| DC transmission is a key technical means to solve our country’s energy and environmental issues and optimize resource allocation across the country.Modular multilevel converters based flexible DC transmission occupies an important position in future power systems.With the improvement of voltage levels and transmission capacity,flexible DC grids are developing towards multi-terminal.DC line protection is of great significance to the safe and stable operation of DC grids,but it is also one of the challenges faced in the practical applications.The flexible DC grid supplied overhead transmission lines has a high failure rate,and it’s inherent low damping makes DC short-circuit faults develop rapidly.The superposition of multi-terminal output exacerbates the damage to the system and internal components of the fault,and puts forward higher requirements for protection.Therefore,in order to improve the insufficients and defects of the existing line protection methods,this paper has carried out work from the aspects of transient characteristic analysis and line protection research.The main research contents of the paper are as follows:(1)The equivalent circuit model of the converter station under unipolar grounding and bipolar short-circuit faults is established to study the fault transient characteristics.Firstly,the topology,main wiring,grounding mode and working principle of the modular multilevel converter are analyzed.The transient characteristics of the flexible DC grid line fault with symmetrical bipolar connection mode is analyzed.Then establish single-ended fault equivalent circuit model of the three transient development stages under different fault situation using the relevant circuit theory,witch including capacitor discharge,inductance freewheeling and AC feed stage.And qualitative analysis and quantitative calculation of fault current and voltage are carried.Looking for suitable fault transient characteristic for protection,laying the theoretical foundation for setting the protection criterion.(2)A single-ended DC line protection method based on the modulus phase plane of instantaneous fault current amplitude is proposed.Taking the flexible DC grid with T-connected lines as the research object,first draw the equivalent circuit of internal and external faults,derive the fault current differential equation,and perform the modulus decomposition of the fault current.Then,combined with the possible fault location and type,the protection action principle and its threshold are set.The ground mode and line mode components of the fault current are used to construct the modulus phase plane,realizing fault recognition and selection.At the same time,the fault threshold is adjusted by constructing a threshold reduction factor,which increases the sensitivity of terminal high-impedance fault.Under the premise of satisfying reliability and rapidity,the protection range is close to the full length of the line,and it has good ability to withstand transition resistance and avoid the influence of system disturbance.(3)A two-terminal protection method for DC line based on the instantaneous power phase plane of the fault component is proposed.Firstly,a multi-terminal flexible DC power grid model is established,the short-circuit fault current and voltage are derived using the loop analysis method,and the instantaneous power of the fault component is calculated.Then the sum and difference characteristics of the instantaneous power of the positive and negative fault components under different fault locations and types are analyzed,and the phase plane is constructed by using the power sum and the power difference to realize protection and fault selection.At the same time,by introducing a low-voltage criterion,the instantaneous power of the fault component is recalculated to eliminate the problem of the protection dead zone at the line exit.This protection method has good selectivity,can protect the entire length of the line,and has a good ability to withstand transition resistance. |
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