## 4 Discussion
### PQ Monitor Allocation through the Prism of Grid Reliability
#### 4.2.1 Flora and Fauna  
As has been determined in Section 3.2.2, flora and fauna are among the five most common primary fault causes by quantity (see Figure 69), and flora is even in second place after ACR events. Meanwhile in Minnesota [57], flora is in third place and fauna in seventh by influence on both SAIFI and SAIDI. However, the result depends not only on geography, grid maintenance, and PQ mitigating devices but also on classification. In Minnesota, first place belongs to the weather (27% SAIFI, 72% SAIDI); however, this group is not formed for overhead power lines in Table 16, because weather (in particular wind) is mutually related with flora. Typical examples of flora interference in overhead power lines are shown in Figure 76. Despite 0.4 kV lines being shown, the examples are also suitable for the cases of both 6 kV and 10 kV, due to similar (identical) poles (including by height).  
A shrub growing beneath a double-circuit three-phase 0.4 kV line is shown in Figure 76a. Many species of tall and fast-growing shrubs (small trees) are native to Lithuania: for example, the typical height of great sallow (_Salix caprea_) is 2-13 m, the typical height of bird cherry (_Prunus padus_) is 3-12 m (sometimes up to 16 m). The importance of each situation depends on end-user criticality and specifics: for example, garden or garage community lightning with sodium-vapor lamps probably is not a priority case. One of necessary preventive measures is vegetation management: in 2019, after vegetation clearance, no failures were recorded in 87.3% of the DSO's overhead lines during the following year (the situation was improved by 22%).  
The risk of falling branches is obvious in the case of the tree tunnel in Figure 76b. On the other hand, these tunnels can be characterized by lower risk of bird impact, in particular large birds such as golden eagle (_Aquila chrysaetos_), whooper swan (_Cygnus cygnus_), mute swan (_Cygnus olor_), or white stork (_Ciconia ciconia_) which breed in open farmlands, in people's neighborhoods, or near marshy wetlands. The wingspan of the golden eagle is up to 230 cm, the whooper swan--up to 245, the mute swan--up to 240, the white stork--up to 215. According to some unofficial data in [69] (p. 7), published by the Lithuanian Ornithological Society, in Lithuania, approximately half of all storks build their nests on electricity poles. According to the \"LIFE Birds on Electrogrid\" final report [70], in Lithuania, 45 thousand bird deaths caused by collisions with the TSO grid are expected annually, and the mortality can be reduced by 1.2-4 times with visibility-increasing measures. For example, during the project period, 112 dead mute swans were found in the Nemunas river, in Kaunas; and photos of both golden eagle and whooper swan killed by collision with the wires can be found in [71]. Please note that the collision by itself does not cause electrical problems--two wires (or points with different electric potential) must be touched simultaneously to cause a short-circuit. There is no research regarding correlation (proportion) between the number of bird collisions and voltage sags/interruptions, but it is reasonable to expect that a reduction of bird mortality will improve PQ. It can be implemented by installing bird collision mitigation measures, also by consideration of migration routes at the power system planning stage.