Fundamentals of Distance Protection

Click to edit Master text styles,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,111,/,GE /,20 September 2024,Fundamentals of Distance Protection,GE Multilin,Outline,Transmission line introduction,What is distance protection?,Non-pilot and pilot schemes,Redundancy considerations,Security for dual-breaker terminals,Out-of-step relaying,Single-pole tripping,Series-compensated 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,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,Typical Protection Schemes,Short Lines,Current differential,Phase comparison,Permissive Overreach Transfer Trip (POTT),Directional Comparison Blocking (DCB),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 Differential,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 Differential,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,),IntendedREACH point,Z,F,1,I*Z,V=I*Z,F,I*Z - V,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,),IntendedREACH point,Z,I*Z,V=I*Z,F,I*Z - V,F,2,What is distance protection?,RELAY,IntendedREACH point,Z,Source Impedance Ratio, Accuracy & Speed,Line,System,Relay,Voltage at the relay:,Fault location,Voltage (%),Voltage change (%),75%,88.24,2.76,90%,90.00,0.91,100%,90.91,N/A,110%,91.67,0.76,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%,3.5370,0.3112,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, V,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,60,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 fault,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/R,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 noise,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 Transients,Distance Element Fundamentals,XL,XC,R,Z1,End Zone,Impedance locus may pass,below the origin of the Z-plane -,this would call for a time delay,to obtain stability,apply delay (fixed or adaptable),reduce the reach,adaptive techniques and better filtering algorithms,CVT Transient Overreach Solutions,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 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 Solution,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 points,Distance Relay Operating Times,Distance Relay Operating Times,20ms,15ms,25ms,30ms,35ms,Distance Relay Operating Times,SLG faults,LL faults,3P faults,Actual maximum reach curves,Relay 1,Relay 3,Relay 2,Relay 4,Maximum Torque Angle,Angle at which mho element has maximum reach,Characteristics with smaller MTA will accommodate larger amount of arc resistance,Traditional,Directional angle lowered and “slammed”,Directional angle “slammed”,Both MHO and directional angles “slammed” (lens),Mho Characteristics,Typical load characteristic impedance,+R,Operate,area,No Operate area,+X,L,+ = LOOKING INTO LINE normally considered forward,Load Trajectory,Reach,Load Swings,Load swing,“Lenticular” Characteristic,Load Swings,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 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 swings,Quadrilateral Characteristics,Ground Resistance (Conductor falls on ground),XL,R,Resultant impedance outside of the mho operating region,Quadrilateral Characteristics,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 - Summary,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 voltage,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,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,S,Memory Polarization,Memory PolarizationImproved Resistive Coverage,Dynamic MHO characteristic for a forward fault,Static MHO characteristic (memory not established or expired),jX,R,Z,L,Z,S,R,L,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 polarization,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 direction,Distance Schemes,Pilot Aided Schemes,No Communication between Distance Relays,Communication between Distance relays,Non-Pilot Aided Schemes,(Step Distance),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 consideration,BUS,BUS,Z1,Z1,Local,Remote,Step Distance Schemes,BUS,BUS,Z1,Z1,End Zone,End Zone,Local,Remote,Step Distance Schemes,BUS,Z1,Z1,Breaker Tripped,BUS,Breaker Closed,Local,Remote,Step Distance Schemes,BUS,Z1,Z1,BUS,Z2 (time delayed),Remote,Local,Step Distance Schemes,Z2 (time delayed),BUS,Z1,BUS,Z2 (time delayed),Step Distance Schemes,Z3 (remote backup),Step Distance Protection,Local Relay Z2,Zone 2 PKP,Local Relay,Remote Relay,Remote Relay Z4,Zone 4 PKP,Over Lap,Distance Relay Coordination,BUS,BUS,Communication Channel,Local Relay,Remote Relay,Need For Pilot Aided Schemes,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,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 Scheme,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 security,Bus,Line,Bus,Zone 1,Zone 1,DUTT Scheme,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),&,Local Trip,Zone 2,Rx PKP,OR,Zone 1,PUTT Scheme,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 provided,POTT Scheme,POTT Scheme,POTT Permissive Over-reaching Transfer Trip,BUS,BUS,End Zone,Communication Channel,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 Scheme,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 Scheme,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 example,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 example,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 faults,Hybrid POTT,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,elements,DCB Scheme,BUS,BUS,End Zone,Communication Channel,Directional Comparison Blocking (DCB),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,Expired,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 Faults,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 returns,DCUB Scheme,BUS,BUS,End Zone,Communication Channel,Directional Comparison Unblocking (DCUB),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 Current,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 Channel,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 RX,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,Expired,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.,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 Communications,Redundant Pilot Schemes,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 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 Attributes,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,Accurate CTs preserve the reverse current direction under weak remote infeed,Security for dual-breaker terminals,Saturation of CT1 may invert the line current as measured from externally summated CTs,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 breaker,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,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,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 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 indiscrimi