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Protection functions
ANSI 51V51V Protection (PVOC): Voltage Restrained OvercurrentWhat 51V protection is, voltage controlled versus voltage restrained overcurrent, how it is set, why it trips and how it is tested on a generator.ANSI 79Automatic Reclosing 79 (RREC): How It WorksAutomatic reclosing 79 explained: dead time, reclaim time, single-pole and three-pole reclosing, setting criteria and why a recloser does not reclose.ANSI 50BFBreaker failure protection 50BF (RBRF): how it worksBreaker failure protection (ANSI 50BF, RBRF): logic, retrip, timer, setting criteria per IEEE C37.119, misoperations and testing.ANSI 87BBus Differential Protection 87B (PDIF): How It WorksBus differential protection 87B explained: high and low impedance schemes, check zone, disconnector replica and why a busbar protection trips or blocks.ANSI 67NDirectional Ground Overcurrent 67N (PTOC + RDIR)How the 67N zero-sequence directional element works: residual current, polarization, angle by neutral grounding, false trips and how it is tested.ANSI 21Distance protection 21 (PDIS): how it worksWhat distance protection 21 is, how it measures impedance, how its zones are set and why it trips or blocks in the field.ANSI 81Frequency protection 81U/81O: how it worksWhat frequency protection 81 (underfrequency 81U and overfrequency 81O) is, how it is used for load shedding and generators, settings and why it trips.ANSI 87GGenerator Differential Protection 87G (PDIF) ExplainedGenerator differential protection 87G: which stator faults it detects and which it misses, how it is set and why it can trip on an external fault.ANSI 87LLine Current Differential 87L (PDIF): How It WorksLine current differential protection 87L: current comparison over a channel, time alignment, line charging current and why it trips or blocks.ANSI 51LR/48Locked Rotor 51LR and Incomplete Sequence 48 ProtectionMotor locked rotor (51LR), prolonged start or incomplete sequence (48) and starts-per-hour (66) protection: how they work, settings and why they trip.ANSI 86Lockout Relay 86: What It Is and How It WorksWhat a lockout relay 86 is, why it blocks closing after internal faults, how it works with 87T, 87B and 50BF, how it is tested and why it operates.ANSI 40Loss of field protection 40: how it worksGenerator loss of field (loss of excitation) protection, ANSI 40: principle, mho and admittance characteristics, settings, misoperations and testing.ANSI 87MMotor Differential Protection 87M (PDIF) ExplainedMotor differential protection 87M: percentage vs self-balancing (core-balance) schemes, when it is justified and why it trips during motor starting.ANSI 46Negative Sequence Protection 46: How It WorksNegative-sequence current protection (ANSI 46): current unbalance in generators, motors and feeders, the I2²t curve, settings, trips and testing.ANSI 59NNeutral Overvoltage 59N (3V0): How It WorksResidual voltage protection 59N: how it detects ground faults on ungrounded systems and high-impedance grounded generators, why it trips, how to test it.ANSI 78Out-of-step protection 78 (PPAM): how it worksWhat out-of-step protection 78 is, how it detects pole slipping, where it is applied, setting criteria and why it trips.ANSI 51Overcurrent 50/51 and 50N/51N (PTOC, PIOC): How It WorksPhase overcurrent 50/51 and ground overcurrent 50N/51N: IEC and IEEE inverse curves, setting criteria, why it trips falsely and how it is tested.ANSI 24Overexcitation 24 V/Hz (PVPH): How It WorksVolts per hertz overexcitation protection (ANSI 24): why it protects transformers and generators, how it is set, why it trips and how it is tested.ANSI 59Overvoltage Protection 59 (PTOV): How It WorksOvervoltage relay 59: where it is applied, how it is set, why it trips on generators, long lines and solar plants, and how it is tested.ANSI 47Phase Sequence Protection 47: How It WorksPhase sequence voltage relay (ANSI 47): reverse rotation and negative-sequence voltage V2 in motors and generators, settings, trips and testing.ANSI 68Power swing blocking 68 (RPSB): how it worksWhat power swing blocking 68 is, how it prevents distance protection trips during swings, how it is set, why it fails and how it is tested.ANSI 87NRestricted earth fault 87N/64REF: how it worksRestricted earth fault (REF) protection for transformers and generators: high and low impedance, settings, misoperations and testing.ANSI 32Reverse power protection 32 (PDOP): how it worksWhat reverse power protection (ANSI 32) is, why it protects the generator prime mover and the interconnection, how it is set and why it trips.ANSI 81RROCOF 81R (PFRC): Rate of Change of Frequency ExplainedROCOF 81R explained: where it is used, its limits for islanding detection, setting criteria and why it trips solar plants and distributed generation.ANSI 25Synchronism Check 25 (RSYN): What It Is and How It WorksWhat synchronism check (ANSI 25) compares before a breaker closes, how sync check and dead-line modes are set and why it blocks the close command.ANSI 49Thermal Overload Protection 49 (PTTR): How It WorksThermal overload relay (ANSI 49, PTTR): thermal model, why not every overload should trip, 49 versus 51 for overload, settings and why it trips.ANSI 63 / 26 / 49Transformer Buchholz 63, Pressure and Temperature 26/49How transformer mechanical protections work: Buchholz relay 63, sudden pressure relay, pressure relief device, and oil and winding temperature 26/49.ANSI 87TTransformer Differential Protection 87T (PDIF) ExplainedTransformer differential protection 87T: restraint, harmonic blocking, vector group compensation and why it trips when the transformer is energized.ANSI 27Undervoltage Protection 27 (PTUV): How It WorksUndervoltage relay (ANSI 27): where it is used in motors, buses and solar plants, how it is set, why it trips and how it is tested.
Schemes and teleprotection
MT/AT/EATBusbar Protection Schemes: High and Low ImpedanceBusbar protection schemes: high- and low-impedance bus differential, zones per section, check zone, disconnector replica and breaker failure 50BF.MT/AT/EATControlled islanding and system separation schemesWhat controlled system separation and intentional islanding are, which protections form the scheme, how islands are resynchronized, utility and industry.ANSI 85Current Reversal Guard on Parallel LinesWhat current reversal guard is in POTT and DCB schemes, why a healthy parallel line trips without it, and how it is set and tested.ANSI 85DCB scheme: directional comparison blockingHow the DCB scheme works: which element sends the block, the coordination timer, what happens if the channel is lost, typical misoperations and testing.ANSI 85DCUB scheme: directional comparison unblockingWhat the DCUB teleprotection scheme is: permissive trip with a guard signal, unblock window on loss of channel, use with power line carrier, and testing.MT/AT/EATDead zone and stub protection: faults between zonesThe dead zone between breaker and CT, stub protection in breaker-and-a-half and ring buses, how these faults are detected and why the logic fails.ANSI 85Direct transfer trip (DTT): breaker failure and reactorsWhat direct transfer trip (DTT or intertrip) is, when it is used for breaker failure, reactors, transformers and anti-islanding, and what channel it needs.ANSI 85DUTT scheme: direct underreaching transfer tripWhat the DUTT teleprotection scheme is, how zone 1 sends a direct trip to the remote end, what channel it needs and why it is rarely used today.ANSI 85Echo and Weak-Infeed Logic in POTT SchemesHow echo keying and weak-infeed logic work in teleprotection: conditions, voltage supervision, risks such as echo lockup, and testing.BT/MT/AT/EATGenerator protection scheme: zones, backup and trip modesHow the protections of a synchronous generator are combined: internal faults, abnormal operating conditions, system backup and trip modes.AT/EATLine Protection Main 1 and Main 2: How It Is BuiltMain 1 and main 2 line protection: what is duplicated, how the two systems coordinate with backup and breaker failure 50BF, and why the scheme fails.ANSI 85Phase comparison protection: what it is, how it worksPhase comparison line protection: current polarity, half-cycle blocks, stability angle, channel delay and how it differs from line differential 87L.ANSI 85POTT scheme: permissive overreaching transfer tripHow the POTT teleprotection scheme works: which zone keys the permissive, what the remote end needs, echo, current reversal, misoperations and tests.ANSI 85PUTT Scheme: Permissive Underreaching Transfer TripHow a PUTT scheme works: zone 1 keys the permissive signal and the remote end trips if its overreaching element sees the fault. Channel, limits, testing.MTRadial feeder protection: recloser and fuse coordinationHow radial feeder protection is coordinated: substation breaker relay, recloser, sectionalizer and fuses, fuse saving, and why the scheme misoperates.AT/EATRemedial Action Schemes (RAS, SPS): How They WorkWhat remedial action schemes (RAS), SPS and SIPS are: their parts, arming logic, typical actions, why they misoperate and how they are tested.MTReverse Interlocking Busbar Protection in MV SwitchgearReverse interlocking (blocking) busbar protection in medium-voltage switchgear: how it works, setting criteria, GOOSE signals and why it misoperates.AT/EATSingle-pole and three-pole reclosing: secondary arcSingle-pole and three-pole reclosing on transmission lines: secondary arc, dead time, pole discordance, leader and follower in breaker-and-a-half.BT/MT/AT/EATSubstation Protection Schemes: A Guide by ElementHow protection is combined for lines, transformers, buses, generators and feeders, and what changes between high, medium and low voltage.ANSI 85Teleprotection Schemes (85): Types, Channel and SelectionLine teleprotection schemes: DUTT, PUTT, POTT, DCB, DCUB, DTT and phase comparison. What each one sends, which channel it needs and when to use it.MT/AT/EATTransformer protection scheme: 87T, 87N, 63 and backupHow differential, restricted earth fault, mechanical protections, backup overcurrent and the lockout relay 86 combine in a power transformer scheme.MT/ATUnderfrequency load shedding (UFLS): how the scheme worksWhat underfrequency load shedding is, how it is built in stages with 81 and df/dt elements, how it is coordinated with generators and why a scheme fails.ANSI 85Zone acceleration in distance protection: how it worksZone acceleration: the remote end's signal extends zone 1 or bypasses the zone 2 delay. Variants with and without a channel, misoperations and testing.
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