## 4 Case Study and Results  
The application of a hybrid AC/DC system with the proposed security ANM framework is now analysed in an urban UK distribution system. The application of the ANM framework is assessed based on minimum DG curtailment and demand shifting that could be achieved under relevant system contingencies. The test system considered for this study consists of the 11-kV urban 77-bus UK generic system in [32] with a MVDC interface at the end of all feeders of the system, as shown in Figure 6. The distribution system is fed from a 33-kV utility grid through two substation transformers. The hybrid AC/DC distribution system contains 75 AC lines and 7 additional DC lines. Four of the DC lines are considered as the existing tie lines (marked with red solid lines). It is assumed that the system is supported by 23 wind-based DGs with a capacity of 1.73 MW each. The total peak load is 24.274 MW and 4.855 MVar. The off-peak load is considered at 40% of the peak load. The voltage limits are set to 0.97/1.03 p.u. (\(\pm\)3% of the nominal voltage) at all buses and thermal limits as given in [32] are used in this study. These limits are relaxed at short-term emergency ratings of 0.94/1.06 p.u. at all bus voltages and thermal limits are 20% higher than the continuous current ratings of the original case. The parameter of the VSCs are provided in Table 1. The demand and generation profiles were obtained from the historic demand and wind data of central Scotland in 2003, as presented in [25]. The data contain two sets of wind generation profiles (as a percentage of the total generation capacity) and one set of demand profile (as a percentage of the system peak load). The number of coincident hours per year for each condition that occurred in the year is also provided. In this study, we have chosen a subset of the original data used containing only instances of high-generation low-demand and low-generation high-demand for our simulation purposes. The selected critical case scenarios of demand and generation profiles are described in Table 2. \begin{table}
\begin{tabular}{l l} \hline \hline
**Item** & **Parameter Settings** \\ \hline VSC rating & 5 MVA \\ DC voltage & \(\pm\)33 kV \\ Phase reactor, \(x_{\sigma}\) & 0.1 p.u. \\ Constant conductance, \(G_{0}\) & 0.01 p.u. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Converter parameter settings. Figure 6: The modified hybrid AC/DC UK generic distribution system. As described earlier in Section 2, we ran a steady-state security assessment for the original test system (without modifications) to identify all the binding contingencies. Since historic data was used, we only considered the critical case scenarios given in Table 2. For the original UK generic system, there are 19 critical contingency cases given in Table 3. We considered these for the modified hybrid network as well. From Table 3, it is clearly shown that outages of any line in the short feeders (F1 to F4) and tie-lines are non-binding. Three configurations are considered:  
* Base case: Original AC configuration;
* Point-to-point (P2P): VSCs are installed at end of the tie lines with P2P configuration, as proposed in [6; 13];
* MVDC: Modified hybrid configuration, where the MVDC interface acts as a platform for all DGs with internal DC links and loads, as shown in Figure 5. The MVDC configuration is an extension of use of DC links in [6; 13], using the same number of VSC stations. \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**No.** & **Wind 1 (\%)** & **Wind 2 (\%)** & **Load (\%)** & **No. of hours** \\ \hline  
\hline \hline \end{tabular}
\end{table} 
Table 2: Critical case scenarios obtained from [25]. \begin{table}
\begin{tabular}{l c} \hline \hline
**Feeders** & **Outages (lines)** \\ \hline F5 & 2–17, 17–18, 18–19, and 19–20 \\ F6 & 2–28, 28–29, 29–30, and 30–31 \\ F7 & 2–39, 39–40, 40–41, and 41–42 \\ F8 & 2–53, 53–54, 54–55, 55–56, 56–57, 57–58, and 58–59 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Critical contingency cases.