Because of the high energy levels and high voltages involved, there will always be potential hazards involved in working with medium voltage (MV) switchgear. Modern developments are, however, significantly reducing these hazards, helping to make MV switchgear safer while also enhancing its reliability. David McCabe of Eaton’s Electrical Sector explains
Let’s be very clear from the outset, the key to safe working on MV equipment of any kind is appropriate training. MV equipment is not forgiving of mistakes, the consequences of which can be literally lethal. Work on MV equipment must, therefore, only be carried out by authorised and competent personnel who are properly trained and qualified. Having said that, there are practical measures that can be taken to minimise risk. Let’s take a look at some of these.
One hazard associated with MV switchgear is arc flash accidents, which occur when a large electrical current passes through ionised air. These accidents are rare, but when they do occur, they invariably have serious consequences. Arc flash accidents can be triggered in many ways, for example when a circuit breaker fails during a switching operation or when insulation suffers a catastrophic breakdown.
When an arc flash occurs, the temperature in the vicinity of the arc rises almost instantly to around 20,000 ºC. Conductors are vaporised, leading to an explosion that expels molten globules of metal. Persons near to an arc flash accident are at high risk of injury or even death. There are also other hazards associated with arc-related accidents. These include fire, the production of dangerous gases and, with older oil insulated switchgear, the expulsion of burning oil.
The design and construction of MV switchgear has an important influence on its susceptibility to these arc-related accidents. Most modern switchgear is, in fact, internally arc tested, which means that should an arc fault occur, its effects are contained within the enclosure. Nevertheless, the hazards mentioned earlier are very real with older equipment. In addition, although containing the effects of arc faults undoubtedly reduces the danger to people and property, such faults cause considerable damage to the equipment in which they occur.
There are very significant benefits to be gained, therefore, by using modern equipment with moulded insulation. This minimises the risk of arc-related faults, not least because insulation of this type allows good control to be achieved over electric fields, thereby reducing the susceptibility of the insulation to breakdown.
Even in the best of equipment, however, arc-related hazards cannot be entirely eliminated. For this reason, it is important the risks are properly assessed and appropriate protective measures put in place. A comprehensive arc hazard analysis, carried out by experts, is the best way to achieve this. Such an analysis can be expected to lead to recommendations covering arc flash boundaries, safe working distances, practical methods of hazard reduction, personal protective equipment (PPE) and safe working practices.
Let’s now move on to another phenomenon associated with MV switchgear – partial discharge in insulation. This is in itself less dramatic than arc flash, but it is probably the most common cause of unreliability in MV equipment and, if it is not detected and remedied, it can ultimately lead to insulation breakdown and possibly even to arc-related accidents.
Partial Discharge (PD) is defined as a localised electrical discharge that does not completely bridge the electrodes. In simple terms, because the stress created by the high voltages encountered in MV equipment, small currents in the form of sparks start to ‘leak’ through the insulation. Typical triggers for PD are voids and cavities filled with air in poorly cast resin transformers, age-related deterioration of cable insulation, badly made power connections, and contaminants or moisture on the surface of insulation.
A key characteristic of PD is it takes the form of very short current pulses with rise times of the order of nanoseconds. These pulses generate radio frequency signals across wide frequency spectrum from DC to hundreds of megahertz. With suitable apparatus, these signals can be monitored, providing a method of detecting and measuring PD.
Often, such measurements are only made periodically – typically twice a year. This approach, however, has a number of drawbacks. The first is PD activity is usually intermittent, so periodic measurements can miss significant problems. The second is trending PD information is a very valuable tool for predictive diagnostics, and six monthly measurements will not provide sufficient data for meaningful trending. For these reasons and others, continuous monitoring of PD is to be preferred.
Various types of equipment are available for continuous PD monitoring, but not all are equally useful as it is difficult to filter the required RF signal generated by PD from other RF signals produced, for example, by switching operations and power electronic devices. PD monitors must, therefore, be chosen with care and should not only take measurements over an optimised frequency range, but also incorporate advanced technology for noise suppression.
Ideally, the monitors should also be suitable for use with capacitive sensors, which offer the best noise immunity, as well as with radio frequency current transformer (RFCT) sensors, which are able to ‘see’ further into the MV installation because they are more sensitive to lower frequencies.
Up to this point, we have considered electrical safety issues but there is one other class of hazard associated with some types of MV switchgear – specifically those types that use sulphur hexafluoride (SF6) for arc quenching.
It’s worth noting SF6 has very poor environmental credentials – it’s global warming potential is 23,900 times that of CO2, for example – but that is not our main concern here. While SF6 itself is usually considered harmless in the concentrations normally used, that’s far from true of the derivatives formed by the arcs created during switching operations.
These by-products, which include HF, SOF2, SF4 and S2F10, are all toxic. Granted they are produced in relatively small quantities during the normal operation of the switchgear, but they are likely to be present when switchgear is dismantled for maintenance or at the end of its life, and appropriate precautions must be taken during operations of this type. This issue is also one of the many good reasons for avoiding the use of equipment that contains SF6 in new installations, especially when excellent alternatives are available, such as those that make use of modern vacuum technology circuit breakers.
Medium voltage switchgear should always be treated with caution, as it rarely gives second chances if mistakes are made. As we have seen, however, expert support and the latest technology can do much to reduce the hazards of working with MV equipment. Perhaps the best advice, therefore, is to choose suppliers with care, and rely only on those who have proven experience and expertise is the very specialist field of MV technology.