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Mon 4 Feb 2008 04:00 AM

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Testing times

Regular testing and monitoring of critical equipment can prolong the service life of ageing power assets.

Regular testing and monitoring of critical equipment can prolong the service life of ageing power assets.

With the liberalisation of energy markets across the world, the issues of safety and reliability have been thrown to the fore as never before. Power providers are having to up their game as customers grow less tolerant of disruptions to electricity supply, whilst at the same time rising energy demand is putting increased pressure on ageing assets.

Industry experts estimate that there are on average five transformer explosions worldwide each week.

Systems are regularly being stretched to the limit of their capacity and as equipment ages it becomes more likely to fail. Catastrophic failure can have dire consequences, such as blackouts, injury or even death. And repair work can run into hundreds of millions of dollars.

But frequent testing and monitoring of key equipment, like transformers, cable systems and switchgears, can help to detect faults before they occur and reduce the likelihood of outages.

Oil analysis

Transformers are widely considered as the most critical pieces of equipment inside power plants because of the large quantity of oil in contact with high-voltage elements.

Industry experts estimate that there are on average five transformer explosions worldwide each week, with the majority caused by electrical faults, triggered by the deterioration of the insulating medium.

In most power transformers, the conductor windings are insulated by paper impregnated with mineral insulating oil. A typical transformer contains 10-12 tonnes of cellulose-based paper and 45 tonnes of oil.

Overtime, heat, oxygen and moisture can degrade this insulation and weaken its mechanical strength.

But regular oil checks will highlight any changes in the condition of the insulation and give early warning that invasive maintenance, perhaps a purification treatment, is needed.

"One of the most important indicators of the condition of a transformer is the oil as an insulating material," says Martin Baur, president and CEO of Austrian electrical testing firm, BAUR Pruf- und Messtechnik.

"Oil insulation is an image of the transformer quality and mobile oil testing devices are usually kept by utilities to check the quality annually or every six months."

"If the quality is changing, like humidity is entering the oil, or dust or other contaminants are detected in the oil, the transformer is likely to fail and maybe explode."

Portable oil testers measure the electric breakdown strength of the transformer insulants. Other devices may look for changes in the transformer's chemical characteristics by recording water or hydrogen content.

As with any condition monitoring test, for oil analysis to be effective, a database of previous test results needs to be maintained in order to track changes in its condition.

For an even more comprehensive analysis, utilities can send oil samples to a laboratory for testing. As Alan Roberts, business development manager Middle East at EA Technology explains: "We use our lab in the UK to provide oil analysis on samples that are returned here."

"We look for other things that portable devices don't check for; we are trying to identify the current condition of the transformer and also indicate its remnant life."
You can identify the condition of the paper in the transformer and then you can correlate that with how that is likely to degrade overtime to a point where the transformer needs to be replaced.

Extra protection

Preventative maintenance tests aim to extend a transformer's lifecycle beyond the average 40 years. Alongside that though, many utilities are now also seeking added protection for their main transformers, as they are such vital components.

France's Sergi Holding has developed and patented a transformer protector, which prevents tanks from exploding in the event of massive failure. The firm's president, Arnaud Magnier, explains:

"Ten to 15 years ago, we didn't really know why transformers were exploding, so we started to invest a lot in research to try to understand the physical phenomena behind it."

"We discovered that when there is an electrical fault inside a transformer, an electrical arc forms inside the oil, generating a bubble of gas, which makes a dynamic pressure inside the transformer."

"The pressure wave propagates inside the transformer and when this interacts with a weak point in the transformer walls, the tank ruptures and you have direct contact between the gases and the oxygen and at this moment you have fire."

"So we decided to plug a transformer protector on the weak point to depressurise the mixture of oil and gases, and to save the transformer tank."

Weak points can include bolts and manholes. Usually one depressurisation set is put on the tank and the transformer protector then becomes the weak point.

Sergi's protector does not replace standard protection devices, such as circuit-breakers and pressure-relief valves, but complements them. "The difference with the other types of protection is that we are dealing with dynamic pressure," says Magnier.

"The pressure-relief valve also deals with pressure, but it is more dedicated to static pressure inside the transformer, when you have gases, which are generated more slowly. We are not replacing the standard protection, this is an add-on."

The company is seeing strong demand for its product, particularly from European power firms wanting to extend the life of transformers that have been in service for 40-50 years.

In addition to that, though, it is also being used to protect new installations. Whilst ageing is the primary reason for insulation breakdown, sometimes bad design means new transformers explode immediately on startup. Furthermore, external influences such as lightning strikes can also trigger a fault.

Sergi enjoys healthy order inflow from the Middle East as well, particularly from Qatar's Kahramaa.

"We are currently protecting all the new transformers in Qatar," Magnier enthuses. "Last year, we sold something like 150 transformer protectors just in Qatar."

"We are now looking at the feasibility with Kahramaa to protect their existing facilities, and we are also starting to promote the product in UAE, Oman, Kuwait and so on. There is huge demand there."

Partial discharge

But while transformer explosions may grab headlines, less obvious faults on cables and switchgear can also plunge a city into darkness.

"Cable failures can be repaired with a couple of thousand euros or even less, but it can be very costly to utilities and industry if power supply is not delivered for sometime," says Baur.
So cable testing and diagnostic tools are very important to maintaining the underground network. We use loss factor measurement and pinpointing methods, like partial discharge measurement, which are predictive methods, to find faults before the failure occurs.

Partial discharge (PD) activity is a clear sign of deterioration in the integrity of a cable or other high-voltage assets. It manifests itself through internal or surface arcing and noise.

Surface discharge is where tracking occurs across the surface of the insulation, exacerbated by airborne contamination and moisture. Internal discharge takes place inside voids within insulation materials.

EA Technology also carries out partial discharge tests on cables, as Roberts explains: "For cables, we can do a partial discharge profile, where we identify the levels and the location of the discharge along the circuit length. But it does require an outage."

"Generally for the more strategic cables that is not a problem as there are often two cables in parallel feeding the distribution network and you can transfer load from one to another and make one available."

"By taking it out of circuit you can apply a test source to one end of the high-voltage cable and to each phase in the cable you inject a signal, and that signal will reflect off the partial discharge site and down the cable."

"We are working on techniques to detect the discharge activity with the cable energised and that will be possible in the near future."

Other important tests to carry out on cables include corrosion tests to reduce the possibility of water-ingress and water-treeing problems.

When testing aged cable systems, using diagnostic methods such as dissipation factor to detect the presence of water in joints or terminations, Baur is quick to point out that the applied voltage level should not reach the higher limits specified by the manufacturer, as this unnecessarily stresses the equipment and could lead to breakdown in the insulation.

PD testing is also the main technique used for assessing the condition of indoor switchgear, but these tests are non-intrusive.

The wide range of instrumentation available includes portable locators, which immediately detect continuous PD activity, or monitors, which measure intermittent activity over time.

"There is a range of equipment you can use, depending on whether you want to detect continuous or intermittent activity," says Roberts.

"When a client wants to undertake some tests when he goes into the substation he can use equipment to give him an indication on what is happening there and then, but he may not get the full picture."

"For benchmarking purposes, you would usually monitor the switchgear for around one week and that enables the load on the network to go through its normal cycle, as the load comes on and drops off during a normal 24-hour period and certain industrial loads that come on for a day or two.

For switchgear that is showing some signs of distress we can leave a monitor in continuously."
EA Technology supplies a variety of devices that measure PD activity by monitoring ultrasonic emissions and transient earth voltage (TEV) effects.

Ultrasonic noise is characteristic of internal and surface arcing, while TEV effects are small discharges, which occur when insulating materials breakdown.

DEWA has purchased a number of instruments from EA Technology, including the UltraTEV Detector, to enable its staff to carry out PD surveys on switchgear. The UltraTEV Detector won an innovation award last year and is the firm's best-selling product.

The hand-held device uses a traffic light LED system to indicate whether assets are working satisfactorily or whether they require further tests or urgent investigation. It combines ultrasonic and electromagnetic technologies.

Asset confidence

Results from oil analysis or PD activity tests allow power plant operators to judge whether a piece of equipment is safe to remain in service or whether remedial work needs to be undertaken.

Condition monitoring also allows engineers to target maintenance resources on equipment, which is showing signs of vulnerability and is most at risk of failure, thereby averting potential outages or faults.

Similarly, it allows scheduled invasive maintenance to be confidently delayed for operational or financial reasons if tests have shown no deterioration in condition.

To be most effective, though, the results should be considered in conjunction with so-called knowledge rules on how a specific piece of equipment typically performs overtime.

This is known as condition-based risk management, which involves studying the condition of an asset and determining how it will degrade and age in the future, by factoring in known specific risks associated with the asset in order to determine end of life.

Condition-based risk management enables power firms to make better decisions on network investments, in addition to scheduling asset maintenance and replacement more effectively, and many equipment suppliers can provide this service.

Investments in new capacity to satisfy the Middle East's rapidly growing hunger for power are underway, but plants cannot be built over night. Until sufficient new facilities come on stream, existing plants will have to continue to bear the heavier demands put upon them.

Increased stress causes equipment to age faster and raises the risk of breakdown, so the region's energy providers need to channel efforts into prolonging the service life of existing facilities as well, through regularly testing and monitoring critical components.

In this way, extended outages and catastrophic failures should be more avoidable.

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