Environmental Impact of Overhead Power Lines
Environmental impact issues of high voltage (HV) transmission lines are of particular importance in relation to the development of 500-750 kV extra high voltage (EHV) power networks and the development of ultra high voltage (UHV) 1150 kV and above.
The impact of airlines on the environment is extremely diverse. Let's look at it in more detail.
The influence of the electromagnetic field on living organisms. The influence of magnetic and electric fields is usually considered separately. The harmful effect of the magnetic field on living organisms, and above all on people, manifests itself only when a lot. high voltages of the order of 150 — 200 A / m, occurring at distances of up to 1 — 1.5 m from the conductors of overhead lines, and are dangerous when working under voltage.
The main problems for EHV and UHV lines are related to the influence of the electric field created by the overhead line. This field is mainly determined by the phase charges.As the overhead line voltage, the number of conductors in a phase, and the equivalent split conductor radius increase, the phase charge increases rapidly. So, the charge on the phase of the 750 kV line is 5-6 times greater than the charge on one conductor of the 220 kV line, and the 1150 kV line is 10-20 times. This creates an electric field stress under the overhead lines that is dangerous to living organisms.
The direct (biological) effect of the electromagnetic field of the EHV and UHN lines on a person is related to the effect on the cardiovascular, central and peripheral nervous system, muscle tissue and other organs. In this case, changes in pressure and pulse are possible. palpitations, arrhythmias, increased nervous irritability and fatigue. Harmful consequences of a person's stay in a strong electric field depend on the strength of the E field and the duration of its exposure.
Without taking into account the duration of exposure to a person, the permissible electric field strength is:
- 20 kV / m - for hard-to-reach areas,
- 15 kV / m — for uninhabited areas,
- 10 kV / m for intersections,
- 5 kV / m for populated areas.
At a voltage of 0.5 kV / m at the boundaries of residential buildings, a person is allowed to remain in an electric field 24 hours a day throughout his life.
For service personnel of substations and CBN and UVN lines, the permissible duration of periodic and long-term stay in the electric field at voltages at the level of the human head (1.8 m above ground level) is established:
- 5 kV / m — residence time is unlimited,
- 10 kV / m — 180 minutes,
- 15 kV / m — 90 minutes,
- 20 kV / m — 10 minutes,
- 25 kV / m — 5 minutes
Fulfillment of these conditions ensures self-healing of the body within 24 hours without residual reactions and functional or pathological changes.
If it is impossible to limit the time spent by personnel under the influence of an electric field, shielding of workplaces, cable screens over roads, shielding canopies and canopies over control cabinets, vertical screens between phases, removable screens during repair work are used and others. As experiments show, a reliable protective effect is created by bushes 3-3.5 m high and fruit trees 6-8 m high growing below the air line. This is due to the fact that bushes and fruit trees have sufficient conductivity and act as a screen at a height that exceeds the height of a person or the height of vehicles.
The indirect effect of an electric field is included in the occurrence of current or short-term discharges when a person who has good contact with the ground touches isolated objects or, conversely, when a person isolated from the ground touches grounded objects. Such phenomena are explained by the presence of increased potentials and EMFs caused by an electromagnetic field on machines, mechanisms or extended metal objects isolated from the ground.
The discharge current flowing through a person depends on the voltage of the line, the active resistance of the person, the volume and capacitance of the objects relative to the line. Continuous current reaching 1 mA is the «threshold of perception» for most people. At a current of 2-3 mA, fear occurs, at 8-9 mA («release threshold») - pain and muscle cramps. Currents above 100 mA flowing through a person for more than 3 s can be fatal.
Short-term spark discharges, in which a pulsed current flows through a person, even at sufficiently large amplitude values, do not pose a danger to life.
The indicated effects of the electromagnetic field establish certain operating conditions and the possibility of the population remaining in the protective zone of the overhead line, which has borders in the form of parallel lines. The strength of the electric field in the protective zone exceeds 1 kV / m. For overhead lines 330 — 750 kV, the zone is 18 — 40 m from the end phases, for overhead lines 1150 kV — 55 m.
Acoustic noise is one of the manifestations of intense corona on the wires. It is perceived by the human ear in the frequency range from 16 Hz to 20 kHz. Loudness is especially high on lines with a large number (more than five) of phase-separated wires during rain and wet weather. If in heavy rain the noise from the crown merges with the noise of the rain, then in light rainfall it is perceived as the predominant source of noise.
Calculations show that for EHV and UHV lines outside the security zone, the noise level is less than permissible. In the CIS, the maximum permissible sound volume is not standardized.
Radio interference occurs when corona on conductors, partial discharges and corona on insulators and fittings, sparks in the contacts of line fittings. The level of radio interference is affected by the radius of the wires, weather conditions, the condition of the surface of the wires (the presence of pollution, precipitation, etc.). To eliminate radio interference in a shielded tone, the permissible voltage on the surface of the conductor is reduced.
Aesthetic impact of the lines... In areas with a high population density, in addition to the economic and technical problems that arise during the construction of power lines, there are problems with the aesthetic impact of these lines on the environment. This effect is related to dimensions (height) of supports, their architectural forms, with the coloring of all line elements.
For a better visual and aesthetic perception, it is recommended: the choice of supports that meet the requirements of industrial aesthetics and the correct architectural forms, natural coverage (screening) in the form of forests, hills, etc., masking (coloring) of linear elements for reducing their shine, using double-chain supports or supports of different heights.
Withdrawal of land from land use. According to the norms, objects under supports and foundations are subject to permanent withdrawal. The dimensions of these places are equal to the base of the support plus a strip of land 2 m wide on each side. When supported by boys, the perimeter of their base passes through the boy's attachment points to the bases.
In addition to permanent land acquisition, temporary land acquisition is carried out along the route of the line for the construction period, which then enters the protection zone of the overhead line.
The cost of withdrawn land is determined in accordance with the standards for individual regions of the country and is defined as the cost of restoring land with characteristics similar to those of fertility.
The construction of all networks with a voltage of 35 kV and above requires the allocation of land for substations and overhead line supports on average 0.1-0.2 hectares for every 1 MW increase in load. The construction of the power plant leads to the acquisition of land up to 0.1 — 0.3 ha / MW and more.
Large areas are occupied by reservoirs, which determine more than 90% of the land set aside for energy facilities.