## Abbreviations  
ETSHE Electric Thermal Storage Heating Equipment  
DEA Data Development Analysis  
FCM Fuzzy C-means  
J Value function of FCM  
m Membership factor  
K Number of clustering  
U Membership matrix  
Figure 15: Percentage improvement of voltage fluctuation relative to the standard value.  
\begin{tabular}{l l} C & Cluster center \\ \(W_{wind}\) & External wind speed value [m/s] \\ \(\Delta T_{wind}\) & Equivalent temperature of wind speed cooling [\({}^{\circ}\)C] \\ \(\alpha,\beta\) & Wind speed correction coefficients \\ \(S_{solar}\) & External light value \(\left[W/\text{m}^{2}\right]\) \\ \(S_{k}\) & Light conversion coefficient \(\left[W/\left(\text{m}^{2}\ ^{\circ}\text{C}\right)\right]\) \\ \(f_{Tinus_{4}}\) & Total current harmonic distortion \\ \(I_{1}\) & Root mean square value of the fundamental current [A] \\ \(I_{h}\) & Root mean square value of the \(h\)-th harmonic current [A] \\ \(U_{ij}\) & Voltage of node \(i\) at time \(j\) [V] \\ \(U_{N}\) & System standard voltage [V] \\ \(\Delta I\) & Current change caused by the output power change [A] \\ \(Z\) & Equivalent impedance of the two-port network distribution network [\(\Omega\)] \\ \(\rho_{S}\) & Unit price corresponding to the unit heat supply of the electric boiler \\ \(Q_{c}^{\circ}(Q_{S})\) & Rated heat supply of the electric boiler [kWh] \\ \(\rho_{X}\) & Unit price of the heat regenerator [V] \\ \(V_{X}\) & Volume of the heat accumulator [m\({}^{3}\)] \\ \(\rho_{B}\) & The increased equipment investment for each additional unit of the \\ \(\rho_{B}\) & rated flow [V] \\ \(\gamma\) & Safety factor when calculating the rated flow \\ \(C_{1}\) & Conversion constant of kilowatt hours and joules \\ \(C_{W}\) & Specific heat of the heat medium [J/(kg\({}^{\circ}\)C)] \\ \(\rho_{W}\) & Density of the heat medium [kg/m\({}^{3}\)] \\ \(\Delta\tau_{T}\) & Difference in heat storage temperature [\({}^{\circ}\)C] \\ \(\rho_{C}\) & The investment cost of the control device for each additional 1 kW of the \\ \(k_{ins}\) & Equipment installation cost coefficient \\ \(\epsilon,\beta,\tau,\zeta\) & Normalized weight coefficient \\ \(P_{i}\) & Active power of node \(i\) [kW] \\ \(Q_{i}\) & Reactive power of node \(i\) [KVar] \\ \(U_{i}\) & Voltage amplitudes of node \(i\) [V] \\ \(U_{j}\) & Voltage amplitudes of node \(j\) [V] \\ \(G_{ij}\) & Conductance of branch \(ij\) [S] \\ \(B_{ij}\) & Susceptance of branch \(ij\) [S] \\ \(\theta_{ij}\) & Voltage phase angle difference between \(i\)-th and \(j\)-th node \\ \(Q_{q}(t)\) & Heat provided by the heat storage body to the user during time \(t\) [kWh] \\ \(Q_{l,max}\) & Maximum design daily heat load [kW] \\ \(r_{l}(t)\) & The ratio of heat load to maximum value in period t of design day \\ \(Q_{N}(t)\) & Heat stored in the heat storage body at the beginning of time t [kWh] \\ \(\alpha_{an}\) & Sensitivity coefficient \\ \(w_{j}\) & Relative production efficiency \\ \(u_{r}\) & Weight of the \(r\)-th power quality indicator \\ \(v_{i}\) & Weight of the \(i\)-th quality level \\ \(x_{rj}\) & \(r\)-th power quality input indicator in the \(j\)-th DMU \\ \(y_{ij}\) & \(i\)-th quality level output in the \(j\)-th DMU indicator \\ \end{tabular}