Introduction
VLF (very low frequency) cable testing is a technique used to detect incipient defects in medium and high voltage cables. It involves applying a high voltage, low frequency AC signal to the cable and measuring several parameters to quantify it’s condition. The primary standard for this is the IEEE 400.2. This was originally released in 2004, revised in 2013 and most recently in 2024. This blog aims to highlight the changes in the latest version and discuss the impact of those changes.
Background
In the twenty years since IEEE 400.2 was first released, it has become the defacto standard for field testing of medium voltage cables, and has contributed tremendously in reducing cable failures. The key benefits from the standard are:
- Created awareness about the dangers of DC high voltage for XLPE cables
- Upgraded cable testing from simple pass/fail to quantitative assessments
- Provided a robust methodology for evaluating cable condition
Key Changes
- In 2013, Leakage Current and Leakage Current Harmonics were considered as parameters that could be used for the Monitored Withstand Test. These are now excluded and the only permissible parameters are Tan Delta and Partial Discharge.
- Dielectric Spectroscopy has been removed as an optional test.
- 2013 version was only applicable for cables upto 69 kV. It is now applicable for cables upto 138 kV.
- 2013 version referenced to both RMS and Peak values. Now, only RMS is considered.
- For safety, it is now required to reduce the voltage slowly after completion of the test, instead of just switching it off.
- VLF Withstand Voltage Test Levels have now been specified for additional levels, as under:
- For HV cables (≥ 66 kV), the minimum time period for an Acceptance Withstand Test is 60 minutes. Earlier, 30 minutes was allowable.
- 2013 version only required Tan Delta Stability (SDev) calculation at 1.0U0. It is now recommended that SDev also be calculated at 0.5U0. In case a cable is significantly degraded, this would give an early indication before applying any higher voltages.
- 2013 version gave recommended limits for only 4 types of filled cables, viz.:
- Carbon-filled EPR (black)
- Mineral-filled EPR (pink)
- Discharge resistant EPR
- Mineral-filled XLPE
Now, a fifth type has been added:
- Lead-free mineral-filled EPR (white)
- 2013 version stated that tan delta and/or partial discharge values could be used to monitor for different degradation modes during a 30 to 60 minute withstand test. Thus, both would have to be monitored to look for water trees as well as voids. The 2024 version states that monitoring tan delta alone during a 30 to 60 minute withstand test can be used to detect PD activity.
- 2024 version adds a note that just measuring tan delta will not locate a singular defect in an XLPE cable.
- 2013 version recommended VLF PD testing follow IEC 60885-3. The 2024 version recommends that IEEE 400.3 be used as far as possible.
- 2024 version clearly states “PD does not occur in water trees”. This removes an ambiguity which was misinterpreted by some to state that PD could be used for detecting water trees.
- The original 2004 version had stringent limits for tan delta and tan delta tip-up. In the 2013 edition, a separate set of limits was defined for North American cables. In the 2024 edition, the original stringent limits have been removed. Instead, it acknowledges that other countries have developed their own criteria. Specific references are given of France (PILC cables), Germany, Singapore (XLPE cables) and South Korea (XLPE cables).
The South Korean document also acknowledges that long length cables can show low tan delta values even when they have significant water trees. Hence, the criteria specified are for cables upto 1.5 km, and trending is a better methodology for longer cables.
Review of the changes
The changes in IEEE 400.2 are mostly excellent ones.
Improvements in safety can be summarized as under:
- Sudden de-energization is not allowed, reducing the stress on the cable
- Checking Stability at 0.5U0 increases probability of finding weak cables before higher voltage is applied, reducing risk of failure during the test
Improvements in utility:
- Cables of upto 138 kV are now covered
- Specific test voltage levels are specified for 11, 22, 33, 66 & 132 kV cables, which is very useful especially in India, UAE, etc.
- Monitoring tan delta during a 60 minute withstand test gives indication of PD activity. Thus, even less qualified personnel can perform the test and get a clue about voids in the cables.
Improvements in quality:
- Since Monitored Withstand Test must measure tan delta or PD only (leakage current is not acceptable), the results are far more qualitative and useful.
- False concepts are addressed. It is clear now that just tan delta will NOT find a singular defect in a cable, and PD will NOT detect water trees.
One key point that needs to be addressed is the dilution of the permissible limits for the tan delta test. The 2013 edition had Table H.1 (for countries outside North America) which was very strict and ensured high quality. As it now removed, poor quality cables and poor workmanship are likely to creep in, significantly reducing cable reliability. A comparison of the various guidelines is shown below:
We have seen many, many cases where the cables were perfectly healthy as per the new guidelines but defective as per the old ones. Several of these have failed or defects found in them during PD testing. Hence, we strongly recommend that either the IEEE 400.2, 2013 limits or the Singapore/ South Korean limits be used to truly ensure cable longevity.
In conclusion, VLF cable testing continues to be a critical diagnostic tool to help improve the reliability and safety of MV & HV power cables. Use IEEE 400.2 smartly along with other standards to correctly specify the way the test is to be done and the acceptance criteria so as to maximize the benefits.At DIATECH, we provide such professional services using the latest technologies. For more information, contact us or request a quote.
