# A Three-Port Power Electronic Transformer Based on Magnetic Integration
## 3 Key Parameters Design
### MVDC Filter  
At the MVDC port, a small filtering capacitor \(C_{\text{f}}\) is installed to provide low-impedance return path for the HF currents, while a filter inductance \(L_{\text{f}}\) is used to block the HF current. Figure 3: Winding arrangement: (**a**) layered structure; (**b**) stacked structure. Figure 7 demonstrates the flexible power coordination of the proposed PET under three dynamic processes. In process I, the MVDC port maintains constant power (\(-3\) MW, a negative sign indicates that power flow out from PET and a positive sign indicates flows in), which is not affected when the power between MVAC and LVDC ports changes from \(-0.5\) MW to \(+0.5\) MW. Then, in process II, when the coordinated power between MVDC and LVDC ports varies from \(0.5\) MW to \(-0.5\) MW, the MVAC port can also keep constant power (\(-3.5\) MW). Process III further shows that power of the LVDC port is not affected when power between the MVAC and MVDC ports are changed from \(-3\) MW to \(3.5\) MW. During all these dynamic processes, the average SM capacitor voltage is kept at nominal value \(800\) V, indicating that PET energy is balanced. These dynamic simulation results confirmed the flexible and independent power control among three ports. From the current waveforms in Figure 7, it can be seen that HF ripple of the MVDC current is negligibly small (amplitude of \(2.5\)A) comparing with nominal MVDC current of \(167\)A. Additionally, the MVAC current shows that it is not affected by HF components. It can be found that the currents of three ports are continuous and smooth. Figure 8 present the steady-state waveforms with a \(4\) MW MVDC port, \(-3.5\) MW MVAC port, and \(-0.5\) MW LVDC port, respectively. Figure 8a verifies that the arm voltage described in (1) contains \(12\) kV DC, \(8.16\) kV/\(50\) Hz LF, and \(2.4\) kV/\(2\) kHz HF voltage components. The HF voltage component is applied on the primary winding, with \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Simulation Value** & **Experiment Value** \\ \hline Line-to-line RMS voltage of MVAC & \(10\) kV & \(200\) V \\ Rated MVDC voltage & \(24\) kV & \(600\) V \\ Rated LVDC voltage & \(750\) V & \(100\) V \\ Line MVAC inductance \(L_{\mathrm{g}}\) & \(10\) mH & \(2\) mH \\ Rated SM capacitor voltage \(U_{\mathrm{SM}}\) & \(800\) V & \(100\) V \\ MMC SM capacitor C\({}_{\mathrm{SM}}\) & \(5.5\) mF & \(2\) mF \\ Number of SM per arm \(n\) & \(30\) & \(6\) \\ Magnetizing inductance of HF transformer \(L_{\mathrm{m}}\) & \(20\) mH & \(10\) mH \\ Leakage inductance of HF transformer \(L_{\sigma}\) & \(158.2\) uH & \(368\) uH \\ Switching frequency of SM\(f_{\mathrm{s}}\) & \(1\) kHz & \(6\) kHz \\ Frequency of injected HF voltage & \(2\) kHz & \(2\) kHz \\ HF transformer turn ratio \(n\):\(n\):\(1\) & \(3.2\):\(3.2\):\(1\) & \(1\):\(1\) \\ MVDC inductance \(L_{\mathrm{f}}\) & \(600\) uH & \(100\) uH \\ MVDC capacitor C\({}_{\mathrm{f}}\) & \(30\) uF & \(30\) uF \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of proposed PET. Figure 7: Dynamic simulation results of proposed three-port PET. each split winding \(u_{\text{ma}}\) (2.4 kV/2 kHz), which is reflected to secondary winding as \(u_{\text{sa}}\) (750 V/2 kHz). The upper- and lower-arm currents are shown in Figure 8b, which contain the 56A DC, 143A/50 Hz LF, and 76A/2 kHz HF current components. The HF current ripple on magnetizing inductance \(\Delta I_{\text{mag\_PP}}\) = 59.5A coincides with the analyzed value in (9). Figure 8c displays a delay time \(\Delta T\) = 63 \(\upmu\)s (phase shift angle of \(\pi\)/4) between primary and secondary winding voltages, and secondary winding current with an amplitude of \(\Delta I\) = 296A when the LVDC port power is \(-\)0.5 MW. Figure 8d shows that the SM capacitor voltage fluctuation \(\Delta U_{\text{C}}\) = 35 V keeps well below the maximum allowable value as calculated in (12).