# Online Estimation of Short-Circuit Fault Level in Active Distribution Network
## 4 Conclusions  
In this paper, a method was proposed to evaluate the fault level represented by the short-circuit current through the breakers in the distribution networks with DGs. The merits of the proposed method can be summarized as follows.  
1. Different from the conventional offline analysis, the proposed method ensures online estimation of the potential short-circuit fault level. It is adaptive to the present situation in the distribution power networks with more and more DGs, which make the fault current change frequently. Meanwhile, the proposed method is relatively easy to be implemented by either PMUs or the measurements from the protection relaying.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Load Type**} & **Magnitudes of \(\Delta\)U and** & **Errors between Actual and Fault** & \multirow{2}{*}{**Confidence**} \\  & \(\Delta\)**I in Percentage** & **Current Estimation** & \\ \hline \multirow{4}{*}{Motor load} & 2\% & 4.46\% & 30.8\% \\  & 3\% & 4.00\% & 33.3\% \\ \cline{1-1}  & 5\% & 3.07\% & 38.5\% \\ \cline{1-1}  & 10\% & 2.82\% & 53.9\% \\ \hline \multirow{4}{*}{Electric arc furnace} & 2\% & 5.00\% & 23.1\% \\ \cline{1-1}  & 3\% & 4.60\% & 38.5\% \\ \cline{1-1}  & 5\% & 3.66\% & 46.2\% \\ \cline{1-1}  & 10\% & 2.58\% & 61.5\% \\ \hline \hline \end{tabular}
\end{table}
Table 2: Test results for different scenarios.  
Figure 8: The simulation results: (a) the instantaneous voltage at Bf12, (b) the instantaneous current at Bf12, and (c) the actual fault current and the fault level estimation (FLE) value.  
The feasibility of the proposed method depends on the accurate recording of the desirable information. An identification method to detect the disturbance direction helps to distinguish the wanted disturbance. The fuzzy logics are applied to classify the voltage and current changes incurred by the disturbance. Only the qualified disturbances will launch the estimation. The fuzzy logics overcome the difficulty in choosing the deterministic thresholds.  
In addition, the least-square solutions with weighted confidence derived from fuzzy logic identifier help to decrease the estimation errors. The usage of the confidence index in the algorithm takes full advantage of all possible disturbances and disposes with them in the different weights. The results derived from the large disturbance are more accurate, with a larger confidence, as the measurement errors (in percentage) can be reduced. The smaller the disturbances are, the less accuracy and confidence are delivered. However, the small disturbances, which may occur more frequently in the practical distribution networks, are also needed so as to decrease the time intervals to launch the estimation. Therefore, the performance of the algorithm should be balanced between the accuracy and the efficiency.  
The algorithm is successfully embedded into the protective relay, to implement the proposed method in practice. A distribution power network is used to validate the proposed method. The results proved the effectiveness and the accuracy of the presented method. Meanwhile, it is true that PMUs can alternatively provide more accurate data and more rapid data processing for the proposed method, if necessary.  
Conceptualization, X.W. and F.S.; methodology, F.S.; validation, P.W.; formal analysis, Y.W.; writing--original draft preparation, F.S.; writing--review and editing, X.W.; supervision, Y.W.; funding acquisition, F.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Key Laboratory of Control of Power Transmission and Conversion (SJTU) Ministry of Education (2016 AA13) and the National Nature Science Foundation of China (514770929).  
The authors declare no conflict of interest.