Substation Automation Products
Transformer protection RET670 Restricted earth fault protection
1MRG019260
Chapter 04
Contents
© ABB Group September 10, 2015 | Slide 2
Application
Operating characteristics
Logical Diagram
In and output signals
Settings
Monitored data
Application restricted earth fault protection
Faults resulting in large fault currents may cause severe damages to windings and the core
Could also result in high gas pressure damaging the tank
The restricted earth fault protection is fast and sensitive and can detect faults in the winding:
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Chapter 04
in impedance earthed systems
close to the star point in solid earthed systems
The restricted earth fault protection is often used as main protection for all winding faults involving earth
Restricted earth fault protection Application
Low impedance type
Can be used to protect directly or low impedance earthed windings
Zero sequence differential protection
An additional directional comparison feature is included
No need for magnitude or phase angle correction
Internally summation if multi breaker arrangement
Fundamental frequency components
OLTC does not influence the function
Insensitive to inrush and overexcitation
Only danger is eventual CT saturation
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Chapter 04
2nd harmonic restraint included
Chapter 04
Restricted earth fault protection Application
1.
1B
2
3A
Transformer A.
Solidly earthed winding with or without OLTC
B.
Earthed though Z-0 earthing transformer
2.
Autotransformer with or without OLTC
3.
Reactor A.
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1A
Solidly earthed
Restricted earth fault protection Application
Single circuit breaker or
Multiple circuit breaker applications
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summation inside the function
Number of instances
Customized: 0 - 3
Pre-configured (A10, A30, B30, A40, B40, A25):
Included: A10: 1; x30/y40: 2; A25: 0
Optional: y40: 1
Chapter 04
Chapter 04
Operating characteristics Restricted earth fault protection
Calculated or measured residual current on terminal side, 3I0 (Ires)
The differential current, Idiff, is the vector sum:
125
3I0_Wx is the summation of CT1 and CT2 for winding x
3I0_W2 is used in autotransformer applications
Stabilizing current, Ibias
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where
The highest current among all current inputs
Chapter 04
Operating characteristics Restricted earth fault protection
Two fixed slopes defines as:
1st: 70% ; 2nd: 100%
End of slope 1: 125% of IBase
Setting: Minimum operation, Idmin
125
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In percent of IBase: 4.0 - 100.0
IBase
Rated current of the winding or CT rating
For autotransformers the highest winding rated current i.e. low voltage side
Operating characteristics Restricted earth fault protection, Ibias
The bias current stabilizes the protection
Higher currents harder conditions higher probability that Idiff > Idmin due to: CT saturation
Ibias is defined as the maximum current of: curr(a) = MAX [I3PW1CT1] / CTFactorPri1 curr(b) = MAX [I3PW1CT2] / CTFactorPri2 curr(c) = MAX [I3PW2CT1] / CTFactorSec1 curr(d) = MAX [I3PW2CT2] / CTFactorSec2 curr(x) = MAX [I3PW1CT1 + I3PW1CT2] curr(y) = MAX [I3PW2CT1 + I3PW2CT2] curr(z) = IN where the CTFactor = Irated_CT / Irated_W default setting: 1.0 (1.0-10.0)
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Chapter 04
Operating characteristics Restricted earth fault protection, Ibias
Example:
CT1 1000 A
The CTFactor :
gives a possibility to adjust the sensitivity
does not influence curr(x-z)
Single breaker application
CT2 1000 A
Chapter 04
Normally the CT ratio matches the winding and the CTFactor = 1.0
Multiple breaker application
Could be necessary to modify CTFactor
500 A
curr(a) = MAX [I3PW1CT1] / CTFactorPri1
IBase = Irated_W and
Irated_CT > or >> Irated_W
curr(a and/or b) might stabilize to much due to transfer currents (fault or load currents)
Example: CTFactor = 1000/500 = 2.0
curr(b) = MAX [I3PW1CT2] / CTFactorPri2 curr(x) = MAX [I3PW1CT1 + I3PW1CT2]
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Chapter 04
Operating characteristics Directional criterion IN , 3I0, IDiff
The restricted earth fault protection may be very sensitive
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No room for any false unbalance currents
External faults may cause
CT saturation false residual currents false tripping
The stabilizing characteristics helps to stabilize to some extent
Objective:
Positively distinguish between internal and external earth faults
Enhancing the security for heavy external faults
Operating characteristics Directional criterion
Same direction in the neutral for both external and internal faults
External fault
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Current in the neutral serves as directional reference
The currents are 180o out of phase (IED reference direction: towards the object)
Chapter 04
Operating characteristics Directional criterion
Internal fault
The residual currents are approximately in phase
Not measuring the same primary current, IN and 3I0
Setting:
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Relay operate angle (ROA) 60 – 90o
Chapter 04
Chapter 04
Operating characteristics Directional criterion
The directional check is executed if:
within the operate area of the REFcharacteristic
3I0 (terminal side) is at least 3% of IBase
The trip is issued if:
If the check is not executed (small currents) then:
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both 3I0 an IN are within the operating region
the check is not a condition for trip
Chapter 04
Operating characteristics Second harmonic blocking
Current [pu]
Shunt reactor switching 150 Mvar 220 kV 50 Hz
IN 3I0
Cycles
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Prevent unwanted operation on reactor energizing due to
terminal side CT saturation cased by
high DC component
The calculation adaptive (input: 3I0, IN, IBias and timing)
No setting: 2nd harmonic ratio: fixed to 60%
Chapter 04
Simplified logical diagram Restricted earth fault protection 3I0
3I0 < 3% of IBase 3I0 > 3% of Ibase
IN Directional check
IN > 50% of Idmin © ABB Group September 10, 2015 | Slide 16
≥1
&
TRIP
&
START
In- and output signals Restricted earth fault protection
Analog input signals (SMAI group signal output)
Binary input signal
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Neutral point current Residual current - W1 - CT1 - W1 - CT2 - W2 - CT1 - W2 - CT2 Block input
Chapter 04
In- and output signals Restricted earth fault protection
Trip Start Directionality: High Internal fault Second harmonic blocking Residual current W1+W2 (3I0) Neutral current (IN) Bias current Differential current Angle from zero sequence feature Second harmonic ratio
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Chapter 04
Binary output signals
Analog output signals HMI and Disturbance recorder
Chapter 04
Restricted earth fault protection settings Overview
Operation On/Off Base values Pick up level Factors influencing bias
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Chapter 04
Monitored data Restricted earth fault protection
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Tests/Functions status/Differential Protection/LowImpREF(PDIF,87N)
TRIP
START
DIROK – Internal fault
BLK2H – Block due to 2nd harmonic
IRES – Magnitude of residual current (3I0, A)
IN – Magnitude of neutral current (A)
IBIAS – Magnitude of bias current (A)
IDIFF – Magnitude of differential current (A)
ANGLE – Direction angle from zero sequence feature (deg)
I2RATIO – Second harmonic ratio
Chapter 04
© ABB SA-TGroup Training September 10, 2015 | Slide 21
