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Fundamentals of Bus Bar Protection






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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.
Fundamentals of Bus Bar Protection

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Slide 1

Fundamentals ofBus Bar Protection

GE Multilin

Slide 2

Outline

  • Bus arrangements

  • Bus components

  • Bus protection techniques

  • CT Saturation

  • Application Considerations:

    • High impedance bus differential relaying

    • Low impedance bus differential relaying

    • Special topics

Slide 3

Single bus - single breaker

  • Distribution and lower transmission voltage levels

  • No operating flexibility

  • Fault on the bus trips all circuit breakers

Slide 4

Multiple bus sections - single breaker with bus tie

  • Distribution and lower transmission voltage levels

  • Limited operating flexibility

Slide 5

Double bus - single breaker with bus tie

  • Transmission and distribution voltage levels

  • Breaker maintenance without circuit removal

  • Fault on a bus disconnects only the circuits being connected to that bus

Slide 6

Main and transfer buses

  • Increased operating flexibility

  • A bus fault requires tripping all breakers

  • Transfer bus for breaker maintenance

Slide 7

Double bus – single breaker w/ transfer bus

  • Very high operating flexibility

  • Transfer bus for breaker maintenance

Slide 8

Double bus - double breaker

  • High operating flexibility

  • Line protection covers bus section between two CTs

  • Fault on a bus does not disturb the power to circuits

Slide 9

Breaker-and-a-half bus

  • Used on higher voltage levels

  • More operating flexibility

  • Requires more breakers

  • Middle bus sections covered by line or other equipment protection

Slide 10

Ring bus

  • Higher voltage levels

  • High operating flexibility with minimum breakers

  • Separate bus protection not required at line positions

Slide 11

SF6, EHV & HV - Synchropuff

Low Voltage circuit breakers

Bus components

breakers

Slide 12

Disconnect switches & auxiliary contacts

Slide 13

Oil insulated current transformer (35kV up to 800kV)

Gas (SF6) insulated current transformer

Bushing type (medium voltage switchgear)

CurrentTransformers

Slide 14

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

Slide 15

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

Slide 16

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

Interlocking Schemes

Slide 17

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

Overcurrent (unrestrained) Differential

Slide 18

0 V

40 V

10 V

10 V

0 V

20 V

If = 8000 A

2000 A

2000 A

0 A

4000 A

Linear Couplers

ZC = 2  – 20  - typical coil impedance

(5V per 1000Amps => 0.005 @ 60Hz )

59

External

Fault

Slide 19

If = 8000 A

40 V

0 V

10 V

10 V

0 V

20 V

0 A

2000 A

2000 A

0 A

4000 A

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

Internal Bus

Fault

59

Slide 20

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

Slide 21

High Impedance Differential

  • Operating signal created by connecting all CT secondaries in parallel

    • CTs must all have the same ratio

    • Must have dedicated CTs

  • Overvoltage element operates on voltage developed across resistor connected in secondary circuit

    • Requires varistors or AC shorting relays to limit energy during faults

  • Accuracy dependent on secondary circuit resistance

    • Usually requires larger CT cables to reduce errors  higher cost

  • Cannot easily be applied to reconfigurable buses and offers no advanced functionality

Slide 22

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

Percent Differential

Slide 23

Low Impedance Percent Differential

  • Individual currents sampled by protection and summated digitally

    • CT ratio matching done internally (no auxiliary CTs)

    • Dedicated CTs not necessary

  • Additional algorithms improve security of percent differential characteristic during CT saturation

  • Dynamic bus replica allows application to reconfigurable buses

    • Done digitally with logic to add/remove current inputs from differential computation

    • Switching of CT secondary circuits not required

  • Low secondary burdens

  • Additional functionality available

    • Digital oscillography and monitoring of each circuit connected to bus zone

    • Time-stamped event recording

    • Breaker failure protection

Slide 24

Digital Differential Algorithm Goals

  • Improve the main differential algorithm operation

    • Better filtering

    • Faster response

    • Better restraint techniques

    • 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

    • External faults with CT saturation

    • CT secondary circuit trouble (e.g. short circuits)

Slide 25

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

Slide 26

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.

Slide 27

CT Saturation

Slide 28

CT Saturation Concepts

  • CT saturation depends on a number of factors

    • Physical CT characteristics (size, rating, winding resistance, saturation voltage)

    • Connected CT secondary burden (wires + relays)

    • Primary current magnitude, DC offset (system X/R)

    • 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

Slide 29

CT Saturation

No DC Offset

  • Waveform remains fairly symmetrical

With DC Offset

  • Waveform starts off being asymmetrical, then symmetrical in steady state

Slide 30

External Fault & Ideal CTs

  • Fault starts at t0

  • Steady-state fault conditions occur at t1

t1

t0

  • Ideal CTs have no saturation or mismatch errors thus produce no differential current

Slide 31

External Fault & Actual CTs

  • Fault starts at t0

  • Steady-state fault conditions occur at t1

t1

t0

Actual CTs do introduce errors, producing some differential current (without CT saturation)

Slide 32

External Fault with CT Saturation

t2

  • Fault starts at t0, CT begins to saturate at t1

  • CT fully saturated at t2

t1

t0

  • CT saturation causes increasing differential current that may enter the differential element operate region.

Slide 33

Some Methods of Securing Bus Differential

  • Block the bus differential for a period of time (intentional delay)

    • Increases security as bus zone will not trip when CT saturation is present

    • Prevents high-speed clearance for internal faults with CT saturation or evolving faults

  • Change settings of the percent differential characteristic (usually Slope 2)

    • Improves security of differential element by increasing the amount of spurious differential current needed to incorrectly trip

    • Difficult to explicitly develop settings (Is 60% slope enough? Should it be 75%?)

  • Apply directional (phase comparison) supervision

    • Improves security by requiring all currents flow into the bus zone before asserting the differential element

    • Easy to implement and test

    • Stable even under severe CT saturation during external faults

Slide 34

High-Impedance Bus Differential Considerations

Slide 35

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

Slide 36

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.

Slide 37

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.

Slide 38

Electromechanical High Impedance Bus Differential Relays

  • Single phase relays

  • High-speed

  • High impedance voltage sensing

  • High seismic IOC unit

Slide 39

P -based High-Impedance Bus Differential Protection Relays

Operating time: 20 – 30ms @ I > 1.5xPKP

Slide 40

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

Slide 41

High-Impedance Module + Overcurrent Relay

Slide 42

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

Slide 43

Low-Impedance Bus Differential Considerations

Slide 44

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

Slide 45

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

Slide 46

CB 11

CB 12

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

Slide 47

Large Bus Applications

87B phase A

87B phase B

87B phase C

Logic relay

(switch status,

optional BF)

Slide 48

Large Bus ApplicationsFor buses with up to 24 circuits

Slide 49

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 matched

      • Any errors will appear as differential currents

    • Associated feeders must be radial

      • No backfeeds possible

    • Pickup setting must be raised to accommodate any errors

Slide 50

Definitions of Restraint Signals

“sum of”

“scaled sum of”

“geometrical average”

“maximum of”

Slide 51

“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)

“Sum Of” vs. “Max Of” Restraint Methods

Slide 52

Bus Differential Adaptive Approach

Slide 53

DIFL

AND

OR

DIR

OR

87B BIASED OP

AND

SAT

DIFH

Bus Differential Adaptive Logic Diagram

Slide 54

Secondary Current of Faulted Circuit(Severe CT Saturation)

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

Slide 55

Phase Comparison Principle Continued…

Slide 56

CT Saturation

  • Fault starts at t0, CT begins to saturate at t1

  • CT fully saturated at t2

t2

t1

t0

Slide 57

NORMAL

SAT := 0

The differential

saturation

current below the

condition

first slope for

certain period of

time

EXTERNAL

FAULT

SAT := 1

The differential-

The differential

restraining trajectory

characteristic

out of the differential

entered

characteristic for

certain period of time

EXTERNAL

FAULT & CT

SATURATION

SAT := 1

CT Saturation Detector State Machine

Slide 58

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

Slide 59

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)

    • SAT saturation detector flag asserts prior to BIASED PKP bus differential pickup

    • 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

    • BIASED PKP and BIASED OP both assert before DIR asserts

    • CT saturation does not block bus differential

  • More examples available (COMTRADE files) upon request

Slide 60

200

150

~1 ms

100

50

current, A

0

-50

-100

-150

-200

0.06

0.07

0.08

0.09

0.1

0.11

0.12

time, sec

Despite heavy CT

saturation the

external fault current

is seen in the

opposite direction

CT Saturation Example – External Fault

Slide 61

CT Saturation – Internal Fault Example

Slide 62

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

Slide 63

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

Slide 64

Per-Unit Current Definition - Example

  • For Zone 1, 1 p.u. = 3200 AP

  • For Zone 2, 1 p.u. = 5000 AP

Slide 65

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

    • Status signals can be developed in programmable logic to provide additional checks or security as required

    • 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

Slide 66

Configuring the Bus Differential Zone

Bus Zone settings defines the boundaries of the Differential Protection and CT Trouble Monitoring.

  • Configure the physical CT Inputs

    • 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

  • Configure AC Signal Sources

  • Configure Bus Zone with Dynamic Bus Replica

Slide 67

High Breakpoint

Low Breakpoint

Dual Percent Differential Characteristic

High Set (Unrestrained)

High Slope

Low Slope

Min Pickup

Slide 68

Calculating Bus Differential Settings

  • The following Bus Zone Differential element parameters need to be set:

    • Differential Pickup

    • Restraint Low Slope

    • Restraint Low Break Point

    • Restraint High Breakpoint

    • Restraint High Slope

    • 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)

Slide 69

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

Slide 70

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

Slide 71

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

Slide 72

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

Slide 73

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

Slide 74

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.

Slide 75

High Breakpoint

Low Breakpoint

Dual Percent Differential Characteristic

High Set (Unrestrained)

High Slope

Low Slope

Min Pickup

Slide 76

Reconfigurable Buses

Protecting re-configurable buses

Slide 77

Reconfigurable Buses

Protecting re-configurable buses

Slide 78

Reconfigurable Buses

Protecting re-configurable buses

Slide 79

Reconfigurable Buses

Protecting re-configurable buses

Slide 80

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:

    • Both N.O. and N.C. contacts should be used

    • Isolator Alarm should be established and non-valid combinations (open-open, closed-closed) should be sorted out

    • Switching operations should be inhibited until bus image is recognized with 100% accuracy

    • Optionally block 87B operation from Isolator Alarm

  • Each isolator position signal decides:

    • Whether or not the associated current is to be included in the differential calculations

    • Whether or not the associated breaker is to be tripped

Slide 81

Isolator – Typical Open/Closed Connections

Slide 82

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

Switch Status Logic and Dyanamic Bus Replica

  • NOTE: Isolator monitoring function may be a built-in feature or user-programmable in low impedance bus differential digital relays

Slide 83

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:

    • Trip the bus zone as the problem with a CT will likely evolve into a bus fault anyway

    • Do not trip the bus, raise an alarm and try to correct the problem manually

    • Switch to setting group with 87B minimum pickup setting above the maximum load current.

Slide 84

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)

Slide 85

Undervoltage condition

87B operates

CT OK

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)

Slide 86

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

Example Architecture for Large Busbars

Slide 87

Isolator Position

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

Isolator Position

Isolator Position

Auxuliary switches wired here; Isolator Monitoring function configured here

Example Architecture – Dynamic Bus Replica and Isolator Position

Slide 88

BF Initiate & Current Supv.

BF Initiate & Current Supv.

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

BF Initiate & Current Supv.

BF Initiate & Current Supv.

Breaker Failure elements configured here

Example Architecture – BF Initiation & Current Supervision

Slide 89

Trip

Trip

Trip

Trip

Breaker Fail Op

Breaker Fail Op

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

Breaker Fail Op

Example Architecture – Breaker Failure Tripping

Breaker Fail Op command generated here and send to trip appropriate breakers

Slide 90

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.

Slide 91

Questions?

Slide 92

Thanks for the time


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