basics of busbar protection
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Fundamentals ofFundamentals ofBus Bar Bus Bar
ProtectionProtectionGE Multilin
2GE Consumer & Industrial
MultilinApr 13, 2023
Outline
• Bus arrangements• Bus components• Bus protection techniques• CT Saturation• Application Considerations:
High impedance bus differential relaying Low impedance bus differential relaying Special topics
3GE Consumer & Industrial
MultilinApr 13, 2023
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
4GE Consumer & Industrial
MultilinApr 13, 2023
ZONE 1ZONE 2
•Distribution and lower transmission voltage levels
•Limited operating flexibility
Multiple bus sections - single breaker with bus tie
5GE Consumer & Industrial
MultilinApr 13, 2023
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
6GE Consumer & Industrial
MultilinApr 13, 2023
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
7GE Consumer & Industrial
MultilinApr 13, 2023
ZONE 1
ZONE 2
•Very high operating flexibility•Transfer bus for breaker
maintenance
Double bus – single breaker w/ transfer bus
8GE Consumer & Industrial
MultilinApr 13, 2023
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
9GE Consumer & Industrial
MultilinApr 13, 2023
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
10GE Consumer & Industrial
MultilinApr 13, 2023
•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
11GE Consumer & Industrial
MultilinApr 13, 2023
Bus components breakers
SF6, EHV & HV - Synchropuff
Low Voltage circuit breakers
BUS 2
CB 1
BUS 1
ISO 1 ISO 2
ISO 3BYPASS
12GE Consumer & Industrial
MultilinApr 13, 2023
-
+
F1aF1c
Contact Input F1a OnContact Input F1c On
F1b
ISO
LA
TO
R 1
ISOLATOR 1 OPEN
7B 7A
BUS 1
-
+
F1aF1c
Contact Input F1a OnContact Input F1c On
F1b
ISO
LA
TO
R 1
ISOLATOR 1 CLOSED
7B 7A
BUS 1
Disconnect switches & auxiliary contacts
BUS 2
CB 1
BUS 1
ISO 1 ISO 2
ISO 3BYPASS
13GE Consumer & Industrial
MultilinApr 13, 2023
BUS 2
CB 1
BUS 1
ISO 1 ISO 2
ISO 3BYPASS
Current Transformers
Oil insulated current transformer (35kV up to 800kV)
Gas (SF6) insulated current transformer
Bushing type (medium voltage switchgear)
14GE Consumer & Industrial
MultilinApr 13, 2023
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
15GE Consumer & Industrial
MultilinApr 13, 2023
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
16GE Consumer & Industrial
MultilinApr 13, 2023
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
BLO
CK
17GE Consumer & Industrial
MultilinApr 13, 2023
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
18GE Consumer & Industrial
MultilinApr 13, 2023
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
ExternalFault
19GE Consumer & Industrial
MultilinApr 13, 2023
59
Linear CouplersEsec= Iprim*Xm - secondary voltage on relay terminals
IR= Iprim*Xm /(ZR+ZC) – minimum operating current
where,Iprim – primary current in each circuitXm – liner coupler mutual reactance (5V per 1000Amps => 0.005 @ 60Hz )ZR – relay tap impedanceZC – sum of all linear coupler self impedancesIf =
8000 A
0 A
0 V 10 V 10 V 0 V 20 V40 V
2000 A
2000 A
4000 A
0 A
Internal BusFault
20GE Consumer & Industrial
MultilinApr 13, 2023
• 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
21GE Consumer & Industrial
MultilinApr 13, 2023
High Impedance Differential• Operating signal created by
connecting all CT secondaries in parallel
o CTs must all have the same ratioo 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
22GE Consumer & Industrial
MultilinApr 13, 2023
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
5187
nDIF IIII ...21
nRES IIII ...21 nRES IIII ...,,,max 21
23GE Consumer & Industrial
MultilinApr 13, 2023
Low Impedance Percent Differential• Individual currents sampled by protection and summated
digitallyo 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 buseso Done digitally with logic to add/remove current inputs from
differential computationo 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 recordingo Breaker failure protection
24GE Consumer & Industrial
MultilinApr 13, 2023
Digital Differential Algorithm Goals• Improve the main differential algorithm operation
o Better filteringo Faster responseo Better restraint techniqueso 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 saturationo CT secondary circuit trouble (e.g. short circuits)
25GE Consumer & Industrial
MultilinApr 13, 2023
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
26GE Consumer & Industrial
MultilinApr 13, 2023
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
27GE Consumer & Industrial
MultilinApr 13, 2023
CT Saturation
28GE Consumer & Industrial
MultilinApr 13, 2023
CT Saturation Concepts
• CT saturation depends on a number of factorso 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
29GE Consumer & Industrial
MultilinApr 13, 2023
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
30GE Consumer & Industrial
MultilinApr 13, 2023
External Fault & Ideal CTs
• Fault starts at t0
• Steady-state fault conditions occur at t1
t0
t1
Ideal CTs have no saturation or mismatch errors thus produce no differential current
31GE Consumer & Industrial
MultilinApr 13, 2023
External Fault & Actual CTs
• Fault starts at t0
• Steady-state fault conditions occur at t1
t0
t1
Actual CTs do introduce errors, producing some differential current (without CT saturation)
32GE Consumer & Industrial
MultilinApr 13, 2023
External Fault with CT Saturation
• Fault starts at t0, CT begins to saturate at t1
• CT fully saturated at t2
t0
t1
t2
CT saturation causes increasing differential current that may enter the differential element operate region.
33GE Consumer & Industrial
MultilinApr 13, 2023
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
presento 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) supervisiono Improves security by requiring all currents flow into the bus zone
before asserting the differential elemento Easy to implement and testo Stable even under severe CT saturation during external faults
34GE Consumer & Industrial
MultilinApr 13, 2023
High-Impedance
Bus Differential
Considerations
35GE Consumer & Industrial
MultilinApr 13, 2023
High Impedance Voltage-operated RelayExternal 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
36GE Consumer & Industrial
MultilinApr 13, 2023
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.
37GE Consumer & Industrial
MultilinApr 13, 2023
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.
38GE Consumer & Industrial
MultilinApr 13, 2023
Electromechanical High Impedance Bus Differential Relays• Single phase relays• High-speed• High impedance voltage sensing• High seismic IOC unit
39GE Consumer & Industrial
MultilinApr 13, 2023
Operating time: 20 – 30ms @ I > 1.5xPKP
P -based High-Impedance Bus Differential Protection Relays
40GE Consumer & Industrial
MultilinApr 13, 2023
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 to1900 Volts 86 – latching contact preventing the resistors from overheating after the fault is detected
High Impedance Module for Digital Relays
41GE Consumer & Industrial
MultilinApr 13, 2023
High-Impedance Module +
Overcurrent Relay
42GE Consumer & Industrial
MultilinApr 13, 2023
• 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
43GE Consumer & Industrial
MultilinApr 13, 2023
Low-Impedance
Bus Differential
Considerations
44GE Consumer & Industrial
MultilinApr 13, 2023
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
45GE Consumer & Industrial
MultilinApr 13, 2023
• 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
46GE Consumer & Industrial
MultilinApr 13, 2023
• 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
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 1 = Not used• Zone 2 = Not used• Zone 3 = Phase C (12 currents)• Zone 4 = Not used
9-12 Circuit Applications
Medium to Large Bus Applications
47GE Consumer & Industrial
MultilinApr 13, 2023
Large Bus Applications
87B phase A
87B phase B
87B phase C
Logic relay(switch status,optional BF)
48GE Consumer & Industrial
MultilinApr 13, 2023
Large Bus ApplicationsFor buses with up to 24 circuits
49GE Consumer & Industrial
MultilinApr 13, 2023
Summing External CurrentsNot Recommended for Low-Z 87B relays
• Relay becomes combination of restrained and unrestrained elements•In order to parallel CTs:• CT performance must be
closely matchedo Any errors will appear as
differential currents• Associated feeders must be
radialo No backfeeds possible
• Pickup setting must be raised to accommodate any errors
CT-1
CT-2
CT-3
CT-4
I 3 =
0I 2
= 0
I 1 =
Erro
r
IDIFF = Error
IREST = Error
Maloperation ifError > PICKUP
50GE Consumer & Industrial
MultilinApr 13, 2023
Definitions of Restraint Signals
“maximum of”
“geometrical average”
“scaled sum of”
“sum of”nR iiiii ...321
nR iiiin
i ...1
321
nR iiiiMaxi ,...,,, 321
nnR iiiii ...321
51GE Consumer & Industrial
MultilinApr 13, 2023
“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)
52GE Consumer & Industrial
MultilinApr 13, 2023
Bus Differential Adaptive Approach
diffe
rent
ial
restraining
Region 1(low differential
currents)
Region 2(high differential
currents)
53GE Consumer & Industrial
MultilinApr 13, 2023
Bus Differential Adaptive Logic Diagram
DIFL
DIR
SAT
DIFH
OR
AN
D
OR 87B BIASED OP
AN
D
54GE Consumer & Industrial
MultilinApr 13, 2023
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)
55GE Consumer & Industrial
MultilinApr 13, 2023
Phase Comparison Principle Continued…
BLOCK
OPERATE
BLOCK
pD
p
II
Ireal
pD
p
II
Iimag
Ip
ID - I p
External Fault Conditions
OPERATE
BLOCK
BLOCK
pD
p
II
Ireal
pD
p
II
Iimag
Ip
ID - I p
Internal Fault Conditions
OPERATE
OPERATE
56GE Consumer & Industrial
MultilinApr 13, 2023
CT Saturation
• Fault starts at t0, CT begins to saturate at t1
• CT fully saturated at t2
t0
t1
t2
57GE Consumer & Industrial
MultilinApr 13, 2023
CT Saturation Detector State Machine NORMAL
SAT := 0
EXTERNAL
FAULT
SAT := 1
EXTERNALFAULT & CT
SATURATION
SAT := 1
The differentialcharacteristic
entered
The differential-restraining trajectoryout of the differential
characteristic forcertain period of time
saturationcondition
The differentialcurrent below thefirst slope forcertain period oftime
58GE Consumer & Industrial
MultilinApr 13, 2023
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
59GE Consumer & Industrial
MultilinApr 13, 2023
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
60GE Consumer & Industrial
MultilinApr 13, 2023
The bus differentialprotection elementpicks up due to heavyCT saturation
The CT saturation flagis set safely before thepickup flag
Thedirectional flagis not set
The elementdoes notmaloperate
Despite heavy CTsaturation theexternal fault currentis seen in theopposite 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
curr
en
t, A
~1 ms
61GE Consumer & Industrial
MultilinApr 13, 2023
The bus differentialprotection elementpicks up
The saturationflag is not set - nodirectionaldecision required
The elementoperates in10ms
Thedirectionalflag is set
All the fault currentsare seen in one
direction
CT Saturation – Internal Fault Example
62GE Consumer & Industrial
MultilinApr 13, 2023
Applying Low-Impedance Differential Relays for Busbar ProtectionBasic Topics• Configure physical CT Inputs• Configure Bus Zone and Dynamic Bus
Replica• Calculating Bus Differential Element settingsAdvanced Topics• Isolator switch monitoring for reconfigurable
buses• Differential Zone CT Trouble• Integrated Breaker Failure protection
63GE Consumer & Industrial
MultilinApr 13, 2023
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
64GE Consumer & Industrial
MultilinApr 13, 2023
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
65GE Consumer & Industrial
MultilinApr 13, 2023
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 operando 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
66GE Consumer & Industrial
MultilinApr 13, 2023
Configuring the Bus Differential Zone
1. Configure the physical CT Inputso CT Primary and Secondary valueso Both 5 A and 1 A inputs are supported by the UR hardwareo 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.
67GE Consumer & Industrial
MultilinApr 13, 2023
Dual Percent Differential Characteristic
High Breakpoint
Low Breakpoint
Low Slope
High Slope
High Set (Unrestrained)
Min Pickup
68GE Consumer & Industrial
MultilinApr 13, 2023
Calculating Bus Differential Settings• The following Bus Zone Differential element
parameters need to be set:o Differential Pickupo Restraint Low Slopeo Restraint Low Break Pointo Restraint High Breakpointo Restraint High Slopeo 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)
69GE Consumer & Industrial
MultilinApr 13, 2023
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
70GE Consumer & Industrial
MultilinApr 13, 2023
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
71GE Consumer & Industrial
MultilinApr 13, 2023
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
72GE Consumer & Industrial
MultilinApr 13, 2023
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
73GE Consumer & Industrial
MultilinApr 13, 2023
Calculating Bus Differential Settings – High Slope• Defines the percent bias for the restraint currents
IRESTHigh 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
74GE Consumer & Industrial
MultilinApr 13, 2023
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.
75GE Consumer & Industrial
MultilinApr 13, 2023
Dual Percent Differential Characteristic
High Breakpoint
Low Breakpoint
Low Slope
High Slope
High Set (Unrestrained)
Min Pickup
76GE Consumer & Industrial
MultilinApr 13, 2023
NORTH BUS
SOUTH BUS
CT-8
B-5
B-6
CT-5
CT-6
S-5
S-6
B-4CT-4
S-3
S-4
B-3CT-3
S-1
S-2
B-2CT-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
77GE Consumer & Industrial
MultilinApr 13, 2023
NORTH BUS
SOUTH BUS
CT-7
CT-8
B-7
B-5
B-6
CT-5
CT-6
S-5
S-6
B-4CT-4
S-3
S-4
B-3CT-3
S-1
S-2
B-2CT-2CT-1
B-1
C-1 C-2 C-4
C-3 C-5
Protecting re-configurable buses
Reconfigurable Buses
78GE Consumer & Industrial
MultilinApr 13, 2023
NORTH BUS
SOUTH BUS
CT-7
CT-8
B-7
B-5
B-6
CT-5
CT-6
S-5
S-6
B-4CT-4
S-3
S-4
B-3CT-3
S-1
S-2
B-2CT-2CT-1
B-1
C-1 C-2 C-4
C-3 C-5
Protecting re-configurable buses
Reconfigurable Buses
79GE Consumer & Industrial
MultilinApr 13, 2023
NORTH BUS
SOUTH BUS
CT-8
B-5
B-6
CT-5
CT-6
S-5
S-6
B-4CT-4
S-3
S-4
B-3CT-3
S-1
S-2
B-2CT-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
80GE Consumer & Industrial
MultilinApr 13, 2023
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 usedo 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 calculationso Whether or not the associated breaker is to be tripped
81GE Consumer & Industrial
MultilinApr 13, 2023
Isolator – Typical Open/Closed Connections
82GE Consumer & Industrial
MultilinApr 13, 2023
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
83GE Consumer & Industrial
MultilinApr 13, 2023
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 anyway2. Do not trip the bus, raise an alarm and try to
correct the problem manually3. Switch to setting group with 87B minimum
pickup setting above the maximum load current.
84GE Consumer & Industrial
MultilinApr 13, 2023
• 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
85GE Consumer & Industrial
MultilinApr 13, 2023
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
86GE Consumer & Industrial
MultilinApr 13, 2023
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
87GE Consumer & Industrial
MultilinApr 13, 2023
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
Isolato
r Posit
ion
Isolator Position
Isolator Position
Isolator P
osition
Example Architecture – Dynamic Bus Replica and Isolator Position
88GE Consumer & Industrial
MultilinApr 13, 2023
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
BF Initia
te &
Curre
nt Supv.
BF Initiate & Current Supv.
BF Initiate & Current Supv.
BF Initiate & Curre
nt Supv.
Example Architecture – BF Initiation & Current Supervision
89GE Consumer & Industrial
MultilinApr 13, 2023
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 Fail Op command generated here and send to trip appropriate breakers
Breake
r Fail O
p
Breaker Fail Op
Breaker Fail Op
Breaker Fail O
p
Trip
TripTrip
Example Architecture – Breaker Failure TrippingTrip
90GE Consumer & Industrial
MultilinApr 13, 2023
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.
91GE Consumer & Industrial
MultilinApr 13, 2023
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