**5. Verification of the High-Power-Density Converter Design**

After the 5L-3L back-to-back converter prototype was assembled, it was tested under different load conditions to verify two important questions. One was whether the loss estimation was correct or not. Correct loss estimation demonstrated correct topology selection. The other was whether the proposed cooling system structure could dissipate the power loss generated by the converter operation or not.

In order to test the power loss of the converter, each side of the converter was tested in the inverter mode, where the DC link voltage was connected to an 800 V DC source and the AC output was connected to a purely inductive load. The test set is given in Figure 11. The DC-side efficiency was tested by using a power analyzer Norma D 5000 (Fluke, Everett, WA, USA). By using this method, each side of the converter was powered to full-load conditions, while the DC source only provided a small amount of power, which was equal to the power losses on both converters and inductive loads. During the test, by varying the modulation indices *M* of both sides of the converter, the output current was changed from zero to its maximum point. Then, the total power losses were recorded. The power losses of the inductive loads were estimated with the models described in [21,22]. By deducting the power losses on inductive loads, the power losses on converters were finally derived.

**Figure 11.** Experimental setup for the full-power test.

Figure 12a shows tested losses and estimated losses for the 3L-NPC under different load conditions. The results showed that the estimation of the conduction loss was quite precise within the whole-load condition. The estimation of the switching loss showed a slightly larger error than that of the conduction loss. Nevertheless, the accuracy was accepted for topology evaluation. Figure 12b shows tested losses and estimated losses for the 5L-HANPC under different load conditions. The results also showed that the estimation of the power loss was correct.

The thermal system performance was verified by investigating the temperature rise on each negative temperature coefficient (NTC) sensor embedded in IGBT modules. There were totally 18 NTC resistors belonging to 18 IGBT modules. The ambient temperature *T*<sup>a</sup> for testing was 20 ◦C, and the air flow was 120 cubic feet per minute (CFM). The converter was tested under full-load conditions: *V*dc = ±400 V; *I*ac = 100 A. The data received from the NTC temperature sensor are plotted in Figure 13. From the results, it can be found that the distributions of the temperature were similar in all three surfaces. On the other hand, the 3L-NPC modules had the highest temperature rise. This is because the power loss per each module in the 3L-NPC was still larger than the power loss per module in the 5L-HANPC.

**Figure 12.** Comparison of tested and estimated power losses for the (**a**) 3L-NPC and (**b**) 5L-HANPC.

**Figure 13.** Temperature rise on negative temperature coefficient (NTC) sensors of IGBT modules (*V*dc = ±400 V, *I*ac = 100 A).

### **6. Evaluation and Comparison**

With the developed 5L-3L back-to-back converter prototype, not only the power density and the efficiency performance of this prototype could be measured, but the other two possible solutions, which were the ATRU/3L-NPC and the 3L-3L back-to-back converter, could also be evaluated by estimating the weight of each main parts and also estimating the power losses according to the calculation derived in Section 3.

Based on the similar commercially available products on the market, the weight of a 60 kW ATRU was aggressively estimated at 20 kg, which is given in Table 7. On the other hand, the AC-input inductor used in the 3L-3L back-to-back converter should be larger than the inductor used in the 5L-3L back-to-back converter prototype. According to the calculation, an inductor with an electrical inductance of 100 μH and an AC current of 150 A was adopted. The weight of this inductor was also estimated and is given in Table 7.

**Table 7.** Weight estimations for magnetic components used in different configurations.


Besides the magnetic components, the weights of the other parts were also estimated. If the 5L-HANPC was replaced with the 3L-NPC on the rectifier part, the weight differences would mainly come from the elimination of six IGBT modules and flying capacitors and from the increased size of the AC Inductor. The weight distribution of each part for the 3L-3L back-to-back converter is shown in column IV of Table 8. If the 5L-HANPC was replaced with the 60 kW ATRU, the heatsink weight could

be decreased by half since only the inverter part required the heatsink, and also the heat dissipation roughly decreased by half. All 5L IGBT modules and flying capacitors were eliminated. The weights of other components, including the auxiliary supply, copper bars, and control PCBs, was estimated to decrease 1 kg. By adding the weight of the ATRU, the total weight of the ATRU-3L converter is shown in column II of Table 8.


**Table 8.** Power density comparison between three possible system configurations.

Since according to our testing results the calculation of the power losses matched the tested power losses very well, the calculation was proved to be quite accurate. The estimation of the power losses for each converter also followed the results derived in Figures 4 and 5. The power losses of the ATRU were not directly calculated. From the product brochure [12], the maximum efficiency was 98%.

The final comparisons in terms of total weight, power density, and expected efficiency are given in Table 8. In order to clearly show the benefit of the proposed solutions, the weight performance of the traditional ATRU-2L-VSC solution is also given in Column I of Table 8. The weight of a commercially available 50 kW 2L-VSC inverter product for aerospace with liquid cooling was found to be 21.8 kg. Thus, the total weight of the ATRU-2L-VSC system was 41.8 kg. It should be noticed that the ATRU used in the comparison was also only designed for the liquid-cooling system, which actually is not fully available in this application area. An air-cooled ATRU should have an even heavier weight.

The comparative results showed that the 3L-3L back-to-back converter was a better solution in terms of power density among the three potential solutions. The reasons are as following: Firstly, the heavy ARTU was replaced by the power converter, while at the same time a good balance between the number of active components and the volume of reactive components was found. The highest power density achieved was 2.79 kVA/kg, if we chose the apparent power rating of the machine (80 kVA) as the base power. The power density of the proposed 5L-3L back-to-back converter was 2.61 kVA/kg. It was also quite high, which was 30% higher than that of the traditional solution ATRU-2L converter. By considering the power efficiency, the proposed 5L-3L back-to-back converter showed a better performance compared to the 3L-3L back-to-back converter. The efficiency at the rated power of the back-to-back system, including the rectifier and inverter stages, was 95.8%. This is because of the highly efficient 5L converter. Furthermore, power density improvement can be made for this asymmetrical-voltage-level topology. Since the switching losses of the 5L rectifier stage were pretty low, if the higher switching frequency was selected, it could use smaller flying capacitors and smaller AC side filters, which would in turn decrease the total weight of the system.

### **7. Conclusions**

According to our work, it can be concluded that the power density of a converter for a high-power-rating motor drive system in aerospace can be improved by replacing an ATRU with the power converter. By properly designing the heatsink structure, the multisurface cooling system can achieve considerably low weight and small size. Eventually, the heatsink size and weight were limited by the number and area of IGBT modules, if the converter topology had high efficiency. By evaluating the prototype performance, the finally achieved power density of the 5L-3L back-to-back converter prototype was 2.61 kVA/kg, which was 30% higher than that of the traditional solution. The efficiency could be up to 95.8%. The expected power density of the 3L-3L back-to-back converter prototype was 2.79 kVA/kg.

In the high-power motor drive system for aerospace applications, a multilevel-converter-based system showed advantages including fewer power losses and higher power density compared to transformer-based systems. In fact, in other applications with the TeDP architecture, multilevel converters can also provide advanced solutions with similar benefits. More importantly, multilevel-converter-based solutions can provide more flexibility. For example, a multilevel-converter-based rectifier provides a DC mid-point, which can potentially connect to the ground point in aircraft. Properly designing a modulation scheme can alleviate common-mode problems. In addition, when the motor speed is low, a converter-based rectifier can lower the DC bus voltage to minimize the switching loss of the system. With all these benefits, it is believed that in the near future, multilevel-converter-based rectifier–inverter power units will realize their wide applications in TeDP architectures.

**Author Contributions:** C.L. and J.Z. proposed the new back-to-back configuration and developed the program to control the converters. Y.Z. prepared the draft of the paper and carried out the experiment. D.X. contributed his expertise to the proposed subject of research and supervised the work when the first author was in Canada. W.L., H.M., and X.H. contributed their expertise to the proposed subject of research and supervised the work in China. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is sponsored by the National Nature Science Foundation of China (51807176 and U1834205), the National Key Research and Development Program of China (2017YFE0112400) and the Zhejiang Provincial Key Research and Development Program (2018C01SA150059).

**Conflicts of Interest:** The authors declare no conflicts of interest.
