substation protection relays- Autony

What Relays Are Used in a Substation?

The most common relays used in substations include overcurrent relays, earth fault relays, differential protection relays, distance relays, directional overcurrent relays, voltage relays, frequency relays, breaker failure relays and digital protection relays. The selection depends on the equipment being protected, voltage level and protection scheme.

Types of Relays Used in Substations

Relay Type	ANSI	Main Function	Typical Substation Application
Overcurrent Relay	50/51	Phase fault & overload protection	Feeders, transformers
Earth Fault Relay	50N/51N	Ground-fault protection	Feeders, transformers
Directional Overcurrent Relay	67/67N	Directional fault protection	Ring/parallel feeders
Differential Relay	87	Internal fault protection	Transformers, busbars
Distance Relay	21	Line fault protection	Transmission lines
Busbar Differential Relay	87B	Busbar fault protection	HV/MV busbars
Transformer Differential Relay	87T	Transformer internal fault	Power transformers
Voltage Relay	27/59	Under/overvoltage	Busbars, feeders
Frequency Relay	81	Under/overfrequency	Grid/substation
Breaker Failure Relay	50BF	Detect breaker failure	HV/MV switchgear
Auto-Reclosing Relay	79	Automatic reclosing	Transmission/distribution lines
Lockout Relay	86	Master trip/lockout	Transformer, busbar, feeder

What Relays Are Used for Different Substation Equipment?

Relays Used for Transformer Protection

87T
50/51
50N/51N
64REF
49
63 / Buchholz
24

Relays Used for Feeder Protection

50/51
50N/51N
67/67N
79

Relays Used for Busbar Protection

87B
50BF
86

Relays Used for Transmission Line Protection

Relays Used for Capacitor Bank Protection

50/51
59
87V
unbalance protection

Relays Used for Incoming and Outgoing Feeders

50/51
50N/51N
27/59
67

relays in substation protection scheme

Primary and Backup Relays in a Substation

Primary Protection

Primary protection is the first line of defense that operates when a fault occurs, undertaking the core task of rapid fault clearance. It requires absolute speed and selectivity. For example, transformer primary protection typically employs differential protection (87T), which can trip within milliseconds in the event of an internal short circuit.

Backup Protection

Backup protection provides remote or local support when the primary protection or circuit breaker fails to operate, preventing fault escalation. A typical example is feeder overcurrent protection (50/51), whose time delay is coordinated with the primary protection through relay coordination using stepped time intervals, ensuring orderly operation when the primary protection fails.

Integrated Protection System

In a complete substation protection system, primary and backup protections complement each other through setting coordination and timing coordination, ensuring both rapid fault clearing under priority conditions and fallback safeguards for abnormal operating conditions. The collaborative configuration of primary protection relays and backup protection relays forms the cornerstone of protection selectivity, sensitivity, and reliability, and is also the core focus of setting calculations and coordination verification.

Digital and Numerical Relays Used in Modern Substations

Modern substations increasingly adopt microprocessor-based relays to replace conventional electromechanical types. These relays convert analog signals (current, voltage) into digital data and process them using mathematical algorithms, offering high precision and flexible settings through software configuration.

Numerical protection relays represent an advanced generation, employing digital signal processing (DSP) for complex calculations such as harmonic analysis and phasor estimation, enabling adaptive protection strategies.

A key advantage is the multifunction protection relay, which integrates multiple functions (e.g., overcurrent, earth fault, directional, voltage) into a single device, reducing panel space, wiring, and maintenance.

Modern relays are equipped with communication capabilities for integration into digital substation relay systems. They support IEC 61850, ensuring interoperability between devices from different manufacturers, and interface with SCADA systems for remote monitoring, control, and setting adjustments.

Digital relays also provide event recording (time-stamped sequence of operations) and fault recording (detailed waveform data), which are invaluable for post-fault analysis, performance verification, and coordination checks.

In summary, digital and numerical relays combine high-speed protection with advanced communication and recording functions, making them essential components of modern digital substations for reliable protection and intelligent grid management.

ANSI Codes for Relays Used in Substations

ANSI Code	Protection Function	Application
21	Distance Protection	Transmission Line
25	Synchronism Check	Incoming Feeder
27	Undervoltage	Busbar/Feeder
46	Negative Sequence	Motor/Generator
49	Thermal Protection	Transformer/Motor
50	Instantaneous Overcurrent	Feeder
51	Time Overcurrent	Feeder
50N/51N	Earth Fault	Feeder/Transformer
59	Overvoltage	Busbar
67	Directional Overcurrent	Ring Feeder
79	Auto-Reclosing	Transmission Line
81	Frequency	Grid
86	Lockout	Transformer/Busbar
87B	Bus Differential	Busbar
87T	Transformer Differential	Transformer
87L	Line Differential	Transmission Line
50BF	Breaker Failure	Switchgear

How to Select a Relay for a Substation

Selection Factor	Key Considerations
Voltage	Select insulation level and rated parameters based on voltage class; high voltage requires stronger insulation and anti-interference capability.
Equipment	Different equipment requires different protections: transformers use 87T/24/63; feeders use 50/51/67; busbars use 87B; lines use 21/87L; capacitor banks use 59/unbalance protection.
Fault Type	Phase-to-phase faults use overcurrent/distance; earth faults use zero-sequence/earth fault protection; internal faults use differential; overloads use overload protection.
CT/VT	Ensure ratio, accuracy, burden, and wiring match; consider CT saturation characteristics and VT accuracy impact on protection.
Protection Function	Determine primary and backup protection combinations; evaluate whether the relay supports all required functions and programmable logic.
Communication	Assess interface types, protocol support, transmission rates, and redundancy to ensure remote monitoring and data exchange capability.
IEC 61850	Verify support for GOOSE, SV, and MMS services to ensure interoperability and object-oriented architecture compatibility.
SCADA	Confirm seamless integration capability for remote monitoring, control, and data upload (event/fault records).

Substation Protection Relay Applications

Voltage Level	Typical Applications	Common Protection Functions	Description
11kV	Distribution substations, industrial user in-feeds, urban distribution networks	50/51 (overcurrent), 50N/51N (zero-sequence overcurrent), 51G (earth overcurrent), 59 (overvoltage), 27 (undervoltage)	11kV systems are mostly neutral ungrounded or grounded via arc suppression coils, relying primarily on overcurrent and zero-sequence protection. The configuration is relatively simple, with lower speed requirements.
22kV	Regional distribution networks, medium-sized industrial users, mining distribution	50/51, 50N/51N, 67 (directional overcurrent), 79 (reclosing), 59, 27	Compared to 11kV, directional elements are added to handle dual-supply or ring network operation modes. Selectivity requirements are higher, and reclosing functions begin to be deployed.
33kV	High-voltage distribution networks, large industrial users, renewable energy collection stations	50/51, 50N/51N, 67/67N, 79, 59/27, 24 (overexcitation, for transformers)	33kV systems are larger in scale with higher fault current levels, requiring directional protection and reclosing coordination. Transformer protection begins to include differential protection (87T) as primary protection.
66kV	Transmission and distribution networks, urban high-voltage distribution, large substation in-feeds	21/21N (distance protection), 67/67N, 50/51, 87L (pilot differential for critical lines), 87T (transformer differential), 79	66kV falls into the high-voltage transmission category, requiring distance protection as the primary line protection. Pilot differential is used for critical double-circuit lines. Protection coordination is complex, requiring high CT/VT accuracy.
110kV	Transmission networks, system hub substations, large power plant outgoing lines	21/21N (distance), 87L (pilot differential), 87T (transformer differential), 50BF (breaker failure), 86 (trip lockout), 79, 67	110kV is a typical high-voltage transmission system, equipped with multi-zone distance protection as primary protection. Critical lines are configured with pilot differential protection, along with breaker failure protection and trip lockout logic. Protection redundancy and complexity are at the highest level.

General Notes:

Higher voltage levels require more complex protection configurations with greater primary/backup redundancy.
11kV–33kV rely primarily on overcurrent + zero-sequence as the basic protection scheme; high-voltage levels (66kV and above) are centered on distance + differential protection.
Digital/numerical relays supporting IEC 61850 and SCADA communication can be deployed across all voltage levels.
CT/VT accuracy and burden requirements increase with higher voltage levels.

Substation Protection Relay Testing and Commissioning

Testing and commissioning are critical steps to ensure protection devices operate correctly and reliably, verifying that relays, CTs/VTs, circuit breakers, and communication systems function as an integrated and coordinated whole.

Testing Objectives

Verify hardware and software functionality
Confirm correct setting entry
Ensure primary/backup protection coordination
Validate SCADA and IEC 61850 communication
Record test results for future reference

Types of Tests

Test Category	Description
Visual Inspection	Check wiring, terminal tightness, grounding, and labeling
Insulation Resistance Test	Measure insulation between circuits and ground to ensure no leakage
Power Supply Test	Verify DC voltage, polarity, and LED indication
CT/VT Test	Verify polarity, ratio, and wiring correctness
Primary Injection Test	Verify complete loop integrity from CT to relay and trip circuit
Secondary Injection Test	Inject simulated signals to verify accuracy and timing
Protection Function Test	Test each protection element (50/51, 87T, 21, etc.) for correct pickup values
Trip Circuit Test	Verify trip output and auxiliary contact functionality
Alarm and Annunciation Test	Confirm alarms, LEDs, and SCADA notifications operate correctly
Communication Test	Verify IEC 61850 (GOOSE/SV/MMS) and SCADA communication
Event/Fault Recording Test	Verify recording generation, storage, and retrieval
End-to-End Test	Verify communication and coordination for line differential (87L) schemes

Commissioning Procedure

Step	Activity
Preparation	Review drawings and setting sheets; prepare test equipment
Configuration	Upload settings and verify parameters
Functional Testing	Test each protection element individually; record operating values
Logic Testing	Simulate faults to verify protection coordination and interlocking logic
Communication Integration	Verify IEC 61850 and SCADA data exchange
Circuit Breaker Interfacing	Test trip/close operations and auxiliary contact feedback
System Integration	Simulate system faults to verify overall coordination
Documentation	Record results and sign off on commissioning reports

Common Challenges

Challenge	Mitigation
CT Saturation	Use low test currents or compensate with test set settings
Communication Delays	Optimize network configuration; test under load conditions
Incorrect Settings	Double-check against setting sheets; use comparison tools
Wiring Errors	Perform continuity checks, insulation tests, and visual verification
Test Equipment Interference	Use shielding, proper grounding, and isolation measures

Importance

Thorough testing ensures reliability (correct operation), security (no nuisance tripping), coordination (proper grading), and compliance (meeting standards). It is the foundation for safe and reliable substation commissioning and operation.

Substation Protection Relay Products

Transformer Protection Relays

Line Protection Relays

Capacitor & Reactor Protection

General / Multifunction Protection

Busbar Protection / Transfer

Arc Flash Protection

ATN-800H Arc Flash Protection Relay

Communication / Automation / Load Shedding

Our Advantages of Substation Protection Relays

As a professional manufacturer of microcomputer protection relays, we offer a full range of self-developed products for 11kV, 22kV, 33kVand 110kV substations. Our core advantages are as follows:

Full Voltage Adaptation, One Device for Multiple Uses: A single unit supports multiple protection logic switching, suitable for lines, transformers, and grid-connected generators, simplifying selection and spare parts management.
Dual Main and Backup Protection: Hierarchical logic design ensures fast main protection operation with reliable backup time-delay coordination, effectively preventing refusal and maloperation, and complying with international standards.
Industrial-Grade Environmental Resistance: Wide-temperature design with strong anti-electromagnetic interference capability, ensuring stable operation under harsh conditions such as high humidity and heavy dust.
Smart Operation and Easy Commissioning: One-click parameter setting with an intuitive HMI, supporting RS485/Ethernet ports (Modbus and IEC60870-103) for seamless connection to remote monitoring systems.
Customized One-Stop Service: Support for protection logic and cabinet size customization, along with parameter guidance, technical training, and full-cycle after-sales support.

Choose us, and you choose a trusted manufacturer and long-term partner.

FAQ

What relays are used in a substation?

Overcurrent (50/51), earth fault (50N/51N), differential (87), distance (21), directional (67), voltage (27/59), frequency (81), breaker failure (50BF), and numerical multifunction relays.

What is the most common relay used in a substation?

Overcurrent relay (ANSI 50/51) – simple, reliable, cost-effective, and widely used across all voltage levels.

What relay is used for transformer protection?

Primary: 87T (differential). Backup: 50/51, 50N/51N, 64REF, 49 (thermal), 63/Buchholz (gas), 24 (overexcitation).

What relay is used for feeder protection?

50/51, 50N/51N, 67/67N (directional), and 79 (auto-reclosing). For high-voltage feeders, distance (21) is also used.

What relay is used for busbar protection?

87B (busbar differential) as main protection, with 50BF (breaker failure) and 86 (lockout relay) for backup.

What is ANSI 87T?

Transformer differential protection – compares primary and secondary currents. An internal fault creates a difference current, triggering fast tripping for winding faults.

What is ANSI 50/51?

50 = Instantaneous overcurrent (no delay, for fast fault clearing).
51 = Time-delayed overcurrent (with inverse/definite time curves, for backup coordination).

What is the difference between numerical and electromechanical relays?

Aspect	Electromechanical	Numerical
Technology	Moving parts, coils, discs	Microprocessor, DSP
Accuracy	Limited, drifts with age	High, stable
Functions	One per relay	Multifunction
Settings	Taps/dials	Software
Communication	None	IEC 61850, SCADA
Recording	None	Event/fault recording
Maintenance	Regular	Minimal, self-diagnostic
Reliability	Affected by vibration/dust	Highly reliable

Numerical relays are the modern standard; electromechanical are outdated but still in older installations.

General Equipment & System Integration Inquiry

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