DC-side modeling of the modular multilevel battery energy storage system based on carrier phase shift modulation

doi:10.19799/j.cnki.2095-4239.2020.0103

Abstract

Abstract:

The carrier phase shift pulse width modulation (CPS-PWM) is a modulation method widely used in modular multilevel converter-based battery energy storage systems (MMC-BESS). At present, the mathematical modeling of MMC-BESS based on CPS-PWM is mostly concentrated on the AC side of the system, and its analysis of the output voltage waveforms is mostly focused on the frequency domain. The mathematical modeling for the DC side of the system and the time-domain analysis of the output waveforms has not yet been studied. Moreover, the instantaneous equivalent circuit for the DC side of MMC-BESS is still unclear. This study performs a theoretical research on CPS-PWM and the DC side of MMC-BESS. Accordingly, time-domain triangle wave mathematical expressions for the carrier waveforms are established, including the two most commonly used CPS-PWM (i.e., N + 1 and 2N+1 modulations). The intersection point sequence is proposed for a constant modulation wave and the carrier waveforms in a carrier period. The expressions of the intersection time interval and the number of sub-modules opened in a single phase, which are only related to the modulation ratio and the number of sub-modules in an arm, are then obtained. Based on this, an equivalent model of the DC side of MMC-BESS is established. We find that the DC side of the system can be equivalent to two voltage sources and one switch, and its parameters are only related to the modulation ratio and the number of bridge modules. Subsequently, the ripple expression of the DC current in the equivalent switching cycle is calculated based on this model. A method for determining the bridge inductance, which can meet the design requirements for the DC current ripple in the steady state, is also proposed. Finally, a three-phase 48-module MMC-BESS simulation model is built in MATLAB/Simulink. The simulation results of important system parameters, such as the number of open sub-modules, equivalent switching frequency and duty cycle, and DC current ripple, are consistent with the theory, thereby verifying the mathematical model accuracy. The simulation results of the current ripple can meet the design requirements when the method for choosing the bridge inductance is used. This also confirms the method effectiveness.

Key words: modular multilevel battery energy storage system(MMC-BESS), carrier phase shift pulse width modulation(CPS-PWM), DC-side equivalent model, bridge inductance

CLC Number:

TM 461

Kai TIAN, Kun CHEN, Man CHEN, Zhibin LING. DC-side modeling of the modular multilevel battery energy storage system based on carrier phase shift modulation[J]. Energy Storage Science and Technology, 2020, 9(6): 1982-1990.

Figures/Tables 12

Fig.1

Fig.2

Fig.3

Fig.4

Fig.5

Table 1

Parameters of simulation model"

参数	数值
直流侧电压 $U D C$ /V	78
直流侧额定电流 $I N$ /A	10
子模块直流电压 $U B$ /V	12
桥臂子模块数N	8
系统子模块总数	48
桥臂电感 $L a$ /mH	1
反向二极管导通压降/V	0.7
子模块开关频率 $f c$ /Hz	1000

Table 1

Table2

Parameters of mathematic model"

调制方式

调制比D

相开通模块数N_on

等效开关占空比d

等效开关频率

f c

/Hz

三相直流

电流纹波 $Δ I d c$ /A

N+1

0.41

6, 8

0.25

8000

0.84

2N+1

0.41

6, 7

0.50

16000

0.28

Table2

Fig.6

Fig.7

Table 3

Simulation results waveforms"

调制方式	等效开关占空比		直流电流纹波
调制方式	仿真等效开关占空比d	占空比相对误差	三相直流电流纹波 $Δ I d c$ /A	电流纹波相对误差
N+1	0.25	1.96%	0.84	0.87%
2N+1	0.49	1.26%	0.28	0.69%

Table 3

Table 4

Simulation results of current ripple"

调制方式	桥臂电感 $L a$ /mH	三相直流电流纹波 $Δ I d c$ /A
N+1	0.89	0.96
2N+1	0.30	0.96

Table 4

Fig.8

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