Cities and government agencies sometimes develop community facilities—parks, playgrounds, bleachers, trails, bike paths, community gardens and more—in close proximity to transmission lines. Real estate developers are often interested in erecting buildings, parking lots or similar public spaces near or within existing transmission line rights of way. Then, there are property owners who want to “enlarge” their backyard to make room for a shed, swing, garden or fence. Others have facilities that are grandfathered in, but haven’t necessarily been checked for safety since they were built.
This blog post covers the main issues that can arise when metal structures are located in the vicinity of high-voltage power lines.
Evolution in safety practices
In North America, guides such as the IEEE Std. 80 – Guide for Safety in AC Substation Grounding establish best practices for ensuring that people in and around substations are not exposed to dangerous potentials under fault conditions. The Canadian Electrical Code (CEC) provides general guidelines to ensure that distribution class systems are specified and installed in a manner that is safe for the public. Unfortunately, it is hard to define general safety criteria for high-voltage power lines because the physical and electrical parameters can vary widely, not to mention the fact that the risk of an incident occurring when someone is present is usually very low. As such, the safety criteria for transmission class systems are generally left up to individual utilities, in addition to leaving them to deal with consumer requests or complaints.
In 2017, CIGRE B2.56, an international working group, published Ground Potential Rise at Overhead AC Transmission Line Structures During Frequency Faults. This technical document recommends actions based on risk acceptability levels, which is different from the deterministic approach used in the IEEE 80 guide. In North America, the IEEE P2467 working group is currently preparing a guide entitled Evaluating AC Interference on Linear Facilities Co-Located Near Transmission Lines. In addition to increasing vigilance among the various stakeholders, these recent initiatives will help to promote best practices in this field.
Main AC interference phenomena
The electrical voltage of power line conductors creates an electric field (measured in kV/m). The electric field strength (or gradient) decreases rapidly as distance increases from the phase conductor. Overhead lines are specially designed to limit the amount of partial discharge caused by the electric field at the conductor surface. This limits undesirable effects such as crackling on high-voltage power lines, which is a common occurrence on humid days. At ground level, high-voltage lines typically generate larger electric fields due to asymmetrical phase geometry. As a result, any close-by structures that are not grounded may be charged through capacitive coupling. This energy is then discharged when someone touches the structure. For example, under certain conditions, some people may feel an unpleasant discharge when touching a vehicle parked directly under a high-voltage power line. Since transmission line voltage is relatively constant, the electric field is mostly constant for a given line geometry. However, nearby above-ground structures can modify the electric field’s profile and effects.
Current flowing in the line conductors creates a magnetic field (measured in Tesla or Gauss), which also decreases rapidly as distance increases from the phase conductor. If a metal structure runs parallel with the line, the magnetic field can induce a current in it through inductive coupling. Unlike capacitive coupling, inductive coupling can induce a current in structures located at or below ground level (e.g., railway tracks or pipelines). The induced current and corresponding voltage can generate a number of undesirable effects, including corrosion, touch voltage hazards and interferences with electrical equipment. The current in a transmission line depends on daily and seasonal load changes, which continuously modulates the magnetic field and induction effects. For example, the current induced in a railway can sometimes develop a voltage high enough to interfere with the signal control systems.
If a ground fault occurs on a high-voltage power line, the current must return to the source. Ground fault current can travel by conduction through parallel metal structures and ground electrodes in different proportions, depending on the type and location of the fault. Current flowing in a ground electrode creates a potential gradient in the earth around the electrode. The magnitude of this gradient in relation to a distant point on earth is known as the ground potential rise (GPR). People standing near the electrode will experience a difference in potential between their feet (step voltage). Similarly, a person touching a metal structure connected to the ground electrode will experience a potential difference between their hands and their feet (touch voltage). If the voltage is high enough, the person receives a shock. The following figure presents typical shock situations due to ground potential rise as per IEEE standard 80.