Transformer Protection
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Protection Engineering And Research Laboratories Session VII : Transformer Protection 9th
Training on Power System Element Protection, & 17th March, 2007 at L&T Manappakam, Chennai.
Dr. G. Pradeep Kumar
Contents
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
© 2007 Protection Engineering And Research Laboratories
17th March 2007 2
Introduction © 2007 Protection Engineering And Research Laboratories
17th March 2007 3
Introduction Transformer Fault Categories
Winding and terminal faults Sustained or un-cleared external faults
Abnormal operating conditions such as overload, over voltage and over fluxing Core faults
© 2007 Protection Engineering And Research Laboratories
17th March 2007 4
Transformer Operation
I
V
A2
Ø
EP
A1
© 2007 Protection Engineering And Research Laboratories
17th March 2007 5
Transformer Operation
I
V
A2
EP
A1
© 2007 Protection Engineering And Research Laboratories
a2
ES
a1
17th March 2007 6
Transformer Operation
IP
V
A2
EP
A1
© 2007 Protection Engineering And Research Laboratories
a2
IS
ES
a1
17th March 2007 7
Transformer Connections a
A
C1
a2
A2
a1 c1
C
b1
A1
C2 B1
B2
b
B
© 2007 Protection Engineering And Research Laboratories
b2
c2 c
17th March 2007 8
Transformer Connections
“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
Line phase designation is same as winding
Example 1 : Dy11 Transformer
High Voltage Windings
How to connect the windings ?
A Phase Windings B Phase Windings C Phase Windings
A2 B2
C2 © 2007 Protection Engineering And Research Laboratories
Low Voltage Windings
A1
a1
a2
B1
b1
b2
C1
c1
c2
17th March 2007 9
Vector Group Example Dy11 12 11
30°
© 2007 Protection Engineering And Research Laboratories
17th March 2007 10
Vector Group Example Dy11 1. Draw Phase-Neutral Voltage Vectors
A
Line Designation a 30 ° b
C
B c
© 2007 Protection Engineering And Research Laboratories
17th March 2007 11
Vector Group Example Dy11 2. Draw Delta Connection A a
b
C
B
© 2007 Protection Engineering And Research Laboratories
c 17th March 2007 12
Vector Group Example Dy11 3. Draw A Phase Windings A a a2 A2
a1
b
A 1 C
B
© 2007 Protection Engineering And Research Laboratories
c 17th March 2007 13
Vector Group Example Dy11 4. Complete Connections A a C1
A2
a 2 a1
C 2 C
c 1
A 1 B 1
B 2
B
© 2007 Protection Engineering And Research Laboratories
b1
b2
b
c 2 c 17th March 2007 14
Vector Group Example Dy11 5. Winding representation on core
A
B
C
A 2 B 2 C2
A 1
a1
a2
B 1
b1
b2
C1
© 2007 Protection Engineering And Research Laboratories
c 1
c 2
a
b
c
17th March 2007 15
Over Current Protection of Transformer © 2007 Protection Engineering And Research Laboratories
17th March 2007 16
11kV Distribution Transformers Typical Fuse Ratings Transformer rating
Fuse
kVA
Full load current (A)
Rated current (A)
Operating time at 3 x rating(s)
100
5.25
16
3.0
200
10.5
25
3.0
300
15.8
36
10.0
500
26.2
50
20.0
1000
52.5
90
30.0
© 2007 Protection Engineering And Research Laboratories
17th March 2007 17
Distribution Transformers Protection 3.3kV 200/5
1500/5
51
50
51 N
50 N
1MVA 3.3/0.44kV
51 N
64
© 2007 Protection Engineering And Research Laboratories
1500/5
17th March 2007 18
Distribution Transformers Protection 11kV
51
50
64
1000/5
5MVA 11/3.3kV
51 N
64
1000/5
3.3kV © 2007 Protection Engineering And Research Laboratories
17th March 2007 19
Transformer Over Current Protection 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
© 2007 Protection Engineering And Research Laboratories
17th March 2007 20
Instantaneous Over Current Protection Source
LV
HV
50 51 50 set to 1.2 to 1.3 x through fault level © 2007 Protection Engineering And Research Laboratories
17th March 2007 21
Transient Overreach Concerns relay response to offset waveforms (DC transient)
Definition I1 - I2 I2
x 100
I2 I1
D.C
I1 = Steady state rms pick up current I2 = Fully offset rms pickup current
© 2007 Protection Engineering And Research Laboratories
17th March 2007 22
Instantaneous Over Current Protection HV
LV
Source
50
50
50 Inst O/C
51
51
51 Time O/C
© 2007 Protection Engineering And Research Laboratories
17th March 2007 23
Instantaneous Over Current Protection 50 set to 1.2 - 1.3 x through fault level
Relay “Behind” Transformer
Grading Margin
50 Setting © 2007 Protection Engineering And Research Laboratories
Max HV Fault Level (at Transformer) 17th March 2007 24
Instantaneous Over Current Protection
51
51/ 50
HV2
HV1
51
HV1
HV2
LV
Time LV
Current
IF(LV)
IF(HV)
1.2IF(LV) © 2007 Protection Engineering And Research Laboratories
17th March 2007 25
2-1-1 Distribution I 3Ø
I 3Ø
0.866I 3Ø I 3Ø
© 2007 Protection Engineering And Research Laboratories
17th March 2007 26
2-1-1 Distribution
HV relay
0.4 sec LV relay
0.866 I 3Ø © 2007 Protection Engineering And Research Laboratories
I 3Ø 17th March 2007 27
Parallel Transformers Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 28
Parallel Transformer Protection Directional Over Current
51 67
Grid supply
Feeders 51
67
51
51 © 2007 Protection Engineering And Research Laboratories
17th March 2007 29
Parallel Transformer Protection Non-directional Over Current
51 51
Grid supply
Bus Section
Feeders
51
51 © 2007 Protection Engineering And Research Laboratories
51 17th March 2007 30
Parallel Transformer Protection Partial Differential Grid supply
P1
P2
S1 S2
67
67 51
S2 S1 P2
P1
51 51
© 2007 Protection Engineering And Research Laboratories
51
17th March 2007 31
Earth Fault Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 32
Earth Fault in a “DY” Transformer Resistance Grounded 3 p.u. turns
x
IP
PR
1 p.u. turns
R IF
Resistor “R” limits earth fault current to load values
For a fault at “x”;
Fault current IF = x.IFL
Effective turns ratio is 3 : x
Line current on delta side for this fault IP= (x2/ 3).IFL
© 2007 Protection Engineering And Research Laboratories
17th March 2007 33
Over Current Protection on Delta Side of “DY” Transformer If as multiple of IF.L. 1.0 0.9
IF
Fault Current Star Side
0.8 0.7 0.6
51
0.5
Over Current Relay
0.4 0.3
Fault Current Delta Side
0.2 0.1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
© 2007 Protection Engineering And Research Laboratories
1.0
x p.u..
17th March 2007 34
Earth Fault in a “DY” Transformer Solidly Grounded If as multiple of IF.L. 10
Fault Current Star Side
9
IF
8 7 6 5 4 3
Fault Current Delta Side
2 1
x 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 © 2007 Protection Engineering And Research Laboratories
p.u..
17th March 2007 35
Earth Fault in a “YD” Transformer Solidly Grounded If as multiple of IF.L. IF
10
IF
9 8
7 6
IP
IN
IP
5 4 3
2 1
IN
0.1 0.2 0.3 © 2007 Protection Engineering And Research Laboratories
0.4 0.5
0.6 0.7 0.8 0.9 1.0
p.u..
17th March 2007 36
Earth Fault in a “DY” Transformer Resistance Grounded
x2 3 For relay operation, S
I
Differential Relay Setting = IS
x2 e.g. If S 20%, then 20% for operation 3 i.e. x 59% Thus 59% of Y winding is not protected
Differential Relay Setting 10% 20% 30% 40% 50% © 2007 Protection Engineering And Research Laboratories
% of Star Winding Protected 58% 41% 28% 17% 7% 17th March 2007 37
Un-restricted Earth Fault Protection
51N
51
51
51
Provides back-up protection for system Time delay required for co-ordination © 2007 Protection Engineering And Research Laboratories
17th March 2007 38
Un-restricted Earth Fault Protection
51N
51
51 51
51N
Can provide better sensitivity (C.T. ratio not related to full load current Improved “effective” setting)
Provides back up protection for transformer and system
© 2007 Protection Engineering And Research Laboratories
17th March 2007 39
Restricted Earth Fault Protection Star Winding
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
© 2007 Protection Engineering And Research Laboratories
17th March 2007 40
Restricted Earth Fault Protection Star Windings
A B C N LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated © 2007 Protection Engineering And Research Laboratories
17th March 2007 41
Restricted Earth Fault Protection Star Windings A B C N
LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated © 2007 Protection Engineering And Research Laboratories
17th March 2007 42
Restricted Earth Fault Protection Delta Winding
Source
REF
Protected zone
Delta winding cannot supply zero sequence current to system
Stability : Consider max LV fault level Recommended setting : less than 30% minimum earth fault level © 2007 Protection Engineering And Research Laboratories
17th March 2007 43
High Impedance Differential Protected Circuit ZM
RCT
RCT RL
IF
RL
IS
VS
RL
ZM
RST R
RL
Voltage across relay circuit VS = IF (RCT + 2 RL)
Stabilizing resistor (RST) used to limit spill current in the relay
path to less than the relay setting (IS)
RST = (VS/IS) – RR
VK > 2VS = 2 IF (RCT + 2 RL)
© 2007 Protection Engineering And Research Laboratories
17th March 2007 44
High Impedance Differential Non-linear Resistors
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 VP is given by an approximate formula (based on experimental results) VP = 22 [VK (VF - VK)]
Where VF = Max. prospective voltage in the absence of saturation. (= IFmax . Relay circuit total impedance)
Metrosil required if VP > 3kV
© 2007 Protection Engineering And Research Laboratories
17th March 2007 45
Restricted Earth Fault Protection Example 1 1MVA (5%) 11000V 415V
1600/1 RCT = 4.9
80MVA
Calculate : 1)
Setting voltage (VS)
2)
Value of stabilising resistor required
1600/1 RCT = 4.8
RS
3)
Effective setting
4)
Peak voltage developed by CT‟s for internal fault
IS = 0.1 Amp
2 Core 7/0.67mm (7.41/km) 100m Long © 2007 Protection Engineering And Research Laboratories
17th March 2007 46
Restricted Earth Fault Protection Example 1 Earth fault calculation :Using 80MVA base Source impedance = 1 p.u. 1 P.U.
Transformer impedance = 0.05 x 80 = 4 p.u. 1 1 4 I1
Total impedance = 14 p.u. I1 =
4 I2
1 = 0.0714 p.u. 14
Base current = 80 x 106 3 x 415
= 111296 Amps 4 Sequence Diagram
IF I0
© 2007 Protection Engineering And Research Laboratories
= 3 x 0.0714 x 111296 = 23840 Amps (primary) = 14.9 Amps (secondary) 17th March 2007 47
Restricted Earth Fault Protection Example 1 (1) Setting voltage
VS = IF (RCT + 2RL) Assuming “earth” CT saturates, RCT = 4.8 ohms 2RL = 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 (RST) RS = (VS/ IS) – (1/IS2)
Where IS = relay current setting
RS = (93.6/0.1) – (1/ 0.12) = 836 ohms © 2007 Protection Engineering And Research Laboratories
17th March 2007 48
“B” Weber/m2 (Tesla) (multiply by Kv to obtain rms secondary volts)
Restricted Earth Fault Protection Example 1
1.6
Kv
1.2
Ki
Line & Neutral CT
158
0.341
Earth CT
236
0.275
0.8
0.4
0
0.04
0.08
0.12
“H” AT/mm (multiply by Ki to obtain total exciting current in Amps) © 2007 Protection Engineering And Research Laboratories
17th March 2007 49
Restricted Earth Fault Protection Example 1 (3) Effective setting IP = CT ratio x (IS + IMAG) 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 © 2007 Protection Engineering And Research Laboratories
17th March 2007 50
Restricted Earth Fault Protection Example 1 Transformer full load current = 1391 Amps Effective setting = (198/1391) x 100% = 14.2% x rated 4)
Peak voltage = 22[VK (VF - VK)] VF = 14.9 x VS/IS = 14.9 x 936 = 13946 Volts For „Earth‟ CT, VK = 1.4 x 236 = 330 Volts (from graph) VP = 22 [330 (13946 - 330)] = 6kV Thus, non-linear resistor is to be used.
© 2007 Protection Engineering And Research Laboratories
17th March 2007 51
Restricted Earth Fault Protection Parallel Transformer T1
N A B C
Bus Section
T2
415 Volt Switchboard © 2007 Protection Engineering And Research Laboratories
17th March 2007 52
Parallel Transformer CT in Earth Circuit N A B C
T1
51N
Bus Section Open
T2
51N
© 2007 Protection Engineering And Research Laboratories
415 Volt Switchboard
17th March 2007 53
Parallel Transformer CT in Earth & Neutral Circuits T1
NA B C
51N
Bus section open
T2
51N 415 volt switchboard © 2007 Protection Engineering And Research Laboratories
17th March 2007 54
Parallel Transformer Residual CT Connection N A B C
T1
F2
Bus section Will mal-operate if bus section is open for fault at F1 T2
F1 415 volt switchboard No mal-operation for fault at F2 (but setting must be greater than load neutral current) © 2007 Protection Engineering And Research Laboratories
17th March 2007 55
Biased Differential Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 56
Differential Protection
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.
© 2007 Protection Engineering And Research Laboratories
17th March 2007 57
Biased Differential Protection Differential Current
I1
BIAS
BIAS
I1 - I2
OPERATE
I2 RESTRAIN
OPERATE
I1 - I2 (I1+I2)/ 2
Mean Thro‟ Current
Bias = Differential current / Mean through current © 2007 Protection Engineering And Research Laboratories
17th March 2007 58
Biased Differential Protection PROTECTED ZONE HV
LV
R
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
© 2007 Protection Engineering And Research Laboratories
17th March 2007 59
Differential Protection CT Connections P1
P2
A2
A1
A
a1
B
© 2007 Protection Engineering And Research Laboratories
a2
P2
P1
C 17th March 2007 60
Differential Protection Interposing CT Connections P1 S1
P2
A2
A1
a1
a2
P2
S2
S1
S2
S1 P1
P2
R R R
© 2007 Protection Engineering And Research Laboratories
P1 S2
Interposing CT provides
Vector correction
Ratio correction
Zero sequence compensation
17th March 2007 61
Differential Protection Example 2 15MVA 66kV / 11kV
150/5 P1
P2
S1
S2
A2
A1 a1
a2
800/5 P2 S2
P1
S1
Dy1
For the application shown above, consider (1)
Winding full load current
(2)
Effect of tap changer (if any)
(3)
C.T. polarities
© 2007 Protection Engineering And Research Laboratories
17th March 2007 62
Differential Protection Example 2 15MVA 66kV / 11kV
150/5 P1
P2
S1
S2
A2
A1 a1
a2
800/5 P2 S2
P1 S1
Dy1
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 Thus, secondary current = 3 x 4.92 = 8.52A
RATIO CORRECTION IS REQUIRED © 2007 Protection Engineering And Research Laboratories
17th March 2007 63
Differential Protection Example 2 150/5 P1 P2 S1
A2
A1
a1
a2
S2
800/5 P2 P1 S1
??
S2
?? S2
S1
??
R
P1
P2
??
R R
© 2007 Protection Engineering And Research Laboratories
17th March 2007 64
Differential Protection Example 2 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 © 2007 Protection Engineering And Research Laboratories
17th March 2007 65
Differential Protection Example 2 150/5 P1 P2 S1
A2
A1
a1
a2
S2
800/5 P2 P1 S1
4.37A
S2
4.92A S2
S1
(2.56)
R
P1
P2
(5)
R R
© 2007 Protection Engineering And Research Laboratories
17th March 2007 66
Differential Protection Example 2 – Effect of Tap Changer 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 © 2007 Protection Engineering And Research Laboratories
17th March 2007 67
Differential Protection Example 2 – Vector Group Current distribution verification P1
S1
P2
A2
A1
a1
a2
P2
P1
S1
S2
S2
S1 P1
S2
P2
R R R
© 2007 Protection Engineering And Research Laboratories
17th March 2007 68
Differential Protection Example 2 – Vector Group Current distribution verification P1
S1
P2
A2
A1
a1
a2
P2
P1
S1
S2
S2
S1 P1
S2
P2
R R R
© 2007 Protection Engineering And Research Laboratories
17th March 2007 69
Differential Protection Example 2 – Vector Group Current distribution verification P1
S1
P2
A2
A1
a1
a2
P2
P1
S1
S2
S2
S1 P1
S2
P2
R R R
© 2007 Protection Engineering And Research Laboratories
17th March 2007 70
Differential Protection Example 2 – Vector Group Current distribution verification P1
S1
P2
A2
A1
a1
a2
P2
P1
S1
S2
S2
S1 P1
S2
P2
R R R
© 2007 Protection Engineering And Research Laboratories
17th March 2007 71
Combined REF and Differential Protection A2
A1 a1
a2
P1
P2
S1
P1
S1 REF
P2
S2
S2
P2 P1 S1
S2 To differential relay © 2007 Protection Engineering And Research Laboratories
17th March 2007 72
Virtual ICT in Numerical Relays Power transformer
Ratio correction
Vector correction
Virtual ICT
Differential element
© 2007 Protection Engineering And Research Laboratories
Virtual ICT
17th March 2007 73
Stability for Inrush Currents © 2007 Protection Engineering And Research Laboratories
17th March 2007 74
Transformer Magnetizing Characteristics
Twice Normal Flux
Normal Flux
Normal No Load Current
No Load Current at Twice Normal Flux
© 2007 Protection Engineering And Research Laboratories
17th March 2007 75
Transformer Magnetizing Current Steady State V + m
Im
- m
© 2007 Protection Engineering And Research Laboratories
17th March 2007 76
Magnetizing Inrush Current Switch on at voltage zero - No residual flux
Im 2m
V
© 2007 Protection Engineering And Research Laboratories
17th March 2007 77
Effect of Inrush Current on Differential Protection Effect of magnetising current 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 © 2007 Protection Engineering And Research Laboratories
17th March 2007 78
Effect of Inrush Current on Differential Protection Solution 1 : Time delay Allow magnetising current to die away before relay can operate Slows operation for genuine transformer faults
© 2007 Protection Engineering And Research Laboratories
17th March 2007 79
Effect of Inrush Current on Differential Protection 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
© 2007 Protection Engineering And Research Laboratories
17th March 2007 80
Effect of Inrush Current on Differential Protection Solution 3 : Gap measurement technique Inhibits relay operation during magnetising inrush Operate speed for genuine transformer faults unaffected by significant CT saturation
© 2007 Protection Engineering And Research Laboratories
17th March 2007 81
Gap Measurement Technique Typical Magnetising Inrush Waveforms
A B C
© 2007 Protection Engineering And Research Laboratories
17th March 2007 82
Gap Measurement Technique Bias Differential Threshold
Differential comparator
T1 = 5ms
T2 = 22ms Trip
© 2007 Protection Engineering And Research Laboratories
17th March 2007 83
Gap Measurement Technique Stability for Inrush Current Bias differential threshold
Differential comparator
Trip T1 = 5ms
T2 = 22ms
Differential input
Comparator output T1
Trip T2 Reset
© 2007 Protection Engineering And Research Laboratories
17th March 2007 84
Gap Measurement Technique Operation for Internal Faults Bias differential threshold
Differential comparator
Trip T1 = 5ms
T2 = 22ms
Differential input
Comparator output
T1 Trip T2 Reset © 2007 Protection Engineering And Research Laboratories
17th March 2007 85
Auto Transformer Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 86
Auto Transformer Protection High Impedance Differential (a) Earth Fault Scheme A B
C
87
© 2007 Protection Engineering And Research Laboratories
High impedance relay
17th March 2007 87
Auto Transformer Protection High Impedance Differential (b) Phase and Earth Fault Scheme A B C a b c 87 87 87
n © 2007 Protection Engineering And Research Laboratories
17th March 2007 88
Inter-turn Fault Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 89
Inter-turn Fault
E
CT Load Shorted turn
Nominal turns ratio Fault turns ratio Current ratio
- 11,000 / 240 - 11,000 / 1 - 1 / 11,000
Requires Buchholz relay © 2007 Protection Engineering And Research Laboratories
17th March 2007 90
Inter-turn Fault Inter-turn fault current Vs number of turns shorted 10
100 Fault current in short circuited turns
80
8
Fault current 60
6
(multiples of rated current)
Primary input current 40
4
20
2
0
5
10
15
20
Primary current (multiples of rated current)
25
Turns short-circuited (% of winding) © 2007 Protection Engineering And Research Laboratories
17th March 2007 91
Buchholz Relay Installation 3 x internal pipe Conservator diameter (minimum) 5 x internal pipe diameter (minimum)
Oil conservator 3 minimum Transformer
© 2007 Protection Engineering And Research Laboratories
17th March 2007 92
Buchholz Relay Petcock
Counter balance weight
Alarm bucket Mercury switch
Oil level
To oil conservator
From transformer
Trip bucket
Aperture adjuster Drain plug
Deflector plate
© 2007 Protection Engineering And Research Laboratories
17th March 2007 93
Over Flux Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 94
Over Fluxing in Transformers 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) © 2007 Protection Engineering And Research Laboratories
17th March 2007 95
Over Fluxing in Transformers Theory
V=kf Causes
2m m
Low frequency High voltage
Im
Geomagnetic disturbances
Effects Tripping of differential element (Transient over fluxing) Damage to transformers (Prolonged over fluxing) © 2007 Protection Engineering And Research Laboratories
17th March 2007 96
5th Harmonic Restraint Blocks differential element for transient over fluxing 25% Overvoltage condition
43% 5th Harmonic content © 2007 Protection Engineering And Research Laboratories
17th March 2007 97
Over Flux Protection 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
© 2007 Protection Engineering And Research Laboratories
17th March 2007 98
Over Flux Protection
V/f = k 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 © 2007 Protection Engineering And Research Laboratories
17th March 2007 99
Over Flux Protection Application Circuit breaker position repeat relay
RL1-1
AUX relay Lower AVR
DC
DC
Inhibit AVR raise Alarm RL2-1
RL2-2
Alarm
© 2007 Protection Engineering And Research Laboratories
Generator field circuit breaker trip coil
17th March 2007 100
Over Load Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 101
Transformer Overload and Life of Insulation 100
Relative rate of using life
With ambient of 20C, hot spot rise of 78 is design normal. A further rise of 6 C doubles rate of using life.
10
1.0 98 0.1 80
90 100 110 120 130 140 Hot spot temp
© 2007 Protection Engineering And Research Laboratories
C
17th March 2007 102
Over Heating Protection Trip
Iload TD setting
Top oil of power transformer
Alarm
On
Fan control
Iload Off
On
Pump control Off
Temp. indication
Heater Local Thermal replica
Temperature sensing resistor
© 2007 Protection Engineering And Research Laboratories
Remote
17th March 2007 103
Thermal Over Load Protection 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 © 2007 Protection Engineering And Research Laboratories
17th March 2007 104
Thank you © 2007 Protection Engineering And Research Laboratories
17th March 2007 105
Training on Power System Element Protection, & 17th March, 2007 at L&T Manappakam, Chennai.
Dr. G. Pradeep Kumar
Contents
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
© 2007 Protection Engineering And Research Laboratories
17th March 2007 2
Introduction © 2007 Protection Engineering And Research Laboratories
17th March 2007 3
Introduction Transformer Fault Categories
Winding and terminal faults Sustained or un-cleared external faults
Abnormal operating conditions such as overload, over voltage and over fluxing Core faults
© 2007 Protection Engineering And Research Laboratories
17th March 2007 4
Transformer Operation
I
V
A2
Ø
EP
A1
© 2007 Protection Engineering And Research Laboratories
17th March 2007 5
Transformer Operation
I
V
A2
EP
A1
© 2007 Protection Engineering And Research Laboratories
a2
ES
a1
17th March 2007 6
Transformer Operation
IP
V
A2
EP
A1
© 2007 Protection Engineering And Research Laboratories
a2
IS
ES
a1
17th March 2007 7
Transformer Connections a
A
C1
a2
A2
a1 c1
C
b1
A1
C2 B1
B2
b
B
© 2007 Protection Engineering And Research Laboratories
b2
c2 c
17th March 2007 8
Transformer Connections
“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
Line phase designation is same as winding
Example 1 : Dy11 Transformer
High Voltage Windings
How to connect the windings ?
A Phase Windings B Phase Windings C Phase Windings
A2 B2
C2 © 2007 Protection Engineering And Research Laboratories
Low Voltage Windings
A1
a1
a2
B1
b1
b2
C1
c1
c2
17th March 2007 9
Vector Group Example Dy11 12 11
30°
© 2007 Protection Engineering And Research Laboratories
17th March 2007 10
Vector Group Example Dy11 1. Draw Phase-Neutral Voltage Vectors
A
Line Designation a 30 ° b
C
B c
© 2007 Protection Engineering And Research Laboratories
17th March 2007 11
Vector Group Example Dy11 2. Draw Delta Connection A a
b
C
B
© 2007 Protection Engineering And Research Laboratories
c 17th March 2007 12
Vector Group Example Dy11 3. Draw A Phase Windings A a a2 A2
a1
b
A 1 C
B
© 2007 Protection Engineering And Research Laboratories
c 17th March 2007 13
Vector Group Example Dy11 4. Complete Connections A a C1
A2
a 2 a1
C 2 C
c 1
A 1 B 1
B 2
B
© 2007 Protection Engineering And Research Laboratories
b1
b2
b
c 2 c 17th March 2007 14
Vector Group Example Dy11 5. Winding representation on core
A
B
C
A 2 B 2 C2
A 1
a1
a2
B 1
b1
b2
C1
© 2007 Protection Engineering And Research Laboratories
c 1
c 2
a
b
c
17th March 2007 15
Over Current Protection of Transformer © 2007 Protection Engineering And Research Laboratories
17th March 2007 16
11kV Distribution Transformers Typical Fuse Ratings Transformer rating
Fuse
kVA
Full load current (A)
Rated current (A)
Operating time at 3 x rating(s)
100
5.25
16
3.0
200
10.5
25
3.0
300
15.8
36
10.0
500
26.2
50
20.0
1000
52.5
90
30.0
© 2007 Protection Engineering And Research Laboratories
17th March 2007 17
Distribution Transformers Protection 3.3kV 200/5
1500/5
51
50
51 N
50 N
1MVA 3.3/0.44kV
51 N
64
© 2007 Protection Engineering And Research Laboratories
1500/5
17th March 2007 18
Distribution Transformers Protection 11kV
51
50
64
1000/5
5MVA 11/3.3kV
51 N
64
1000/5
3.3kV © 2007 Protection Engineering And Research Laboratories
17th March 2007 19
Transformer Over Current Protection 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
© 2007 Protection Engineering And Research Laboratories
17th March 2007 20
Instantaneous Over Current Protection Source
LV
HV
50 51 50 set to 1.2 to 1.3 x through fault level © 2007 Protection Engineering And Research Laboratories
17th March 2007 21
Transient Overreach Concerns relay response to offset waveforms (DC transient)
Definition I1 - I2 I2
x 100
I2 I1
D.C
I1 = Steady state rms pick up current I2 = Fully offset rms pickup current
© 2007 Protection Engineering And Research Laboratories
17th March 2007 22
Instantaneous Over Current Protection HV
LV
Source
50
50
50 Inst O/C
51
51
51 Time O/C
© 2007 Protection Engineering And Research Laboratories
17th March 2007 23
Instantaneous Over Current Protection 50 set to 1.2 - 1.3 x through fault level
Relay “Behind” Transformer
Grading Margin
50 Setting © 2007 Protection Engineering And Research Laboratories
Max HV Fault Level (at Transformer) 17th March 2007 24
Instantaneous Over Current Protection
51
51/ 50
HV2
HV1
51
HV1
HV2
LV
Time LV
Current
IF(LV)
IF(HV)
1.2IF(LV) © 2007 Protection Engineering And Research Laboratories
17th March 2007 25
2-1-1 Distribution I 3Ø
I 3Ø
0.866I 3Ø I 3Ø
© 2007 Protection Engineering And Research Laboratories
17th March 2007 26
2-1-1 Distribution
HV relay
0.4 sec LV relay
0.866 I 3Ø © 2007 Protection Engineering And Research Laboratories
I 3Ø 17th March 2007 27
Parallel Transformers Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 28
Parallel Transformer Protection Directional Over Current
51 67
Grid supply
Feeders 51
67
51
51 © 2007 Protection Engineering And Research Laboratories
17th March 2007 29
Parallel Transformer Protection Non-directional Over Current
51 51
Grid supply
Bus Section
Feeders
51
51 © 2007 Protection Engineering And Research Laboratories
51 17th March 2007 30
Parallel Transformer Protection Partial Differential Grid supply
P1
P2
S1 S2
67
67 51
S2 S1 P2
P1
51 51
© 2007 Protection Engineering And Research Laboratories
51
17th March 2007 31
Earth Fault Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 32
Earth Fault in a “DY” Transformer Resistance Grounded 3 p.u. turns
x
IP
PR
1 p.u. turns
R IF
Resistor “R” limits earth fault current to load values
For a fault at “x”;
Fault current IF = x.IFL
Effective turns ratio is 3 : x
Line current on delta side for this fault IP= (x2/ 3).IFL
© 2007 Protection Engineering And Research Laboratories
17th March 2007 33
Over Current Protection on Delta Side of “DY” Transformer If as multiple of IF.L. 1.0 0.9
IF
Fault Current Star Side
0.8 0.7 0.6
51
0.5
Over Current Relay
0.4 0.3
Fault Current Delta Side
0.2 0.1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
© 2007 Protection Engineering And Research Laboratories
1.0
x p.u..
17th March 2007 34
Earth Fault in a “DY” Transformer Solidly Grounded If as multiple of IF.L. 10
Fault Current Star Side
9
IF
8 7 6 5 4 3
Fault Current Delta Side
2 1
x 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 © 2007 Protection Engineering And Research Laboratories
p.u..
17th March 2007 35
Earth Fault in a “YD” Transformer Solidly Grounded If as multiple of IF.L. IF
10
IF
9 8
7 6
IP
IN
IP
5 4 3
2 1
IN
0.1 0.2 0.3 © 2007 Protection Engineering And Research Laboratories
0.4 0.5
0.6 0.7 0.8 0.9 1.0
p.u..
17th March 2007 36
Earth Fault in a “DY” Transformer Resistance Grounded
x2 3 For relay operation, S
I
Differential Relay Setting = IS
x2 e.g. If S 20%, then 20% for operation 3 i.e. x 59% Thus 59% of Y winding is not protected
Differential Relay Setting 10% 20% 30% 40% 50% © 2007 Protection Engineering And Research Laboratories
% of Star Winding Protected 58% 41% 28% 17% 7% 17th March 2007 37
Un-restricted Earth Fault Protection
51N
51
51
51
Provides back-up protection for system Time delay required for co-ordination © 2007 Protection Engineering And Research Laboratories
17th March 2007 38
Un-restricted Earth Fault Protection
51N
51
51 51
51N
Can provide better sensitivity (C.T. ratio not related to full load current Improved “effective” setting)
Provides back up protection for transformer and system
© 2007 Protection Engineering And Research Laboratories
17th March 2007 39
Restricted Earth Fault Protection Star Winding
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
© 2007 Protection Engineering And Research Laboratories
17th March 2007 40
Restricted Earth Fault Protection Star Windings
A B C N LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated © 2007 Protection Engineering And Research Laboratories
17th March 2007 41
Restricted Earth Fault Protection Star Windings A B C N
LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated © 2007 Protection Engineering And Research Laboratories
17th March 2007 42
Restricted Earth Fault Protection Delta Winding
Source
REF
Protected zone
Delta winding cannot supply zero sequence current to system
Stability : Consider max LV fault level Recommended setting : less than 30% minimum earth fault level © 2007 Protection Engineering And Research Laboratories
17th March 2007 43
High Impedance Differential Protected Circuit ZM
RCT
RCT RL
IF
RL
IS
VS
RL
ZM
RST R
RL
Voltage across relay circuit VS = IF (RCT + 2 RL)
Stabilizing resistor (RST) used to limit spill current in the relay
path to less than the relay setting (IS)
RST = (VS/IS) – RR
VK > 2VS = 2 IF (RCT + 2 RL)
© 2007 Protection Engineering And Research Laboratories
17th March 2007 44
High Impedance Differential Non-linear Resistors
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 VP is given by an approximate formula (based on experimental results) VP = 22 [VK (VF - VK)]
Where VF = Max. prospective voltage in the absence of saturation. (= IFmax . Relay circuit total impedance)
Metrosil required if VP > 3kV
© 2007 Protection Engineering And Research Laboratories
17th March 2007 45
Restricted Earth Fault Protection Example 1 1MVA (5%) 11000V 415V
1600/1 RCT = 4.9
80MVA
Calculate : 1)
Setting voltage (VS)
2)
Value of stabilising resistor required
1600/1 RCT = 4.8
RS
3)
Effective setting
4)
Peak voltage developed by CT‟s for internal fault
IS = 0.1 Amp
2 Core 7/0.67mm (7.41/km) 100m Long © 2007 Protection Engineering And Research Laboratories
17th March 2007 46
Restricted Earth Fault Protection Example 1 Earth fault calculation :Using 80MVA base Source impedance = 1 p.u. 1 P.U.
Transformer impedance = 0.05 x 80 = 4 p.u. 1 1 4 I1
Total impedance = 14 p.u. I1 =
4 I2
1 = 0.0714 p.u. 14
Base current = 80 x 106 3 x 415
= 111296 Amps 4 Sequence Diagram
IF I0
© 2007 Protection Engineering And Research Laboratories
= 3 x 0.0714 x 111296 = 23840 Amps (primary) = 14.9 Amps (secondary) 17th March 2007 47
Restricted Earth Fault Protection Example 1 (1) Setting voltage
VS = IF (RCT + 2RL) Assuming “earth” CT saturates, RCT = 4.8 ohms 2RL = 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 (RST) RS = (VS/ IS) – (1/IS2)
Where IS = relay current setting
RS = (93.6/0.1) – (1/ 0.12) = 836 ohms © 2007 Protection Engineering And Research Laboratories
17th March 2007 48
“B” Weber/m2 (Tesla) (multiply by Kv to obtain rms secondary volts)
Restricted Earth Fault Protection Example 1
1.6
Kv
1.2
Ki
Line & Neutral CT
158
0.341
Earth CT
236
0.275
0.8
0.4
0
0.04
0.08
0.12
“H” AT/mm (multiply by Ki to obtain total exciting current in Amps) © 2007 Protection Engineering And Research Laboratories
17th March 2007 49
Restricted Earth Fault Protection Example 1 (3) Effective setting IP = CT ratio x (IS + IMAG) 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 © 2007 Protection Engineering And Research Laboratories
17th March 2007 50
Restricted Earth Fault Protection Example 1 Transformer full load current = 1391 Amps Effective setting = (198/1391) x 100% = 14.2% x rated 4)
Peak voltage = 22[VK (VF - VK)] VF = 14.9 x VS/IS = 14.9 x 936 = 13946 Volts For „Earth‟ CT, VK = 1.4 x 236 = 330 Volts (from graph) VP = 22 [330 (13946 - 330)] = 6kV Thus, non-linear resistor is to be used.
© 2007 Protection Engineering And Research Laboratories
17th March 2007 51
Restricted Earth Fault Protection Parallel Transformer T1
N A B C
Bus Section
T2
415 Volt Switchboard © 2007 Protection Engineering And Research Laboratories
17th March 2007 52
Parallel Transformer CT in Earth Circuit N A B C
T1
51N
Bus Section Open
T2
51N
© 2007 Protection Engineering And Research Laboratories
415 Volt Switchboard
17th March 2007 53
Parallel Transformer CT in Earth & Neutral Circuits T1
NA B C
51N
Bus section open
T2
51N 415 volt switchboard © 2007 Protection Engineering And Research Laboratories
17th March 2007 54
Parallel Transformer Residual CT Connection N A B C
T1
F2
Bus section Will mal-operate if bus section is open for fault at F1 T2
F1 415 volt switchboard No mal-operation for fault at F2 (but setting must be greater than load neutral current) © 2007 Protection Engineering And Research Laboratories
17th March 2007 55
Biased Differential Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 56
Differential Protection
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.
© 2007 Protection Engineering And Research Laboratories
17th March 2007 57
Biased Differential Protection Differential Current
I1
BIAS
BIAS
I1 - I2
OPERATE
I2 RESTRAIN
OPERATE
I1 - I2 (I1+I2)/ 2
Mean Thro‟ Current
Bias = Differential current / Mean through current © 2007 Protection Engineering And Research Laboratories
17th March 2007 58
Biased Differential Protection PROTECTED ZONE HV
LV
R
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
© 2007 Protection Engineering And Research Laboratories
17th March 2007 59
Differential Protection CT Connections P1
P2
A2
A1
A
a1
B
© 2007 Protection Engineering And Research Laboratories
a2
P2
P1
C 17th March 2007 60
Differential Protection Interposing CT Connections P1 S1
P2
A2
A1
a1
a2
P2
S2
S1
S2
S1 P1
P2
R R R
© 2007 Protection Engineering And Research Laboratories
P1 S2
Interposing CT provides
Vector correction
Ratio correction
Zero sequence compensation
17th March 2007 61
Differential Protection Example 2 15MVA 66kV / 11kV
150/5 P1
P2
S1
S2
A2
A1 a1
a2
800/5 P2 S2
P1
S1
Dy1
For the application shown above, consider (1)
Winding full load current
(2)
Effect of tap changer (if any)
(3)
C.T. polarities
© 2007 Protection Engineering And Research Laboratories
17th March 2007 62
Differential Protection Example 2 15MVA 66kV / 11kV
150/5 P1
P2
S1
S2
A2
A1 a1
a2
800/5 P2 S2
P1 S1
Dy1
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 Thus, secondary current = 3 x 4.92 = 8.52A
RATIO CORRECTION IS REQUIRED © 2007 Protection Engineering And Research Laboratories
17th March 2007 63
Differential Protection Example 2 150/5 P1 P2 S1
A2
A1
a1
a2
S2
800/5 P2 P1 S1
??
S2
?? S2
S1
??
R
P1
P2
??
R R
© 2007 Protection Engineering And Research Laboratories
17th March 2007 64
Differential Protection Example 2 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 © 2007 Protection Engineering And Research Laboratories
17th March 2007 65
Differential Protection Example 2 150/5 P1 P2 S1
A2
A1
a1
a2
S2
800/5 P2 P1 S1
4.37A
S2
4.92A S2
S1
(2.56)
R
P1
P2
(5)
R R
© 2007 Protection Engineering And Research Laboratories
17th March 2007 66
Differential Protection Example 2 – Effect of Tap Changer 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 © 2007 Protection Engineering And Research Laboratories
17th March 2007 67
Differential Protection Example 2 – Vector Group Current distribution verification P1
S1
P2
A2
A1
a1
a2
P2
P1
S1
S2
S2
S1 P1
S2
P2
R R R
© 2007 Protection Engineering And Research Laboratories
17th March 2007 68
Differential Protection Example 2 – Vector Group Current distribution verification P1
S1
P2
A2
A1
a1
a2
P2
P1
S1
S2
S2
S1 P1
S2
P2
R R R
© 2007 Protection Engineering And Research Laboratories
17th March 2007 69
Differential Protection Example 2 – Vector Group Current distribution verification P1
S1
P2
A2
A1
a1
a2
P2
P1
S1
S2
S2
S1 P1
S2
P2
R R R
© 2007 Protection Engineering And Research Laboratories
17th March 2007 70
Differential Protection Example 2 – Vector Group Current distribution verification P1
S1
P2
A2
A1
a1
a2
P2
P1
S1
S2
S2
S1 P1
S2
P2
R R R
© 2007 Protection Engineering And Research Laboratories
17th March 2007 71
Combined REF and Differential Protection A2
A1 a1
a2
P1
P2
S1
P1
S1 REF
P2
S2
S2
P2 P1 S1
S2 To differential relay © 2007 Protection Engineering And Research Laboratories
17th March 2007 72
Virtual ICT in Numerical Relays Power transformer
Ratio correction
Vector correction
Virtual ICT
Differential element
© 2007 Protection Engineering And Research Laboratories
Virtual ICT
17th March 2007 73
Stability for Inrush Currents © 2007 Protection Engineering And Research Laboratories
17th March 2007 74
Transformer Magnetizing Characteristics
Twice Normal Flux
Normal Flux
Normal No Load Current
No Load Current at Twice Normal Flux
© 2007 Protection Engineering And Research Laboratories
17th March 2007 75
Transformer Magnetizing Current Steady State V + m
Im
- m
© 2007 Protection Engineering And Research Laboratories
17th March 2007 76
Magnetizing Inrush Current Switch on at voltage zero - No residual flux
Im 2m
V
© 2007 Protection Engineering And Research Laboratories
17th March 2007 77
Effect of Inrush Current on Differential Protection Effect of magnetising current 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 © 2007 Protection Engineering And Research Laboratories
17th March 2007 78
Effect of Inrush Current on Differential Protection Solution 1 : Time delay Allow magnetising current to die away before relay can operate Slows operation for genuine transformer faults
© 2007 Protection Engineering And Research Laboratories
17th March 2007 79
Effect of Inrush Current on Differential Protection 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
© 2007 Protection Engineering And Research Laboratories
17th March 2007 80
Effect of Inrush Current on Differential Protection Solution 3 : Gap measurement technique Inhibits relay operation during magnetising inrush Operate speed for genuine transformer faults unaffected by significant CT saturation
© 2007 Protection Engineering And Research Laboratories
17th March 2007 81
Gap Measurement Technique Typical Magnetising Inrush Waveforms
A B C
© 2007 Protection Engineering And Research Laboratories
17th March 2007 82
Gap Measurement Technique Bias Differential Threshold
Differential comparator
T1 = 5ms
T2 = 22ms Trip
© 2007 Protection Engineering And Research Laboratories
17th March 2007 83
Gap Measurement Technique Stability for Inrush Current Bias differential threshold
Differential comparator
Trip T1 = 5ms
T2 = 22ms
Differential input
Comparator output T1
Trip T2 Reset
© 2007 Protection Engineering And Research Laboratories
17th March 2007 84
Gap Measurement Technique Operation for Internal Faults Bias differential threshold
Differential comparator
Trip T1 = 5ms
T2 = 22ms
Differential input
Comparator output
T1 Trip T2 Reset © 2007 Protection Engineering And Research Laboratories
17th March 2007 85
Auto Transformer Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 86
Auto Transformer Protection High Impedance Differential (a) Earth Fault Scheme A B
C
87
© 2007 Protection Engineering And Research Laboratories
High impedance relay
17th March 2007 87
Auto Transformer Protection High Impedance Differential (b) Phase and Earth Fault Scheme A B C a b c 87 87 87
n © 2007 Protection Engineering And Research Laboratories
17th March 2007 88
Inter-turn Fault Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 89
Inter-turn Fault
E
CT Load Shorted turn
Nominal turns ratio Fault turns ratio Current ratio
- 11,000 / 240 - 11,000 / 1 - 1 / 11,000
Requires Buchholz relay © 2007 Protection Engineering And Research Laboratories
17th March 2007 90
Inter-turn Fault Inter-turn fault current Vs number of turns shorted 10
100 Fault current in short circuited turns
80
8
Fault current 60
6
(multiples of rated current)
Primary input current 40
4
20
2
0
5
10
15
20
Primary current (multiples of rated current)
25
Turns short-circuited (% of winding) © 2007 Protection Engineering And Research Laboratories
17th March 2007 91
Buchholz Relay Installation 3 x internal pipe Conservator diameter (minimum) 5 x internal pipe diameter (minimum)
Oil conservator 3 minimum Transformer
© 2007 Protection Engineering And Research Laboratories
17th March 2007 92
Buchholz Relay Petcock
Counter balance weight
Alarm bucket Mercury switch
Oil level
To oil conservator
From transformer
Trip bucket
Aperture adjuster Drain plug
Deflector plate
© 2007 Protection Engineering And Research Laboratories
17th March 2007 93
Over Flux Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 94
Over Fluxing in Transformers 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) © 2007 Protection Engineering And Research Laboratories
17th March 2007 95
Over Fluxing in Transformers Theory
V=kf Causes
2m m
Low frequency High voltage
Im
Geomagnetic disturbances
Effects Tripping of differential element (Transient over fluxing) Damage to transformers (Prolonged over fluxing) © 2007 Protection Engineering And Research Laboratories
17th March 2007 96
5th Harmonic Restraint Blocks differential element for transient over fluxing 25% Overvoltage condition
43% 5th Harmonic content © 2007 Protection Engineering And Research Laboratories
17th March 2007 97
Over Flux Protection 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
© 2007 Protection Engineering And Research Laboratories
17th March 2007 98
Over Flux Protection
V/f = k 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 © 2007 Protection Engineering And Research Laboratories
17th March 2007 99
Over Flux Protection Application Circuit breaker position repeat relay
RL1-1
AUX relay Lower AVR
DC
DC
Inhibit AVR raise Alarm RL2-1
RL2-2
Alarm
© 2007 Protection Engineering And Research Laboratories
Generator field circuit breaker trip coil
17th March 2007 100
Over Load Protection © 2007 Protection Engineering And Research Laboratories
17th March 2007 101
Transformer Overload and Life of Insulation 100
Relative rate of using life
With ambient of 20C, hot spot rise of 78 is design normal. A further rise of 6 C doubles rate of using life.
10
1.0 98 0.1 80
90 100 110 120 130 140 Hot spot temp
© 2007 Protection Engineering And Research Laboratories
C
17th March 2007 102
Over Heating Protection Trip
Iload TD setting
Top oil of power transformer
Alarm
On
Fan control
Iload Off
On
Pump control Off
Temp. indication
Heater Local Thermal replica
Temperature sensing resistor
© 2007 Protection Engineering And Research Laboratories
Remote
17th March 2007 103
Thermal Over Load Protection 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 © 2007 Protection Engineering And Research Laboratories
17th March 2007 104
Thank you © 2007 Protection Engineering And Research Laboratories
17th March 2007 105
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