## 2 Overview of the Basic Concepts of Wind Farm Design and Optimization
### Adverse Factors and Conflicting Objectives
#### 2.2.2 Wind Farm Infrastructure  
One of the most important prerequisites for a wind project is that there should be enough electrical, civil engineering and telecommunication infrastructure, so the project will not incur in large additional expenses [74; 75; 76] by building auxiliary infrastructure, which in turn might lead to economical unviability. The wind farm electrical infrastructure is mainly composed of substations containing step-up transformers, Medium Voltage Alternate Current (MVAC) collection lines and High Voltage Alternate Current (HVAC) or High Voltage Direct Current (HVDC) transmission lines. Typically, each WT contains a transformer that increases the generated voltage (e.g., 220 V\(\sim\)3 kV) to the MVAC collection system voltage (e.g., 3 kV\(\sim\)60 kV). The MVAC collection system is in turn connected to a large capacity transformer that raises the voltage to the HVAC or HVDC transmission system voltage (e.g., 69 kV\(\sim\)161 kV) [77]. In practice, several WTs can be connected to the HVAC or HVDC transmission system by sharing a common MVAC collection line circuit. This strategy has proven to reduce the cable length requirements at the expense of increasing the cable size (cross sectional area) in order to meet capacity constraints [78; 79; 80; 81]. The MVAC collection system layout depend on the wind farm layout, where WTs can usually be interconnected in a modular fashion, while accounting for the power lines capacity, to produce a combined power output which can be either increased or decreased through the addition or subtraction of WTs at the different stages of the electric network implementation. For security reasons, all the electrical components are typically over dimensioned (e.g., by 30% above the expected rated capacity) since they are very sensitive to variations in the local climate and to electrical instabilities derived from the main electrical system, among other factors, which in turn can produce significant impacts on their performance [82]. The cable size is typically determined based on an economic analysis that in turn depends on the amount of current the cable can transport without overheating, which is a consequence of the Joule-based power losses. Fixed costs are higher for large cross sectional cables, but Joule losses are lower since they have lower electric resistance [55, 77, 78, 79, 80, 81, 83]. The size and location of the wind farm might lead to the problem of finding suitable electrical interconnection points, which must have enough capacity to handle the energy delivered from the wind farm. In cases where the distance between the electrical interconnection point and the wind farm is large (e.g., around 100 km [55, 84]) HVDC transmission lines may become more technologically and/or economically convenient than the conventional HVAC transmission lines [85]. However, the decision of using HVDC transmission lines is stressed by the fact that it is generally difficult to get permission and funds to build new overhead lines and/or substations and, although losses are low for HVDC transmission lines, there are added losses due to the Voltage Source Converter (VSC) components, which are typically difficult to quantify. The decision of using one type of transmission line or another (AC or DC) [85] is mostly based on the amount of reactive power generation and on the amount of Joule-based power losses, which depends on the transmission line length, transmission voltage and the rated power among other factors [78, 80, 82, 85, 86, 87, 88, 89, 90, 91]. Therefore, interconnections between the wind farm and the main electric system and interconnections between WTs within the wind farm must be planned with the aim of reducing reactive power generation, power losses and total costs. The available civil engineering infrastructure should provide route access (e.g., through highways, paved/unpaved roads, tunnels, bridges, railways, _etc._) to the wind farm site.