VLF Diagnostic Cable Testing
- July 26, 2019
- Posted by: Dominique
- Categories: Cable Testing and Diagnostics, Partial Discharge Testing
Very low frequency (VLF) diagnostic testing is the application of an AC sinusoidal waveform, generally applied at 0.1 Hz or lower, to assess the quality of electrical insulation in high capacitive loads, such as cables. A major advantage of VLF is that when comparing the voltage application at power frequency at 60 Hz, there is a 600x lower power and current requirement when testing the same cable at the same voltage level. Hence the size of the generator required to produce high voltages can be significantly reduced, allowing for more portability and accessibility of the equipment in field testing environments. As VLF is a pure sinusoidal waveform, application of this voltage mimics the power frequency waveform, and therefore, does not subject the cable to a waveform it would not experience under service conditions. For example, the application of DC voltages on polymeric cables may cause trapped space charges, which when reapplying AC power frequency following a test, could lead to a failure while in service. The added benefit of applying a VLF sinusoidal waveform is the ability to conduct cable diagnostic techniques, such as VLF withstand testing, VLF tan delta diagnostics, and VLF partial discharge diagnostics.
VLF Withstand Testing
A VLF withstand test is the application of VLF test voltages on a cable that are significantly higher than what the cable would experience during normal operating conditions. The elevated test voltages allow for weak points and pre-damaged areas, such as areas of partial discharge (PD) or poor workmanship during splicing, installation, etc., to break down during the test, rather than have this occur while cables are in service. There are many worldwide standards that incorporate the use of VLF voltages as a voltage waveform that is suitable to be used for withstand testing of cables, including IEC 60502, CENELEC HD 620, IEEE 400, and IEEE 400.2 and in North America the IEEE standards are the most important.
The IEEE Std 400.2 standard is a very easy to interpret guide for conducting VLF withstand tests on shielded power cables rated 5 – 69 kV. Recommended voltage levels are given 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 3 times the rated phase to ground voltage of the cable system.
In order to cause the weak points of a cable to fail during a withstand test, the voltages should be applied for a fairly long time, between 15 and 60 minutes. The test time depends largely on the age of the circuit and what type of test is conducted. The recommended minimum test time for aged cable circuits is 30 minutes. 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.
Essentially VLF withstand testing is a “go/no-go” or “pass/fail” test, and will only give information on whether a failure has occurred or not. If failures have not occurred during the withstand test, it is impossible to estimate the remaining integrity or health of the cable or how long the cable may remain in service without failing. This is where applying tan delta diagnostics or partial discharge diagnostics can be applied as a complementary procedure together with VLF withstand.
More detailed information on VLF withstand testing can be found under The Basics of VLF Testing.
HVT’s Complete Range of VLF Generators:
| Frida | Viola | PHG 80 Portable |
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VLF Tan Delta Diagnostics
Tan delta measurements are a precise and non-destructive method to provide important information on the extent of ageing in cable insulation. The test applies an AC sinusoidal waveform at 0.1 Hz frequency and measures the degree of real power dissipation or losses in the cable system. These losses could include the presence of water trees in XLPE cables, moisture in joints and terminations, and the presence of partial discharges.
Following up on the recommended test voltages that IEEE 400.2 provides, this standard also helped to create the tan delta monitored withstand test (TD-MWT). This is a VLF withstand test while monitoring the tan delta characteristics of the cable. Typically, the test is divided up into two phases, the voltage ramp-up phase and the voltage hold phase. During the ramp-up phase, tan delta is measured at 0.5U0 increments, in which 6-10 measurements are conducted at each increment. Mean tan delta, tan delta stability (standard deviation), and differential tan delta (tip-up; value between 0.5U0 and 1.5U0) are measured. During the hold phase at 2U0, tan delta is continuously monitored. This method has the added benefit that the overall test time can be reduced to 15 – 30 minutes if the tan delta values remain stable for at least 15 minutes and no failure occurs. Monitoring of tan delta also has the benefit that a lower test voltage can be applied to the cable under test.
IEEE 400.2 also conducted statistical analysis of thousands of measurement results from the Cable Diagnostic Focused Initiative by NEETRAC. This helped to supply condition assessment criteria, or threshold values, pertaining to different cable types, such as XLPE, EPR (with different filler material), and PILC.
Comparing to the simple withstand test, a VLF tan delta diagnostic test can give a user a very simple and time-efficient testing solution in order to realize a condition-based and cost-optimized maintenance plan. Conducting trend analysis and comparing measurements over time will help a user see whether a cable is deteriorating and should be replaced soon, or whether the cable may remain in service for a longer time, effectively cutting high costs associated with unnecessary replacements.
More detailed information on VLF tan delta diagnostics can be found under VLF Tan Delta Testing.
HVT’s Complete Range of VLF Generators:
All of the VLF generators we offer have the ability to program test routines and condition assessment criteria for tan delta testing. The Frida TD and Viola TD, with integrated tan delta measuring function, have the recommended IEEE 400.2 test routine and condition assessment criteria for all cable types pre-programmed on the instruments. These instruments are truly plug n’ play – once connected to the device under test, the measurement and analysis is fully automatic.
| Frida TD | Viola TD | PHG 80 TD |
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VLF Partial Discharge Diagnostic Testing
Partial discharges (PD) are evidence of a degrading insulation system, which could lead to very costly repairs and can predictively lead to an electrical breakdown of high voltage apparatus. These occur in areas of the insulation subjected to higher local electric field concentrations, such as in a cavity or a sharp protrusion in the cable dielectric or splice. It is important to note that tan delta is a global characteristic being measured, meaning if there are regions in the cable with a high density of partial discharge, these areas could not be located using only tan delta diagnostics. However, using partial discharge diagnostics with source allocation, these areas of PD could be located during a VLF withstand test.
More detailed information on partial testing can be found under The Basics of Partial Discharge Testing.
Similar to the TD-MWT, IEEE 400.2 also allows for PD to be measured as a monitored characteristic in a partial discharge monitored withstand test (PD-MWT). In combination with a VLF generator, the PD charge magnitude can be measured in pC giving an indication of the severity of the PD seen in the cable. In general, a high PD magnitude means a weak point in the cable is evident that could quickly lead to a failure. The PD site can be located with equipment that allows for “time of flight” PD measurements, based upon time domain reflectometry (TDR). By analyzing the time it takes for the PD signal to travel a certain length of the cable, direct allocation of PD activity in cable segments and accessories, such as joints and terminations, is enabled.
More detailed information on the “time of flight” PD measurements can be found under “Time of Flight” Partial Discharge Measurements.
A major advantage of VLF diagnostics using equipment supplied by HV TECHNOLOGIES Inc. is the ability to provide an all in one test called a full monitored withstand test (Full-MWT). Rather than only measuring one monitored characteristic during a withstand test, both tan delta and partial discharge can be measured simultaneously, as shown below.
A Full-MWT gives a complete evaluation of the cable condition and better detection of hidden faults. For example, if moisture is present in the cable or joint, the tan delta will initially be high, but a drying effect will be seen after a while due to the voltage application (reduction of tan delta). After a while the area of moisture may produce PD signals that can be measured and located. The Full-MWT effectively reduces the test duration for cables in good condition by up to 75% compared to a simple withstand test. Furthermore, the ability to measure the diagnostic characteristics simultaneously would reduce a tan delta and PD diagnostic test by 50%, rather than having to conduct the tests sequentially. As a result, operating costs associated with test personnel and the downtime of the tested cable can be effectively reduced. A detailed paper Simultaneous PD and TD Testing was written by HVT for the 2018 IEEE T&D Conference discussing the ability to reduce testing time and realize condition-based and cost-optimized maintenance plans.
HVT’s Complete Range of VLF Partial Discharge Diagnostic Instruments:
| PD-TaD 62 | PD-TaD 80 |
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PD level measurement according to IEC 60270
PD source localization and display of PD activity over the cable length
Coupling capacitor incl. measuring impedance and PD measuring unit in one device
Menu guided control software
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