Tan Delta and Capacitance Test on Transformer

Herewith some information relating to Tan Delta (Power Factor) and Capacitance Test on transformers from our perspective. With regards to your statement relating to whether Tan Delta testing could have any adverse effect on transformer health, the following:

- This test is a diagnostic insulation assessment tool that is a measurement of the fundamental AC electrical characteristics of insulation. These electrical characteristics could provide an indication of moisture, insulation deterioration, destructive agents, or ionization that can affect the dielectric strength and serviceability of the insulation.

- The test is non-destructive, however, it is important that the appropriate test voltage is selected before applying a test voltage to any equipment. It is recommended that the rated phase to phase voltage for windings below 12kV (phase to phase) on small distribution transformers is not exceeded.

- Tan Delta / Capacitance tests are normally performed as part of factory acceptance testing, pre-commissioning testing, on-site condition monitoring test, and also as part of Condition Assessment testing following electrical disturbances.

With regards to the failure that was experienced on the kVA dry-type transformer, there could be various reasons for the failure and as with most transformer failures, this may require a thorough investigation to ascertain the root cause. Repair of dry transformers are in most cases only possible via the OEM as this relates to the unique cast resin winding technology of each manufacturer.

Hengfeng supply professional and honest service.

It depends on ones point of view.

Elevated voltages (above normal) on a weak cable can cause cable failure at one of those weak points. AC high voltage testing at elevated voltages (at 50/60Hz or VLF testing) can cause weak elements in a cable to fail. &#;It has nothing to do with the frequency of the applied voltage waveform and everything to do with the pressure - that is the voltage, applied.  

As long as one follows the correct standards for withstand testing and the cable fails this test, then depending on your interpretation of word "destructive", you have exposed and detected a severe weakness in the cable, that would very likely have failed in normal service. The big difference is that a failure under test has very limited current and therefore power injected into the fault, while a unplanned failure under service, can cause catastrophic and collateral damage to other devices in the area of the failure and customer interruptions etc.
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&#;Note that based on extensive research performed by a large university, they found that about 3% of cables may fail a withstand test if one applies the correct withstand voltages, so don't think that every cable you perform a withstand test will fail. Only those that have very severe defects will run to failure.

Also, learn more about VLF and Hipot testing here.

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The length of cable you can test depends on the cable being tested and the capabilities of the test instrument being used.

Regarding the cable, what is of primary importance is the capacitance of the cable. This value can be obtained from the cable's data sheet or by asking the cable manufacturer. This is normally given as per foot or per meter. Simply multiply this value by the distance of the cable in question and you have your capacitance.
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Next look at the capabilities of the test instrument and the voltage and frequency that you will want to apply to the cable.
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Example:

&#;The HVA34 is one of the most popular VLF units on the market.
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The HVA34 can test 0.5uF (0.5E-6 F) at full voltage (24kV RMS) & max. freq. (0.1Hz)
The cable that is to be tested has a capacitance of 100pF (100E-12 F) per foot.
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What is the max. length of cable that can be tested using the HVA34 at full voltage and frequency?

So. . . 0.5E-6 / 100E-12 = feet of cable!

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Remember that if a lower frequency is selected, such as 0.05Hz, this will be 10,000 feet of cable (half frequency, double the length).

Also note that if the voltage is decreased, the length of cable can also be increased. If for example 16kV is applied, then a cable of about 11,000 feet can be tested.

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Regarding short cables, our HVA series equipment can test cables as short as a few feet/meters as long as the cable(s) is properly terminated and the shield is grounded.

Test leads generally get a lot of abuse during their lifetime and are often subject to damage and possible failure.

While having a broken test lead can lead to some frustrating troubleshooting to find the guilty culprit, there are some basic tests that can be done to either help eliminate or to blame the test lead for a possible faulty test result.
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Using a conventional multimeter in Ohm mode, check the resistance of the main conductor and shield.

&#;Note: The test lead should be removed from the test equipment and obviously not energized for these tests to be carried out.
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HVA28TD, HVA34TD, HVA45TD: The main conductor should have less than a few Ohms from end to end to show continuity.
Questions regarding the shield continuity should be addressed by calling HV Diagnostics.
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HVA30/34: The main conductor should have less than a few Ohms from one end to the other end - that is full continuity.
The Shield should show continuity (less than a few ohms).

For more 240V AC Tan Delta Testerinformation, please contact us. We will provide professional answers.

HVA60: Depending on the version of test lead, the main conductor should have approximately 10kOhms, 2.6kOhms or 1.2kOhms from end to end.
The Shield should show continuity (less than a few ohms).

HVA90: The main conductor should read approximately 1.2kOhm from end to end.
The Shield should show continuity (less than a few ohms).

HVA120: The main conductor should read approximately 1.2kOhm from end to end.
The Shield should show continuity (less than a few ohms).

Normally the COM port is automatically selected by the Windows operating system to work and communicate with our instruments.

If you are having communication issues, first make sure you are using the correct RS232 Serial cable that we supplied.

There are many versions of serial cables on the market that do not work with our equipment. Even if you have a cable that fits mechanically, it will not necessarily work. Also, we recommend using the USB to serial adapter interface that we supplied with this serial cable.
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In the HVA Control Center Software, under the &#;Settings&#; tab, ensure that the &#;Select Com Port&#; is set to &#;AUTO&#;.
Make sure you have the latest software.
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For HVA Series VLF/DC units (NOT VLF/TD), check the menu item &#;Instrument Settings&#; to ensure the USB Flash is set to &#;NO&#; to activate serial communication.
(USB Flash should be set to YES when downloading reports to USB memory sticks)
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Finally, if the above does not work, in the Device Manager of the Windows operating system, check the COM port allocated to that device. Once you know the COM port number assigned to that device, you need to make sure the HVA Software uses that same com port number to link correctly to the instrument.
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Please contact us if you need the latest HVA Control Center software and we will provide a free download link.

The length limitation is dependent on two parameters: the cable's capacitance (that is length dependent) and the load capability of the test instrument being used for the test.

&#;How short?
For a Tan Delta test the capacitance should be a minimum of 5,000pF to have enough current to run a test. To find the minimum length, divide 5,000pF by the capacitance per foot of your cable (contact manufacturer). On average, the capacitance per foot is 100pF/ft. The resulting number will give you the minimum needed to test.

Example: 5,000pF / (100pF/ft.) = 50ft. of cable.

How long?
Each unit has its own specifications so check your unit manual for its capabilities. For example, our HVA28TD can test up to approximately 5,000ft. (0.5uF) at maximum voltage and maximum frequency. Once the load exceeds that capacitance(0.5uF), the frequency will automatically drop to accommodate the load. You must pay attention to this frequency drop with Tan Delta diagnostics because you need to maintain 0.1Hz to evaluate a cable with Tan Delta.  

Note: 1 mile = 5,280 ft.

Yes! HV Diagnostics, Inc. offers NETA Accredited Training Services for VLF, Tan Delta, and Partial Discharge diagnostics using our world-class high voltage electrical test equipment.

Our training is a combination of both theory and practical. We have extensive application knowledge that extends well beyond our instruments. Students will learn how to correctly operate HV Diagnostics test equipment and properly setup our instruments to various electrical termination interfaces. We offer both in-person and online courses for VLF and Tan Delta training.

Due to its specialized nature, we only provide in-person training for Partial Discharge as part of a three-day intensive course that includes VLF / TD.
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For more about on our Training Services and a peek into our state-of-the-art Training Center, click here! NETA Accredited Training Services for VLF, Tan Delta, and Partial Discharge diagnostics.

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*Note to New Equipment Users:
To provide the required level of professional support with HV Diagnostics test equipment, we strongly recommend new users complete our training course(s). Incorrect use of equipment and/or testing application can lead to negative user experience, unsatisfactory results, incorrect data, and probable damage to the equipment.

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If you are looking for more details, kindly visit 260V AC Tan Delta Tester.