(CT) and voltage transformers (PT) for various electrical performance indicators. It is the core equipment ensuring accurate grid metering and reliable relay protection operation.
By 2026, with the increased demand for equipment status awareness in smart grids, this instrument has evolved from a single "volt-ampere characteristic test" to an intelligent terminal with full-parameter automatic diagnosis, wide-band frequency analysis, and edge computing capabilities. The following is a detailed analysis based on the latest technical standards:
1. Core Functions and Importance
CT (Current Transformer) Inspection:
Excitation Characteristics (Volt-Ampere Characteristics): Assess core quality, calculate saturation point, ensure no saturation under high short-circuit fault currents, and correct protection operation.
Ratio and Polarity: Verify correct multiplier and consistent direction (to prevent differential protection misoperation).
DC Resistance and Burden: Check winding integrity and secondary circuit load capacity.
Demagnetization: Eliminate core residual magnetism and restore measurement accuracy.
PT (Voltage Transformer) Inspection:
Excitation Characteristics: Evaluate insulation condition and core performance.
Ratio, Polarity, and Vector Group: Ensure correct phase relationship for metering and protection circuits.
Secondary Burden and Impedance: Verify load capacity.
Application Scenarios: New equipment commissioning acceptance, periodic preventive tests, fault troubleshooting (e.g., analysis after protection misoperation), and metering dispute arbitration.
2. Working Principle and Technical Architecture
Modern test sets adopt a variable frequency power supply + high-precision sampling + digital signal processing (DSP) architecture:
Variable Frequency Signal Source: Built-in high-power variable frequency power supply with output frequency adjustable from 5Hz to 400Hz (or even higher). By changing frequency, high-frequency excitation can be simulated at lower voltages, thereby extrapolating the excitation characteristics at power frequency, significantly reducing the external power capacity required for testing.
Fully Automatic Closed-Loop Control:
The instrument automatically raises voltage/current and collects voltage (U) and current (I) in real-time.
Automatically fits the excitation curve using algorithms such as least squares, accurately calculating knee point voltage (
saturation coefficient, 10% error curve, and other key parameters.
Wide-Band Frequency Analysis:
In addition to power frequency (50/60Hz), it can perform frequency response characteristic tests to analyze the transmission characteristics of transformers at different frequencies, which is particularly important for evaluating electronic transformers or broadband protection transformers.
Equivalent Circuit Modeling:
Automatically constructs the equivalent circuit model of the transformer based on test data
3. New Technology Trends in 2026
One-Key Fully Automatic Diagnosis:
No need for manual setting of complex voltage step increments or knee point judgment. The user only needs to input nameplate parameters, and the instrument automatically executes the complete process of "DC resistance -> ratio -> polarity -> excitation -> demagnetization -> burden", reducing the time to a few minutes.
Intelligent Criteria: Built-in 2026 national standards (GB) and industry standards (DL/T) database, automatically determines "Pass/Fail" results and provides specific analysis of exceeded items.
No External Voltage Regulator and Current Booster:
Single unit integrates high-power supply with maximum output voltage up to 0~3000V (or even higher) and maximum output current up to 0~30A (or even 60A), fully meeting the testing needs of all types of transformers up to 500kV, completely eliminating bulky external equipment.
Support for Electronic Transformers:
For electronic transformers (ECT/EVT) in smart substations, equipped with fiber optic interface (IEC 61850-9-2 protocol), can directly read digital outputs for ratio error, phase error, and delay tests.
Cloud Collaboration and Big Data:
Historical Trend Analysis: Automatically retrieves historical test data of the equipment, generates "health trend charts", and provides early warning of insulation aging or inter-turn short circuit risks.
Familial Defect Comparison: Connects to the cloud via 5G/Wi-Fi, compares test results with the "family library" of equipment from the same manufacturer and batch, quickly identifying batch quality hazards.
Portability and Protection:
Weight controlled within 10kg, military-grade rugged design, suitable for harsh field environments.
Equipped with reverse charging protection and high-voltage induced voltage protection to prevent damage from primary side induced high voltage.
4. Detailed Explanation of Key Test Items
Table
Test Item Purpose 2026 Technical Highlights
Excitation Characteristics Measure volt-ampere curve, find knee point, calculate 10% error curve Automatic fitting, no manual plotting; supports high-frequency extrapolation to power frequency.
Ratio Test Verify current/voltage transformation ratio Wide range automatic ranging, accuracy up to 0.05 class; supports multi-tap automatic scanning.
Polarity Check Confirm same-name terminal direction Graphical display of waveform phase, intuitive judgment, prevents misjudgment.
DC Resistance Test Detect winding continuity and contact condition Four-wire high-precision measurement, automatic temperature correction.
Demagnetization Eliminate core residual magnetism Automatically executes standard demagnetization process of "voltage rise - voltage drop - reverse", ensuring thorough demagnetization.
Burden Test Measure secondary circuit impedance Virtual burden method, no need for external high-power resistor box.
Transient Characteristics (For TPY/TPZ class) Measure transient factor Simulates DC component decay, evaluates transient performance.
5. Key Selection Indicators
Output Voltage: Recommended ≥1500V (covering 220kV and below), high-end requires ≥3000V (covering 500kV).
Output Current: Recommended ≥10A (to meet large cross-section winding testing), high-end requires ≥30A.
Measurement Accuracy: Voltage/current 0.05 class, ratio 0.05 class, phase 0.1 degree.
Frequency Range: 10Hz ~ 500Hz (the wider, the better, to adapt to different core materials).
Interface Functions: Whether it supports IEC 61850 fiber optic interface (for smart substations).
Data Processing: Built-in storage capacity, USB/Bluetooth/Wi-Fi transmission, report printing functions.
6. Brief Operation Process
Wiring:
Disconnect the transformer secondary side from other equipment.
Connect the test set's voltage/current output lines to the secondary winding (pay attention to polarity).
If testing primary current flow, thread the primary side (some high-end models support primary side injection).
Parameter Setting:
Enter transformer nameplate information (model, ratio, accuracy class, capacity, etc.).
Select test items (usually choose "automatic test").
Start Test:
Press the start button, and the instrument automatically completes all sub-items.
The screen displays volt-ampere curve, error curve, and intermediate results in real-time.
Result Analysis:
The instrument automatically generates conclusions (Pass/Fail).
Check whether the 10% error curve meets the system short-circuit capacity requirements.
Report Generation:
Save data, print on-site or upload to the production management system (PMS) via APP.
Restoration: Remove wiring, restore secondary circuit connections, and clean up the site.
7. Safety Precautions
Open Circuit Danger: When testing CT, it is strictly forbidden to open the secondary circuit (although the test set has internal protection, external wiring must be secure) to prevent high voltage endangering personnel and equipment.
Grounding Protection: The instrument casing and non-test terminals of the transformer must be reliably grounded.
Induced Voltage Protection: When testing in an operating substation, pay attention to induced voltage from adjacent live equipment, and use shielded wires if necessary.
Discharge: After testing, especially after high-voltage excitation tests, wait for the instrument to automatically discharge completely before removing wires.
8. Mainstream Brands and Models in 2026
Domestic Leaders: Wuhan Kaidi Zhengda (KDFA series), Shanghai Sute (SUTE-FA), Baoding Tianwei (TW-FA), Guodian Huamei. These brands have fully popularized "cloud-edge collaboration" functions by 2026.
International Brands: OMICRON (CP CU 1 / CT Analyzer) —— industry benchmark, known for extremely high accuracy and powerful software analysis; Megger, Doble.
Typical Model Features: Such as KDFA-8000 Pro (supports fiber optic testing of electronic transformers), OMICRON CT Analyzer 3.0 (built-in AI fault diagnosis engine).
Summary: The transformer characteristic test set is the "stethoscope" for power system safety. By 2026, equipment not only tests faster and with higher accuracy, but also achieves full lifecycle health management of transformers through digital means, making it an indispensable key equipment for building a strong smart grid.
