The major problems associated with extra high voltage transmission lines are following:
1. Corona loss and radio interference :
Since transmission line voltage level is a governing factor in the corona loss, it is more acute in EHV lines. This loss further increases bad weather conditions.
The power line also disturbs the radio and TV broadcasting. To reduce the corona loss, the spacing between the conductors and conductor diameter can be increased. Herc, if conductor spacing is increased, the cost of line support increased highly due to an increment in span.
The diameter of a conductor can be increased by using a hollow conductor or Aluminum conductor steel reinforced (ACSR) conductor. But the cost of both these types of conductors is high and their handling is also difficult. Wind and snow loading is also higher with these conductors.
Bundled conductors offer an economical solution for minimizing corona loss and radio interference for EHV lines. These conductors have filler material or air space inside, such that the overall diameter is increased. sometimes tow or more conductors per phase are used in the case of EHV lines.
2. Line support:
EHV lines have larger mechanical loading on towers due to bundled conductors, larger air and ground clearance. It has also considerable dynamic forces acting on it due to broken conductors etc. Common transmission line towers with fabricated steel are used.
The tower cost is varied from 30% to 50% of the total cost of the line. So, better and cheaper tower designs must be evolved to affect the economy.
Recent design for line supports include the following points:
(i) Composite design using standard structure steel for lightly loaded equipment and high tensile steel for highly loaded equipment.
(ii) Use of guyed aluminum lower (instead of steel).
(iii) Standard wooden pole structure,
(iv) Suspension tower employing reinforced concrete tubes.
(v) H-frame structure supported by steel.
3. Insulation requirements :
The magnitude of line voltage surges determines the required line insulation level of the transmission line. These surges may be due to internal causes or due to external causes.
The order of lightning surge is near about 1000 to 1100 kV and the order of switching surge is near to 3.5 times the normal voltage. So, in the EHV transmission line, as the operating voltage is more than 400 kV, the switching surge predominates the lightning surges. Hence, the insulation is done according to the switching surges.
Therefore, the insulation requirements are much costlier when the voltages increase.
4. Erection difficulties :
The erection of EHV transmission lines causes a wide range of problems requiring extremely skillful workmanship and transportation of structural materials.
5. Power station and substation equipment:
For EHV transmission, the size and rating of a generator, transformer, circuit breakers, conductor diameter, relays, etc. have to be increased. Surge arresters of high voltage are to be used in power stations.
All these effects cause handling and transportation problems of equipment as the substations are situated in remote areas.
6. High short circuit current :
Due to the increase in voltage of EHV transmission, short circuit currents are very high.
For such fault, the circuit breaker of high capacity is required which has a higher cost.
7. Surge arrestor:
Gapless metal oxide arresters are required instead of gap type silicon carbide arresters for both lightning and switching impulse duties because in EHV transmission the switching overvoltages are predominant.
8. Single pole switching and auto reclosing :
In the case of EHV transmission, single-pole switching and auto reclosing are required for maintaining transient stability. In the case of any fault at a single line, if single-pole switching is available instead of multipole there are two lines present in the circuit which supplies the required load power. At the time of transients, the auto recloser switch opens and after some time it recloses and maintains the supply.
9. Surface voltage gradient :
For EHV lines, the surface voltage gradient is high by which corona lines losses increases. Due to the high surface voltage gradient, the conductor attracts the dust particles and it causes the further increment of corona losses.
10. Shunt reactors :
In EHV transmission, shunt reactors for compensation are required to absorb the excessive reactive power generated by line capacitances. It will increase the cost of the line.
11. High electrostatic field :
In EHV transmission, a high electrostatic field is generated. This field is harmful to humans, animals, and plants. So, a sufficient height of conductor by pole must be maintained in this case.
12. Bundled conductor :
The transmission line using bundled conductor becomes heavier. Hence, the tower must be stronger to withstand the weight of the bundled conductor. Also, the cost of bundled conductors is high.
13. Insulation coordination :
In EHV transmission, the insulation coordination should be based on switching surges. since switching surges are more predominant in EHV.
14. Series capacitors :
The use of series compensation also introduces a few problems. Some of these problems are
(i) Sub-synchronous resonance:
The series capacitor introduces a sub-synchronous frequency (proportional to the square root of compensation) in the system. In some cases, this frequency may interact with steam turbine generator shaft and give rise to high tensional stresses.
In hydro turbine generators, the risk of sub-synchronous resonance is small because the tensional frequencies are about 10 Hz or even less
(ii) Line protection:
Series compensation can lead to false operation of the distance relays of the line protection if the degree of compensation and capacitor location are not proper. To ensure the correct operation of the distance protection, this series compensation is limited to 40% (when the installation is located at the middle of the line) and 30% (when the installation is located at one-third of the line).
(iii) Ferroresonance:
When an unloaded or lightly loaded transformer is energized through a series of compensated lines ferroresonance may occur. The frequency of oscillation is an integral multiple of the system frequency.
This can be suppressed by using shunt resistors across the capacitor or by short-circuiting the capacitor temporarily through an isolator or a bypass breaker.
(v) High recovery voltage:
Series capacitor produces high recovery voltages across the circuit breaker contacts which may damage the circuit breaker.
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