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ntroduction: Relay protection devices serve as the "safety guards" of the power system, rapidly isolating faulty equipment during system faults to ensure safe and stable grid operation. The relay protection tester (also known as a relay test set or microcomputer relay protection tester) is the dedicated tool used to verify whether these safety guards are functioning reliably. Whether it is pre-commissioning testing of newly installed protection devices or periodic inspection of in-service protection devices, the relay protection tester is an essential tool for protection device commissioning and regular inspection at power utilities. This article provides a comprehensive analysis of relay protection testers from five dimensions: working principles, technical classifications, core specifications, selection guidelines, and application trends.
A relay protection tester is a microprocessor-based dedicated device that outputs controllable voltage and current signals with adjustable amplitude, frequency, and phase to test the operating behavior of electrical relays, relay protection devices, and automatic safety devices. It is primarily used for testing and researching various types of protection relays and protection systems, including electromagnetic, integrated circuit, and microprocessor-based types across all voltage levels, enabling flexible control of voltage, current amplitude, and phase frequency.
In simple terms, the function of a relay protection tester is to simulate various power system fault conditions by applying specific voltage and current signals to the protection device under test, observing whether the protection device operates correctly according to preset settings, thereby verifying whether the protection device's functionality is intact and its settings are accurate.
According to the definition in the power industry standard DL/T 624 "Technical Specifications for Microcomputer-Based Test Devices for Relaying Protection," the standard name for the relay protection tester is "microcomputer-based test device for relaying protection." The latest version of this standard is DL/T 624-2023, issued on May 26, 2023, and effective from November 26, 2023, serving as the core basis for the design, production, and inspection of relay protection testers. In addition, DL/T 1153-2025 "Calibration Specification for Relay Protection Testers" was issued on December 18, 2025, and became effective from June 18, 2026, specifying the metrological characteristics, calibration conditions, calibration items, and calibration methods for relay protection testers, applicable to the calibration of analog, digital, and analog-digital integrated relay protection testers.
The basic working principle of a relay protection tester can be summarized in four stages: signal synthesis, power amplification, signal output, and feedback detection.
Signal Synthesis: The digital signal processor (DSP) inside the instrument synthesizes the digital waveforms of the required voltage and current signals according to the preset test plan. Modern high-end testers use 16-bit DACs (digital-to-analog converters) , capable of generating high-density sine waves with 2000 points per cycle for the fundamental frequency, delivering excellent waveform quality.
Power Amplification: The synthesized digital signals are converted from digital to analog and then amplified to the required voltage and current levels through a power amplifier. Power amplification technology is a core differentiator between generations of testers. Early products used OCL amplifiers, which were bulky (approximately 25kg), had narrow dynamic ranges, and offered limited accuracy. Modern products employ digital power amplifier technology , which is compact, lightweight, and highly efficient, representing the development direction of relay protection testers.
Signal Output: The amplified voltage and current signals are applied to the protection device under test through test leads. Mainstream products can output up to 6 voltage phases and 6 current phases, flexibly combinable to achieve various output modes including conventional 4-voltage-3-current, 6-voltage, 6-current, and more.
Feedback Detection: The instrument simultaneously captures the protection device's trip signals (contact status), recording the time interval from signal application to protection operation with millisecond precision, thereby determining whether the protection device's operating characteristics meet requirements.
| Type | Output Configuration | Applications |
|---|---|---|
| Single-Phase Relay Tester | Single-phase voltage + single-phase current | Simple relay verification, gradually being phased out |
| Three-Phase Relay Tester | 3-phase voltage + 3-phase current | Conventional line protection, overcurrent protection, etc. |
| Six-Phase Relay Tester | 6-phase voltage + 6-phase current | Transformer differential protection, auto-transfer switch (ATS) devices, busbar protection, and other complex protection schemes |
Six-phase relay testers can output up to 6 voltage phases and 6 current phases, flexibly combinable to achieve conventional 4-voltage-3-current, 6-voltage-only, 6-current-only, and 12-channel output modes. Current channels can be connected in parallel for higher output, with six-phase parallel output reaching up to 180A. The additional channels can serve as zero-sequence current sources or as compensation current for differential protection testing.
Portable Type: Compact and lightweight, designed for on-site rapid testing, suitable for field testing at substations, power plants, and other mobile applications.
Bench-top or Rack-mount Type: Suitable for laboratory or fixed workstations, offering more comprehensive functionality.
The technological evolution of relay protection testers can be divided into three stages: basic type with limited functionality and accuracy; standard type with multi-phase output, state sequence, and other essential functions; and intelligent type integrating GPS synchronized testing, automatic vector diagram generation, fault playback, and AI-assisted analysis capabilities.
Accuracy Class: According to DL/T 624 requirements, equipment used for relay protection testing at 110kV and above must have accuracy not lower than Class 0.2; for routine testing at 10kV and below distribution networks, Class 0.5 devices are acceptable. High-precision testers can achieve output accuracy of 0.1%.
Output Capability: Mainstream products offer per-phase voltage output of 120–125V and per-phase current output of 30–40A, with three-phase parallel output reaching 120A and six-phase parallel output reaching 180A. Some products are equipped with an independent 110V/220V DC auxiliary power supply.
Time Measurement Accuracy: Time measurement accuracy is one of the key specifications of relay protection testers, with modern products achieving accuracy down to 0.01 milliseconds.
Output Frequency Range: Testers must be capable of outputting signals at various frequencies to test frequency relays and simulate system frequency deviation conditions. High-end products offer frequency resolution as fine as 0.001Hz.
The core principle of selection is to meet current testing needs while leaving room for future development. The following five aspects should be considered:
Identify the Type of Protection Device to Be Tested: This is the first step in selection. If only conventional line protection needs testing, a three-phase relay tester is sufficient. However, if transformer differential protection, automatic transfer switch (ATS) devices, busbar protection, or other complex protection schemes are involved, a six-phase relay tester is required for simultaneous multi-channel electrical signal output.
Confirm the Required Accuracy Class: Determine the required accuracy based on the voltage level of the protection device under test. For 110kV and above systems, select Class 0.2 equipment; for 10kV and below distribution networks, Class 0.5 equipment is acceptable. Also ensure the equipment can pass calibration by a legal metrology institution.
Assess Whether Output Capability Is Sufficient: Choose output capability based on the maximum current requirements at the site. If testing high-current relays or performing group tests is required, select a model with higher per-phase output capability (e.g., 40A per phase).
Evaluate Functional Completeness: Does it support advanced functions such as state sequences, harmonic superposition, and fault playback? Does it have GPS synchronization triggering capability for dual-ended testing? Is it equipped with automatic vector diagram generation to visually display phase relationships between electrical quantities? Does it support USB connectivity and data export?
Consider Field Usability: Portable products are more suitable for frequent site-to-site movement; handheld products offer over 8 hours of battery life; large-screen Chinese interfaces and touchscreen operation significantly improve work efficiency.
Relay protection testing is the "first line of defense" for power grid safety. Whether during pre-commissioning of new substations, after renovation of aging equipment, or during periodic inspection of in-service equipment, relay protection testers are indispensable tools.
In the case of the State Grid Tianjin Jizhou Company's 35kV Tungsten Mine Substation renovation project, technicians used an integrated relay protection secondary circuit tester to successfully complete the equipment upgrade and commissioning. The tester underwent four generations of product iteration over three years, effectively solving the challenge of lengthy power outages during aging substation renovations.
In substation field commissioning, relay protection testers can perform the following critical tasks: setting verification to confirm the accuracy of protection device settings; group testing to simulate various fault types (phase-to-phase short circuits, single-phase-to-ground faults, etc.) and test the complete protection logic from start to trip; differential protection percentage restraint curve scanning to automatically scan transformer and generator-transformer unit differential protection curves; and auto-transfer switch (ATS) testing to simulate bus voltage loss conditions and verify automatic backup power switching logic.
Currently, relay protection testers are rapidly evolving toward multi-protocol integration, high-dynamic response, and AI-driven analysis. In the future, testers will leverage artificial intelligence algorithms for self-learning capabilities, automatically identifying protection device types, generating test plans, and providing intelligent analysis of test results with fault prediction capabilities. Meanwhile, the global digital relay protection tester market continues to grow, with global sales reaching $90.17 million in 2025. Traditional labor-intensive and time-consuming testing methods are being replaced by automated test systems.
Wuhan Kedi Zhongwei Electric Co., Ltd. is a high-tech enterprise integrating R&D, production, and sales of power testing equipment, instruments, and online power monitoring systems. The company's core products cover 15 major categories with over 300 products, including relay protection testers, series resonant test systems, DC high voltage generators, loop resistance testers, transformer testing instruments, cable fault locators, and more. Products are widely used in power grids, power generation, renewable energy, railways, petrochemicals, metallurgy, and other fields, and are exported to Southeast Asia, the Middle East, Africa, South America, and other overseas markets.
Leveraging the concentration of higher education institutions in Wuhan, the company maintains close technical cooperation with Huazhong University of Science and Technology, Wuhan University, Wuhan High Voltage Research Institute, and other research institutions, employing a team of professionals in high voltage, computer science, and electronics. Kedi Zhongwei consistently adheres to the business philosophy of "technology leadership, service first," providing customers with full-cycle services from selection consulting and on-site training to after-sales maintenance.
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