# A Three-Port Power Electronic Transformer Based on Magnetic Integration
## 3 Key Parameters Design
### Experiment Verification  
To further verify the proposed PET, a downscaled prototype with six SMs per arm, has been built as in Figure 9a. The ferrite magnetic core chosen for integrated transformers is UU66/55/40. The inner and outer loop control algorithms are implemented in the TMS320F28377D DSP, while an EP3C25Q240 FPGA is used to perform the modulation scheme. As shown in Figure 9b, the prototype was tested at the condition of 5 kW/600 V MVDC port, 3.5 kW/200 Vrms MVAC port and 1.5 kW/100 V LVDC port. Compared with simulations, the experimental values of MVAC inductance \(L_{\text{g}}\), magnetizing inductance \(L_{\text{m}}\), MVDC inductance \(L_{\text{f}}\) and SM capacitors are smaller due to downscaled voltages and currents. The leakage inductance is larger because the LVDC power is only 1.5 kW. In addition, since there are only six SMs in each arm, the SM switching frequency is higher  
Figure 8: Steady-state simulation results: (**a**) arm voltages and transformer winding voltages; (**b**) arm currents and magnetizing current; (**c**) transformer winding voltages and secondary winding current; (**d**) six arm SM voltages; (**e**) secondary winding current with double LF fluctuation; (**f**) secondary winding HF current after applying the proposed modulation. (6 kHz) in experiments. Detailed experiment parameters are listed in Table 1. Experimental results in Figure 10a verifies that the arm voltage contains 300 V DC, 163 V/50 Hz LF and 100 V/2 kHz HF components. The HF voltages applied on primary (\(u_{\text{ma}}\)) and secondary (\(u_{\text{sa}}\)) winding can be observed as 100 V/2 kHz. Similarly, Figure 10b shows that the upper- and lower-arm currents contain 2.8A DC, 7.1A/50 Hz LF, and 5.2A/2 kHz HF components. The HF current ripple on magnetizing inductance \(\Delta I_{\text{mag\_PP}}=5\)A also agrees with the analyzed value as calculated in (9). The MVAC current waveform confirms that it has not been affected by HF components. Figure 10c shows a phase shift angle of \(\pi/12\) (\(\Delta T=20\)\(\upmu\)s) between primary and secondary winding voltages, and the secondary winding current amplitude is \(\Delta I=5.4\) A, when LVDC power is 1.5 kW. It can be found in Figure 10d that the SM capacitor voltage fluctuation \(\Delta U_{\text{C}}=5\) V keeps well below the maximum allowable value as calculated in (12). After applying the proposed modulation scheme, the current stress of 18A [see Figure 10e] is reduced to below 10A [see Figure 10f], and the soft switching characteristic is guaranteed as the double LF fluctuation has been eliminated. As shown in Figures 8 and 10, there are some differences in values between experiments and simulations. However, the experiment matches the simulation well in principle. In fact, there are some switching spikes (at MMC equivalent switching frequency of \(f_{\text{SW}}=n\times f_{\text{S}}\)) existing in the HF current waveform \(i_{\text{sa}}\), which are caused by switching of MMC SMs. As shown in Figures 8c and 10c, the switching spikes are negligibly small because the leakage inductance \(L_{\sigma}\) has a high impedance for switching frequency (30 kHz and 36 kHz for simulation and experiment, respectively), and these detailed switching spikes can be found in the zoom-in waveform of \(i_{\text{sa}}\). Figure 9: Prototype: (**a**) photograph of the laboratorial three-port PET prototype; (**b**) circuit diagram of the test platform. Figure 11 further provides the experimental verifications under dynamic power exchange condition to illustrate the stability. The MVAC/MVDC ports dynamic power exchange results are shown in Figure 11a, while the LVDC/MVDC ports dynamic power exchange results are shown in Figure 11b. In Figure 11a, LVDC port implements constant power control, while MVAC port dynamically adjusts power and the MVDC port performs PET energy balance control. During 1.0-8.5 s, the power between MVAC and MVDC ports changes from \(-1\) kW to \(+3.5\) kW, the LVDC port maintains constant power of 1.5 kW, the voltages and currents of all ports are smooth.