IDAX series of insulation diagnostic analyzers
Fastest system in marketplace
By using a multi-frequency test signal at low frequencies, the cumulative measuring time is reduced, and eliminates the need for combining frequency and time domain measurements to quicken the test
Reliable measurements in high interference environments
By measuring entirely in the frequency domain, EMI is minimized
Automated individual temperature correction (ITC)
For accurate comparison with reference data/tests
Dedicated test procedures
For power transformers, bushings, and current transformers
About the product
The IDAX is an insulation diagnostic analyzer based on DFR (Dielectric Frequency Response), also known as FDS (Frequency Domain Spectroscopy). DFR technology is an established test procedure in laboratories that, in an innovative effort by Megger, has been adapted for field use in the IDAX range of instruments.
DFR is the measurement of capacitance and losses (tan delta or power factor) over multiple frequencies. The measured DFR curve is dependent on insulation geometry, moisture, oil conductivity, and temperature. By advanced curve fitting to the reference material model, the IDAX calculates moisture content in solid insulation, the oil’s conductivity at 25 °C reference temperature, and tan delta/power factor at 20 °C reference temperature.
In these calculations, ITC (Individual Temperature Correction), another important Megger innovation, is used to translate test data from the test object temperature to the reference temperatures. The IDAX software incorporates an ITC-corrected frequency sweep designed explicitly to assess instrument transformers and bushings.
Thanks to a novel approach to the combination of time and frequency domain data, the IDAX provides the shortest measurement time in the marketplace for a full DFR measurement from 1 kHz to 10 μHz. Separate reference models are fitted to each data set (time or frequency) prior to transformation and combination, which eliminates the risk of artefacts introduced by approximations or transformation of incomplete data sets.
The IDAX is exceedingly easy to use, with an automated test flow and presentation of results that uses an easy-to-understand ‘traffic light’ system.
The IDAX DFR method is now part of international guides and standards, e.g., Cigre TB 254, Cigre TB 414, Cigre TB 445, Cigre TB 775, IEEE C57.152-2013, IEEE C57.161-2018.
The IDAX is available in multiple versions:
- IDAX300 – A compact and light three-channel input (red, blue, and ground), three-terminal (generator, measure, and guard), and one ammeter instrument for use with an external computer that runs the IDAX diagnostic software.
- IDAX300/S – As IDAX 300 but with two ammeters for two simultaneous measurements.
- IDAX350 – As IDAX 300/S but housed in a rugged and waterproof case with an on-board computer that can also be used to control other Megger instruments.
- IDAX322 - AS IDAX 300/S but with built-in 2 kV amplifier for higher signal-to-noise ratio in low capacitive test objects. Ideal for field testing bushings.
For extended applications, the IDAX interfaces seamlessly with VAX high voltage amplifiers; VAX020 for 2 kV and VAX220/230 for 20/30 kV (on request).
Technical specifications
- Test type
- Capacitance and dissipation/power factor
FAQ / Frequently Asked Questions
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.
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 %.
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.
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.
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.
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.
Further reading and webinars
Troubleshooting
There are a few possible reasons and countermeasures for this:
1. The generator output is earthed/grounded.
You should:
- Check the measurement setup and disconnect the ground.
- Change the measurement configuration if you cannot disconnect the terminal of the test object from ground.
2. The generator output is connected to a measuring electrode (input or ground).
You should:
- Check the measurement setup.
- Disconnect measuring or guard electrodes from the generator output.
- Don't connect the generator output to either measuring or guard electrodes.
3. High stray capacitances to ground are present or the test object has a high capacitance.
You should:
- Lower the highest frequency used in measurement.
- Lower the test voltage.
4. If you try to use an old version of the IDAX software (version 3.2 or earlier), but the firmware in the IDAX is for the IDAX software 4.0 or newer, the IDAX software does not understand the incapability and it usually results in error 347.
Please check the IDAX software and if you are using version 3.2 or earlier, upgrade to 4.0 or newer (this new software will automatically upgrade the firmware if necessary).
Values of capacitance measured for different configurations are in disagreement. This includes the UST, GST-Guard, and GST-Ground. When performing a UST measurement, the measuring electrode is connected together with the ground electrode, or is connected to ground:
You should:
- Check the measurement setup and make sure that the measuring electrode is connected to a non-grounded terminal of the test object and that the ground electrode is connected to ground.
- Check cable connectors for damage.
- Measure the resistance between the chassis and guard electrode. It should be 1.2 to 1.4 ohms. If resistance is lower than this, there is a short-circuit in the instrument.
If the measured capacitance is below the limit specified in C-file by MinSpecimenC, then possible reasons and countermeasures include:
- The measured capacitance is higher than 10 pF. However, the specimen size is very small which results in a low value of capacitance:
- Change the limit set by MinSpecimenC to an approximately 10 % lower value than the measured capacitance.
- Select another measurement configuration, if possible.
- If the measured capacitance is lower than 10 pF, then most likely, there is no contact with the test specimen:
- Check connections with the specimen for loose contacts.
- Check the measurement cables for damage.
For more information of actual measured capacitance, please see Message Window.
A measured capacitance above the limit specified in the test plan by MaxSpecimenC is usually due to the large size of a test object, resulting in high values of capacitance:
- Change the limit set by MaxSpecimenC to an approximately 10 % higher value than the measured capacitance.
- Select another measurement configuration, if possible.
- A decrease in test voltage allows for measuring at higher frequencies
If the measured DC current exceeds the limits set in the test plan by MaxDCCurrent, then the most common reason is too low a resistance between the measurement electrode and guard. For example, measuring a UST configuration between high and low voltage windings of a two-winding transformer, the low voltage winding has too low an impedance to ground (inductive voltage transformer connected, internal damage of transformer, neutral connected to ground via a Peterson coil). For a GST measurement, the same applies to guard electrodes, i.e., a guard electrode with too low a resistance to ground may introduce DC currents.
Make sure that the floating electrode has a high resistance to ground. If that’s not possible, use another setup (e.g., measure to ground without use of guard).
It is possible to increase the limit level for DC current in the Measurement Template, but only when the difference is very small and all other possibilities are excluded.
If the measured interference or hum current exceeds the limits set in the test plan by MaxHumCurrent, then the level of interference is very high. Try to reduce the interference level by:
- Disconnecting the still connected busbars that pick up interference.
- Selecting another setup, e.g., a CHG+CHL is much less influenced by interference compared to CHG.
- As a last option, it is possible to increase the limit for hum current in the Measurement Template.
Interpreting test results
Megger’s IDAX software provides an analysis of moisture content, oil conductivity, and temperature corrected, line frequency PF/DF test results. It is important that you supply the insulation temperature of the asset under test for an accurate assessment.
For a new transformer, the moisture content in the solid insulation is commonly targeted to be less than 0.5 % by weight. As the transformer gets older, the moisture content will typically increase around 0.05 % per year for a sealed conservator transformer and by approximately 0.2 % per year for free-breathing transformers. In an old and/or severely deteriorated transformer, the moisture content can be greater than 4 %. The graph below provides moisture interpretation criteria by Megger and different standards bodies. In agreement between them is that moisture content above 2 % in a transformer requires attention.
Recommended criteria for assessment of water, given by percent by weight, in the solid insulation of transformers.
These acceptance criteria are somewhat ‘broad-brush’. Generally, for higher voltage class transformers, less percentage moisture by weight contamination can be tolerated.
The criticality of addressing a wet transformer is also elevated when the transformer is excessively loaded. When coupled with exposure to higher temperatures, such as those resulting from overloading, the transformer insulation may age rapidly. In addition, moisture awareness is a critical data point for system operators who may otherwise unwittingly cause a transformer winding failure through emergency switching and loading, if these activities result in an increase in temperature that exceeds a wet transformer’s bubble inception temperature.
User guides and documents
FAQ / Frequently Asked Questions
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).