# Fault Analysis and Protection Principle for the Distribution Networks Integrated with PV and BESS
## 2 Fault Characteristics of the Distribution Networks with PV and BESS
### FRT Control Strategy of PV and BESS  
#### 2.1.1 Symmetric Faults  
According to the current grid connection standard GB/T 34120-2023 [22], \"Technical requirements for power conversion system of electrochemical energy storage system\", during symmetric faults, the PV and BESS should operate in FRT mode and inject positive sequence reactive current into the network to support the voltage at the PCC. Since the inverters used in both the PV and the BESS adopt the constant power control method, their control strategies during the fault period are similar. The details are as follows: During symmetric faults, similar to the conventional negative sequence current suppression control strategy, priority is typically given to controlling reactive power to provide voltage support for the system. Since the PV and BESS controls the reactive power magnitude by adjusting the reactive current component, when a fault occurs causing a voltage variation at the PCC, the positive sequence reactive current reference value \(I_{q,ref}^{+}\) can be adjusted based on the voltage sag level at the PCC of the PV and BESS system to regulate the reactive power delivered by the PV and BESS, with specific values specified in the grid connection standards. Meanwhile, due to the output current limitations of PV and BESS, the positive sequence active current reference value \(I_{d,ref}^{+}\) is determined by \(I_{q,ref}^{+}\), as shown in Equation (1).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**System Parameters** & **Parameter Value** \\ \hline Rated voltage/frequency of the AC system & 10 kV/50 Hz \\ AC voltage of the converter & 0.4 kV \\ DC voltage of the converter & 1.0 kV \\ Rated capacity of the PV & 5 MVA \\ Rated capacity of the BESS power station & 1 MVA \\ Capacities of loads Load1 and Load2 & 2.5 MVA \\ Power factors of loads Load1 and Load2 & 0.87 \\ Length of line AB & 4 km \\ Lengths of lines AC and CD & 5 km \\ Unit Impedance of the line & 0.26 + j0.42 \(\Omega\)/km \\ Equivalent impedance under the maximum & j0.45 \(\Omega\) \\ operating mode of the system & j0.63 \(\Omega\) \\ Equivalent impedance under the minimum & j0.63 \(\Omega\) \\ operating mode of the system & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Specific parameters of the distribution network with PV and BESS.  
Figure 1: Structural diagram of a distribution network at a certain location in Tianjin connected with PV and BESS.  
\[\left\{\begin{array}{ccc}I_{q,ref}^{+}=\left\{\begin{array}{ccc}0&\frac{\sqrt{3}| \boldsymbol{u}^{+}|}{U_{N}}>0.9\\ 2\times(0.9-\frac{\sqrt{3}|\boldsymbol{u}^{+}|}{U_{N}})\times I_{N}&0.35<\frac{ \sqrt{3}|\boldsymbol{u}^{+}|}{U_{N}}<0.9\\ 1.1\times I_{N}&\frac{\sqrt{3}|\boldsymbol{u}^{+}|}{U_{N}}\leq 0.35\end{array}\right.\end{array}\right. \tag{1}\]  
Among them, \(\boldsymbol{U}^{+}\) is the phase voltage vector at PCC; \(U_{N}\) is the rated line voltage of the distribution network; \(I_{N}\) is the rated phase current of the PV and the BESS output; \(I_{dref.normal}^{+}\) is the positive sequence active power current command value given by the DC voltage outer loop when the system is not in a fault state; \(k_{pdc}\) and \(k_{idc}\) are the proportional and integral coefficients of the DC voltage outer loop in the constant power control mode; \(U_{dc.ref}\) and \(U_{dc}\) are the reference and actual values of the DC voltage under constant power control mode respectively; \(I_{\text{lim}}\) is the output current limit.