I. Equipment Overview
The transformer short-circuit impedance tester is a specialized testing device used to detect the deformation and displacement conditions of the windings of power transformers. During transportation, transformers may be subjected to mechanical shocks, or during operation, if there is an export short-circuit fault, the internal windings are prone to deformation and displacement, which can cause changes in insulation distance and a decrease in mechanical strength. In severe cases, it may even lead to insulation breakdown, inter-phase short circuits, and other safety accidents.
Short-circuit impedance testing is a non-destructive detection method. The equipment accurately measures the impedance characteristics of the transformer windings at different frequencies, determines the mechanical integrity of the windings, and provides reliable data for transformer condition maintenance and fault diagnosis.
II. Working Principle and Technical Foundation
2.1 Basic Definition of Short-Circuit Impedance
Transformer short-circuit impedance refers to the equivalent impedance presented by the primary side when the secondary side is short-circuited. In the industry, it is usually expressed as a percentage of impedance voltage.
The basic principle of short-circuit impedance testing: Short-circuit the low-voltage side winding of the transformer, apply the rated frequency AC test voltage to the high-voltage side, and make the current on the high-voltage side reach the rated value. At this point, the ratio of the measured voltage to the rated voltage is the impedance voltage percentage.
2.2 Circuit Composition and Deformation Determination Logic
From the circuit perspective, transformer short-circuit impedance includes resistance components and reactance components. Among them, the reactance component mainly reflects the leakage magnetic flux path of the winding and is closely related to structural parameters such as winding geometric dimensions, inter-winding distances, and core structure.
When the winding undergoes radial or axial deformation, the leakage magnetic flux path will change, and the leakage reactance value will also fluctuate. Staff can compare the measured short-circuit impedance value with the factory marking value and historical test data to determine whether the winding has a deformation defect.
III. Equipment Technical Introduction
3.1 Core Technology
This type of tester is equipped with digital signal processing technology, which can simultaneously complete the sampling and calculation of voltage, current, active power, power factor, etc. The equipment uses lock-in phase technology to track the frequency of the test power supply in real time, avoiding interference to measurement accuracy caused by frequency fluctuations. At the same time, the equipment can maintain high measurement accuracy even in low power factor conditions, and this performance is also a core indicator for judging the quality of the tester.
3.2 Basic Composition of the Instrument
The transformer short-circuit impedance tester is mainly composed of the main unit, test cables, current clamp, short-circuit connection block, etc. The main unit integrates a voltage regulation power supply module, signal acquisition unit, data processing unit, and human-computer interaction interface. The structure has high integration and is suitable for on-site testing scenarios.
3.3 Functional Characteristics
(1) Flexible Measurement Mode
The equipment supports single-phase measurement and three-phase measurement modes. In the three-phase measurement mode, one connection can complete the full set of tests for a three-phase transformer, and the instrument can automatically switch the measurement phase, accurately calculating the short-circuit impedance values of phases A, B, and C, and generating the error percentage of each phase compared to the factory marking value; in the single-phase measurement mode, it is suitable for single-phase transformers or special wiring conditions, and the test needs to be completed by manually changing the wiring three times.
(2) Automatic Conversion Function
Traditional short-circuit impedance tests require applying rated current, which has a high requirement for the test power supply capacity. Modern testers adopt low-current test automatic conversion technology, only requiring inputting a few amperes of test current, and combining the transformer’s inherent parameters, the measurement results can be automatically converted to standard values under rated current conditions, significantly reducing the requirements for power supply capacity in on-site testing and simplifying the test process.
(3) Temperature Compensation Mechanism
Temperature changes will alter the DC resistance of the transformer winding, indirectly affecting the resistance component of the short-circuit impedance. The tester has a professional temperature compensation algorithm. After the staff inputs the real-time temperature of the winding, the equipment can automatically convert the test results to the standard reference temperature, ensuring the accuracy of the measured data compared to the factory data.
(4) Complete Data Management Capability The equipment is equipped with a large-capacity non-volatile memory, which can store hundreds of sets of test data and waveform files. The stored data can be printed out on-site using the built-in thermal printer of the device, or exported to a computer via the USB interface for in-depth analysis. Some devices can directly generate standard format test reports, effectively improving the efficiency of on-site operations.
IV. On-site Testing Procedures and Implementation Key Points
4.1 Safety Preparation
Before testing, it is necessary to confirm that the tested transformer is completely de-energized, and that the high and low voltage sides have completed grounding discharge operations; remove the external leads of the transformer to achieve complete isolation of the equipment from the power grid; inspect the surrounding environment of the test site, eliminate potential safety hazards, and ensure the safety of the test operation.
4.2 Wiring Operation
Use a dedicated short-circuit wiring block to reliably short-circuit the low-voltage winding of the transformer, ensuring tight contact and the minimum contact resistance; connect the voltage test line and current test line of the tester to the corresponding terminals on the high-voltage side of the transformer, strictly distinguish the voltage and current circuits, and separate the wiring to prevent induction interference and affect the test accuracy.
4.3 Parameter Setting
After the equipment is powered on and preheated, enter the parameter setting interface, accurately input the transformer nameplate information, including rated capacity, rated voltage, rated current, short-circuit impedance nominal value, tap changer position, real-time winding temperature, etc.; select the type of test power supply, which is divided into internal power supply and external power supply, and simultaneously set the target test current value.
4.4 Test Execution
After confirming that the wiring is correct and the parameter settings are accurate, start the test program. The instrument automatically adjusts the output voltage to make the test current reach the preset standard. When the data reading stabilizes, it automatically records the measurement information. After the test is completed, the equipment automatically calculates the short-circuit impedance value, the deviation percentage between the measured value and the nameplate value, and displays it on the human-machine interaction interface.
4.5 Result Judgment Criteria
The operating condition of the transformer winding can be preliminarily determined based on the test deviation value. The general reference criteria are as follows:
1. The deviation between the measured short-circuit impedance value and the factory value, historical test value is within ±2% range, it is determined that the winding is in good condition;
2. The deviation is within the range of ±2% to ±3%, it is determined that the winding has slight deformation, and daily monitoring needs to be strengthened;
3. The deviation reaches ±3% or above, it is determined that the winding has obvious deformation and serious defects, and it is recommended to disassemble and inspect or return for repair.
Note: The above criteria are general reference standards. The actual determination needs to be combined with the voltage level of the transformer and the relevant regulations of the industry.
Post time: May-14-2026