P R I N C I P L E S A N D A P P L I C ATI O N S PROTECTIVE R E L A Y I N G (2025)

Protective Relays

A review on protective relays' developments and trends

One of the most complex disciplines in electrical engineering is power system protection which requires not only the proper understanding of the different components of a power system and their behaviours but also a good knowledge and analysis of the abnormal circumstances and failures that can occur in any element of a power system. Moreover, the rapid changing and development in relays principles as well as in their technologies are additional factors that oblige those people working in the field to expand and update continuously their knowledge. In this paper, we shed light in the evolution of protective relays since the onset of electrical energy to currently. We try also to foresee the future prospects and trends in this area.

Relay models for protection studies

2003 IEEE Bologna Power Tech Conference Proceedings,, 2003

Power system protection relays have changed significantly during the last ten years. Processor based designs are now highly reliable integrated protection, control and monitoring systems that significantly outperform the suite of relays and control equipment from the electromechanical and static era. Technological advances and utility restructuring ensures protection remains an immensely challenging field, especially for young IT literate electrical power engineers.

Relay protection devices functionality comparative analysis. Sergiy M. Radimov, Valerii P. Plis (Herald of Advanced Information Technology, Vol. 6 No. 3)

Relay protection devices functionality comparative analysis, 2023

With the development of technology, there has been an evolution in the electrical power industry, replacing conventional electromechanical relays with more advanced devices. Multifunctional microprocessor relay protection terminals became such innovations. This transition marked a new era in the field of monitoring and control of electrical systems. One of the strategic tasks of the energy sector is the comprehensive technical re-equipment and reconstruction of relay protection and automation systems with a focus on maximum automation of dispatch control operations. Solving this problem is impossible without the use of microprocessor devices. The purpose of this work is to conduct a comparative analysis of relay protection devices based on electromechanical relays, electronic components and microprocessor devices, review and compare their characteristics. The work presents the advantages and problems of using microprocessor-based relay protection and automation devices in modern substations. The stages of complexity of relay protection and automation systems from electromechanics to a digital substation are shown. The general trends in the formation of the concept of "Smart Grid" and the main directions for creating intelligent electrical power systems are considered.

Application guide for the choice of protective relays.pdf

Abnormal conditions other than short circuits,, 8 A-c tripping, 335 Angle-impedance relay, 79 for tripping on 1088 of synchronism, 362 Angle of maximum torque, adjustment,, 57 of power relays, 52, 55 of shortcircuit relays, 55 Arcs, effect on distance relays, 345 resistance, 302 Arc -furnace -transformer protection, 270 ASA accuracy classification,, of current transformers,, 121 of potential transformers, 133 Attenuation, carrier-current, 378 Automatic reclosing, see Reclosing, automatic Auxiliaries,, station, protection of, 229 see also Motor protection Back-up relaying, defined, 6 effect of intermediate current sources, 347 for bus protection, 275, 291 generator external fault,227 reversed third zone, 175, 349 transformer external fault, 264, 268 with pilot relaying, 378, 386 Blind spot in pilot relaying, 94 Blinder with directional-comparison relaying, 386, 390 Blocking pilot, 88, 91 Blocking terminal, with a-c wire-pilot relaying, 375 I N D E X Capacitance potential device, equivalent circuit, 135 non-linear burdens, 139 standard accuracy, 137 standard rated burdens, 136 Capacitor, series, effect on distance relaying, 367 Capacitor tripping, 335 Carrier-current attenuation, 378 Carrier-current-pilot relaying, see Pilot relaying, carrier-current Circuit breaker, by-passing, 292 standard capacities, 3 Circulating-current pilot relaying, 92, 94 Cold-load pickup, 334 Cold-load restoration, 334 Compensated voltage, see Transformer compensation Constant-product characteristic, 38 Contact definitions, 17 Contact races in pilot relaying, 91 Continuous pilot, 109 Control spring, 17 Conventions, vector, 53 Corrosion, effect of polarity on, 19 Coupling-capacitor description, 134 Coupling-capacitor insulation coordination, 142 Coupling-capacitor potential device, Bee Capacitance potential device Current-balance relay, directional type, 62 for line protection, 330 overcurrent type, 58 Current biasing, for mho relay, 82 for offset impedance relay, 77 to avoid distance-relay misoperation on arcs, 346 Current compensation for ground distance relays,, 360 Current switching for distance relays, 368 Current transformers,, accuracy calcula , 113 ASA accuracy classification, 121 burden, 114 for generator differential relaying, 198 ratiocorrection-factor curves,, 116 secondaryexcitation curves,, 117 grounding the secondaries of differentially connected CT's, 291 overvoltage in secondary, 124 polarity and connections, 126 proximity effect, 126 Current transformers, requirements for pilot relaying, 377 secondary leakage reactance, 117, 119, 120 transient errors, 124, 278, 318 types, 113 zero-phase-sequencecurrent shunt, -130, 249 D-c offset, effect on induction relays, 32, 39 overreach of distance relays, 82, 350 overreach of overcurrent relays, 308 time constant, 279 D-c relays, single-quantity, 22 directional, 24, 49 Differential relays, 63 see also Percentagedifferential relays Directional-comparison relaying, for bus protection, 277 principle of operation, 106 see also Line protection with pilot relays Directional control, of electromagnetic relays,, 23 of single-quantity induction relays,, 32,57,310,313 Directional-overcurrent relay, 57 Directional relays,, a-c types, 33, 52 connections,, 52 power, 52 short circuits, 56 characteristics on R-X diagram, 74 d-c types,, 49 use of shunts, 52 effect of mutual induction on ground relays, 393 electromagnetic-attraction type, 24 groundrelay polarization, 151, 326 misoperating tendencies, 314 negative-phase-aequence type, 330 operating characteristics,, 37 response of polyphase relays to positive and negative phase sequence,, 183 response of ainglephase relays to short circuits,, 187 Distance relays,, current and voltage switching, 368 effect of power swings and lose of synchronism,, 181 effect of wye-delta transformer between relay and fault, 172 electronic type, 369 ground-relay connections, 360 impedance seen during faults,, 167 Generator protection, overvoltage, 217 potential-transformer fuse blowing, 228 prime mover, 230 station auxiliary, 229 stator overheating, 216 stator short circuit, 195 calculation of CT errors, 198 ground faults, sensitive, 208 ground faults in unit generators, 209 overcurrent relays for, 215 turn-to-turn faults, 204 unbalanced phase currents, 221 vibration, 225 Ground-distance-relay connections, 360 Ground-fault neutralizer, effect on line relaying, 321 to mitigate the effect of a fault, 2 Grounding protective relay for trans former protection, 263 Ground preference, 91, 108 Ground resistance, 303 Grounding-tranaformer protection, 268 Hsrmonic-current restraint, for distance relays,, 357 for transformer differential relays, 257 Holding coil, 18 Impedance diagram, see R-X diagram Impedance relay, characteristic on R-X diagram, 72 for line protection, 340 general, 70 see also Distance relays Induction-cup and induction-loop structures, 30, 31 Induction-type relay, directional, 31 general characteristics,, 26 single-quantity, 31 structures, 29 torque production, 26 Insulating transformer for pilot-wire circuits,, 98 Intermittent pilot, 109 Line protection with distance relays,, adjustment of distance relays,, 341 area, effect of, 345 blocking tripping on 1088 of synchro , 304 choice between impedance, reactance, and mho,, 340 connections of ground distance relays,, 360 Distance relays,, use of low-tension voltage, 145, 148 see also Line protection with distance relays Distribution-circuit protection, see Line protection with overcurrent relays Drop-out defined, 17 Electric arc-furnace-transformer protection, 270 Electromagnetic-attraction relay, directional, 24 general characteristics,, 16 single-quantity, 22 Electronic relay, directionalcomparison pilot, 396 distance, 369 I N D E X Evaluation of protective relaying, 12 Expulsion protective gaps, effect of, on distance relays, 367 External-fault back-up relaying, see Back-up relaying Fail ures,, electrical, see Faults False residual current, 318 Fault bus, 275 Faults, mitigation of effects of, 2 prevention of, 2 probability of, effect on practice, 11 see also Short circuits Fire, protection against, 230 Fire-pump-motor protection, 230 Footing resistance, tower-, 303 Frequency, compensation of relays for changes in, 49 effect on induction relays,, 32, 39 Frequency-converter protection, see Generator protection Fundamental principles of protective relaying, 4 Fuse, coordinating with a, 335 Fuse blowing, potential-tranaformer, effect on distance relays,, 361 effect on generator relays,, 228 Generator protection, bearing overheating, 228 external-fault back-up, m field ground, 218 loss of excitation, 223 1088 of synchronism,, 218 miscellaneous,, 228 motoring, 225 open circuits,, 215 over excitation,, 225 over speed,, 226 Line protection with distance relays, current and voltage switching, 368 expulsion protective gaps, effect of, 367 fuse blowing, effect of, 361 electronic relays, 369 intermediate current sources, effect of, 347 low-tension current, use of, 356 low-tension voltage, use of, 352 magnetizing inrush, effect of, 359 overreach, 351, 360 purposeful tripping on loss of synchronism, 361 reclosing, automatic, 366 series capacitor, effect of, 367 see also Distance relays Line protection with overcurrent relays, a-c and capacitor tripping, 335 adjustment of ground vs. phase re lays, 316 adjustment of inverse-time-overcur rent relays, 297 arc and ground resistance, 302 directional feature, 310 fuses, coordination with, 335 ground faults in ungrounded systems, detection of, 319 ground-fault neutralizers, effect of, 321 instantaneous overcurrent relays,, use of, 306 inverseness,, choice of, in relay char acteristics, 305 limiting ground-fault-current magni tude, effect of, 317 loop circuits,, effect on relay adjust , 303 misoperation prevention of aingle directional-overcurrent re during ground faults, 314 negative-phase sequence ground di Line trap, 100 Linear couplers, 284 Load shedding, 334, 363 Locking in, with generator differential relaying, 202 with transformer differential relaying, 251 Loss-of-excitation protection, 223 Loss-of-field protection, 223 Loss of synchronism, characteristics on RX diagram, 177 derivation of relay current and voltage, 176 effect on distance relays, 181 generator protection, 218 trip-blocking relay, 304, 390 tripping relay, 361 Low-tension current for distance relays, 356 Low-tension voltage, for directional relaying, 386 for distance relaying, 148, 352 general, 145 Magetizing-current inrush, effect on distance relays, 359 effect on transformer differential relays, 254 in parallel transformer banks, 259 Maximum torque, angle of, adjustment, 57 power relays, 52, 55 short-circuit relays, 55 Memory action, described, 83 effect of voltage source location, 144 Mho relay, characteristics on R-X diagram, 81 for line protection, 340 operating characteristic, 80 Microwave-pilot relaying, see Pilot relaying, microwave Minimum pickup of directional relays,, 38 Mixing transformer for wire-pilot relaying, 96 I N D E X Modified-impendance relay, 77 Motor protection, field ground, 237 fire-pump, 230 1088 of excitation, 237 1088 of synchronism, 236 rotor overheating, 236 stator overheating, 232 stator short circuit, 230 unattended motors,, 230 under voltage,, 237 Multiterminal-line protection, with a-c wire-pilot relaying, 376 with directional-comparieon pilot relaying, 387 with phasecomparison pilot relaying, 382 Mutual induction, effect of, on direc relays,, 393 from power circuit to pilot wires,, 88 Neutralizing transformers for wire-pilot circuits,, 99 Normally blocked trip circuit, 109 Open phase, effect of, on directional relays, 323, 325 equivalent circuits for, 323 protection of generators against, 215 Operating principles,, basic, electromagneticattraction relays,, 16 Operating principles, directional type, 24 singlequantity type, 22 induction relays: directional type, 33 singlequantity type, 31 Operation indicator, 17 Operator vs. protective relays, 11 Opposed-voltage pilot...

The Art and Science of Protective relaying

Abnormal conditions other than short circuits,, 8 A-c tripping, 335 Angle-impedance relay, 79 for tripping on 1088 of synchronism, 362 Angle of maximum torque, adjustment,, 57 of power relays, 52, 55 of shortcircuit relays, 55 Arcs, effect on distance relays, 345 resistance, 302 Arc -furnace -transformer protection, 270 ASA accuracy classification,, of current transformers,, 121 of potential transformers, 133 Attenuation, carrier-current, 378 Automatic reclosing, see Reclosing, automatic Auxiliaries,, station, protection of, 229 see also Motor protection Back-up relaying, defined, 6 effect of intermediate current sources, 347 for bus protection, 275, 291 generator external fault,227 reversed third zone, 175, 349 transformer external fault, 264, 268 with pilot relaying, 378, 386 Blind spot in pilot relaying, 94 Blinder with directional-comparison relaying, 386, 390 Blocking pilot, 88, 91 Blocking terminal, with a-c wire-pilot relaying, 375 I N D E X Capacitance potential device, equivalent circuit, 135 non-linear burdens, 139 standard accuracy, 137 standard rated burdens, 136 Capacitor, series, effect on distance relaying, 367 Capacitor tripping, 335 Carrier-current attenuation, 378 Carrier-current-pilot relaying, see Pilot relaying, carrier-current Circuit breaker, by-passing, 292 standard capacities, 3 Circulating-current pilot relaying, 92, 94 Cold-load pickup, 334 Cold-load restoration, 334 Compensated voltage, see Transformer compensation Constant-product characteristic, 38 Contact definitions, 17 Contact races in pilot relaying, 91 Continuous pilot, 109 Control spring, 17 Conventions, vector, 53 Corrosion, effect of polarity on, 19 Coupling-capacitor description, 134 Coupling-capacitor insulation coordination, 142 Coupling-capacitor potential device, Bee Capacitance potential device Current-balance relay, directional type, 62 for line protection, 330 overcurrent type, 58 Current biasing, for mho relay, 82 for offset impedance relay, 77 to avoid distance-relay misoperation on arcs, 346 Current compensation for ground distance relays,, 360 Current switching for distance relays, 368 Current transformers,, accuracy calcula , 113 ASA accuracy classification, 121 burden, 114 for generator differential relaying, 198 ratiocorrection-factor curves,, 116 secondaryexcitation curves,, 117 grounding the secondaries of differentially connected CT's, 291 overvoltage in secondary, 124 polarity and connections, 126 proximity effect, 126 Current transformers, requirements for pilot relaying, 377 secondary leakage reactance, 117, 119, 120 transient errors, 124, 278, 318 types, 113 zero-phase-sequencecurrent shunt, -130, 249 D-c offset, effect on induction relays, 32, 39 overreach of distance relays, 82, 350 overreach of overcurrent relays, 308 time constant, 279 D-c relays, single-quantity, 22 directional, 24, 49 Differential relays, 63 see also Percentagedifferential relays Directional-comparison relaying, for bus protection, 277 principle of operation, 106 see also Line protection with pilot relays Directional control, of electromagnetic relays,, 23 of single-quantity induction relays,, 32,57,310,313 Directional-overcurrent relay, 57 Directional relays,, a-c types, 33, 52 connections,, 52 power, 52 short circuits, 56 characteristics on R-X diagram, 74 d-c types,, 49 use of shunts, 52 effect of mutual induction on ground relays, 393 electromagnetic-attraction type, 24 groundrelay polarization, 151, 326 misoperating tendencies, 314 negative-phase-aequence type, 330 operating characteristics,, 37 response of polyphase relays to positive and negative phase sequence,, 183 response of ainglephase relays to short circuits,, 187 Distance relays,, current and voltage switching, 368 effect of power swings and lose of synchronism,, 181 effect of wye-delta transformer between relay and fault, 172 electronic type, 369 ground-relay connections, 360 impedance seen during faults,, 167 Generator protection, overvoltage, 217 potential-transformer fuse blowing, 228 prime mover, 230 station auxiliary, 229 stator overheating, 216 stator short circuit, 195 calculation of CT errors, 198 ground faults, sensitive, 208 ground faults in unit generators, 209 overcurrent relays for, 215 turn-to-turn faults, 204 unbalanced phase currents, 221 vibration, 225 Ground-distance-relay connections, 360 Ground-fault neutralizer, effect on line relaying, 321 to mitigate the effect of a fault, 2 Grounding protective relay for trans former protection, 263 Ground preference, 91, 108 Ground resistance, 303 Grounding-tranaformer protection, 268 Hsrmonic-current restraint, for distance relays,, 357 for transformer differential relays, 257 Holding coil, 18 Impedance diagram, see R-X diagram Impedance relay, characteristic on R-X diagram, 72 for line protection, 340 general, 70 see also Distance relays Induction-cup and induction-loop structures, 30, 31 Induction-type relay, directional, 31 general characteristics,, 26 single-quantity, 31 structures, 29 torque production, 26 Insulating transformer for pilot-wire circuits,, 98 Intermittent pilot, 109 Line protection with distance relays,, adjustment of distance relays,, 341 area, effect of, 345 blocking tripping on 1088 of synchro , 304 choice between impedance, reactance, and mho,, 340 connections of ground distance relays,, 360 Distance relays,, use of low-tension voltage, 145, 148 see also Line protection with distance relays Distribution-circuit protection, see Line protection with overcurrent relays Drop-out defined, 17 Electric arc-furnace-transformer protection, 270 Electromagnetic-attraction relay, directional, 24 general characteristics,, 16 single-quantity, 22 Electronic relay, directionalcomparison pilot, 396 distance, 369 I N D E X Evaluation of protective relaying, 12 Expulsion protective gaps, effect of, on distance relays, 367 External-fault back-up relaying, see Back-up relaying Fail ures,, electrical, see Faults False residual current, 318 Fault bus, 275 Faults, mitigation of effects of, 2 prevention of, 2 probability of, effect on practice, 11 see also Short circuits Fire, protection against, 230 Fire-pump-motor protection, 230 Footing resistance, tower-, 303 Frequency, compensation of relays for changes in, 49 effect on induction relays,, 32, 39 Frequency-converter protection, see Generator protection Fundamental principles of protective relaying, 4 Fuse, coordinating with a, 335 Fuse blowing, potential-tranaformer, effect on distance relays,, 361 effect on generator relays,, 228 Generator protection, bearing overheating, 228 external-fault back-up, m field ground, 218 loss of excitation, 223 1088 of synchronism,, 218 miscellaneous,, 228 motoring, 225 open circuits,, 215 over excitation,, 225 over speed,, 226 Line protection with distance relays, current and voltage switching, 368 expulsion protective gaps, effect of, 367 fuse blowing, effect of, 361 electronic relays, 369 intermediate current sources, effect of, 347 low-tension current, use of, 356 low-tension voltage, use of, 352 magnetizing inrush, effect of, 359 overreach, 351, 360 purposeful tripping on loss of synchronism, 361 reclosing, automatic, 366 series capacitor, effect of, 367 see also Distance relays Line protection with overcurrent relays, a-c and capacitor tripping, 335 adjustment of ground vs. phase re lays, 316 adjustment of inverse-time-overcur rent relays, 297 arc and ground resistance, 302 directional feature, 310 fuses, coordination with, 335 ground faults in ungrounded systems, detection of, 319 ground-fault neutralizers, effect of, 321 instantaneous overcurrent relays,, use of, 306 inverseness,, choice of, in relay char acteristics, 305 limiting ground-fault-current magni tude, effect of, 317 loop circuits,, effect on relay adjust , 303 misoperation prevention of aingle directional-overcurrent re during ground faults, 314 negative-phase sequence ground di Line trap, 100 Linear couplers, 284 Load shedding, 334, 363 Locking in, with generator differential relaying, 202 with transformer differential relaying, 251 Loss-of-excitation protection, 223 Loss-of-field protection, 223 Loss of synchronism, characteristics on RX diagram, 177 derivation of relay current and voltage, 176 effect on distance relays, 181 generator protection, 218 trip-blocking relay, 304, 390 tripping relay, 361 Low-tension current for distance relays, 356 Low-tension voltage, for directional relaying, 386 for distance relaying, 148, 352 general, 145 Magetizing-current inrush, effect on distance relays, 359 effect on transformer differential relays, 254 in parallel transformer banks, 259 Maximum torque, angle of, adjustment, 57 power relays, 52, 55 short-circuit relays, 55 Memory action, described, 83 effect of voltage source location, 144 Mho relay, characteristics on R-X diagram, 81 for line protection, 340 operating characteristic, 80 Microwave-pilot relaying, see Pilot relaying, microwave Minimum pickup of directional relays,, 38 Mixing transformer for wire-pilot relaying, 96 I N D E X Modified-impendance relay, 77 Motor protection, field ground, 237 fire-pump, 230 1088 of excitation, 237 1088 of synchronism, 236 rotor overheating, 236 stator overheating, 232 stator short circuit, 230 unattended motors,, 230 under voltage,, 237 Multiterminal-line protection, with a-c wire-pilot relaying, 376 with directional-comparieon pilot relaying, 387 with phasecomparison pilot relaying, 382 Mutual induction, effect of, on direc relays,, 393 from power circuit to pilot wires,, 88 Neutralizing transformers for wire-pilot circuits,, 99 Normally blocked trip circuit, 109 Open phase, effect of, on directional relays, 323, 325 equivalent circuits for, 323 protection of generators against, 215 Operating principles,, basic, electromagneticattraction relays,, 16 Operating principles, directional type, 24 singlequantity type, 22 induction relays: directional type, 33 singlequantity type, 31 Operation indicator, 17 Operator vs. protective relays, 11 Opposed-voltage pilot...

power system protection

An electrical power system consists of generators, transformers, transmission and distribution lines. Short circuit and other abnormal conditions often occur on a power system. The heavy current associated with short circuits is likely to cause damage to the equipment if suitable protective relays and circuit breakers are not provided for the protection of each section of the power system. A protective system includes circuit breakers, transducers (CTs and VTs), and protective relays to isolate the faulty section of the power system from the healthy sections. The function of a protective relay is to detect and locate a fault and issue a command to the circuit breaker to disconnect the faulty element. The conventional protective relays are either of electromechanical or static type. The electromechanical relays suffer from several drawbacks such as high burden on instrument transformer, high operating time, contact problem etc. The static relays also suffer from a number of disadvantages such as inflexibility, inadaptability to changing system conditions and complexity. The functions of electromechanical protection systems are now being replaced by microprocessor-based digital protective relays, sometimes called "numeric relays". The increased growth of power system both in size and complexity has brought about the need for fast and reliable relays to protect major equipment and to maintain system stability. The concept of digital protection employing computers which shows much promise in providing improved performance has evolved during the past two decades. Digital computer can easily fulfil the protection requirements of modern power system without difficulties. With the development of economical, powerful and sophisticated microprocessor, there is a growing interest in developing microprocessor-based protective relays which are more flexible because of being programmable and are superior to conventional relays. The objective of this paper is to give a comparative review of microprocessor-based protective relays.

REVIEW OF MICROPROCESSOR BASED PROTECTIVE RELAYS

An electrical power system consists of generators, transformers, transmission and distribution lines. Short circuit and other abnormal conditions often occur on a power system. The heavy current associated with short circuits is likely to cause damage to the equipment if suitable protective relays and circuit breakers are not provided for the protection of each section of the power system. A protective system includes circuit breakers, transducers (CTs and VTs), and protective relays to isolate the faulty section of the power system from the healthy sections. The function of a protective relay is to detect and locate a fault and issue a command to the circuit breaker to disconnect the faulty element. The conventional protective relays are either of electromechanical or static type. The electromechanical relays suffer from several drawbacks such as high burden on instrument transformer, high operating time, contact problem etc. The static relays also suffer from a number of disadvantages such as inflexibility, inadaptability to changing system conditions and complexity. The functions of electromechanical protection systems are now being replaced by microprocessor-based digital protective relays, sometimes called "numeric relays". The increased growth of power system both in size and complexity has brought about the need for fast and reliable relays to protect major equipment and to maintain system stability. The concept of digital protection employing computers which shows much promise in providing improved performance has evolved during the past two decades. Digital computer can easily fulfil the protection requirements of modern power system without difficulties. With the development of economical, powerful and sophisticated microprocessor, there is a growing interest in developing microprocessor-based protective relays which are more flexible because of being programmable and are superior to conventional relays. The objective of this paper is to give a comparative review of microprocessor-based protective relays.

Commissioning of Protective Relay Systems Commissioning of Protective Relay Systems

—Performing tests on individual relays is a common practice for relay engineers and technicians. Most utilities have a wide variety of test plans and practices. However, properly commissioning an entire protection system, not just the individual relays, presents a challenge. This paper suggests a process for performing consistent and thorough commissioning tests through many sources: breaking out relay logic into schematic drawings; using SER, metering, and event reports from relays; simulating performance using end-to-end testing and lab simulations; and utilizing other tools, including synchrophasor measurements. We examine and suggest approaches for commissioning several applications: distribution bus protection, short line protection using communications-aided tripping, main-tie-main scheme, line and transformer differential protection. Finally, we propose that, while 100% commissioning certainty may not be possible, we can approach 100% by integrating event report analysis to validate our commissioning strategy.