Modular multilevel converters have become one of the most promising topology structures in medium to high voltage and high capacity converters due to their advantages such as modular structure, low device voltage requirements, low output harmonics, and high error tolerance. They have gradually been promoted and applied to renewable energy generation systems. At the same time, modular multi-level converters also provide rich interfaces for energy storage devices in renewable energy generation systems, which can form energy storage modular multi-level converters, which is more conducive to system integration and coordinated control. This article conducts extensive and in-depth research on modular multilevel converter system modeling, operational characteristics analysis, and capacitor voltage balance control and pulsation suppression after integrating battery energy storage systems. The main research content and results are summarized as follows:
(1) Modular multilevel converter AC port equivalent circuit model considering internal coupling is proposed, which can more accurately describe the operating characteristics of the AC port, reduce the difficulty of system analysis and design, and design high-performance power control strategies based on this model, reveal low-frequency oscillation mechanisms, and improve system output performance.
On the basis of considering the internal coupling and the internal coupling caused by circulation control, the modular multilevel converter AC port is equivalent to the structure of the equivalent impedance of a two-level converter series connection. By analyzing the frequency characteristics of the equivalent series impedance, the influence of modular multilevel converter internal coupling on its AC port is revealed, and an accurate AC port model is obtained. Then, the influence of factors such as loop control on the equivalent series impedance was analyzed, and a simplified expression for the equivalent series impedance was obtained, which is the first order link of RC parallel connection. The resistance R is related to the loop control, and the capacitance C is related to the submodule capacitance. Both steady-state and dynamic simulation results indicate that the proposed equivalent circuit model can accurately reflect the operating characteristics of the AC output port. Moreover, the proposed equivalent circuit model can be obtained based on the main circuit and control system parameters of the modular multilevel converter system, without the need for complex modeling processes, effectively reducing the difficulty of system analysis and design, and facilitating practical engineering applications. Finally, based on the proposed equivalent circuit model, an AC loop control strategy was designed to improve the decoupling control performance of active and reactive power; Revealed the mechanism of low-frequency oscillation during independent operation of modular multilevel converter, and pointed out that low-frequency oscillation can be suppressed through loop control.
(2) Established modular multilevel converter steady-state and dynamic models based on harmonic state space theory, obtained accurate steady-state solutions for each variable, and proposed a secondary circulation control scheme that comprehensively considers multiple performance indicators; Obtained the mutual influence relationship and stability boundary of each controller, and proposed a simplified design method for controller parameters.
Firstly, a steady-state model of modular multilevel converter based on HSS theory was established, and accurate steady-state solutions of internal variables were obtained, providing a basis for designing main circuit parameters and evaluating the economy and superiority of the system. Analyzed the impact of secondary circulation control on multiple performance indicators such as capacitor voltage ripple, bridge arm current peak, and bridge arm current effective value, and provided an optimized control scheme for secondary circulation that comprehensively considers multiple performance indicators, which is beneficial for improving the system. The results indicate that: 1) modular multilevel converter AC port control does not affect the dynamic characteristics of internal capacitor voltage and circulating current; 2) The circulating current control will affect the stability of phase capacitor voltage and bridge arm capacitor voltage control, and the larger the circulating current controller parameters, the poorer the stability of capacitor voltage, which is opposite to the simplified model results; 3) There is a negative correlation between the phase capacitor voltage control and the parameters of the inter bridge capacitor voltage controller. When designing parameters, comprehensive consideration should be given to ensure good dynamic performance of the capacitor voltage. Based on the precise analysis results of the HSS model, a simplified design method for controller parameters is proposed that is more convenient for practical engineering applications. Finally, the accuracy of the steady-state and dynamic analysis results in this paper was verified through sufficient experiments and simulations.
(3) Propose an evaluation method for the balancing ability of modular multilevel converters in battery energy storage systems submodules, and obtain the balancing ability of submodules under different capacitor voltage balancing control strategies; Propose a capacitor voltage closed-loop equalization control scheme for adjusting the port voltage of the submodule, which improves the equalization ability of the submodule.
Firstly, the concept of sub module power imbalance was defined, and the balancing ability of the sub modules was evaluated using the allowable range of imbalance in the system, which served as the design basis for SOC balancing controller parameters. Analyzed the sub module balancing ability of different port voltage regulation methods in capacitor voltage closed-loop balancing control, and the results showed that the square wave voltage regulation scheme has greater sub module balancing ability than the AC/DC voltage regulation scheme. However, the closed-loop equalization control of capacitor voltage is constrained by modulation, and the equalization ability of submodules rapidly decreases at high modulation ratios, making it difficult to meet the demand for rapid SOC equalization. Therefore, a sub module port voltage regulation scheme under closed-loop control of capacitor voltage is proposed, which expands the output range of sub module port voltage and ensures high sub module equalization ability even at high modulation ratios. Unlike capacitor voltage closed-loop equalization control, when using capacitor voltage sorting equalization, each submodule is not independent of each other and the equalization ability of the submodules cannot be analyzed separately. Therefore, it is proposed to treat the bridge arm submodules as a whole, evaluate the sum of their balancing capabilities, and use their average values to design the parameters of the SOC balancing controller. Finally, the accuracy of the evaluation method for sub module balancing capability and the effectiveness of the proposed improved control strategy were verified through detailed simulation and experimental results.
(4) A battery connection method based on an isolated three port DC/DC converter was proposed, and a control strategy was designed that can simultaneously suppress capacitor voltage pulsation and transmit battery power.
The impact of introducing a battery energy storage system on capacitor voltage pulsation in conventional battery access methods was analyzed, and the results showed that the submodule capacitance required by modular multilevel converters in battery energy storage systems was much greater than that of modular multilevel converter with the same power capacity. A battery connection method based on an isolated three port DC/DC converter to modular multilevel converter is proposed, with the upper bridge arm submodule, lower bridge arm submodule, and battery unit connected to three ports respectively. By controlling the distribution of battery power between bridge arms, the fundamental frequency component in the circulating current can be suppressed, and the voltage fluctuation of the upper and lower bridge arm capacitors can be balanced; By transmitting the fundamental frequency pulsation power between the upper and lower bridge arms, the fundamental frequency pulsation component of the capacitor voltage can be eliminated, thereby effectively reducing the capacitance voltage pulsation component. Analyzed the working principle of the proposed battery connection method, established a corresponding mathematical model, and based on this, proposed a control scheme that can simultaneously suppress capacitor voltage pulsation and transmit battery power, and designed corresponding controller parameters. Finally, a modular multilevel converters in battery energy storage systems simulation model and experimental platform based on a three-port DC/DC converter were built, and the proposed topology and control strategy were verified in detail. The experimental and simulation results show that the proposed battery connection method and control strategy can significantly reduce the amplitude of capacitor voltage ripple and achieve good battery power control performance.
We have conducted in-depth research on several issues related to the application of modular multilevel converter in energy storage systems, and have achieved some phased research results. The following research aspects can be considered for the next step:
(1) By configuring redundant modules, modular multilevel converter can achieve fault-tolerant operation and improve system reliability. After integrating the energy storage unit, the modular multilevel converter submodule faults are divided into two types: energy storage side faults and modular multilevel converter side faults, and the energy storage type modular multilevel converter has a certain ability to operate with energy storage unit faults. In addition, the removal of faulty submodules will also affect the power balance between each phase, bridge arm, and submodule. Therefore, it is necessary to study fault-tolerant operation strategies suitable for energy storage modular multilevel converter.
(2) For modular multilevel converters in battery energy storage systems using CPS-PWM, the main factor limiting the equalization ability of submodules is submodule overmodulation. By injecting a certain third harmonic into the AC output voltage, it can to some extent avoid submodule overmodulation and inevitably improve the equalization ability of the submodule. Therefore, it is necessary to study the impact of third harmonic injection on the equalization ability of submodules, and propose optimization methods for the equalization ability of submodules based on this.
(3) When the battery is connected to modular multilevel converter through an isolated three port DC/DC converter, the three winding isolation transformer needs to bear a larger voltage. For situations with lower voltage levels, such as 10kV, the design of isolation transformers is relatively easy and the economic performance of the system is guaranteed. But as the voltage level increases, the requirements for insulation of isolation transformers will also increase, and the economy of the system will also decrease. Therefore, it is necessary to conduct in-depth analysis on the economic performance of battery connection methods based on isolated three-port converters at different voltage levels.
(4) The construction of the modular multilevel converters in battery energy storage systems experimental platform based on an isolated three-port DC/DC converter is not yet perfect. Only some sub modules use battery connection methods based on the isolated three-port converter, while other sub modules still use conventional battery connection methods. Experimental verification mainly focuses on the control of battery power and the suppression of capacitor voltage pulsation, without sufficient dynamic experimental testing. Therefore, it is necessary to establish a comprehensive modular multilevel converters in battery energy storage systems experimental platform based on isolated three-port converters in subsequent work, and conduct sufficient stable and dynamic experimental testing.
(5) The research in this article mainly focuses on the control problem of modular multilevel converters in battery energy storage systems, and only considers the battery unit as a controlled current source, without considering the battery response speed in practical applications, nor analyzing the SOH attenuation factor and equilibrium control of the battery, which is crucial for the practical application of battery energy storage systems.