IDAX series of insulation diagnostic analysers

Suggested stop frequency versus insulation temperature (℃):

• 0-4.9℃ - 0.1MHz
• 5-9.9℃ - 0.2MHz
• 10-19.9℃ - 0.5MHz
• 20-29.9℃ - 1MHz
• 30-44.9℃ - 2MHz
• 45-59.9℃ - 5MHz
• > 60℃ - 10MHz

It is critical to record the insulation temperature/apparatus temperature when performing an IDAX measurement. The test object temperature should in most cases not be assumed to be the ambient temperature.

For transformers, the test object temperature should be your closest approximation to the oil or winding temperature. If the transformer has winding temperature gauges, this temperature should be used as your apparatus temperature. If the transformer does not have winding temperature gauges then you can record the top oil temperature and bottom oil temperature; an average of the two temperatures can be used as your apparatus temperature. This temperature should then be entered in the “App. temp.” box under the “Measurement Sequence” tab in the “Results” window.

By measuring the impedance at one point, i.e. at a specific frequency and amplitude, parameters such as tan delta/power factor, capacitance and resistance can be calculated. The impedance of a sample is measured by applying a voltage across the sample. This voltage will generate a current through the sample. By accurately measuring the voltage and the current, the impedance can be calculated.

Moisture assessment of a transformer’s solid insulation by DFR is more accurate than taking an oil sample for a moisture content test. The latter often results in over estimations of water-in-paper content. However, an oil sample can be taken while the transformer is still energised. A DFR test is performed when a transformer is de-energised. An on-line moisture monitor installed in the transformer provides ‘anytime’ trending of moisture but requires an outage to install and is, thereafter, tracking only one transformer. Therefore, this approach is relatively expensive. As an asset owner, the course of ensuing action that you plan to take under various scenarios should inform the method you choose to assess moisture in your transformers. If one wants an accurate assessment of the moisture content in their transformer’s solid insulation so that they can determine whether or not to process the unit, DFR is an excellent choice.

DFR (direct frequency response) and SFRA (sweep frequency response analysis) are two very different tests. They are often confused because they both make measurements at many different frequencies.

DFR assesses how the losses in insulation behave as frequency changes.

SFRA assesses the propagation of a voltage signal through a winding at many different frequencies. An SFRA test provides a mechanical assessment of a transformer.

The time required to complete a DFR test depends upon the asset being tested, its temperature and, in the case of a transformer, its condition. The better the transformer’s insulation health, the longer the test requires. However, a DFR test using the IDAX will generally be less than 20 minutes. For a bushing, a DFR test takes less than 5 minutes.

DFR test curves are provided for all measurements. Depending on the asset, additional, discrete test results are reported. For example, a transformer report includes the moisture content of the transformer’s solid insulation, the conductivity of its liquid insulation, and the transformer overall insulation’s 50/60 Hz tan delta or power factor value. When testing a bushing, the percent dissipation factor or power factor value is reported at three different frequencies.

No, it’s very different. DFR testing is a series of tan delta tests carried out over a range of frequencies. The frequencies used are much lower than those used for SFRA – typically 1 mHz (millihertz!) to 1 kHz. The results are usually presented as a capacitance and/or dissipation factor/power factor curve. When used in conjunction with insulation modelling, they provide invaluable information about the condition of the transformer’s insulation system, particularly the moisture content of cellulose insulation and the oil conductivity. 

The measuring techniques used are similar, but, as the name implies, narrowband DFR uses a much more restricted range of frequencies – usually from around 1 Hz to 500 Hz. Also, the results are analysed directly rather than by using modelling techniques. It takes much less time to carry out a narrowband DFR test than a full DFR test – around two minutes compared with upwards of twenty minutes or more – but the narrowband test doesn’t provide the estimated moisture content for the cellulose insulation. However, a narrowband DFR test indicates problems earlier than traditional power factor/tan delta tests performed only at power frequency. It also confirms that seemingly good power factor/tan delta values really are good and allows the transformer’s individual temperature correction (ITC) factor to be determined.

DFR stands for Dielectric Frequency Response. The test is also known as FDS (Frequency Domain Spectroscopy). DFR is a measurement technique in which capacitance and losses (expressed as dissipation factor/tan delta or power factor) are measured over multiple frequencies to assess the insulation condition in test objects, such as power transformers, bushings, and instrument transformers. 

DFR technology is an established test procedure in laboratories that, in an innovative effort by Megger, has been adapted for field use through the IDAX range of instruments. In transformers, bushings, and instrument transformers, issues are only sometimes visible under conditions where it is easy to perform diagnostic tests (typically, at ambient temperature and line frequency). Instead, problems are generally best exposed at higher temperatures or closer to the operational limits of the objects.  

Unfortunately, temperature is not easily or efficiently controlled in a field test environment. The strength of a DFR test is that tan delta or power factor is the basis for its measurements. Tan delta or power factor is primarily a function of insulation system geometry, ageing byproducts, moisture, liquid insulation conductivity, frequency, and temperature. By applying knowledge about these relationships, we can assess ageing byproducts, moisture, and conductivity in the frequency domain via DFR rather than in the much more difficult-to-control temperature domain. 

Therefore, DFR makes it easy to find problems in the insulation under conveniently achieved conditions in the field.

A traditional response may have been to sample the oil from the transformers and determine the sample’s moisture content by dissolved gas analysis (DGA) or the Karl Fischer titration method. However, this approach has some shortcomings. One is that the oil content of a typical HV CT is small, so repeated sampling to monitor moisture ingress into the CT over some time is not practical. Another limitation is that the DGA and Karl Fischer tests determine the moisture content of the oil but cannot be depended upon to provide accurate information about the moisture content of the solid insulation (usually paper) in the CT, which is often implicated in thermal runaway leading to catastrophic failure. A better option for determining the moisture content of HV CTs is to use Dielectric Frequency Response (DFR).