Robotics redefine transformer inspection3 March 2019
Transformers are a key part of the power infrastructure but many of them are very old, the average age of US transformers is for example estimated to be about 40 years. Robotic inspection of oil-filled power transformers has potential for utilities and industrial operators to enhance safety and reduce the duration of outages at the same time as improving collaboration with remote experts and decision-makers. By Jamie Stapleton Digital Leader – Transformers, ABB
There have been great advances in recent years in the non-invasive condition monitoring of oil-filled transformers. While these digital solutions are extremely helpful for identifying possible faults and avoiding failures, nothing beats a visual inspection when an operator needs to know the exact location or extent of a fault or check a critical transformer’s condition after a lightning strike, for example.
However, physical transformer inspection can require an outage of three or more days for a large unit, as well as a team of specialists. In preparation, the mineral oil inside the transformer must be drained away and stored. Then, once the transformer has cooled down and is safe for a technician to enter, a confined space team with emergency medical training needs to be on hand to support the technician while he or she enters the oily, narrow and delicate interior to capture photography and inspect manually. The transformer must then be refilled before it can re-start service.
Recognising that most operators want to maximise asset availability by minimising the duration of outages, ABB has developed an alternative technique using a free-swimming submersible robot, called TXplore. There is no need to drain down the oil or send a technician into the oily, sensitive and hazardous interior and the approach also eliminates any potential mishaps during oil handling.
As a result, the service helps operators to control risk of injury, damage to the transformer, impact on the environment and maximise the availability of critical assets.
Two technicians and one day
A team of two technicians can deliver the service during an outage lasting less than a day.
One technician releases the robot into the transformer through an access hatch on the topside. The second technician then takes over as pilot and guides the robot using the type of controller used in computer gaming. Control signals and data communications are carried between the controller and the robot via a wireless link.
Bright LED lighting and multiple on-board cameras capture clear images and video locally. The photography and video can also be shared in near real-time with utility engineers and transformer experts anywhere in the world. As a result, the operators can tap into the knowledge of leading transformer experts and keep asset owners informed. At the same time, the asset operators can ask operators to take a closer look at components such as bushings, windings, leads, tap changer, core top, core support and insulation.
Accessing hard-to-reach areas
Space is tight inside transformers, so the robot measures only 18 x 20 x 24 cm to ensure that it can go anywhere inside the transformer where a person could see – and therefore match the performance.
It has a smooth casing and all-round visibility via feeds from its cameras to avoid catching on internal components and a wireless data link means there is no risk of an umbilical becoming tangled.
The robot can be adapted for the many different types of oil that have been used over the years to insulate and cool the interior of transformers. Its buoyancy can be adjusted for use in mineral oil, ester-based or silicone fluids. Transformer oil typically becomes darker in colour over the years as moisture, acidic compounds and sludges build up and particles become suspended. Therefore, to pierce through even the darkest oil, the robot is fitted with bright LED lights and this enables the multiple on-board cameras to pick up detailed images.
Some transformers that were installed before the late 1980s may include PCBs (polychlorinated biphyenyls), toxic substances that are strictly controlled.
Their presence is identified by testing of a pre-inspection oil sample, which also determines the correct buoyancy setting. If PCBs are present, then the inspection team makes arrangements for handling of hazardous products and licenced disposal of cleaning materials and any packaging or other materials that come into contact with oil containing PCBs.
Protecting oil integrity
Another important consideration for traditional or robotic transformer inspections is maintaining the quality of the oil. Changes to transformer oil can affect its dielectric strength, which is the measure of its insulating capability, and in turn can affect the performance and health of the unit. As a result, it is vital to ensure that any objects that come into contact with the oil do not introduce contamination or react with the oil in any way.
Recognising this, the designers of the TXplore robot carried out extensive validation and testing of the robot, its materials and subsystems. Its outer shell is based on a high-performance polymer that minimises the likelihood of electrical coupling, chemical reaction or structural damage while protecting interior components from high temperatures and pressures.
Testing of prototypes was carried out under various temperature and pressure conditions over 96 hours to demonstrate leakage protection. In addition, spatial and depth navigation was tested thoroughly in seven different types of oil to check for stability of the robot itself and also of the images captured.
The robot’s post-inspection cleaning was even tested after being used in heavily contaminated oil from a field transformer. After cleaning, the prototype was immersed in fresh oil for an extended period. Testing of oil samples showed no contamination of the new oil, demonstrating that the cleaning procedures established as part of the service will protect the transformers where the service is used.
The team also addressed another potential side effect experienced with some submersible remotely operated vehicles (ROVs), where bubbles form on propellers and propeller shrouds due to changes in pressure causing cavitation. Extensive stroboscopic testing of the propeller was carried out, with a particular focus on areas that are prone to cavitation on other ROVs, such as the leading edge of the propeller blade or the gap between the propeller and the shroud. Testing at all speeds in different fluids showed that no gas bubbles formed.
Further testing evaluated the quality and accuracy of photographs and video images under various conditions and temperatures, as well as the data connectivity for remote experts. Redundant radio systems in the robot and the local control console ensure that navigation can continue even when communication is impaired.
Finally, a series of test inspections was carried out, with the TXplore operators able to reduce inspection time to less than two hours. For the asset owner/operator this represents a total outage of less than one day.
The service successfully captured clear images, which were shared in real time and in a post-inspection report.
Inspections already carried out
The very first TXplore inspections have already been completed successfully, in the USA. ABB demonstrated the robot at its own power transformer factory in St Louis, as well as on a transformer at a critical substation serving one of its own high-voltage testing facilities.
A further demonstration in a test tank gave a North American utility confidence to proceed with a pilot test on a transformer that was more than 50 years old. This project provided verification of the robot’s mobility, handling, visualisation capabilities and potential to collaborate with local and remote experts. It also confirmed that oil handling and draining down is not essential to gaining insight into a transformer’s condition.
Since then, service teams in several countries, including the UK, have taken delivery of their own service robots and have received in-depth training on the commercial and technical aspects of service delivery. This ensures consistency at every step from taking and testing pre-inspection samples, through the launch and piloting of the robot on the day of the inspection, as well as data transmission and collaboration, to post- inspection cleaning and delivery of reporting.
The future is safe and digital
For operators of power transformers, robotic inspection has the potential to grow in importance. The robot’s system architecture has been designed so that new functions can be added over time as the technology develops and demand from end users grows.
In addition, the digital record of the inspection can be used as a baseline for future inspections. Future inspections can follow an identical path inside the transformer and capture images from the same angles with the same lighting. In turn, this will give an accurate picture and straightforward comparison for transformer experts to measure the effects of time on critical assets.
For the present, the main benefit to utility and industrial operators is that robotic inspection of transformers helps them to minimise both risk to personnel and the environment, and return transformers to service with minimal disruption. It can also help them control the complexity and human cost of transformer inspection.