The Basics of VLF Testing
- June 20, 2018
- Posted by: Dominique
- Category: Cable Testing and Diagnostics
Very low frequency (VLF) withstand testing is the application of an AC sinusoidal waveform, generally at 0.01 – 0.1 Hz, to assess the quality of electrical insulation in high capacitive loads, such as cables. During the test cables are subjected to a test voltage significantly higher than what they experience during normal operating conditions. The higher test voltage allows for weak points or pre-damaged areas within the cable to breakdown during the test, rather than while they are in service. Essentially, the VLF withstand test is a “go or no-go” test, otherwise known as a “pass/fail” test.
What are the advantages of VLF over power frequency?
A good basic equation to know is: P = 2πfCV2
where f = applied test frequency, C = capacitance of the test object, and V = applied test voltage.
Already from this equation it can be seen that decreasing the applied test frequency will decrease the power required to apply a voltage, and since P = IV (where I = current), the amount of current required to apply a voltage is also lowered.
From the above relationship, when comparing VLF 0.1 Hz to power frequency (60 Hz), there is a 600x lower power and current requirement when testing a cable at the same test voltage. This has the added benefit that the size of the test instrument can be significantly reduced to allow for a very portable high voltage tester. An example of which is the Frida VLF Tester, which can generate test voltages up to 24 kVRMS or 34 kVpeak and weighs only 22 kg (48 lbs).
What are the causes of cable failure during a VLF withstand test?
When a cable is subjected to a considerably higher test voltage than what it typically sees in service, any defects in the cable will see higher stress levels that may grow within the insulation. This is a phenomenon known as “treeing” and these trees arise at stress enhancements where there are voids, protrusions, contaminants, or water trees. The term “treeing” stems from their branch-like structure resembling a tree.
Electrical trees are channels of carbonization that arise from partial discharge activity within the insulation. Once an electrical tree grows big enough and bridges the electrodes of the cable system, a breakdown of the cable insulation is created. Water trees are tree-like structures that form from the electrochemical interaction of the electric field and water ingress within the cable. Their growth is extremely slow, but they act as stress enhancements, which can help to initiate an electrical tree. Below is a photograph of a water tree growing into an electrical tree.
What are the recommended test voltage levels and testing times?
The IEEE Std 400.2-2013: IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (less than 1 Hz) was introduced to give an easy to interpret guide for conducting VLF withstand tests on shielded power cables rated 5 – 69 kV. Below is an overview of the recommended voltage levels that should be applied during installation, acceptance, and maintenance testing of medium voltage distribution cables depending on the cable system rating (phase to phase voltage). Generally, VLF withstand testing calls for testing the cables up to 3U0, where U0 is the rated phase to ground voltage.
VLF testing times should last between 15 and 60 minutes, depending on the age of the circuit and what type of test is conducted. For example, a minimum test time of 30 minutes is recommended for aged cable circuits. Extending the time to 60 minutes should be considered for particularly important circuits, such as feeder circuits. For installation and/or acceptance tests, the minimum recommended time is 60 minutes.
The times recommended for VLF withstand testing stem from studies conducted on tree growth rate on partial discharge defects in XLPE cable systems. According to IEEE Std 400: IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems, differences can be seen in the channel tree growth rate between power frequency, VLF sinusoidal, and VLF cosine-rectangular, and the fastest tree growth rate is achieved by applying VLF sinusoidal. A channel tree growth rate with a 3U0 test voltage at 0.1 Hz VLF sinusoidal on field-aged XLPE cables is 10.9 – 12.6 mm per hour (mm/h). A typical 15 kV medium voltage cable in USA has an insulation thickness of 0.22” (5.6 mm), and therefore, during a VLF test time of 30 minutes all defects within the cable should grow to failure. When comparing this figure to power frequency AC or 0.1 Hz VLF Cos-Rectangular, the tree growth rate is only 2.2 – 5.9 and 3.4 – 7.8 mm/h, respectively. As a result, 0.1 Hz VLF sinusoidal is the ideal frequency and waveform for cable withstand testing. Potential failures should happen during the actual test so that repairs can be made immediately. Failures in the cable during service result in higher costs for the utilities and are a nuisance for the power consumer.
Why is withstand testing with DC voltage not recommended?
Testing of cables was traditionally conducted with DC voltages in the past, which was also known as DC hipoting. Once polymeric cables (XLPE, EPR, etc.) were introduced into the electric grid and readily being tested with DC, a considerable rise in premature failures occurred. These failures were being attributed to trapped space charge within the defects of the insulation due to enhanced charge migration in one direction (DC electric field). Once testing was completed and AC power frequency was reapplied in service, a significant field enhancement could occur at these defects leading to a fast-growing electrical tree and subsequent failure. An illustration of the space charge effect after DC testing a XLPE cable is shown below.
HVT’s Range of VLF Testers:
|Frida||Viola||PHG 80 Portable|