Support for the IDAX series of insulation diagnostic analysers
There are a few possible reasons and countermeasures for this:
1. The generator output is earthed/grounded.
- 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).
- 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.
- 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:
- 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
Software and firmware updates
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
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.
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.
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.
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.
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.