How to Perform Loss Detection on Transformer Iron Core

1、 Transformer factory/new manufacturing stage: precise quantitative detection of iron core loss

This stage of testing is a laboratory level precision test, which measures the core core loss through no-load testing and includes pre testing of iron loss for silicon steel sheet raw materials. The data serves as the core indicator for determining the factory qualification of transformers and meets the requirements of GB/T 1094.2 Power Transformers Part 2: Temperature Rise of Liquid Immersed Transformers.

Pre inspection of iron loss in silicon steel sheet raw materials

Before making the iron core, the raw material iron loss of the silicon steel sheets (oriented/non oriented) used is first tested. A silicon steel sheet iron loss tester is used to cut standard samples according to GB/T 2521 Cold rolled Grain Oriented, Non oriented Magnetic Steel Strip (Sheet). At the rated frequency (50Hz/60Hz) and rated magnetic flux density, the specific iron loss of each silicon steel sheet is directly measured, and raw materials with excessive iron loss are removed to control the basic loss of the iron core from the source.

Iron core assembly no-load test (core iron loss detection)

After the overall assembly of the transformer is completed, a no-load test of the winding open circuit is carried out. The no-load loss measured by this method minus a small amount of no-load stray loss is the actual total loss of the iron core.

① Test preparation: Open all windings on the secondary side of the transformer, connect the primary winding to the rated frequency AC power supply, check whether the grounding of the transformer core is reliable (single point grounding), whether the core clamp is fastened in place, and eliminate the influence of assembly defects on losses;

② Voltage test: Apply voltage to the primary winding step by step through a voltage regulator until the rated phase voltage/line voltage is reached. After stabilizing the voltage, use a precision power analyzer and no-load test bench to measure the no-load power (no-load loss) and no-load current of the transformer;

③ Data correction and calculation: The no-load loss includes core loss, a small amount of winding copper loss, and stray loss of structural components. The non core loss part needs to be deducted according to the national standard formula to obtain the pure iron core loss value. At the same time, the iron loss data under different magnetic flux densities should be recorded, and the iron loss magnetic flux density curve should be drawn;

④ Result judgment: Compare the measured core loss with the design value and national standard limit value. If the measured value exceeds the tolerance, it is necessary to investigate assembly problems such as excessive gap between silicon steel sheets, insulation damage between sheets, and insufficient core stacking.

Core single piece (unassembled) loss detection

If it is necessary to separately detect the loss of the iron core body (without winding), a dedicated test bench for iron core loss is used. A temporary excitation winding is wound in the iron core window, and a rated frequency excitation current is applied to the iron core to achieve the rated magnetic flux density. By testing the input power of the excitation circuit, the loss of the iron core body can be directly measured, which is suitable for separate quality verification after the assembly of the iron core.

2、 Transformer in service stage: on-site non dismantling inspection of iron core loss

In service transformers cannot be disassembled, and the detection core consists of no-load loss retesting and loss anomaly correlation detection. The focus is on comparing the factory data to determine whether the core loss has increased. Combined with auxiliary testing to locate the cause of the loss anomaly (such as multi-point grounding of the core, aging of inter chip insulation, local short circuit of the core, etc.), the detection is a power outage non disassembly operation, suitable for on-site preventive testing and fault diagnosis in substations.

On site no-load loss retest

① Test preparation: Power off the transformer and ensure safe isolation. Remove the high and low voltage busbar connections, open all secondary windings, clean up the electromagnetic interference sources around the transformer (such as adjacent operating equipment), connect the testing instrument with shielded wires to reduce the impact of on-site interference on data;

② Pressure test: Due to the capacity limitation of the on-site power supply, low voltage no-load test (applying 10%~50% of the rated voltage) or rated voltage no-load test can be used. The no-load loss and no-load current can be measured by a portable variable frequency power supply and a precision power meter. The low voltage test needs to be converted to the core loss at the rated magnetic flux density according to the law that the magnetic loss is proportional to the square of the magnetic flux density;

③ Data comparison analysis: Compare the measured and converted iron core loss with the factory test data and historical test data. If the increase in loss exceeds 10%~15%, it is judged as abnormal iron core loss and further defect investigation is required.

Auxiliary positioning detection of abnormal iron core loss

After discovering abnormalities in the no-load loss retest, the core defect of the iron core was located through the following special tests to indirectly verify the reason for the increase in loss, which is the core supporting means of on-site inspection:

① Core grounding current test: Using a clamp ammeter and core grounding current tester, measure the current of the core grounding lead wire. During normal operation, the single point grounding current of the core is generally less than 100mA. If the current is greater than 1A, it indicates that there are multiple points of grounding in the core, resulting in the formation of circulating currents between the pieces and additional eddy current losses, which is the most common cause of increased core losses;

② Core insulation resistance test: Use a megohmmeter (2500V) to measure the insulation resistance between the core and the clamp, and between the core and the oil tank. If the insulation resistance is less than 100M Ω, it indicates that the insulation between the iron chips is aging, damaged, or the core is damp, causing a short circuit between the chips and generating local eddy current losses;

③ Local magnetic leakage detection: A portable magnetic leakage detector is used to scan the position of the iron core corresponding to the outer wall of the transformer oil tank. If an abnormal increase in local magnetic leakage is detected, it indicates that there is a local short circuit or insufficient stacking of the iron core at that position, resulting in concentrated local losses;

④ Oil spectrum analysis: Combined with transformer oil spectrum detection, if the content of hydrocarbon gases such as methane and ethane in the oil abnormally increases and there is no winding overheating characteristic, it can prove the fault of local overheating and increased loss of the iron core.

Precautions for on-site inspection

① On site testing should be carried out after the transformer is completely powered off and fully discharged, with proper grounding protection to prevent electric shock;

② If there is strong electromagnetic interference on site, a variable frequency power supply should be used to apply the test voltage, or a filtering device should be added to avoid interference causing loss and distortion of measurement data;

③ The iron core loss converted from low voltage is an estimated value. If accurate judgment is required, it is recommended to return the transformer to the factory for no-load testing at rated voltage.

3、 Online monitoring of iron core loss (normalized loss monitoring of in-service transformers)

For large power transformers (220kV and above), an online monitoring system can be used to achieve normalized monitoring of core losses, without the need for power outages, and real-time monitoring of loss changes:

By installing voltage transformers and current transformers on the high and low voltage sides of the transformer, real-time voltage and current signals are collected, and no-load losses (core losses) are calculated through backend algorithms;

Synchronize monitoring of parameters such as the grounding current of the iron core, surface temperature of the oil tank, and dissolved gases in the oil, and construct a multidimensional data model. When indicators such as iron core loss and grounding current exceed the threshold, the system automatically alarms and promptly detects abnormal trends in iron core loss;

Online monitoring data can be linked with the substation's integrated automation system to provide data support for transformer status maintenance, avoiding serious faults such as core overheating and insulation breakdown caused by abnormal core loss.