06_transformerprotection
TRANSCRIPT
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TransformerProtection
Protection ApplicationHandbook
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Transformer protectionPower transformers in a power system
400 kV AC Transmission
Generation
130 kV Subtransmission
Distribution
M
MV
LV
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Transformer protection
General
A Power Transformer is a very valuable and vitallink in a Power Transmission system
Many sizes and types of power transformer
Transformer faults may cause substantial losses
.The repair time may be long
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Transformer protection
General
A fast protection system for transformer isessential to minimize the damage in case of aninternal fault with suitable back-up protection
scheme to take care of uncleared system faults
Faults in a Transformer occur due to insulation
breakdown, ageing of insulation, overheating dueto over excitation, oil contamination and leakageor reduced cooling
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Transformer protection
Different vector group connections and ways of neutral connection
Y d + z
YN d11 + zn
Y y
YN yn0 (d1)
Y d
YN d1
D y
D yn11
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Transformer protectionDifferent winding arrangements
Autotransformer+ tertiary
e.g.. YN Auto d1
2-winding transf.
+ Auxiliarytransformer
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Transformer protection
Particular power transformer applications
HVDC
Static VARCompensator
M
Motor drive
UV
On- loadtap-changer
Generator-transformer
unit
x x
Double breaker
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Transformer protection
Overload Capability
It is possible to overload power transformers
Older transformers may withstand 140% continuously
Overloading and loss of cooling causes overheating
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Transformer protection
Withstand capabilty for External Faults
System Shunt Faults
Transformer
reactances
Fault current Permitted fault
duration
% multiple of ratedcurrent seconds
4 25 2
5 20 3
6 16.6 47 14.2 5
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Transformer protection
Types of Internal Faults
Earth faults
Short-circuits
Inter turn Faults
Core Faults Tank Faults
Reduced cooling
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Transformer protection
Abnormal Conditions
Overload
Over voltage
Reduced system voltage
Overexcitation
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Transformer protectionDifferential Protection
Classical application of differential protection
Interposing current transformers for phase shift andtransformation ratio correction
Protection zone, bushing CTs
Influence from OLTC Harmonic restrained differential protections
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Transformer protectionTransformer differential protection. Current bias characteristics
I 1
I2
Id min
1 5 10
1
5
10
I
High
Trips
Blocks
Differential current |I1+I2 |
Unrestrained
< 25
Irestrain(|I1 |+ |I2 |)/2
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Transformer protection
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Transformer protection
Inrush Current
Inrush currents flow in connection with energisation
Inrush currents may also flow at voltage recovery
Sympathy inrush current
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Transformer protection
Inrush Current
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Transformer protection
Factors that Affect the Inrush Current
The source impedance
The size of the transformer
The location energized winding
The point of wave when the switch closes The magnetic properties of the core
The remanence of the core
The use of pre-insertion resistors
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Transformer protectionConsequences of Inrush Current
The inrush current has a large DC component that may
saturate the CTs
DC saturation of phase and neutral CTs may cause afalse residual current and a false differential current
There is a risk that sensitive differential protections,residual overcurrent protections and neutral pointovercurrent protections may operate incorrectly
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Transformer protection
Magnetizing current at overexcitation
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Transformer protection
Differential protection for
Autotransformer
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Transformer protectionGround Fault Currents
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Transformer protectionEarth Fault Protection
Restricted earth fault protection
High-impedance restricted earth fault protections Low-impedance restricted earth fault protections
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Transformer protection
Restricted Earth fault protection
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Transformer protectionThermal overload protection
Time
0,63
Rise of temperature
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Transformer protection
Over excitation protection
Overexcited transformers become overheatedand damaged
Especially required for Generator Transformers
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Transformer protection
Overexcitation Protection
iE= 4.44fNABmax
f = the system frequency [Hz],
N = the number of turns on the core leg [1],
A = the area of the core leg [m2] and
Bmax = the peak value of the flux density [T]
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Transformer protection
Over excitation Protection
(B /B) = V fV / f
n
n n
max
Bn = the rated peak value of the flux density [T], V = the RMS value of the terminal voltage [V],
Vn
= the rated RMS value of the terminal voltage [V], and
fn = the rated system frequency [Hz]
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Transformer protection
Over excitation protection
Definite
V/Hz inverse
t
V/Hz
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Transformer protection
Time overcurrent relays
Used on all feeding circuits of transformer
Provide back up for internal faults
Also provide back up for system faults Instantaneous high element
f
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Transformer protection
Residual Overcurrent relays
Used on all feeding circuits of transformer
Provide back up for internal faults
Also provide back up for system faults
Instantaneous high element used
T f t ti
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Transformer protection
Impedance Protection
Back-up protection of the local busbars
Back-up protection of the outgoing feeders
Back-up protection of the busbar at the remote end
Plain underimpedance protection/Distance protection
T f t ti
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Transformer protection
Monitors
Monitors are very important devices which detect faults and
abnormal service conditions which may develop into fault.
T f t ti
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Transformer protection
Oil and Gas Monitoring
Sudden gas pressure protection
Buchholz protection Oil level monitoring
T f t ti
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Transformer protection
Temperature Monitoring
The oil thermometer
The winding thermometer
T f t ti
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Transformer protection
Protective Relays Used ( Transformers > 5
MVA)
Gas detector relay ( Buchholz) Over load protection
Thermal relays
Temperature monitoring relays
Over current protection
Ground fault protection
Differential protection
Interturn faults
Pressure relay for tap changer
Oil level monitor
Transformer protection
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Transformer protection
Protective Relays Used ( Transformers IF ( RCT + 2 RL)
VK > 2 VR
Transformer protection
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Phase overcurrent relays The pick up current seting must be set above highest possible load
current
It shall also serve as overload protection In many cases it can be considered that overload capacity is 1.4 times
rated power
The lowest setting value can be written as
I pickup = 1.2 . I max/K
It should also be able to detect all faults within the zone
I pickup < 0.7. I scmin
The high set relay should be set so as to be selective wrt other relays
I high > 1.2. K t .I scmax
K t is transient overreach and I scmax is largest fault current on lowvoltage side
Time setting should be set as to get shortest fault time with maintainedselectivity
Transformer protection
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Failure to Reset of an Overcurrent Protection
The relay does not reset
t
I
I stI reset
Transformer protection
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Inverse Time Characteristics Selectivity
Fault current
Time
t
Transformer protection
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Residual over current relays
Earth fault current is fed from the transformer in case ofphase to earth fault in the system. Therefore there is aneed
to trip the transformer in case of such fault. In most cases itshould be considered as back up to line protection. Currentsetting is usually low. Time setting should be long to beselective wrt line protection.
Transformer protection
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System Transformer with Impedance Protection
Z