Gas leaks are often invisible to the naked eye and can occur at any point in a pipeline or other infrastructure. Depending on the gas in question, a leak can involve anything from a loss of profit to an extreme danger to the safety of people and the health of the environment. Since a gas leak in an unexpected location can be difficult or nearly impossible to detect with tools such as gas sniffers (sniffers), many utilities are turning to optical gas imaging (OGI) to better detect leaks.
One utility company that has adopted the use of optical gas imaging technology is ČEPS a.s., based in Prague, Czech Republic. It is the only Czech transmission system operator and is responsible for the maintenance and upgrading of 39 substations with 68 transformers. Almost all circuit breakers, current transformers, voltage transformers and gas-insulated substations in the distribution network use sulfur hexafluoride (SF6 gas) as an insulator.
SF6 gas is the most widely used insulator in high-voltage equipment around the world. However, it is also a very potent greenhouse gas, so any leakage in equipment can jeopardize the continuity of power distribution and have consequences for the environment. To limit these negative impacts, utilities can detect SF6 leaks with an optical gas imaging camera such as the FLIR G306.
"Detecting SF6 leaks at an early stage helps to avoid breakdowns and ensures continuity in power distribution," explains Milan Sedláček, head of the High Voltage Maintenance Department at ČEPS a.s.
"We tested an optical gas imaging camera on a current transformer that was leaking SF6 and immediately realized its potential," says Sedláček. "We had tried unsuccessfully to detect the leak by other means, but with the optical gas imaging camera we managed to find out where the problem was. After this surprisingly successful demonstration, we bought the camera and have not regretted that decision. It has proven to be very effective even after only a few months of use."
GAS SNIFFERS VERSUS OGI CHAMBERS
Since SF6 is colorless, odorless and non-flammable, it is almost impossible to detect with the naked eye. A widely used tool for detecting this invisible gas is the so-called "sniffer", a device that measures the concentration of a given gas in a single location and generates a reading of the concentration in parts per million (ppm).
While these tools are useful, their application is limited, according to Sedláček. "A sniffer only detects gas leaks at one point. This means that it is very easy to miss a leak. The current transformer we used to test the optical gas detection camera is a good example. We knew it was leaking, as we needed to refill it with SF6 about every six to eight months, but we could not locate the leak with the sniffers. With the camera we were able to locate the leak very quickly," he explains.
SF6 leakage in electrical equipment can occur due to installation errors, alterations during planned maintenance or failure of sealing parts due to aging. The most common leak paths in electrical distribution equipment are flanges, bushings, rupture discs and valve stems.
"As long as leaks are confined to places where they are expected to occur, sniffers can be very useful, but leaks often occur in unexpected places," Sedláček explains. "In the case of the current transformer we used to test the optical gas imaging camera, the leak occurred in the current transformer head material, not in a place where two parts were joined, where a leak would be suspected," Sedláček continues. "We would never have been able to find the leak location with a leak detector. Sniffers only provide a spot measurement, while the optical gas imaging camera provides an overview of the entire current transformer or other piece of equipment being inspected."
ADVANTAGES OF OGI
According to Sedláček, the main advantage of optical gas imaging over other methods is the detection range. "With a sniffer you have to be within a few millimeters of the leak site to detect the SF6 leak, but we have found that the optical gas imaging camera can detect small leaks at six meters, so it can be used safely while the equipment is under load. This means no downtime is required for the inspection, which is a big advantage for us."
Another advantage is faster inspection speed. "Using sniffers to detect gas leaks takes a lot of time. You have to physically hold the sensor next to every possible leak site," explains Sedláček. "With the optical gas imaging camera, you can scan an entire piece of equipment at once. And because it has similar dimensions and weight to a normal video camera, it is easy to use in the field."
A special camera feature that Sedláček often uses is the High Sensitivity Mode (HSM): an image subtraction video processing technique that effectively improves the thermal sensitivity of the camera. HSM subtracts a percentage of the individual pixel signals of the video stream's frames from the subsequent frames. This enhances the differences between frames, making leaks stand out more clearly in the resulting video.
According to Sedláček, the decision to invest in an optical gas imaging camera was a wise one. "It allows us to find and therefore repair gas leaks quickly, which saves money that would otherwise be spent on refilling with SF6, and does so without the need for downtime while we find the leaks," he says.
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