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This document provides an overview of fundamentals of distance protection for transmission lines. It discusses types of transmission lines and typical protection schemes used based on line length. It then describes what distance protection is and challenges in relay design related to transients. The document outlines considerations for distance relay characteristics, polarization, and schemes including non-pilot and pilot schemes. It discusses redundancy, security, out-of-step relaying and series compensation.

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Fundamentals of Distance Protection GE Multilin
2 / GE / April 11, 2023 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
3 / GE / April 11, 2023 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.
4 / GE / April 11, 2023 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 < 0.5
5 / GE / April 11, 2023 Typical Protection Schemes Short Lines • Current differential • Phase comparison • Permissive Overreach Transfer Trip (POTT) • Directional Comparison Blocking (DCB)
6 / GE / April 11, 2023 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
7 / GE / April 11, 2023 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
8 / GE / April 11, 2023 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 F1 I*Z V=I*ZF I*Z - V
9 / GE / April 11, 2023 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*ZF I*Z - V F2
10 / GE / April 11, 2023 What is distance protection? RELAY Intended REACH point Z
11 / GE / April 11, 2023 Source Impedance Ratio, Accuracy & Speed Lin e System Relay Voltage at the relay: SIR f f V V PU LOC PU LOC N R   ] [ ] [ 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 0.76
12 / GE / April 11, 2023 Source Impedance Ratio, Accuracy & Speed Lin e System Relay Voltage at the relay: SIR f f V V PU LOC PU LOC N R   ] [ ] [ 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
13 / GE / April 11, 2023 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 C1 C2 2 3 5 6 1 4 7 High Voltage Line Secondary Voltage Output 8
14 / GE / April 11, 2023 Challenges in relay design > Transients: – High frequency – DC offset in currents – CVT transients in voltages C1 C2 2 3 5 6 1 4 7 High Voltage Line Secondary Voltage Output 8 CVT output 0 1 2 3 4 steady-state output -60 -40 -20 0 20 40 power cycles voltage, V 60
15 / GE / April 11, 2023 Challenges in relay design -0.5 0 0.5 1 1.5 -100 -80 -60 -40 -20 0 20 40 60 80 100 Voltage [V] -0.5 0 0.5 1 1.5 -3 -2 -1 0 1 2 3 4 5 Current [A] vA vB vC iA iB ,iC -0.5 0 0.5 1 1.5 -100 -50 0 50 100 Reactance comparator [V] power cycles SPOL SOP Sorry… Future (unknown) > In-phase = internal fault > Out-of-phase = external fault
16 / GE / April 11, 2023 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
17 / GE / April 11, 2023 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
18 / GE / April 11, 2023 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
20 / GE / April 11, 2023 -10 -5 0 5 10 -5 0 5 10 15 Reactance [ohm] Resistance [ohm] 18 22 26 30 34 42 44 Actual Fault Location Line Impedance Trajectory (msec) dynamic mho zone extended for high SIRs Impedance locus may pass below the origin of the Z-plane - this would call for a time delay to obtain stability
21 / GE / April 11, 2023 > apply delay (fixed or adaptable) > reduce the reach > adaptive techniques and better filtering algorithms CVT Transient Overreach Solutions
22 / GE / April 11, 2023 > 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
23 / GE / April 11, 2023 The outer zone 1: > is fixed at the actual reach > applies certain security delay to cope with CVT transients Delayed Trip Instantaneous Trip R X The inner zone 1: > has its reach dynamically controlled by the voltage magnitude > is instantaneous CVT Transients – Adaptive Solution
24 / GE / April 11, 2023 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
25 / GE / April 11, 2023 Distance Relay Operating Times
26 / GE / April 11, 2023 Distance Relay Operating Times 20ms 15ms 25ms 30ms 35ms
27 / GE / April 11, 2023 Distance Relay Operating Times SLG faults LL faults 3P faults
28 / GE / April 11, 2023 0 5 10 15 20 25 30 0 10 20 30 40 50 60 70 80 90 100 Maximum Rach [%] SIR Actual maximum reach curves Relay 1 Relay 3 Relay 2 Relay 4
29 / GE / April 11, 2023 Maximum Torque Angle • Angle at which mho element has maximum reach • Characteristics with smaller MTA will accommodate larger amount of arc resistance
30 / GE / April 11, 2023 Traditional Directional angle lowered and “slammed” Directional angle “slammed” Both MHO and directional angles “slammed” (lens) Mho Characteristics
31 / GE / April 11, 2023 Typical load characteristic impedance +R Operate area No Operate area +XL + = LOOKING INTO LINE normally considered forward Load Trajectory Load Swings
32 / GE / April 11, 2023 Load swing “Lenticular” Characteristic Load Swings
33 / GE / April 11, 2023 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.
34 / GE / April 11, 2023 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
35 / GE / April 11, 2023 Quadrilateral Characteristics
36 / GE / April 11, 2023 Ground Resistance (Conductor falls on ground) R Resultant impedance outside of the mho operating region Quadrilateral Characteristics
37 / GE / April 11, 2023 Mho Quadrilatera l 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
38 / GE / April 11, 2023 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
39 / GE / April 11, 2023 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)
40 / GE / April 11, 2023 Memory Polarization jX R Dynamic MHO characteristic for a reverse fault Dynamic MHO characteristic for a forward fau Impedance During Close-up Faults Static MHO characteristic (memory not established or expired) ZL ZS
41 / GE / April 11, 2023 Memory Polarization Memory Polarization…Improved Resistive Coverage Dynamic MHO characteristic for a forward fault Static MHO characteristic (memory not established or expired) jX R ZL ZS RL
42 / GE / April 11, 2023 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
43 / GE / April 11, 2023 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
44 / GE / April 11, 2023 Distance Schemes Pilot Aided Schemes No Communication between Distance Relays Communication between Distance relays Non-Pilot Aided Schemes (Step Distance)
45 / GE / April 11, 2023 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 ZL • Zone 2: – Set to protect remainder of line – Overreaches into adjacent line/equipment – Minimum reach setting 120% of ZL – 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
46 / GE / April 11, 2023 Z1 Z1 Local Remote Step Distance Schemes
47 / GE / April 11, 2023 Z1 Z1 End Zone End Zone Local Remote Step Distance Schemes
48 / GE / April 11, 2023 Z1 Z1 Breaker Tripped Breaker Closed Local Remote Step Distance Schemes
49 / GE / April 11, 2023 Z1 Z1 Z2 (time delayed) Remote Local Step Distance Schemes Z2 (time delayed)
50 / GE / April 11, 2023 Z1 Z2 (time delayed) Step Distance Schemes Z3 (remote backup) …
51 / GE / April 11, 2023 Step Distance Protection
52 / GE / April 11, 2023 Local Relay – Z2 Zone 2 PKP Local Relay Remote Relay Remote Relay – Z4 Zone 4 PKP Over Lap Distance Relay Coordination
53 / GE / April 11, 2023 Communication Channel Local Relay Remote Relay Need For Pilot Aided Schemes
54 / GE / April 11, 2023 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)
55 / GE / April 11, 2023 Distance-based Pilot Protection
56 / GE / April 11, 2023 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
57 / GE / April 11, 2023 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
58 / GE / April 11, 2023 Bus Line Bus Zone 1 Zone 1 DUTT Scheme
59 / GE / April 11, 2023 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)
60 / GE / April 11, 2023 Bus Line Bus Zone 1 Zone 2 Zone 2 Zone 1 To protect end of line & Local Trip Zone 2 Rx PKP OR Zone 1 PUTT Scheme
61 / GE / April 11, 2023 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
62 / GE / April 11, 2023 Bus Line Bus Zone 1 Zone 2 Trip Line Breakers OR t Rx Tx AND (Z1) (Z1) o Zone 1 Zone 2 Zone 2 Zone 1 POTT Scheme
63 / GE / April 11, 2023 POTT Scheme POTT – Permissive Over-reaching Transfer Trip End Zone Communication Channel
64 / GE / April 11, 2023 Local Relay Remote Relay Remote Relay FWD IGND Ground Dir OC Fwd OR Local Relay – Z2 ZONE 2 PKP Local Relay FWD IGND Ground Dir OC Fwd OR TRIP Remote Relay – Z2 POTT TX ZONE 2 PKP POTT RX Communicatio n Channel POTT Scheme
65 / GE / April 11, 2023 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
66 / GE / April 11, 2023 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
67 / GE / April 11, 2023 Local Relay Open Remote Relay Remote FWD IGND POTT TX Remote – Z2 Communication Channel POTT RX OPEN POTT TX Communication Channel POTT RX TRIP POTT Scheme Echo example
68 / GE / April 11, 2023 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
69 / GE / April 11, 2023 Bus Line Bus Zone 1 Zone 2 Zone 2 Zone 1 Zone 4 Local Remote Weak system Hybrid POTT
70 / GE / April 11, 2023 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
71 / GE / April 11, 2023 Bus Line Bus Zone 1 Zone 2 Zone 2 Zone 1 Local Remote DCB Scheme
72 / GE / April 11, 2023 End Zone Communication Channel Directional Comparison Blocking (DCB)
73 / GE / April 11, 2023 Directional Comparison Blocking (DCB) Internal Faults Local Relay Remote Relay Local Relay – Z2 Zone 2 PKP TRIP Timer Start FWD IGND GND DIR OC Fwd OR Dir Block RX NO TRIP Expired
74 / GE / April 11, 2023 Local Relay Remote Relay Remote Relay – Z4 Zone 4 PKP REV IGND GND DIR OC Rev OR DIR BLOCK TX Local Relay – Z2 Zone 2 PKP Dir Block RX Communication Channel FWD IGND GND DIR OC Fwd OR TRIP Timer Start No TRIP Directional Comparison Blocking (DCB) External Faults
75 / GE / April 11, 2023 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
76 / GE / April 11, 2023 Bus Line Bus Trip Line Breakers Tx1 (Un-Block) Forward Forward Tx2 (Block) Forward Rx2 Rx1 t o AND t o AND AND AND Lockout (Block) (Un-Block) DCUB Scheme
77 / GE / April 11, 2023 End Zone Communication Channel Directional Comparison Unblocking (DCUB)
78 / GE / April 11, 2023 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
79 / GE / April 11, 2023 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
80 / GE / April 11, 2023 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
81 / GE / April 11, 2023 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
82 / GE / April 11, 2023 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.
83 / GE / April 11, 2023 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
84 / GE / April 11, 2023 Redundant Pilot Schemes
85 / GE / April 11, 2023 • 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
86 / GE / April 11, 2023 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
87 / GE / April 11, 2023 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.
88 / GE / April 11, 2023 Security for dual-breaker terminals Accurate CTs preserve the reverse current direction under weak remote infeed
89 / GE / April 11, 2023 Security for dual-breaker terminals Saturation of CT1 may invert the line current as measured from externally summated CTs
90 / GE / April 11, 2023 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
91 / GE / April 11, 2023 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.
92 / GE / April 11, 2023 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.
93 / GE / April 11, 2023 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.
94 / GE / April 11, 2023 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.
95 / GE / April 11, 2023 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 trip
96 / GE / April 11, 2023 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 current
97 / GE / April 11, 2023 Series-compensated lines E Xs SC XL Infinte Bus Benefits of series capacitors: • Reduction of overall XL 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 miles
98 / GE / April 11, 2023 Series-compensated lines E Xs SC XL Infinte Bus 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 errors
99 / GE / April 11, 2023 Series-compensated lines Series Capacitor with MOV
100 / GE / April 11, 2023 Series-compensated lines
101 / GE / April 11, 2023 Series-compensated lines Dynamic Reach Control
102 / GE / April 11, 2023 Series-compensated lines Dynamic Reach Control for External Faults
103 / GE / April 11, 2023 Series-compensated lines Dynamic Reach Control for External Faults
104 / GE / April 11, 2023 Series-compensated lines Dynamic Reach Control for Internal Faults
105 / GE / April 11, 2023 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 transformer
106 / GE / April 11, 2023 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 transformer
107 / GE / April 11, 2023 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 point
108 / GE / April 11, 2023 > 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 Attributes
109 / GE / April 11, 2023 > 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 Attributes
110 / GE / April 11, 2023 > 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 Attributes
111 / GE / April 11, 2023

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In this document
Powered by AI

Overview of distance protection and its significance in transmission lines including its basic principles and challenges.

Information on typical operating times for distance relays under different fault conditions.

Characteristics of distance relay responses under load conditions and how they interact with faults.

Overview of various distance protection schemes including directional and quadrilateral characteristics.

Discusses communication schemes for distance protection, focusing on pilot-aided protection, their attributes and benefits.Emphasizes the importance of accurate current measurement in ensuring the reliability of protection systems.

Addresses out-of-step conditions in transmission systems and methods of maintaining stability.

Discusses the complexities presented by series capacitors in transmission lines and solutions for effective distance protection.

Details desirable attributes for distance relays, emphasizing flexibility and adaptability for effective operation.

transmission-line-protection.ppt

  • 1.
  • 2.
    2 / GE / April11, 2023 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
  • 3.
    3 / GE / April11, 2023 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.
  • 4.
    4 / GE / April11, 2023 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 < 0.5
  • 5.
    5 / GE / April11, 2023 Typical Protection Schemes Short Lines • Current differential • Phase comparison • Permissive Overreach Transfer Trip (POTT) • Directional Comparison Blocking (DCB)
  • 6.
    6 / GE / April11, 2023 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
  • 7.
    7 / GE / April11, 2023 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
  • 8.
    8 / GE / April11, 2023 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 F1 I*Z V=I*ZF I*Z - V
  • 9.
    9 / GE / April11, 2023 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*ZF I*Z - V F2
  • 10.
    10 / GE / April11, 2023 What is distance protection? RELAY Intended REACH point Z
  • 11.
    11 / GE / April11, 2023 Source Impedance Ratio, Accuracy & Speed Lin e System Relay Voltage at the relay: SIR f f V V PU LOC PU LOC N R   ] [ ] [ 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 0.76
  • 12.
    12 / GE / April11, 2023 Source Impedance Ratio, Accuracy & Speed Lin e System Relay Voltage at the relay: SIR f f V V PU LOC PU LOC N R   ] [ ] [ 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
  • 13.
    13 / GE / April11, 2023 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 C1 C2 2 3 5 6 1 4 7 High Voltage Line Secondary Voltage Output 8
  • 14.
    14 / GE / April11, 2023 Challenges in relay design > Transients: – High frequency – DC offset in currents – CVT transients in voltages C1 C2 2 3 5 6 1 4 7 High Voltage Line Secondary Voltage Output 8 CVT output 0 1 2 3 4 steady-state output -60 -40 -20 0 20 40 power cycles voltage, V 60
  • 15.
    15 / GE / April11, 2023 Challenges in relay design -0.5 0 0.5 1 1.5 -100 -80 -60 -40 -20 0 20 40 60 80 100 Voltage [V] -0.5 0 0.5 1 1.5 -3 -2 -1 0 1 2 3 4 5 Current [A] vA vB vC iA iB ,iC -0.5 0 0.5 1 1.5 -100 -50 0 50 100 Reactance comparator [V] power cycles SPOL SOP Sorry… Future (unknown) > In-phase = internal fault > Out-of-phase = external fault
  • 16.
    16 / GE / April11, 2023 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
  • 17.
    17 / GE / April11, 2023 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
  • 18.
    18 / GE / April11, 2023 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
  • 19.
  • 20.
    20 / GE / April11, 2023 -10 -5 0 5 10 -5 0 5 10 15 Reactance [ohm] Resistance [ohm] 18 22 26 30 34 42 44 Actual Fault Location Line Impedance Trajectory (msec) dynamic mho zone extended for high SIRs Impedance locus may pass below the origin of the Z-plane - this would call for a time delay to obtain stability
  • 21.
    21 / GE / April11, 2023 > apply delay (fixed or adaptable) > reduce the reach > adaptive techniques and better filtering algorithms CVT Transient Overreach Solutions
  • 22.
    22 / GE / April11, 2023 > 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
  • 23.
    23 / GE / April11, 2023 The outer zone 1: > is fixed at the actual reach > applies certain security delay to cope with CVT transients Delayed Trip Instantaneous Trip R X The inner zone 1: > has its reach dynamically controlled by the voltage magnitude > is instantaneous CVT Transients – Adaptive Solution
  • 24.
    24 / GE / April11, 2023 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
  • 25.
    25 / GE / April11, 2023 Distance Relay Operating Times
  • 26.
    26 / GE / April11, 2023 Distance Relay Operating Times 20ms 15ms 25ms 30ms 35ms
  • 27.
    27 / GE / April11, 2023 Distance Relay Operating Times SLG faults LL faults 3P faults
  • 28.
    28 / GE / April11, 2023 0 5 10 15 20 25 30 0 10 20 30 40 50 60 70 80 90 100 Maximum Rach [%] SIR Actual maximum reach curves Relay 1 Relay 3 Relay 2 Relay 4
  • 29.
    29 / GE / April11, 2023 Maximum Torque Angle • Angle at which mho element has maximum reach • Characteristics with smaller MTA will accommodate larger amount of arc resistance
  • 30.
    30 / GE / April11, 2023 Traditional Directional angle lowered and “slammed” Directional angle “slammed” Both MHO and directional angles “slammed” (lens) Mho Characteristics
  • 31.
    31 / GE / April11, 2023 Typical load characteristic impedance +R Operate area No Operate area +XL + = LOOKING INTO LINE normally considered forward Load Trajectory Load Swings
  • 32.
    32 / GE / April11, 2023 Load swing “Lenticular” Characteristic Load Swings
  • 33.
    33 / GE / April11, 2023 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.
  • 34.
    34 / GE / April11, 2023 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
  • 35.
    35 / GE / April11, 2023 Quadrilateral Characteristics
  • 36.
    36 / GE / April11, 2023 Ground Resistance (Conductor falls on ground) R Resultant impedance outside of the mho operating region Quadrilateral Characteristics
  • 37.
    37 / GE / April11, 2023 Mho Quadrilatera l 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
  • 38.
    38 / GE / April11, 2023 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
  • 39.
    39 / GE / April11, 2023 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)
  • 40.
    40 / GE / April11, 2023 Memory Polarization jX R Dynamic MHO characteristic for a reverse fault Dynamic MHO characteristic for a forward fau Impedance During Close-up Faults Static MHO characteristic (memory not established or expired) ZL ZS
  • 41.
    41 / GE / April11, 2023 Memory Polarization Memory Polarization…Improved Resistive Coverage Dynamic MHO characteristic for a forward fault Static MHO characteristic (memory not established or expired) jX R ZL ZS RL
  • 42.
    42 / GE / April11, 2023 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
  • 43.
    43 / GE / April11, 2023 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
  • 44.
    44 / GE / April11, 2023 Distance Schemes Pilot Aided Schemes No Communication between Distance Relays Communication between Distance relays Non-Pilot Aided Schemes (Step Distance)
  • 45.
    45 / GE / April11, 2023 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 ZL • Zone 2: – Set to protect remainder of line – Overreaches into adjacent line/equipment – Minimum reach setting 120% of ZL – 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
  • 46.
    46 / GE / April11, 2023 Z1 Z1 Local Remote Step Distance Schemes
  • 47.
    47 / GE / April11, 2023 Z1 Z1 End Zone End Zone Local Remote Step Distance Schemes
  • 48.
    48 / GE / April11, 2023 Z1 Z1 Breaker Tripped Breaker Closed Local Remote Step Distance Schemes
  • 49.
    49 / GE / April11, 2023 Z1 Z1 Z2 (time delayed) Remote Local Step Distance Schemes Z2 (time delayed)
  • 50.
    50 / GE / April11, 2023 Z1 Z2 (time delayed) Step Distance Schemes Z3 (remote backup) …
  • 51.
    51 / GE / April11, 2023 Step Distance Protection
  • 52.
    52 / GE / April11, 2023 Local Relay – Z2 Zone 2 PKP Local Relay Remote Relay Remote Relay – Z4 Zone 4 PKP Over Lap Distance Relay Coordination
  • 53.
    53 / GE / April11, 2023 Communication Channel Local Relay Remote Relay Need For Pilot Aided Schemes
  • 54.
    54 / GE / April11, 2023 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)
  • 55.
    55 / GE / April11, 2023 Distance-based Pilot Protection
  • 56.
    56 / GE / April11, 2023 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
  • 57.
    57 / GE / April11, 2023 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
  • 58.
    58 / GE / April11, 2023 Bus Line Bus Zone 1 Zone 1 DUTT Scheme
  • 59.
    59 / GE / April11, 2023 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)
  • 60.
    60 / GE / April11, 2023 Bus Line Bus Zone 1 Zone 2 Zone 2 Zone 1 To protect end of line & Local Trip Zone 2 Rx PKP OR Zone 1 PUTT Scheme
  • 61.
    61 / GE / April11, 2023 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
  • 62.
    62 / GE / April11, 2023 Bus Line Bus Zone 1 Zone 2 Trip Line Breakers OR t Rx Tx AND (Z1) (Z1) o Zone 1 Zone 2 Zone 2 Zone 1 POTT Scheme
  • 63.
    63 / GE / April11, 2023 POTT Scheme POTT – Permissive Over-reaching Transfer Trip End Zone Communication Channel
  • 64.
    64 / GE / April11, 2023 Local Relay Remote Relay Remote Relay FWD IGND Ground Dir OC Fwd OR Local Relay – Z2 ZONE 2 PKP Local Relay FWD IGND Ground Dir OC Fwd OR TRIP Remote Relay – Z2 POTT TX ZONE 2 PKP POTT RX Communicatio n Channel POTT Scheme
  • 65.
    65 / GE / April11, 2023 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
  • 66.
    66 / GE / April11, 2023 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
  • 67.
    67 / GE / April11, 2023 Local Relay Open Remote Relay Remote FWD IGND POTT TX Remote – Z2 Communication Channel POTT RX OPEN POTT TX Communication Channel POTT RX TRIP POTT Scheme Echo example
  • 68.
    68 / GE / April11, 2023 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
  • 69.
    69 / GE / April11, 2023 Bus Line Bus Zone 1 Zone 2 Zone 2 Zone 1 Zone 4 Local Remote Weak system Hybrid POTT
  • 70.
    70 / GE / April11, 2023 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
  • 71.
    71 / GE / April11, 2023 Bus Line Bus Zone 1 Zone 2 Zone 2 Zone 1 Local Remote DCB Scheme
  • 72.
    72 / GE / April11, 2023 End Zone Communication Channel Directional Comparison Blocking (DCB)
  • 73.
    73 / GE / April11, 2023 Directional Comparison Blocking (DCB) Internal Faults Local Relay Remote Relay Local Relay – Z2 Zone 2 PKP TRIP Timer Start FWD IGND GND DIR OC Fwd OR Dir Block RX NO TRIP Expired
  • 74.
    74 / GE / April11, 2023 Local Relay Remote Relay Remote Relay – Z4 Zone 4 PKP REV IGND GND DIR OC Rev OR DIR BLOCK TX Local Relay – Z2 Zone 2 PKP Dir Block RX Communication Channel FWD IGND GND DIR OC Fwd OR TRIP Timer Start No TRIP Directional Comparison Blocking (DCB) External Faults
  • 75.
    75 / GE / April11, 2023 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
  • 76.
    76 / GE / April11, 2023 Bus Line Bus Trip Line Breakers Tx1 (Un-Block) Forward Forward Tx2 (Block) Forward Rx2 Rx1 t o AND t o AND AND AND Lockout (Block) (Un-Block) DCUB Scheme
  • 77.
    77 / GE / April11, 2023 End Zone Communication Channel Directional Comparison Unblocking (DCUB)
  • 78.
    78 / GE / April11, 2023 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
  • 79.
    79 / GE / April11, 2023 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
  • 80.
    80 / GE / April11, 2023 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
  • 81.
    81 / GE / April11, 2023 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
  • 82.
    82 / GE / April11, 2023 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.
  • 83.
    83 / GE / April11, 2023 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
  • 84.
    84 / GE / April11, 2023 Redundant Pilot Schemes
  • 85.
    85 / GE / April11, 2023 • 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
  • 86.
    86 / GE / April11, 2023 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
  • 87.
    87 / GE / April11, 2023 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.
  • 88.
    88 / GE / April11, 2023 Security for dual-breaker terminals Accurate CTs preserve the reverse current direction under weak remote infeed
  • 89.
    89 / GE / April11, 2023 Security for dual-breaker terminals Saturation of CT1 may invert the line current as measured from externally summated CTs
  • 90.
    90 / GE / April11, 2023 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
  • 91.
    91 / GE / April11, 2023 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.
  • 92.
    92 / GE / April11, 2023 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.
  • 93.
    93 / GE / April11, 2023 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.
  • 94.
    94 / GE / April11, 2023 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.
  • 95.
    95 / GE / April11, 2023 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 trip
  • 96.
    96 / GE / April11, 2023 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 current
  • 97.
    97 / GE / April11, 2023 Series-compensated lines E Xs SC XL Infinte Bus Benefits of series capacitors: • Reduction of overall XL 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 miles
  • 98.
    98 / GE / April11, 2023 Series-compensated lines E Xs SC XL Infinte Bus 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 errors
  • 99.
    99 / GE / April11, 2023 Series-compensated lines Series Capacitor with MOV
  • 100.
    100 / GE / April11, 2023 Series-compensated lines
  • 101.
    101 / GE / April11, 2023 Series-compensated lines Dynamic Reach Control
  • 102.
    102 / GE / April11, 2023 Series-compensated lines Dynamic Reach Control for External Faults
  • 103.
    103 / GE / April11, 2023 Series-compensated lines Dynamic Reach Control for External Faults
  • 104.
    104 / GE / April11, 2023 Series-compensated lines Dynamic Reach Control for Internal Faults
  • 105.
    105 / GE / April11, 2023 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 transformer
  • 106.
    106 / GE / April11, 2023 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 transformer
  • 107.
    107 / GE / April11, 2023 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 point
  • 108.
    108 / GE / April11, 2023 > 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 Attributes
  • 109.
    109 / GE / April11, 2023 > 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 Attributes
  • 110.
    110 / GE / April11, 2023 > 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 Attributes
  • 111.