I. Basic Working Principle
The core measurement of this device employs either the volt-ampere method or the bridge balance method, with specialized optimization tailored to the high inductance characteristics of transformer windings.
Constant Current Source Excitation: The device features an integrated high-precision constant current source capable of delivering stable DC test currents across multiple ranges, commonly including 1A, 3A, 5A, 10A, and 50A. The current range is selected based on winding resistance and equipment rated capacity—higher capacity and lower resistance windings require higher current outputs to ensure sufficient measurement resolution.
Voltage Drop Sampling: A high-precision analog-to-digital converter samples the steady-state voltage drop across the winding terminals, and resistance values are calculated using Ohm’s Law. To minimize errors caused by lead resistance, the device typically adopts a four-terminal connection method, keeping the current output loop and voltage sampling loop independent.
Fast Inductance Charging: To mitigate transient effects caused by winding inductance, the device incorporates automatic rapid discharge and constant-current regulation algorithms, significantly reducing stabilization time and improving field operation efficiency.
II. Functional Architecture and Core Features
Modern digital models integrate multiple intelligent functions beyond basic measurement capabilities:
Wide Range Auto-Switching: Automatically identifies the resistance range and switches between micro-ohm to kilo-ohm scales without manual adjustment.
Temperature Compensation and Correction: Equipped with interfaces for PT100 temperature sensors, it continuously monitors ambient or oil temperature and automatically converts measured data to standard reference temperatures (20°C or 75°C), eliminating temperature-induced measurement errors.
Demagnetization and Residual Magnetism Removal: Built-in automatic demagnetization programs generate attenuated AC pulses or reverse DC pulses after measurement to eliminate residual magnetism in the core, preventing interference with subsequent tests.
Data Management and Storage: Features large storage capacity and communication interfaces, enabling on-site printing of test reports and centralized data organization and management post-testing.
Safety Protection Mechanisms: Integrated overvoltage, overcurrent, and reverse polarity protection circuits, along with automatic discharging and alarm functions. After testing, the device automatically discharges stored energy in the winding. Disassembly of test connections is prohibited until the discharge indicator light turns off, ensuring operational safety.
III. Selection Criteria
Selecting the appropriate equipment specifications is crucial for ensuring measurement accuracy and compatibility. Key considerations include:
(1) Test Current Selection
The smaller the resistance being tested, the higher the required test current. Recommended selection guidelines are as follows:
- Resistance ≤10 mΩ (common in low-voltage sides of large power transformers): Use devices with 20A–50A current ranges.
- Resistance 10 mΩ–1 Ω (typical for medium-voltage windings): Use devices with 10A–20A current ranges.
- Resistance ≥1 Ω (common in high-voltage windings and high-voltage sides of distribution transformers): Devices with 1A–5A ranges meet accuracy requirements.
(2) Matching Test Object Parameters
Ensure the device’s maximum output current can handle the load capacity, maintaining stable constant current even under highly inductive loads. For transformers equipped with on-load tap changers, prioritize models supporting fast tap scanning and data memory functions, enabling efficient multi-tap testing with automatic data saving.
(3) Portability and Environmental Adaptability
Field conditions are often challenging; select devices with IP54 or higher protection ratings and shock-resistant enclosures. Models featuring large-screen graphical LCD displays are preferred, allowing clear viewing of real-time data and status information even under bright outdoor lighting.
IV. Key Points for On-Site Operation
Even with accurate equipment, improper operation can still result in data distortion. Pay close attention to the following during field operations:
Proper Wiring: Strictly use the four-terminal connection method. Voltage sampling clips should be connected to the inner side of the winding terminals, while current output clips connect to the outer side. These two sets of wires must not cross or share contact points to avoid introducing contact resistance errors. Discharge Operation: The winding has inductive energy storage characteristics. After measurement, wait until the equipment completes discharging. During discharge, strictly prohibit touching or removing test clamps and connections. Only proceed with disconnecting wires after the device indicates that discharging is complete or the discharge indicator light turns off. For high-capacity equipment, repeated discharges may be necessary to ensure all residual charge is fully released.
Instrument Preheating and Calibration: After powering on a high-precision instrument, it is recommended to preheat for 5 to 10 minutes to allow internal reference sources and constant current sources to reach thermal stability. If the instrument has been idle for an extended period or has experienced significant environmental temperature changes, perform self-calibration before use.
Interference Mitigation: Place the testing area away from strong electromagnetic interference sources. Use shielded twisted-pair cables for voltage sampling, ensuring the shield layer is grounded at only one end.
V. Common Abnormalities and Troubleshooting Methods
Long-term unstable readings and continuous drift
Cause: High winding inductance and insufficient charging time; poor contact of test clamps.
Troubleshooting: Increase the test current range and wait until the charging state stabilizes; clean the clamp contacts and securely reattach them.
Measurement values are significantly too high or too low.
Cause: Failure to use the four-terminal connection method as required; abnormal output from the constant current source.
Troubleshooting: Reconnect according to specifications; calibrate the equipment using a standard resistor.
Equipment triggers protection alarm.
Cause: Short circuit or open circuit in the test line, or inter-turn short circuit in the tested winding.
Troubleshooting: Check the integrity of the test line; separately measure phase data and comprehensively assess the winding condition.
Abnormally long discharge time
Cause: High-capacity, high-inductance windings store more energy, which is normal.
Troubleshooting: Wait patiently for the discharge to complete; do not forcibly remove the circuit.
This device is a crucial testing tool for assessing the condition of transformer windings, and the selection of equipment and adherence to operating procedures directly determine the accuracy of test data. During on-site operations, it is essential to select instruments with appropriate specifications based on the capacity and winding resistance of the equipment under test, strictly follow operational requirements such as four-terminal wiring and safe discharge practices, and effectively utilize intelligent functions like temperature correction and data storage. Relying on stable equipment performance and standardized procedures enables accurate data acquisition, providing reliable support for evaluating the operational status of internal current-carrying systems in transformers.
Post time: Jul-17-2026