Transformer Protection

Protection EngineeringAnd Research Laboratories Session VII : Transformer Protection Dr. G. Pradeep Kumar Training on Power System Element Protection, 9 th & 17 th March, 2007 at L&T Manappakam, Chennai. © 2007 Protection Engineering And Research Laboratories 2 17 th March 2007  Introduction  Over current protection  Parallel transformer protection  Earth faults protection  Biased differential protection  Auto transformer protection  Inter-turn fault protection  Over flux protection  Over load protection Contents © 2007 Protection Engineering And Research Laboratories 3 17 th March 2007 Introduction © 2007 Protection Engineering And Research Laboratories 4 17 th March 2007  Winding and terminal faults  Sustained or un-cleared external faults  Abnormal operating conditions such as overload, over voltage and over fluxing  Core faults Transformer Fault Categories Introduction © 2007 Protection Engineering And Research Laboratories 5 17 th March 2007 A1 Ø I A2 V E P Transformer Operation © 2007 Protection Engineering And Research Laboratories 6 17 th March 2007 A1 I A2 V E P a1 a2 E S Transformer Operation © 2007 Protection Engineering And Research Laboratories 7 17 th March 2007 A1 I P A2 V E P a1 a2 E S I S Transformer Operation © 2007 Protection Engineering And Research Laboratories 8 17 th March 2007 A B C C2 C1 A2 A1 B2 B1 a b c c2 c1 a2 a1 b1 b2 Transformer Connections © 2007 Protection Engineering And Research Laboratories 9 17 th March 2007  “Clock Face” numbers refer to position of low voltage phase- neutral vector with respect to high voltage phase neutral vector  Line connections made to highest numbered winding terminal available B1 B2 b2 b1 a2 a1 A1 A2 C1 C2 c2 c1 High Voltage Windings Low Voltage Windings A Phase Windings B Phase Windings C Phase Windings  Line phase designation is same as winding  Example 1 : Dy11 Transformer  How to connect the windings ? Transformer Connections © 2007 Protection Engineering And Research Laboratories 10 17 th March 2007 30° 11 12 Vector Group Example Dy11 © 2007 Protection Engineering And Research Laboratories 11 17 th March 2007 1. Draw Phase-Neutral Voltage Vectors 30 ° A C B a c b  Line Designation  Vector Group Example Dy11 © 2007 Protection Engineering And Research Laboratories 12 17 th March 2007 2. Draw Delta Connection A C B a c b Vector Group Example Dy11 © 2007 Protection Engineering And Research Laboratories 13 17 th March 2007 3. Draw A Phase Windings A C B a c b a2 a1 A2 A 1 Vector Group Example Dy11 © 2007 Protection Engineering And Research Laboratories 14 17 th March 2007 4. Complete Connections A C B a c b a 2 a1 A2 A 1 C1 C 2 B 1 B 2 b2 b1 c 1 c 2 Vector Group Example Dy11 © 2007 Protection Engineering And Research Laboratories 15 17 th March 2007 5. Winding representation on core c 1 c 2 b1 b2 a1 a2 a b c A B C C2 C1 B 2 B 1 A 2 A 1 Vector Group Example Dy11 © 2007 Protection Engineering And Research Laboratories 16 17 th March 2007 Over Current Protection of Transformer © 2007 Protection Engineering And Research Laboratories 17 17 th March 2007 Transformer rating Fuse kVA 100 200 300 500 1000 5.25 10.5 15.8 26.2 52.5 16 25 36 50 90 3.0 3.0 10.0 20.0 30.0 Full load current (A) Rated current (A) Operating time at 3 x rating(s) 11kV Distribution Transformers Typical Fuse Ratings © 2007 Protection Engineering And Research Laboratories 18 17 th March 2007 51 3.3kV 200/5 50 51 1MVA 3.3/0.44kV 1500/5 1500/5 64 N 50 N 51 N Distribution Transformers Protection © 2007 Protection Engineering And Research Laboratories 19 17 th March 2007 11kV 50 51 5MVA 11/3.3kV 1000/5 1000/5 64 51 N 64 3.3kV Distribution Transformers Protection © 2007 Protection Engineering And Research Laboratories 20 17 th March 2007 Requirements  Fast operation for primary short circuits  Discrimination with downstream protections  Operation within transformer withstand  Non-operation for short or long term overloads  Non-operation for magnetising inrush Transformer Over Current Protection © 2007 Protection Engineering And Research Laboratories 21 17 th March 2007 Source HV LV 50 51 50 set to 1.2 to 1.3 x through fault level Instantaneous Over Current Protection © 2007 Protection Engineering And Research Laboratories 22 17 th March 2007 Concerns relay response to offset waveforms (DC transient) Definition I 1 - I 2 I 2 x 100 I 1 = Steady state rms pick up current I 2 = Fully offset rms pickup current I 1 I 2 D.C Transient Overreach © 2007 Protection Engineering And Research Laboratories 23 17 th March 2007 Source LV HV 50 50 Inst O/C 50 Time O/C 51 51 51 Instantaneous Over Current Protection © 2007 Protection Engineering And Research Laboratories 24 17 th March 2007 50 set to 1.2 - 1.3 x through fault level Grading Margin Relay “Behind” Transformer 50 Max HV Setting Fault Level (at Transformer) Instantaneous Over Current Protection © 2007 Protection Engineering And Research Laboratories 25 17 th March 2007 HV2 51 51/ 50 51 LV HV1 Time LV HV1 HV2 Current I F(LV) 1.2I F(LV) I F(HV) Instantaneous Over Current Protection © 2007 Protection Engineering And Research Laboratories 26 17 th March 2007 2-1-1 Distribution I 3Ø I 3Ø I 3Ø I 3Ø 0.866 © 2007 Protection Engineering And Research Laboratories 27 17 th March 2007 I 3Ø 0.866 I 3Ø 0.4 sec LV relay HV relay 2-1-1 Distribution © 2007 Protection Engineering And Research Laboratories 28 17 th March 2007 Parallel Transformers Protection © 2007 Protection Engineering And Research Laboratories 29 17 th March 2007 Grid supply Feeders 51 51 67 67 51 51 Parallel Transformer Protection Directional Over Current © 2007 Protection Engineering And Research Laboratories 30 17 th March 2007 Grid supply Feeders 51 51 51 51 51 Bus Section Parallel Transformer Protection Non-directional Over Current © 2007 Protection Engineering And Research Laboratories 31 17 th March 2007 67 67 51 51 51 51 P1 P1 S1 S1 S2 S2 P2 P2 Grid supply Parallel Transformer Protection Partial Differential © 2007 Protection Engineering And Research Laboratories 32 17 th March 2007 Earth Fault Protection © 2007 Protection Engineering And Research Laboratories 33 17 th March 2007 1 p.u. turns \3 p.u. turns x I P PR R I F Earth Fault in a “DY” Transformer Resistance Grounded  Resistor “R” limits earth fault current to load values  For a fault at “x”; Fault current I F = x.I FL  Effective turns ratio is \3 : x  Line current on delta side for this fault I P = (x 2 / \3).I FL © 2007 Protection Engineering And Research Laboratories 34 17 th March 2007 _ I F 51 Over Current Relay I f as multiple of I F.L. 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Fault Current Star Side Fault Current Delta Side 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 x p.u.. Over Current Protection on Delta Side of “DY” Transformer © 2007 Protection Engineering And Research Laboratories 35 17 th March 2007 _ I F 10 9 8 7 6 5 4 3 2 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 x p.u.. I f as multiple of I F.L. Earth Fault in a “DY” Transformer Solidly Grounded Fault Current Star Side Fault Current Delta Side © 2007 Protection Engineering And Research Laboratories 36 17 th March 2007 _ I F I P I N 10 9 8 7 6 5 4 3 2 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 _ p.u.. I f as multiple of I F.L. I N I P I F Earth Fault in a “YD” Transformer Solidly Grounded © 2007 Protection Engineering And Research Laboratories 37 17 th March 2007 I Differential Relay Setting = I S _ protected not is winding Y of 59% Thus 59% i.e. operation for 20% 3 then 20%, If e.g. operation, relay For 3 2 S S 2 > > = I I > I = I x x x Differential Relay Setting % of Star Winding Protected 10% 58% 20% 41% 30% 28% 40% 17% 50% 7% Earth Fault in a “DY” Transformer Resistance Grounded © 2007 Protection Engineering And Research Laboratories 38 17 th March 2007 51 51 51 51N  Provides back-up protection for system  Time delay required for co-ordination Un-restricted Earth Fault Protection © 2007 Protection Engineering And Research Laboratories 39 17 th March 2007  Can provide better sensitivity (C.T. ratio not related to full load current  Improved “effective” setting)  Provides back up protection for transformer and system 51 51 51 51N 51N Un-restricted Earth Fault Protection © 2007 Protection Engineering And Research Laboratories 40 17 th March 2007 Protected Zone REF  Relay only operates for earth faults within protected zone.  Uses high impedance principle.  Stability level : usually maximum through fault level of transformer Restricted Earth Fault Protection Star Winding © 2007 Protection Engineering And Research Laboratories 41 17 th March 2007 A B C N LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated Restricted Earth Fault Protection Star Windings © 2007 Protection Engineering And Research Laboratories 42 17 th March 2007 A B C N Restricted Earth Fault Protection Star Windings LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated © 2007 Protection Engineering And Research Laboratories 43 17 th March 2007 Protected zone REF Source  Delta winding cannot supply zero sequence current to system  Stability : Consider max LV fault level  Recommended setting : less than 30% minimum earth fault level Restricted Earth Fault Protection Delta Winding © 2007 Protection Engineering And Research Laboratories 44 17 th March 2007 Protected Circuit R Z M R CT R L Z M R CT R L I F I S R ST V S R L R L High Impedance Differential  Voltage across relay circuit  V S = I F (R CT + 2 R L )  Stabilizing resistor (R ST ) used to limit spill current in the relay path to less than the relay setting (I S )  R ST = (V S /I S ) – R R  V K > 2V S = 2 I F (RCT + 2 RL) © 2007 Protection Engineering And Research Laboratories 45 17 th March 2007  During internal faults the high impedance relay circuit offers an excessive burden to the CT‟s.  A very high voltage develops across the relay circuit and the CT‟s.  Can cause damage to insulation of CT, secondary winding and relay.  Magnitude of peak voltage V P is given by an approximate formula (based on experimental results) V P = 2\2 \[V K (V F - V K )]  Where V F = Max. prospective voltage in the absence of saturation. (= I Fmax . Relay circuit total impedance)  Metrosil required if V P > 3kV High Impedance Differential Non-linear Resistors © 2007 Protection Engineering And Research Laboratories 46 17 th March 2007 1MVA (5%) 11000V 415V 1600/1 R CT = 4.9O 80MVA R S 1600/1 R CT = 4.8O I S = 0.1 Amp 2 Core 7/0.67mm (7.41O/km) 100m Long Calculate : 1) Setting voltage (V S ) 2) Value of stabilising resistor required 3) Effective setting 4) Peak voltage developed by CT‟s for internal fault Restricted Earth Fault Protection Example 1 © 2007 Protection Engineering And Research Laboratories 47 17 th March 2007 Earth fault calculation :- Using 80MVA base Source impedance = 1 p.u. Transformer impedance = 0.05 x 80 = 4 p.u. 1 Total impedance = 14 p.u. I 1 = 1 = 0.0714 p.u. 14 Base current = 80 x 10 6 \3 x 415 = 111296 Amps I F = 3 x 0.0714 x 111296 = 23840 Amps (primary) = 14.9 Amps (secondary) 1 P.U. 1 4 I 1 4 I 2 4 I 0 Sequence Diagram Restricted Earth Fault Protection Example 1 © 2007 Protection Engineering And Research Laboratories 48 17 th March 2007 (1) Setting voltage V S = I F (R CT + 2 RL ) Assuming “earth” CT saturates, R CT = 4.8 ohms 2 RL = 2 x 100 x 7.41 x 10 -3 = 1.482 ohms Setting voltage = 1.49 (4.8 + 1.482) = 93.6 Volts (2) Stabilising Resistor (R ST ) R S = (V S/ I S ) – (1/I S 2 ) Where I S = relay current setting R S = (93.6/0.1) – (1/ 0.1 2 ) = 836 ohms Restricted Earth Fault Protection Example 1 © 2007 Protection Engineering And Research Laboratories 49 17 th March 2007 1.6 1.2 0.8 0.4 0.04 0.12 0.08 0 “H” AT/mm (multiply by Ki to obtain total exciting current in Amps) “ B ” W e b e r / m 2 ( T e s l a ) ( m u l t i p l y b y K v t o o b t a i n r m s s e c o n d a r y v o l t s ) Kv Ki Line & 158 0.341 Neutral CT Earth CT 236 0.275 Restricted Earth Fault Protection Example 1 © 2007 Protection Engineering And Research Laboratories 50 17 th March 2007 (3) Effective setting I P = CT ratio x (I S + ¿I MAG ) Line & Neutral CTs Flux density at 93.6V = 93.6/158 = 0.592 Tesla From graph, mag. Force at 0.592 Tesla = 0.015 AT/mm Mag. Current = 0.015 x 0.341 = 0.0051 Amps ‘Earth’ CT Flux density at 93.6V = 93.6/236 = 0.396 Tesla From graph, mag. Force at 0.396 Tesla = 0.012 AT/mm Mag. Current = 0.012 x 0.275 = 0.0033 Amps Thus, effective setting = 1600 x (0.1 + [(4 x 0.0051) + 0.0033]) = 197. 92A Restricted Earth Fault Protection Example 1 © 2007 Protection Engineering And Research Laboratories 51 17 th March 2007 Transformer full load current = 1391 Amps Effective setting = (198/1391) x 100% = 14.2% x rated 4) Peak voltage = 2\2\[V K (V F - V K )] V F = 14.9 x V S /I S = 14.9 x 936 = 13946 Volts For „Earth‟ CT, V K = 1.4 x 236 = 330 Volts (from graph) V P = 2\2 \[330 (13946 - 330)] = 6kV Thus, non-linear resistor is to be used. Restricted Earth Fault Protection Example 1 © 2007 Protection Engineering And Research Laboratories 52 17 th March 2007 A N T1 Bus Section T2 415 Volt Switchboard C B Restricted Earth Fault Protection Parallel Transformer © 2007 Protection Engineering And Research Laboratories 53 17 th March 2007 Parallel Transformer CT in Earth Circuit A N T1 Bus Section Open T2 415 Volt Switchboard C B 51N 51N © 2007 Protection Engineering And Research Laboratories 54 17 th March 2007 Parallel Transformer CT in Earth & Neutral Circuits Bus section open 415 volt switchboard 51N T2 T1 N A B C 51N © 2007 Protection Engineering And Research Laboratories 55 17 th March 2007 Bus section 415 volt switchboard T1 N A B C F2 F1 Will mal-operate if bus section is open for fault at F1 No mal-operation for fault at F2 (but setting must be greater than load neutral current) T2 Parallel Transformer Residual CT Connection © 2007 Protection Engineering And Research Laboratories 56 17 th March 2007 Biased Differential Protection © 2007 Protection Engineering And Research Laboratories 57 17 th March 2007  Phase differential protection may be justified for larger transformers (generally > 5MVA).  Provides fast operation on any winding  Measuring principle :  Based on the same circulating current principle as the restricted earth fault protection  However, it employs the biasing technique, to maintain stability for heavy through fault current  Biasing allows mismatch between CT outputs.  Biasing is essential for transformers with tap changing facility.  Another important requirement of transformer differential protection is immunity to magnetising in rush current. Differential Protection © 2007 Protection Engineering And Research Laboratories 58 17 th March 2007 BIAS BIAS I 2 I 1 OPERATE I 1 - I 2 Differential Current OPERATE RESTRAIN Mean Thro‟ Current (I 1 +I 2 )/ 2 I 1 - I 2 Bias = Differential current / Mean through current Biased Differential Protection © 2007 Protection Engineering And Research Laboratories 59 17 th March 2007 LV R HV PROTECTED ZONE  Correct application of differential protection requires CT ratio and winding connections to match those of transformer.  CT secondary circuit should be a “replica” of primary system.  Account for;  Difference in current magnitude  Phase shift  Zero sequence currents Biased Differential Protection © 2007 Protection Engineering And Research Laboratories 60 17 th March 2007 P1 P2 A2 A1 a1 a2 P2 P1 A B C Differential Protection CT Connections © 2007 Protection Engineering And Research Laboratories 61 17 th March 2007 Interposing CT provides  Vector correction  Ratio correction  Zero sequence compensation R R R P1 P2 A2 A1 a1 a2 P2 P1 S2 S1 P1 P2 S1 S2 S1 S2 Differential Protection Interposing CT Connections © 2007 Protection Engineering And Research Laboratories 62 17 th March 2007 S2 S1 P1 P2 A1 A2 a2 a1 P2 15MVA 66kV / 11kV 150/5 800/5 P1 Dy1 S1 S2  For the application shown above, consider (1) Winding full load current (2) Effect of tap changer (if any) (3) C.T. polarities Differential Protection Example 2 © 2007 Protection Engineering And Research Laboratories 63 17 th March 2007 S2 S1 P1 P2 A1 A2 a2 a1 P2 15MVA 66kV / 11kV 150/5 800/5 P1 Dy1 S1 S2 Assuming no tap changer Full load currents :- 66kV : 131 Amp = 4.37 A (sec.) 11kV : 787 Amp = 4.92 A (sec.) However, require 11kV C.T.‟s to be connected in A Thus, secondary current = \3 x 4.92 = 8.52A RATIO CORRECTION IS REQUIRED Differential Protection Example 2 © 2007 Protection Engineering And Research Laboratories 64 17 th March 2007 Differential Protection Example 2 150/5 800/5 ?? ?? ?? ?? R R R P1 P2 A2 A1 a1 a2 P2 P1 S2 S1 P1 P2 S1 S2 S1 S2 © 2007 Protection Engineering And Research Laboratories 65 17 th March 2007  It is a normal practice to connect 11kV C.T.‟s in “Star” and utilise a “Star”/”Delta” interposing C.T. (this method reduces lead VA burden on the line C.T.‟s).  Current from 66kV side = 4.37 A  Thus, current required from “Delta” winding of ICT = 4.37A  Current input to “Star” winding of ICT = 4.92 A  Required ICT ratio = 4.92 / (4.37 /\3) = 4.92 / 2.52  Can also be expressed as  5 / 2.56 Differential Protection Example 2 © 2007 Protection Engineering And Research Laboratories 66 17 th March 2007 Differential Protection Example 2 150/5 800/5 4.37A 4.92A (5) (2.56) R R R P1 P2 A2 A1 a1 a2 P2 P1 S2 S1 P1 P2 S1 S2 S1 S2 © 2007 Protection Engineering And Research Laboratories 67 17 th March 2007  Assume 66kV +5%, -15%  Interposing C.T. ratio should be based on mid tap position  Mid Tap (-5%) = 62.7 kV  Primary current (@15 MVA) = 138 Amp  Secondary current = 4.6 Amp  Interposing C.T. ratio required = 4.92 / (4.6/ \3) = 4.92 / 2.66  May also be expressed as : 5 / 2.7  As compared with 5 / 2.56 based on nominal voltage Differential Protection Example 2 – Effect of Tap Changer © 2007 Protection Engineering And Research Laboratories 68 17 th March 2007 R R R P1 P2 A2 A1 a1 a2 P2 P1 S2 S1 P1 P2 S1 S2 S1 S2 Differential Protection Example 2 – Vector Group Current distribution verification © 2007 Protection Engineering And Research Laboratories 69 17 th March 2007 R R R P1 P2 A2 A1 a1 a2 P2 P1 S2 S1 P1 P2 S1 S2 S1 S2 Differential Protection Example 2 – Vector Group Current distribution verification © 2007 Protection Engineering And Research Laboratories 70 17 th March 2007 R R R P1 P2 A2 A1 a1 a2 P2 P1 S2 S1 P1 P2 S1 S2 S1 S2 Differential Protection Example 2 – Vector Group Current distribution verification © 2007 Protection Engineering And Research Laboratories 71 17 th March 2007 R R R P1 P2 A2 A1 a1 a2 P2 P1 S2 S1 P1 P2 S1 S2 S1 S2 Differential Protection Example 2 – Vector Group Current distribution verification © 2007 Protection Engineering And Research Laboratories 72 17 th March 2007 To differential relay S2 S1 P1 P2 REF S2 S1 P1 P2 S2 S1 P1 P2 a2 a1 A1 A2 Combined REF and Differential Protection © 2007 Protection Engineering And Research Laboratories 73 17 th March 2007 Power transformer Differential element Virtual ICT Vector correction Ratio correction Virtual ICT Virtual ICT in Numerical Relays © 2007 Protection Engineering And Research Laboratories 74 17 th March 2007 Stability for Inrush Currents © 2007 Protection Engineering And Research Laboratories 75 17 th March 2007 Twice Normal Flux Normal Flux Normal No Load Current No Load Current at Twice Normal Flux Transformer Magnetizing Characteristics © 2007 Protection Engineering And Research Laboratories 76 17 th March 2007 Steady State + u m - u m u V I m Transformer Magnetizing Current © 2007 Protection Engineering And Research Laboratories 77 17 th March 2007 Switch on at voltage zero - No residual flux u 2u m V I m Magnetizing Inrush Current © 2007 Protection Engineering And Research Laboratories 78 17 th March 2007  Appears on one side of transformer only  Seen as fault by differential relay  Normal steady state magnetising current is less than relay setting  Transient magnetising inrush could cause relay to operate Effect of magnetising current Effect of Inrush Current on Differential Protection © 2007 Protection Engineering And Research Laboratories 79 17 th March 2007  Allow magnetising current to die away before relay can operate  Slows operation for genuine transformer faults Solution 1 : Time delay Effect of Inrush Current on Differential Protection © 2007 Protection Engineering And Research Laboratories 80 17 th March 2007 Solution 2 : 2nd (and 5th) harmonic restraint  Makes relay immune to magnetising inrush  Slow operation may result for genuine transformer faults if CT saturation occurs Effect of Inrush Current on Differential Protection © 2007 Protection Engineering And Research Laboratories 81 17 th March 2007  Inhibits relay operation during magnetising inrush  Operate speed for genuine transformer faults unaffected by significant CT saturation Solution 3 : Gap measurement technique Effect of Inrush Current on Differential Protection © 2007 Protection Engineering And Research Laboratories 82 17 th March 2007 Typical Magnetising Inrush Waveforms A C B Gap Measurement Technique © 2007 Protection Engineering And Research Laboratories 83 17 th March 2007 Differential comparator T1 = 5ms T2 = 22ms Trip Bias Differential Threshold Gap Measurement Technique © 2007 Protection Engineering And Research Laboratories 84 17 th March 2007 Bias differential threshold Trip Differential comparator T1 = 5ms T2 = 22ms Differential input Comparator output Trip Reset T2 T1 Gap Measurement Technique Stability for Inrush Current © 2007 Protection Engineering And Research Laboratories 85 17 th March 2007 Differential input Comparator output Trip Reset T2 T1 Gap Measurement Technique Operation for Internal Faults Trip Differential comparator T1 = 5ms T2 = 22ms Bias differential threshold © 2007 Protection Engineering And Research Laboratories 86 17 th March 2007 Auto Transformer Protection © 2007 Protection Engineering And Research Laboratories 87 17 th March 2007 (a) Earth Fault Scheme A B C High impedance relay 87 Auto Transformer Protection High Impedance Differential © 2007 Protection Engineering And Research Laboratories 88 17 th March 2007 (b) Phase and Earth Fault Scheme A B C a b c 87 87 87 n Auto Transformer Protection High Impedance Differential © 2007 Protection Engineering And Research Laboratories 89 17 th March 2007 Inter-turn Fault Protection © 2007 Protection Engineering And Research Laboratories 90 17 th March 2007 Nominal turns ratio Fault turns ratio Current ratio - 11,000 / 240 - 11,000 / 1 - 1 / 11,000 Requires Buchholz relay CT E Shorted turn Load Inter-turn Fault © 2007 Protection Engineering And Research Laboratories 91 17 th March 2007 Inter-turn fault current Vs number of turns shorted 100 80 60 40 20 0 5 10 15 20 25 10 8 6 4 2 Fault current in short circuited turns Primary input current Fault current (multiples of rated current) Primary current (multiples of rated current) Turns short-circuited (% of winding) Inter-turn Fault © 2007 Protection Engineering And Research Laboratories 92 17 th March 2007 5 x internal pipe diameter (minimum) 3 x internal pipe diameter (minimum) Transformer 3 minimum Oil conservator Conservator Buchholz Relay Installation © 2007 Protection Engineering And Research Laboratories 93 17 th March 2007 Petcock Counter balance weight Oil level From transformer Aperture adjuster Deflector plate Drain plug Trip bucket To oil conservator Mercury switch Alarm bucket Buchholz Relay © 2007 Protection Engineering And Research Laboratories 94 17 th March 2007 Over Flux Protection © 2007 Protection Engineering And Research Laboratories 95 17 th March 2007  Generator transformers and grid transformers  Usually only a problem during run-up or shut down, but can be caused by loss of load / load shedding etc.  Flux · V/ f  Effects of over fluxing, increase in,  Magnetizing current  Winding temperature  Noise and vibration  Overheating of laminations and metal parts (caused by stray flux) Over Fluxing in Transformers © 2007 Protection Engineering And Research Laboratories 96 17 th March 2007  Low frequency  High voltage  Geomagnetic disturbances  Tripping of differential element (Transient over fluxing)  Damage to transformers (Prolonged over fluxing) 2u m I m Causes Effects V = k f u Over Fluxing in Transformers Theory u m © 2007 Protection Engineering And Research Laboratories 97 17 th March 2007  Blocks differential element for transient over fluxing 25% Overvoltage condition 43% 5th Harmonic content 5 th Harmonic Restraint © 2007 Protection Engineering And Research Laboratories 98 17 th March 2007  Protective relay responds to V/f ratio  Measured from voltage input  Two stage protection Stage 1 - lower A.V.R. or alarm Stage 2 - Trip field or transformer  Definite time of Inverse time Over Flux Protection © 2007 Protection Engineering And Research Laboratories 99 17 th March 2007 Trip and alarm outputs for clearing prolonged over fluxing Alarm : Definite time characteristic to initiate corrective action Trip : IDMT or DT characteristic to clear over-fluxing condition Settings Pick-up 1.5 to 3.0 i.e. 110V x 1.05 = 2.31 50Hz DT setting range 0.1 to 60 seconds Over Flux Protection V/f = k u © 2007 Protection Engineering And Research Laboratories 100 17 th March 2007 DC Circuit breaker position repeat relay AUX relay Lower AVR Inhibit AVR raise Alarm Generator field circuit breaker trip coil RL2-2 Alarm RL2-1 RL1-1 DC Over Flux Protection Application © 2007 Protection Engineering And Research Laboratories 101 17 th March 2007 Over Load Protection © 2007 Protection Engineering And Research Laboratories 102 17 th March 2007 90 100 110 120 130 140 Hot spot temp° C 0.1 1.0 10 100 Relative rate of using life  With ambient of 20°C, hot spot rise of 78 ° is design normal.  A further rise of 6° C doubles rate of using life. 98° 80 Transformer Overload and Life of Insulation © 2007 Protection Engineering And Research Laboratories 103 17 th March 2007 I load TD setting Top oil of power transformer Heater Thermal replica Temperature sensing resistor Remote Temp. indication Local Off On Off On Alarm Trip Pump control Fan control I load Over Heating Protection © 2007 Protection Engineering And Research Laboratories 104 17 th March 2007  Thermal replica for 1Ø or 3Ø protection.  Adjustable time constant : 5 mins. to 160 mins.  Current settings : Thermal : 0.4 IN to 1.175 IN Instantaneous : 2 IN to 25 IN  Thermal state indication  Separately adjustable trip and alarm settings Thermal Over Load Protection © 2007 Protection Engineering And Research Laboratories 105 17 th March 2007 Thank you