Fundamentals of Bus Bar Protection GE Multilin. 2 GE Consumer & Industrial Multilin 2-Jun-14...

Fundamentals ofFundamentals ofBus Bar Bus Bar

ProtectionProtectionGE Multilin

2GE Consumer & Industrial

MultilinApr 10, 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

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

ZONE 1ZONE 2

•Distribution and lower transmission voltage levels

•Limited operating flexibility

Multiple bus sections - 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

Double bus - single breaker with bus tie

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

ZONE 1

ZONE 2

•Very high operating flexibility•Transfer bus for breaker

maintenance

Double bus – single breaker w/ transfer bus

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

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

•Higher voltage levels

•High operating flexibility with minimum

breakers

•Separate bus protection not required at

line positions

Ring bus

Bus components breakers

SF6, EHV & HV - Synchropuff

Low Voltage circuit breakers

ISO 1 ISO 2

ISO 3BYPASS

F1aF1c

Contact Input F1a OnContact Input F1c On

ISOLATOR 1 OPEN

F1aF1c

Contact Input F1a OnContact Input F1c On

ISOLATOR 1 CLOSED

Disconnect switches & auxiliary contacts

ISO 1 ISO 2

ISO 3BYPASS

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)

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

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

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

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

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

ExternalFault

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 V 10 V 10 V 0 V 20 V40 V

2000 A

4000 A

Internal BusFault

• 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

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

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

nDIF IIII ...21

nRES IIII ...21 nRES IIII ...,,,max 21

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

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)

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

copper

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

copper

CT Saturation

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

CT Saturation

Ratio Current CT Current

No DC Offset• Waveform remains

fairly symmetrical

With DC Offset• Waveform starts off

being asymmetrical, then symmetrical in steady state

External Fault & Ideal CTs

• Fault starts at t0

• Steady-state fault conditions occur at t1

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

External Fault & Actual CTs

• Fault starts at t0

• Steady-state fault conditions occur at t1

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

External Fault with CT Saturation

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

• CT fully saturated at t2

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

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

High-Impedance

Bus Differential

Considerations

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

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.

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.

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

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

P -based High-Impedance Bus Differential Protection 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 to1900 Volts 86 – latching contact preventing the resistors from overheating after the fault is detected

High Impedance Module for Digital Relays

High-Impedance Module +

Overcurrent Relay

• 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

Low-Impedance

Bus Differential

Considerations

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

• 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

• 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

• 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

Large Bus Applications

87B phase A

87B phase B

87B phase C

Logic relay(switch status,optional BF)

Large Bus ApplicationsFor buses with up to 24 circuits

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

IDIFF = Error

IREST = Error

Maloperation ifError > PICKUP

Definitions of Restraint Signals

“maximum of”

“geometrical average”

“scaled sum of”

“sum of”nR iiiii ...321

nR iiiin

i ...1

nR iiiiMaxi ,...,,, 321

nnR iiiii ...321

“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)

Bus Differential Adaptive Approach

restraining

Region 1(low differential

currents)

Region 2(high differential

currents)

Bus Differential Adaptive Logic Diagram

OR 87B BIASED OP

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)

Phase Comparison Principle Continued…

OPERATE

ID - I p

External Fault Conditions

OPERATE

ID - I p

Internal Fault Conditions

OPERATE

CT Saturation

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

• CT fully saturated at t2

CT Saturation Detector State Machine NORMAL

SAT := 0

EXTERNAL

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

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

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

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

time, sec

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

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

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

Per-Unit Current Definition - Example

Current Channel

Primary

Secondary

F1 3200 A 1 A 1

F2 2400 A 5 A 1

F3 1200 A 1 A 1

F4 3200 A 1 A 2

F5 1200 A 5 A 2

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

• 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

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.

Dual Percent Differential Characteristic

High Breakpoint

Low Breakpoint

Low Slope

High Slope

High Set (Unrestrained)

Min Pickup

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)

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

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

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

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

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

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.

Dual Percent Differential Characteristic

High Breakpoint

Low Breakpoint

Low Slope

High Slope

High Set (Unrestrained)

Min Pickup

NORTH BUS

SOUTH BUS

B-4CT-4

B-3CT-3

B-2CT-2

C-1 C-2 C-4

C-3 C-5

Protecting re-configurable buses

Reconfigurable Buses

NORTH BUS

SOUTH BUS

B-4CT-4

B-3CT-3

B-2CT-2CT-1

C-1 C-2 C-4

C-3 C-5

NORTH BUS

SOUTH BUS

B-4CT-4

B-3CT-3

B-2CT-2CT-1

C-1 C-2 C-4

C-3 C-5

NORTH BUS

SOUTH BUS

B-4CT-4

B-3CT-3

B-2CT-2

C-1 C-2 C-4

C-3 C-5

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

Isolator – Typical Open/Closed Connections

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

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.

• 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

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

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

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

Isolator Position

Isolator P

osition

Example Architecture – Dynamic Bus Replica and Isolator Position

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

nt Supv.

BF Initiate & Current Supv.

BF Initiate & Curre

nt Supv.

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 Fail Op command generated here and send to trip appropriate breakers

Breake

r Fail O

Breaker Fail Op

Breaker Fail O

TripTrip

Example Architecture – Breaker Failure TrippingTrip

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.

Fundamentals of Bus Bar Protection GE Multilin. 2 GE Consumer & Industrial Multilin 2-Jun-14...

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