Reactors

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Shunt reactors in thermal power plant

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Reactors

  1. 1. Reactors in power system M.G.Morshad / ACM ( Elect.) Transformer Mtce. Division / TS II
  2. 2. References 1. REPORT - SYSTEM PLANNING & PROJECT APPRAISAL DIVISION – CEA 2. LOAD GENERATION BALANCE REPORT 2010-11 – CEA 3. INSTALLED CAPACITY - CEA 4. JOURNAL - TECH NEWS – AREVA 5. HV SHUNT REACTOR SECRETS FOR PROTECTION ENGINEERS - Zoran Gajić , Birger Hillström, Fahrudin Mekić ABB Sweden, Västerås, Sweden 6. IEEE Guide for the Protection of Shunt Reactors 7. APPLICATION OF NUMERICAL RELAYS FOR HV SHUNT REACTOR PROTECTION – Z. Gajić, B. Hillstrőm, M. Kockott , ABB Automation Technologies ,Sweden 8. TRANSIENTS DUE TO SWITCHING OF 400 KV SHUNT REACTOR Ivo Uglešić, Sandra Hutter University of Zagreb Miroslav Krepela Siemens Božidar Filipović- Grčić Croatian National Electricity Franc Jakl University of Maribor 9. IS 5553 ( Part 2) 1990 10. IS 2026 ( Part 1 & 3) 11. Installation & commissioning of shunt reactor - BHEL
  3. 3. Reactors in power system REACTORS Series REACTOR Shunt REACTOR PURPOSE To reduce short circuit current PURPOSE To reduce over voltage Z Z Acts as inductive load and reduce high voltage by absorbing MVAR. Acts as inductance and opposes the flow of short circuit current. AREA OF APPLICATION Tie Lines AREA OF APPLICATION Bus , Lines, Tertiary winging of ICT
  4. 4. Classification of shunt reactors Shunt Reactors Dry Type (system voltage Below 72.5 KV) Oil immersed Type (system voltage 72.5 KV & above) Core less Gapped Core Air Core •Star connected with neutral grounding • Range 30 to 300 MVAR •Connected at the terminals of transmission line • Delta connected • Range below 30 MVAR •Connected at the tertiary winding of transformer Switch on / off type Permanently connected type with thyristor controlled
  5. 5. Back ground for installing shunt reactor
  6. 6. Reason for high grid voltage in Southern grid during off peak period – As per CEA report Availability 28450 MW Peak Load demand 34224 MW Off Peak Load demand 13000 MW Installed Capacity 44220 MW Grid voltage higher than rated (Max 1.05PU or 441KV) Grid voltage lower than rated (Max 0.95PU or 399 KV) 11Hrs 15Hrs 22 Hrs 03 Hrs
  7. 7. Effect of high grid voltage High over voltage cause – • Difficulty in regulating load flow through HVDC line • Difficulty in synchronization inter grid transmission line • instability in generator due to operation of generator in under excitation zone near the pole slip region. • Increase in line loss Present practice to overcome overvoltage situation • Keeping all 64 Nos Reactors (56 Nos Line &8 Nos Bus) are in service during off peak period • Switching off all lightly loaded lines
  8. 8. Extract of PGCIL report (2006) • With existing reactors and opening of the lines as per existing practice , the study reports indicate high voltage profile throughout the grid (the voltage ranges between 416 kV and 445 kV). • Addition of 15 numbers of reactors of 63 MVAR each is not adequate to control the voltages under acceptable limits even with some of the transmission lines switched off. • Even large generating stations like Ramagundam, Neyveli, Vijayawada, Raichur TPS are not able to hold their voltages as these are crossing reactive power absorption limit. As such these stations may also be considered for installation of bus reactors. • With 10 more reactors (making it 25 ), the results indicate that when lightly loaded transmission lines are out of service, the voltages at various buses are generally controlled and are less than 420 kV and with the above lines in, the voltages are higher going up to 431 kV. • Therefore, it is concluded that provision of large number of reactors are required to control the high voltages situation in the grid.
  9. 9. Calculation of grid reactive power About 27 numbers of 63 MVAR shunt reactor are required in southern grid to absorb reactive power for bringing down grid voltage from 441 KV to 416 KV during off peak period Formula Data System Voltage Skv 420 KV Fault Current level FkA 40 KA Short Ckt MVA Scc= 1.732 X Skv X FkA 29097 MVA Max Bus voltage V1 = 441 KV or (441/420) PU 1.05 PU Acceptable Bus voltage V2 = 416 KV or (416/420)PU 0.99 PU Total reactive power Sr = Scc{ ( V2 – V1)/V1} 1763 MVAR Standard capacity Sst 63 MVAR Nos of reactor required Sr / Sst 27 Nos
  10. 10. Recommended location for additional 25 Nos reactors
  11. 11. Advantages of installing reactor Technical 1. Limited voltage rise on transmission lines at the time of light loads or after load shedding 2. Prevention of self excitation on generator on leading PF load. 3. Reduction of over voltage on sound phases during a line to ground fault. 4. Reduction of switching over voltage due to initial charging of lines. Commercial 1. Being a capital item, investment on this equipment will be adjusted in the fixed cost portion of tariff so that entire amount ( capital + interest ) will return within the operating period of the reactors.
  12. 12. Basic operating principle
  13. 13. Basic operating principle System Voltage, (V) 1. Initial charging current (i) produces pulsating flux in the coil 2. Induced voltage (e) = L (di / dt) is produced in opposite direction of the coil due to pulsating flux 3. As a result of two opposite voltages current equal to (V-e) / R passes through the coil Shunt Reactor acts as inductive load when it is connected to Bus/Line and draws current for active & reactive load. The reactive portion of current creates pulsating flux in the core and the power required for this purpose is known as reactive power (KVAR). KVAR = Current X system voltage X SinΦ The active portion of current causes I2R loss and the power loss due to heating is known as active power (KW). KW = Current X system voltage X Cos Φ Induced Voltage, (e) Initial charging current. Operating current (V-e) / R. System Voltage Operating current (I) Active current (I x Cos Φ ) to active load (KW) Reactivecurrent(IxSinΦ) toreactiveload(KVAR) Φ
  14. 14. Operating function of shunt reactors (System voltage – Induced voltage) 1. Current drawn by each phase = (Resistance per phase winding) 2. Induced voltage (e) depends upon the construction and magnetization characteristic of the core 3. Shunt reactor is a device with the fixed impedance value. Therefore the individual phase current is directly proportional to the applied phase voltage (i.e. I=U/Z). 4. For balance three phase current (Ir + Iy + Ib = 0), no current flows through the neutral. 5.Only during fault when phase current becomes un balance (Ir + Iy + Ib ≠ 0), current less than 1 PU passes through the neutral. It is due to the fact that positive sequence reactance(X1) is more or less equal to zero sequence reactance (x0) in five limbs core configuration (Normally X1/Xo = 0.9). 6. During operation heat is produced as a result of copper loss ( due top I2R) and core Loss ( due to Hysteresis and eddy current) 7. Typical total core & copper loss (KW)= rating of reactor (MVAR) x 0.2% 8. Typical core loss & copper loss = 75% & 25% of total loss respectively Ir Iy Ib
  15. 15. Induced voltage and Magnetizing characteristic of the core
  16. 16. Characteristics of the induced voltage The magnitude of the inductive voltage (e) developed in the reactor coil due to pulsating flux depends upon - 1. Flux density in the core ( Concentration of flux in the core ) 2. Magnetization characteristic of the core ( Ability of the core to produce flux) 1. Flux density in the core •The coil provided with iron core always have higher flux concentration than the coil without core (Air core). •As higher concentration of pulsating flux creates higher induced voltage (e) in the coil, iron core is used for higher voltage (> 72.5KV ) Reactor and air core is used for lower voltage (< 72.5KV ) Reactor Reactor - 72.5 KV and above •Star connected with solid / reactor grounding •Oil cooled , gapped core •Rating 30 to 300 MVAR •Directly connected to bus / transmission line Reactor - Below 72.5 KV •Delta connected •Air cooled , Air core •Rating below 30 MVAR •Directly connected to tertiary winding of transformer
  17. 17. 2. Magnetization characteristic V – I linearity range •The flux produced by the core remains proportional to the current passes through the coil till the core gets saturated. •After saturation of core it can not produce flux further with the increase of current. •Because of this magnetic saturation of the core, reactor coil can not develop inductive voltage (e) further. •In such condition impedance of the coil becomes lower with respect to the applied voltage and higher current drawn by the winding causing high I2R loss ( heat generation) or earth fault.
  18. 18. Components of Reactor WTI OTI 1. 400KV,1250 Amps OIP HV bushings (3 Nos) 2. 145 KV ,1250 amps neutral bushing (1No) 3. Air cell type conservator tank with silica gel breather 4. Radiator banks (8 Nos) with ONAN type cooling 5. Buchholtz relay, Pressure relief valve and sudden pressure valve for protection 6. Floor mounted marshalling box for providing WTI,OTI and required control circuit for protection 7. Provision of line side and neutral side CT terminals in each phase 8. Two numbers treated earth pit for earthing neutral through steel flat 9. Two number earth grid terminals for earthing reactor tank and other metal structure.
  19. 19. Components of reactor Five limbed core construction is adopted to achieve high zero sequence impedance. In addition to the three gapped core limbs with windings, there are two continuous outer return limbs. The two unwound side limbs help in achieving zero sequence impedance approximately equal to the positive sequence impedance
  20. 20. Gapped core construction is preferred for high system voltages over coreless construction due to the high energy density that can be achieved in gapped core construction The core sections between consecutive air gaps are moulded in epoxy resin to prevent movement between individual laminations. The spacers forming the air gaps are blocks of ceramics with a high modules of elasticity and the whole stacking of core modules is cemented together during the assembly to form a solid column without possibility of rocking , or rubbing between individual parts. The core segments are of radial laminated configuration. The radial laminations prevent fringing flux from entering flat surfaces of core steel which would result in eddy current overheating and hot spots.
  21. 21. Interleaved disc winding has been used for rated voltages 220 KV and above. This type of winding configuration provides better impulse voltage distribution. For lower voltage classes a continuous disc winding or a multi layer helical winding are used. M 6M 5M 4 Loss in Watt / Kg CRGO Steel StripHi-B grade Thickness 0.3- 0.5 mm , Flux Density 1.6 Tesla Frequency 50 Hz, Gray colour ,E Carlite insulating laminated core 23 M0H ZDKH 0.89 1.11 0.74 0.66 0.57 0.97 27 M0H Grading of core according to loss
  22. 22. Vibration in reactor 1. As a result of magnetization of core, a magnetic attraction force (F=107 xB2 )/8π N / m2 ) approximately equal to (107 x1.62)/8π N/m2 or 104 Ton / m2 is produced in the air gap between the core sections. 2. This force pulsates at double frequency (2 X 50 Hz = 100 Hz) due to sinusoidal flux of frequency 50 Hz. 3. Because of this double frequency pulsating force in the core sections, high vibration and noise is observed in reactor during operation. To reduce the vibration within the limit ( 200 micron) following measures are taken during construction – 1. Air gap are filled with ceramic materials 2.
  23. 23. Earthing of Reactor Treated earth pit To earthing gridTo earthing grid LA LA LA 1. Neutral earthing – To provide return path for the fault / unbalance current, the neutral of the reactor is grounded to two separately treated earth pit 2. Tank earthing – To avoid heating of tank due to circulation of eddy current as result of voltage build up on tank due to continuous passing of fractional portion of main flux through it, the potential of the tank is made zero by connecting with earth grid. 3. Steel structure earthing – To avoid damaging of steel structure from lighting strike, entire steel structure is grounded through earth grid.
  24. 24. Cooling of reactor Approximate heat generation during operation of reactor due to core and copper loss = 0.2% x MVAR rating = 120 KW Core loss due to hysteresis & eddy current is about 75% of total loss i.e. 0.75X120 = 90 KW Copper loss due to I2 R loss is about 25% of total loss i.e. 0.25X120 = 30 KW The loss of 120 KW is converted to heat 50000 Litre of oil absorb the heat and rise the top oil temperature to maximum 45 Deg C 8 nos radiator banks reduces oil temperature to 15 Deg C by ONAN cooling process.
  25. 25. Significance of parameters 1. Capacity (S) – As reactor is used as inductive load which only absorb reactive power, it capacity is given in MVAR . Rated Voltage 1 PU ( 420 KV) 1.05 PU (441KV) Rated Capacity 63 MVAR 69.45 MVAR Current ( MVARx100)/(1.732xKV) 86.6 Amps 90.92 Amps 2. Impedance / Positive sequence Impedance (X1) – It is the per phase AC resistance of the winding which decides the magnitude of per phase current. X1 = U / I = {420000/(1.732X86.6)} =2800 Ohms 3. Zero sequence Impedance (X0) – It is the AC resistance of the neutral path which decides the magnitude of earth fault current through neutral. The value of X0 depends on the construction of core. For five limbs core, the value of X0 lays between 90% and 100% of positive sequence impedance (X1). Therefore X0 = 0.9 . X1 = 0.9 X 2800 =2520 Ohms. Because of high zero sequence impedance earth fault current is restricted within 1 PU 4. Winding resistance (R) – It is the per phase DC resistance of the winding which decides copper loss (I2R). Measured winding resistance per phase = -2.570 Ohms at 38 Deg C 5. Total loss – It is the total active power consumed by the reactor and converted in to heat. Total loss (core and copper loss) = 110KW 6. Power Factor – CosΦ = 110 / 63000 = 0.17% 110KW 630000KVAR630000KVA Φ
  26. 26. Operation
  27. 27. Position of Bus reactors in ts ii Stage I Gen (3 X 210MW) Stage II Gen (4 X 210MW) 400KV Bus 400KV Extension Bus Reactor II Tie Line to TS II Expn ICT Power Grid Feeder Power Grid Feeder Tie Line 230KV Bus Mines Feeder State Grid Feeder Tie Line to TS I Reactor I Power Grid Feeder TS II Expansion Gen (2 X 250MW) 400KV Bus
  28. 28. voltage compensation methods in power system Switchyard Bus 395 – 415KV R E C C A P Delivers reactive power to boost up voltage Absorb reactive power to reduce bus voltage Increasing tap to deliver reactive power for increasing bus voltage Decreasing tap to absorb reactive power for decreasing terminal voltage Over excitation to deliver reactive power for increasing terminal voltage Under excitation to absorb reactive power for decreasing terminal voltage Capacitive loading Inductive loading Current Leading MVAR Lagging MVARCurrent 105% Grid Voltage (445KV) 100% Grid Voltage (420KV) 95% Grid Voltage (380KV) 1. Reduce excitation 2. Reduce GT tap 3. Switch on reactor 4. Disconnect lines 5. Reduce Gen load 1. Increasing excitation 2. Increase GT tap 3. Switch on Capacitor 4. Reduce gen load
  29. 29. Operation of bus reactor Switching ON : Whenever bus voltage goes to 4% higher than rated voltage i.e. 400 x104/100 = 416 KV Switching Off : Whenever bus voltage goes to 2% less than rated voltage i.e. 400x98/100 = 392 KV However switching On & Off are to be carried out as per the direction of SRLDC  With the switching on of bus Reactor, Station MVAR will increase depending upon the grid voltage  Since Reactor is a fixed impedance equipment , when grid voltage is lower , reactor will absorb lower current and generate lower MVAR Load.  Similarly when grid voltage is higher , reactor will absorb higher current and generate higher MVAR Load. GRID VOLTAGE REACTOR CURRENT REACTIVE LOAD 400 KV 85 Amps 60 MVAR 420 KV 87 Amps 63 MVAR 441 KV 90 Amps 69 MVAR
  30. 30. Observation of parameters during switching on reactor Switchyard parameters Time Grid voltage Station MVAR Reactor current ( As per Meter) Reactor MVAR ( As per Meter) Reactor II (SL No 6007011 Location 17th Bay) Before charging 10:05 Hrs 413 KV 295 MVAR 0 0 After charging 10:15 Hrs 411KV 335 MVAR 85 Amps 63 MVAR Reactor I (SL No 6007012 Location 16th Bay) Before charging 12:10 Hrs 414 KV 254 MVAR 0 0 After charging 12:17 Hrs 411KV 331 MVAR 87 Amps 63 MVAR UCB IV Parameters Time Gen Terminal Voltage MVAR Bus Voltage PF Reactor II (SL No 6007011 Location 17th Bay) Before charging 10:15 Hrs 15.9 KV 50 MVAR 410 KV 0.98 After charging 10:06 Hrs 15.9 KV 55 MVAR 405KV 0.98 Reactor I (SL No 6007012 Location 16th Bay) Before charging 12:10 Hrs 16 KV 40 MVAR 408 KV 0.99 After charging 12:17 Hrs 16 KV 60 MVAR 405 KV 0.99 Site Parameters Running Hours OTI WTI Ambient Temp Noise & Vibration Label Reactor II (SL No 6007011 Location 17th Bay) Switched On: 10:13 Hrs Switched Off: 11:15 Hrs 1Hour 30 Deg C 34 Deg C 28 Deg C Normal Reactor I (SL No 6007012 Location 16th Bay) Switched On: 12:15 Hrs Switched Off: 13:10 Hrs 1 Hour 33 Deg C 34 Deg C 33 Deg C Normal
  31. 31. Behavior of shunt reactor during operation
  32. 32. Switching on of shunt reactor Typical inrush current 3 to 5.5 times of rated current. Due to closing of breaker poles in three phases at different point of cycle, unsymmetrical current is developed in three phases which persist for approximately 1 sec Closing point of BRK for Lowest inrush current . Closing point of BRK for Highest inrush current . As a result of unsymmetrical current, 3rd harmonic current passes through the neutral that may cause spurious tripping on earth fault. Due to unsymmetrical current, DC off set current is produced which decrease slowly because of low loss in reactor and may cause saturation of CT N R Y B
  33. 33. waveform of inrush current with dc off set current during switching in DC Off set current Wave form of 3rd harmonic current through neutral during Switching in
  34. 34. Typical problem during switching in One of the principal difficulties with shunt reactor protection scheme is false tripping during reactor energizing. This false tripping typically occurs within some hundreds of millisecond or even 1 to 2 seconds after closing of circuit breaker. It also happens randomly and not with every reactor switching attempt Most of the time, it trips on Restricted Ground Fault protection / Differential Protection / Ground fault protection during switching in  It should be noted that HV shunt reactors are typically switched in and out at least once per day or even more often depending on the power system loading patterns.  During switching in of shunt reactor relatively high and long lasting dc current component appears in one or more phases. This current waveform moves the operating point of CT magnetic core on the hysteresis curve in one direction and when the dc component diminish it leaves the main CT with certain level of residual (i.e. remnant) flux.  During normal operation reactor current is always around 1pu and therefore of a relatively low magnitude, which is never big enough to move the operating point towards the origin.  Therefore when next switching attempt comes, depending on the moment of switching, residual flux in the CT core can increase or decrease. Thus this mechanism will sooner or later cause CT saturation during reactor switch in operation.  This CT saturation then causes problems for protective relays, which lose the correct information about the primary current and therefore cause false operation of protective relays. Cause for tripping during switching in
  35. 35. Switching off During switching off operation of reactor high transient over voltage is developed due to breaking of inductive current. This switching transients are inversely proportional to the shunt reactor rated power Typical over voltage in 400 KV reactors * ( Sav – Steepness of voltage) Switching transients overvoltage can be reduced considerably by installing surge reactor and control switching operation i.e switching off during zero crossing. Frequent transients overvoltage due to switching off operation always have the impact on the dielectric life of reactor and breakers.
  36. 36. Electrical faults in shunt reactors
  37. 37. Phase to ground fault at line side Faults in shunt reactor Internal fault External fault Phase to ground fault Phase to ground fault at neutral side Phase to phase short circuit fault Inter turn short circuit fault Over load due to over voltage and harmonics
  38. 38. Phase to earth fault - out side the reactor • •Shunt reactor is a device with the fixed impedance value. Therefore the individual phase current is directly proportional to the applied phase voltage (i.e. I=U/Z). During external fault voltage of the faulty phase becomes lower than other phases and a result of that unbalance is created in the phase current. Because of unbalance phase current , zero seq. current less than 1 PU passes through the neutral. N R Y B Zero seq current (<1 PU) Un balance in phase current Unbalance in phase current due to external earth fault Low zero sequence current through neutral due to unbalance in the phase current
  39. 39. Phase to earth fault - at the line side 1. Short circuit current flow through the line side faulty phase and causes unbalance in the phase current 2. Zero sequence current typically 1 PU flow through the neutral due to unbalance phase current N R Y B Zero seq current Un balance in phase current
  40. 40. Phase to earth fault – at the neutral side Rated current passes through the line side phase High current passes through neutral side due to transformer action N R Y B Rated current in the line side
  41. 41. Turn to turn short Shunt reactor winding impedance is approximately proportional to the square of the number of active turns. Short circuit between some number of turns will cause the decrease of the winding impedance only in the faulty phase and corresponding small raise of the shunt reactor neutral point current. Currents during turn-to-turn fault are of the small magnitude and they will not produce any sufficient unbalance voltage. Sufficient unbalance voltage is produced only when number of turn-to-turn short is high. In such condition it is possible to detect turn to turn fault with the help of sensitive directional zero seq relay connected on the HV side of the reactor.
  42. 42. Summary – protection CAUSES EFFECTS RESULTS IN PROTECTION ACTUATES Switching ON Unsymmetrical inrush current •Zero sequence 3rd harmonic current through the neutral •Saturation of CT due to slow decaying of DC offset current •Restricted Earth fault with time delay •Diff protection Switching OFF Transient over voltage High voltage stress on the dielectric of the reactor & circuit breaker. No protection is recommended for this purpose. External phase to ground fault Lower than rated current through the faulty phase •Unbalance in three phase line current •Low zero sequence current ( 1PU) passes through the neutral •Line side residual current protection •Differential protection Internal phase to ground fault at line side High current at line side in the faulty phase Unbalance in three phase line current •Low zero sequence current 1PU) passes through the neutral •Line side residual current protection • line side over current protection •Differential protection Internal phase to ground fault at neutral side High current at line neutral side •Rated current at line side phases •High current at neutral side •Neutral side over current protection •Differential protection Internal Phase to phase short circuit As the chance of this fault is very remote due constructional feature of the reactor , protection for this fault is not recommended. Inter turn fault Low magnitude voltage unbalance Small rise in neutral side current No protection is recommended since the magnitude of fault is very low
  43. 43. Electrical Protection& relay scheme
  44. 44. Reactor protection PURPOSE OF SHUNT REACTOR PROTECTION The purpose of the protection relaying is to disconnect the reactor and limit damage in case of internal short circuits, earth faults, inter turn faults and over voltage or over load. The reactor forms certain impedance for rated frequency, and as it is shunt connected, as over load may be caused by over voltage or harmonics in voltage and current. PROTECTION DEVICES INBUILT OR MOUNTED ON REACTOR a) Oil immersed reactor usually have a gas detector and oil surge detector (Buchholz alarm & trip devices), which are excellent for detecting internal faults. b) Temperature monitors for oil & winding provide good over load protection. c) Pressure relief device is provided to safe guard the reactor from high pressures. REACTOR DIFFERENTIAL PROTECTION It is widely used as instantaneous protection for short circuit faults with in the differential zone. this is treated as main-1 protection for reactor. It can be of high impedance type or of a sensitive current stabilized type. High impedance differential protection relays require an equal CT turns ratio on the phase and neutral side. Sensitivity is 5% of nominal reactor CT current. BACK-UP PROTECTION A variety of relays are available a) Over current & earth fault protection. ( 50, 50N, 51, 51N, 67, 67N – any combination of these) b. Under impedance / distance ( z<)(21r). c. Neutral displacement protection (un>) RESTRICTED EARTH FAULT PROTECTION If, for some reason, a sensitive differential protection not chosen, a restricted earth fault protection can be utilized. LINE PROTECTION – I, LINE PROTECTION – II
  45. 45. CBIP Guidelines on shunt reactor Protection Reactor Differential protection I) Shall be Triple Pole Type. ii) Have an operating current sensitivity at least 10% of nominal current. iii) Shall be tuned with system frequency. iv) Have an operating time not grater than 30 m sec at 5 times of setting. v) Have a suitable non-linear resistor to limit the peak voltage during in-zone faults in case of high impedance type. vi) Shall be high or low impedance Principle type. Reactor REF Protection. I) shall be single Pole. ii) Have an operating current sensitivity at least 10% of nominal current. iii) Shall be tuned with system frequency. iv) Have a suitable non-linear resistor to limit the peak voltage during in-zone faults in case of high impedance type. v) Shall be high or low impedance Principle type. vi) Connection of restricted earth fault protection on the neutral side shall be from residually connected Bushing CTs or from the ground side CT. Reactor Backup Protection (Impedance type) I) Shall be Triple pole type. ii) Shall be single step Polarized ‘MHO’ or Impedance Distance relay suitable for Measuring Phase to Ground and Phase to Phase to faults. iii) Shall grounds a Characteristic angle between 60-80 deg. iv) Shall have adjustable definite time delay with setting range of 0.2 to 2.0 sec. v) Shall have a suitable range for covering 60% of Reactor impedance. vi) Typical setting : Reach - 60% of Reactor Impedance, Time setting - 1 sec OR Reactor Backup Protection (Definite Time O/L & E/F). i) Shall be single stage Definite Time 3 Pole, Over Current relay with adjustable current and Time. ii) Shall be connected for 2 O/C and 1 E/F connection and shall be non-directional with reset ratio and low Transient Overreach. iii) Typical settings of o/c relays are: Current Setting- 1.3 x Rated current , Time setting - 1 sec
  46. 46. Protection of bus reactor for double bus & transfer bus scheme 51N 21 87U Bus I Bus II Transfer bus 87BB1 R E A C T O R II 51N 21 87U 87BB1 R E A C T O R I 87BB287BB2 To Bus Bar protection To Bus Bar protection
  47. 47. 1. Differential protection U1 V1 W1 U2 V2 W2 N DP 87 1s1 1s2 1s1 1s2 1s1 1s2 1s1 1s2 1s1 1s2 1s1 1s2 CT Specification 1. Ratio: 200/1A 2. Class: PS 3. Knee point voltage: 200V 4. Magnetizing current: 40 mA 5. Secondary resistance: 1 Ohms Purpose :Internal / external phase to ground fault. Line side CT 1 Neutral side CT 1
  48. 48. Typical Relay connection for Differential protection Type of relay : High impedance differential relay Setting : operating current sensitivity at least 10% of nominal current. operating time not grater than 30 m sec at 5 times of setting.
  49. 49. 2. Residual earth fault protection U1 V1 W1 U2 V2 W2 N Instantaneous Residual over Current Relay (50N) Or AC Time residual Over Current relay (51N) 1s1 1s2 1s1 1s2 1s1 1s2 CT Specification 1. Ratio: 200/1A 2. Class: PS 3. Knee point voltage: 200V 4. Magnetizing current: 40 mA 5. Secondary resistance: 1 Ohms Assigned Protection 1. External phase to ground fault.( Unbalance phase current) 2. Internal phase to ground fault at line side .( Unbalance phase current) 3. Circuit breaker pole discrepancy.( Unbalance phase current) Typical relay setting •Set low set to 20% with time delay in between0.6s and 1s or even longer. •Use 2nd harmonic blocking. •Set high set to 175% with time delay of 0.1s.
  50. 50. 4. Back up impedance protection Assigned Protection Internal phase to ground fault at line side Typical relay setting •Set low set to 130% with time delay in between 0.6s and 1s. •Set high set to 250% with time delay of 0.1s. U1 V1 W1 U2 V2 W2 N 21R – 3Ph, REACTOR BACKUP IMPEDANCE RELAY OF SUITABLY SHAPED CHARECRESTICS EITHER SINGLE / DOUBLE ZONE TYPE 1s1 1s2 1s1 1s2 1s1 1s2 CT Specification 1. Ratio: 200/1A 2. Class: PS 3. Knee point voltage: 200V 4. Magnetizing current: 40 mA 5. Secondary resistance: 1 Ohms
  51. 51. Typical Relay connection for back up impedance protection
  52. 52. 5. Line protection main & backup U1 V1 W1 Line Side U2 V2 W2 Neutral Side N Bus Bar protection I & II 1s1 1s2 1s4 1s3 500/1A 1000/1A 2000/1A 1s1 1s2 1s4 1s3 500/1A 1000/1A 2000/1A 1s1 1s2 1s4 1s3 500/1A 1000/1A 2000/1A Assigned Protection Bus bar protection
  53. 53. Electrical Test on reactor Prepared by M. G. Morshad / Additional Chief Manager ( Elect.) Transformer Maintenance Division Thermal Power Station II Neyveli Lignite Corporation Ltd
  54. 54. Electrical test as per IS 5553 Routine Test ( To confirm the operating criteria) 1. Measurement of WR 2. Measurement of IR & PI 3. Measurement of impedance by bridge methods 4. Measurement of loss and current at rated voltage and ambient temperature 5. Isolation test Dielectric Test ( To confirm the dielectric strength of the insulation) 1. Separate source voltage withstand test at 230 KV for one minute 2. Induced over voltage withstand test with PD indication at 364 KV AC (1.5/√3 PU ) for 30 minutes during which the PD level shall not exceed 500pc 3. Full wave lighting impulse voltage withstand test at 1300 KVp on line terminal 4. Switching impulse voltage withstand test at 1050 KVp on line terminal Type Test ( To confirm the design criteria) 1. Temperature rise test along with DGA before and after test 2. Full wave lighting impulse voltage withstanding test at 550 KVp on neutral terminal 3. Measurement of zero sequence reactance 4. Measurement of acoustic noise level 5. Magnetizing curve test / knee voltage measurement 6. Measurement of capacitance and tan delta between winding and tank Special Test ( To confirm design and operating criteria ) 1. FRA test 2. DGA test before and after electrical test 3. Jacking test on reactor tank 4. Vacuum test on reactor tank 5. Oil leak test 6. Snap back test on HV bushing
  55. 55. Measurement of WR Purpose - To measure DC resistance per phase of coil for calculating I2R loss in the coil, which in turns decides the temperature rise. The measurement also shows whether the winding joints are in order and the windings are correctly connected. Measuring methods - 1. Only at the stable value of current (I), corresponding voltage (V) value is taken for measuring the value of resistance ( R = V/I). 2. Applied current must not be higher than 15% of the rated current 3. % error in measurement increases with increases of applied current due to increases in I2R loss at higher current. 4. Value to be measured between (R – N) , (Y-N), and (B-N), and all the measured value must be equal. N R Y B Resistance measuring Kit Voltage lead Current lead R-N Y-N B-N Room Temp ( 37 Deg C) 75 Deg C 2.570 Ω 2.570 Ω 2.570 Ω 2.570 Ω 2.570 Ω 2.570 Ω
  56. 56. Measurement of IR Minimum IR value Below 6.6 KV 6.6 - 11KV 22 – 33 KV Above 66 KV K = 1.00 30 Deg C 200 MΩ 400 MΩ 500 MΩ 600 MΩ K = 1.65 40 Deg C 121 MΩ 242 MΩ 303 MΩ 363 MΩ K = 2.60 50 Deg C 77 MΩ 153 MΩ 192 MΩ 230MΩ K = 4.20 60 Deg C 47 MΩ 95MΩ 119 MΩ 142 MΩ K = 6.6 70 Deg C 30 MΩ 60 MΩ 75 MΩ 90 MΩ K = 10.5 80 Deg C 19 MΩ 38 MΩ 47 MΩ 57MΩ (Polarization Index) PI Value = (15 minutes IR / 60 minutes IR ) Less than 1 Dangerous Above 1 to 1.1 Poor Above 1.1 to 1.25 Questionable Above 1.25 to 2.0 Fair Above 2 Good Purpose - To ascertain minimum insulation strength (IR Value) and dryness level (PI Value) of the winding required to charge the reactor. N R Y B 5 KV IR measuring Kit (Megger) Procedure – Measure IR and PI value between (R+Y+B +N) – (Tank + E) with 5 KV Megger and confirm the minimum IR and PI value as per the table given below. _ +
  57. 57. Isolation test Purpose - To ascertain that the reactor core is insulated from the tank and core frame. G CL Core BoltCore clamp Core Tank CL – connected to core lamination CC – Connected to core clamp G – Connected to tank ( Earth ) CC Procedure : 1.Disconnect the closing link that connects the two terminals CL-G. 2.Connect the tank with earth 3. Use a Megger and measure IR value between CL and CC + G by applying 3.5 KV for 1 minute 4. The measured IR value shall be minimum 1000 kohms ( 1 M Ohms) 5. There is no general requirement on the insulation level CC-G .
  58. 58. Measurement of impedance by bridge methods Purpose : To measure the per phase impedance ( AC resistance) of the winding which controls the flow of current through the windings. Impedance (Z) = √[(Resistance) 2 + ( Reactance)2 ] = 420KV / 86Amps Reactor windings Temp (0 C) U = 100/5 C4 (μF) M3 (mH) CN (pF) Lx (H) = (M3/U)X(C4/CN)x1000 X (Ω) =2∏fLx U 38 20.00 8.98966 1.0032 50.915 8.856355604 2783.43 V 38 20.00 8.96700 1.0032 50.915 8.834031621 2776.41 W 38 20.00 8.96210 1.0032 50.915 8.829204282 2774.89 Where U = CT Ratio ( 100/5) M3 = Mutual inductance = 1.0032 mH CN= standard capacitance = 50.915 pF C4= measured bridge capacitance in pF X= calculated impedance of the winding in Ohms
  59. 59. Measurement of loss and current at rated voltage and ambient temperature Purpose: To measure the loss ( core & copper loss) in reactor at rated operating condition. Reactors windings Temp (0 C) V = 420/√3 KV R4 ( KΩ) C4 (μF) ω = 2∏f Tan delta = 1000/ (C4xR4xω) X (Ω) Loss ( KW) at rated voltage = V2 *(Tan Delta/X*1000) U 38 242.49 210.15 8.98966 314.29 0.0016842350 2783.43 35.58 V 38 242.49 201.15 8.96700 314.29 0.0017640389 2776.41 37.36 W 38 242.49 332.75 8.96210 314.29 0.0010669585 2774.89 22.61 Phase Voltage (KV) X ( Ohms) Phase current = KV*1000/X ( C ) Rated Amps ( R ) Calculated loss at rated voltage (KW) Calculated loss at rated current = KW X ( R/C)2 U 242.49 2783.43 87.12 86.60 35.58 35.16 V 242.49 2776.41 87.34 86.60 37.36 36.73 W 242.49 2774.89 87.39 86.60 22.61 22.20 Total loss at 38Deg C at rated current = (35.16+36.73+22.20) KW = 94.09 KW
  60. 60. Purpose of dielectric test Voltage level as per IS 2072 Normally reactors are operated at the rated operating voltage. During its operation it is exposed to various transient over voltages like power frequency over voltage (1.5 x BIL, due to system over voltage), Lighting impulse (due to lighting), switching impulse (due to switching off). To avoid abrupt failure of insulation due to these transients, insulation is designed considering all the aspect. Dielectric test confirms the capability of the insulation to withstand these transient overvoltage which is subjected to the reactor during its service life. Operating voltage KV rms Highest system voltage (BIL) KV rms Power frequency voltage (KV rms) Switching Impulse (KV Peak ) Lighting impulse (KV Peak ) 0.415 1.1 3 - - 3.3 3.6 10 - 20/40 6.6 7.2 20 - 40/60 11 12 28 - 60/75 15 17.5 38 - 75/95 24 50 - 95/125 33 36 70 - 145/170 52 95 - 250 66 72.6 140 - 325 123 185/230 - 450/550 145 230/275 - 550/650 170 230/275/325 - 550/650/750 230 245 325/370/395 - 750/850/950 300 395/460 750/850 950/1050 362 460/510 850/950 1050/1175 400 420 570/360 950/1050 1300/1425
  61. 61. Separate source voltage withstand test at 230 KV for one minute PURPOSE : To verify the operating voltage withstanding capacity of the minor insulation ( paper ) used in line terminals and windings. NR Y B 50 Hz, AC Generator Method Test voltage from a 50 Hz, sinusoidal source is applied between (HV +N) and (E + Tank) through a step up transformer Maximum test voltage ( 400 KV /1.732) or 230 KV rms Duration of test 60 sec Measurement of test voltage Direct reading for RMS type voltmeter or (Reading / √2) for Peak type voltmeter (KV) Confirmation The test is declared to be successful if the test voltage does not collapse during the test. KV
  62. 62. Induced over voltage withstand test with PD indication at 364 KV AC (1.5/√3 PU ) for 30 minutes during which the PD level shall not exceed 500pc ( Method 2) PURPOSE : To verify the power frequency voltage withstanding capacity of the minor insulation (Paper ) used in line terminals & windings and the maximum level of PD observed during the test. Method Test voltage from a 160Hz ( to avoid saturation of core), sinusoidal source is applied at line terminals through a step up transformer keeping neutral terminal and tank grounded. PD is measured by PDD connected with impedance ( z) & capacitors as shown in the fig Maximum test voltage ( 1.5 x 420KV /1.732) or 364 KV rms Duration of test 30 minutes in steps at various voltages level as shown below Measurement of test voltage Direct reading for RMS type voltmeter or (Reading / √2) for Peak type voltmeter (V) Confirmation The test is declared to be successful if the test voltage does not collapse during the test. and the PD level is observed within 500pC 364KV 420KV 364KV 5 Minutes 5 Sec 30 Minutes N RYB 50 Hz, AC Generator KV Z PDD
  63. 63. Full wave lighting impulse voltage withstand test at 1300 KVp on line terminal PURPOSE : To verify the impulse voltage withstanding capacity of the major insulation ( pressboard) used between the windings, line terminals caused by lighting strike. N R Y B Impulse Generator 0.1 Ώ Recorder - + Method Test voltage from an impulse generator is applied at line terminals keeping neutral grounded through 0.1 Ώ resister and other terminals directly grounded. The sequence for applying impulse is - one impulse of a voltage between 50% and 75 % of the full test voltage, and three subsequent impulses at full voltage Test voltage 1300 KV peak with Front time Duration of test T1 = 1,2μs ± 30% and Time to half-value T2 = 50 μs ± 20% Confirmation The test is successful if the test voltage does not collapse during the test.
  64. 64. Switching impulse voltage withstand test at 1050 KVp on line terminal N R Y B Impulse Generator 5 0 0 Ώ Recorder - + Method Test voltage from an impulse generator is applied at line terminals keeping neutral grounded through 500 Ώ resister and other terminals directly grounded. The sequence for applying impulse is - one impulse of a voltage between 50% and 75 % of the full test voltage, and three subsequent impulses at full voltage Test voltage 1050 KV peak Duration of test Front time Tp> 100μs , Time above 90% Td> 200μs and Time to the first zero passage T0> 500μs ( preferably 1000 μs ) Confirmation The test is successful if the test voltage does not collapse during the test. PURPOSE : To verify the impulse voltage withstanding capacity of the major insulation (Pressboard) between the winding ,line terminals caused by switching operation.
  65. 65. Type Test 1. Temperature rise test along with DGA before and after test 2. Full wave lighting impulse voltage withstanding test at 550 KVp on neutral terminal 3. Measurement of zero sequence reactance 4. Measurement of acoustic noise level 5. Magnetizing curve test / knee voltage measurement 6. Measurement of capacitance and tan delta between winding and tank
  66. 66. Temperature rise test The purpose of the measurement is to check that the temperature rises of the oil and the windings do not exceed the limits agreed on or specified by the standards. (T1)Hot / top oil temp Cold winding resistance at (T) 38 deg C R1 2.569 Ohms Hot winding resistance after switched off (to be derived from graph) R2 2.743 Ohms Ambient temperature after switched off Ta 34.23 Deg C Calculated winding temp at Time of S/D Tw = {R2/R1 (235+T)R1} – 235 (2.743/2.569) x ( 235+ 38) – 235 = 56.49 Deg C Average oil temp at S/D T1 55.30 – ½(55.30 – 4425) = 49.22 Deg c Winding temp gradient Tg = (Tw – T1 ) 56.49 – 49.22 = 7.27 Deg c Average oil temp rise Td = (T1 - Ta) 49.22 – 34.23 = 14.99 Deg C Corrected winding temperature Tc = (Tg + Td) 7.27+ 14.99 = 22.26 Deg C Winding temperature rise (Tc – Ta) Temp rise Time Steady state temp T 55..30 / B 44.25 T 35.00 / B 33.00 8 Hours
  67. 67. Full wave lighting impulse voltage withstanding test at 550 KVp on neutral terminal NR Y B Impulse Generator 5 0 0 Ώ Recorder - + Method Test voltage from an impulse generator is applied at neutral terminals keeping other terminals grounded through 500 Ώ resister and tank directly grounded. The sequence for applying impulse is - one impulse of a voltage between 50% and 75 % of the full test voltage, and three subsequent impulses at full voltage Test voltage 550 KV peak Duration of test T1 = 1,2μs ± 30% and Time to half-value T2 = 50 μs ± 20% Confirmation The test is successful if the test voltage does not collapse during the test. PURPOSE : To verify the impulse voltage withstanding capacity of the major and minor insulation (Pressboard and paper) used in neutral terminals and side of the winding caused by switching operation.
  68. 68. Measurement of zero sequence reactance N R Y B 50 Hz, AC Generator V A A Purpose : To measure the AC resistance (Impedance) of neutral path which controls the earth fault current (zero sequence current) through neutral during internal or external earth fault. Method •Two phase AC supply is applied between (R+Y+B) and( N) through a step up transformer which is fed by a generator. •Applied voltage is increased till 70% of the rated current flow through the neutral . Zero seq impedance {(3 x Applied voltage) / Neutral current} x { rated frequency / test frequency}. Current through neutral = 62.5 Amps Applied Voltage = 56.40 KV Test Frequency = 49.80 Hz Zero Sequence Impedance = [(3 x 56400)/ 62.5] x [50.0/49.8] = 2718.07 Ohms
  69. 69. Measurement of acoustic noise level and vibration 1 Meter 2 MeterMicrophone for picking up noise The purpose of the sound level measurement is to check that the sound level of the reactor meets the specification requirements given in relevant standards . A sound spectrum analyses is used for measuring sound level. The sound spectrum indicates the magnitude of sound components as a function of frequency. The sound pressure level is the measured at various points around at a distance (D) of 30 cm for ONAN or 2 m for ONAF cooling system spaced at an interval (X) of 1 meter. Limit Sound level Vibration level Within 200 microns Within 81db
  70. 70. Magnetizing curve test / knee voltage measurement Test procedure • Each phase of the reactor is charged one by one with direct current. • When the maximum test current reached, supply is switched off and the reactor winding is short-circuited simultaneously by DC current breaker. • The decaying current in the circuit is registered by a computer assisted data acquisition system. • The saturation curve Flux (Ø) / Flux (Ø) nom. versus I / Inom is then determined using the formula. Imax Maximum measured current Calculation Imin Minimum measured current L(I) = (IxR) / (di/dt) I nom Nominal AC current / √ 2 Ø nom = I nom x L mean I mean Calculated inductance from I min to I nom Ø (I ) = ∫( Imin L(I)) +(Imean x L mean) Ø nom Nominal flux at I nom R Circuit resistance (RL + Rs+ Rc) I / I nom Ø / Ø nom 2 2
  71. 71. Measurement of capacitance and tan delta between winding and tank Angle δ Capacitance (pF) [(I/2 f V π) x Cos δ ] Tan δ = Sin δ =Cos Φ(PF) Condition of the insulation 0.0 Deg (I / 3140) x 1.000 0 Pure capacitor 0.5 Deg (I / 3140) x 0.999 0.002 Very good 0.5 Deg (I / 3140) x 0.999 0.004 Good 0.5 Deg (I / 3140) x 0.999 0.006 Fairly good 0.5 Deg (I / 3140) x 0.999 0.007 Acceptable 0.5 Deg (I / 3140) x 0.999 0.008 Not acceptable 90.0 Deg 0 1.0 Pure resistance N R Y B 10 KV Tan Delta Kit Purpose : To ascertain the condition of the solid insulation of the windings. + + + + - - - - I Ic I Ir Ir V δ Φ I = total current drawn by the capacitor formed between winding and tank and the value of capacitance is [(I/2 f V π) x Cos δ ] (pF) Ic = Capacitor charging current Ir = Current flow through the capacitor due to impurities / disintegration of the insulation between winding and tank. As this current is in phase with applied voltage, it is dissipated in heat. Measured Capacitance = 9887 pF, Tan δ = 0.0031 at temp 39 Deg C
  72. 72. Special Test 1. FRA test 2. DGA test before and after electrical test 3. Vacuum test on reactor tank 4. Oil leak test 5. Snap back test on HV bushing
  73. 73. FRA test Purpose- Frequency Response Analysis (FRA) is carried out to detect displacement (or movement) of the windings. Usually the first measurement in the factory is used as a fingerprint. Results of later measurements are compared with the first one in the factory. The software controlled sine wave generator produces output voltage of max. 4 Vrms with frequency range of 50 Hz to 1 MHz. It has 75 Ω output impedance. Input impedance is 75 Ω. Voltage from the generator is applied to the one transformer terminal (one winding end) and response voltage is measured on another terminal (the other winding end).
  74. 74. FRA test report Impedance value Z in kΩ versus frequency is plotted on the diagram with indication of terminals with applied and response voltage. Or attenuation A (or damping) in dB (20 log (Uoutput / Uinput ) versus frequency is plotted on the diagram with indication of terminals with applied and response voltage. The reactor is said to be healthy if no deviation is observed between the results taken in factory and field
  75. 75. DGA test 5 gms of silver nitrate (AgNO3) dissolved in 100 ml distilled water A week solution of ammonia in water is slowly added to 100 ml of solution 1, until a white curdled precipitate which forms first disappears in the mixture. Chemical analysis of gas : The gas analyser loaded with these solution is connected to the top pet cock. Small quantities of gas collected in the gas relay (Bucholtz relay) is allowed to pass through the two solutions.
  76. 76. Vacuum test on reactor tank Oil leak test
  77. 77. Snap back test on HV bushing Purpose : Snap back test was carried out on the above bushing to determine the natural frequency and damping factor. Following equipment were used to conduct this test. 1: Piezoelectric accelerometer B & K 4371. 2: PL 202 Real Time FFT Analyzer Methods : Two nos. Piezoelectric accelerometer were mounted 90 degree apart at the bottom of the bushing, one in the direction of applied force (X) and other 90 degree to the applied force (Y). A force of 250 Kg was applied at the top and then it was cut-off. The resulting vibrations were recorded on the FFT. The recorded signals were analyzed on FFT Analyzer to determine the natural frequency and damping factor of the bushing. X Y FFT X direction Y direction Natural frequency From FFT reading 3.25 3.25 Damping factor [(100 / 2 π n) * log (Y 1 / Yn+1)] Where: n = No. of cycle Y n+1 = Amplitude of (n+1) cycle peak Y1 = Amplitude of 1st cycle peak 1.421% 2.03% The test is successful if no evidence of physical damage is observed on the bushing after the test
  78. 78. Final Observations
  79. 79. Though oil immersed, shunt reactor and power transformer are viewed alike, there are distinct differences between construction and operating characteristics of these two devices. As NLC is going to install two numbers 63 MVAR bus reactor for the first time in TS II and the operating and maintenance staffs are not properly exposed to its operating data, following information need to be collected from any southern grid thermal power plant ( not from substation) presently operating with similar capacity bus reactors for successful and trouble free operation of reactor in TS II – Average number of switching operation of the reactor per day Numbers of operating hours achieved since commissioning Numbers of forced / planned shutdown taken after commissioning Protection co-ordinations and its settings Number of false/actual tripping, if any, since commissioning and its reasons Maximum & minimum bus voltage for switching in and out of reactors Reduction in leading MVAR and bus voltage after switching in of the reactor Any abnormalities observed in generator excitation during switching in/out  Average reactor current, winding and oil temperature Any failure of parts like bushings, LA, gaskets etc since commissioning Remarks of the operating staff on the performance of the reactors
  80. 80. Thank you

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