The power sector is growing rapidly nowadays. Therefore, we need to increase the capacity of electrical transmission systems to fulfil the rapidly growing demands. The most basic way to increase the transmission capacity is to construct new transmission lines. However, nowadays it become almost impossible to find new corridors for new transmission lines. The increased environmental constraints and public opposition are mainly responsible for that. Even if we find a new corridor, the cost of ROW is so high that it is not practical to construct a new transmission line.
Therefore, instead of using a new corridor, we may use the existing corridor by enhancing the capacity of the existing transmission line. We can do that by using higher capacity conductors, increasing the transmission voltage level, adding circuits to the existing line, and converting the line to HVDC, etc.
HTLS for Increasing Transmission Capacity
Operating Temperature Limits of Conventional Overhead Transmission Conductor
HTLS conductors are the most economical solution among the options mentioned above. We conventionally use ACSR conductors for transmission lines. We also commonly use AAAC conductors for transmission lines. These conductors constitute about 30% to 40% of the total cost of overhead EHV transmission lines. Previously, manufacturers designed conventional ACSR to operate at a maximum temperature of 75°C. However, currently, some manufacturers design ACSR and AAAC conductors for maximum operating temperatures of 85°C and 95°C, respectively. Actually, beyond this thermal limit, the conductors undergo annealing. Annealing of hard-drawn aluminium in conventional ACSR weakens the mechanical strength. A hard-drawn aluminum conductor significantly loses its strength above 93⁰C. Hence, these temperature limits restrict the ampacity of these conductors.
Obviously, we can increase the ampacity of the conductors by increasing their cross-sectional area. However, stringing heavier and thicker conductors requires mechanical upgrading of the existing transmission line infrastructure.
To use the existing systems, we can use HTLS conductors. HTLS conductors are High Performance Conductors. HTLS technology nearly doubles the ampacity of a conductor compared to ACSR and AAAC. However, HTLS maintains similar physical properties compared to ACSR and AAAC. The physical properties include weight and cross-section. HTLS also does not compromise the mechanical strength of the conductor even at elevated temperatures. Therefore, we can easily replace the existing ACSR with HTLS without any major modification in the existing transmission tower structures.
Operating Temperature Limits of HTLS Conductors
Normal HTLS conductors can operate continuously up to 150⁰ C. Some of these conductors can also operate continuously up to 250⁰C without any degradation in mechanical or electrical properties. Due to their high operating temperature limits, these conductors can typically carry 1.5–2 times more current than ACSR conductors with the same cross-sectional area.
We can also consider High Performance Conductors, like HTLS, for new transmission lines. Depending on the HTLS type, its cost may be 1.5–5 times higher than that of conventional ACSR or AAAC conductors. Additionally, the cost of extra ohmic loss at elevated temperature, the cost of specialised conductor accessories, and the cost of de-stringing of existing conductor are also to be considered. Moreover, the current rating of terminal equipment at substations also needs to be examined and enhanced if required.
Other Options for Increasing Transmission Capacity
As transmission capacity (MVA) is the product of voltage (kV) and current (A). So, we can increase transmission capacity by increasing transmission voltage levels, current-carrying capacity of conductors, or both. There are some options available for increasing transmission capacity. Each of these techniques has its unique cost-benefit ratio.
Use of Higher Size Conductors
This is the most straightforward method of increasing the ampacity of the overhead transmission lines. However replacing the existing conductors with thicker and heavier conductors is the most impractical approach. Because the stringing of higher size conductors in the same line requires strengthening or replacing the existing transmission towers. The redesigned towers may require new foundations, because the existing foundations may not be suitable for the newly designed towers.
Circuit Addition
Another solution for enhancing line capacity is the addition of a circuit to a line. We can construct a double circuit or multicircuit line in the corridor of a single-circuit line. Also, we can upgrade a double-circuit transmission line to a multicircuit line. However, a certain tower structure is not suitable for attaching an additional circuit to it. Therefore, to accommodate more circuits in the same corridor, all the towers along with their foundations need to be reconstructed. Also, the width of the right-of-way may need to be increased for the purpose. Widening the corridor can sometimes be very difficult or even impossible.
Increasing Transmission Voltage Levels
Increasing transmission capacity by raising the transmission voltage level is very effective. Because enhancing transmission voltage levels reduces the energy losses. However, this approach requires a considerable expenditure to construct higher voltage-rated substations at the line ends. Additionally, to maintain increased ground clearance and increased phase-to-phase clearance for the higher voltage level, the line requires taller support structures. Obviously, this adds significant expenditures. Also, the insulation cost increases for higher-voltage transmission lines.
Often, at higher voltage levels, to limit corona effects, we have to increase the effective cross-sectional area of conductors irrespective of their current-carrying capacity. This means that the conductor’s current-carrying capacity may be sufficient, but we may still need to increase its cross-sectional area. So, we normally use larger-diameter conductors or bundled conductors at high voltage levels. This also adds extra cost.
Using HVDC
To increase the ampacity of a transmission line, we can use an HVDC transmission system. However, HVDC requires AC–DC converter stations at both ends of the line. These converter stations are very expensive. Therefore, HVDC is not always a common or economical solution for increasing transmission capacity.
Enhancing Surge Impedance Loading (SIL)
For very long transmission lines, the power transfer capability mainly depends on the surge impedance loading (SIL). To optimize SIL, we must optimize the conductor spacing, configuration, and line design.
In practice, engineers increase SIL by adding more sub-conductors per phase. They also increase the effective cross-sectional area of the bundle conductor. Using asymmetrical bundle conductors can also help. In some cases, reducing the phase-to-phase distance improves surge impedance loading.
