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Non-pilot and pilot schemes Redundancy considerations Security for dual-breaker terminals Out-of-step relaying Single-pole tripping Series-compensated linesTransmission Lines: Transmission Lines A Vital Part of the Power System : Provide path to transfer power between generation and load Operate at voltage levels from 69kV to 765kV Deregulated markets, economic, environmental requirements have pushed utilities to operate transmission lines close to their limits.Transmission Lines: Transmission Lines Classification of line length depends on: Source-to-line Impedance Ratio (SIR), and Nominal voltage Length considerations: Short Lines: SIR > 4 Medium Lines: 0.5 < SIR < 4 Long Lines: SIR < 0.5Typical Protection Schemes Short Lines: Typical Protection Schemes Short Lines Current differential Phase comparison Permissive Overreach Transfer Trip (POTT) Directional Comparison Blocking (DCB)Typical Protection Schemes Medium Lines: Typical Protection Schemes Medium Lines Phase comparison Directional Comparison Blocking (DCB) Permissive Underreach Transfer Trip (PUTT) Permissive Overreach Transfer Trip (POTT) Unblocking Step Distance Step or coordinated overcurrent Inverse time overcurrent Current DifferentialTypical Protection Schemes Long Lines: Typical Protection Schemes Long Lines Phase comparison Directional Comparison Blocking (DCB) Permissive Underreach Transfer Trip (PUTT) Permissive Overreach Transfer Trip (POTT) Unblocking Step Distance Step or coordinated overcurrent Current DifferentialWhat is distance protection?: What is distance protection? For internal faults: IZ – V and V approximately in phase ( mho ) IZ – V and IZ approximately in phase ( reactance ) RELAY ( V,I ) Intended REACH point Z F 1 I*Z V=I*Z F I*Z - VWhat is distance protection?: What is distance protection? For external faults: IZ – V and V approximately out of phase ( mho ) IZ – V and IZ approximately out of phase ( reactance ) RELAY ( V,I ) Intended REACH point Z I*Z V=I*Z F I*Z - V F 2What is distance protection?: What is distance protection? RELAY Intended REACH point ZSource Impedance Ratio, Accuracy & Speed: Source Impedance Ratio, Accuracy & Speed Line System Relay Voltage at the relay: Consider SIR = 0.1 Fault location Voltage (%) Voltage change (%) 75% 88.24 2.76 90% 90.00 0.91 100% 90.91 N/A 110% 91.67 0.76Source Impedance Ratio, Accuracy & Speed : Source Impedance Ratio, Accuracy & Speed Line System Relay Voltage at the relay: Consider SIR = 30 Fault location Voltage (%) Voltage change (%) 75% 2.4390 0.7868 90% 2.9126 0.3132 100% 3.2258 N/A 110% 3.5370 0.3112Challenges in relay design: Challenges in relay design Transients: High frequency DC offset in currents CVT transients in voltages CVT output 0 1 2 3 4 steady-state output power cycles -30 -20 -10 0 10 20 30 voltage, VChallenges in relay design: Challenges in relay design Transients: High frequency DC offset in currents CVT transients in voltages CVT output 0 1 2 3 4 steady-state output -60 -40 -20 0 20 40 power cycles voltage, V 60Challenges in relay design: Challenges in relay design -0.5 0 0.5 1 1.5 -100 -50 0 50 100 Reactance comparator [V] power cycles S POL S OP Sorry… Future (unknown) In-phase = internal fault Out-of-phase = external faultTransient Overreach: Transient Overreach Fault current generally contains dc offset in addition to ac power frequency component Ratio of dc to ac component of current depends on instant in the cycle at which fault occurred Rate of decay of dc offset depends on system X/RZone 1 and CVT Transients: Zone 1 and CVT Transients Capacitive Voltage Transformers (CVTs) create certain problems for fast distance relays applied to systems with high Source Impedance Ratios (SIRs): CVT-induced transient voltage components may assume large magnitudes (up to 30-40%) and last for a comparatively long time (up to about 2 cycles) 60Hz voltage for faults at the relay reach point may be as low as 3% for a SIR of 30 the signal may be buried under noiseZone 1 and CVT Transients: CVT transients can cause distance relays to overreach. Generally, transient overreach may be caused by: overestimation of the current (the magnitude of the current as measured is larger than its actual value, and consequently, the fault appears closer than it is actually located), underestimation of the voltage (the magnitude of the voltage as measured is lower than its actual value) combination of the above Zone 1 and CVT TransientsPowerPoint Presentation: Distance Element Fundamentals XL XC R Z1 End ZonePowerPoint Presentation: Impedance locus may pass below the origin of the Z-plane - this would call for a time delay to obtain stabilityCVT Transient Overreach Solutions: apply delay (fixed or adaptable) reduce the reach adaptive techniques and better filtering algorithms CVT Transient Overreach SolutionsCVT Transients – Adaptive Solution: Optimize signal filtering: currents - max 3% error due to the dc component voltages - max 0.6% error due to CVT transients Adaptive double-reach approach filtering alone ensures maximum transient overreach at the level of 1% (for SIRs up to 5) and 20% (for SIRs up to 30) to reduce the transient overreach even further an adaptive double-reach zone 1 has been implemented CVT Transients – Adaptive SolutionCVT Transients – Adaptive Solution: The outer zone 1: is fixed at the actual reach applies certain security delay to cope with CVT transients The inner zone 1: has its reach dynamically controlled by the voltage magnitude is instantaneous CVT Transients – Adaptive SolutionDesirable Distance Relay Attributes: Desirable Distance Relay Attributes Filters : Prefiltering of currents to remove dc decaying transients Limit maximum transient overshoot (below 2%) Prefiltering of voltages to remove low frequency transients caused by CVTs Limit transient overreach to less than 5% for an SIR of 30 Accurate and fast frequency tracking algorithm Adaptive reach control for faults at reach pointsDistance Relay Operating Times: Distance Relay Operating TimesDistance Relay Operating Times: Distance Relay Operating Times 20ms 15ms 25ms 30ms 35msDistance Relay Operating Times: Distance Relay Operating Times SLG faults LL faults 3P faultsPowerPoint Presentation: Actual maximum reach curves Relay 1 Relay 3 Relay 2 Relay 4Maximum Torque Angle: Maximum Torque Angle Angle at which mho element has maximum reach Characteristics with smaller MTA will accommodate larger amount of arc resistanceMho Characteristics: Traditional Directional angle lowered and “slammed” Directional angle “slammed” Both MHO and directional angles “slammed” (lens) Mho CharacteristicsPowerPoint Presentation: Typical load characteristic impedance +R Operate area No Operate area +X L + = LOOKING INTO LINE normally considered forward Load Trajectory Reach Load SwingsPowerPoint Presentation: Load swing “Lenticular” Characteristic Load SwingsPowerPoint Presentation: Load Encroachment Characteristic The load encroachment element responds to positive sequence voltage and current and can be used to block phase distance and phase overcurrent elements.Blinders: Blinders Blinders limit the operation of distance relays (quad or mho) to a narrow region that parallels and encompasses the protected line Applied to long transmission lines, where mho settings are large enough to pick up on maximum load or minor system swingsQuadrilateral Characteristics: Quadrilateral CharacteristicsPowerPoint Presentation: Ground Resistance (Conductor falls on ground) XL R Resultant impedance outside of the mho operating region Quadrilateral CharacteristicsPowerPoint Presentation: Mho Quadrilateral Better coverage for ground faults due to resistance added to return path Lenticular Used for phase elements with long heavily loaded lines heavily loaded Standard for phase elements JX R Distance Characteristics - SummaryDistance Element Polarization: Distance Element Polarization The following polarization quantities are commonly used in distance relays for determining directionality: Self-polarized Memory voltage Positive sequence voltage Quadrature voltage Leading phase voltageMemory Polarization: Memory Polarization Positive-sequence memorized voltage is used for polarizing: Mho comparator (dynamic, expanding Mho) Negative-sequence directional comparator (Ground Distance Mho and Quad) Zero-sequence directional comparator (Ground Distance MHO and QUAD) Directional comparator (Phase Distance MHO and QUAD) Memory duration is a common distance settings (all zones, phase and ground, MHO and QUAD)Memory Polarization: Memory Polarization jX R Dynamic MHO characteristic for a reverse fault Dynamic MHO characteristic for a forward fault Impedance During Close-up Faults Static MHO characteristic (memory not established or expired) Z L Z SMemory Polarization: Memory Polarization Memory Polarization…Improved Resistive Coverage Dynamic MHO characteristic for a forward fault Static MHO characteristic (memory not established or expired) jX R Z L Z S R LChoice of Polarization: Choice of Polarization In order to provide flexibility modern distance relays offer a choice with respect to polarization of ground overcurrent direction functions: Voltage polarization Current polarization Dual polarizationGround Directional Elements: Ground Directional Elements Pilot-aided schemes using ground mho distance relays have inherently limited fault resistance coverage Ground directional over current protection using either negative or zero sequence can be a useful supplement to give more coverage for high resistance faults Directional discrimination based on the ground quantities is fast : Accurate angular relations between the zero and negative sequence quantities establish very quickly because: During faults zero and negative-sequence currents and voltages build up from very low values (practically from zero) The pre-fault values do not bias the developing fault components in any directionPowerPoint Presentation: Distance Schemes Pilot Aided Schemes No Communication between Distance Relays Communication between Distance relays Non-Pilot Aided Schemes (Step Distance)Step Distance Schemes: Step Distance Schemes Zone 1: Trips with no intentional time delay Underreaches to avoid unnecessary operation for faults beyond remote terminal Typical reach setting range 80-90% of Z L Zone 2: Set to protect remainder of line Overreaches into adjacent line/equipment Minimum reach setting 120% of Z L Typically time delayed by 15-30 cycles Zone 3: Remote backup for relay/station failures at remote terminal Reaches beyond Z2, load encroachment a considerationPowerPoint Presentation: BUS BUS Z1 Z1 Local Remote Step Distance SchemesPowerPoint Presentation: BUS BUS Z1 Z1 End Zone End Zone Local Remote Step Distance SchemesPowerPoint Presentation: BUS Z1 Z1 Breaker Tripped BUS Breaker Closed Local Remote Step Distance SchemesPowerPoint Presentation: BUS Z1 Z1 BUS Z2 (time delayed) Remote Local Step Distance Schemes Z2 (time delayed)PowerPoint Presentation: BUS Z1 BUS Z2 (time delayed) Step Distance Schemes Z3 (remote backup) …Step Distance Protection: Step Distance ProtectionDistance Relay Coordination: Local Relay – Z2 Zone 2 PKP Local Relay Remote Relay Remote Relay – Z4 Zone 4 PKP Over Lap Distance Relay CoordinationPowerPoint Presentation: BUS BUS Communication Channel Local Relay Remote Relay Need For Pilot Aided SchemesPilot Communications Channels: Pilot Communications Channels Distance-based pilot schemes traditionally utilize simple on/off communications between relays, but can also utilize peer-to-peer communications and GOOSE messaging over digital channels Typical communications media include: Pilot-wire (50Hz, 60Hz, AT) Power line carrier Microwave Radio Optic fiber (directly connected or multiplexed channels)Distance-based Pilot Protection: Distance-based Pilot ProtectionPowerPoint Presentation: Pilot-Aided Distance-Based Schemes DUTT – Direct Under-reaching Transfer Trip PUTT – Permissive Under-reaching Transfer Trip POTT – Permissive Over-reaching Transfer Trip Hybrid POTT – Hybrid Permissive Over-reaching Transfer Trip DCB – Directional Comparison Blocking Scheme DCUB – Directional Comparison Unblocking SchemeDirect Underreaching Transfer Trip (DUTT): Direct Underreaching Transfer Trip (DUTT) Requires only underreaching (RU) functions which overlap in reach (Zone 1). Applied with FSK channel GUARD frequency transmitted during normal conditions TRIP frequency when one RU function operates Scheme does not provide tripping for faults beyond RU reach if remote breaker is open or channel is inoperative. Dual pilot channels improve securityPowerPoint Presentation: Bus Line Bus Zone 1 Zone 1 DUTT SchemePermissive Underreaching Transfer Trip (PUTT): Permissive Underreaching Transfer Trip (PUTT) Requires both under (RU) and overreaching (RO) functions Identical to DUTT, with pilot tripping signal supervised by RO (Zone 2)PowerPoint Presentation: & Local Trip Zone 2 Rx PKP OR Zone 1 PUTT SchemePermissive Overreaching Transfer Trip (POTT): Permissive Overreaching Transfer Trip (POTT) Requires overreaching (RO) functions (Zone 2). Applied with FSK channel: GUARD frequency sent in stand-by TRIP frequency when one RO function operates No trip for external faults if pilot channel is inoperative Time-delayed tripping can be providedPowerPoint Presentation: POTT SchemePowerPoint Presentation: POTT Scheme POTT – Permissive Over-reaching Transfer Trip BUS BUS End Zone Communication ChannelPowerPoint Presentation: Local Relay Remote Relay Remote Relay FWD I GND Ground Dir OC Fwd OR Local Relay – Z2 ZONE 2 PKP Local Relay FWD I GND Ground Dir OC Fwd OR TRIP Remote Relay – Z2 POTT TX ZONE 2 PKP POTT RX Communication Channel POTT SchemePowerPoint Presentation: POTT TX 4 POTT TX 3 POTT TX 2 POTT TX 1 A to G B to G C to G Multi Phase Local Relay Remote Relay POTT RX 4 POTT RX 3 POTT RX 2 POTT RX 1 Communications Channel(s) POTT SchemePowerPoint Presentation: Local Relay Remote Relay POTT TX ZONE 2 OR GND DIR OC FWD Communication Channel TRIP GND DIR OC REV GND DIR OC REV POTT RX Start Timer Timer Expire GND DIR OC FWD POTT Scheme Current reversal examplePowerPoint Presentation: Local Relay Open Remote Relay Remote FWD I GND POTT TX Remote – Z2 Communication Channel POTT RX OPEN POTT TX Communication Channel POTT RX TRIP POTT Scheme Echo exampleHybrid POTT: Hybrid POTT Intended for three-terminal lines and weak infeed conditions Echo feature adds security during weak infeed conditions Reverse-looking distance and oc elements used to identify external faultsPowerPoint Presentation: Hybrid POTTDirectional Comparison Blocking (DCB): Directional Comparison Blocking (DCB) Requires overreaching (RO) tripping and blocking (B) functions ON/OFF pilot channel typically used (i.e., PLC) Transmitter is keyed to ON state when blocking function(s) operate Receipt of signal from remote end blocks tripping relays Tripping function set with Zone 2 reach or greater Blocking functions include Zone 3 reverse and low-set ground overcurrent elementsPowerPoint Presentation: DCB SchemePowerPoint Presentation: BUS BUS End Zone Communication Channel Directional Comparison Blocking (DCB)PowerPoint Presentation: Directional Comparison Blocking (DCB) Internal Faults Local Relay Remote Relay Local Relay – Z2 Zone 2 PKP TRIP Timer Start FWD I GND GND DIR OC Fwd OR Dir Block RX NO TRIP ExpiredPowerPoint Presentation: Local Relay Remote Relay Remote Relay – Z4 Zone 4 PKP REV I GND GND DIR OC Rev OR DIR BLOCK TX Local Relay – Z2 Zone 2 PKP Dir Block RX Communication Channel FWD I GND GND DIR OC Fwd OR TRIP Timer Start No TRIP Directional Comparison Blocking (DCB) External FaultsDirectional Comparison Unblocking (DCUB): Directional Comparison Unblocking (DCUB) Applied to Permissive Overreaching (POR) schemes to overcome the possibility of carrier signal attenuation or loss as a result of the fault Unblocking provided in the receiver when signal is lost: If signal is lost due to fault, at least one permissive RO functions will be picked up Unblocking logic produces short-duration TRIP signal (150-300 ms). If RO function not picked up, channel lockout occurs until GUARD signal returnsPowerPoint Presentation: DCUB SchemePowerPoint Presentation: BUS BUS End Zone Communication Channel Directional Comparison Unblocking (DCUB)PowerPoint Presentation: Directional Comparison Unblocking (DCUB) Normal conditions Local Relay Remote Relay GUARD1 TX GUARD1 RX Communication Channel GUARD2 TX GUARD2 RX NO Loss of Guard FSK Carrier FSK Carrier NO Permission NO Loss of Guard NO Permission Load CurrentPowerPoint Presentation: Directional Comparison Unblocking (DCUB) Normal conditions, channel failure Local Relay Remote Relay GUARD1 TX GUARD1 RX Communication Channel GUARD2 TX GUARD2 RX FSK Carrier FSK Carrier Loss of Guard Block Timer Started Loss of Guard Block Timer Started Load Current NO RX NO RX Block DCUB until Guard OK Expired Block DCUB until Guard OK Expired Loss of ChannelPowerPoint Presentation: Directional Comparison Unblocking (DCUB) Internal fault, healthy channel Local Relay Remote Relay GUARD1 TX GUARD1 RX Communication Channel GUARD2 TX GUARD2 RX FSK Carrier FSK Carrier Loss of Guard Permission TRIP1 TX Local Relay – Z2 Zone 2 PKP TRIP1 RX TRIP2 TX TRIP Remote Relay – Z2 ZONE 2 PKP TRIP Z1 TRIP2 RXPowerPoint Presentation: Directional Comparison Unblocking (DCUB) Internal fault, channel failure Local Relay Remote Relay GUARD1 TX GUARD1 RX Communication Channel GUARD2 TX GUARD2 RX FSK Carrier FSK Carrier TRIP1 TX Local Relay – Z2 Zone 2 PKP NO RX TRIP2 TX TRIP Remote Relay – Z2 ZONE 2 PKP TRIP Z1 NO RX Loss of Guard Loss of Channel Loss of Guard Block Timer Started Duration Timer Started ExpiredRedundancy Considerations: Redundancy Considerations Redundant protection systems increase dependability of the system: Multiple sets of protection using same protection principle and multiple pilot channels overcome individual element failure, or Multiple sets of protection using different protection principles and multiple channels protects against failure of one of the protection methods. Security can be improved using “voting” schemes (i.e., 2-out-of-3), potentially at expense of dependability. Redundancy of instrument transformers, battery systems, trip coil circuits, etc. also need to be considered.PowerPoint Presentation: BUS BUS End Zone Communication Channel 1 Communication Channel 2 Loss of Channel 2 AND Channels: POTT Less Reliable DCB Less Secure OR Channels: POTT More Reliable DCB More Secure More Channel Security More Channel Dependability Redundant CommunicationsRedundant Pilot Schemes: Redundant Pilot SchemesPilot Relay Desirable Attributes: Integrated functions: weak infeed echo line pick-up (SOTF) Basic protection elements used to key the communication: distance elements fast and sensitive ground (zero and negative sequence) directional IOCs with current, voltage, and/or dual polarization Pilot Relay Desirable AttributesPilot Relay Desirable Attributes: Pre-programmed distance-based pilot schemes: Direct Under-reaching Transfer Trip (DUTT) Permissive Under-reaching Transfer Trip (PUTT) Permissive Overreaching Transfer Trip (POTT) Hybrid Permissive Overreaching Transfer Trip (HYB POTT) Blocking scheme (DCB) Unblocking scheme (DCUB) Pilot Relay Desirable AttributesSecurity for dual-breaker terminals: Security for dual-breaker terminals Breaker-and-a-half and ring bus terminals are common designs for transmission lines. Standard practice has been to: sum currents from each circuit breaker externally by paralleling the CTs use external sum as the line current for protective relays For some close-in external fault events, poor CT performance may lead to improper operation of line relays.Security for dual-breaker terminals: Security for dual-breaker terminals Accurate CTs preserve the reverse current direction under weak remote infeedSecurity for dual-breaker terminals: Security for dual-breaker terminals Saturation of CT1 may invert the line current as measured from externally summated CTsSecurity for dual-breaker terminals: Security for dual-breaker terminals Direct measurement of currents from both circuit breakers allows the use of supervisory logic to prevent distance and directional overcurrent elements from operating incorrectly due to CT errors during reverse faults. Additional benefits of direct measurement of currents: independent BF protection for each circuit breaker independent autoreclosing for each breakerSecurity for dual-breaker terminals: Security for dual-breaker terminals Supervisory logic should: not affect speed or sensitivity of protection elements correctly allow tripping during evolving external-to-internal fault conditions determine direction of current flow through each breaker independently: Both currents in FWD direction internal fault One current FWD, one current REV external fault allow tripping during all forward/internal faults block tripping during all reverse/external faults initially block tripping during evolving external-to-internal faults until second fault appears in forward direction. Block is then lifted to permit tripping.Single-pole Tripping: Single-pole Tripping Distance relay must correctly identify a SLG fault and trip only the circuit breaker pole for the faulted phase. Autoreclosing and breaker failure functions must be initiated correctly on the fault event Security must be maintained on the healthy phases during the open pole condition and any reclosing attempt.Out-of-Step Condition: Out-of-Step Condition For certain operating conditions, a severe system disturbance can cause system instability and result in loss of synchronism between different generating units on an interconnected system.Out-of-Step Relaying: Out-of-Step Relaying Out-of-step blocking relays Operate in conjunction with mho tripping relays to prevent a terminal from tripping during severe system swings & out-of-step conditions. Prevent system from separating in an indiscriminate manner. Out-of-step tripping relays Operate independently of other devices to detect out-of-step condition during the first pole slip. Initiate tripping of breakers that separate system in order to balance load with available generation on any isolated part of the system.Out-of-Step Tripping: Out-of-Step Tripping The locus must stay for some time between the outer and middle characteristics Must move and stay between the middle and inner characteristics When the inner characteristic is entered the element is ready to tripPower Swing Blocking: Power Swing Blocking Applications: Establish a blocking signal for stable power swings (Power Swing Blocking) Establish a tripping signal for unstable power swings (Out-of-Step Tripping) Responds to: Positive-sequence voltage and currentSeries-compensated lines: Series-compensated lines Benefits of series capacitors : Reduction of overall X L of long lines Improvement of stability margins Ability to adjust line load levels Loss reduction Reduction of voltage drop during severe disturbances Normally economical for line lengths > 200 milesSeries-compensated lines: Series-compensated lines SCs create unfavorable conditions for protective relays and fault locators: Overreaching of distance elements Failure of distance element to pick up on low-current faults Phase selection problems in single-pole tripping applications Large fault location errorsSeries-compensated lines Series Capacitor with MOV: Series-compensated lines Series Capacitor with MOVSeries-compensated lines : Series-compensated linesSeries-compensated lines Dynamic Reach Control: Series-compensated lines Dynamic Reach ControlSeries-compensated lines Dynamic Reach Control for External Faults: Series-compensated lines Dynamic Reach Control for External FaultsSeries-compensated lines Dynamic Reach Control for External Faults: Series-compensated lines Dynamic Reach Control for External FaultsSeries-compensated lines Dynamic Reach Control for Internal Faults: Series-compensated lines Dynamic Reach Control for Internal FaultsDistance Protection Looking Through a Transformer: Distance Protection Looking Through a Transformer Phase distance elements can be set to see beyond any 3-phase power transformer CTs & VTs may be located independently on different sides of the transformer Given distance zone is defined by VT location (not CTs) Reach setting is in sec, and must take into account location & ratios of VTs, CTs and voltage ratio of the involved power transformerTransformer Group Compensation: Transformer Group Compensation Depending on location of VTs and CTs, distance relays need to compensate for the phase shift and magnitude change caused by the power transformerSetting Rules: Setting Rules Transformer positive sequence impedance must be included in reach setting only if transformer lies between VTs and intended reach point Currents require compensation only if transformer located between CTs and intended reach point Voltages require compensation only if transformer located between VTs and intended reach point Compensation set based on transformer connection & vector group as seen from CTs/VTs toward reach pointDistance Relay Desirable Attributes: Multiple reversible distance zones Individual per-zone, per-element characteristic: Dynamic voltage memory polarization Various characteristics, including mho, quad, lenticular Individual per-zone, per-element current supervision (FD) Multi-input phase comparator: additional ground directional supervision dynamic reactance supervision Transient overreach filtering/control Phase shift & magnitude compensation for distance applications with power transformers Distance Relay Desirable AttributesDistance Relay Desirable Attributes: For improved flexibility, it is desirable to have the following parameters settable on a per zone basis: Zero-sequence compensation Mutual zero-sequence compensation Maximum torque angle Blinders Directional angle Comparator limit angles (for lenticular characteristic) Overcurrent supervision Distance Relay Desirable AttributesDistance Relay Desirable Attributes: Additional functions Overcurrent elements (phase, neutral, ground, directional, negative sequence, etc.) Breaker failure Automatic reclosing (single & three-pole) Sync check Under/over voltage elements Special functions Power swing detection Load encroachment Pilot schemes Distance Relay Desirable AttributesPowerPoint Presentation: Questions? 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