# Topology and Control Strategy of PV MVDC Grid-Connected Converter with LVRT Capability  
[PERSON]  
1 Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China;  
[EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (X.H. ); [EMAIL_ADDRESS] (Y.W. );  
[EMAIL_ADDRESS] (H.L. );  
2 School of Electronics, Electrical and Communication Engineering, University of Chinese Academy of Sciences, Beijing 100190, China 2  
[PERSON]  
1 Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China;  
[EMAIL_ADDRESS] (Y.Z. ); [EMAIL_ADDRESS] (X.H. ); [EMAIL_ADDRESS] (Y.W. );  
[EMAIL_ADDRESS] (H.L. );  
[EMAIL_ADDRESS] (H.L. );  
zhouyu@mail.iee  
Plenty of research works on isolated dc-dc converters have been carried out, but most of them are not fit for MVDC application because of limited ratings of the semiconductors. Series-connected active semiconductors are a solution for MVDC application [8], but it is difficult to keep voltage balance on the switches, especially in high frequency. Modular multilevel dc-dc converter is one solution for MVDC output. A high-frequency-link dc-dc converter based on modular multilevel topology is proposed in [9; 10]. However, the voltages of these topologies are stepped up only by transformer and the boosting capacity is limited. As the capacity increases above megawatt level, design and manufacturing of high-frequency transformer (HFT) become more difficult in terms of insulation, cooling [11]. Topology of dc-dc modules with input parallel and output series (IPOS) configuration is another approach for MVDC application. The output voltages of the modules are connected in series to realize the MVDC output of the converter. The capacity and voltage level of the dc-dc modules are greatly reduced relative to the converter. Thereby the difficulty of designing and manufacturing the dc-dc module and HFT is reduced. Dc-dc modules are the core in an IPOS converter. Different module topologies will lead to different characteristics of the converter. In order to realize high-voltage output and increase the power density of the converter, medium-voltage dc-dc module is required to reduce the number of modules. Dual-Active-Bridge (DAB) is a popular topology in solid state transformer. The voltage gain range is widened by optimized modulation method [12], but it is not fit for high-voltage gain application because of low-input voltage utilization and limitation of boosting capacity. High-voltage gain can be achieved in isolated boost topology [13]. However, it cannot work when the output voltage is lower than input voltage, LVRT cannot be realized. In addition, a startup circuit on the high-voltage side is required in boost topology. An isolated buck-boost converter can operate on either buck or boost mode to widen the output voltage range [14]. Not only high-voltage gain, but also wide-voltage range is achieved by the isolated buck-boost converter. Some isolated buck-boost converters are derived by merging a buck converter with a boost converter. A buck cascades a DAB to form a buck-boost topology in [15; 16]. A switched-capacitor-based submodule cascades a DAB to form a buck-boost in [17]. The cascaded two stages decrease the efficiency and increase the cost of the converter. The semi-active rectifier-based isolated buck-boost converters are developed in [18; 19]. A three-level full-bridge isolated buck-boost converter is proposed in [20] to increase the output voltage. However, active switches should be included on both the primary and secondary sides of the transformer to achieve boost and buck mode in the existing isolated buck-boost converters. The voltage rating of active switches limits the output voltage level of the converter. Several works are done on the dc converter to avoid overcurrent when voltage dip occurs. A practical solution of dc-dc converter based on switched capacitor is proposed in [21]. The switched capacitor can disconnect from the MVDC grid effectively as dc breaker to avoid capacitor discharge during LVRT.