Transformer Protection PRESENTED BY PROF VG PATEL Wednesday

Transformer Protection PRESENTED BY PROF. VG PATEL Wednesday, March 10, 2021 VG PATEL 1

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION ØINTRODUCTION: v. Transformer is an important link in power system between generating station and transmission sub station, between transmission sub transmission and sub transmission sub station, between sub transmission sub station and primary distribution sub station, finally between primary distribution and the consumers. v. Capacity few KVA to hundreds of MVA. v. Variety of constructional and operating features. Wednesday, March 10, 2021 VG PATEL 2

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION OCCURRENCE OF THE FAULT IN THE TRANSFORMER IS RELATIVELY LESS DUE TO: v SIMPLE, STATIC CONSTRUCTION WITHOUT ANY ROTATING PARTS v TOTALLY ENCLOSED AND OIL IMMERSED v BUT FOR ANY SYSTEM FAULT, FAULT CURRENT FLOWS THROUGH TRANSFORMER Wednesday, March 10, 2021 VG PATEL 3

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION Main Objectives of the protective scheme for Transformers • There is no fix rule that the transformers above particular capacity must have the comprehensive protection and others can have simple protection. • The objectives are 1 Minimize the losses 2 Maximize the up time 3 Minimize the loss of property 4 Minimize the possibility of threat to human life • The damage done is directly proportional to the square of the current and is directly proportional to the time for which the fault current flows Wednesday, March 10, 2021 VG PATEL 4

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION FAULT WITHSTAND LEVEL Transformer reactance (%) 4 5 6 7 Fault current (Multiple of rating) 25 20 16. 6 14. 2 Permitted fault duration (Seconds) 2 3 4 5 LARGE FAULT CURRENT PRODUCES SEVERE MECHANICAL STRESSES IN THE TRANSFORMER. Wednesday, March 10, 2021 VG PATEL 5

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION PROTECTIVE / MONITORING DEVICES Transformer Protecti Transformer rated betn on rated above rated below 500 KVA 5 MVA 500 KVA and 5 MVA x x Differential Restricted Earth Fault x x Back up Over current x Back up Earth Fault x Transformer Over Fluxing x x Wednesday, March 10, 2021 VG PATEL 6

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION PROTECTIVE / MONITORING DEVICES Protection Fuses Buchholz Relay, PRV, OSR Oil and Winding Temperature Indicators Oil Level Monitoring Equipments Wednesday, March 10, 2021 Transformer rated betn 500 Transformer rated above 5 KVA and 5 rated below MVA 500 KVA x x x x VG PATEL 7

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION Conditions and Protection Philosophy INTERNAL • Winding Phase-Phase , Phase-Ground faults Differential (87 T), Overcurrent (51, 51 N) Restricted ground fault protection (87 RGF) • Winding inter-turn faults Differential (87 T), Buchholz relay, • Core insulation failure, shorted laminations Differential (87 T), Buchholz relay, sudden pressure relay • Tank faults Differential (87 T), Buchholz relay and tank-ground protection • Over fluxing Volts/Hz (24) EXTERNAL • Overloads Thermal (49) Over voltage(59) • Overfluxing Volts/Hz (24) • External system short circuits Time overcurrent (51, 51 G), Instantaneous overcurrent (50, 50 G) Wednesday, March 10, 2021 VG PATEL 8

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION Potential causes for abnormality ØMechanical failure ØElectrical failure ØInsulation failure ØOperation of equipments beyond recommended limits Wednesday, March 10, 2021 VG PATEL 9

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION IMPORTANT NOTE: By providing excellent protection to transformer, one can not prevent fault from occurring, faults are going to occur. Wednesday, March 10, 2021 VG PATEL 10

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION 10 categories of currents in the transformer 1) Rated current 2) Full load current 3) Load current (R L C) 4) No load current (Exciting current) – I 0 * 5) Inrush current * 6) Magnetizing current * 7) Eddy current * 8) Short circuit current 9) Fault current Over and above these 10 categories, depending upon internal fault, differential or unbalance current also flows. Protection provided for which current? Wednesday, March 10, 2021 VG PATEL 11

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION FAULTS IN SYSTEM EQUIPMENT OVER HEAD LINES % OF TOTAL EQUIPMENT FAULTS % OF TOTAL FAULTS 50 CTs AND PTs 2 10 CONTROL EQUIPMENTS 3 SWITCHGEARS 15 MISCELLANEOUS 8 TRANSFORMERS 12 CABLES Wednesday, March 10, 2021 VG PATEL 12

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION Fault Statistics • Single phase to earth • Two phases to earth 80% 10% • Phase to phase faults 5% • Three phase faults 5% • The probability of line faults caused by lightning are 0 -1/2 faults/ 100 km / year. Wednesday, March 10, 2021 VG PATEL 13

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION Following particulars of Trans. are necessary to be known while selecting the protection scheme for a power Transformer. v. KVA or MVA rating v. Voltage ratio v. Winding connections. v. Per unit / per cent reactance v. Neutral point earthing resistance v. Value of system earthing resistance. Wednesday, March 10, 2021 VG PATEL 14

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION v. Whether indoor or outdoor v. Dry or oil filled. v. With or without conservator. v. Length and area of cross section of connecting leads between CTs and relay panel (Pilot wires). v. Fault level at the power transformer terminals. v. Network diagram showing the position of transformer in the system network. Wednesday, March 10, 2021 VG PATEL 15

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION v Since failure of a power transformer will cause large interruption, it is essential that excellent protection should be provided. v Should not assume that proper protection will prevent occurrence of faults in the transformer; faults are bound to occur. v Protection schemes help us to detect these faults and save a transformer from excessive damage by automatically disconnecting it from mains very quickly. Wednesday, March 10, 2021 VG PATEL 16

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION • The faults in transformer can be caused by failure of insulating material due to dust, moisture, voids, weakening of winding due to external short circuits, aging, etc. • The over voltage due to switching and lightning strokes. L. A. s provided at the transformer terminals, spark over at about 80% of rated insulation level and protect the transformer against over voltages. Wednesday, March 10, 2021 VG PATEL 17

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION Protection schemes TO AVOID FIRE AND CATACOPIC FFECT, EXCELLENT PROTECTION IS A MUST. Protection scheme of transformer helps us to identify the abnormalities and isolate the faulty portion, save a transformer from excessive damage and protect the remaining system against further damage and tripping by automatically disconnecting transformer from mains very quickly. Wednesday, March 10, 2021 VG PATEL 18

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION Protection schemes Purpose of Protection System v. Minimize damage v. Leave unaffected equipment in-service v. Maintain equipment operating limits v. Maintain electrical system stability Wednesday, March 10, 2021 VG PATEL 19

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION Protection schemes • Generally following protection schemes are employed: v. Differential relaying v. Restricted earth fault relay v. Buchholz relay v. Over current protection v. Over fluxing protection v Only simplest protection such as fuses or molded case circuit breakers can be justified for transformer of lowe rrating. Wednesday, March 10, 2021 VG PATEL 20

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION FOLLOWING SAFETY AND MONITORING DEVICES ARE EMPLOYED v Fluid level gauge v Pressure switch v Sudden pressure relay v Pressure relief valve v Fluid temperature indicator v Smoke detector etc. Wednesday, March 10, 2021 VG PATEL 21

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION [A] ELECTRICAL FAULTS CAUSE IMMEDIATE DAMAGES • • Phase to phase & phase to ground faults. Terminal faults at H. V & L. V. terminals. H. V. & L. V. windings. Short circuit between the turns of L. V. or H. V. windings. • Earth fault on tertiary winding, etc. Wednesday, March 10, 2021 VG PATEL 22

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION [B] NON ELECTRICAL OR INCIPIENT FAULTS v. Core faults v. Break down of insulation of core bolts v. Core lamination insulation v. Bad electrical joints v. All the above cause localized heating & there by generation of gas (Decomposition of insulation oil) v Failure of coolant leading to overheating of transformer Wednesday, March 10, 2021 VG PATEL 23

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION v. Failure of cooling system. v. Leakage of oil, tank fault. v. Regulator faults, bad load sharing among transformers operating in parallel can cause overheating due to circulating current. v. Fault in tap changers. Group [B] faults are not serious in their incipient stage but might develop into major and serious faults in course of time. Such faults should be detected and removed as early as possible. Wednesday, March 10, 2021 VG PATEL 24

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION v. It should be noted that protection against faults of Group [A] are not capable to detect faults of group[B]. v. Means applicable to detect the faults of group[B] cannot quickly detect faults of group[A]. v. As such, means adopted for protection against faults of group[A] and [B] should not be treated as substitutes but as supplementary to each other. Wednesday, March 10, 2021 VG PATEL 25

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION There are certain tests, which after rectifying the value nearer to permissible limit, minimizes the failure of transformers due to internal fault. DGA test: DGA will indicate heating, burning, sparking/arcing, loose joint, etc. inside the xmer. * Capacitance Tan Delta / P. I. & A. I. tests: Capacitance Tan Delta and P. I. & A. I. values will indicate insulation condition of transformer. * • Oil sample test: As per IS - 1866. Also, the trend of change in the oil gives useful information (can be known from its colour and odor) Oil sample test will indicate oil quality, which is directly affecting transformer health and life both. Wednesday, March 10, 2021 VG PATEL 26

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION Maintenance head will have data ready to decide maint. of transformer including oil filtration, replacement of oil or periodic overhaul (P. O. H. ) of transformer at site. Though the maintenance head of electrical department regularly carries out above tests for transformer health monitoring, to some extent, internal faults can be kept within control but there is no control on specific external faults. The faults are beyond control of maintenance / operation team and they do occur. Wednesday, March 10, 2021 VG PATEL 27

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION a. Over current Protection b. Earth fault Protection c. Main transformer Buchholz Relay d. O. L. T. C. Buchholz relay e. Restricted Earth Fault Relay f. Differential Relay g. Winding temperature & oil Temperature trip relay I. Unit protection relay j. Back up protection relay Wednesday, March 10, 2021 VG PATEL 28

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION HARMONIC CONTENT IN INRUSH CURRENT • Biased to provide stability during heavy through faults. • Second harmonic restraint to present operation by magnetizing inrush current. • An instantaneous high-set Element. Wednesday, March 10, 2021 VG PATEL 29

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION MAGNETIZING INRUSH CURRENT Wednesday, March 10, 2021 VG PATEL 30

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION HARMONIC CONTENT IN INRUSH CURRENT HARMONIC TYPICAL HARMONIC COMPONENT VALUE COMPONENT TYPICAL VALUE DC 55% 5 th 4. 1% 2 nd 63% 6 th 3. 7% 3 rd 26. 8% 7 th 2. 4% 4 th 5. 1% Wednesday, March 10, 2021 VG PATEL 31

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION The magnetizing inrush phenomenon produces current input to the energized winding which has no equivalent on the other side of the transformer. The normal bias is therefore not effective. Since, the phenomenon is transient, stability can be maintained by providing small time delay, which can be achieved by various means: --Induction pattern relay of IDMT type. --Instantaneous relay shunted by a fuse link, so called kick-fuse. --Harmonic restraint. Wednesday, March 10, 2021 VG PATEL 32

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION If damage to important transformers is to be minimized, it is essential to clear faults without delay. The inrush current wave form is different. The inrush current contains all orders of harmonics but these are not equally suitable for providing bias. The study of this subject is complex as the wave form depends on the degree of saturation and on the grade of iron in the core. Wednesday, March 10, 2021 VG PATEL 33

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION The principal conclusions can be summarized as follows; (a) D. C. or offset component (zero harmonics) A uni-directional component will usually be present in the inrush current of a single phase transformer and in the principal inrush current of a three phase unit. However, if at the instant of switching, the residual flux for any phase is equal to the flux, which would exist in the steady state at that point on the voltage wave, then the transient disturbance should take place on that phase. Wednesday, March 10, 2021 VG PATEL 34

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION Large inrush current will flow in the other two phases, corresponding to high peak flux values established in these phase cores. If the uni-direction current component were used to stabilize a differential system, some sort of cross-phase biasing would be required because of this effect. Since many fault current wave forms will have initial offset, delay in Wednesday, March 10, 2021 VG PATEL 35

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION (b) Second harmonics This component is present in all inrush waveforms in which mirror image symmetry can be found about certain ordinates. Second harmonics is always present as long as the uni-directional component of flux exists. Normal fault current do not contain second or other even harmonics. Wednesday, March 10, 2021 VG PATEL 36

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION Output component of a CT which is energized in to steady state saturation will also contain odd harmonics but not even harmonics. But when the CTs are saturated by the transient component of the fault current, the resulting saturation is not symmetrical and even harmonics are introduced in to the output current. The second harmonics is therefore an attractive basis for a stabilizing bias against inrush effect, but care must be taken to ensure that CTs should be sufficiently large not to delay relay operation by Wednesday, March 10, 2021 VG PATEL 37

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION the harmonics produced by transient saturation. The differential current is passed through a filter which extracts the second harmonics; this component is then applied to produce a restraining quantity sufficient to overcome the operating tendency. Wednesday, March 10, 2021 VG PATEL 38

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION (c) Third harmonics The third harmonics is also present in the inrush current. When windings are connected in delta, the line currents are the difference of 2 phase currents. The sustained third harmonic component is quite likely to be produced by CT saturation under heavy in-zone fault conditions. All this means that the third harmonic is not desirable means of stabilizing a protective system against inrush effects. Wednesday, March 10, 2021 VG PATEL 39

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION (d) Higher harmonics All other harmonics are theoretically present in inrush current but the relative magnitude diminishes rapidly as the order of harmonic increases. The differential relays are restrained if the magnetizing current contains 30 % fifth harmonic (which can happen because of temporary over voltage condition). There may be 5 % of fourth harmonic in a given inrush current. Modern alternative to harmonic bias is inrush detection and over fluxing detection. Wednesday, March 10, 2021 VG PATEL 40

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION TYPES OF DIFFERENTIAL PROTECTION OF POWER XMER v. Unbiased inverse definite minimum time delay relay (induction type) v. Percentage bias differential relay (induction type) v. High speed differential relay with harmonic restraint feature, and with high set unit. v. High impedance type differential relay (for auto x’mer only) Wednesday, March 10, 2021 VG PATEL 41

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION ØUnbiased IDMT relay • When power x’mer do not have tap change equipment, the C. T. s on both side windings are properly matched electrically and magnetically which allows the use unbiased relay. Wednesday, March 10, 2021 VG PATEL 42

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION The main disadvantages of this scheme are: • Need to adopt a high current setting due to transformer mismatch. • Longer duration of fault clearance. • Relay mal operate under external fault when small unbalance under normal condition will be multiplied to a great extent on through fault. Wednesday, March 10, 2021 VG PATEL 43

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION Percentage bias differential relay • This type of relay has bias windings to provide stability on external faults, and an inherent time delay to over come magnetizing inrush currents. Differential current X 100 % Bias = through current. • Bias settings normally provided in diffl. relays vary form 15 to 50%. However, self biasing diffl. relays are also available in which the bias is automatically adjusted to the current settings. Wednesday, March 10, 2021 VG PATEL 44

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION • The percentage bias settings provided in these relays from 15 to 50 %. • The operating current settings are normally 40 to 100% • Operating time is usually adjustable from 0. 1 to 0. 3 sec. • Disadvantage of this relay is its long operating time on internal faults. Wednesday, March 10, 2021 VG PATEL 45

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION • In order to obtain the lowest possible operating time on internal fault without losing stability on magnetizing inrush, the following additional devices can be used with percentage bias differential relays of induction type. –Voltage blocking relay which isolates the tripping circuit of differential relay during charging of x’mer and –Time delay relay having delay on reset which isolates the tripping circuit of differential relay during charging of x’mer. Wednesday, March 10, 2021 VG PATEL 46

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION • The disadvantage of the above two schemes is that the x’mer fault if any, present at the time of charging of x’mer can be detected only after the blocking feature in the tripping circuit of the diffl. relay is removed. Wednesday, March 10, 2021 VG PATEL 47

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION HIGH SPEED DIFF. RELAY WITH HARMONIC RESTRAINT FEATURE • To achieve high speed operation on internal faults without sacrificing stability on magnetizing inrush, harmonic restraint feature is incorporated in diffl. relay. • This feature is based on the fact that 2 nd harmonic component predominant in the magnetizing inrush current of x’mer Wednesday, March 10, 2021 VG PATEL 48

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION • The differential relay filters out the second harmonic component and uses it for restraining its operation. • This type of relay is also provided with percentage bias for stability on external faults. • Normal bias settings vary from 15 to 50%. • Operating time of this type relays is usually less than 40 milliseconds at twice the current setting. • The operating current setting generally varies from 10 -50%. Wednesday, March 10, 2021 VG PATEL 49

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION • These relays are also provided with highset instantaneous over current relay units have a fixed setting of above 8 times the rated current. • High speed differential relays with harmonic restraint feature are generally recommended for all x’mers above 15 MVA Wednesday, March 10, 2021 VG PATEL 50

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION RESTRICTED EARTH FAULT PROTECTION Ir N Iy Ib Wednesday, March 10, 2021 VG PATEL 51

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION RESTRICTED EARTH FAULT PROTECTION • Provides sensitive, instantaneous earth fault protection within the protected zone of the transformer. • Operates on circulating current principle. • CAG 14 – high impedance relay is used with external stabilizing resistor for the stability against through faults. • Operating coil of the relay is connected with l. Circuit in series to immune harmonic produced by CT saturation. Wednesday, March 10, 2021 VG PATEL 52

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION A simple over current and earth fault system will not give good protection cover for a star/ delta connected primary winding, particularly if the neutral is earthed through impedance. The degree of protection is very much improved by the application of a REF protection. The residual current of 3 CTs is balanced against the output of a CT in neutral conductor. The relay is of the high impedance type. Wednesday, March 10, 2021 VG PATEL 53

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION The system is operative for faults on the star winding and will remain stable for all faults out side this zone. A high impedance instantaneous relay with low setting covers good percentage of winding. REF protection is often applied even when the neutral is solidly earthed. For a delta connected or unearthed star winding, a high impedance relay can be used giving fast operation and phase fault stability. Both windings of a transformer can be protected separately with REF protection, thereby providing high-speed protection against E/F. Wednesday, March 10, 2021 VG PATEL 54

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION The system is operative for faults on the star winding and will remain stable for all faults out side this zone. A high impedance instantaneous relay with low setting covers good percentage of winding. REF protection is often applied even when the neutral is solidly earthed. For a delta connected or unearthed star winding, a high impedance relay can be used giving fast operation and phase fault stability. Both windings of a transformer can be protected separately with REF protection, thereby providing high-speed protection against E/F. Wednesday, March 10, 2021 VG PATEL 55

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION RESTRICTED EARTH FAULT PROTECTION Ir N Iy Ib Wednesday, March 10, 2021 VG PATEL 56

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION REF PROTECTION FOR RESISTANCE EARTHED SYSTEM Wednesday, March 10, 2021 VG PATEL 57

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION BACK UP EARTH FAULT PROTECTION Provided where power transformers with secondary star connected and connected to transmission line. Ir N Iy Ib R Wednesday, March 10, 2021 VG PATEL 58

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION BACKUP OVER CURRENT PROTECTION • A FAULT EXTERNAL TO A TRANSFORMER RESULTS IN OVERLOAD / TEMP. RISE / MECHANICAL FORCES. • TRANSFORMER MAY FAIL IF THE FAULT IS NOT CLEARED PROMPTLY. Wednesday, March 10, 2021 VG PATEL 59

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION BACKUP OVER CURRENT PROTECTION Ir IR Iy N IY Ib IB O/C O/C E/F Wednesday, March 10, 2021 VG PATEL 60

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION BACKUP OVER CURRENT PROTECTION • RELAY SETTINGS ALLOWS SYSTEM CO -ORDINATION. • TIME DELAY IS HIGH. • INSTANTANEOUS ELEMENT IS PROVIDED FOR PROTECTION AGAINST HEAVY INTERNAL FAULTS. • SEPARATE SET OF CTs (CORE) IS PROVIDED. Wednesday, March 10, 2021 VG PATEL 61

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION TRANSFORMER OVER FLUXING PROTECTION • E = 4. 44 * * f * T • E/f • OVER FLUXING INCREASES MAGNETIZING CURRENT AND IRON / CORE LOSS WHICH IN TURN OVERHEATS CORE AND DAMAGES INSULATION OF LAMINATION, CORE BOLT AND WINDING INSULATION. –SUCH CONDITION OCCURS WHEN TRANSFORMER IS CONNECTED TO THE GENERATOR AND EHV CIRCUIT BRAKER IS OFF (PRIOR TO SYNCHRONIZATION) –EXCITATION IS MADE ON AT REDUCED SPEED OF T. G. SET. AVR IS FAULTY AND CAUSES AN OVER VOLTAGE. Wednesday, March 10, 2021 VG PATEL 62

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION TRANSFORMER OVER FLUXING PROTECTION • GTT 11 RELAY IS PROVIDED. • SETTING IS DONE AT 1. 12 TIMES RATED V/F RATIO WITH TIME DELAY OF 60 SECS. Wednesday, March 10, 2021 VG PATEL 63

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION EXCITING VOLTAGE VS EXCITATION CURVE 10% Vk Vk 50% Iek EXCITATION CURRENT Ie Wednesday, March 10, 2021 VG PATEL 64

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION BUCHHOLZ RELAY • BUCHHOLZ RELAY PROVIDES UNIVERSAL RES-PONSE TO ALL TYPES OF FAULTS WITHIN THE TRANSFORMER. – PROVIDES PROTECTION AGAINST: – MINOR / INCIPIENT FAULTS SUCH AS • HOT SPOT ON THE CORE DUE TO S/C OF LAMINATION. • CORE BOLT INSULATION FAILURE. • FAULTY JOINTS. – INTER TURN FAULTS OR OTHER WINDING FAULTS INVOLVING LOWER POWER INFEEDS. – MAJOR WINDING FAULTS LIKE EARTH FAULTS OR INTERPHASE FAULTS. Wednesday, March 10, 2021 VG PATEL 65

TRANSFORMER ENCYCLOPAEDIA TRANSFORMER PROTECTION BUCHHOLZ RELAY INSTALLATION GUIDE LINES: • Buchholz relay is installed in the pipe connecting the transformer tank and the oil conservator. • The angle of inclination of the axis of the pipe with the horizontal plane should be between 10 o to 11 o. • The nominal pipe bore diameter as per IS recommendation is; 25 mm. Trans. up to 1000 k. VA. 50 mm. Trans. Between 1 MVA & 10 MVA. 80 mm. Trans. above 10 MVA. Wednesday, March 10, 2021 VG PATEL 66

TRANSFORMER ENCYCLOPAEDIA OPEN FORUM Wednesday, March 10, 2021 67

TRANSFORMER ENCYCLOPAEDIA THANQ Wednesday, March 10, 2021 68