Jan. 13, 2025
This article delves into the significance of conducting Tan Delta (Power Factor) and Capacitance tests on transformers, providing insights based on our expertise. Addressing concerns about the potential negative implications of Tan Delta testing on transformer integrity:
- Tan Delta testing serves as an essential diagnostic tool for assessing insulation quality. It measures critical AC electrical properties of insulation, which can help indicate moisture presence, insulation degradation, and harmful substances that may compromise dielectric strength and reliability.
- The procedure is non-invasive; nevertheless, selecting the correct test voltage before commencing testing is crucial. We advise caution when testing small distribution transformers with windings rated below 12kV (phase to phase) to avoid exceeding their rated phase-to-phase voltage.
- Tan Delta and Capacitance tests are typically performed during factory acceptance, pre-commissioning, routine condition monitoring, and following electrical disturbances as part of Condition Assessment evaluations.
Concerning the recent failure experienced with a kVA dry-type transformer, multiple causes could be attributed to this failure. Like most transformer malfunctions, it often necessitates a comprehensive investigation to determine the underlying problem. Repairs for dry-type transformers generally need to be performed by the OEM due to the specialized cast resin winding technology employed by different manufacturers.
Hengfeng provides professional, reliable services.
Testing a weakened cable with elevated voltages can indeed lead to failures at vulnerabilities. High voltage AC testing at frequencies of 50/60Hz or VLF may cause the weak sections of a cable to fail. The emphasis lies not in the applied voltage waveform's frequency but rather in the voltage pressure applied.
It’s crucial to adhere to established standards for withstand testing. If a cable fails, it could indicate a significant defect, which might have resulted in failure during normal operation. A failure during testing is generally less serious, as it involves minimal current and power flow, unlike an unanticipated failure in service that can lead to extensive damage and interruptions.
Research from a reputable university indicates that only about 3% of cables are likely to fail when subjected to correctly applied withstand voltages; hence, not every cable will fail during testing.
For further insights on VLF and Hipot testing, click here.
The test cable length depends on both the cable capacitance and the specifications of the testing equipment. To find the capacitance, consult the cable’s data sheet or the manufacturer. Generally, this is measured in pF per foot or per meter. Multiply that value by the cable length to obtain total capacitance.
For example:
The HVA34 is a popular VLF testing unit.
It can handle 0.5uF (0.5E-6 F) at a maximum voltage of 24kV RMS and frequency of 0.1Hz.
If testing a cable with a capacitance of 100pF (100E-12 F) per foot,
Maximum cable length = 0.5E-6 / 100E-12 feet.
If a lower frequency, such as 0.05Hz, is selected, the length capability increases to 10,000 feet. Similarly, reducing the voltage applied also extends the testing capability. For instance, if 16kV is applied, a cable length of approximately 11,000 feet can be tested.
Note: With the HVA series, testing can be performed on cables as short as a few feet/meters if they are correctly terminated and grounded.
Test leads endure significant wear and tear over their service life, leading to potential damage. A malfunctioning lead complicates troubleshooting; however, basic tests can reveal faults in the test leads which may affect results.
Make sure to disconnect the lead from the testing device and ensure it is not energized.
For HVA28TD, HVA34TD, HVA45TD: The main conductor should display low resistance (a few Ohms maximum) end-to-end for continuity. Inquiries regarding shield continuity should be directed to HV Diagnostics.
For HVA30/34: Expect low resistance across the main conductor indicating full continuity, along with continuity in the shield (below a few Ohms).
For additional 240V AC Tan Delta Tester information, please contact us for professional support.
For HVA60: Resistance values should be approximately 10kOhms, 2.6kOhms, or 1.2kOhms, with shield continuity at low resistance.
For HVA90 and HVA120, the main conductor should show about 1.2kOhm continuity with low shield resistance.
Generally, the Windows operating system auto-selects the COM port for interfacing with our instruments. If you encounter issues:
First, verify that you are using the correct RS232 serial cable supplied by us.
Various serial cables exist, and compatibility is crucial, even if a cable physically fits. It’s advisable to utilize the USB to serial adapter provided along with the cable.
In HVA Control Center Software, within the Settings tab, set 'Select Com Port' to 'AUTO'.
Ensure that your software is up to date.
For HVA Series VLF/DC units (excluding VLF/TD), check the Instrument Settings to confirm USB Flash is set to 'NO' for serial communication (set to 'YES' when downloading reports to USB).
Lastly, check the Device Manager in Windows for the COM port assigned to your device. Match this port with the HVA Software to ensure proper linkage to the instrument.
Contact us to get the latest HVA Control Center software; we’ll provide a free download link.
How short?
A minimum capacitance of 5,000pF is necessary to conduct a viable Tan Delta test. To compute the minimal cable length, divide 5,000pF by the capacitance per foot (contact your cable manufacturer). Typically, the capacitance is around 100pF/ft, yielding:
Example: 5,000pF / (100pF/ft.) = 50ft. of cable.
How long?
Each testing unit has distinct specifications; refer to your instrument manual for its maximum capabilities. For instance, the HVA28TD can test up to approximately 5,000ft. (0.5uF) at peak voltage and frequency. Beyond that, the frequency will automatically adjust based on the load. It is vital to maintain a frequency of 0.1Hz for effective Tan Delta diagnostics.
Note: 1 mile = 5,280 ft.
Yes, HV Diagnostics, Inc. provides NETA Accredited Training Services for VLF, Tan Delta, and Partial Discharge diagnostics utilizing our advanced high voltage testing equipment.
Our training encompasses both theoretical and practical components, imparting comprehensive knowledge extending beyond just equipment operation. Students will learn to effectively use HV Diagnostics instruments and configure them for various electrical terminations. We offer both in-person and online courses for VLF and Tan Delta training.
For Partial Discharge, we conduct in-person training as part of a specialized three-day intensive course that includes VLF/TD.
To learn more about our Training Services and to view our premier Training Center, click here! NETA Accredited Training Services for VLF, Tan Delta, and Partial Discharge diagnostics.
*Important Note for New Equipment Users:
To ensure optimal support while using HV Diagnostics testing equipment, it's highly recommended that new users complete our training courses. Improper use of equipment may lead to negative experiences, unsatisfactory outcomes, inaccurate data, and potential damage to instruments.
If you seek more details, kindly check 260V AC Tan Delta Tester.
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