The requirement for more reliable and cost effective power networks continues, with downtime measured (and penalized) in minutes as well as seconds. Over the past 20 years, there has been increasing realization that the uncontrollable external impact of wildlife is responsible for a far greater proportion of unplanned outages than previously acknowledged. As a result, wildlife and asset protection has become an area of growing interest given that reducing unplanned outages equates to improved efficiency. Consider, for example:
• The cost of even a single outage can prove extremely high;
• Potential fines for not adequately protecting protected wildlife are also increasing;
• Estimates of wildlife outages/year are in the tens of thousands;
• The global cost of wildlife induced outages is estimated at over US$10 bln each year;
• 50% of wildlife-induced outages are caused by birds.
Along with better understanding of these challenges has come added pressure from environmental bodies and these concerns now mesh with those of electricity suppliers where, for them, reliability is the critical factor.
This article, based on a contribution by Don Barrett of TE Connectivity, deals with applications where wildlife can interact with electrical networks. These problems are better defined and potential solutions are provided that employ modern materials and design to enhance reliability while also protecting wildlife.
Every power provider has suffered periodic disruptions from unplanned outages of one type or other. These have traditionally been classified as Weather, Wildlife or Unknown, with limited knowledge of the proportions due to each. Now, with greater knowledge, such unplanned outages are better categorized as due to:
1. Wildlife (birds & animals)
2. Weather (storms, wind, lightning)
3. Vegetation (tree branches)
4. Human intervention (accidental or deliberate)
5. Unknown or not yet categorized
Broadly speaking, problems created by wildlife at substations and on overhead lines fall into one of two main categories: bridging and wildlife guano pollution flashover. While the result – a system trip and possible arc flashover – may be the same, the way it was brought about and the best solution differs in each case. So do the remedial products that can be applied to the equipment being compromised.
Bridging is where a bird or animal makes contact between phases or between phase and ground, leading to a short circuit. For example, large birds can easily cause problems across all distribution voltages at substations and on lines. Phase bridging usually results in the creature being electrocuted and how it falls will determine whether the auto-reclosing system operates successfully or not. Often the bird or animal will fall away and allow the auto-recloser to re-energize the circuit. Although this may not normally require investigation by emergency crews, it usually leaves unnoticed but tell-tale evidence of damage.
Although the circuit may operate normally, a series of such events can lead to progressive damage that becomes worse with each successive trip and that finally causes hardware failure, e.g. where each trip burns out a single strand of conductor eventually dropping the line. Moreover, if the dead creature falls between its points of contact and remains there, this will result in the circuit not being able to be re-energized until an emergency crew removes the fault source. At least, this type of event can be logged with some certainty of what happened. By contrast, when the bird or animal falls to the ground or on top of a transformer, it is likely to be removed by a predator and no evidence will remain.
Most bridging problems occur on MV systems (typically < 36 kV) designed such that bare conductors, bushings, busbar systems and other equipment have ‘air-spaced’ clearances that operate without issue under normal conditions. Clearances are typically up to 40 cm at substations and about 1m for overhead lines to allow for swinging conductors. Unfortunately, these clearances mean that a wide range of birds and animals can cause problems at substations and on lines. It should be noted that, although line conductors are generally 1 m apart, the most common failure mode is between the cross-arm (usually grounded) and the conductor – a typical clearance of only 30 to 40 cm.
Medium and large birds perch on a cross-arm and bridging usually occurs during landing or take-off. In more unusual cases, depending on location, wildlife such as bears, possums or even snakes climb poles and cause similar problems.