Research On Inverter-type Microgrid Short-circuit Fault Characteristic Analysis And Fault Current Limit Protection

With the vigorous development of new energy, microgrid as the key technologies to realize large-scale applications of new energy and renewable energy distributed generation is paid more attention. Microgrid that mainly combines the various types of distributed new energy to provide a reliable and stable power supply to the nearest load is low-voltage grid. Differented from the traditional power grid, distributed micro-generation of microgrid transports power energy to the users through power electronics interfaces. The new features of such micro-generation inverters are low inertia and fast response. And short circuit fault is one of the biggest threats to the reliable operation of microgrid. These new features of inverter-type micro-generation make short circuit fault characteristics and transient stability of microgrid exhibit different characteristics. When the short circuit fault occurs, how to achieve the smooth transition of microgrid safely to a new stable state is the urgent problem to be solved.First of all, constant power, constant frequency and constant voltage and droop control,these three different control strategies working mechanism of microgrid mico-generation are summarized and compared. Then, the short-circuit characteristics of different control micro-generation are analyzed, the advantages and disadvantages of three different control strategies are summarized. The characteristics of microgrid fault is not only related to the characteristics of distributed micro-generation, but also closely related to fault type, fault location and operation modes of microgrid. Therefore, microgrid based multi-inverters is built, the short-circuit characteristics of different fault types, different short circuit positions and microgrid operation modes are simulated and analyzed.Secondly, according to the characteristics of the microgrid, the line power transmission model of P-8 equation is derived, and the characteristic curve is drawn. Furthermore, according to the principle of different control strategies of micro-generation, P-8 equation of three different micro-generation control that are the constant power, constant voltage constant frequency and droop control are derived. Then, distribution law of P-8 curve of the constant power, constant voltage constant frequency and droop control in P-8 model of the microgrid line power transmission is analyzed, the main factors of transient stability of three different control strategies of are summarized.Finally, according to the characteristics of short circuit fault current and the power quality problems of microgrid, the composite system of fault current limiting and power quality is proposed, the optimal installation position of the composite system is established. The topology structure and working mechanism of the system are described. And the current limiting function is optimized, The shunt current limiter of the shunt transformer is matched with the series impedance of the series transformer to limit fault current, reasonable fault current limiting is realized. In addition, the control strategy of the fault current limit and power quality composite system is studied, The power quality problems such as voltage sag, voltage and current harmonics can be solved when the microgrid is not in trouble, when short-circuit fault occurs in microgrid, the short-circuit current can be quickly and reasonably limited, which improves the efficiency of the composite system.