## 2 Active Distribution Network Communication Demand Analysis
### The \(\mu\)MPMU Application
Based on the description and method given in a previous study [27], conventional state estimation generally uses SCADA data for parameter identification of lines and transformers. However, the SCADA data failed to provide a uniform timestamp across the entire network. Thus, it is impossible to obtain a strictly time-synchronized data section. In addition, the parameter identification based on the SCADA data needs to use the global measurement value to determine the global parameters, while there is a defect of residual pollution. All those limitations can be resolved by deploying uMPMUs into the distribution network. The measurement of the uMPMU is based on the global synchronous clock, which is able to achieve a high time precision and data accuracy that's required by online parameter identification of the power distribution system. Also, it can solve the problem of high-precision dynamic measurement that is difficult to achieve by the traditional power distribution terminal (FTU, DTU, fault indicator, etc.) in the distribution network. The following is an example of the transformation ratio and parameter calculation of the three-winding transformer to illustrate the benefits of introducing the uMPMU for parameter calculation. Power transmission is performed by the high-voltage side and the medium-voltage side of the three-winding transformer, and reactive power compensation is done by the low-voltage side. Its equivalent circuit and phasor relationship are shown in Figure 2. \(Z_{1}\), \(Z_{2}\), and \(Z_{3}\) represent the impedances of the high-voltage, medium-voltage, and low-voltage side windings, respectively, Zm is the excitation impedance, \(\alpha\) represents the angle between \(\dot{I_{1}}\) and \(\dot{U_{2}}\), \(\varphi_{2}\) represents the angle between \(\dot{I_{2}}\) and \(\dot{U_{2}}\). Approximately, the angle between the excitation current \(\dot{I_{3}}\) and the low-voltage side winding current \(\dot{I_{m}}\) and \(\dot{U_{2}}\) is \(90^{\circ}\), so that the calculation formula of the three-winding transformation ratio can be obtained:  
\[k=\frac{I_{2}}{I_{1}}\frac{\cos\varphi_{2}}{\cos\alpha} \tag{1}\]  
According to the calculated actual ratio, the medium voltage side electrical quantity and parameters are reduced to the high voltage side, then  
\[\Delta U=\overset{\bullet}{U_{1}}-\overset{\bullet}{U_{2}}=\overset{\bullet} {I_{1}}Z_{1}+\overset{\bullet}{I_{2}}Z_{2} \tag{2}\]  
Ignoring the influence of the branch resistance, let the angles of \(\dot{I_{1}}\) and \(\dot{I_{2}}\) between \(\Delta U\) be \(\theta_{1}\) and \(\theta_{2}\), respectively, and solve the above equation. \[\left\{ \begin{array}{l}X_{1}=\frac{\Delta U}{I_{1}\cos\theta_{1}}\frac{ \Delta U}{\theta_{2}-I_{1}\sin\theta_{1}}\\ X_{2}=\frac{I_{1}\cos\theta_{1}}{I_{2}\cos\theta_{2}}X_{1}\end{array} \right. \tag{3}\]  
By using the phasors of voltage and current with synchronous time stamping of uMPMU to solve the above formula, parameters such as transformer ratio and winding reactance can be computed. Figure 2: (**a**) Equivalent circuit for three-winding transformer with tap changer; (**b**) phasor relationship of three-winding transformer.