| As an important component of the energy internet,the energy router can realize the energy transmission of multiple power ports,meet diversified energy dispatching requirements,and maintain the stability of the power system.The energy routing system can be regarded as a large energy router device,and the system usually has the fault characteristics such as no natural zero crossing current and fast current rising speed in the field of medium and high voltage DC transmission.Therefore,this thesis conducts research in the hybrid DC energy routing system,and proposes corresponding solutions to the DC line fault that may occur in the process of DC transmission.This thesis first introduces the working principle and topology structure of the energy routing system,which is mainly composed of line commutated converter(LCC)and modular multilevel converter based on battery energy storage system(MMC-BESS).The mathematical models of LCC and MMC are derived,and combined with the system working principle,the corresponding steady-state control strategies for LCC,MMC and energy storage unit are designed respectively.Then the effectiveness of each control is verified by simulation.Secondly,the characteristic of LCC and MMC under DC fault conditions is analyzed,and then an improved hybrid DC circuit breaker(DCCB)is designed to clear the fault current and protect the system safety,and the working process and parameter design are studied in detail.The fault detection method based on the rate of change of voltage(ROCOV)is introduced,through the simulation verification of two DC fault types of single-pole grounding and double-pole short-circuit,it can be concluded that the proposed DCCB can cut off the fault line and effectively clear the fault current,but cannot guarantee the stability of MMC AC side.Aiming at the problem of AC power grid instability at the receiving end during DC fault,this thesis proposes a power switching control of energy storage unit,which is coordinated with the improved hybrid DCCB to alleviate the impact of the DC fault on the AC side.In addition,the state of charge(SOC)three-level balance control is added to the energy storage unit to ensure the coordinated operation between different batteries,and the transient response characteristics of DC fault are analyzed.The system simulation model is built to verify the feasibility of the proposed fault ride-through coordinated control scheme,and the incomplete power compensation of energy storage unit is also analyzed.The proposed scheme is compared with the existing scheme through a design example,which further proves the advantages of the proposed scheme.Finally,in order to further verify the correctness and rationality of the proposed scheme,a small experimental platform of MMC and improved hybrid DCCB was built.In addition,considering the experimental conditions and safety issues,the experimental verification was completed together with RTDS hardware-in-the-loop platform. |
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