Microprocessor-Based Protective Relays – Complete Guide for Power System Engineers & Equipment Buyers

Many old substations require upgrading. Conventional electromechanical protective relays are being widely replaced with advanced Microprocessor-Based Protective Relays. This document covers the functions, installation solutions and commissioning tests of such relays in detail to offer valuable guidance for global customers.

Microprocessor-Based Protective Relays VS Electromechanical Protective Relays

What are microprocessor-based protective relays?

Microprocessor-based protective relays are digital intelligent devices with embedded microprocessors. They sample circuit current, voltage and frequency, process signals internally and issue trip/alarm signals upon system faults.

What is an electromechanical protective relay?

Electromechanical protective relays, also known as electromagnetic protective relays, are traditional protection devices widely adopted in early power systems. They operate purely through electromechanical components including electromagnetic coils, iron cores, mechanical discs, springs and metal contacts.

Comparison Between Microprocessor-Based & Electromechanical Protective Relays

Microprocessor-based relays are digital intelligent units with embedded chips, featuring multi-function integration, high precision, fault recording and remote communication. Electromechanical relays rely on electromagnetic mechanical structures. They have single protection functions, low accuracy, easy mechanical wear and no data transmission capability.

Item	Microprocessor-Based Protective Relays	Electromechanical Protective Relays
Working Principle	Digital chip calculation & logic judgment	Electromagnetic coil & mechanical action
Functions	Integrated overcurrent, differential, earth fault, etc.	Only one single protection function per unit
Accuracy	High measurement & setting precision	Low precision, large error margin
Data Function	Fault recording, event logging	No data storage or recording
Communication	Support remote monitoring & parameter adjustment	No communication interface
Service Life	Long, few wearing parts	Short, prone to mechanical aging
Cost	Higher initial cost, low maintenance cost	Low purchase price, frequent maintenance

Microprocessor-Based Protection Relay

Working Process of Microprocessor-Based Protective Relays

Step	Simplified English Description
1. Signal Acquisition	Gather real-time analog (current, voltage) and digital signals (breaker/switch status) via CTs and VTs.
2. A/D Conversion	Convert analog signals into CPU-readable digital data to reduce signal distortion and improve monitoring precision.
3. CPU Processing	The main CPU sorts and analyzes digital data, and executes embedded protection algorithms for real-time judgment.
4. Protection Algorithm Calculation	Compare real-time operating data with preset thresholds; activate protection logic when overcurrent, overvoltage, earth fault or other abnormalities occur.
5. Trip Output & Alarm	Output trip signals to disconnect faulty circuits and trigger local & remote alarms after fault confirmation.

Main Components

A complete microprocessor-based protection relay consists of six core functional modules, working collaboratively to ensure stable and reliable device operation:

• 1. CPU Module
• 2. Analog Input Module
• 3. Digital I/O Module
• 4. Communication Module
• 5. Power Supply Module
• 6. HMI Module

6 Core Functions of Microcomputer Protection

Microprocessor-based protective relays, DSP secondary devices, protect lines, transformers, motors and grids. More intelligent, precise and versatile than electromagnetic relays, they integrate protection, measurement, control, event recording, communication and time synchronization for grid monitoring and maintenance.

Protection Function (Core Function)

As the most fundamental function of microprocessor-based protective relays, the protection function monitors the operating status of equipment in real time, quickly identifies faults and abnormalities through built-in algorithms, isolates faults in a timely manner via tripping and alarm signals, prevents equipment damage and large-scale power outages, and ensures the safe and stable operation of the power system.

Measurement Function (Real-time Monitoring Function)

The measurement function of microprocessor-based protective relays serves as the basic acquisition module of the device. It collects analog quantities via transformers and calculates various electrical parameters with high precision after analog-to-digital conversion, providing real-time data support for operation monitoring, dispatch adjustment and fault analysis.

Control Function (Equipment Regulation Function)

The control function of microprocessor-based protective relays enables automatic operation of power equipment. It mainly controls primary equipment including circuit breakers and disconnectors to complete normal closing and opening. Local and remote control of switchgears can be realized via remote commands, improving the automation and safety of operation and maintenance while reducing risks of manual misoperation.

Event Recording Function (Fault Tracing Function)

Event recording mainly stores three categories of data:

1. Fault recording data, including fault occurrence time, fault type, electrical quantities, protection actions and trip/reclose status, to fully reproduce the entire fault process;
2. Abnormal operation logs covering voltage out-of-limit, abnormal load, device alarms and other abnormal signals;
3. Operation logs such as local/remote closing & opening, parameter modification and device reset.

All records can be queried and exported for analyzing equipment defects and weak links of the power grid.

Communication Function (Data Interaction Function)

The communication function of microprocessor-based protective relays supports mainstream power communication protocols including IEC61850, IEC60870-5-103 and Modbus.

It uploads real-time measurement data, fault information, alarm signals and device operating status, and receives remote commands such as circuit breaker switching, setting value modification and parameter configuration.

Time Synchronization Function (Time Unification Function)

The time synchronization function unifies the time of all secondary equipment in the substation and corrects cumulative clock errors of relays, ensuring precise and consistent timestamps for all operation, fault and action records, which lays the foundation for accurate accident tracing and data analysis.

The microprocessor-based protective relays support multiple synchronization modes including GPS/Beidou satellite timing, IRIG-B code timing and NTP network timing, with synchronization accuracy ranging from milliseconds to microseconds.

Classification of Protective Relays

According to different protection objects and application scenarios, microprocessor-based protective relays are divided into six mainstream types, covering all power system protection demands:

Feeder Protection Relay

It is the most widely used protection device in power distribution systems, mainly applied to distribution feeders, ring main units, and switchgear panels of 11kV, 22kV, 33kV projects.

Its core ANSI functions include 50/51 overcurrent protection, 50N/51N earth fault protection, 27 undervoltage, 59N protection relay , and 81 protection relay, effectively ensuring the safe operation of distribution lines.

Motor Protection Relay

Specially designed for industrial motor equipment, widely used in pump, fan, compressor, and crusher protection in mining, chemical, and water treatment industries.

It integrates professional motor protection functions such asthermal protection relay, 46 negative sequence relay , 48 locked rotor, and 50/51 overcurrent, solving common motor burnout and abnormal operation problems.

Transformer Protection Relay

Used for safety protection of power transformers and distribution transformers, with core functions including differential protection relay 87t, 50/51 overcurrent protection, REF residual current protection, transformer thermal protection relay, and 63 gas protection.

It is the primary guarantee for the safe and stable operation of power transformation equipment.

Generator Protection Relay

Suitable for hydropower, thermal power, solar, and wind farm generator sets, integrating differential protection relay 87g, directional over current relay protection, 40 excitation loss, 46 negative sequence, 64 grounding, and 81 frequency protection functions.

It adapts to the complex operating conditions of new energy power plants and ensures reliable grid-connected operation of generators.

Busbar Protection Relay

Equipped with core 87b protection relay function, applied to medium and high voltage switchgears and HV substations.

It can quickly isolate busbar short-circuit faults, avoid large-scale power outages caused by busbar faults, and improve the stability of the whole station power supply.

Line Protection Relay

Mainly used for transmission lines and sub-transmission systems, supporting distance relay protection, 67 protection relay, and 87L line differential protection.

It is suitable for long-distance power transmission projects and provides reliable line fault protection.

Every Engineer Should Know: ANSI Codes of Protection Relays

ANSI numbers for protection relays are the core part of this standard system.

Mastering common ANSI protection relay codes is essential for relay selection, commissioning, and maintenance. The most frequently used ANSI function codes are sorted as follows:

ANSI Code	Function Description
21	Distance Protection
25	Synchronism Check
27	Undervoltage Protection
32	Directional Power Protection
46	Negative Sequence Protection
49	Thermal Overload Protection
50	Instantaneous Overcurrent Protection
51	Time Overcurrent Protection
59	Overvoltage Protection
67	Directional Overcurrent Protection
79	Auto Reclosing
81	Frequency Protection
87	Differential Protection
50BF	Breaker Failure Protection

The above covers an introduction to common ANSI numbers for protection relays. If you wish to learn more, please refer to professional technical documents.

Installation Methods of Numerical Relays

Numerical protection relays support multiple flexible installation methods, adapting to different on-site equipment layout and environmental requirements:

Embedded mounting on HV switchgear

Front panel for status check, fault alarm and local operation. Internal secondary wiring and communication plugs, dustproof and anti-interference. Suitable for 10kV/35kV mid-mounted cabinets, RMUs, HV inlet/outlet cabinets.

Central panel mounting

Standard size: 800×600×2260, customizable as required. Protection, measurement and communication devices embedded on panel front. Terminal blocks, DC power and switches installed below with separated strong/weak current wiring for easy commissioning and maintenance. For 35kV+ substation & switching station control rooms.

Local mounting for box transformers

PV/wind box transformer controllers adopt embedded or wall-mounted installation on LV cabinets. Fitted with temperature-humidity controllers and dehumidifiers to resist outdoor large temperature difference, dust and moisture. Also applicable to pump stations and factory distributed power equipment.

Factory Testing of Numerical Protection Relays

Qualified numerical protection relays must pass strict factory tests to ensure that all performance indicators meet IEC international standards, including routine factory tests and type tests.

Combined Routine Factory & Protection Relay Tests

Hardware Inspection

Including visual appearance inspection and internal wiring verification to eliminate defective hardware and wiring errors.

Functional Testing

Comprehensive detection of device input, output, and logic functions to ensure normal operation of basic functions.

Protection Relays Testing

Accurate verification of protection pickup values, operating time, and trip logic to ensure protection accuracy and responsiveness.

Communication Testing

Test the compatibility and stability of Modbus, IEC61850, IEC104 and other protocols to ensure smooth data communication.

Test List for Microprocessor-Based Protection Relays Before Commissioning

No.	Test Item	Core Test Content
1	Visual & Insulation Inspection	Check device appearance, wiring and terminal fastening; test insulation resistance of secondary circuits to ground
2	Power Supply Function Test	Verify AC/DC operating voltage range, power-on restart and power-loss alarm functions
3	Analog Sampling Accuracy Calibration	Calibrate amplitude and phase measurement accuracy of 3-phase voltage, current, zero/negative sequence signals
4	Protection Setting Group Verification	Verify operating value, reset value and operating time delay of each protection function according to the setting sheet
5	Digital Signal & Logic Circuit Test	Check binary input and remote control output signals; verify trip linkage, blocking and interlock logic
6	Circuit Breaker Whole Group Transmission Test	Complete full-process linkage test of opening/closing, trip circuit, anti-pumping and auto-reclosing with circuit breaker
7	Communication & SCADA Matching Test	Verify device communication connectivity; check consistency of telemetry, telesignalisation, remote control and fault recording data with SCADA
8	Fault Recording & Alarm Function Test	Simulate various faults to verify event records, fault recording and device abnormal alarm functions are normal
9	Whole System Linkage Test	Simulate actual fault conditions to verify no refusal to operate or mal-operation of the whole protection logic
10	On-Load Test After Primary Energization	After primary equipment is energized, check phase sequence, amplitude, phase and power flow direction of current and voltage; confirm no differential current and abnormal alarms

How to Choose the Right Numerical Protection Relay

Reasonable relay selection is the key to ensuring power system safety and reducing project costs. The six-step scientific selection method is suitable for all overseas power projects and industrial scenarios:

Step 1: Identify Protected Equipment Type

Confirm the protection object (feeder, motor, transformer, generator, capacitor bank) to match the corresponding professional relay type.

Step 2: Determine Project Voltage Level

Match relay specifications according to project voltage including 400V, 6kV, 11kV, 22kV, 33kV, 66kV, 110kV.

Step 3: Define Required ANSI Functions

Select targeted protection functions according to actual project demands, avoid configuring redundant functions, and control project costs.

Step 4: Confirm Communication Protocol Requirements

Select compatible protocols (IEC61850, Modbus, IEC104, DNP3) according to on-site SCADA and automation system types.

Step 5: Evaluate On-Site Environmental Conditions

Select high-temperature resistant, moisture-proof, anti-corrosion or anti-vibration models according to high temperature, high humidity, coastal salt fog, mining dust and other harsh environments.

Step 6: Reserve Future Expansion Space

Prioritize relays with expandable I/O interfaces and upgradable protocols to adapt to subsequent substation automation upgrading and function expansion.

Why More International Customers Are Choosing Chinese Numerical Protection Relays

Chinese numerical protection relays have core competitive advantages in global market competition:

1. Cost-Effective Pricing: Under the premise of consistent international IEC/ANSI standard performance, the product price is more competitive, effectively reducing overall project investment costs.

2. Full International Standard Compliance: Fully compliant with IEC60255, IEC61000 and other international standards, supporting IEC61850 core protocol, meeting global project access requirements.

3. Fast Delivery & Flexible Customization: Chinese manufacturers have efficient production and supply chains, with short delivery cycles. They support personalized function adjustment and customized development to adapt to special requirements of different regional projects.

4. Professional Global Technical Support: Provide full-cycle services including pre-sales scheme design, on-site commissioning guidance, and after-sales operation maintenance, solving customer technical problems efficiently.

5. Equivalent Core Performance: The protection accuracy, response speed, communication stability, and service life of high-quality Chinese relays are completely comparable with international first-line brands, realizing high-quality replacement of imported equipment.

Frequently Asked Questions (FAQ)

Q1: How long does a numerical relay last?

A: Qualified industrial-grade numerical protection relays have a design service life of more than 10 years. With regular inspection and maintenance, stable operation for 15–20 years can be realized, far exceeding the service life of traditional electromechanical relays.

Q2: What communication protocols should I choose?

A: For new digital substations, prioritize IEC61850 protocol; for conventional industrial distribution projects, Modbus TCP/RTU is the most cost-effective choice; for old substation renovation, IEC104 and DNP3.0 are compatible with most old SCADA systems.

Q3: Can numerical relays replace old electromechanical relays directly?

A: Yes. Most numerical relays support compatible installation size and logic docking, which can directly replace traditional electromechanical relays without large-scale transformation of on-site panels and circuits.

Q4: How often should relay testing be performed?

A: It is recommended to complete full functional testing during installation and commissioning; routine inspection and partial testing every year; comprehensive calibration and testing every 3–5 years to ensure protection accuracy.

Q5: Is IEC61850 necessary for my project?

A: IEC61850 is a mandatory standard for new smart digital substations. For conventional industrial power distribution projects, Modbus protocol can meet daily operation and monitoring demands, which is more cost-effective.

Q6: What is the difference between feeder, motor, and transformer protection relays?

A: Feeder relays focus on line overcurrent and earth fault protection; motor relays focus on thermal overload, locked rotor, and negative sequence protection; transformer relays focus on differential and gas protection, with targeted function configurations for different protection objects.

Q7: Which ANSI functions are mandatory for a 22kV feeder?

A: The mandatory core functions include ANSI 50/51 overcurrent protection, 50N/51N earth fault protection, 27 undervoltage and 59 overvoltage protection, and 79 auto reclosing function.

Q8: How can I verify relay settings before energization?

A: Complete secondary injection testing and logic verification before energization, check the matching degree of setting values and on-site operating conditions, and confirm the correctness of trip and alarm logic.

Conclusion

With 13 years of working experience in the power relay protection industry, I would like to share my personal views on the selection of microprocessor-based protective relays:

1. Product selection shall take technical requirements as the primary principle. Only when the microprocessor-based protective relays fully meet the on-site operating conditions can other factors be considered.

2. Prioritize mature old models rather than newly launched products when selecting microprocessor-based protective relays. New products have not been verified by long-term market and on-site operation. Although the mature models have slight deficiencies in technical performance, their operational stability is fully guaranteed.

3. If microprocessor-based protective relays fail shortly after initial commissioning, the problem is mostly caused by unqualified product quality, while only a few cases result from on-site factors.

4. Most communication abnormalities of the on-site background power monitoring system are caused by non-standard layout and wiring of communication cables during the construction process, rather than faults of the microprocessor-based protective relays themselves.