High-Frequency-Link-Based Reactive-Power Optimal Control for Modular Multi-Active Bridge Converter
Authors: Haoyu Wang; Shiqi Ji; Di Mou; Wenhao Xie; Yangbin Zeng; Zhengming Zhao
Extended Abstract:
Modular multi-active bridge (MMAB) converters, with inherent advantages of galvanic isolation, modularity, and high power density, can provide multiple dc ports for multifarious power conversion through high-frequency links (HFLs), making them attractive for integrating distributed renewable energies, storage equipment, and time-varying dc loads into distribution grids. However, because power flows are strongly coupled among ports through the HFL, the traditional single phase-shift (SPS) control, which is widely applied for its simplicity in regulating port voltages and powers, suffers from limited control degrees of freedom. Under conditions with mismatched voltage conversion ratios and light loads, SPS control causes some ports to partly lose zero-voltage-switching (ZVS) operation and produces large root-mean-square (RMS) inductance currents, leading to unfavorable efficiency performance.
To independently handle each port with optimal capabilities, this letter proposes a dual-loop optimal control for MMAB converters to achieve the decoupling of active and reactive powers. A unified frequency-domain model of MMAB and multi-active bridge (MAB) converters is first established without reliance on accurate model parameters, and the multiport system is separated into multiple subsystems with independent control loops. Conditioning circuits implemented with FPGA-based direct digital synthesis, analog multipliers, and voltage-frequency converters are designed to extract the magnitude and phase information of high-frequency components, which is used to eliminate the fundamental reactive power and thereby optimize conducting and switching characteristics through reducing the overall RMS currents. By regarding each port as a subsystem regardless of the coupling through the HFL, the proposed reactive-power optimal control (RPOC) operates online without dependence on accurate modeling and can be easily extended to any multiport system with HFLs. Finally, the effectiveness of the proposed strategy is validated on a hardware-in-the-loop-based four-port MMAB converter. Experimental results show that, across load conditions ranging from light to relatively high loads, RPOC reduces the overall RMS currents significantly (nearly 50%) and achieves ZVS operation in all ports, confirming improved system efficiency and performance.
Additional Information:
The proposed method has been furtherly used in our multi-port MW level solid state transformer (SST), highly improving the overall efficiency of the converter. The developed multi-port SST has been deployed in data center, university campus, industry campus, etc. with at least four ports including 10 kVac, 10 kVdc, 750 Vdc and 380 Vdc.
