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— TEXAS A&M, 72ND ANNUAL CONFERENCE FOR PROTECTION RELAY ENGINEERS
Modern Design Principles for Numerical Busbar Differential Protection
Mike KockottABB Inc.
Wednesday, March 27, 2019COLLEGE STATION, TX
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Slide 2
Co-author acknowledgement
Zoran Gajić ABB ABHamdy Faramawy VästeråsLi He SwedenKlas KoppariLee Max
March 28, 2019
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Slide 3
Basic principles
Based on differential current measurement:
– out-of-zone fault, currents balance (i.e. Σ i =0)
– sum of currents in = sum of currents out
– in-zone fault, currents no longer balance (i.e. Σ i >>0)
– sum of currents in ≠ sum of currents out
March 28, 2019
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Slide 4
History
Circulating current differential (1900s)
High impedance differential (1940s)
Percentage restrained differential (1960s)
First generation numerical differential (late 80s / early 90s)
March 28, 2019
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Slide 5
Measurement
Traditionally, bus differential relays measure the secondary currents of magnetic core CTs
‒ non-linear devices
‒ needs to be taken into account in the measurement design of bus differential relays
Example: external fault
‒ primary currents will always be balanced
‒ on secondary side, this same balance may not be measured
March 28, 2019
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Slide 6
Measurement
CT saturation
‒ occurs for high primary current conditions
‒ no longer able to transform primary current
‒ when fully saturated, a CT is no longer a current source, but instead has just a resistance value
Remanence
‒ random parameter that can improve or reduce ability to transform primary current
March 28, 2019
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Slide 7
Measurement
Secondary dc transient
‒ occurs on interruption of primary current
‒ takes the form of an exponentially decaying dc current
‒ caused by discharge of stored energy
The non-linear phenomena of magnetic core CTs have the tendency to cause unwanted operation of bus differential relays
‒ so each type of bus differential relay (high impedance, numerical, etc.) must have the means to overcome in its measurement design this non-linear behaviour
March 28, 2019
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Slide 8
High impedance
All CT secondary circuits are galvanically connected
‒ used to overcome the non-linear phenomena
‒ a high resistance RHIGH is placed in series with the operating element
March 28, 2019
RLRL
RL
RHIGHMetrosil
I>
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Slide 9
High impedance
External (through flow) fault
‒ full CT saturation
‒ the maximum differential branch currentIdiffmax = Iunbmax*
‒ to ensure stability Idiffmax
< set I> pickup
March 28, 2019
RLRL
I>
RL
RCTRHIGHMetrosil(RL+RCT)max
RHIGH+(RL+RCT)max
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Slide 10
High impedance
Criteria to be met:
‒ CT secondary loop resistance
‒ must be low (with a similar value in all bays)
‒ CT requirements
‒ due to lack of any restraint quantity, all CTs must have the same ratio and magnetising characteristics
Requirements incur additional expense
Quite reliable and very sensitive
Gives operating times of less than one cycle for internal faultsMarch 28, 2019
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Slide 11
Percentage restrained
Developed to lessen the restrictions on CTs imposed by high impedance
All CT secondary circuits are galvanically connected
Again, it is this galvanic connection between the CT circuits that is used to overcome the non-linear phenomena of the CTs
March 28, 2019
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Slide 12
Percentage restrained
All measurement decisions are based on these three quantities
‒ operation (measurement) of the relay does not depend on the number of connected bays to the protected zone
March 28, 2019
≈ iIN≈ iD
≈ iOUT
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Slide 13
Percentage restrained
Rl = the total saturated CT secondary loop resistance (RL+RCT)
Rd = the relay differential branch resistance
March 28, 2019
RlRd
Id
Iin
Rl
Rl+RdId = Iin
operate
Idiff
Iin
stable
Stability for external faults, even with fully saturated CT, is guaranteed for differential current below the characteristic
OperateIdiff > x*Iin
StableIdiff ≤ x*Iin
x > Rl
Rl+Rd
Iout
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Slide 14
Percentage restrained
Improvements over high impedance differential
‒ less severe CT requirements
‒ can accommodate different CT ratios using auxiliary current transformers
‒ can tolerate other relays on the same CT core
‒ allows much higher resistance to be included in the secondary circuits of the main CTs
Fast operating times for internal faults
‒ detect 1 - 3ms, trip output 9 - 13ms
March 28, 2019
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Slide 15
Shortcomings of earlier generations
Unable to detect open CT
Required interposing CTs to match main CT ratios (not possible for high impedance)
Double bus (single CB) – required switching in CT circuits, bistable relays to replicate disconnector status for zone selection, trip output selection
Check zone – required to ensure stable operation for open CT (requires one set of CT cores for ZA/ZB, and a separate set of CT cores for CZ); overcome problems with disconnector auxiliary contacts
March 28, 2019
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Slide 16
Numerical
Analog inputs are galvanically isolated
Each analog input quantity is sampled and converted to a numerical number – these numerical numbers are used in the algorithms
‒ it is therefore not possible to copy and directly re-use the analog technique
March 28, 2019
RL
RELAY
CT notsaturated
RL
RCT
RELAY
CTsaturated
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Slide 17
Numerical
The analog inputs are galvanically isolated
The secondary circuit loop resistance is no longer the important factor
The critical factor now is the time available to make the measurement and decision, i.e. the short period of time, within each cycle, when the CTs are not saturated [the CTs must be able to correctly reproduce the current for a minimum time within each cycle before saturation of the CT begins]
For practical protection class CTs, time to saturation, even under extremely heavy CT saturation, is around 2ms – the design criterion used for the numerical algorithm
March 28, 2019
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Slide 18
Numerical
March 28, 2019
i1 i2 i3
i7i6i5i4
Differential Protection Zone
DifferentialProtection Zone
iin
id
ioutBus =>
iin = max(SP, SN)iout = min(SP, SN)
RMS values of the differential, incoming and outgoing currents are calculated over the last power system cycle (1 cycle long moving window)
Have IIN, IOUT, ID & iin, iout, id for the algorithms
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Slide 19
Numerical
March 28, 2019
Measured signal relationships
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Slide 20
Numerical
Internal fault
External fault without CT saturation
External fault with CT saturation
March 28, 2019
IinIout
Id
IinIout
Id
IinIout
Id
critical criterion is the time available to make the measurement [initial correct reproduction of the current before CT saturation occurs]
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Slide 21
Numerical
Zone selection
‒ adapt dynamically to changing topology for multi-zone applications (using auxiliary contacts of primary apparatuses)
‒ connection of bay currents to the appropriate 87B zone
‒ automatic zone merging (e.g. zone interconnection on double bus applications, closing of bus-sectionalizing disconnectors, etc.)
‒ selective tripping i) of only the bay CBs connected to the faulty zone, and ii) of only the bay CBs connected to the same zone as a bay CB that has failed to trip (breaker failure)
March 28, 2019
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Slide 22
Numerical
Zone selection
‒e.g. complex busbar arrangement ‒ connections
March 28, 2019
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Slide 23
Real time digital simulator testing
IED with 6 differential zones
‒ functional testing
‒ faults at different locations
‒ different switchgear configurations
‒ verify selectivity (zone selection, zone merging, etc.)
March 28, 2019
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Slide 24
Real time digital simulator testing
‒ dynamic testing
‒ primary time constants 100, 200, 350ms; fault inception angles 0°(maximum DC offset), 30°, 60°, 90°, 120°, 150°; different combinations of main CT ratios; different remanence levels
‒ evaluate behaviour during different transient conditions (e.g. CT saturation, evolving faults, AR onto permanent external fault, etc.)
March 28, 2019
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Slide 25
Real time digital simulator testing
March 28, 2019
Network model – functional testing
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Slide 26March 28, 2019
Functional tests –correct switchgear input status
Real time digital simulator testing
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Slide 27March 28, 2019
Functional tests –incorrect switchgear input status
Real time digital simulator testing
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Slide 28
Real time digital simulator testing
Evolving fault L1 (phase A) external to L2 (phase B) internal
March 28, 2019
Full test scope:
‒evolving external to internal, internal to external, after 5, 10, 15, 20ms
Z1
Z2
C1 C2 C5 C3 C4
X XX
X XO O O O
O
Z3
Z4
L2
L1
Load Source Load Source
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Slide 29
Real time digital simulator testing
Internal L1L2 (phase AB) fault with CT saturation
March 28, 2019
Z1
Z2
C1 C2 C5 C3 C4
X XX
X XO O O O
O
Z3
Z4
L2
L1
Load Source Load Source
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Slide 30March 28, 2019
Real time digital simulator testing
Open CTZ1
Z2
C1 C2 C5 C3 C4
X XX
X XO O O O
OL2
L1
Source Load Load Source
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Slide 31
Real time digital simulator testing
Breaker failure following external fault
March 28, 2019
Z1
Z2
C1 C2 C5 C3 C4
X XX
X XO O O O
O
Z3
Z4
L2
L1
Load Source Load Source
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Slide 32March 28, 2019
Real time digital simulator testing
Network model used – external fault with CT saturation
Transformer
3
IED
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Slide 33March 28, 2019
Real time digital simulator testing
External fault with CT saturation
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Slide 34
Conclusion
Numerical busbar differential protection with 6 zones, verified
– software based zone selection / merging
– fast operation for internal faults
– correct operation for internal faults with CT saturation, evolving faults, etc.
– complete stability for external faults with CT saturation
– correct timeous detection of an open CT secondary circuit
Provides additional features like disturbance and event recording, built-in circuit breaker failure protection, IEC61850 communication
March 28, 2019