transformer protection calculations
TRANSCRIPT
8/10/2019 Transformer Protection Calculations
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SLIDE 1
Transformer
Protection
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SLIDE 2
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SLIDE 3
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SLIDE 4
maximum fuse rating or
circuit breaker setting
125% of full load current
480
240/120V
set ≤ 2·5Ifl
set ≤ 1·25Ifl
Maximum fuse rating
or circuit breaker
setting 125% of fullload current
Set≤
2.5 I fl
Set ≤ 1.25 I fl
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SLIDE 5
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SLIDE 6
fuse rating
< 2·5Ifl
Relay set < 3IflSI
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SLIDE 7
r
y
b
R Y B
r
y
b
Differential Protection
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SLIDE 8
When applying relays for the protection of
transformers, the following points must be
considered:
•
Line currents present on either side
of star delta transformers will differ in
magnitude and/or phase.
• Any sudden change in the
energisation of the transformer will
produce an inrush magnetising current into
or out of one winding. This can occur when
energising a previously disconnectedtransformer or when a sudden appreciable
drop in transformer load occurs.
• Delta connected windings do not
provide a path for zero sequence currents
during earth fault conditions.
• Tap changers used for voltage
regulation purposes cause the turns ratioand hence the voltage and current
magnitudes to vary.
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SLIDE 9
Magnetising Inrush for a Small Transformer
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SLIDE 10
Relaypoint
1
2
1
1
1
0
Relaypoint
Fault current flow in transformers
Generator Relay
0
point
1
1
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SLIDE 11
External fault
Stability
Internal fault
Operation
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SLIDE 12
Current Transformer Connections and Secondary Current Flow
External fault: maloperation of relay
Consider and modify connections for correct operation
Internal Fault: Operation of relay
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SLIDE 13
10
9
8
7
6
5
4
3
2
1
2 4 6 8 10 12 14 16 18 20
Bias Setting
40%
30%
20%
Operating
current I2 (pu)
Bias current I1 (pu)
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SLIDE 14
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SLIDE 17
Winding arrangement & current distribution
3I
I
I
2I
I I
I
3I
I
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SLIDE 18
C.T. Connections
O.C. Relays
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SLIDE 19
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SLIDE 20
87
ZEBZEA
A B
Vf
R ACT RLA RLB RBCT
RR
If
IR
High impedance differential
(Principle of operation)
Saturation has the effect of lowering the excitation
impedance and is deemed to have taken place in CT Buntil, at the limit, the shunt impedance becomes zero and
the CT has no output.
Vf = If (RLB + RB CT)
Current through relay IR = Vf / (RR + RLB + RB CT)
If RR is small IR ≈ If which is not acceptable
If RR is large IR is reduced and without incurring
signif icant error
IR can be reduced below any specified relay setting
RR
)CTBRLBR(.f I
RRf V
RI
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SLIDE 21
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SLIDE 22
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SLIDE 23
Percentage of Transformer Winding
Protected Against Earth Faults
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SLIDE 24
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SLIDE 25
Buchholz Relay
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SLIDE 26
(3)
( 3 )
86
87
52 - 2
**
63
49
*
52-1
HV bus
151 G
51 G
connection
51
( 3 )
trips 52 - 2
trips 52 - 1
( 2 )
( 1 )
(1)
50G
connection
(3)
50/51/51N
(1)50N
Protection diagram
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SLIDE 27
R
Y
B
HV CTs
LV CTs
Auto transformer
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SLIDE 28
Operating
Bias coils
coil
Three-winding transformer
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SLIDE 29
B
POWER TRANSFORMER
B
R
r
R
y
Differential Protection
B B
E / F Relay
Core Balance CT
R
b
B B
E / F Relay
EarthingTransformer
Aux. CT
Biased dif ferential & earth fault protection
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SLIDE 30
600/1
13.8 kV 132 kV
EF
displacement
Circuit breaker
Winding temp.
WT
Trip coilTC
CB
Restricted earth
SEF
Neutral
Standby
NDearthfault
leakage
Earth FaultREL
NDR
TR
20/1
Trip Relay
BTBucholz trip
OvercurrentOC
TR
CB6000/20
1000/5
B.T. W.T.
144 MVA
RRR protectiondifferential
SEF
R
R
REL
OC / EF
R
R R TC
CB
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SLIDE 31
Differential Protection with Supplementary
Earth Fault Protection
Interposing CTs
Ratio 1 / 0.577
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SLIDE 32
Typical Arrangement of Differential Protectionwith Supplementary Earth Fault Protection
Interposing CTs
Ratio 1 / 0.577
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SLIDE 33
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SLIDE 34
Differential Characterist ics
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SLIDE 35
(a)
(b)
Transformer Feeder Circuits
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SLIDE 36
Restricted earth fault
Tripping relay
_
N O
+
+
_
B
Y
R
B T
_FTS
W TB G
H.V. Balanced
E/F often used
_
Directional o/c & E/F
_
R Y B
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SLIDE 37
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SLIDE 40
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SLIDE 41
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SLIDE 42
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SLIDE 43
11kV
1.8A
200 / 1875A
1750A
875A
R
8.8A
4.4A
0.9A
66kV
4.4A
1500 / 0.577
9090A
9090A
Y B
7.0A
3.5A
0.9A
3.5A
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r
y
b
R Y B
r
y
b