Infrared Thermal Imaging Inspection of Electrical Transmission and Distribution Power Lines.
Electric Utility Power Systems e,g. BC Hydro, Canada consist of the three major components such as the power generation, power transmission and power distribution. Utility companies do a lot planned maintenance, various repairs and reconstruction works to provide customers with the uninterruptible power supply. One of many problems of the electrical systems, including the transmission and power lines could be a bad connection or corrosion. It increases resistance and the current flow produces excessive temperature which usually might be the cause of unplanned shutdowns. Therefore, utility companies do preventive maintenance inspections and other type of the system condition assessment methods. Nowadays, the Infrared Thermal Imaging Technology is a main part of the preventive inspections.
Let’s consider the advantages and disadvantages of the Infrared technology applied for Transmission and Distribution (T&D) power lines (outdoor inspection) inspections.
Benefits of Infrared Thermal Imaging Inspections:
- Non-contact, non-destructive, quick scan and finding problems.
- IR camera scans hot spots, visualizes, analyzes t ° of an object& its t ° rise.
- Can locate, identify the level of the problem, and establish the time frame in which the equipment should be repaired.
- Reduced equipment downtime; reduced cost of maintenance.
Excellent tool for predictive maintenance inspections.
Possible Disadvantages of Infrared Surveys. Generally, possible inconvenience or disadvantages of IR surveys might be:
- Load factors.
- Weather/atmospheric factors.
- Emissivity factors
Corona effect or discharge survey.
- Wind cooling effect, no IR survey is recommended if the wind speed is above 8m/s.
- Rain, especially heavy rain can changes the surface temperature of the inspected object.
- Sunlight: Solar load, heating and reflections.
Note: Some latest Flir Cameras may not be affected by atmospheric conditions.
- You cannot measure the line/phase actual loads during the inspection unlike the indoor electrical equipment. Before inspection, the T&D lines load should be agreed with the client as load can be changed any time.
- During the inspection period, the line load should be at least 40-45% of the nominal rate.
- The optimal load could be between the 70 and 75% of the nominal rate.
- If the load is more than 90% of the nominal rate, it can be difficult to spot overheated places as the line could be hot.
Client should provide the line load data recorded frequently during the inspection.
The line items to inspect are typically: Conductors, Insulators, Clips, various sleeve connectors and etc.Emissivity factor (EF) of the listed above items are different and could be distorted due to wear, damage, corrosion and etc. For example:
- New conductors (EF) might be around ε=0.23-0.25, while old and polluted could be (EF) ε=0.95
- New joints (EF) might be from ε=0.25 to 0.3
- Greasy black overheated parts of conductors and joints (EF) -0.97.
The emissivity table of power line components is varied from the one source to another.
Infrared inspection of Corona discharges.
Corona effect (CE) is a shining discharge from conductors and insulators due to ionization of the air, where the electrical field exceeds a critical value.
- CE emits Ultraviolet (UV) radiation, a little heat and sounds and ultrasounds.
- Depending on the wind speed, direction and the corona intensity IR camera may or may not detect the anomaly.
IR camera is not an ideal tool to scan the corona discharges since it based on the infrared radiation, while corona emits Ultraviolet (UV) radiation on the different range and a little heat.
Some recommendations for Infrared Inspection of T& D Power Lines:
- Use a proper, high resolution Infrared camera, set its parameters correctly.
- Identify the lines components to be inspected, find out their realistic emissivity in advance and create own table to use that during the inspection.
- Measure the distance, outside &reflected temperature, air humidity and wind speed.
- Do IR surveys before sunrise, after sunset and clouded days or use the latest Flir cameras.
- Take IR pictures at different angles to avoid light reflectance
- Follow the Standard for Infrared Inspection of Electrical Systems and Safety rules.
- Use the nominal/actual load rates data and wind correction factors for analyzing and reporting.
Use the Electrical Systems Absolute Temperature Criteria formula for analyzing.
Possible Solutions for T& D Power Lines Assessments. Corona Effect (CE) Survey Options:
- Ultrasound Technology that translates CE ultrasounds into the audible range.
A Corona camera (for example, DayCor® Technology cameras) is non-destructive testing technology used in day time, can block sunlight, receive Corona UV signals only and visualize that. There are several of portable, vehicle mounted and airborne cameras options.
Conclusion: The combination of IR and Corona Cameras might be the best solution for Power Lines Assessments.
- IR camera can detect equipment bad connections, hot spots, Tem. rise -ΔT and visualize.
Corona camera can detect corona discharges and visualize.
Rick Turdubay, Level II Certified Infrared Thermographer, BS in Electrical Engineering.
Read more about our Infrared Thermal Imaging services HERE.