# Grid Nodes Selection Strategies for Power Quality Monitoring
## 4 Problems and Future Prospects
### Sophisticated Attention to Every PQ Event
The authors do not give enough attention to device types and measurement requirements. Those dimensions must be added to future allocation strategies: cost-efficient selection of criteria should be ensured by considering monitor type (its features and technical characteristics), PQ events target group (individually for each monitor type regarding propagation mechanism), voltage level, electrical power system features, technical condition of substation, remote communication aspects, national legislation, policy (including responsibility sharing, penalties), etc. Let us discuss one of the most important monitor characteristics--sampling frequency. It highly correlates with the price. Obviously, a high sampling rate is required for fast processes and transients monitoring. In addition, a higher sampling rate is important for precise waveform extraction which can be used in the future, in advanced algorithms such as wavelet transform. The classification of voltage transients is proposed and their typical characteristics are given in IEEE Std 1159; we prefer the IEEE standard to the insufficiently informative EN 50160 and EMC standards. Voltage transients are classified into two main groups--impulsive and oscillatory. Impulsive transients are classified into nanosecond, microsecond, and millisecond: their typical (rise) time constants--5 ns, 1 \(\upmu\)s, and 0.1 ms, typical duration--less than 50 ns, 50 ns-1 ms, more than 1 ms, respectively [3]. Spectral content of impulse transients cannot be determined due to the Heisenberg uncertainty principle; thus, time constants are given instead of spectral content. Oscillatory transients are classified into low, medium, and high frequency. Typical characteristics: spectral content of low frequency oscillations is up to 5 kHz, duration--0.3-5 ms, magnitude--0-4 p.u. ; medium frequency oscillations--5-500 kHz, 20 \(\upmu\)s, 0-8 p.u. ; high frequency oscillations--0.5-5 MHz, 5 \(\upmu\)s, 0-4 p.u. [3]. In addition, in Figure 7, spectrum calculations of Figure 6 transients are presented. Two spectral components of commutation process are clearly seen in Figure 7a: fundamental grid frequency and free oscillation frequency of the transient examined. According to IEEE Std 1159, this transient is treated as a low frequency oscillation. On the contrary, the peaks are not seen in Figure 7b. There are various and scarcely determined reasons for such occurrence: limitations due to the Heisenberg uncertainty principle (either an oscillation is too short or transient is impulsive), spectrum aliasing (simulation sampling frequency is too low for transient), impact of spectral leakage determined by window function (in given example, rectangular window is used), noise influence, higher grid impedance for higher frequency components, features of measurement circuit, etc. Along with FT, other methods  
Figure 6: **(a) Phase-to-ground voltage transient (after 0.05 s) caused by 10 kV power line commutation (switching on); (b) Phase-to-ground voltage oscillations in 330 kV 100 km length power line end (busbar), where strong generation presents, caused by two-phase-to-ground short-circuit at the other end of the same line (after 0.05 s). Sampling time is \(10^{-6}\) s. The simulation has been performed with model of chosen region of Lithuanian power system. **  
will potentially be used in the future for short signals, for example, short-time Fourier transform. Currently, only free oscillations monitoring seems to be massively solvable in practice through a prism of sampling frequency (at least partly), since it does not require unrealizable sampling. Many analyzers, along with casual parameters monitoring, have separated a high sampling frequency short-memory channel. However, since a very high sampling rate is required for impulses, limitations regarding the dynamic range of casual PQ monitor remain: according to IEEE Std 1159, the typical magnitude of oscillations is up to 8 p.u., but the nature of impulses is not well studied; thus, it is hard to predict the real values.