HV assets: 1 Hz testing for the full picture
The challenges of traditional insulation testing
Accurate knowledge of the insulation condition of critical HV assets such as transformers and bushings is essential if unplanned downtime and network outages are to be avoided.
A common way of assessing insulation condition is to measure power factor/dissipation factor (PF/DF) at line frequency (50/60 Hz) to confirm that the results obtained are within acceptable limits.
This is a useful procedure, but unfortunately, conclusions based only on these results may be unreliable.
There are two reasons for this.
- The first is that results are highly temperature dependent and, with conventional instruments, applying temperature correction that’s accurate and reliable is difficult.
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The second and even more important reason is that line frequency PF/DF tests are relatively insensitive to changes in insulation condition: in many cases, the results don’t start to change until the insulation is severely degraded.
The solution: 1 Hz testing
One way of dealing with the limitations of line frequency PF/DF testing is to trend results over a period of months or even years.
This allows small but developing changes to be identified. This approach only works, however, if historical results are available; it also requires a considerable level of expertise to interpret the trend data to determine whether or not any changes are significant.
A much better solution is to supplement line frequency tests with PF/DF tests carried out at a frequency of 1 Hz.
Because testing at this frequency is less influenced by capacitive effects, it is up to ten times more sensitive to changes in the condition of insulation.
Furthermore, it provides results that are easy to interpret immediately, without the need for trending.
There’s an additional benefit when testing is performed with Megger instruments: Individual Temperature Correction (ITC).
For this, the instrument automatically carries out a series of PF/DF tests at frequencies from 1 Hz to 505 Hz. Based on the results, it calculates an accurate and dependable temperature correction factor for the specific asset under test, and it applies this factor to both 1 Hz and line frequency test results.
Key advantages of 1 Hz testing
- The combination of 20 ºC temperature-corrected 1 Hz and line frequency measurements is an effective tool for identifying early and advanced insulation degradation, which could ultimately lead to insulation failure in HV components such as transformers and bushings.
- Discrepancies between the results from the line frequency and 1 Hz PF/DF tests, when the results are properly corrected to 20 ºC, is strongly indicative of progressive failure of the insulation.
- For this reason, the combination of results increases confidence in the insulation assessment, thereby providing asset managers and maintenance specialists with data they can rely on to ensure reliable operation of their HV assets and, in many cases, to extend the service lives of costly critical components.
Megger’s 1 Hz testing solutions
If you already use Megger instruments, you may have everything you need to perform 1 Hz testing.
This functionality is available with dedicated DFR instruments in the IDAX300 insulation diagnostic analyser series, and also with the equipment used for 10 kV line frequency PF/DF testing, such as the DELTA4000 power factor/tan delta tester or TRAX + TDX multifunction transformer and substation test system.
In all cases, the ITC and 1 Hz tests use the same connections as the normal 10 kV line frequency tests, so it’s only necessary to make the connections once for the complete suite of tests.
Conclusion
1 Hz testing represents a significant advancement in insulation condition assessment for HV assets.
By providing enhanced sensitivity to insulation changes and offering easy-to-interpret results, this method enables asset managers and maintenance specialists to make informed decisions about their critical equipment.
Combined with Megger's Individual Temperature Correction technology, 1 Hz testing offers a comprehensive solution for early detection of insulation issues, potentially extending asset lifespans and reducing the risk of unexpected failures.
Don't miss the opportunity to enhance your asset management practices. Explore the power of 1 Hz testing today.
