Assessing condition of overhead transmission conductors

Inspection: See to foresee

Unexpected conductors’ failure could lead to safety hazards, loss of supply and possible environmental damages and capital losses. Overhead conductors must withstand daily and extreme outstanding electrical and mechanical loading events and should maintain sufficient clearances to avoid phase to ground or phase-to-phase flashovers. Alongside the overhead transmission lines, the overhead ground wires shield the phase conductors from lightning strokes. They must also resist daily and extreme loading events without failure and must maintain sufficient clearance from the phase conductors that they shield. The condition of thousands of kilometers of overhead line conductors and ground wires is a critical reliability factor and must be routinely inspected, preferably by helicopters and drones, to assess their condition, plan preventive maintenance, and minimize the risk of damage caused by failure.

Aging and degradation

After a long period of service conductors age and their material properties degrade. Material degradation deteriorates their mechanical and electrical strengths and increases the probability of failure, compared to unaged conductors. The Australian study found that 30% of conductors’ failures were attributed to corrosion, 19% to splice/joint failures, 14% to fretting fatigue, 9% to ties and 8% to electrical connection failure as seen in the below chart. To minimize the risk of damage caused by conductors’ failure, utilities normally replace conductors that may have reached the end of their service life.

Percentage of different causes of failure [1].

The predictive maintenance methodology

Determining the remaining service life of conductors has significant economic consequences and can be complicated. Comprehensive maintenance strategies have been adopted to reduce them, such as predictive and preventive maintenance. The predictive methodology relies on three legs:

• Practical documented gathered experience

• Information on the current asset condition, and

• Technical knowledge

Practical gathered experience is a collection of inspection reports and tagged video clips and photos taken by complementary technologies that together provide an encompassing portrait of all existing faults. Media is valuable for evidence, reference and comparison. Media can be dissected at the office by professionals revealing data that has been overlooked during inspection. Information on the current condition derives from the latest inspection reports. Technical knowledge is necessary for analyzing, trending, decision making and predicting end of service life. Technical knowledge helps classify criticality and helps management prioritize actions.

Why aerial inspection

The inspection technologies deployed can include combinations of ultraviolet (UV) HD cameras to detect voltage related faults that discharge corona partial discharge (PD) and arcing; visual HD cameras to detect physical damage and corrosion; and thermal imaging to detect current related faults that create hot spots. Detection is always localized and cannot provide an automatic preview of a whole circuit, which may span over 100 km, hence the need for aerial inspections. Aerial inspections use helicopters with stabilized gimbals loaded with combinations of HD sensors and laser range finders. Helicopters can scan hundreds of kilometers per day and collect concurrently detailed information by multiple sensors. Unmanned aerial vehicles (UAVs) are also used effectively to inspect overhead circuits, although they are confined to certain distances and duration. Still, drones are more efficient than the alternative foot patrol. Today drones can carry combinations of compact UV, infrared (IR) and visual cameras, as for example DJI m300 with Ofil’s micROM HD corona camera and an IR senor in a vertical arrangement.

Left: Stabilized gimbal with 5 Sensor; Middle: DJI m300 carrying a DayCor® camera and IR camera. Right: Foot patrol handheld camera and binoculars.

Visual inspection

Visual inspection is the most commonly used form of inspection for all transmission line problems. It is a subjective assessment of the conductor or overhead ground wire condition. High-definition video cameras with outstanding resolution provide clear view of existing

• corrosion

• broken outer strands

• surface discoloration possible from internal corrosion

• flashed surface

• excessive sag and

• overgrown vegetation.

Normally, this visual inspection takes place from the ground or from a helicopter. In any case, the only conductor degradation that can be detected is that which is visible on the outside of the conductor or overhead ground wire. Degradation that occurs internal to the conductor or within the confines of attachments to the conductors cannot be easily detected during a routine visual inspection. In cases of very severe internal corrosion, corrosion byproducts may become visible on the conductor surface. However, unless the defect has become severe, detection of such a defect through visual inspection is difficult. Similarly, broken strands may not be detectable until the defect becomes severe. Defects such as high resistance joints cannot be detected using visual inspections.

Top: A sample of corrosion on an overhead ground wire. The internal strand is exposed in the photograph to show that the corrosion is primarily on the exterior surfaces of the wire. Source: EPRI Bottom: Exposed steel core strands from two identical conductor samples, one sample showing corrosion on the steel core and the other showing a corrosion-free steel core. The outer surface of the aluminum strands looks the same for both conductors, which is an indication of the difficulty in identifying steel core corrosion by field observation. A laboratory sample testing is usually performed. Source: EPRI

Melted conductor at the joint splice. The melted wires decrease the contact resistance and therefore this failure cannot be detected by IR camera. Source: VNL Veiki [4]

Hot spots detection by IR cameras

Thermal imaging with an infrared camera is commonly used to detect conductor joints, midspan joint spirals that join two ends of conductors. These spirals give mechanical strength and conduct current. When damaged, a high contact resistance develops due to oxidized surface or incorrect mounting and result in elevated temperature and IR signature. This high resistance is normally an indication that the joint is deteriorating and will eventually experience a mechanical failure. Yet, upon further deterioration these joints can melt, and their resistance is reduced as well as their mechanical strength. Then only a UV camera can detect the fault due to local UV discharge.

In theory thermal imaging can be used to detect any conductor degradation that yields an increase in conductor resistance such as broken conductor strands as well as bad joints. However, it has generally been found by EPRI that field application of thermal imaging to detect broken conductor strands outside of joints is not effective without extensive training in IR thermography and great care in its field application.

Failed spiral midspan joint removed from power lines. Source: VNL Veiki [4]

Left: Improper installation. Right: Oxidation of surface — corrosion. Source: VNL Veiki [4]

Corona PD detection with UV cameras

High-frequency PDs have been identified in the literature as the main symptom of degraded overhead transmission lines of medium to high voltage. Corona PD is both an indication of faults and an active degrading factor. One of the best and beneficial outcomes of detecting corona is the fact that corona is emitted during early stages of faults when they can still be repaired. Corona PD is generated due to excessive, usually unplanned, electric field stress. The phenomenon takes place in air on the surface of energized conductors where ionizations processes take place forming a local conductive region not high enough to cause a complete electrical breakdown. Corona discharge is usually formed in irregular surfaces, highly curved regions, small diameter wires, sharp corners, projecting points, or in edges of metal surfaces. It generates audible noise and radio-frequency interferences, abrasive acid compound such as ozone and is undesirable to human health. The amount of heat dissipated by corona is negligible.

Corona discharge is always accompanied with losses:

• Energy loss

• Material loss

• Quality loss

• Properties loss

• Reliability loss

Energy loss is the outcome of electrical discharge to air, material loss is the outcome of corrosive processes from acids compounds created during ionization that affect the mechanical and electrical properties of conductors. Quality loss refers to the reduced functionality of components subjected to corona. Properties loss is, for example, loss of dielectric and hydrophobicity properties. Reliability loss is incurred by defects such as conductors’ broken strands, surface pitting and roughness, bird caging, broken splice joint, loose line dampers and other factors that lead to corona discharge. Aging processes are accompanied by nuisance to the environment as audio noise and radio interference. The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line, which may cause inductive interference with neighboring communication lines.

Source: VNL Veiki [4]

Corona cameras detect during daytime invisible UV radiation (in the solar blind range 240 nm to 280 nm) and can pinpoint the emitting sources, leading inspectors directly, in real time and at no time, to the suspected faulty components. These digital cameras such as Ofil’s DayCor are based on UV core technology. They take advantage of the fact that during ionization processes UV photons are being discharged. But the discharge is local and faint, and therefore these cameras deploy image intensifiers to enable remote detection of the discharge and their sensitivity to UV is critical. The DayCor cameras, which are commonly used by utilities worldwide for inspection of overhead transmission lines can detect broken and protruding stands, bird caging, surface scarring, loose splice/joint, loose ties, dirt, unfit conductors’ radii, broken dampers, bad design, improper installations and more.

Detected by UV cameras: Middle: Conductor damaged by lightning flash. Right: Bird caging. Source: VNL Veiki [4]

EPRI field guide to corona detection, referring to raised stands and bird caging of conductors, detected with DayCor II corona camera. [5]

Overhead conductor failure modes

Overhead conductor failure modes, such as aging, broken conductors, excessive sagging, broken shield wires and excessive shield wire sag, are affected by environmental conditions. Environmental conditions accelerate corrosive processes that lead to loss of reduced mechanical and electrical strength. Winds and wind gust might lead to phase-to-phase clashing, fatigue and broken stands. Lightning can flash conductors and lead to annealing, and moisture and heat can encourage growth of moth on the conductors’ surface. Water ingress can create water trees in the conductors; ice can affect the conductors’ tensile properties. Overheating can affect sagging and more. Additional failure modes originate from vandalism, frequent reclosers, vegetation and animals. It is the cumulative effects of degradation mechanisms that lead to what is called conductor aging.

Conclusions

Any comprehensive investigation on conductors’ condition, investigating their failure modes and degradation mechanisms depend on testing and inspecting. The key parameters to extend conductor lifetime, reduce maintenance cost of replacing, refurbishing and incurred revenue loss, is asset management that relies on predictive maintenance. Aerial inspection, seemingly costly, is the most cost-effective method to cover long-distance inspection in less time with combinations of remote sensing technologies. The available technologies are highly sensitive and smart and can be applied to determine the stage of degradation of conductors and overhead ground wires. The quality of data collected depends on the selected sensors and on the stabilizing airborne platform. Manufacturers such as Ofil can advise on the recommended platforms and sensors.

[1] Overhead Conductor Condition Monitoring Milestone Report 1 Prepared by Dr Lakshitha Naranpanawe, Dr. Hui Ma and Professor Tapan Saha, Power & Energy Systems Research Group School of Information Technology & Electrical Engineering, The University of Queensland Brisbane, Queensland, December 2018

[2] Havard, D.G., Bissada, M.K., Fajardo, C.G., Horrocks, Meale, J.R., Motlis, J, Tabatabai, M., and Yoshiki Gravelsins, K.S., “Aged ACSR Conductors: Part II – Prediction of Remaining Life”, IEEE Transactions on Power Delivery, Vol. 7, No. 2, April 1992, IEEE, New York, NY.

[3] Parameters that Influence the Aging and Degradation of Overhead Conductors, EPRI, Palo Alto, CA: 2003. 1001997.

[4] Dr. Otto Gunter, Dr. Laszlo Varga, “Laboratory and Field Test With UV and IR Camera for Determination The Reason of Damage of Overhead Line Clamps and Conductors”, UGM 2005, USA

[5] Guide to Corona and Arcing Inspection of Overhead Transmission Lines, EPRI, Palo Alto, CA: 2001. 1001910.