## 4 Discussion
### PQ Monitor Allocation through the Prism of Grid Reliability
#### 4.2.2 PQ Mitigation Techniques  
Firstly, as has been mentioned before, one of the most obvious preventive measures is the increase in the percentage of underground cable lines. European country data for 2018 can be found in [50] (p. 80): for example, in Lithuania, the share in the LV grid was 32.4%, and in the MV grid--27.3%. Underground cables do not have visual impact, which is desirable in urban environments. However, it is clear a priori that both repair and fault location search processes are significantly more complex in comparison with overhead power lines. Secondly, an efficient PQ mitigation system is one of the ultimate aspirations of all PQ research (in particular this paper). However, despite many papers, a lot of questions remain about PQ mitigation devices, their application (allocation strategy), classification, etc. For example, [13] uses a classification into single power supply and dual power supply equipment, and investigates features of the following devices: dynamic voltage restorer--DVR, solid-state transfer switch--SSTS, static synchronous compensator--STATCOM, static VAR compensator--SVC, unified PQ conditioner--UPQC, and uninterruptible power supply--UPS. Each device has its own unique features; thus, the mitigation efficiency will also be selective and depend on voltage sag parameters (e.g., voltage level, duration, energy, rise time) [5]: for example, STATCOM (with IGBTs) has a shorter response time than SVC (with thyristors).  
Let us introduce an example of PQ mitigation research (Figure 77), performed with Siemens PSS/E software tool. Voltage sag mitigation of capacitor banks is compared with a flywheel system under the following situation: (1) the device is installed on the 10 kV line of a 10/0.4 kV substation, which feeds a manufacturing plant of polymer film packages; (2) the investigated node is the other end of the same 10 kV line in the 110/10 kV substation;  
Figure 76: Typical examples of fauna interference with the public grid in Lithuania. (**a**) A shrub beneath a 0.4 kV line; (**b**) A 0.4 kV line in the forested area.  
(3) a three-phase fault occurs at the selected farther node of the 110 kV grid. The fault duration is 0.15 s. Voltage sag/interruption avoidance is a critical issue for this industrial plant, because an outage will not only deteriorate the quality of the plastic, but also will lead to the production line's equipment clogging. Please note that a short-circuit at another 110 kV node will cause a different (depth) voltage sag, but the nature of the mitigation process will be similar.  
In Figure 77, it is seen that the investigated flywheel system mitigates the voltage sag by up to 0.1 p.u. (from 0.45 p.u. up to 0.55 p.u.). Even such a small mitigation has some importance and increases interruption avoidance probability; however, currently, this remains a direction for further research. Meanwhile, the capacitor bank is not efficient due to the following reasons: (1) a relatively long response time required for the activation of all levels (to achieve the highest possible capacitance); (2) the power of capacitor is proportional to the voltage square:  
\[W=\frac{CU^{2}}{2}, \tag{35}\]  
where: \(C\)--capacitance, F; \(U\)--voltage, V.