IDAX series of insulation diagnostic analysers

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.

Any time that you can perform an entire measurement in only FDS (frequency domain spectroscopy) mode, you have a better, more reliable measurement. A true frequency domain method that speeds up measurement time is accomplished by applying several frequencies simultaneously, and extracting measurement data from the applied frequencies simultaneously. Because the voltage level of each frequency needs to be reduced, the measurements are a bit more sensitive to AC interference, but DC current interference will not affect the measurements. This multi-frequency approach is an advance over the older approach of combining FDS with PDC, which is more sensitive to AC interference and also quite sensitive to DC interference.

The time savings of the multi-frequency method are significant. For example, using three frequencies simultaneously reduces measurement time by about 40 %.

When testing low capacitance objects like bushings and current transformers, the IDAX paired with the VAX020 provides a test voltage of up to 2 kV, giving an excellent signal-to-noise ratio and measurement up to 1 kHz. A unique single-material version of ITC brings test results to a reference temperature regardless of the test object temperature. IDAX supports OIP, RIP, RBP, OIP CT, and user-defined materials.

With the 20/30 kV amplifiers (VAX220), the IDAX can be used to assess the status of XLPE cables. Frequency sweeps are done at 25 %, 50 %, 75 %, and 100 % of service phase-to-ground voltage, and by comparing DFR curves, water treeing can be detected. DFR makes it possible to separate the characteristic response of water trees from the influence of accessories and creep currents.

The most commonly used voltage level is 140 V RMS, which is enough to measure a transformer’s CHL insulation in most conditions. However, in situations where there is a high level of interference or when measuring CH or CL (of a transformer), reactors, bushings, and current transformers (CTs), a 140 V RMS signal does not provide a high enough measurement signal-to-noise ratio to get meaningful results. A higher test voltage, like the IDAX 322’s 1400 V RMS/2000 V peak, will improve the measurement accuracy and is recommended in these instances.

The benefit of having two available metering systems in one instrument is the unique advantage of testing two capacitances simultaneously. For example, the IDAX can test two HV bushings at the same time. It can also simultaneously measure both of a three-winding transformer’s interwinding insulation systems, e.g., CLH and CLT.

No other instrument is capable of simultaneous measurement in the frequency domain. In some dual-channel instruments, both channels share a single ammeter. With these instruments, either half as many measurements are made (i.e., less accuracy), or no timesaving is realised compared to performing two separate tests.

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.

Moisture ingress is a relatively common problem with bushings and current transformers that can have catastrophic consequences; bushing malfunction is the cause of 17 % of all transformer failures and up to 70 to 80 % of all transformer fires. A failing bushing is also likely to explode, potentially damaging the entire substation.

Performing traditional tan delta or power factor tests at line frequency is not enough, as such testing can give false positive results; only through DFR can you access the true status of a bushing or instrument transformer. In addition to catching high moisture levels, DFR has successfully detected traces of partial discharges in HV and EHV bushings.

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.

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).

One of the most critical applications for the IDAX is determining the moisture content in transformer insulation. Moisture in the insulation significantly accelerates the ageing process. Moisture can cause bubbles between windings, resulting in catastrophic failures. The IDAX provides reliable moisture assessments in one test. You can perform the test at many temperatures; it can take upwards of 20 minutes or more, depending on the condition and temperature of the test object.

Maintenance (or replacement) decisions about a transformer should be informed by the unit's insulation condition and expected loading. Adding just a few operational years to the predicted end-of-life for a transformer (or generator, or cable) by optimising its working condition based on reliable diagnostic data means substantial cost savings for the equipment owner.

A transformer owner can also use FDS technology to assess the condition and ageing of the insulation in bushings, CTs, VTs, and other components. Numerous ongoing research projects at institutes and universities worldwide are adding experience and value to users of the IDAX.

Moisture that accumulates in the insulating system of a power transformer affects several properties:

• Limits the loading capability as higher moisture levels decrease the bubble inception temperature
• Lowers the dielectric strength of the oil, which has a direct effect on the insulation properties
• Ages the cellulose insulation, lessening its mechanical strength as a consequence

DFR by IDAX is the only reliable method to determine the humidity in power transformers without decommissioning or disassembly. Typically, single-frequency tan delta/power factor tests can give incorrect results due to temperature effects, and oil analysis is unreliable as moisture mainly resides in the solid insulation. In a power transformer application, the IDAX uses a unique two-material model and ITC to accurately calculate humidity, oil conductivity, and tan delta/power factor.

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.