Control Strategy of CLLC-Based Interface Converter Between AC/DC Buses under Unbalanced Condition

doi:10.12204/j.issn.1000-7229.2021.11.003

Abstract

Abstract:

Aiming at the problem of double-frequency pulsation of DC bus voltage in microgrid under unbalanced AC voltage condition, a control strategy for two-stage AC/DC interface converter with CLLC DC transformer is proposed, which can ensure the three-phase currents balance and suppress the voltage pulsation of DC bus under balanced and unbalanced AC bus voltage. Firstly, the power transmission characteristics of the AC/DC interface converter under unbalanced condition are analyzed, and the control strategy to suppress the negative-sequence current at the AC side is designed. Secondly, the fundamental equivalent model of CLLC DC transformer is established, and its voltage gain and input impedance characteristics are analyzed. On this basis, considering the input voltage pulsation characteristics of CLLC DC transformer under unbalanced conditions, the optimal parameter design of CLLC DC transformer is carried out and a control strategy based on the feedforward of pulsation voltage is proposed to suppress the voltage pulsation of DC bus. Finally, the Matlab/Simulink simulation results show that the proposed control strategy can ensure the three-phase currents balance and suppress the voltage pulsation of the DC bus under both balanced and unbalanced AC bus voltage.

Key words: DC microgrid, voltage imbalance, AC/DC interface converter, CLLC DC transformer, control strategyy

CLC Number:

TM46

KONG Jiansheng, REN Chunguang, QIN Yue, ZHANG Baifu, YANG Yugang, HAN Xiaoqing. Control Strategy of CLLC-Based Interface Converter Between AC/DC Buses under Unbalanced Condition[J]. ELECTRIC POWER CONSTRUCTION, 2021, 42(11): 23-33.

Figures/Tables 17

Fig.1 Two-stage bi-directional isolated AC/DC interface converter

Fig.2 Control block diagram of front-stage AC/DC converter

Fig.3 Fundamental equivalent model of CLLC-DCT

Fig.4 Relation between voltage gain M and ω*, Q (k=4)

Fig.5 Relation between voltage gain M and ω*, k (Q=0.2)

Fig.6 Characteristic curves of input impedance under different values of Q at a given k

Table 1 Design indices of CLLC-DCT

Fig.8 Voltage gain curve when Lm= 2.4 mH, k=8

Fig.9 Input impedance characteristic curve when Lm= 2.4 mH, k =8

Fig.10 Control block diagram of CLLC-DCT

Table 2 Parameters of three-phase full-bridge AC/DC converter

Fig.11 Waveforms of switch drive and capacitor voltage on the primary side of CLLC-DCT with full load or light load

Table 3 Control strategies for different simulations

Fig.12 The waveform of simulation 1

Fig.13 The waveform of simulation 2

Fig.14 The waveform of simulation 3

Fig.15 The waveform of simulation 4

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