21955068-High-Low-Impedance-BusBar-Protection.ppt

Fundamentals of
Bus Bar Protection
GE Multilin

2
GE Consumer & Industrial
Multilin
29-Oct-22
Outline
• Bus arrangements
• Bus components
• Bus protection techniques
• CT Saturation
• Application Considerations:
 High impedance bus differential relaying
 Low impedance bus differential relaying
 Special topics

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1 2 3 n-1 n
ZONE 1
- - - -
• Distribution and lower transmission voltage levels
• No operating flexibility
• Fault on the bus trips all circuit breakers
Single bus - single breaker

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ZONE 1
ZONE 2
• Distribution and lower transmission voltage levels
• Limited operating flexibility
Multiple bus sections - single breaker with
bus tie

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ZONE 1
ZONE 2
• Transmission and distribution voltage levels
• Breaker maintenance without circuit removal
• Fault on a bus disconnects only the circuits being connected
to that bus
Double bus - single breaker with bus tie

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ZONE 1
MAIN BUS
TRANFER BUS
• Increased operating flexibility
• A bus fault requires tripping all breakers
• Transfer bus for breaker maintenance
Main and transfer buses

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ZONE 1
ZONE 2
• Very high operating flexibility
• Transfer bus for breaker maintenance
Double bus – single breaker w/ transfer bus

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ZONE 1
ZONE 2
• High operating flexibility
• Line protection covers bus section between two CTs
• Fault on a bus does not disturb the power to circuits
Double bus - double breaker

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ZONE 1
ZONE 2
• Used on higher voltage levels
• More operating flexibility
• Requires more breakers
• Middle bus sections covered by line or other equipment
protection
Breaker-and-a-half bus

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• Higher voltage levels
• High operating flexibility with minimum breakers
• Separate bus protection not required at line positions
B1 B2
TB1
L1 L2
L3 L4
TB1
Ring bus

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Bus components breakers
SF6, EHV & HV - Synchropuff
Low Voltage circuit breakers
BUS 2
CB 1
BUS 1
ISO 1 ISO 2
ISO 3
BYPASS

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29-Oct-22
-
+
F1a
F1c
Contact Input F1a On
Contact Input F1c On
F1b
ISOLATOR
1
ISOLATOR 1 OPEN
7B 7A
BUS 1
-
+
F1a
F1c
Contact Input F1a On
Contact Input F1c On
F1b
ISOLATOR
1
ISOLATOR 1 CLOSED
7B 7A
BUS 1
Disconnect switches & auxiliary contacts
BUS 2
CB 1
BUS 1
ISO 1 ISO 2
ISO 3
BYPASS

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29-Oct-22
BUS 2
CB 1
BUS 1
ISO 1 ISO 2
ISO 3
BYPASS
Current Transformers
Oil insulated current transformer
(35kV up to 800kV)
Gas (SF6) insulated current
transformer
Bushing type (medium
voltage switchgear)

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Protection Requirements
High bus fault currents due to large number of circuits
connected:
• CT saturation often becomes a problem as CTs may not be sufficiently
rated for worst fault condition case
• large dynamic forces associated with bus faults require fast clearing
times in order to reduce equipment damage
False trip by bus protection may create serious problems:
• service interruption to a large number of circuits (distribution and sub-
transmission voltage levels)
• system-wide stability problems (transmission voltage levels)
With both dependability and security important, preference is
always given to security

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Bus Protection Techniques
• Interlocking schemes
• Overcurrent (“unrestrained” or “unbiased”)
differential
• Overcurrent percent (“restrained” or “biased”)
differential
• Linear couplers
• High-impedance bus differential schemes
• Low-impedance bus differential schemes

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Interlocking Schemes
• Blocking scheme typically
used
• Short coordination time
required
• Care must be taken with
possible saturation of feeder
CTs
• Blocking signal could be sent
over communications ports
(peer-to-peer)
• This technique is limited to
simple one-incomer
distribution buses
50
50 50 50 50 50
BLOCK

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Overcurrent (unrestrained) Differential
• Differential signal formed by
summation of all currents feeding
the bus
• CT ratio matching may be
required
• On external faults, saturated CTs
yield spurious differential current
• Time delay used to cope with CT
saturation
• Instantaneous differential OC
function useful on integrated
microprocessor-based relays
51

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59
Linear Couplers
ZC = 2  – 20  - typical coil impedance
(5V per 1000Amps => 0.005 @ 60Hz )
If = 8000 A
40 V 10 V 10 V 0 V 20 V
2000 A 2000 A 4000 A
0 A
0 V
External
Fault

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59
Linear Couplers
Esec= Iprim*Xm - secondary voltage on relay terminals
IR= Iprim*Xm /(ZR+ZC) – minimum operating current
where,
Iprim – primary current in each circuit
Xm – liner coupler mutual reactance (5V per 1000Amps => 0.005 @ 60Hz )
ZR – relay tap impedance
ZC – sum of all linear coupler self impedances
If = 8000 A
0 A
0 V 10 V 10 V 0 V 20 V
40 V
2000 A 2000 A 4000 A
0 A
Internal Bus
Fault

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• Fast, secure and proven
• Require dedicated air gap CTs, which may not be used for
any other protection
• Cannot be easily applied to reconfigurable buses
• The scheme uses a simple voltage detector – it does not
provide benefits of a microprocessor-based relay (e.g.
oscillography, breaker failure protection, other functions)
Linear Couplers

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High Impedance Differential
• Operating signal created by
connecting all CT secondaries in
parallel
o CTs must all have the same ratio
o Must have dedicated CTs
• Overvoltage element operates on
voltage developed across resistor
connected in secondary circuit
o Requires varistors or AC shorting
relays to limit energy during faults
• Accuracy dependent on secondary
circuit resistance
o Usually requires larger CT cables to
reduce errors  higher cost
Cannot easily be applied to reconfigurable buses and
offers no advanced functionality
59

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Percent Differential
• Percent characteristic used
to cope with CT saturation
and other errors
• Restraining signal can be
formed in a number of
ways
• No dedicated CTs needed
• Used for protection of re-
configurable buses
possible
51
87
n
DIF I
I
I
I 


 ...
2
1
n
RES I
I
I
I 


 ...
2
1  
n
RES I
I
I
I ...,
,
,
max 2
1


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Low Impedance Percent Differential
• Individual currents sampled by protection and summated digitally
o CT ratio matching done internally (no auxiliary CTs)
o Dedicated CTs not necessary
• Additional algorithms improve security of percent differential
characteristic during CT saturation
• Dynamic bus replica allows application to reconfigurable buses
o Done digitally with logic to add/remove current inputs from differential
computation
o Switching of CT secondary circuits not required
• Low secondary burdens
• Additional functionality available
o Digital oscillography and monitoring of each circuit connected to bus zone
o Time-stamped event recording
o Breaker failure protection

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Digital Differential Algorithm Goals
• Improve the main differential algorithm operation
o Better filtering
o Faster response
o Better restraint techniques
o Switching transient blocking
• Provide dynamic bus replica for reconfigurable bus bars
• Dependably detect CT saturation in a fast and reliable manner,
especially for external faults
• Implement additional security to the main differential algorithm to
prevent incorrect operation
o External faults with CT saturation
o CT secondary circuit trouble (e.g. short circuits)

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Low Impedance Differential (Distributed)
• Data Acquisition Units (DAUs)
installed in bays
• Central Processing Unit (CPU)
processes all data from DAUs
• Communications between DAUs
and CPU over fiber using
proprietary protocol
• Sampling synchronisation
between DAUs is required
• Perceived less reliable (more
hardware needed)
• Difficult to apply in retrofit
applications
52
DAU
52
DAU
52
DAU
CU
copper
fiber

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Low Impedance Differential (Centralized)
• All currents applied to a single
central processor
• No communications, external
sampling synchronisation
necessary
• Perceived more reliable (less
hardware needed)
• Well suited to both new and
retrofit applications.
52 52 52
CU
copper

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CT Saturation

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CT Saturation Concepts
• CT saturation depends on a number of factors
o Physical CT characteristics (size, rating, winding resistance,
saturation voltage)
o Connected CT secondary burden (wires + relays)
o Primary current magnitude, DC offset (system X/R)
o Residual flux in CT core
• Actual CT secondary currents may not behave in the same manner as
the ratio (scaled primary) current during faults
• End result is spurious differential current appearing in the summation
of the secondary currents which may cause differential elements to
operate if additional security is not applied

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CT Saturation
Ratio Current CT Current
Ratio Current CT Current
No DC Offset
• Waveform remains fairly
symmetrical
With DC Offset
• Waveform starts off being
asymmetrical, then
symmetrical in steady
state

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External Fault & Ideal CTs
• Fault starts at t0
• Steady-state fault conditions occur at t1
differential
restraining
t0
t1
Ideal CTs have no saturation or mismatch errors thus
produce no differential current

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External Fault & Actual CTs
• Fault starts at t0
• Steady-state fault conditions occur at t1
differential
restraining
t0
t1
Actual CTs do introduce errors, producing some differential
current (without CT saturation)

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External Fault with CT Saturation
• Fault starts at t0, CT begins to saturate at t1
• CT fully saturated at t2
differential
restraining
t0
t1
t2
CT saturation causes increasing differential current that
may enter the differential element operate region.

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Some Methods of Securing Bus Differential
• Block the bus differential for a period of time (intentional delay)
o Increases security as bus zone will not trip when CT saturation is present
o Prevents high-speed clearance for internal faults with CT saturation or
evolving faults
• Change settings of the percent differential characteristic (usually Slope 2)
o Improves security of differential element by increasing the amount of
spurious differential current needed to incorrectly trip
o Difficult to explicitly develop settings (Is 60% slope enough? Should it be
75%?)
• Apply directional (phase comparison) supervision
o Improves security by requiring all currents flow into the bus zone before
asserting the differential element
o Easy to implement and test
o Stable even under severe CT saturation during external faults

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High-Impedance
Bus Differential
Considerations

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High Impedance Voltage-operated Relay
External Fault
• 59 element set above max possible voltage developed across
relay during external fault causing worst case CT saturation
• For internal faults, extremely high voltages (well above 59
element pickup) will develop across relay

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High Impedance Voltage Operated Relay
Ratio matching with Multi-ratio CTs
• Application of high impedance differential relays with CTs of
different ratios but ratio matching taps is possible, but could
lead to voltage magnification.
• Voltage developed across full winding of tapped CT does not
exceed CT rating, terminal blocks, etc.

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High Impedance Voltage Operated Relay
Ratio matching with Multi-ratio CTs
• Use of auxiliary CTs to obtain correct ratio matching is also
possible, but these CTs must be able to deliver enough voltage
necessary to produce relay operation for internal faults.

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Electromechanical High Impedance Bus
Differential Relays
• Single phase relays
• High-speed
• High impedance voltage sensing
• High seismic IOC unit

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Operating time: 20 – 30ms @ I > 1.5xPKP
P -based High-Impedance Bus Differential
Protection Relays

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RST = 2000 - stabilizing resistor to limit the current
through the relay, and force it to the lower impedance CT
windings.
MOV – Metal Oxide Varistor to limit the voltage to
1900 Volts
86 – latching contact preventing the resistors from
overheating after the fault is detected
High Impedance Module for Digital
Relays

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High-Impedance Module
+
Overcurrent Relay

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• Fast, secure and proven
• Requires dedicated CTs, preferably with the same CT ratio
and using full tap
• Can be applied to small buses
• Depending on bus internal and external fault currents, high
impedance bus diff may not provide adequate settings for
both sensitivity and security
• Cannot be easily applied to reconfigurable buses
• Require voltage limiting varistor capable of absorbing
significant energy
• May require auxiliary CTs
• Do not provide full benefits of microprocessor-based relay
system (e.g. metering, monitoring, oscillography, etc.)
High Impedance Bus Protection - Summary

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Low-Impedance
Bus Differential
Considerations

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P-based Low-Impedance Relays
• No need for dedicated CTs
• Internal CT ratio mismatch compensation
• Advanced algorithms supplement percent differential
protection function making the relay very secure
• Dynamic bus replica (bus image) principle is used in
protection of reconfigurable bus bars, eliminating the need
for switching physically secondary current circuits
• Integrated Breaker Failure (BF) function can provide
optimal tripping strategy depending on the actual
configuration of a bus bar

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• Up to 24 Current Inputs
• 4 Zones
• Zone 1 = Phase A
• Zone 2 = Phase B
• Zone 3 = Phase C
• Zone 4 = Not used
• Different CT Ratio Capability for
Each Circuit
• Largest CT Primary is Base in
Relay
2-8 Circuit Applications
Small Bus Applications

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• Relay 1 - 24 Current Inputs
• 4 Zones
• Zone 1 = Phase A (12 currents)
• Zone 2 = Phase B (12 currents)
CB 12
CB 11
• Different CT Ratio Capability for Each Circuit
• Largest CT Primary is Base in Relay
• Relay 2 - 24 Current Inputs
• 4 Zones
• Zone 3 = Phase C (12 currents)
9-12 Circuit Applications
Medium to Large Bus Applications

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Large Bus Applications
87B phase A
87B phase B
87B phase C
Logic relay
(switch status,
optional BF)

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Large Bus Applications
For buses with up to 24 circuits

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Summing External Currents
Not Recommended for Low-Z 87B relays
• Relay becomes combination
of restrained and unrestrained
elements
•In order to parallel CTs:
• CT performance must be closely
matched
o Any errors will appear as
differential currents
• Associated feeders must be radial
o No backfeeds possible
• Pickup setting must be raised to
accommodate any errors
CT-1
CT-2
CT-3
CT-4
I
3
=
0
I
2
=
0
I
1
=
Error
IDIFF
= Error
IREST
= Error
Maloperation if
Error > PICKUP

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Definitions of Restraint Signals
“maximum of”
“geometrical average”
“scaled sum of”
“sum of”
n
R i
i
i
i
i 



 ...
3
2
1
 
n
R i
i
i
i
n
i 



 ...
1
3
2
1
 
n
R i
i
i
i
Max
i ,...,
,
, 3
2
1

n
n
R i
i
i
i
i 



 ...
3
2
1

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“Sum Of” vs. “Max Of” Restraint Methods
“Sum Of” Approach
• More restraint on external faults;
less sensitive for internal faults
• “Scaled-Sum Of” approach takes
into account number of connected
circuits and may increase
sensitivity
• Breakpoint settings for the percent
differential characteristic more
difficult to set
“Max Of” Approach
• Less restraint on external faults;
more sensitive for internal faults
• Breakpoint settings for the percent
differential characteristic easier to
set
• Better handles situation where one
CT may saturate completely (99%
slope settings possible)

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Bus Differential Adaptive Approach
differential
restraining
Region 1
(low differential
currents)
Region 2
(high differential
currents)

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Bus Differential Adaptive Logic Diagram
DIFL
DIR
SAT
DIFH
OR
AND
OR
87B BIASED OP
AND

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Phase Comparison Principle
• Internal Faults: All fault (“large”) currents are approximately in
phase.
• External Faults: One fault (“large”) current will be out of phase
• No Voltages are required or needed
Secondary Current of
Faulted Circuit
(Severe CT Saturation)

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Phase Comparison Principle Continued…
BLOCK
OPERATE
BLOCK








 p
D
p
I
I
I
real








 p
D
p
I
I
I
imag
Ip
ID - Ip
External Fault Conditions
OPERATE
BLOCK
BLOCK








 p
D
p
I
I
I
real








 p
D
p
I
I
I
imag
Ip
ID - Ip
Internal Fault Conditions
OPERATE
OPERATE

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CT Saturation
• Fault starts at t0, CT begins to saturate at t1
• CT fully saturated at t2
differential
restraining
t0
t1
t2

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CT Saturation Detector State Machine
NORMAL
SAT := 0
EXTERNAL
FAULT
SAT := 1
EXTERNAL
FAULT & CT
SATURATION
SAT := 1
The differential
characteristic
entered
The differential-
restraining trajectory
out of the differential
characteristic for
certain period of time
saturation
condition
The differential
current below the
first slope for
certain period of
time

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CT Saturation Detector Operating
Principles
• The 87B SAT flag WILL NOT be set during internal faults,
regardless of whether or not any of the CTs saturate.
• The 87B SAT flag WILL be set during external faults,
regardless of whether or not any of the CTs saturate.
• By design, the 87B SAT flag WILL force the relay to use
the additional 87B DIR phase comparison for Region 2
The Saturation Detector WILL NOT Block the Operation of
the Differential Element – it will only Force 2-out-of-2
Operation

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CT Saturation Detector - Examples
• The oscillography records on the next two slides were captured from a
B30 relay under test on a real-time digital power system simulator
• First slide shows an external fault with deep CT saturation (~1.5 msec of
good CT performance)
o SAT saturation detector flag asserts prior to BIASED PKP bus
differential pickup
o DIR directional flag does not assert (one current flows out of zone),
so even though bus differential picks up, no trip results
• Second slide shows an internal fault with mild CT saturation
o BIASED PKP and BIASED OP both assert before DIR asserts
o CT saturation does not block bus differential
• More examples available (COMTRADE files) upon request

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The bus differential
protection element
picks up due to heavy
CT saturation
The CT saturation flag
is set safely before the
pickup flag
The
directional flag
is not set
The element
does not
maloperate
Despite heavy CT
saturation the
external fault current
is seen in the
opposite direction
CT Saturation Example – External Fault
0.06 0.07 0.08 0.09 0.1 0.11 0.12
-200
-150
-100
-50
0
50
100
150
200
time, sec
current,
A
~1 ms

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The bus differential
protection element
picks up
The saturation
flag is not set - no
directional
decision required
The element
operates in
10ms
The
directional
flag is set
All the fault currents
are seen in one
direction
CT Saturation – Internal Fault Example

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Applying Low-Impedance Differential
Relays for Busbar Protection
Basic Topics
• Configure physical CT Inputs
• Configure Bus Zone and Dynamic Bus Replica
• Calculating Bus Differential Element settings
Advanced Topics
• Isolator switch monitoring for reconfigurable buses
• Differential Zone CT Trouble
• Integrated Breaker Failure protection

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Configuring CT Inputs
• For each connected CT circuit enter Primary rating and
select Secondary rating.
• Each 3-phase bank of CT inputs must be assigned to a
Signal Source that is used to define the Bus Zone and
Dynamic Bus Replica
Some relays define 1 p.u. as the maximum
primary current of all of the CTs connected in the
given Bus Zone

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Per-Unit Current Definition - Example
Current
Channel
Primary Secondary Zone
CT-1 F1 3200 A 1 A 1
CT-2 F2 2400 A 5 A 1
CT-3 F3 1200 A 1 A 1
CT-4 F4 3200 A 1 A 2
CT-5 F5 1200 A 5 A 2
CT-6 F6 5000 A 5 A 2
• For Zone 1, 1 p.u. = 3200 AP
• For Zone 2, 1 p.u. = 5000 AP

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Configuration of Bus Zone
• Dynamic Bus Replica associates a status signal with each
current in the Bus Differential Zone
• Status signal can be any logic operand
o Status signals can be developed in programmable logic
to provide additional checks or security as required
o Status signal can be set to ‘ON’ if current is always in the
bus zone or ‘OFF’ if current is never in the bus zone
• CT connections/polarities for a particular bus zone must be
properly configured in the relay, via either hardwire or
software

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Configuring the Bus Differential Zone
1. Configure the physical CT Inputs
o CT Primary and Secondary values
o Both 5 A and 1 A inputs are supported by the UR hardware
o Ratio compensation done automatically for CT ratio differences up to 32:1
2. Configure AC Signal Sources
3. Configure Bus Zone with Dynamic Bus Replica
Bus Zone settings defines the boundaries of the Differential
Protection and CT Trouble Monitoring.

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Dual Percent Differential Characteristic
High
Breakpoint
Low
Breakpoint
Low Slope
High Slope
High Set
(Unrestrained)
Min Pickup

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Calculating Bus Differential Settings
• The following Bus Zone Differential element parameters need to be set:
o Differential Pickup
o Restraint Low Slope
o Restraint Low Break Point
o Restraint High Breakpoint
o Restraint High Slope
o Differential High Set (if needed)
• All settings entered in per unit (maximum CT primary in the zone)
• Slope settings entered in percent
• Low Slope, High Slope and High Breakpoint settings are used by the CT
Saturation Detector and define the Region 1 Area (2-out-of-2 operation
with Directional)

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Calculating Bus Differential Settings –
Minimum Pickup
• Defines the minimum differential current required for
operation of the Bus Zone Differential element
• Must be set above maximum leakage current not zoned off
in the bus differential zone
• May also be set above maximum load conditions for added
security in case of CT trouble, but better alternatives exist

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Low Slope
• Defines the percent bias for the restraint currents from
IREST=0 to IREST=Low Breakpoint
• Setting determines the sensitivity of the differential element
for low-current internal faults
• Must be set above maximum error introduced by the CTs in
their normal linear operating mode
• Range: 15% to 100% in 1%. increments

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29-Oct-22
Low Breakpoint
• Defines the upper limit to restraint currents that will be
biased according to the Low Slope setting
• Should be set to be above the maximum load but not more
than the maximum current where the CTs still operate
linearly (including residual flux)
• Assumption is that the CTs will be operating linearly (no
significant saturation effects up to 80% residual flux) up to
the Low Breakpoint setting

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29-Oct-22
High Breakpoint
• Defines the minimum restraint currents that will be biased
according to the High Slope setting
• Should be set to be below the minimum current where the
weakest CT will saturate with no residual flux
• Assumption is that the CTs will be operating linearly (no
significant saturation effects up to 80% residual flux) up to
the Low Breakpoint setting

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29-Oct-22
High Slope
• Defines the percent bias for the restraint currents IRESTHigh
Breakpoint
• Setting determines the stability of the differential element
for high current external faults
• Traditionally, should be set high enough to accommodate
the spurious differential current resulting from saturation
of the CTs during heavy external faults
• Setting can be relaxed in favour of sensitivity and speed as
the relay detects CT saturation and applies the directional
principle to prevent maloperation
• Range: 50% to 100% in 1%. increments

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29-Oct-22
Calculating Unrestrained Bus Differential
Settings
• Defines the minimum differential current for unrestrained
operation
• Should be set to be above the maximum differential current
under worst case CT saturation
• Range: 2.00 to 99.99 p.u. in 0.01 p.u. increments
• Can be effectively disabled by setting to 99.99 p.u.

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Dual Percent Differential Characteristic
High
Breakpoint
Low
Breakpoint
Low Slope
High Slope
High Set
(Unrestrained)
Min Pickup

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29-Oct-22
NORTH BUS
SOUTH BUS
CT-8
B-5
B-6
CT-5
CT-6
S-5
S-6
B-4
CT-4
S-3
S-4
B-3
CT-3
S-1
S-2
B-2
CT-2
CT-1
B-1
C-1 C-2 C-4
C-3 C-5
CT-7
B-7
Protecting re-configurable buses
Reconfigurable Buses

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NORTH BUS
SOUTH BUS
CT-7
CT-8
B-7
B-5
B-6
CT-5
CT-6
S-5
S-6
B-4
CT-4
S-3
S-4
B-3
CT-3
S-1
S-2
B-2
CT-2
CT-1
B-1
C-1 C-2 C-4
C-3 C-5

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NORTH BUS
SOUTH BUS
CT-7
CT-8
B-7
B-5
B-6
CT-5
CT-6
S-5
S-6
B-4
CT-4
S-3
S-4
B-3
CT-3
S-1
S-2
B-2
CT-2
CT-1
B-1
C-1 C-2 C-4
C-3 C-5

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NORTH BUS
SOUTH BUS
CT-8
B-5
B-6
CT-5
CT-6
S-5
S-6
B-4
CT-4
S-3
S-4
B-3
CT-3
S-1
S-2
B-2
CT-2
CT-1
B-1
C-1 C-2 C-4
C-3 C-5
CT-7
B-7

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Isolators
• Reliable “Isolator Closed” signals are needed for the Dynamic
Bus Replica
• In simple applications, a single normally closed contact may
be sufficient
• For maximum safety:
o Both N.O. and N.C. contacts should be used
o Isolator Alarm should be established and non-valid combinations
(open-open, closed-closed) should be sorted out
o Switching operations should be inhibited until bus image is recognized
with 100% accuracy
o Optionally block 87B operation from Isolator Alarm
• Each isolator position signal decides:
o Whether or not the associated current is to be included in the
differential calculations
o Whether or not the associated breaker is to be tripped

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Isolator – Typical Open/Closed
Connections

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Isolator Open
Auxiliary
Contact
Isolator Closed
Auxiliary
Contact
Isolator Position Alarm Block Switching
Off On CLOSED No No
Off Off LAST VALID After time delay
until
acknowledged
Until Isolator
Position is valid
On On CLOSED
On Off OPEN No No
NOTE: Isolator monitoring function may be a built-in feature or user-
programmable in low impedance bus differential digital relays
Switch Status Logic and Dyanamic Bus
Replica

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Differential Zone CT Trouble
• Each Bus Differential Zone may a dedicated CT Trouble
Monitor
• Definite time delay overcurrent element operating on the
zone differential current, based on the configured Dynamic
Bus Replica
• Three strategies to deal with CT problems:
1. Trip the bus zone as the problem with a CT will likely
evolve into a bus fault anyway
2. Do not trip the bus, raise an alarm and try to correct
the problem manually
3. Switch to setting group with 87B minimum pickup
setting above the maximum load current.

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• Strategies 2 and 3 can be accomplished by:
 Using undervoltage supervision to ride through the period
from the beginning of the problem with a CT until declaring a
CT trouble condition
 Using an external check zone to supervise the 87B function
 Using CT Trouble to prevent the Bus Differential tripping (2)
 Using setting groups to increase the pickup value for the 87B
function (3)
Differential Zone CT Trouble

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Differential Zone CT Trouble – Strategy #2
Example
• CT Trouble operand is used to rise an alarm
• The 87B trip is inhibited after CT Trouble
element operates
• The relay may misoperate if an external fault
occurs after CT trouble but before the CT trouble
condition is declared (double-contingency)
87B operates
Undervoltage condition
CT OK

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Example Architecture for Large Busbars
Dual (redundant) fiber with
3msec delivery time between
neighbouring IEDs. Up to 8
relays in the ring
Phase A AC signals and
trip contacts
Phase B AC signals and
trip contacts
Phase C AC signals and
trip contacts
Digital Inputs for isolator
monitoring and BF

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Phase A AC signals wired
here, bus replica configured
here
Phase B AC signals wired
here
Phase C AC signals wired
here
Auxuliary switches wired here;
Isolator Monitoring function
configured here
Example Architecture – Dynamic Bus
Replica and Isolator Position

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here, current status
monitored here
monitored here
monitored here
Breaker Failure
elements configured
here
Example Architecture – BF Initiation &
Current Supervision

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monitored here
monitored here
monitored here
Breaker Fail Op command
generated here and send to trip
appropriate breakers
Trip
Trip
Trip
Example Architecture – Breaker Failure
Tripping Trip

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IEEE 37.234
• “Guide for Protective Relay Applications to Power
System Buses” is currently being revised by the K14
Working Group of the IEEE Power System Relaying
Committee.

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