Research on Startup of DC Transformer for Electric Vehicle Cluster Grid-Connection

doi:10.12096/j.2096-4528.pgt.22099

Abstract

Abstract:

Modular multilevel direct current transformer (MMDCT) provides a feasible solution to realize the efficient and stable integration of electric vehicle charging and discharging cluster into urban DC distribution network. However, there are impulse currents caused by capacitor charging and transformer inrush current in its starting process. Therefore, a fast pre-charging strategy based on compound frequency control was proposed. The independent control of direct current component and intermediate frequency component was used to realize simultaneous charging of the primary and secondary sides. The double closed-loop control was adopted in the primary side, while the variable-step phase shift peak current control was adopted in the secondary side, which solved the contradiction between charging speed and inrush current. Furthermore, a quantitative comparative analysis of various pre-charging strategies was carried out. The effectiveness and feasibility of the proposed pre-charging strategy were verified through simulation. The results show that the magnitude of the inrush current and the charging time are effectively controlled by the proposed method.

Key words: electric vehicle, direct current transformer (DCT), pre-charging, compound frequency control

CLC Number:

TK 01

Jun JIA, Weihao FAN, Zhipeng LÜ, Jianguang YAO, Shan ZHOU, Jian WANG, Jintao ZHANG. Research on Startup of DC Transformer for Electric Vehicle Cluster Grid-Connection[J]. Power Generation Technology, 2023, 44(6): 875-882.

Figures/Tables 15

Fig. 1 Topology structure of MMDCT

Fig. 2 Equivalent circuit of the primary side charging

Fig. 3 Equivalent circuit of the secondary side charging

Fig. 4 Block diagram of pre-charging strategy based on composite frequency control

Fig. 5 Block diagram of voltage and current double closed loop control

Fig. 6 Block diagram of submodule capacitor voltage balancing control

Fig. 7 Block diagram of variable step control

Fig. 8 Working curve of pre-charging strategy

Fig. 9 Working curve of voltage and current on the secondary side

Tab.1 Peak of inrush current on the transformer

策略	冲击电流峰值i_peak
策略1	$V d c 1 4 f a c (L p + L k)$
策略2	$V d c 1 φ 1 m 2 π f a c (L p + L k)$
本文策略	$n π v C - k T φ 1 v C o 2 π f a c (L p + L k)$

Tab.1 Peak of inrush current on the transformer

策略	冲击电流峰值i_peak
策略1	$V d c 1 4 f a c (L p + L k)$
策略2	$V d c 1 φ 1 m 2 π f a c (L p + L k)$
本文策略	$n π v C - k T φ 1 v C o 2 π f a c (L p + L k)$

Tab. 2 Simulation parameters of MMDCT

参数	数值
原边直流电压V_dc1/kV	20
原边桥臂子模块数n	40
原边子模块电容电压稳态值V_C/V	500
原边子模块电容C_SM/μF	1 000
桥臂电感L_p/mH	5
变压器变比k_T	20∶3
变压器漏感L_k/mH	20
变压器频率f_ac/Hz	800
副边输出滤波电容C_o/μF	2 000
副边额定输出电压V_dc2/kV	3
载波频率f_c/Hz	3 200
变步长控制模块允许最大电流值/A	150

Fig. 10 Peak curves of inrush current on the transformer of the proposed strategy

Fig. 11 Peak curves of inrush current on the transformer

Fig. 12 Simulation waveforms of pre-charging strategy

Tab. 3 Simulation results of pre-charging strategy

	充电时间/ms	原边冲击电流/A
策略1	750	432
策略2	950	215
本文策略	620	122

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