HV transmission lines inevitably pass through mountainous areas where forest and brush fires are a constant threat. This has made wildfires an important factor in line trips and outages. Occurrence of wildfires is both regional and seasonal. During dry months, mountains and hilly areas are only sparsely populated with vegetation and at greatest risk. Causes of wildfires include human activity as well as natural factors such as lightning. Another cause, not always appreciated, is sunlight reflecting off broken glass scattered on the ground. During certain times of year, mountain fires will become one the dominant hazards affecting the safe operation of overhead lines.
In most of China, for example, spring is the dry season where mountainous areas are especially prone to wildfire. In the first quarter of one recent year, wildfires accounted for nearly half of all line fault trips on transmission lines of 220 kV and higher within the system operated by the China Southern Power Grid. That year, transmission lines in one region alone, i.e. the Kunming Power Supply Bureau in Yunnan Province, had to contend with 66 different fires, which together lead to 15 trips on lines from 110 kV to 500 kV. During this same period, emergency outages had to be taken on 20 lines. Similarly, a few years ago, mountain fires led to 66 trips on lines of 110 kV and higher within the China Southern Power Grid and accounted for 2.3% of total line trips that year. The situation was made worse by the fact that there is typically a comparatively low success rate in line reclosing in the case of wildfires, which can also cause significant damage to transmission structures and equipment. This makes it even more difficult to restore power, thereby affecting the safe and steady operation of the grid.
To better understand the mechanisms by which wildfires can affect external insulation as well as air gap discharges on HV lines, research institutes in countries such as the U.S., Canada, Brazil and Mexico have performed extensive simulations and other studies. In China, such work has been carried out at universities including Tsinghua University, Wuhan University, and the University of Science & Technology of China. For example, several years ago the Graduate School at the Shenzhen Campus of Tsinghua University co-operated with the China Southern Power Grid in research at its Kunming Test Base. There, testing related to issues of how mountain fires impact the characteristics of air gap discharges. Findings from such tests and simulations performed by research institutes in China and abroad have been essentially the same. The fire sources being simulated included timber, tree branches, sugar cane as well as piled timber with the addition of other flammable materials to make it burn similar to natural vegetation. Related laboratory testing has used oil as the main fuel.
In regard to discharge gap, a variety of electrode types have been used, including rod-plane, rod-rod, line-plane, etc. Test line sections have also been used during such research, with fires set directly below the conductor to best study their impact on discharges of the conductor gap. Because the air gap types adopted by different research institutes are often different as are the fuels used to simulate wildfire, test results are also quite different. Nevertheless, all findings are consistent in that they demonstrate that mountain fires can have great impact on withstand/discharge voltage of the air gap. In contrast to normal operating conditions, discharge voltage drops significantly with the most dangerous case being that the flames become so big that they run directly through the air gap.
Additional research has been done on the specific mechanisms by which mountain fires impact air gap discharge. Initial results showed that factors such as high flame temperature, increase in electrons and ion concentration from the flames, influence of ash and smoke dust on electric field, etc. can all lead to a decrease in air gap discharge voltage. These are all issues worthy of further research.
Because many transmission lines run through areas where human presence is sporadic, it is important to monitor them closely so as to quickly raise the alarm in the event of wildfire. In China, for example, utilities have taken measures to this effect, such as real-time monitoring using cameras installed in those areas prone to fire. They also conduct inspections of vulnerable line sections using helicopters or unmanned aerial vehicles. In addition to emergency measures to extinguish fires quickly in the event of outbreak, it is also necessary to know how best to respond to wildfires near transmission lines. For example, special emergency measures can be taken such as decreasing voltage on affected DC lines or, in extreme cases, taking an outage on AC lines.
Professor Guan Zhicheng
(retired) Tsinghua Universty, Shenzhen Campus