## 7 Economic Analysis of Solution Cost
### Economic Optimisation of the Solution  
Obviously, choosing the optimal number and size of breakers is an important task, but it is not an easy one. On the one hand, if too many breakers are installed, the initial investment cost will raise significantly. On the other hand, a high number of breakers divide the full plant in smaller areas with a lower energy consumption per test. Moreover, although a higher number of breakers would be required, dividing the plant into smaller sectors will lead to breakers with less power cutting capability and, therefore, cheaper ones. Expression (3) gives a relationship between power and price of the required breakers:  
\[C_{Brkr}=\alpha P_{N}+\beta \tag{3}\]  
where \(C_{Brkr}\) is the unitary cost in E, \(P_{N}\) the rated power in \(kW\) and \(\alpha\) and \(\beta\) are two parameters to establish a linear regression about the cost of breakers in the working range of the selected scenario.  
In order to calculate the optimal amount of breakers, an optimisation analysis is going to be carry out in a selected application. For a typical USPVP application using power solar inverters of 1 MW, \(\alpha\) and \(\beta\) have been estimated according to current market prices [68; 69; 70; 71; 72; 73; 74] as 1.475 and 325, respectively. Additionally, several costs could be taken into account, but only the exposed above are relevant to this study. These costs are represented by the initial investment and the energy consumption during the inspection process. The investment in breakers (\(C_{0}\)) can be formulated as  
\[C_{0}=C_{Brkr}N_{Brkr} \tag{4}\]  
where \(N_{Brkr}\) is the amount of breakers, the variable to optimise.  
Regarding electricity power consumption, current legislation specifies that two inspections must be performed at 10% and 100% of \(I_{sc}\). This cost can be calculated by means of expression (5),  
\[C_{Insp}=\frac{C_{Energy}(P_{10\%I_{sc}}+P_{100\%I_{sc}})t}{N_{Brkr}} \tag{5}\]  
where the inspection energy cost per MWp of PV panels installed (\(C_{Insp}\)) depends on the energy price (\(C_{Energy}\)), the power consumed during both tests (\(P_{10\%I_{sc}}\) and \(P_{100\%I_{sc}}\)) and the time required to inspect 1 MWp-equivalent-area; moreover, it is inversely proportional to the number of breakers used. Electricity cost is \(\sim\)10 cent EUR/kWh [75; 76], the energy consumption has been taken from Table 2, and the inspection time using unmanned vehicle techniques is estimated to be \(\sim\)13 min/MWp.  
These costs have been calculated for a one-year period, assuming a plant expected life of 25 years. Furthermore, increasing plant automatisation would improve the capacity to carry out the maintenance tasks, being able to perform inspections more frequently. In this case, it is assumed that an inspection is completed monthly. In order to show the costs on an annual basis, the initial investment (\(C_{0}\)) has been divided for the 25 years of expected plant life; inspection energy cost (\(C_{Insp}\)) has been multiplied by 12 annual inspections; and finally, total costs are the sum of both of them. Selected costs are represented in Figure 13, where the minimum cost is found installing 4 or 5 breakers per PV-panel-\(MWp\), which, according to expressions (3) and (4), it represents an investment of 2775 EUR.  
Accordingly to these estimations, the price to adapt a new USPVP to EL inspection can be calculated as the addition of pre-charge cost (135 EUR) and breaker cost (2775 EUR), which gives a result of 2910 EUR. On the other hand, it should be noted that inverters already have a switch to disconnect the panels; so the extra cost will be 2910 EUR minus the cost of 1-MW-breaker (1800 EUR), which is an extra cost of 1110 EUR per MWp. Major studies assert that the average installation cost of a large PV plant is 1112 EUR/kW [5]. Based on this information, it can be concluded that an increase of 0.10% on the initial investment would allow the on-site EL maintenance of the PV plant.