UME looks at the issues to consider when specifying switchgear
The continuing expansion of electrical distribution networks places pressure on the equipment that keeps them safe and reliable. Faults or failure can cause localised blackouts, damage to property, or serious injury and death to operational staff.
This is the case with switchgear, which must be selected for installation on a case-by-case basis, to ensure it will work appropriately, within the required environment. And although failures are rare, manufacturers advise against complacency. The most frequent problem is a flashover voltage when electricity jumps from one conductor to another creating an arc.
If the breaker insulation fails to absorb this arc an explosion can occur. Therefore it is imperative to inspect, test and maintain all switchgear and other substation equipment on a regular basis.
Frequent testing and monitoring can help to detect deterioration in switchgear equipment before faults occur and prevent outages. Partial discharge (PD) monitoring is the main technique used to monitor the health of high-voltage switchgear.
PD activity manifests itself through internal or surface arcing and noise, and is a sign that the insulating medium is failing. Portable detectors that record ultrasonic noise and transient earth voltage effects can indicate whether assets are working satisfactorily or whether they require further tests or urgent investigation. In addition, for gas-insulated installations regular pressure checks are vital to protect against leaks and a breakdown in the integrity of the insulation.
Checks on switchgear and substations should also include visual inspections for evidence of sooting and distortion on enclosures, the detection of smells and high temperatures, corrosion leaks and so on. The immediate environment also needs regular inspection such as external walls in case of water ingress and fences for signs of unauthorised access or interference.
During the selection process there are several things to consider, the most important of which is safety.
“Because people are operating [the equipment] it has to be safe and it has to be reliable,” said Praveen Kashyap, manager - Technical Sales Support & Estimation for Eaton Electric. “Also, you are relying on some automation, if it is not reliable there can be some very difficult situations where protection has to work and it doesn’t.”
So, safety and reliability in switchgear product selection are absolutely essential. Looking for established performance track records are an obvious clue as to what works and what doesn’t. But apart from the performance qualities of the equipment itself, the environment in which it is placed must also be considered.
The equipment specified for any given situation will change depending upon the surrounding environmental conditions. This can make housings or enclosures for the switchgear especially important and a key consideration in the Middle East is proximity to the sea. Hot coastal locations have a brutal effect on equipment, particularly metal.
“Whether you are putting something in Ajman or Al Ain makes a big difference,” said Kashyap. “When buying equipment, you must know where it will be installed. How much moisture, heat and salt it can withstand is the deciding factor.”
Polycarbonate and glass-fibre enclosures are reasonable protective solutions, as is the possible use of stainless steel. Combinations of filters and air conditioning units included in larger installations can also keep the environment under control and regulate the temperature. The degree of protection offered can be indicated by an ‘IP’ rating.
What this all boils down to is operational sustainability. The lifecycle of the product has to be long enough to justify the cost and the long–term support must be available from the manufacturer, so replacement parts will be accessible and any regulatory upgrades can be carried out, without having to start from scratch.
Flexibility also plays a part here too. The ability to extend a switchgear installation has to be planned in at the initial installation phase. Both the selection of equipment and the allocation of space are important, if further additions or adjustments may be required at a later date. If this is a possibility, then selecting equipment that can be expanded is absolutely essential.
A decisive factor, both for longevity and operational costs, is the insulation used, whether air, oil, gas or solid. The choice of insulation is also important for the environmental impact of the switchgear.
The emergence of this as a significant consideration is linked to the most common gas insulation, SF6, and its position as a greenhouse gas. There is a growing debate about the use of SF6 gas in the power industry.
The advantages of gas-insulated switchgear over air-insulated installations are obvious in that using SF6 gas as an insulating medium reduces the clearance distance needed between active and non-active parts of switchgear, meaning less space is required to house the substation. Manufacturers say these systems occupy just 5-20% of the space used by the air-insulated design.
On top of that, gas-insulated substations are less sensitive to air pollution and to interference from salt and sand particles than substations open to the elements.
“The insulation determines the size of the switchgear,” said Kashyap. “A lot of polluting substances are being used in medium voltage switchgear. SF6 gas is good a product from a performance point of view – it has good properties for insulation and for quenching the arc – but is corrosive and a very potent greenhouse gas.
“While it is okay to use at the higher end of the network, as you go downstream it is spread over a very large area and can become difficult to manage.”
It is possible to replace SF6 with vacuum technology when it comes to insulating the circuit-breakers in medium-voltage switchgear. And as environmental awareness grows in the region the vacuum option is expected to gain in popularity.
The use of vacuum technologies for arc interruption in circuit-breakers is not new: the technology has been around for several decades and over the years improvements in design have made the technology more practical. The size of a 15 kV vacuum interrupter bottle has shrunk from 180 mm to 50 mm diameter during the past 40 years.
Meanwhile, modern sealing techniques ensure that units retain their vacuum for more than 30 years and in the event of a problem, the leaks are harmless to the environment.
The danger of SF6 is leakage, which damages the atmosphere and renders the switchgear inoperative. While the right maintenance procedure can ensure that doesn’t happen, this is most likely to be carried out upstream in the power chain and be managed by a utility.
Further downstream, MV secondary stations are the last parts of the distribution network. Their importance is that they deliver power to the transformers located in buildings or just outside buildings in package stations.
Secondary stations number in the thousands and are usually spread-out over the length and breadth of a city, with roughly 250-300 for every primary station. For example, a city with 50 primary stations would have around 15 000 secondary stations. In view of their large number, it becomes impractical, not to mention very expensive, to carry out regular maintenance.
Kashyap says the alternative is cast-resin switchgear. It offers the same space saving as gas-insulated switchgear, but without the potential environmental issues.
“We are dedicated to non-SF6 switchgear for the downstream market, right through to consumer level, where we use cast resin,” he said. “It is solid, very stable and in terms of electrical strength it makes the switchgear equally compact. It is very safe and reliable.”
However, while SF6 is easy to make and doesn’t require a great deal of clever technology, that is not the case with cast resin. Cast resin requires well developed technology and the ability to make a trouble-free cast-resin molding. These factors introduce a cost element that isn’t present with the simpler options.
“There is a definitely a cost, because it is a good technology,” said Kashyap. “This cost is not something which has to be taken as a basic cost of the material, because unless the user takes into account the benefits he has and the operational cost saving, he will never buy a more expensive [option].”
If a long-term view is taken, the cost savings in operational maintenance do deliver rewards quite quickly.
On an expected product life of more than 30 years, cast-resin can produce a 7% per annum savings through reduced operational costs. This equates to a payback period of six to seven years for the additional capital expenditure required up front.
But some switchgear producers play down the negative aspects of SF6 and its potential threat to the environment, after all it has been the technology of choice for many years, and if used correctly with appropriate controls and safety measures it poses little immediate danger.
“Latest studies in the EU show that SF6 in switchgear is not harmful considering the small volume used in switchgear,” said ABB’s Abdelwahab Yacoub. “SF6 gas is inert, but precautions must be taken in case of internal arc in the switchgear. SF6 gas is applied in much larger quantities in industry rather than in switchgear.”
Safety and efficiency are at the forefront of all developments in the switchgear sector and like most industries suppliers are constantly working to improve control and protection by making best use of the latest technologies.
Smarter designs are just around the corner, as Yacoub concluded: “Switchgear design has improved a lot in recent years. We talk today about intelligent switchgear using bay control units for communication, control, protection and metering.
In the future more wireless communication solutions are likely to be integrated into this very steady industry and product range.
Purchasing decision: five questions to ask
Will the switchgear protect the network and the operators who use it?
Will it always work without fault or shut down when required?
Can the switchgear be expanded if necessary?
What’s the operational lifespan of the switchgear and the cost associated with that lifespan?
Is the switchgear safe for and protected from the environment?
The protection of enclosures against ingress of dirt or against the ingress of water is defined in IEC529 (BSEN60529:1991). An enclosure which protects equipment against ingress of particles will also protect a person from potential hazards within that enclosure.
The degrees of protection are most commonly expressed as “IP” followed by two numbers, eg: IP65, where the numbers define the degree of protection. The first digit (foreign bodies protection) shows the extent to which the equipment is protected against particles, or to which people are protected from enclosed hazards.
The second digit (water protection) indicates the extent of protection against water.
So an enclosure rated IP66 would be intended for general purpose indoor and outdoor use primarily to provide a degree of protection against corrosion, windblown dust and rain, splashing water, and hose-directed water; and to be undamaged by the formation of ice on the enclosure.
Sulphur hexafluoride (SF6)
SF6 is a man-made, highly potent greenhouse gas (one of six) with an estimated atmospheric life of 3 200 years and a global warming potential (GWP) 23 000 times that of CO2.