Tema 07 - High Low Impedance Busbar Protection 092

8/10/2019 Tema 07 - High Low Impedance Busbar Protection 092

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Fundamentals of

Bus Bar Protection

GE Multilin


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GE Consumer & IndustrialMultilin

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Outline

Bus arrangementsBus componentsBus protection techniquesCT SaturationApplication Considerations:

High impedance bus differential relayingLow impedance bus differential relayingSpecial topics


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Distribution and lower transmission voltage levels

No operating flexibility Fault on the bus trips all circuit breakers

Single bus - single breaker


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Distribution and lower transmission voltage levels

Limited operating flexibility

Multiple bus sections - single breaker withbus tie


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Transmission and distribution voltage levels Breaker maintenance without circuit removal

Fault on a bus disconnects only the circuits being connected

to that bus

Double bus - single breaker with bus tie


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Increased operating flexibility A bus fault requires tripping all breakers

Transfer bus for breaker maintenance

Main and transfer buses


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Very high operating flexibility Transfer bus for breaker maintenance

Double bus single breaker w/ transfer bus


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High operating flexibility Line protection covers bus section between two CTs

Fault on a bus does not disturb the power to circuits

Double bus - double breaker


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Used on higher voltage levels

More operating flexibility

Requires more breakers

Middle bus sections covered by line or other equipment

protection

Breaker-and-a-half bus


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Higher voltage levels

High operating flexibility with minimum breakers

Separate bus protection not required at line positions

Ring bus


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Bus componentsbreakers

SF6, EHV & HV - Synchropuff

Low Voltage circuit breakers


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12GE Consumer & Industrial

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Disconnect switches & auxiliary contacts


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Protection Requirements

High bus fault currents due to large number of circuitsconnected:

CT saturation often becomes a problem as CTs may not be sufficientlyrated for worst fault condition case

large dynamic forces associated with bus faults require fast clearingtimes 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 isalways given to security


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Bus Protection Techniques

Interlocking schemes Overcurrent (unrestrained or unbiased)

differential

Overcurrent percent (restrained or biased)differential

Linear couplers

High-impedance bus differential schemes

Low-impedance bus differential schemes


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Interlocking Schemes

Blocking scheme typically

used Short coordination time

required

Care must be taken with

possible saturation of feederCTs

Blocking signal could be sentover communications ports(peer-to-peer)

This technique is limited tosimple one-incomerdistribution buses


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Overcurrent (unrestrained) Differential

Differential signal formed bysummation of all currents feedingthe bus

CT ratio matching may berequired

On external faults, saturated CTsyield spurious differential current

Time delay used to cope with CTsaturation

Instantaneous differential OC

function useful on integratedmicroprocessor-based relays


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59

Linear Couplers

ZC= 2 20 - typical coil impedance

(5V per 1000Amps => 0.005 @ 60Hz )

If = 8000 A

40 V 10 V 10 V 0 V 20 V

2000 A 2000 A 4000 A0 A

0 V

ExternalFault


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59

Linear CouplersEsec= Iprim*Xm - secondary voltage on relay terminals

IR= Iprim*Xm /(ZR+ZC) minimum operating current

where,Iprim primary current in each circuitXmliner coupler mutual reactance (5V per 1000Amps => 0.005 @ 60Hz )ZR relay tap impedance

ZC sum of all linear coupler self impedances

If = 8000 A

0 A

0 V 10 V 10 V 0 V 20 V

40 V

2000 A 2000 A 4000 A0 A

Internal BusFault


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Fast, secure and proven

Require dedicated air gap CTs, which may not be used forany other protection

Cannot be easily applied to reconfigurable buses The scheme uses a simple voltage detector it does notprovide benefits of a microprocessor-based relay (e.g.oscillography, breaker failure protection, other functions)

Linear Couplers


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Percent Differential

Percent characteristic usedto cope with CT saturationand other errors

Restraining signal can be

formed in a number ofways

No dedicated CTs needed

Used for protection of re-

configurable busespossible


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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 differentialcharacteristic during CT saturation

Dynamic bus replica allows application to reconfigurable buseso Done digitally with logic to add/remove current inputs from differentialcomputation

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 toprevent incorrect operation

o External faults with CT saturationo CT secondary circuit trouble (e.g. short circuits)


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Low Impedance Differential (Distributed)

Data Acquisition Units (DAUs)

installed in bays

Central Processing Unit (CPU)processes all data from DAUs

Communications between DAUsand CPU over fiber usingproprietary protocol

Sampling synchronisationbetween DAUs is required

Perceived less reliable (morehardware needed)

Difficult to apply in retrofitapplications


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Low Impedance Differential (Centralized)

All currents applied to a singlecentral processor

No communications, externalsampling synchronisationnecessary

Perceived more reliable (lesshardware needed)

Well suited to both new andretrofit applications.


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CT Saturation


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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 asthe ratio (scaled primary) current during faults

End result is spurious differential current appearing in the summationof the secondary currents which may cause differential elements tooperate if additional security is not applied


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CT Saturation

No DC Offset

Waveform remains fairlysymmetrical

With DC Offset

Waveform starts off beingasymmetrical, thensymmetrical in steadystate


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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 thusproduce no differential current

l l l


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External Fault & Actual CTs

Fault starts at t0 Steady-state fault conditions occur at t1

t0

t1

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

E l F l i h CT S i


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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 thatmay 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 orevolving faults

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

o Improves security of differential element by increasing the amount ofspurious differential current needed to incorrectly trip

o Difficult to explicitly develop settings (Is 60% slope enough? Should it be75%?)

Apply directional (phase comparison) supervision

o Improves security by requiring all currents flow into the bus zone beforeasserting 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 Voltage-operated RelayExternal Fault

59 element set above max possible voltage developed acrossrelay during external fault causing worst case CT saturationFor internal faults, extremely high voltages (well above 59

element pickup) will develop across relay


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High Impedance Voltage Operated RelayRatio matching with Multi-ratio CTs

Use of auxiliary CTs to obtain correct ratio matching is alsopossible, but these CTs must be able to deliver enough voltagenecessary to produce relay operation for internal faults.


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Electromechanical High Impedance BusDifferential Relays

Single phase relays

High-speed

High impedance voltage sensing

High seismic IOC unit


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Hi h I d M d l


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High-Impedance Module+

Overcurrent Relay


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Fast, secure and proven

Requires dedicated CTs, preferably with the same CT ratioand 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 forboth sensitivity and security

Cannot be easily applied to reconfigurable buses

Require voltage limiting varistor capable of absorbingsignificant energy

May require auxiliary CTs

Do not provide full benefits of microprocessor-based relaysystem (e.g. metering, monitoring, oscillography, etc.)

High Impedance Bus Protection - Summary


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Low-Impedance

Bus Differential

Considerations


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P-based Low-Impedance Relays

No need for dedicated CTs

Internal CT ratio mismatch compensation

Advanced algorithms supplement percent differential

protectionfunction making the relay very secure

Dynamic bus replica (bus image) principle is used inprotection of reconfigurable bus bars, eliminating the need

for switching physically secondary current circuits

Integrated Breaker Failure (BF) function can provide

optimal tripping strategy depending on the actual

configuration of a bus bar


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Up to 24 Current Inputs 4 Zones

Zone 1 = Phase A Zone 2 = Phase B Zone 3 = Phase C Zone 4 = Not used

Different CT Ratio Capability forEach Circuit

Largest CT Primary is Base inRelay

2-8 Circuit Applications

Small Bus Applications


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Relay 1 - 24 Current Inputs

4 Zones Zone 1 = Phase A (12 currents) Zone 2 = Phase B (12 currents) Zone 3 = Not used Zone 4 = Not used

CB 12CB 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


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Large Bus Applications

87B phase A

87B phase B

87B phase C

Logic relay

(switch status,

optional BF)

L B A li ti


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Large Bus ApplicationsFor buses with up to 24 circuits

S i E t l C t


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Summing External CurrentsNot Recommended for Low-Z 87B relays

Relay becomes combinationof restrained and unrestrainedelements

In order to parallel CTs:

CT performance must be closelymatched

o Any errors will appear asdifferential 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

maximum of

geometrical average

scaled sum of

sum of


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ff l d h


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Bus Differential Adaptive Approach

ff l d


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Bus Differential Adaptive Logic Diagram

DIFL

DIR

SAT

DIFH

OR

AND

O

R87B BIASED OP

AND

h i i i l


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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 ofFaulted Circuit

(Severe CT Saturation)

h i i i l i d


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Phase Comparison Principle Continued

CT S i


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CT Saturation

Fault starts at t0, CT begins to saturate at t1 CT fully saturated at t2

t0

t1

t2

CT S t ti D t t St t M hi


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CT Saturation Detector State Machine

NORMAL

SAT := 0

EXTERNAL

FAULT

SAT := 1

EXTERNAL

FAULT & CT

SATURATION

SAT := 1

The differential

characteristic

entered

The differential-

restraining trajectory

out of the differential

characteristic for

certain period of time

saturation

condition

The differential

current below the

first slope for

certain period of

time

CT S t ti D t t O ti


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CT Saturation Detector OperatingPrinciples

The 87B SAT flag WILL NOTbe set during internal faults,regardless of whether or not any of the CTs saturate.

The 87B SAT flag WILLbe set during external faults,

regardless of whether or not any of the CTs saturate. By design, the 87B SAT flag WILLforce the relay to use

the additional 87B DIR phase comparison for Region 2

The Saturation Detector WILL NOT Block the Operation ofthe Differential Element it will only Force 2-out-of-2Operation

CT S t ti D t t E l


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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 ofgood CT performance)

o SAT saturation detector flag asserts prior to BIASED PKP busdifferential 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

CT S t ti E l E t l F lt


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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

current,A

~1 ms

CT S t ti I t l F lt E l


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CT Saturation Internal Fault Example

A l i L I d Diff ti l


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Applying Low-Impedance DifferentialRelays for Busbar Protection

Basic 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

C fi i CT I t


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Configuring CT Inputs

For each connected CT circuit enter Primary rating andselect Secondary rating.

Each 3-phase bank of CT inputs must be assigned to aSignal Source that is used to define the Bus Zone andDynamic Bus Replica

Some relays define 1 p.u. as the maximumprimary current of all of the CTs connected in the

given Bus Zone

Per Unit Current Definition Example


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Per-Unit Current Definition - Example

CurrentChannel

Primary Secondary Zone

CT-1 F1 3200 A 1 A 1

CT-2 F2 2400 A 5 A 1

CT-3F3 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

Configuration of Bus Zone


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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 logicto provide additional checks or security as required

o Status signal can be set to ON if current is always in thebus zone or OFF if current is never in the bus zone

CT connections/polarities for a particular bus zone must beproperly configured in the relay, via either hardwire or

software

Configuring the Bus Differential Zone


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Configuring the Bus Differential Zone

1. Configure the physical CT Inputs

o CT Primary and Secondary values

o Both 5 A and 1 A inputs are supported by the UR hardware

o Ratio compensation done automatically for CT ratio differences up to 32:1

2. Configure AC Signal Sources3. Configure Bus Zone with Dynamic Bus Replica

Bus Zone settings defines the boundaries of the Differential

Protection and CT Trouble Monitoring.

Dual Percent Differential Characteristic


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HighBreakpoint

Low

Breakpoint

Low Slope

High Slope

High Set

(Unrestrained)

Min Pickup

Calculating Bus Differential Settings


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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 CTSaturation 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 foroperation of the Bus Zone Differential element

Must be set above maximum leakage current not zoned offin the bus differential zone

May also be set above maximum load conditions for addedsecurity 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 fromIREST=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 bebiased according to the Low Slope setting

Should be set to be above the maximum load but not morethan the maximum current where the CTs still operatelinearly (including residual flux)

Assumption is that the CTs will be operating linearly (nosignificant saturation effects up to 80% residual flux) up tothe Low Breakpoint setting



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Calculating Bus Differential Settings High Breakpoint

Defines the minimum restraint currents that will be biasedaccording 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 tothe Low Breakpoint setting


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Calculating Unrestrained Bus Differential


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Calculating Unrestrained Bus DifferentialSettings

Defines the minimum differential current for unrestrainedoperation

Should be set to be above the maximum differential currentunder 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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HighBreakpoint

Low

Breakpoint

Low Slope

High Slope

High Set

(Unrestrained)

Min Pickup

Reconfigurable Buses


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Protecting re-configurable buses




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Isolators


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Isolators Reliable Isolator Closed signals are needed for the Dynamic

Bus Replica

In simple applications, a single normally closed contact maybe 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 recognizedwith 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

Isolator Typical Open/Closed


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Isolator Typical Open/ClosedConnections

Switch Status Logic and Dyanamic Bus


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Isolator OpenAuxiliaryContact

Isolator ClosedAuxiliaryContact

Isolator Position Alarm Block Switching

Off On CLOSED No No

Off Off LAST VALID After time delayuntilacknowledged

Until IsolatorPosition 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

Replica


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Example Architecture Dynamic Bus


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Phase AAC signals wiredhere, bus replica configuredhere

Phase BAC signals wiredhere, bus replica configuredhere

Phase CAC signals wiredhere, bus replica configured

here

Auxuliary switches wired here;

Isolator Monitoring function

configured here

p yReplica and Isolator Position

Example Architecture BF Initiation &


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Phase AAC signals wiredhere, current statusmonitored here

Phase BAC signals wiredhere, current statusmonitored here

Phase CAC signals wiredhere, current status

monitored here

Breaker Failureelements configuredhere

Example Architecture BF Initiation &Current Supervision

Example Architecture Breaker Failure


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Phase AAC signals wiredhere, current statusmonitored here

Phase BAC signals wiredhere, current statusmonitored here

Phase CAC signals wiredhere, current status

monitored here

Breaker Fail Op commandgenerated here and send to tripappropriate breakers

Trip

TripTrip

Example Architecture Breaker FailureTripping Trip

IEEE 37.234


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IEEE 37.234

Guide for Protective Relay Applications to PowerSystem Buses is currently being revised by the K14Working Group of the IEEE Power System RelayingCommittee.


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Tema 07 - High Low Impedance Busbar Protection 092

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