When you work on medium voltage switchgear, your first and overriding priority will always be to maximise safety. But what can you do to protect yourself against hidden hazards that are not readily apparent? Damon Mount of Megger looks into this issue and offers some useful suggestions.
If you’re professionally involved with MV switchgear, you’ll know that there are many ways in which it can fail. You’ll also know that the most common is insulation breakdown, and that the results can be catastrophic. Even a modest fault in an MV installation releases enough energy to wreck equipment and, if the circumstances are truly unfortunate, to kill. Which means that when you’re working with or even near MV equipment, you want to be as sure as you possibly can be that such a fault is not likely to occur.
Unfortunately, equipment that’s about to fail rarely gives you an advance warning. Or does it? In fact, insulation breakdowns are very often preceded by partial discharge (PD) events. If you can detect and monitor these events, you will, therefore, have a strong indication of equipment and cables that may soon fail. You can then investigate further and, if necessary, take the equipment out of service before a really serious problem develops.
At the very least, you’ll know that it’s a good idea to keep your distance from the suspect equipment if you want to stay safe. That’s reason enough for owners and operators of MV systems to insist that PD checks are always carried out on their equipment before any work takes place on or near it.
That’s all very well, but how do you carry out these surveys? Equipment to detect PD is nothing new but, for the most part, it has been designed not only to detect PD but also to investigate and characterise it. That’s excellent in its place, but the consequence of all this functionality is that the equipment requires skill to operate and to correctly interpret the results. Also, if you invest in this equipment, it will undoubtedly serve you well in demanding applications, but if you only intend to use it for MV switchgear surveys, you’ll be paying for a lot of features you’ll never use.
What you really need is a compact instrument – ideally handheld – that is very easy to use and that provides clear, unambiguous go/no-go results. You’ll need an instrument that works in a live environment, so that you don’t need to take equipment out of service to carry out your surveys. You won’t, however, need sophisticated functionality – you can always bring in other equipment when you want to investigate potential problems more fully – but you will need versatility, so that you can deploy the widest possible range of PD detection techniques. A PD detector can offer the versatility you need by employing multiple sensors – both internal and external – so let’s take a look at some of the possibilities.
You will find an internal acoustic sensor – essentially a microphone that listens for the characteristic noise produced by partial discharges – is a good choice for PD testing on air-insulated MV switchgear and equipment that’s in direct line of sight. If you need more flexibility in where you can place the sensor, you can achieve this by using an external acoustic sensor that connects to the detector via a cable.
For fully enclosed switchgear, you’ll want a contact probe that is sensitive to vibrations produced by partial discharge, while for switchgear where the components are not visually accessible – mostly vacuum and GIS equipment – an internal TEV (transient earth voltage) sensor will pick up RF radiation from the PD via the switchgear’s metallic enclosure. An external TEV sensor performs a similar function but will also help you to localise the PD source.
If you have a PD detector that allows you to connect an HFCT (high-frequency current transformer) sensor, this will usefully extend its functionality to include simple on-line PD surveying of MV cables. Finally, a parabolic acoustic sensor will let you carry out outdoor surveys to detect, for example, corona and surface discharge in terminations, CTs, PTs and isolators.
Let’s remember now that if you’re going to be routinely carrying out quick pre-work safety surveys, your PD detector must be fast and easy to use, so what should you need to do to configure it appropriately for the type of sensor in use? The answer is nothing. The detector should automatically set itself up for the type of internal sensor you select, or for the type of external sensor you plug in. This not only saves you time and trouble, but also makes it near impossible for you to make mistakes that might produce misleading results.
And what about those results? You may have heard – or even know from your own experience – that PD results can sometimes require skill and experience to interpret. But you don’t need that level of detail for a safety survey. So how about an instrument that provides you with ‘traffic light’ results? Green for all is OK, amber for caution – consider investigating further, and red for danger – further investigation definitely needed.
Of course, as you gain experience, you may well want to be able to alter the thresholds for amber and red indications to suit your own specific requirements, and you’ll almost definitely want a little more detail in the results so you can record and trend them. A phase-pattern PD display option will also be useful, as it provides a very useful way to distinguish between true partial discharge effects and random noise.
Hopefully by now you’re convinced of the benefits of routinely carrying out PD surveys on MV equipment, especially before you start work on or near it, and so you’ll be interested to know that a PD survey instrument that meets all of the requirements we’ve discussed has recently been introduced to the market. This is the new PD Scan from Megger.
In fact, this innovative device does rather more. It includes, for example, a camera that allows photographs to be easily attached to test reports, and which can also be used to read QR codes to identify equipment and documentation. It has a humidity and temperature sensor so that you can easily add these key parameters to your data, and there’s provision for you to download your results easily into a PC and quickly generate comprehensive reports.
For all of its versatility and functionality, this instrument is as simple to use as a smartphone. It has just three buttons, and a large colour touchscreen that is intuitive in use and shows only those options you need to deal with the current operation, rather than forcing you to plough through a rat’s nest of complicated menus.
