Transcript Fundamentals of Bus Bar Protection

Fundamentals of
Bus Bar Protection
GE Multilin
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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Multilin
7-Jul-15
Single bus - single breaker
ZONE 1
1
2
3
----
n-1
n
• Distribution and lower transmission voltage levels
• No operating flexibility
• Fault on the bus trips all circuit breakers
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Multiple bus sections - single breaker with
bus tie
ZONE 1
ZONE 2
• Distribution and lower transmission voltage levels
• Limited operating flexibility
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Double bus - single breaker with bus tie
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
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Main and transfer buses
MAIN BUS
ZONE 1
TRANFER BUS
• Increased operating flexibility
• A bus fault requires tripping all breakers
• Transfer bus for breaker maintenance
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Double bus – single breaker w/ transfer bus
ZONE 1
ZONE 2
• Very high operating flexibility
• Transfer bus for breaker maintenance
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Double bus - double breaker
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
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Breaker-and-a-half bus
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
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Ring bus
L1
L2
TB1
B1
B2
TB1
L3
L4
• Higher voltage levels
• High operating flexibility with minimum breakers
• Separate bus protection not required at line positions
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Bus components
breakers
BUS 1
BUS 2
ISO 1
ISO 2
Low Voltage circuit breakers
CB 1
ISO 3
BYPASS
SF6, EHV & HV -
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Synchropuff
7-Jul-15
Disconnect switches & auxiliary contacts
BUS 1
BUS 1
BUS 2
ISO 2
ISOLATOR 1
ISO 1
+
7B
7A
ISOLATOR 1 OPEN
F1a
F1c
F1b
Contact Input F1a On
Contact Input F1c On
-
CB 1
ISO 3
BYPASS
ISOLATOR 1
BUS 1
+
7B
7A
ISOLATOR 1 CLOSED
F1a
F1c
F1b
-
Contact Input F1a On
Contact Input F1c On
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Multilin
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Current Transformers
BUS 1
BUS 2
ISO 1
ISO 2
Gas (SF6) insulated current
transformer
Oil insulated current transformer
(35kV up to 800kV)
CB 1
ISO 3
BYPASS
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 subtransmission 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
BLOCK
50
50
50
50
50
50
• 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
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Overcurrent (unrestrained) Differential
•
•
51
•
•
•
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
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Linear Couplers
ZC = 2  – 20  - typical coil impedance
(5V per 1000Amps => 0.005 @ 60Hz )
40 V
10 V
10 V
0V
20 V
0V
59
External
Fault
If = 8000 A
2000 A
2000 A
0A
4000 A
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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 Internal Bus
Fault
40 V
0V
0A
10 V
2000 A
10 V
2000 A
0V
0A
59
20 V
4000 A
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Linear Couplers
•
•
•
•
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)
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High Impedance Differential
• Operating signal created by
connecting all CT secondaries in
parallel
CTs must all have the same ratio
o Must have dedicated CTs
o
59
• 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
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Percent Differential
•
•
87
51
•
•
I DIF  I1  I 2  ...  I n
I RES  I1  I 2  ...  I n
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 reconfigurable buses
possible
I RES  max  I1 , I 2 , ..., I n 
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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)
52
52
DAU
52
DAU
CU
copper
fiber
DAU
• 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
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Low Impedance Differential (Centralized)
52
52
52
CU
• 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.
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
No DC Offset
• Waveform remains fairly
symmetrical
Ratio Current
CT Current
With DC Offset
Ratio Current
CT Current
• Waveform starts off being
asymmetrical, then
symmetrical in steady
state
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differential
External Fault & Ideal CTs
t1
t0
restraining
• Fault starts at t0
• Steady-state fault conditions occur at t1
Ideal CTs have no saturation or mismatch errors thus
produce no differential current
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differential
External Fault & Actual CTs
t1
t0
restraining
• Fault starts at t0
• Steady-state fault conditions occur at t1
Actual CTs do introduce errors, producing some differential
current (without CT saturation)
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External Fault with CT Saturation
differential
t2
t1
restraining
• Fault starts at t0, CT begins to saturate at t1
• CT fully saturated at t2
t0
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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P -based High-Impedance Bus Differential
Protection Relays
Operating time: 20 – 30ms @ I > 1.5xPKP
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High Impedance Module for Digital
Relays
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
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High-Impedance Module
+
Overcurrent Relay
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High Impedance Bus Protection - Summary
• 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.)
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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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Small Bus Applications
2-8 Circuit Applications
• 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
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Medium to Large Bus Applications
9-12 Circuit Applications
• Relay 1 - 24 Current Inputs
• 4 Zones
• Zone 1 = Phase A (12 currents)
• Zone 2 = Phase B (12 currents)
• Zone 3 = Not used
• Zone 4 = Not used
• Relay 2 - 24 Current Inputs
• 4 Zones
• Zone 1 = Not used
• Zone 2 = Not used
• Zone 3 = Phase C (12 currents)
• Zone 4 = Not used
• Different CT Ratio Capability for Each Circuit
• Largest CT Primary is Base in Relay
CB 11
CB 12
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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-1
I1 = Error
CT-2
I2 = 0
CT-3
I3 = 0
CT-4
IDIFF = Error
IREST = Error
Maloperation if
Error > PICKUP
• 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
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Definitions of Restraint Signals
iR  i1  i2  i3  ...  in
1
iR   i1  i2  i3  ...  in 
n
iR  n i1  i2  i3  ...  in
iR  Max i1 , i2 , i3 ,..., in 
“sum of”
“scaled sum of”
“geometrical average”
“maximum of”
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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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differential
Bus Differential Adaptive Approach
Region 2
(high differential
currents)
Region 1
(low differential
currents)
restraining
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Bus Differential Adaptive Logic Diagram
AND
DIFL
OR
OR
DIR
AND
SAT
87B BIASED OP
DIFH
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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…
External Fault Conditions
 Ip
imag 
 ID  I p





 Ip
imag 
 ID  I p

OPERATE
BLOCK
ID - Ip
Internal Fault Conditions
Ip
 Ip 

real 
 ID  I p 






OPERATE
BLOCK
ID - Ip
 Ip 

real 
 ID  I p 


Ip
BLOCK
BLOCK
OPERATE
OPERATE
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CT Saturation
differential
t2
t1
t0
restraining
• Fault starts at t0, CT begins to saturate at t1
• CT fully saturated at t2
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CT Saturation Detector State Machine
NORMAL
SAT := 0
The differential
current below the
first slope for
certain period of
time
saturation
condition
EXTERNAL
FAULT
SAT := 1
The differential
characteristic
entered
EXTERNAL
FAULT & CT
SATURATION
The differentialrestraining trajectory
out of the differential
characteristic for
certain period of time
SAT := 1
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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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CT Saturation Example – External Fault
200
150
current, A
100
~1 ms
50
0
-50
-100
-150
-200
0.06
0.07
0.08
0.09
0.1
0.11
0.12
time, sec
The bus dif f erential
protection element
picks up due to heav y
CT saturation
The
directional f lag
is not set
The CT saturation f lag
is set saf ely bef ore the
pickup f lag
The element
does not
maloperate
Despite heavy CT
saturation the
external fault current
is seen in the
opposite direction
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CT Saturation – Internal Fault Example
The bus dif f erential
protection element
picks up
The saturation
f lag is not set - no
directional
decision required
All the f ault currents
are seen in one
direction
The element
operates in
10ms
The
directional
f lag is set
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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
CT-2
CT-3
CT-4
F1
3200 A
1A
1
F2
2400 A
5A
1
F3
1200 A
1A
1
F4
3200 A
1A
2
CT-5
CT-6
F5
1200 A
5A
2
F6
5000 A
5A
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
Bus Zone settings defines the boundaries of the Differential
Protection and CT Trouble Monitoring.
1. Configure the physical CT Inputs
o
o
o
CT Primary and Secondary values
Both 5 A and 1 A inputs are supported by the UR hardware
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
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Dual Percent Differential Characteristic
High Set
(Unrestrained)
High Slope
Low Slope
High
Breakpoint
Min Pickup
Low
Breakpoint
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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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Calculating Bus Differential Settings –
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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Calculating Bus Differential Settings –
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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Calculating Bus Differential Settings –
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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Calculating Bus Differential Settings –
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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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 Set
(Unrestrained)
High Slope
Low Slope
High
Breakpoint
Min Pickup
Low
Breakpoint
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Reconfigurable Buses
C-3
C-5
NORTH BUS
S-1
B-1
S-5
S-3
B-5
CT-7
CT-1
CT-2
B-2
CT-3
B-3
CT-4
CT-5
B-4
B-7
CT-6
CT-8
B-6
S-2
S-6
S-4
SOUTH BUS
C-1
C-2
C-4
Protecting re-configurable buses
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Reconfigurable Buses
C-3
C-5
NORTH BUS
S-1
B-1
CT-1
S-5
S-3
B-5
CT-2
B-2
CT-3
CT-4
B-3
CT-7
B-4
CT-5
B-7
CT-6
CT-8
B-6
S-2
S-6
S-4
SOUTH BUS
C-1
C-2
C-4
Protecting re-configurable buses
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Reconfigurable Buses
C-3
C-5
NORTH BUS
S-1
B-1
CT-1
S-5
S-3
B-5
CT-2
B-2
CT-3
CT-4
B-3
CT-7
B-4
CT-5
B-7
CT-6
CT-8
B-6
S-2
S-6
S-4
SOUTH BUS
C-1
C-2
C-4
Protecting re-configurable buses
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Reconfigurable Buses
C-3
C-5
NORTH BUS
S-1
B-1
S-5
S-3
B-5
CT-7
CT-1
CT-2
B-2
CT-3
B-3
CT-4
CT-5
B-4
B-7
CT-6
CT-8
B-6
S-2
S-6
S-4
SOUTH BUS
C-1
C-2
C-4
Protecting re-configurable buses
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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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GE Consumer & Industrial
Multilin
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Isolator – Typical Open/Closed
Connections
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GE Consumer & Industrial
Multilin
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Switch Status Logic and Dyanamic Bus
Replica
Isolator Open
Auxiliary
Contact
Isolator Closed
Auxiliary
Contact
Isolator Position
Alarm
Block Switching
Off
On
CLOSED
No
No
Off
Off
LAST VALID
Until Isolator
Position is valid
On
On
CLOSED
After time delay
until
acknowledged
On
Off
OPEN
No
No
NOTE: Isolator monitoring function may be a built-in feature or userprogrammable in low impedance bus differential digital relays
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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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GE Consumer & Industrial
Multilin
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Differential Zone CT Trouble
• 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)
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GE Consumer & Industrial
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Differential Zone CT Trouble – Strategy #2
Example
87B operates
Undervoltage condition
CT OK
• 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)
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GE Consumer & Industrial
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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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GE Consumer & Industrial
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Example Architecture – Dynamic Bus
Replica and Isolator Position
Phase A AC signals wired
here, bus replica configured
here
Phase B AC signals wired
here, bus replica configured
here
Phase C AC signals wired
here, bus replica configured
here
Auxuliary switches wired here;
Isolator Monitoring function
configured here
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GE Consumer & Industrial
Multilin
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Example Architecture – BF Initiation &
Current Supervision
Phase A AC signals wired
here, current status
monitored here
Phase B AC signals wired
here, current status
monitored here
Phase C AC signals wired
here, current status
monitored here
Breaker Failure
elements configured
here
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GE Consumer & Industrial
Multilin
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Example Architecture – Breaker Failure
Trip
Tripping
Trip
Phase A AC signals wired
here, current status
monitored here
Phase B AC signals wired
here, current status
monitored here
Trip
Trip
Phase C AC signals wired
here, current status
monitored here
Breaker Fail Op command
generated here and send to trip
appropriate breakers
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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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