. Fault Current = 20 x 874.96 (RCT + 2 RL) © ABB Group Septem ber 25. KPV = 2 x 52.48x1000 /( 1.48 kA. Hence.e.5 kA while system STC is 52.79 = 17. Fault current considering trans impedance is 17. rated current / Prim. the through fault current shall be limited to 20 times the rated current.496 (RCT + 2 RL) = 35 (RCT + 2 RL) Thus. Therefore.48 (RCT + 2 RL) = 104. Rated current) = 52. core size will be considerably large & costly. Also. IF = (reflected current on sec. © ABB Group Septem ber 25. KPV required = 2 x 17.498 kA i. KPV by system fault current will be 3 times higher(104. side) = 52.8 kA x ( Sec. 2012 | Slide 23 CT – Theory & Application FAULT CURRENT CALCULATION FOR DIFFERENTIAL PROTECTION Calculation of KPV considering system fault level: At 5% Impedance. always requires Mumetal 400kV CT always requires Mumetal © ABB Group Septem ber 25. 0. 2012 | Slide 26 .5Cl.5Cl. the magnetic field gets generated around the conductor •If we reverse the supply.2 /0. 2012 | Slide 25 CT – Theory & Application •When you pass current through the Conductor. 0. When ratio is low or Burden is high or Accuracy is stringent than indicated above. Mumetal is foreseen. 0. 0. 0. 20VA.2 S Cl.CT – Theory & Application Design Criteria for Metering Core: How much can we offer? Without Mumetal 500/1 220 1000/1 220 500/1 66 & 132 1000/1 66 & 132 Possible Ratios Voltage Rating Burden/Accuracy 30VA. Cost increases to 15times. 10VA. the magnetic field changes the direction © ABB Group Septem ber 25.5Cl. 15VA.5Cl. L . 2012 | Slide 28 . 2012 | Slide 27 CT – Theory & Application Primary Insulation Insulation build up in new design of IMB 420 differs from the old design used in IMBD 145 and 245.10 7 [N. m. s s s s s Ly Main foil #y C 45 C 34 C 23 C 04 C 05 C 12 C 03 C 02 C 01 Main foil #x Lp Lx © ABB Group Septem ber 25.I 2 . A] D © ABB Group Septem ber 25.CT – Theory & Application Inner side of Hair pin Conductor F = ELECTRODYNAMIC FORCES ARE REPELLING IN NATURE DUE TO SAME NATURE OF FLUX DIRECTION D 1 2 L F F F 2.I 1 . CT – Theory & Application Primary Insulation It consists of a large number of main foils with 4 partial foil of the same length between each main foil. This can give problems in partial discharge test and in wet switching impulse test for an IMB 420. Axial step for each group of partial foils is the same. 2012 | Slide 29 CT – Theory & Application Primary Insulation New build up. This insulation build up is sensitive to the radial dielectric stress between the main foils and balancing of radial stress can give high axial stress between the foil edges. 2012 | Slide 30 . Main foil Partial foils Main foil © ABB Group Septem ber 25. © ABB Group Septem ber 25. Calculated voltage stresses for the new insulation build show moderate levels compared to the old build up.00 Foil num ber © ABB Group Septem ber 25.50 0.40 0.20 0. By this capacitance and voltage stress are easily controlled than in the old build up. 2012 | Slide 32 . 2012 | Slide 31 CT – Theory & Application Primary Insulation Stress levels for SIWL Ax ia l stre ss a t SIW L 1050 kV 0.70 0.CT – Theory & Application Primary Insulation The new insulation build up for IMB 420 consists of 4 main foils with 20 partial foils between each main. LIWL and AC test voltage are shown below. In this insulation build up the length of the partial foils are different and the axial steps are also different.30 0.10 0. © ABB Group Septem ber 25. Stress levels for SIWL.60 0. 0 0 2 0 . 2012 | Slide 34 .00 4.0 0 5 .0 0 3 5 . 2012 | Slide 33 CT – Theory & Application Primary Insulation Stress levels for AC Test Voltage: 630 kV for 1 minute Radial stress at AC test voltage 630 kV 20.00 0 .00 Foil number © ABB Group Septem ber 25.00 6.00 8.0 0 2 5 .00 12.00 Fo il n u m b e r © ABB Group Septem ber 25.00 18.0 0 3 0 .00 16.00 0.00 2.00 14.CT – Theory & Application Primary Insulation Stress levels for LIWL Ra d ial s t r e s s a t L IW L 1 4 2 5 k V 4 5 .0 0 1 5 .00 10.0 0 1 0 .0 0 4 0 . Tank Oil Level Ind. Cores © ABB Group Septem ber 25. Pri. Terminal Conn. Insulation Insulator P2 Terminal Box Bottom Tank Sec. 2012 | Slide 36 . 2012 | Slide 35 IMB 420 CT – Inner Details © ABB Group Septem ber 25. Head Expan.IMB CT – Construction Nitrogen Gas/ Metal Bellow Pri. Conductor P1 Pri. This moisture can deteriorate the Dielectric Strength of the primary insulation. Why Evacuation?? It is nothing but applying vaccuum to the assembled CT to remove atmospheric gases which are trapped during assembly.Manufacturing Cycle – IMB CT Primary Insulation Assembled Primary Drying Secondary Winding Tanking & Secondary Assembly. Oil & Gas Filling Quartz Filling Testing Insulator & Connection Head Assembly Packing & Despatch © ABB Group Septem ber 25. Expansion Tank Assly Evacuation. being Hygroscopic in nature. Hence this moisture has to be removed. 2012 | Slide 37 CT – Construction Why Drying?? To remove moisture from the insulating paper. 2012 | Slide 38 . has the tendency to absorb moisture from the atmosphere. Paper. © ABB Group Septem ber 25. 2012 | Slide 39 CT – Construction Primary Conductor Carries current through the CT Made of Aluminium. This process is called Impregnation. Insulation in a special room with strict control over humidity. © ABB Group Septem ber 25. Copper when high current densities are involved Primary Insulation Provides isolation between High Voltage and Low Voltage Electrical Grade Insulation paper. 2012 | Slide 40 Impregnation in vaccuum to avoid formation of air pockets in the insulation .CT – Construction Why Oil filling and pressing?? Pressing of oil is done after filling to ensure that oil is absorbed into the innermost part of insulation imparting the desired Dielectric Strength of the primary insulation. Why Waiting and Soaking?? This is an extended impregnation without applying oil pressure to ensure that no pockets of insulation are left without oil. temperature and dust levels Drying by special process combining high temperature and high vaccuum © ABB Group Septem ber 25. CT – Manufacturing IMB Primary Winding © ABB Group Septem ber 25. 2012 | Slide 41 CT – Manufacturing Secondary Winding Pri+Sec Oven Loading © ABB Group Septem ber 25. 2012 | Slide 42 . it allows slow heating / cooling of the CT. 2012 | Slide 44 .CT – Manufacturing Insulator Assembly + Sec Wiring Oil + Nitrogen Filling © ABB Group Septem ber 25. the diffusion of moisture through OilQuartz layers becomes slow over years Received from vendor in treated form and free from Magnetic particles Treated at Works for removing moisture before filling Filled into the CT at a temperature of 80 oC © ABB Group Septem ber 25. 2012 | Slide 43 CT – Construction Quartz in IMB CT Provides mechanical strength for the conductors Reduces the quantity of oil Improves the quality of oil by absorbing moisture Being a bad conductor of heat. Hence it eliminates the possibility of gas generation due to rapid cooling of CT During CT in service. 2012 | Slide 46 .Oil level indicator 3. Active part 9.Sec core 5. Base 10.CT – Construction Oil Cushioning Compensates Volumetric changes in Oil due to Temperature Variations IMB CT uses Nitrogen Gas/Metal Bellow Top Sealing All Gaskets are under/ below Oil level giving positive indication for leakage Enhances life of gaskets © ABB Group Septem ber 25.Terminal Block 11. Upper flange 7.Dome 2.Oil level Indicator © ABB Group Septem ber 25. 2012 | Slide 45 CT – Theory & Application LIVE TANK CURRENT TRANSFOEMER DESIGN 1. Upper tank 6. Insulator 8. Primary terminals 4. 2012 | Slide 48 . 2012 | Slide 47 CT – Construction DIFFERENCES BETWEEN Live Tank Design Dead Tank Design Weight of heavy cores rests on metal body & so primary insulation is not affected Weight of heavy cores is borne by paper insulation producing mechanical stresses Heat from secondary winding is dissipated directly eliminating interturn failures due to hot spots Heat from secondary winding is dissipated through thick insulation increasing interturn failures Suitable for currents high currents above 2000 Amps Suitable for high currents above 2000 Amps Fault current passes through the primary winding safely upto the earthed bottom tank Special precaution must be taken to pass fault current from top chamber to earth Main insulation at bottom Main Insulation at top Longer Primary length Short Primary length Weight at bottom Weight at Top © ABB Group Septem ber 25.CT – Construction PPHV-IT Constructional Differences DEAD TANK LIVE TANK SECONDARY CORES PRIMARY ACTIVE INSULATION CENTRE of GRAVITY SECONDARY LEADS SECONDARY CORES © ABB Group Septem ber 25. 2012 | Slide 49 CT – Construction Sealing with Metallic Bellow Bellow separates transformer oil from the external environment. There is no necessity of changing of oil or filtering it.Poor thermal stability Symmetrical construction and thus facilitates uniform stress distribution Non-symmetrical axis of insulation resulting into uneven stress distribution No chance of exposure of insulation to gas as the active pri ins starts below pri terminals Insulation of N2 filled CTs may get exposed to gas during expansion / contraction of oil leading to failure Substantially high level of oil above main insulation Comparatively low level of oil above main insulation © ABB Group Septem ber 25. No possibility of chemical reaction with oil. hence lesser possibility of building excessive pressure inside the transformer. The advantages of this sealing are as under: It allows the expansion and contraction of transformer oil volume as the bellow is free to expand and contract. Metallic bellows are superior to rubber bellows for their mechanical strength and most suitable for equipments designed for horizontal transportation. 2012 | Slide 50 .Difficult site handling - .Better thermal stability Uneven Temp Gradient . This reduces the aging effect increasing the life of the transformer.CT –Construction DIFFERENCES BETWEEN Live Tank Design High Centre of Gravity Dead Tank Design Low Centre of Gravity - .Failure during earthquakes - . There is no chance of gas or atmosphere coming in contact with oil.Instability Additional stability Ease of site handling Better seismic withstand capability Comparatively High dynamic withstand capability Low dynamic withstand capability during fault current Smooth Temp Gradient. © ABB Group Septem ber 25. This method is easy and economical. which may produce partial discharge. absorbed gas is evolved in form of gas bubbles and gets aligned across primary. The expansion chamber on top of the CT is evacuated first applying vacuum and then the vacuum is filled with dry Nitrogen gas. 2012 | Slide 52 . In our IMB CT this gas bubble formation is prevented by quartz layers & saves primary insulation from such hazards. excessive pressure may be developed and N2 may get absorbed in the oil. If expansion chamber is not adequately large to take care of expansion. © ABB Group Septem ber 25.CT – Construction Sealing With N2 Cushion This traditional arrangement is very common. The chamber is then sealed to avoid breathing with outer atmosphere. 2012 | Slide 51 Manufacturing Highlights Quality Control Raw Material Primary Insulation / Secondary Winding Drying & Impregnation Assembly Oil Filling Final CT © ABB Group Septem ber 25. In our IMB CT the expansion chamber is larger & gas pressure is maintained as 1 bar guage Under low pressure during oil contraction. CT – Routine Tests Tests as per IEC: 60044-1 and IS: 2705 Verification of Terminal Markings and Polarity Power Frequency Withstand Test on Primary and Secondary Partial Discharge Measurement Inter-turn Over Voltage Test Determination of Errors © ABB Group Septem ber 25. 2012 | Slide 54 . 2012 | Slide 53 CT – Type Tests Tests as per IEC: 60044-1 and IS: 2705 Short Time Current Test Temperature Rise Test Lightning Impulse Test Switching Impulse Test Radio Interference Voltage Test Determination of Errors Facilities for conducting all Type Tests on CTs except Short Time current Test (ERDA / CPRI) and Seismic test (IIT Madras / University of Roorkee) are available at our works. © ABB Group Septem ber 25. IMB 245. Transportation – IMB 73. – IMB 73. 2012 | Slide 56 . IMB 145 Hor. IMB 145.CT – Special Tests Tests as per IEC: 60044-1/ IS: 2705 Chopped Lightning Impulse Test Measurement of Capacitance and Dielectric Dissipation Factor Multiple Chopped Impulse Test on Primary Special tests as prescribed in CIGRE have also been conducted © ABB Group Septem ber 25. IMB 420 © ABB Group Septem ber 25. 2012 | Slide 55 CT – Packing and Transportation Generally all CTs are packed in Steel Cases Vertical Trans. even and raised ground (platform) in Vertical direction only © ABB Group Septem ber 25. Unpacking & Storage Unloading of CTs should be carried out using a crane of requisite capacity (Details in Instruction Manual) When received at site. the CT is to be first inspected for any damages or leakages They are to be unpacked and stored on a hard.8 Quality Fine Thread with a Tightening Torque of 250 Nm minimum © ABB Group Septem ber 25. 2012 | Slide 57 Checks Before Installation Visual Check for any sign of Leaks or breakage Check Oil level Mechanical Check that the ’D3’ terminal meant for Tan Delta measurement is properly earthed Check that Earthing of tank is done by Copper or suitable strips and properly tightened with M10 bolts of 4.8 Quality Fine Thread with a Tightening Torque of 30 Nm minimum Check levelling of Structure by Spirit level Ensure fixing of CT on Structure with M16 bolts of 8. 2012 | Slide 58 .CT – Unloading. the terminals meant for highest ratio are shorted. For comparison with future measurement during Maintenance Values should be < 0. 2012 | Slide 60 .5 Sec. Measure with Resistance meter and avoid DC Source Refer Instruction Manual 1HYN520002 for IMB for complete details © ABB Group Septem ber 25. if possible preferably by connecting resistor equivalent to the rated burden to restrict short time current Ensure that the cores which are in use should have their balance terminals left open Ensure contact surfaces of CT Primary Terminations thoroughly cleaned and apply good quality contact grease like Servogen and brush contact surfaces with wire mesh. 2012 | Slide 59 Checks Before Commissioning Continuity Test/ or Rough Ratio Measurement: @To ensure internal connections of Primary and Secondary are intact Tan Delta at 5 or 10 kV @ To ensure that ‘D3’ connections from inside Tank are intact. Apply thin layer of contact grease before making Primary Connections Ensure the fixing of CT Primary terminations with Terminal Connectors by M12 bolts of 4.Checks Before Installation Electrical Ensure that in case of idle cores.8 Quality Fine Thread with a Tightening Torque of 55 Nm minimum © ABB Group Septem ber 25. Resistance Measurement @ Values should match with Rating or Test Certificate. Unpacking and Storage Checks before Installation & Commissioning Care and Troubleshooting © ABB Group Septem ber 25. 2012 | Slide 62 . Type and Special Tests Packing and Transportation Unloading.Capacitor Voltage Transformer Product Range Theory & Application Construction and Manufacturing Routine. 2012 | Slide 61 Product Range Capacitor Voltage Transformers (IEC:60186/ 5/IEC:60358/IS:3156/IS:9348) 60044- Type WN 36 / WN 73 / WN 145 / WP 245 / WS 420N2 This is the mixed di-electric design of CVTs The insulation consists of a combination of paper and Polypropylene film (Mixed Dielectric) This design has been indigenously developed Metal Bellow used to compensate for oil expansion © ABB Group Septem ber 25. 2012 | Slide 63 CVT – Theory & Application What is a CVT used for ? Measurement of Voltage Isolation between High Voltage & Low Voltage. FR circuit © ABB Group Septem ber 25. Insulator PT. HV Choke. Inputs to Relay/Protection systems PLCC (Power line Carrier Communication) Main Parts of a CVT Capacitor Part EMU - Capacitor Stack.sion © ABB Group Septem ber 25.Product Range Coupling Capacitors (IEC:60358/IS:9348) Type KN 36 / KN 73 / KN 145 / KP 245N2 This is the mixed di-electric design of CCs The insulation consists of a combination of Paper and Polypropylene film (Mixed Dielectric) This design has been indigenously developed Uses metal bellow to compensate for oil expan . 2012 | Slide 64 . 6 V/V2 = C2/C = 6. The charging current will be very low giving accurate output C2 e.22 VA 2) Any attempt to load this system results into uneven voltage distribution between C1 and C2 and inaccurate output © ABB Group Septem ber 25.9 kV point of view by connecting intermediate PT This will fetch effective burden e.6 Therefore.) LIMITATIONS: 1) To tap 110 V from C2 is therefore a problem as C2 V2=110 V effective burden fed by this is only 0.9 kV V/V2 = C2/C = 1200 Therefore.g.e. adequate to handle from insulation C1 V1=131.04 thousand pF © ABB Group Septem ber 25. 20 kV.g.8 lakh pF (This is 180 times larger Cap.CVT – Theory & Application Design Principles V e. C= 4400 pF V2=20 kV V1/V2 = C2/C1 = 5.g. 200 VA and above. 2012 | Slide 66 . C2 = 52. C2 = 29. C= 4400 pF V1/V2 = C2/C1 = 1199 C1 V1=131. V = 132 kV. V = 132 kV. 2012 | Slide 65 CVT – Theory & Application Design Principles HOW TO OVERCOME? V Tap the voltage at suitable medium voltage i. 8 = 30mA HV Choke adjusted to give Zero Phase Angle Error at 50Hz © ABB Group Septem ber 25.CVT – Theory & Application C1 LT IP RT C2 VP=20kV/ 3 BURDEN VS=110/ 3 © ABB Group Septem ber 25.72/181.72 A IP= 4.72/PT VOLTAGE RATIO = 4. 2012 | Slide 68 .72/20000/110=4. 2012 | Slide 67 CVT – Theory & Application ISXCE VP VS Ø ISRT ISXLT IS CE=C1+C2 -BURDEN ANGLE Ø-PHASE ANGLE BETWEEN VP & VS IS-BURDEN/VS e. 300/110/ 3=4.g. for the applied voltage from 80% to 120%.5 Hz Applied Voltage 0.CVT – Theory & Application Accuracy of CVT at different Frequency CVT is a tuned device and hence the errors are prone to variations with frequency.5 Hz ( i. 2012 | Slide 69 CVT – Theory & Application Frequency Variation Total ( ) Burden on CVT 0.5 Hz to 51.5 Hz 250 VA 49 Hz to 51 Hz 300 VA 49.50 Hz +/. The standards specify that errors of Metering winding are to be guaranteed from 25 % to 100% of the rated burden and for a frequency range of 49. 2012 | Slide 70 .2 Class for a variation in the burden from 25% to 100% of the rated Burden.2 Class 60 VA 50VA 40 VA 30VA 80% to 120% 80% to 120% 80% to 120% 80% to 120% © ABB Group Septem ber 25. We have carried out measurements on a 220 kV.5 Class 150 VA 48 Hz to 52 Hz 200 VA 48. for the frequency variation as against the different total burden connected on the CVT as indicated in Table below.e.5/0. 4400 pF CVT as a case study for variation in Accuracy as against a variation in frequency of the prevailing incoming supply at different time.5 to 50. we concluded the Accuracy Test results as follows: The Accuracy of a CVT remains within 0. From such measurements.5 Hz to 50. © ABB Group Septem ber 25.1 %). 5 Hz has increased by + 0.5 Cl & 60 VA/0.CVT – Theory & Application Total ( ) Burden on 220 kV CVT: 300 VA/0.35%.5 Hz is .the Accuracy falls with in 0.5 Hz has increased by .5 Class Accuracy for full and simultaneous burden measured at 49.2 Cl © ABB Group Septem ber 25.5 Hz is . 300 VA/0.2 %Class.0.2class for 10 VA Burden This clearly indicates that the accuracy of CVT at all the three frequencies remains well within the limits of 0.15 for 50 Hz frequency Accuracy for 1/4th burden measured at 49.2% and at 50.0.05 for 50 Hz frequency at 50. refer vertical dark lines corresponding to burden of 300 VA & power factor 0.0.5 Hz has reduced by . which indicates error limit from zero burden to 100% burden.0. at 50 Hz is .05 for 50 Hz frequency © ABB Group Septem ber 25. 2012 | Slide 71 CVT – Theory & Application Pl. From above curves.8.5% /0.1 for 50 Hz frequency at 50.g. following can be inferred: e. Errors measured at Total (full) burden of 300VA at different frequencies are respectively: At 49.12%. Looking to Vector length (Vertical dark line) . 2012 | Slide 72 .5 Hz has reduced by + 0.0. © ABB Group Septem ber 25. in real life situation due to huge load requirement frequency always tend to go down to 49 Hz for 8 to 10 months of year & under frequency relay will operate at 48.5 Hz resulting into cascade tripping. However CVT can cater same accuracy at 49 & 51 Hz due to reduced burden requirement However.5 to 50.50 Hz +/.e.1 %). 2012 | Slide 74 .CVT – Theory & Application CVT is a tuned device and hence the errors are prone to variations with frequency. 2012 | Slide 73 CVT – Theory & Application Numbers of Secondary Windings:Number of Secondary Windings may be 2 or 3 depending upon the requirement. The standards specify that errors of Metering winding are to be guaranteed from 25 % to 100% of the rated burden and for a frequency range of 49. Secondary winding Burdens will add together to impose high burden on CVT and affect the accuracy. if customer desires so © ABB Group Septem ber 25. Hence system runs between 49 & 48.5 or 0.2 with low VA burden.5 Hz as a unavoidable evil as cut off load will raise political issues Where as frequency to be more than 50 Hz never happens & even if it happens it is controlled automatically by governor provided in generator Hence solution is found by tuning CVT at 49 Hz so that it gives same accuracy at 50 & 48 Hz. If metering is required for energy billing then it may be separated with assigning it with precision grade of 0. This is unlike core burdens on CT.5 Hz ( i. Other winding may be of class 1 or 3 for indicating Metering/Protection. CVT – Theory & Application Voltage Factor The voltage factor is dependent upon the system earthing condition. Terminal Metal Bellow Insulator Cap. 2012 | Slide 75 CVT – Construction Pri. Under fault conditions. © ABB Group Septem ber 25. Stack EMU Epoxy PT Spark Gap Terminal Box 20 kV Bushing HF Bushing © ABB Group Septem ber 25. the voltage appearing across open delta winding is 3 times the rated sec voltage. To achieve high VF core flux density has to be reduced substantially. 2012 | Slide 76 . which increases the cost. Adequate VF must be provided OR there is danger of failure during fault on the system which produce transients of high voltage nature. in isolated neutral system. Stack Top Lead Mtg. Elements Bottom Lead Insulation Split Pin © ABB Group Septem ber 25. 2012 | Slide 78 .CVT – Construction – Cap. Stand Cap. 2012 | Slide 77 CVT – Construction – EMU Bottom Tank PT Resistor Capacitor FR Choke HV Choke © ABB Group Septem ber 25. 2012 | Slide 79 Manufacturing Cycle – CVT Coil Winding EMU Assembly Stack Assembly Pressing & Soldering EMU Testing CVT Testing Finishing Drying & Impregnation Stack Calibration & Testing Capacitor Part Assly. 2012 | Slide 80 .Schematic FR Ckt Characteristic V1 I m C1 p 50 Freq Step Down PT V2 Vs HV Choke C2 FR Ckt Spark Gap Resistor Sealed Unit Protects the EMU from overvoltages © ABB Group Septem ber 25. Packing & Despatch Oil Filling © ABB Group Septem ber 25.CVT . CVT – Manufacturing Capacitor Coil Winding Coil Stacking Stack Soldering © ABB Group Septem ber 25. 2012 | Slide 82 . 2012 | Slide 81 CVT – Manufacturing EMU Assembly CVT Assembly Insulator Assly On Stack © ABB Group Septem ber 25. BBMB) SUCH PROBLEMS ARE UNRELEVANT AND UNKNOWN FOR CVTs CVTs ARE EVEN PREFERRED WHERE PLCC IS NOT IN USE. SO CAPACITOR PART OF CVT OFFERS LOW IMPEDANCE RESULTING INTO UNIFORM DIST. 2012 | Slide 83 CVT – Advantages over PT CVT PT PTs ARE INDUCTIVE DEVICES. AND COILS GET FURTHER STRESSED. DURING IMPULSE TEST. 2012 | Slide 84 . BURDEN REQTS. © ABB Group Septem ber 25. HENCE PTs ARE SUSCEPTIBLE TO FAILURES (NEXT TO LA) ALL CVTs ARE PROVIDED WITH INTENTIONAL FR SUPPRESSION CKTs HENCE WELL PROTECTED AGAINST FR OSCILLATIONS OUTPUT: 500 VA OUTPUT: 200 . IMPULSE WAVE SHOWS PRESENCE OF HIGH FREQ WHEN ANALYSED WITH FOURIER SERIES. VOLTAGE STRESS AT START OR END ARE HIGH. PTs COME INTO RESONANCE WITH GRADING CAPACITORS PROVIDED WITH CBs WHICH CREATE TRANSIENT/PERMANENT OVERVOLTAGE LEADING TO FAILURE (ref. HENCE THEY ARE INSULATED MORE. CVTs USE CAPACITOR COILS IN SERIES FOR STEPPING DOWN VOLTAGE. PROBLEM IS MORE APPARENT FOR HIGHER VOLTAGES. VOLTAGE DIST. THIS DECREASES INTER TURN CAP. HAVE DRASTICALLY REDUCED TO 50 VA (ref.CVT – Advantages over PT CVT MODULAR CONSTRUCTION EASY TO TRANSPORT EASY TO HANDLE EASY TO ERECT PT SINGLE AND BULKY UNIT SIMILAR FR DAMPING CKTs ARE NOT PROVIDED. HENCE DESIGN BECOMES DIFFICULT AND ONLY OPTIMUM IS POSSIBLE. PGCIL) ON ACCOUNT OF DIGITAL METERS AND RELAYS NO SUCH APPLICATION POSSIBLE CAN BE USED AS COUPLING CAPACITOR FOR PLCC NON UNIFORM ELECTRICAL STRESS DISTRIBUTION DUE TO INTER-TURN & INTER-LAYER CAPACITANCE UNIFORM ELECTRICAL STRESS DISTRIBUTION THUS MORE RELIABLE DESIGN © ABB Group Septem ber 25. IS UNIFORM ACROSS EACH COIL.300 VA IN PRESENT CONTEXT. the CVT is to be first inspected for any damages or leakages They are to be unpacked and stored on a hard. 2012 | Slide 85 CVT – Unloading and Unpacking Unloading of CVTs should be carried out using a crane of requisite capacity (Details in Instruction Manual) When received at Site. even and raised ground (platform) in Vertical direction only © ABB Group Septem ber 25. 2012 | Slide 86 .CVT – Packing & Transportation Generally all CVTs are packed in Steel Cases All CVTs are transported Vertically Ensure that CVTs are not made Horizontal during emergency transshipment © ABB Group Septem ber 25. ensure the earthing of ’HF’ terminal through PLCC device Check that Earthing of tank is done by Copper or suitable strips and properly tightened with M10 bolts of 4.Checks before Installation Visual level Mechanical Check for any sign of leaks or breakage Check Oil Check that the ’HF’ terminal meant for C & Tan Delta measurement is properly earthed In case of use with PLCC.8 Quality Fine Thread with a Tightening Torque of 350 Nm min. © ABB Group Septem ber 25. 2012 | Slide 88 . 2012 | Slide 87 Checks Before Commissioning Electrical Continuity Test/ or Rough Ratio Measurement @ To ensure internal connections of Capacitor Part and EMU are intact C & Tan Delta at 5 or 10 kV @ To ensure that Divider connections from inside are intact @ For comparison with future measurement during Maintenance @Tan Delta should be < 0.5% @ For ‘C’ – refer Manual Refer Instruction Manual 1HYN525002 for complete details © ABB Group Septem ber 25.8 Quality Fine Thread with a Tightening Torque of 30 Nm minimum Check levelling of Structure by Spirit level Ensure fixing of CVT on Structure with M18 bolts of 8. the reason for fault shall be investigated and rectified before fuses of 16 A rating are replaced Higher rating of fuse than required will not protect the CVT and lower rating will add burden and affect accuracy © ABB Group Septem ber 25.Care for CTs and CVTs Carry out regular Visual Inspection of Oil Level Clean the insulator periodically. Acceptable values are: IMB 145 – 2 %. on de-energized CT. IMB 245 – 1 % : IMB 420 – 0. the tightening of Primary and Ground Connections shall be checked with recommended Tightening Torques. Measure the Primary contact resistance: IMB – 200 Ohms max If possible.5 % © ABB Group Septem ber 25.8 % : CVTs – 0. 2012 | Slide 89 Care for CTs and CVTs Tan Delta values found beyond the Acceptable Limits in CTs call for further investigation by Oil Sampling for DGA and check of moisture content Abrupt changes in Tan Delta are alarming than its absolute values higher but constant In case of Fuse Failures in CVTs. 2012 | Slide 90 . Hot Line Washing can be used No Oil Change or Oil Filteration is required as the CTs and CVTs are hermetically sealed During Annual Inspection. carry out online checks for any Hot Spot at Primary Connections by ThermoVision Camera Tan Delta should be measured every year and recorded. © ABB Group Septem ber 25. 2012 | Slide 91 ~ Trouble Shooting for CTs and CVTs Minor Leakage from Sealing Joints In case of leakage from any part like Insulator and Tank Cover. This will make the core free of Residual flux and it is now ready to function as desired. the joint may require replacement. Relay operation sequence Logbook record of voltage and frequency before fault . carry out tightening of nuts successfully by about 1/6th of a turn until uniformly tightened If leakage still persists. they are required to be Demagnetized in order to let them perform accurately after the fault is cleared. The following method may be adopted Gradually excite the core upto its Output Voltage and reduce it to zero. 2012 | Slide 92 Disturbance Recorder.Care for CTs and CVTs Incase of saturation of CT core during fault. The factory or nearest Regional office should be contacted Major Irregularities/ Damage/ Faults Inform the factory or nearest Regional office with atleast the following details alongwith Site and Commissioning history: Problem observed with background of earlier Care records alongwith details given in the Name Plate © ABB Group Septem ber 25. Repeat it 3 – 4 times. 2012 | Slide 93 .© ABB Group Septem ber 25.