Transformer protection.ppt
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TRANSFORMER PROTECTION Target Audiance
• Electrical Engineers • Protection Engineers • Test & Commissioning Engineers • Power System Engineers • Utility Engineers • Electrical Technicians
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TRANSFORMER PROTECTION Introduction Transformers are a critical and expensive component of the power system. Due to long lead time for repair and replacement of transformers, a major goal of transformer protection is limiting the damage to a faulted transformer. The comprehensive transformer protection provided by multiple function protective relays is appropriate for critical transformers of all applications.
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TRANSFORMER PROTECTION
The type of protection for the transformers varies depending on the application and importance of the transformer. Transformers are protected primarily against faults and overloads. The type of protection used should minimize the time of disconnection for faults within the transformer and to reduce the risk of catastrophic failure to simplify eventual repair.
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TRANSFORMER PROTECTION
Any extended operation of the transformer under abnormal condition such as faults or overloads compromises the life of the transformer, which means adequate protection should be provided for quicker isolation of the transformer under such condition.
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TYPES OF FAULTS
The various possible transformer faults are, 1. Overheating 2. Winding faults 3. Open circuits 4. External faults 5. Over fluxing
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OVERHEATING
They are due to Sustained overloads and short circuits Failure of cooling system Protection The thermal overload relays and temperature relays, sounding the alarm are used The thermocouples or resistance temperature indicators are also provided near the winding. These are connected in a bridge circuits. When the bridge balance gets disturbed for more than the permissible duration circuit breaker trips. 11
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WINDING FAULTS The winding faults are called internal faults. The overheating or mechanical shocks deteriorates the winding insulation. If the winding insulation-is weak, there is a possibility of Phase to phase faults Earth faults Inter turn faults Persistent fault would have a possibility of oil-fire. PROTECTION Differential protection Over current protection is also used as a backup protection For earth fault protection, the restricted earth fault protection system, neutral current relays or leakage to frame protection system is used.
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OPEN CIRCUIT
The open circuit in one of the three phases causes the undesirable heating of the transformer. Open circuits are much harmless compared to other faults. Transformers can be manually disconnected from the system during such faults.
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THROUGH FAULTS Through faults are the external faults which occur outside the protected zone. These are not detected by the differential protection. If faults persists the transformer gets subjected to thermal and mechanical stresses which can damage the transformer. PROTECTION The overcurrent relays with undervoltage blocking, zero sequence protection and negative sequence protection are used to give protection The setting of the overcurrent protection not only protects transformer but also covers the station busbar and portion of a transmission line.
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OVERFLUXING The flux density in the transformer core is proportional to the ratio of the voltage to frequency i.e. V/f Transformers are designed to work with certain value of flux density in the core. In the generator transformer unit, if full excitation is applied before generator reaches its synchronous speed then due to high V/f the overfluxing of core may occur. PROTECTION V/f relay called volts/hertz relay is provided to give the protection against overfluxing operation.
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Other faults Tap changer faults High voltage surges due to lightning Switching surges Incipient faults
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Importance of Protection Detect and isolate fault Maintain system stability To limit the damage in adjacent equipment Minimize fire risk Minimize risk to personnel
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Transformer Connections
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Transformer Connections a a2
A C1
A2
C2 C B1
A B C
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A2 B2
C2
A1 B1 C1
> Transformer Protection
a1 b1 c1
c1
A1 B2 B
a2
b2 c2
a1 b1
b2
b
c2 c a b
c
“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.
Transformer Vector Groups
Phase displacement
Yy0 Dd0 Zd0
Phase displacement
Yy6 Dd6 Dz6
Lag phase displacement
Yd1 Dy1 Yz1
Lead phase displacement
Yd11 Dy11 Yz11
Group 1 0 Group 2 180 Group 3 30 Group 4 30
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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 : Dy 11 Transformer High Voltage Windings A Phase Winding s B Phase Winding s C Phase Winding s
A2 B2 C2
Low Voltage Windings
a1
a2
B1
b1
b2
C1
c1
c2
A1
Question : How to connect windings ?
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Overcurrent Protection
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Transformer Overcurrent 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
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Use of Instantaneous Overcurrent Protection
HV
→
LV
50 51 50 set to 1.2 - 1.3 x through fault level 24
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Transient Overreach Concerns relay response to offset waveforms (DC transient) Definition
I1 - I2 x 100 I2
I2 I1
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I1 = Steady state rms pick up current I2 = Fully offset rms pickup current
Instantaneous High Set Overcurrent Relay Applied to a Transformer
5 1 LV
5 1 HV 1
5 1 HV 2
HV1 Tim e
HV2
LV
IF(LV)
IF(HV)
1.2IF(LV) 26
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Curren t
Current Distribution
I3Ø
I3Ø
0.866 I3Ø I3Ø
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HV relay
0.4 sec LV relay
0.866 I3Ø I3Ø 28
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Parallel Transformers Directional Relays (1)
51 Grid supply
67 Feeders
51
67 51 33
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51
Parallel Transformers Directional Relays (2)
51 Grid supply
51
Bus Section
Feeders
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51 34
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51
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Earth Fault Protection
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Transformer Earth Faults 3 p.u. turns 1 p.u. turns
x
IP
PR
Protective Relay
R
ResistorlimitsE/Fcurrentto fullloadvalues
IF
Thus,primarycurrent, P
2 . F.L. 3
For a fault at : Faultcurrent . F.L. Effectiveturnsratio 3 :
If C.T. ratio (on primary side) is based on full load current of transforme r, then C.T. secondary
2 circuit 3 37
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x .F.L. 3
Overcurrent Relay Sensitivity to Earth Faults (1) If as multiple of IF.L. 1.0 0.9
IF
0.8
Star Side
0.7 0.6
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0.5
Overcurrent Relay
0.4
Delta Side
0.3 0.2 0.1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Overcurrent Relay Setting > IF.L. 38
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p.u..
Overcurrent Relay Sensitivity to Earth Faults (2) If as multiple of IF.L. 10 9 8
IF
Star Side
7 6
51
5
Overcurrent Relay
4 3 2
Delta Side
1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 p.u..
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Overcurrent Relay Sensitivity to Earth Faults (3) If as multiple of IF.L. 10 IF
IF
9 8 7 6
IP
IN
IP
5 4 3 2
IN
1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
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p.u..
Earth Fault on Transformer Winding
I=
2 3
For relay operation, I > IS
I
e.g. If IS = 20%, then > 20% for operation 3 2
Differential Relay Setting = IS
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i.e. > 59% Thus 59% of
winding is not protected
Differential Relay Setting 10%
% of Star Winding Protected 58%
20%
41%
30%
28%
40%
17%
50%
7%
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Unrestricted Earthfault Protection (1)
51 N
5 1
Provides back-up protection for system Time delay required for co-ordination
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5 1
5 1
Unrestricted Earth fault Protection
51 N
51 N
5 1
5 1
5 1
Can provide better sensitivity (C.T. ratio not related to full load current) (Improved “effective” setting) Provides back up protection for transformer and system 43
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Star Winding REF
Protected Zone
RE F
Relay only operates for earth faults within protected zone. Uses high impedance principle. Stability level : usually maximum through fault level of transformer 44
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Restricted E/F Protection
A B C N
LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated
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Restricted E/F Protection
A B C N
LV restricted E/F protection trips both HV and LV breaker Recommended setting : 10% rated
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Delta Winding Restricted Earth Fault
Source
Protected zone REF
Delta winding cannot supply zero sequence current to system Stability : Consider max LV fault level Recommended setting : less than 30% minimum earth fault level 47
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High Impedance Principle Protected Circuit Z M
RCT
RCT RL
IF
M
IS
VS RL
RL
Z
RS RT
Voltage Across Relay Circuit
RL
VS = IF (RCT + 2RL)
Stabilising resistor RST limits spill current to IS (relay setting) VS RST = - RR where RR = relay burden IS CT knee point VKP = 2VS = 2IF (RCT + 2RL) 48
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The Use Of Non-Linear Resistors (Metrosils)
During internal faults the high impedance relay circuit constitutes an excessive burden to the CT’s. A very high voltage develops across the relay circuit and the CT’s. Causes 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 = 2 2VK (VF - VK) Where VF = Swgr. Fault Rating in amps x Z of relay circuit CT ratio @ setting Metrosil required if VP > 3kV 49
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Non-Linear Resistors (Metrosils) IOP RST VM
VS RR
Metrosil Characteristic V = CI
Suitable values of C & chosen based on : Max secondary current under fault conditions Relay setting voltage 50
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REF Protection Example
1MVA (5%) 11000V 415V
1600/1 RCT = 4.9
Calculate : 1) Setting voltage (VS)
80MV A
2) Value of stabilising resistor required 3) Effective setting 1600/1 RCT = 4.8
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RS
MCAG14 IS = 0.1 Amp 2 Core 7/0.67mm (7.41/km) 100m Long
4) Peak voltage developed by CT’s for internal fault
Solution (1) Earth fault calculation :Using 80MVA base Source impedance = 1 p.u. Transformer impedance = 0.05 x 80 = 4 p.u. 1 p.u.
1 P.U.
Total impedance = 14 1
4 I1 = 1 = 0.0714 p.u. 14
I1
Base current = 80 x 106 3 x 415
4
= 111296 Amps
I2 IF 4 I0 Sequence Diagram 52
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= 3 x 0.0714 x 111296 = 23840 Amps (primary) = 14.9 Amps (secondary)
Solution (2)
(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 = 14.9 (4.8 + 1.482) = 93.6 Volts
(2) Stabilising Resistor (RS) RS = VS - 1 IS IS2
Where IS = relay current setting
RS = 93.6 - 1 = 836 ohms 0.1 0.12
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Solution (4)
Peak voltage = 22VK (VF - VK) VF = 14.9 x VS = 14.9 x 936 = 13946 Volts IS For ‘Earth’ CT, VK = 1.4 x 236 = 330 Volts (from graph) VPEAK = 22 x 330 x (13946 - 330) = 6kV Thus, Metrosil voltage limiter will be required.
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Traditional Large Transformer Protection Package 33K V
200/5
51 5 0
10MVA 33/11KV 600/5
51 N
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600/5 5/5A
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Differential Protection
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Differential Protection Overall 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. It is essential for transformers with tap changing facility. Another important requirement of transformer differential protection is immunity to magnetising in rush current. 57
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Biased Differential Scheme
Differential Current
I1
BIAS
OPERATE
BIAS I 2
I1 I2
OPERATE
I1 I2
RESTRAIN
I1 + I2 2
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Mean Through Current
Differential Protection
HV
PROTECTED ZONE
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. Consider : (1) (2) (3) 59
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Difference in current magnitude Phase shift Zero sequence currents
Differential Connections And polarity markings
P1
P2
A2
A1
A
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a1
B
a2
C
P2
P1
Use of Interposing CT P1
S1
P2
A2
A1 a1
P1
S2
R
R
S1 P1
R
Interposing CT provides : Vector correction Ratio correction Zero sequence compensation > Transformer Protection
P2
S2
S2
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a2
P2
S1
Interposing current transformers
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Interposing current transformers
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Interposing current transformers
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Differential Protection 150/5 P2 P1 S1
15MVA 66kV / 11kV A2
A1 a1
a2
800/5 P2 P1 S2
S2
S1
Dy1
Given above: Need to consider (1) Winding full load current (2) Effect of tap changer (if any) (3) C.T. polarities Assuming no tap changer Full load currents:66kV: 131 Amp = 4.37 Amps secondary 11kV: 787 Amp = 4.92 Amps secondary However, require 11kV C.T.’s to be connected in Thus, secondary current = 3 x 4.92 = 8.52A
RATIO CORRECTION IS REQUIRED 81
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Differential Protection 800/5
150/5 P1 S1
P2
A2
A1 a1
a2
P2
P1
S2
S2
4.37A
4.92A
S1
S2 P1
(2.56)
R
S1
P2
(5)
R R
It is usual to connect 11kV C.T.’s in and utilise a / interposing C.T. (this method reduces lead VA burden on the line C.T.’s) Current from 66kV side = 4.37 Amp Thus, current required from winding of int. C.T. = 4.37 Amp Current input to winding of int. C.T. = 4.92 Amp Required int C.T. ratio = 4.92 / 4.37 = 4.92 / 2.52 3 May also be expressed as : 5 / 2.56 82
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Effect of Tap Changer
e.g. 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 Compared with 5 / 2.56 based on nominal voltage
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Combined Differential and Restricted Earth Fault Protection A1 a1
A2
a2
P1
P2
S1
P1
P2
S1 S2
REF
S2
P2 P1 S1 S2
To differential relay 84
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Three Winding Transformer 63MV A 132KV
300/5
25MV A 11KV
1600/5
50MV A 33KV
1000/5 4.59
5.51
10.33
2.88
5
2.88
5
All interposing C.T. ratio’s refer to common MVA base (63MVA) 85
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Traditional Use of Interposing CT Dy1(-30°)
Yd11(+30°)
R
R R
Interposing CT provides : Vector correction Ratio correction Zero sequence compensation 86
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Integral Vectorial and Ratio Compensation Power transformer
Numeric Relay
Ratio correction
Vectorial correction Virtual interposing CT
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Differential element
Virtual interposing CT
Transformer Magnetising Characteristic Twice Normal Flux
Normal Flux
Normal No Load Current No Load Current at Twice Normal Flux 88
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Magnetising Inrush Current Steady State
+ m
V Im
- m
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Magnetising Inrush Current Switch on at Voltage Zero - No residual flux
Im
2m
V
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Transformer 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
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Transformer Differential Protection Effect of Magnetising Inrush
SOLUTION 1 : TIME DELAY
Allows magnetising current to die away before relay can operate
Slow operation for genuine transformer faults
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Transformer Differential Protection Effect of Magnetising Inrush
SOLUTION 2 : 2ND (and 5TH) HARMONIC RESTRAINT
Makes relay immune to magnetising inrush Slower operation may result for genuine transformer faults if CT saturation occurs
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Typical Magnetising Inrush Waveforms
A B C
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Inter-Turn Fault Protection
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Inter-Turn Fault
CT E Load Shorted turn
Nominal turns ratio Fault turns ratio Current ratio
- 11,000 / 240 - 11,000 / 1 - 1 / 11,000
Requires Buchholz relay
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Interturn Fault Current / Number of Turns Short Circuited 100
10 Fault current in short circuited turns
80 Fault current (multiples of rated current)
8
60
6 Primary input current
40
4
20
2
0
5
10
15
20
25
Turn short-circuited (percentage of winding)
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Primary current (multiples of rated current)
Non-electrical Protection Buchholz Protection Pressure Relief Protection Sudden Pressure Protection Winding Temperature Protection Oil Temperature Protection Oil Level Abnormal Protection
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Buchholz Protection
The function of the relay is to detect an abnormal condition within the tank and send an alarm or trip signal.
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Buchholz operate when:
Gas produced from the transformer due to fault An oil surge from the tank to the conservator A complete loss of oil from the conservator (very low oil level)
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Pressure Relief Device
Transformers’ last line of defense against internal pressure
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Sudden Pressure Protection
Used to detect sudden increase in gas pressure caused by internal arcing Set to operate before pressure relief device
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Oil Temperature Protection
Typically sealed spiral-bourdon-tube dial indicators with liquidfilled bulb sensor
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Buchholz Relay Installation
3 x internal pipe diameter (minimum)
Conservator
5 x internal pipe diameter (minimum)
Oil conservator 3 minimum Transformer
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Buchholz Relay
Petcock Alarm bucket
Counter balance weight
Mercury switch
Oil level
To oil conservator
From transformer
Trip bucket
Aperture adjuster Drain plug
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Deflector plate
Overfluxing Protection
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Transformer protection
Over fluxing protection Over fluxing arises principally from the following system conditions. High system voltage Low system frequency Geomagnetic disturbances The latter result in low frequency earth currents circulating through a transmission system. 107
> Transformer Protection
Overfluxing Generator transformers 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 overfluxing :
Increase in magnetising current Increase in winding temperature Increase in noise and vibration Overheating of laminations and metal parts (caused by stray flux)
Protective relay responds to V/f ratio
Stage 1 - lower A.V.R. Stage 2 - Trip field
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Overfluxing Basic Theory
V = kf
2m
m
CAUSES
Low frequency High voltage Geomagnetic disturbances EFFECTS
Tripping of differential element (Transient overfluxing) Damage to transformers (Prolonged overfluxing)
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Ie
V/Hz Overfluxing Protection
V K f Trip and alarm outputs for clearing prolonged overfluxing Alarm : Definite time characteristic to initiate corrective action Trip : IDMT or DT characteristic to clear overfluxing 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
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> Transformer Protection
V/Hz Characteristics Enables co-ordination with plant withstand characteristics
t = 0.8 + 0.18 x K (M - 1)2 1000 K = 63 K = 40 K = 20 K=5 K=1
100 Operating time (s)
10 1
1
1.1
1.2
1.3
1.4
M = V Hz Setting 111
> Transformer Protection
1.5
1.6
Thermal Overload Protection
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Over temperature Generally regarded as overload protection also deals with failure of or interference with pumps and fans or shutting of valves to pumps Winding hot spot temperature is the main issue, but both oil and winding temperature are usually measured and used to: … initiate an alarm … trip circuit breakers … control fans and pumps
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> Transformer Protection
Over temperature Two temperatures must be monitored:
Winding temperature (‘WTI’) -(short thermal τ) this can rise rapidly, without much of an increase in oil temperature
Oil temperature (‘OTI’) -(long thermal τ) this can rise slowly to a critical point without an unacceptable winding temperature increase
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> Transformer Protection
Typical alarm and trip levels
winding alarm - 90ºC to 110ºC „ winding trip - 110ºC to 135ºC „ oil alarm - 80ºC to 95ºC „ oil trip - 95ºC to 115ºC 115
> Transformer Protection
Overheating Protection
I load
Alarm
TD setting Top oil of power transformer
Trip
I load
On
Fan control Off On
Pump control
Off
Heater
Temp. indication Local
Thermal replica
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> Transformer Protection
Temperature sensing resistor
Remote
Typical Transformer Protection to Feeders
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> Transformer Protection
Protection of Parallel Transformer Feeders
Higher voltage busbar Z
OC
OC
Z
FTS
FTS REF
REF
DP
DP Bh
WT
WT
Bh
REF
2
1
DOC
REF
SBEF 2 stage
SBEF 2 stage
OC
OC
Load Load 118
> Transformer Protection
Lower voltage busbar
1
DOC
2
Transformer Feeders FEEDE R
PW
PILOT S
P W
For use where no breaker separates the transformer from the feeder. Transformer inrush current must be considered. Inrush is a transient condition which may occur at the instant of transformer energisation. Mag. Inrush current is not a fault condition Protection must remain stable. MCTH provides a blocking signal in the presence of inrush current and allows protection to be used on transformer feeders. 119
> Transformer Protection
Transformer Feeder Protection Dy11 A B
P1 S1
P2 S2
C
23 24 25 26 27 28
I
i
II
ii
III
iii
MCTH
MBCI
27 28
MFAC 14
19 17
17
PILOTS
18 19
18 17 19 1
+ MVTW 02
3
13 14
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> Transformer Protection
S1
A B C
17
19
P1 S2
MCTH 23
17
23 24 25 26 27 28
P2
MBCI 18
24 25 26 27 28 23 24 25 26 27 28
Protection of Transformer Feeders
Power transformer
P540 Scheme
Ratio correction Vectorial correction
Virtual interposing CT
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> Transformer Protection
Virtual interposing CT
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