Relay Protection Technology Of Microgrid Based On Impedance Characteristics

Microgrid contains multiple distributed generation(DG),which is different from the single source radiation structure of traditional distribution network.Microgrid has become a complex multi-source system.The power flow distribution of normal operation,the size and direction of short-circuit current have changed.After the distributed generation is connected to the microgrid,the traditional current protection will refuse to operate,and the protection is easy to lose its selectivity.However,the existing microgrid protection does not fully adapt to the complex structure and control strategy of microgrid.In order to ensure the protection does not mal-operation and improve the protection selectivity,a microgrid protection based on impedance characteristics is proposed.Most of the distributed generation in microgrid is Inverter-Interfaced Distributed Generator,and the characteristics of Inverter-Interfaced Distributed Generator affected by control strategy are complex.Therefore,the microgrid with inverter-Interfaced Distributed Generator is studied.Firstly,the impedance characteristics of the interfaced distributed generators are analyzed,and the conclusion that the impedance phase angle of the interfaced distributed generation varies from 0°～360° is obtained,and the specific range of the impedance amplitude of the interfaced distributed generation is obtained.The relationship between the impedance amplitude of the interfaced distributed generation and the amplitude of the system side power supply is analyzed.Then,combined with the characteristics that the impedance is less affected by the operation mode and the fault components is less affected by the load current and transition resistances the phase angle and amplitude characteristics of the measured impedance on both sides of the protected line are analyzed.The principle of microgrid protection based on impedance characteristics is proposed and a complete implementation scheme is designed.The influence of data asynchrony on the proposed protection principle is analyzed theoretically.By setting the off-line impedance value sent to the opposite side when the two side protections are not started,the protection can work normally in the case of data asynchrony.It is theoretically analyzed that the proposed protection principle is not affected by DG access location and penetration in microgrid.A typical microgrid simulation model is built in PSCAD to verify the effectiveness of the protection scheme.The simulation results show that the proposed protection scheme can achieve no maloperation of external fault and reliable operation of internal fault when the data on both sides are not synchronized,and the protection performance is not affected by DG/0 permeability,protected line length,DG fault current limit and transition resistance of microgrid.Finally,the hardware platform of microgrid protection device based on impedance characteristics is designed,and the software design of microgrid protection based on impedance characteristics is completed on the basis of the hardware platform,including the software process design of analog data acquisition and sampled-data transmission,sequence component decomposition design,protection startup design,fault component impedance calculation design.At the same time,the communication design of microgrid protection based on impedance characteristics is completed.The microgrid protection scheme based on impedance characteristics studied in this paper can realize the accurate location of microgrid line fault and improve the technical level of microgrid relay protection,which has important engineering practical value.