Testing underground – and undersea – high-voltage power cables is always a challenging task and has often involved the use of huge lorry-mounted test sets. As Damon Mount of Megger explains, however, recent developments mean that much smaller, easily transportable test sets can now provide results that are accurate, detailed and dependable.
Portable high voltage testing
In recent years, the increased focus on renewable energy sources and the growth of national and international power transmission networks has meant that a lot of buried HV (high-voltage) cable, usually operating at around 132 kV, has been installed, in both on-shore and off-shore locations. This growth in the use of buried HV cables is will undoubtedly continue, and this will place increasing pressure on cable testing resources.
To ensure that buried cables and their associated joints and terminations meet design targets, and to confirm the quality of the workmanship of the cable installers, testing is essential both before the cable is first energised and after subsequent changes or repairs. Tests typically include time-domain reflectometry (TDR) fingerprinting, sheath testing and high voltage withstand testing. The current trend is to use these tests principally as an aid to realising maximum asset life and stability of client returns from the installation.
Two key standards, IEC 60840 for cables rated between 30 kV and 150 kV, and IEC 62067 for cables rated above 150 kV, require that the test voltage is selected from a table within the standard, and is agreed between the cable installation contractor and their client. The test is to be carried out with a test voltage having a substantially sinusoidal wave shape, at a frequency between 20 Hz and 300 Hz. In other words, the test should be carried out using a wave shape and frequency close to the operational wave shape and frequency.
However, there is a get-out option, which in this case allows soak testing at the normal operating voltage of the system to be substituted for high voltage testing. In fact, when space is limited at the test site or access routes are restricted, this substitution may seem unavoidable. This is because the resonant test techniques commonly used for evaluating HV cables require large amounts of energy to sustain the resonant circuit and to overcome capacitive and inductive losses.
As a result, resonant test sets are large, commonly requiring an articulated lorry trailer to transport them. In fact, for longer cables, it may be necessary to connect several test sets in parallel to achieve the required power output, further increasing costs and inconvenience. In addition, there is inevitably a need for a substantial power source in the form of a large generator set mounted on yet another articulated lorry trailer.
Fortunately, in recent years a technique known as damped ac (DAC) testing has been developed to the point where it is now a realistic, practical and commercially viable alternative to resonant testing. CIGRE is actively working to formally recognise this type of testing for HV cables. Megger is at the forefront of these developments, and has recently introduced new HV DAC test sets that are available in 200 kV and 300 kV versions. These are very much smaller than resonant test sets – they pack easily into a large Sprinter-type van – yet they can test cables that present loads up to 8 µF.
This leads to an enormously important benefit: it is no longer necessary to rely on hired equipment to test HV cables, with all the associated problems of availability and cost. Instead, with the new DAC test sets, it is feasible for the cable operator, or at least a local service provider, to purchase their own test equipment. This means that more testing can be carried out without major concerns over additional costs and that testing can be performed at short notice. In addition, with a DAC test set, testing can be carried out in locations that can’t be reached by resonant test sets, reducing the need to rely on the rather inadequate soak test approach.
But why are DAC test sets so much smaller and more convenient? The answer is that resonant test sets continually pump energy into the cable at the resonant frequency (between 20 Hz and 300 Hz) to maintain the test voltage. In contrast, the DAC test set fires energy into the cable on a repetitive cycle, allowing the voltage to decay between shots.
While it is ideal to carry out tests at the normal operating frequency of the circuit under test, it is worth noting that there is a well-established compromise, written into standards, which applies to medium voltage cables operating between 11 kV and 33 kV. This is VLF (very low frequency) testing, which is usually carried out at a frequency of 0.1 Hz. This frequency has been chosen in the light of extensive research; it offers the best compromise between effectiveness of test and efficiency in terms of size, weight cost and ease of deployment of the test equipment. In the same way, DAC testing can realistically be described as the effective, efficient and economical way of testing HV cables operating at 132 kV and higher.
Partial discharge testing
Having seen that DAC testing is a viable alternative to resonance test techniques, it is now worth considering the additional benefits of partial discharge (PD) testing used in conjunction with voltage withstand testing. It is a recognised fact that no withstand test, whether it uses dc (the least useful!), VLF, resonant or DAC techniques, will reveal all of the possible failure modes of an installed cable. In contrast, PD testing carried out in conjunction with high voltage testing will reveal those failure modes not detected by high voltage testing alone.
The DAC test method actually facilitates fast, effective and detailed PD evaluation and, with the latest software, the calculation of results and generation of reports are taken care of automatically. It is fair to say that newly manufactured cables, termination systems and joints are, in themselves, unlikely to fail on test. The main objective of cable testing is therefore to reveal where human intervention has created a problem. This may, for example, be deficient system design, badly manufactured joints or terminations, or simply poor workmanship. In other words, the key function of testing is to evaluate built system quality.
Conventional sheath testing will reveal general cable pulling and burial problems, and modern sheath testers such as the Megger MFM10 are highly accurate and readily portable instruments. In addition, they incorporate easy-to-use reporting software that allows contractors to confirm the quality of their work and clients to keep accurate records. However, in the jointing and termination of HV cables, which are highly skilled operations, PD testing can reveal much more about the state of the asset.
In a perfect new installation, there will be no PD, but in an older installation that has been extended or modified it is realistic to aim for a quantifiable improvement or a minimum level of PD. PD testing is primarily of benefit in evaluating joint and termination quality, so that both the contractor and client can be sure that the asset will realise its full working lifespan.
VLF and PD testing in the medium voltage sector
In the medium voltage (MV) 11 kV to 33 kV sector, PD testing offers exactly the same benefits to the system operator, and has the same objective of exposing all possible system faults. VLF testing is now rapidly becoming accepted in the UK as the preferred method for MV cable testing, a position that it has held in many other markets for decades. Development work is, however, still continuing to help users get even more out of this invaluable testing technology.
This has led to Megger introducing its TDS-NT range of test sets, which make use of the highly advantageous cosine-square VLF test wave shape, and integrate the new DAC technology for detailed analysis. The well-established Cosine Square wave shape has leading and trailing edges that closely match those of a 50 Hz sine wave, and it is this feature of the waveform that allows it to be used for accurate and reliable PD testing. It is even possible to save time by testing the cable for PD while the VLF withstand test is being carried out. TDS-NT test sets can also use their on-board HV power source to generate the DAC waveform for even more detailed PD analysis.
The ingenious TDS-NT test sets not only combine VLF, DAC and PD testing in a single convenient unit, they also use the same evaluation software as the Megger HV DAC test system to rapidly produce accurate, dependable and detailed results.
Conclusion
Testing buried HV and MV cables is never likely to be a trivial task, but developments in techniques and equipment are making it much easier and much less costly. As a result, cable operators can now use testing as an invaluable and cost-effective tool not only for fault finding, but also to ensure that they get the maximum life and valuable from their assets.
