Understanding 1 Hz testing: The science behind improved insulation assessment
The integrity of insulation is paramount in high-voltage (HV) electrical equipment. To assess insulation condition, engineers have relied on line-frequency dissipation factor (LF DF) testing for decades.
However, recent advances have shown that supplementing this traditional method with 1 Hz testing can provide a more comprehensive and accurate evaluation.
Let's look at the theory behind this innovative approach.
Dielectric response in frequency domain
To understand 1 Hz testing, we first need to understand the concept of dielectric response.
When we apply a sinusoidal signal to an insulation system, we can measure various dielectric properties, including capacitance, complex permittivity, and conductivity.
The total current density comprises two components:
- One in phase with the applied field (resistive component)
- One 90° leading the applied field (capacitive).
These measurements give us valuable insights into the insulation's condition without causing any damage.
In particular, the ratio of imaginary to real components of the complex permittivity ε ̂, known as the dissipation factor or tan delta (δ), is crucial in assessing insulation health.
ε ̂=ε^' (ω)-iε"(ω)
tanδ (ω)=(ε" (ω) )/(ε^' (ω) )
Temperature matters: The Arrhenius equation
Unfortunately, tan delta measurements are temperature dependent, and one of the challenges in insulation testing is accounting for temperature variations.
The Arrhenius equation shows how temperature affects the dielectric response:
L=ln(f_2 )-ln(f_1)=-E_a/κ_B (1/T_2 -1/T_1 )
Where Ea is the activation energy, kB is the Boltzmann constant, and T is the temperature in Kelvin.
Using this equation, it is possible to normalise measurements to a reference temperature (typically 20°C). This correction is vital for accurate comparisons over time or between different assets.
The effect of temperature on an oil-impregnated paper (OIP) sample is shown in Figure 1.
Line-frequency dissipation factor: The traditional approach
Traditionally, insulation assessment has relied on line-frequency DF testing performed at 50 or 60 Hz. While this method is useful, it has limitations.
Line-frequency DF values can sometimes remain stable even when insulation is degrading, which means that the test results fail to reveal the early signs of problems.
Additionally, as already noted, the results are highly temperature-dependent, making accurate interpretation challenging without proper correction.
1 Hz testing: A complementary approach
By measuring the dissipation factor at 1 Hz as well as at line frequency, we gain several advantages:
1. Increased sensitivity: 1 Hz measurements are up to ten times more sensitive to changes in insulation condition than LF tests.
2. Early detection: Problems that might be missed by line-frequency DF testing alone can often be identified with 1 Hz measurements.
3. Immediate interpretation: 1 Hz results are often easier to interpret without the need for long-term trending.
The lowest-losses frequency concept
To fully appreciate the value of 1 Hz testing, we need to understand the concept of lowest-losses frequency in the dielectric response of oil-paper insulation.
As we sweep through different frequencies, we observe a transition point (ωr) where the dielectric response shifts from a relatively linear low-loss system to a higher-loss region with greater dispersion.
Temperature changes cause this lowest-losses frequency to shift. Higher temperatures push it to higher frequencies, while lower temperatures move it lower.
This shift is crucial to understand because changes in the dielectric response curve's vertical or horizontal axis can indicate a change in the insulation's condition, as seen in Figure 2.
By measuring at both line frequency and 1 Hz, we can better capture this behaviour and gain a more complete picture of the insulation's health.
Conclusion
The combination of line-frequency and 1 Hz DF testing, coupled with proper temperature correction, provides a powerful tool for assessing the condition of HV equipment insulation.
This approach offers increased sensitivity, earlier problem detection, and more reliable results across various temperature conditions.
As we continue to push the boundaries of electrical power systems, these advanced testing methods become increasingly necessary for maintaining reliability and extending the life of valuable assets.
In other posts, we explore practical applications of 1 Hz technology and how it's revolutionising maintenance practices in the field.
Stay tuned to learn how 1 Hz testing makes a difference in real-world scenarios, from transformers to bushings and beyond!
