The major problems associated with EHV transmission are discussed below:
Problem # 1. Corona Loss and Radio Interference:
The corona is not only a source of power loss but it is also a source of interference with radio and television. The corona loss depends on various factors such as system frequency, system voltage, air conductivity, air density, conductor radius, conductor surface, load conditions, atmospheric conditions etc. The problem is more acute in case of EHV transmission.
When the electric field at the surface of an energized conductor exceeds 2-3 kV/mm, audible and sometimes visible corona discharge takes place, causing a loss of power and radiation of electrical noise. Corona loss varies through the year depending upon weather conditions. The corona loss under bad weather conditions may be as high as 100 times the fair weather condition loss.
Corona inception voltage gradient is an important parameter for conductor design. For limiting the corona loss, audible noise and radio interference it is necessary to limit the electric stress at the surface of the conductor to 1.8 kV/mm (rms), preferably to 1.5 kV/mm (rms).
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The corona loss can be reduced either by increasing the spacing between the conductors or the diameters of conductors but the spacing between the conductors cannot be increased to a large extent. Large diameter conductors (hollow conductors or ACSR conductors) have been used to bring down the corona loss and radio interference but the cost of manufacture of such conductors is high and their handling is both difficult and expensive.
Another difficulty with the use of such conductors is large wind and snow loadings. However, use of ACSR conductors is quite economical for line voltage up to 400 kV. Corona and radio-interference are also kept within permissible limits for such lines with this type of conductors. Bundled conductors, are invariably employed for EHV (400 kV or higher) lines.
The use of bundled conductors reduces the voltage gradient in the vicinity of line and thus, reduces the possibilities of corona discharge. Although the bundled conductors are used on EHV transmission lines primarily to reduce corona loss and radio-interference, but they have several other advantages.
The choice of configuration and cross-section of ac conductors with reference to corona considerations is generally above the economic cross-section based on thermal considerations or transient stability considerations i.e. the design of ac conductors based on corona limitations gives a cross-section much larger than that with respect to economical power transfer limit imposed by stability limit.
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The corona loss is quite large in EHV transmission lines. For example, in a twin- Moose bundled conductor, 2 × 235 mm2 (copper equivalent area) at 1.618 kV/mm of the 400 kV line, designed by UPSEB (Now U.P. P.C.L.) the annual corona loss per km of line is about 3,700 kWh. If the bundled-conductors were not having been employed in EHV lines, then corona loss would have been comparatively very high.
It is quite difficult to estimate corona loss with reasonable accuracy; as such loss depends on weather conditions. Attempts are made towards designing the transmission lines such that the fair weather corona loss is kept low and bad weather conditions do not seriously affect the line performance.
When bundled conductors are employed, spacers have to be installed at intervals of 80-100 m in each span to maintain the spacing between sub-conductors. This is necessary to prevent the conductors from getting stuck-up due to wind forces or electromagnetic induction introduced by heavy currents.
The increase in SIL with the increase in the spacing between the sub-conductors is small and, therefore, the intra bundle spacings are not very critical and 0.45 m is the most commonly used value.
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Transmission lines with bundled conductors need somewhat stronger towers. It is due to the fact that bundled conductors are subjected to large wind and snow loadings.
Problem # 2. Heavy Supporting Structures and Erection Difficulties:
EHV transmission lines have large mechanical loading on towers because of use of bundled conductors, large air and ground clearances, considerable dynamic forces due to broken conductors etc. Transmission line towers with fabricated steel members are usually employed in EHV transmission. A typical standard suspension tower for a 500 kV line is shown in Fig. 13.2.
The transmission lines are made more wind-resistant as they are to bear out the wind pressures during storms and cyclones.
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Recent designs for line supports include any of the following concepts:
(i) Composite design employing standard structural steel for lightly loaded members and high tensile steel for heavily loaded members.
(ii) H-frame structures supported by steel guy cables.
(iii) Guyed aluminium towers in place of steel towers.
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(iv) Suspension towers using reinforced concrete tubes.
Problems of transportation and erection arise as the supporting structures are to be transported over long distances and high standard workmanship is required for erection of EHV transmission lines.
Problem # 3. Insulation Requirements:
The level of insulation required depends upon the magnitude of likely voltage surges due to internal causes (switching operations) or due to external causes (lightning etc.) The lines are usually protected against lightning etc. by use of ground wire and rapid auto-reclosing circuit breakers.
The ground wire, the line insulation and the tower footing are properly coordinated for adequate lightning protection. Switching surges are, however more dangerous as they may cause overvoltage’s of 2-3 times the normal operating voltage. With the developments in the design of relay-breaker systems, however it is possible to control and minimise switching over-voltages.
Line insulation is designed to take care of switching overvoltage’s, temporary overvoltage’s and atmospheric overvoltage’s. The insulation level of a transmission line is based on the switching surge expectancy on the system. The maximum switching surge overvoltage to the ground is taken as 2.5 pu and the insulation is designed for this voltage. In addition adequate protection against atmospheric overvoltage’s is provided.