A protection scheme is the combination of relays, instrument transformers, communication channels, logic and breakers that protects a complete element of the network. Each element (line, transformer, bus, generator, feeder) has its own scheme, with main protection, backup and breaker failure, and above them sit the system schemes: load shedding, area separation and remedial actions.
What a protection scheme is
A protection function detects something: an impedance inside the zone, a differential current, a low frequency. A protection scheme is what turns those functions into a decision on the network: which current transformers (CTs) and voltage transformers (VTs) feed each relay, how the functions back each other up, which channel reaches the remote terminal, which breakers open and what is blocked afterwards.
Every scheme answers four questions:
- Zone: which element it protects and where its boundaries are (the CTs). See protection zones.
- Main and backup: who clears first and who acts if something fails, with or without redundancy depending on the voltage level.
- Breaker failure: what happens if the breaker does not open (50BF).
- After the trip: reclosing, lockout with the 86 relay or transfer.
This guide is the map of the series. Each section links to the detailed scheme for each element.
Schemes by element
Transmission and subtransmission lines
The line is protected by distance, line differential or both, supported by communication channels with the remote terminal. At high and extra-high voltage two independent main protections are installed. See line protection: main 1 and main 2, teleprotection schemes and single-pole and three-pole reclosing. The reference guide is IEEE C37.113 (2025 edition).
Power transformers
The scheme combines the differential, restricted earth fault, the transformer’s own mechanical protection, backup overcurrent on each winding and a lockout relay that opens all the breakers. See the transformer protection scheme. Guide: IEEE C37.91.
Buses
A bus fault is the most severe in the substation because it affects every connected bay. It is protected by a high- or low-impedance differential, zones per section and a check zone, or at medium voltage by blocking schemes between relays. See busbar protection schemes and reverse interlocking bus protection. Guide: IEEE C37.234.
Generators
The generator combines protection against internal faults, abnormal conditions (loss of field, unbalance, reverse power, overexcitation) and system conditions, with tripping modes that decide what opens: unit breaker, field, turbine. See the generator protection scheme. Guide: IEEE C37.102.
Distribution feeders
A radial feeder is protected in cascade: relay at the substation, recloser on the main trunk, sectionaliser and fuses on the laterals. See the radial feeder protection scheme. Guide: IEEE C37.230.
Blind spots between zones
Between the breaker and its CT, or in the section left energised when a line disconnector opens in a breaker-and-a-half bay, no main protection sees the fault properly. See dead zone and stub protection.
System schemes
Some schemes do not protect a piece of equipment but the stability of the network:
- Underfrequency load shedding: settings and stages defined by the system operator.
- Area separation and islanding: splitting the network in a controlled way before it collapses.
- Remedial action schemes (SPS/RAS): predefined automatic actions for contingencies identified in studies.
How the schemes coordinate with each other
The schemes of the different elements are not independent:
- The 50BF of any bay trips all the adjacent breakers feeding the fault (those on its bus or, in breaker-and-a-half, the neighbour in the diameter) and, if the bay is a line, sends a transfer trip to the remote terminal.
- The bus differential usually blocks reclosing of the connected lines.
- The transformer 86 relay blocks closing of all its breakers until someone resets it.
- The system schemes must be coordinated with generator protection: if frequency falls, load shedding must act before the frequency protection of the units.
Protection by voltage level
The job of a protection is the same at any level: detect the fault and isolate only the affected element. What changes with voltage is the consequence of slow clearing. At high and extra-high voltage a poorly cleared fault can compromise the stability of the interconnected system; at medium voltage it mainly affects the customers of one circuit. That is why the scheme gains redundancy and speed as the voltage rises.
Voltage level classes
Each standard draws the lines differently. A common reference in IEEE practice, based on ANSI C84.1:
| Class | Range (phase-to-phase, AC) |
|---|---|
| Extra-high voltage (EHV) | Above 230 kV |
| High voltage (HV) | Above 100 kV up to 230 kV |
| Medium voltage (MV) | Above 1 kV up to 100 kV |
| Low voltage (LV) | Up to 1 kV |
IEC calls everything above 1 kV AC high voltage, and many grid codes set their own limits. Use the classification that applies to the project.
What each level carries
- Extra-high voltage: two independent main protections, redundant teleprotection, single-pole reclosing, bus differential with check zone, 50BF on every breaker and system schemes.
- High voltage: depending on the transmission owner, one or two main protections (distance or 87L with teleprotection), 67N for resistive faults, reclosing, bus differential, 50BF and pole discordance.
- Medium voltage: coordinated 50/51 and 50N/51N overcurrent, directional elements where there are loops or distributed generation, reclosing on overhead networks, reclosers and fuses, blocking schemes in switchgear, arc flash protection, 81 for load shedding and 27/59.
- Low voltage and industry: breaker trip units, motor and generator relays, automatic transfer schemes and internal load shedding when the plant can be islanded.
| Aspect | High and extra-high voltage | Medium voltage |
|---|---|---|
| Main line protection | 21 and 87L with teleprotection | 50/51 and 50N/51N |
| Redundancy | Main 1 and main 2, 50BF | One relay per circuit, upstream backup |
| Neutral | Usually solidly or effectively grounded | Solid, resistance, compensated or ungrounded |
| Ground fault | High current | Can be small: core-balance CT and sensitive 67N |
| CTs | Transient-performance classes or classes defined by knee-point voltage, depending on the relay | Class P protection CTs are usually enough |
| Tests | Functional and end-to-end with teleprotection | Secondary injection and functional |
Redundancy: main 1 and main 2
At high and extra-high voltage the main protection is duplicated, often with different principles (distance and line differential), and the CT cores, voltage circuits, DC supplies, trip coils and teleprotection channels are also separated. Duplicating relays that share the same battery or the same CT core gives no real redundancy. At medium voltage the usual practice is a single protection per circuit, backed up by the upstream one.
Clearing times
In transmission, maximum fault clearing times are set by the system operator from the stability studies, and they drive the whole scheme: relay speed, type of teleprotection, breaker interrupting time and the 50BF setting. In distribution, coordination with downstream devices and the thermal limit of transformers and cables govern. The criteria are explained in coordination and setting criteria.
Neutral grounding changes ground fault protection
At medium voltage the way the neutral is grounded decides how a ground fault is detected. With a solidly grounded neutral the ground fault current can be of the order of the three-phase current and ground overcurrent is enough. With resistance grounding it is limited and 51N has to be more sensitive. With an ungrounded or compensated neutral the current is small and sensitive directional functions fed by a core-balance CT and residual voltage are needed. Applying to an ungrounded network the ground setting meant for a solidly grounded one leaves faults unseen. See also zero-sequence current.
Keep in mind that relays do not protect against overvoltages: surge arresters and insulation coordination limit them; relays clear faults.
How each manufacturer implements it
Siemens SIPROTEC (DIGSI), SEL (AcSELerator QuickSet), Hitachi Energy and ABB Relion (PCM600), GE Vernova Multilin (EnerVista) and MiCOM / Easergy (GE Vernova and Schneider Electric; MiCOM S1, Easergy Studio) offer relays per element: line, transformer, bus, generator and feeder. The difference lies in how the scheme is built: each brand’s own programmable logic, IEC 61850 GOOSE messages or conventional wiring. The same scheme is written very differently in each brand.
Why a scheme fails
Most misoperations do not come from a poorly designed function but from the scheme: a CT on the wrong core, a zone that does not overlap, a blocking signal that never arrives, a disconnector whose auxiliary contact does not report its position, an active settings group different from the one studied. The first step is to rebuild the sequence of events of all relays with a common time reference.
How a scheme is tested
Each function is tested by secondary injection, but the scheme is only proven by functional tests that cover the complete path: from the CT to the trip coil, including blocking signals, the 86, reclosing and messages between relays. On lines it is completed with end-to-end tests.
Do you have a scheme that tripped when it should not have, or a new one to configure from scratch? At Tiza Group we review the events and configure the relays remotely. See our remote relay configuration service or contact us.
Related guides
- Fundamentals of electrical protection
- Protection zones, main and backup protection
- Coordination and setting criteria
- Transformer protection scheme
- Busbar protection schemes
- Generator protection scheme
- Radial feeder protection scheme
- Line protection: main 1 and main 2
References
- IEEE C37.113-2025, Guide for Protective Relay Applications to Transmission Lines.
- IEEE C37.91-2021, Guide for Protecting Power Transformers.
- IEEE C37.234-2021, Guide for Protective Relay Applications to Power System Buses.
- IEEE C37.102-2023, Guide for AC Generator Protection.
- IEEE C37.230-2020, Guide for Protective Relay Applications to Distribution Lines.
- IEEE C37.119-2025, Guide for Power System Circuit Breaker Failure Protection.
- IEEE C37.104-2022, Guide for Automatic Reclosing on AC Distribution and Transmission Lines.
- IEEE C37.250-2020, Guide for Engineering, Implementation, and Management of System Integrity Protection Schemes.
- ANSI C84.1, Electric Power Systems and Equipment – Voltage Ratings (60 Hz).
- CIGRE TB 140 (WG 34.01, 1999), Reliable fault clearance and back-up protection.
- J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
What is the difference between a protection function and a protection scheme?
A function is an algorithm that detects a condition, such as transformer differential 87T or distance 21. The scheme is the whole set that protects an element: which functions are used, with which instrument transformers, how they back each other up, which breakers they trip and which communication channels they use.
Does every substation need duplicated main protection?
No. Full duplication is common at extra-high voltage and on critical high-voltage elements, as required by the transmission owner and the system operator. At medium voltage the usual practice is one main protection with local or remote backup.
How are voltage levels classified?
It depends on the standard. In IEC practice anything above 1 kV AC is high voltage. ANSI C84.1 and IEEE use classes: low voltage up to 1 kV, medium voltage above 1 kV up to 100 kV, high voltage above 100 kV up to 230 kV and extra-high voltage above 230 kV. Many grid codes set their own limits; use the one that applies to the project.