| For the new power system with the characteristics of "double height" and "double randomness",energy storage has become an important technology and basic equipment to smooth the double power fluctuation of "source-load" and improve the flexible regulation ability,security and stability of the system.Due to the low voltage level of energy storage units,multiple energy storage units are integrated in series through their inverters,which is an effective form of high-voltage and large capacity energy storage.For the series inverter type energy storage system,the frequency synchronization between each energy storage inverter and the power grid is the precondition for the safe,reliable and stable operation of the system.However,the series inverter energy storage system has the characteristics of strong coupling,multi-scale and nonlinear,so the coupling mechanism is complex and unclear.The synchronization control which take into account the robustness,reliability,power supply quality,battery life and communication cost of the system is still a problem to be solved.At present,the synchronization of series system mostly depends on high bandwidth communication network and centralized controller,so that the communication failure or synchronization clock deviation can easily lead to system instability.Therefore,it is of great significance to reveal the self synchronization mechanism of the series inverter energy storage system under on/off grid modes.The decentralized framework and robust self synchronization control strategy of the system should be further studied,which can get rid of the high dependence on real-time communication.Thus,the power supply reliability and quality of the system are improved,and the development of the high-voltage series inverter energy storage system is promoted.The main work and innovations of this paper are summarized as follows.1)Off-grid mode self-synchronization control of single-phase series inverter energy storage system.Aiming at the problem of differential regulation of existing off-grid self-synchronization control,the power transmission model of series inverter system is established.Based on the coupling relationship between output power and voltage phase angle,the frequency self-synchronization mechanism between units is revealed.Utilizing the common current characteristics of each unit of the series structure,a decentralized control strategy is proposed which takes into account both frequency selfsynchronization and self-restoration.Based on the singular perturbation theory and Lyapunov stability theory,the stability of the proposed decentralized control is proved.The simulation and experimental results show that the proposed method can realize frequency self-synchronization,frequency self-recovery and power sharing without communication.2)Grid-connected mode self-synchronization control of single-phase series inverter energy storage system.To address the poor anti-interference ability of the existing decentralized control under the grid-connected mode,based on the analysis of the coupling characteristics of the system steady-state voltage and the grid voltage,it is pointed out that the grid voltage fluctuation will affect the existence and uniqueness of the system balance point under the existing decentralized control framework.For this reason,a robust decentralized self synchronization control method is proposed,which realizes the control of the system balance point by constructing a voltage correction mechanism.On the basis of ensuring the self synchronization of the system frequency,the flexible adjustment ability of power and power factor is greatly improved.The simulation and experimental results under power grid voltage and load fluctuation show that the system has good disturbance rejection capability and robust performance.3)Self-synchronization control of three-phase series inverter energy storage system.In order to solve the problem that the three-phase system is difficult to realize global self-synchronization under phase fluctuation and load imbalance disturbance,the self-synchronization mechanism of each phase subsystem and the coupling characteristics between phases are deeply analyzed.A decentralized synchronization framework based on masterslave coordinated control is proposed.The framework realizes the global frequency synchronization and three-phase voltage balance of the system by establishing a coordination mechanism between the master units.The master units of each phase adjust the voltage amplitude and phase of the corresponding phase subsystem based on the voltage information of the system,which can ensure the voltage balance between the system phases.Each slave unit in the phase only relies on the local information to realize the self-synchronization of each phase subsystem.The stability of the system is proved based on singular perturbation theory and Lyapunov stability theory.The simulation and experimental results under nonideal conditions such as phase disturbance and unbalanced load show that the proposed decentralized master-slave control method makes the system have good power supply reliability and quality.4)Off-grid and grid-connected self-synchronization control strategies with considering SOC balance.To solve the problems of system safety and life decay caused by the inconsistency of SOC,the relationship between the output power of energy storage unit and its own SOC are derived.Based on the analysis of the transmission mechanism of the series system,the self-synchronization control strategies are proposed to ensure the global SOC consistency of the system under both off-grid and grid-connected modes.Firstly,a dual time scale self-synchronization control method of SOC self equalization is proposed for the series inverter energy storage system under off-grid mode.The self-synchronization control law is adaptively modified by the local SOC information,so that the system SOC equalization and frequency synchronization are realized without any communication.Secondly,for the grid-connected mode,a distributed SOC observer based synchronization control strategy considering the SOC balance and virtual synchronous generator is proposed.It can ensure synchronization and SOC balance of each units,and provide the system with appropriate inertial support,which can improve the stability,reliability and security of the system.Finally,based on the singular perturbation theory and small signal modeling,the stability of the closed-loop system and the convergence of the SOC equalization algorithm are analyzed.Simulation and experimental results prove the effectiveness of the proposed synchronization control strategy with considering SOC balance. |
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