1 Hz testing: Revolutionising insulation assessment in HV equipment
Staying ahead of potential failures is crucial in high-voltage (HV) equipment maintenance. Traditional line-frequency dissipation factor (LF DF) testing has long been the industry standard way of assessing insulation conditions, but it doesn’t provide a complete solution.
As discussed in our Understanding 1 Hz Testing post, 1 Hz testing offers enhanced sensitivity to insulation changes and easier result interpretation when compared with traditional LF DF testing.
This makes 1 Hz testing an invaluable tool for maintaining critical HV equipment. Let's explore its applications across different types of assets.
Bushings
Bushings are essential components in HV systems, providing insulated pathways for conductors.
There are various types of bushings, including:
- Oil-impregnated paper (OIP)
- Resin-impregnated paper (RIP)
- Resin-impregnated synthetic (RIS)
1 Hz testing has proved to be particularly effective for bushing assessment.
When used in conjunction with LF DF testing, it provides a comprehensive picture of insulation health.
For OIP bushings, here's a quick guide to interpreting 1 Hz DF results at 20°C:
| OIP Bushing Insulation Condition | 1 Hz PF at 20 °C |
| Excellent | 0.2 – 0.4 |
| Good | 0.4 – 0.75 |
| Aged | 0.75 – 1.25 |
| Investigate | > 1.25 |
This enhanced sensitivity of 1 Hz testing allows earlier detection of insulation issues, potentially preventing costly failures and extending bushing life.
1 Hz testing is particularly effective in detecting early-stage insulation degradation in bushings, which can occur due to factors like partial discharge, thermal ageing, or moisture ingress.
This early detection capability is invaluable because bushing failures can be catastrophic, potentially leading to transformer damage, fires and widespread outages.
Moreover, 1 Hz testing can help to differentiate between various types of insulation issues.
For instance, it can help with deciding whether the issue is moisture contamination or general insulation ageing, which is useful because different issues may require different remediation strategies.
Transformers
The longevity and reliability of transformers depends critically on insulation condition.
While LF DF testing has long been the go-to method for assessing transformer insulation, 1 Hz testing offers significant advantages, particularly in detecting moisture and early-stage degradation.
For oil impregnated paper (OIP) transformers, here's how to interpret 1 Hz DF results at 20°C:
| OIP Transformer Insulation Condition | 1 Hz PF at 20 °C |
| Excellent | 0.2 – 0.5 |
| Good | 0.5 – 1.0 |
| Aged | 1.0 – 1.75 |
| Investigate | > 1.75 |
Importantly, 1 Hz testing can detect moisture problems that might be missed by LF DF testing alone.
This capability is crucial, as moisture is a primary factor in insulation degradation and can significantly shorten a transformer's lifespan.
Another significant advantage of 1 Hz testing for transformers is its ability to provide insights into the condition of the different insulation components.
Transformers contain various insulation systems - from the main tank insulation to tap changer insulation - each with its own ageing characteristics.
1 Hz testing, especially when combined with a Dielectric Frequency Response (DFR) analysis, can help identify which insulation components are degrading.
This allows maintenance interventions to be targeted more precisely. For instance, it might reveal that while the main tank insulation is in good condition, the tap changer insulation is degrading, allowing maintenance teams to focus their efforts where they're most needed.
Instrument Transformers (CTs, VTs, and CVTs)
Current transformers (CTs), voltage transformers (VTs), and capacitive voltage transformers (CVTs) play crucial roles in metering, protection, and control.
However, they often lack built-in monitoring systems, making regular testing essential.
1 Hz testing is particularly valuable for these items due to their small capacitance (usually less than 800 pF).
This low capacitance can make traditional testing methods less reliable, especially in the presence of electromagnetic interference (EMI).
For CTs and CVTs, 1 Hz DF values below 0.3% at 20°C generally indicate good condition.
However, the insulation volume in these devices is typically dominated by paper, making them particularly sensitive to moisture ingress.
The application of 1 Hz testing to instrument transformers allows:
- More accurate assessment of insulation condition
- Earlier detection of moisture ingress or insulation degradation
- Better planning of maintenance or replacement activities
1 Hz testing is especially useful for older instrument transformers and those in harsh environmental conditions.
These devices often have limited or no provision for oil sampling, making traditional oil analysis difficult or impossible.
In such cases, 1 Hz testing provides a non-invasive way to assess insulation health. This is a key benefit with sealed units where any breach could compromise the integrity of the insulation system.
In summary, 1 Hz testing can help identify potential issues in instrument transformers before they affect measurement accuracy or protection system reliability.
Conclusion
1 Hz testing of HV equipment is a significant advance in insulation assessment technology. By providing earlier and more sensitive detection of insulation issues, this method allows asset managers and maintenance teams to make better informed decisions.
Whether it's detecting moisture in a new transformer, identifying early-stage degradation in a bushing or assessing the condition of a hard-to-monitor instrument transformer, 1 Hz testing has proved its worth time and again.
As we continue to rely more heavily on our electrical infrastructure, tools like 1 Hz testing make it possible to move from reactive maintenance to proactive asset management, thereby enhancing the reliability and longevity of our power systems.
In this other post, we explore real-world case studies that demonstrate the practical impact of 1 Hz testing and see how this technology is making a difference in the field.
