Recently, Spain's Galicia Region has experienced spectacular growth in power-generation capacity, particularly in the sector of wind generation. As a result, consumers in this region are changing their role to power producers and exporters — a situation that has lead to the reversal of power flows in the existing high-voltage (HV) system.

This effect is most noticeable in the northwest zone in the Spanish province of La Coruña, where the installation of an additional 244.2 MVA of wind and gas-fired cogeneration projects offering fixed-price energy to the utility has overloaded Union Fenosa's (UF) existing 66-kV distribution system. To provide a HV system able to export the planned two-phase increase in generating capacity, the utility had to redesign and increase the capacity of the existing system to provide an export capability of 631.4 MVA.

Phase 1: 219.2 MVA_(wind)
+ 40.2 MVA_(cogen)

Phase 2: 372 MVA^(wind)

Feasibility Study

To export the planned additional generating capacity, UF considered two alternative methods of increasing the capacity between Tambre II 220/66-kV substation and Santiago II 220/66-kV substation:

• Constructing a 220-kV overhead line between the two substations

• Increasing the transmission capacity of the existing 66-Kv overhead line (Tambre-Santiago II) in service, until the point where it crossed the existing 220-Kv overhead line (Meson-Santiago II — a double-circuit line built in 1984 that has continued in operation as a single circuit). This alternative gave UF two possible solutions to analyze:

• Use of conductors with high-transmission capacity. The incremental transmission capacity gained through the use of special conductors (layers of aluminium alloy around steel-alloy core) permits higher conductor operating temperatures to increase the transfer capacity without a significant increase in conductor sag. A solution would allow UF to maintain statutory conductor ground clearance and use the existing lattice towers. However, as the increased transfer capacity was insufficient, and the conductor load losses were unacceptably high, this possible solution was rejected.

• Upgrading nominal voltage. The increase in power-transfer capability achieved by upgrading the nominal voltage of the existing from 66-Kv to 220-Kv would increase the transmission capacity by 233%. More importantly, the existing conductor and the majority of the tangent line supports could be used on the section to the intersection with the existing 220-Kv Mesón-Santiago II transmission line. Figure 1 shows the UF high-voltage network for the northwest zone before and after the circuit upgrading.

Reasons to Upgrade the Circuit

Realization Period. This factor proved to be an important consideration because of the commitments acquired with the wind-power facility and the administration. The construction of a new 220-Kv line requires:

• Studying a new line trace for possible construction.

• Receiving approval of the project by the administration.

• Obtaining servitude permissions with the landlords and expropriations of land in the affected zones.

  Selecting circuit uprating eliminates or minimizes three procedures:

• The trace study was eliminated.

• The approval of the project by the administration was reduced as this alternative had greatly reduced environmental impact compared with the construction of a new line.

• The servitude permissions and the land expropriations are minimized being the landlords for the existing line.

Environmental Impact. The construction of a new line corridor would require tree clearance, new accesses and site damage by line-construction machinery through an area of high ecological interest and dense forests that is without urbanized development. The severe environmental impact would pose serious difficulties in obtaining line permissions. However, the use of a compact design and the use of the existing supports in the 66-Kv line (with short vain) permits the use of the existing corridor, quite minor when compared with a conventional solution of a new 220-Kv line, with the corresponding decrease of the environmental impact.

Table 1. Characteristics of 220-kV overhead line.

Nominal voltage kV	220
Maximum voltage kV	245
Frequency Hz	50
Length of 66-kV section km (miles)	19.44 (12.1)
Length of 220-kV section km (miles)	7.27 (4.5)
Total line length km (miles)	26.71 (16.6)
Condor conductor — Aluminium Core Steel Reinforced (ACSR)	454.5 mm2 (0.70 in2)
Fiber-optic cable	OPGW-48 F
Power-transfer capacity MVA	307
Number of circuits 66-kV section	1
Number of circuits 220-kV section	2

Table 2. Electrical characteristics.

Critical impulse flashover 1,2/50 ms (kV peak)	1.165
Low-frequency flashover (kV rms)	525
RIV 0,5 MHz/300 W/141 kV (dB/1 mV)	30
Leakage distance (mm)	6.125

Economic Cost. By increasing the power-transfer capability of the existing line, there is a 40% economical saving compared with the construction of a new 220-Kv circuit. This saving is attributable to reduced servitude and expropriations costs, reduced material and construction costs.

220-Kv Line Characteristics

Table 1 shows the most important physical characteristics of the 220-Kv overhead line (Tambre II-Santiago II). The first section consists of increasing the electric-transmission capacity of the existing 66-Kv overhead line (Tambre I-Santiago II) until the intersection with the existing 220-Kv overhead line (Mesón-Santiago II). The 66-Kv line Tambre I-Santiago II was built with lattice towers and ACSR Condor conductor in 1984 interconnecting the city of Tambre with the Santiago II 220/66-Kv substation. The majority of the existing towers (54) constructed with separate footing foundations (46) or monobloc foundations (8) were satisfactory for the 220-kV compact superstructure, but 24 towers were changed to satisfy the ground clearance and mechanical loading specification for the 220-kV circuit.

The 21 lattice towers on the Meson-Santiago II existing 220-kV double-circuit line required no additional work.

Electrical Characteristics of Line

The most important electrical and radio-electric insulation characteristics are included in Table 2. The insulators selected for this line were from three types: composite braced line post with articulated system in the tangent supports on the first section; composite suspension/tension insulators for the angle supports; and glass suspension insulators for the remaining supports in the second section. The housing and the core of the composite insulators contain silicon rubber and E glass with resins.

The grading of electric field in the critical zones of the insulators and the aging design test made by the manufacturers were subject to HV laboratory testing. The existing conductors on the 66-kV line (ACSR Condor) proved to be compatible for its usage at 220 kV, without corona-effect problems on the surface of the conductor.

Mechanical Characteristics of Line

The validation of the 66-kV tower body with the 220-kV compact superstructure in the first section of the line was made with a metallic structures program. As a result of this study, the majority of the tangent supports (54) were adequate to accommodate the compact superstructure, but 24 towers required change.

Table 3. Mechanical characteristics for the second section of the 220-kV line.

Conductor	ACSR Condor
Maximum tense (daN)	3.189
CHS (cold hour stress)	20%
Maximum microstrain (ms)	150

The compact superstructure design offers a composite braced line post with articulated system that has the following advantages:

• Elimination of longitudinal loads when the tower is subject to unbalanced loads. The braced line post is designed to rotate until the longitudinal tension of the conductor in both sides of the span is equal.

• Reduction of the torsion efforts applied to the tangent support in the event of a broken conductor. The braced line post rotates 90 degrees releasing conductor tension and applying the load to two points on the tower body compared with the conventional system where the same load is applied in the end of the crossarm.

• A compact design allows a reduction in the distance between cross-arms and an overall height reduction compared with conventional towers.

• Elimination of bending stresses in the insulators. With the articulated system, the horizontal post will work in compression and the brace in tension. The use of a 220-kV line post without brace or with a fixed brace would not be viable.

• Improved assembly. The use of composite insulators instead of ceramic insulators facilitates the complicated assembly in the first section of the line. This resulted in an advantage as assembly had to be completed quickly in short periods of time.

No re-tensioning of the conductor was necessary in the first section of the line as the distance between the suspension and anchorage points of the conductor remained unchanged after the substitution of the superstructures. The absence of international standards for the mechanical testing of line post insulators with articulated system motivated the development of techniques for laboratory checking of the mechanical specifications of the line post insulators.

Prior to fabricating the 54 units of 220-kV superstructures, a prototype was made (Fig. 3). Various tests checked the assembly of all the accessories and insulators. The existing supports in the second section of the line were 220-kV double-circuit lattice towers. The mechanical characteristics for the second circuit strung on these towers with an available second circuit where the conductor was ruling and stringing.

220-kV Line Construction

The most significant activities in the construction of the first section of the line (increasing the nominal voltage from 66 kV to 220-kV) were:

• The refurbishment of 54 towers of the 66-kV circuit with monobloc foundation superstructure by a new 220-kV compact structure (Fig. 4). The change process on towers with separate footing foundations is shown in Fig. 5.

• Replacement of all angle and tension supports in the existent 66-kV line (17 units) and of those tangent supports where the conductor to ground clearance was insufficient (7 units).

• Stringing and tensioning the new OPGW fiber-optic cable.

All these activities were carried out during weekend working periods and with punctual 8-hour interruptions in energy transmission, after which, the circuit continued in service at 66 kV.

The assembly of angle and tension supports involved the replacement of the complete tower erected partially on the existing foundations. The fabrication of the new 220-kV compact superstructure was dimensionally excellent, making the connection of the superstructure to the existing tower body relatively easy. Conductor stringing and sagging was straightforward as the suspension and anchorage points were unchanged.

The installation of the second circuit, including the new OPGW on the second section (existing 220-kV line), was completed while the existing circuit remained energized.

Considerable Results

Nowadays, obtaining administrative authorizations and rights-of-way permissions from land owners on the route of a proposed new overhead lines is becoming increasing difficult. Therefore, one solution is to increase the power-transfer capacity of an existing circuit. This type of project provides additional capacity by increasing the nominal voltage uses, a methodology that employs conventional and new engineering techniques.

The most important and significant benefits that UF achieved as a result of this project were:

• 233% increase in the power transfer capability of the existing circuit.

• 40% cost saving with respect to the construction of a new line.

• Environmental impact reduction.

• Considerable reduction in the construction time — the 220-kV line was in service within three months. (The construction of a new 220-kV line has a minimum period of construction of two years because of trace study, topography, project, agreement with the landlords, administrative authorizations and construction.)

The new 220-kV line Tambre II-Santiago II circuit commenced operation in January 2000.

David Vindel Cottereau graduated from the Polytechnic University of Comillas (ICAI) in electrical engineering in 1991. Cottereau is manager of the Design, Innovation and New Technologies Department in Union Fenosa Distribution, responsible for the new designs, specification and construction standards in transmission and distribution.

Ramón Morales Arquero graduated from the Polytechnic University of Madrid (ETSII) in electrical engineering in 1995. Arquero is in the Electric Department of Soluziona Ingenieria in the new design, specification and construction standards of high-voltage transmission lines and substations as project manager.

Javier Sacristán Heras graduated from the Polytechnic University of Madrid (ETSII) in electrical and electronic engineering in 1995. Heras works in the Planning Department of Union Fenosa Distribution performing long-range planning studies, providing operations support, determining the electric transmission system expansion and reinforcements, and developing load forecasts.