By Sheela Johnson
This case study elaborates various Options & Alternatives considered in techno-commercial evaluation, for construction of a tie line between two Gas Insulated Substations (GIS) of power generating plants that are 2.5kM apart. The power generating plants, are already commissioned and are in operation. Each generating plant has independant startup power from grid, however a tie line was required for ensuring reliability of startup power. Typically, Tie-lines are provided for sharing and transfering power between substations. Establishing a Tie-Line between two Extra High Voltage (EHV) ,Gas Insulated Substations(GIS) required careful study of many electrical parameters, which have direct bearing on its design and cost. For arriving at the rating of the tie line, load flow studies were conducted and startup power requirements were computed.
Tie line had to be constructed with no discontinuity of service of existing substations, overhead power lines and generating plants. The approach to site, construction facilities and accessibility required for implementing the Tie line were studied.
A techno- commercial evaluation of various options that were studied for establishing the tie line are discussed below:
Option -1:- Gas-insulated transmission lines/ bus duct (GIL/GIBD) solution for high power interconnections
Gas Insulated Transmission technology was evaluated, since the right of way (ROW) required is less when compared to overhead lines. For a rated current, GIL with SF6 gas insulation will require lesser space and is suitable in restricted areas. However due to restrictions in routing of GIL in an existing operational plant due to interferences with foundations of existing structures, crossing of roads, tunnels and trenches, this option was ruled out. Also this option was not cost-effective.
Option -2 Overhead line (OHL) with Lattice or Monopole Towers
The conventional Overhead line (OHL) built on lattice type structure is proven and universally adopted method of power evacuation or tie line connection between the units. Generally in generating stations power line connections to switchyard are by OHL within the plant area. This is basically because of easy erection, maintenance, routing & cost effectiveness. The most common problem with OHL was due to right of way (ROW) and corridor width issue. Conventional Overhead line (OHL) built on lattice type structure and monopole were compared in this option. Locating towers within an operational plant by avoiding crossing of existing EHV lines which were already charged, was the most challenging task. Tower Spotting study was carried out taking into account various configuration of line, line types, height, top hamper width, length of insulator assembly, minimum clearances, etc. Site survey was conducted to identify feasible routes and to collect data on existing facilities.
Monopole type towers were also studied as it required lesser right of way when compared to lattice type towers. Seismic Analysis was carried out for comparison of base shear & base moment for wind & seismic loads acting on monopole/ lattice type towers. However locating monopole towers in restricted areas was not feasible due to its large foundation requirements and interference with existing structure/building foundations.
By selecting line conductors based on the standard sizes used by utilities, the current carrying capacities, voltage, loading limits and impedances were optimized. Prevailing new technologies of insulator for the required basic insulation level of transmission line was studied.
Sag Tension studies were performed to achieve optimum conductor tension and sag. The most optimum span was arrived at by using software.
Option-3 Buried EHV cable
Buried cable option was studied for the route where the Tie line was crossing existing 400kV line. It was observed that cable option was economical since the tower heights required for crossing existing OH lines was very high.
The sizing of EHV cable for both Aluminium and Copper conductors was performed considering loss factors, earthing methods, installation methods etc. Suitability checks of existing GIS termination for terminating cables were also carried out.
Alternative routes that were studied for establishing the tie line are discussed below:
Alternative -1 In this alternative, routing of Tie line was planned to avoid crossing of existing 400kV power lines evacuating power from operational units. The route available was through the gap between two buildings, which would avoid shut down of power lines. However, the clearance was not adequate to construct either lattice structure or monopole tower OH line. In this alternative routing of tie line using cable was also studied. The route length of buried cable in this alternative was high and therefore this alternative was not selected.
Alternative -2 The route planned was along the boundary wall of the site, away from the both the substations. In this route crossing of 400kV existing power lines could not be eliminated. For crossing existing lines a strung bus above existing lines was studied. The study involved detailed calculation of Sag Tension for constructing the tie line with a strung bus above the existing 400kV lines. This alternative was discarded due to substantial increase in height of towers and due to the necessity for outage of existing lines.
Alternative -3 Similar to Alternative 2 the route planned was along the boundary wall of the site away from both substations. However in this route crossing of 400kV existing power lines was with a strung bus below existing 400kV lines. In this alternative the outage of existing lines could be avoided & height of towers was less. However due to long route length, constructability issues because of rough terrain and inaccessibility, this alternative was not economically feasible.
Alternative-4 For reducing the number of towers it was decided to route the Tie line closer to both substations. In this alternative, for crossing of 400kV lines it was decided to go with underground cable whereby outage of the existing line was not required. Accessibility of existing site roads and constructability was easy in this case since the route was within the plant. With a combination of overhead transmission line & buried cable installed for crossing existing OH lines this alternative was best suited and was selected.
Based on detailed techno-commercial evaluation carried out for different options and alternate routes, it was observed that routing of the tie line with a combination of overhead line and buried cable closer to substations without resorting to outage of existing lines would be most economical solution.
TCE has significant experience in offering engineering and project management services for constructing tie lines. TCE can conduct all studies required to provide a comprehensive solution from survey, design, installation and to commissioning of Tie Lines.