1 Hz testing in action: Real-world applications
In our previous posts, we've explored the science behind 1 Hz testing and its applications in various types of high voltage (HV) equipment.
Now let's look at real-world scenarios where this technology has made a significant difference.
These applications demonstrate how 1 Hz testing is changing insulation assessment and improving maintenance practices in the field.
Field experience is central to validating new technologies, and 1 Hz testing is no exception.
The following case studies showcase how this method has provided critical insights that traditional line-frequency dissipation factor (LF DF) testing alone might have missed.
We’ll examine three diverse scenarios where 1 Hz testing proved its worth.
Commissioning new 69 kV RIP bushings
During the commissioning of new 69 kV resin-impregnated paper (RIP) bushings, a series of tests were conducted before energisation to ensure proper functioning.
LF DF tests were performed at 3°C, and the results initially appeared to be within acceptable limits according to CIGRE guidelines for new RIP bushings.
However, a surprising discrepancy emerged when 1 Hz testing was applied with a DELTA4000, along with Individual Temperature Correction (ITC) to normalise results to 20°C.
Two bushings (Y1 and Y3) showed significantly higher DF values at 1 Hz than the third bushing (Y2).
Upon discussing these results with the commissioning team, it was revealed that the Y bushings had been improperly seated during transport, and water had been observed in their protective wrapping.
This information, combined with the 1 Hz test results, led to the bushings being sent back for inspection, repair, and drying.
This case demonstrates how 1 Hz testing can detect insulation issues that traditional LF DF testing alone may miss, especially in new equipment where problems might not be expected.
New transformer – 16 MVA 138 kV with elevated moisture
A new 16 MVA 138 kV transformer was tested after assembly and before energisation. Ensuring the dryness of solid insulation is crucial for transformer longevity and reliability.
LF DF values, when corrected to 20°C using ITC, appeared excellent. However, the 1 Hz DF results told a different story.
While still in the "good" range, they were higher than expected for a new transformer.
Full-spectrum Dielectric Frequency Response (DFR) testing confirmed the presence of 1.6% moisture in the solid insulation, far exceeding the acceptable level of 10 ppm (0.001%) suggested in IEEE Std. C57.106.
Subsequent oil analysis corroborated the 1 Hz and DFR findings, confirming elevated moisture levels.
This case highlights the sensitivity of 1 Hz testing in detecting moisture issues in new equipment. It identified the need for drying procedures before energisation, potentially extending the transformer's service life.
EHV Capacitive Voltage Transformer (CVT) – 765 kV
During routine maintenance on a 765 kV substation, a small oil stain was observed on the surface of a B-phase CVT C1-1 stack.
LF DF results for this CVT were higher than those of sister units but not alarmingly so. Based on LF DF alone, further action might not have been taken.
However, 1 Hz DF testing clearly indicated significant insulation degradation in the C1-1 stack. Based on the 1 Hz results, the unit was removed from service for investigation. Upon disassembly, a puncture was found in the C1-1 stack, allowing oil to leak.
Without the insights from 1 Hz testing, this issue might have progressed to a catastrophic failure, potentially affecting adjacent equipment, the environment and the safety of personnel.
Benefits of 1 Hz testing
These applications highlight several common themes:
- Enhanced sensitivity: In all cases, 1 Hz testing detected issues that were either missed or not clearly identified by LF DF testing alone.
- Early detection: From moisture in a new transformer to degradation in a CVT, 1 Hz testing allowed early identification of problems, enabling proactive maintenance.
- Comprehensive assessment: When used with LF DF testing and other diagnostic tools, 1 Hz testing provides a more complete picture of insulation health.
- Cost savings: By identifying issues early, 1 Hz testing helps to avoid costly failures and extends equipment life, potentially saving millions in replacement and downtime costs.
Conclusion
These real-world examples demonstrate the value of incorporating 1 Hz testing into regular maintenance routines for HV equipment.
By providing earlier and more sensitive detection of insulation issues, this technique helps asset managers and maintenance teams to make informed decisions, potentially extending equipment life and avoiding costly failures.
As we rely more heavily on our electrical infrastructure, technologies like 1 Hz testing are becoming increasingly important. They allow us to move from reactive maintenance to proactive asset management, enhancing the reliability and longevity of our power systems.
The future of insulation assessment looks bright, with 1 Hz testing leading towards more accurate, reliable, and efficient maintenance practices.
As this technology continues to prove its worth in the field, we can expect to see its adoption become more widespread, contributing to a more resilient and reliable electrical grid for all.
