Protection strategy for DC multi-microgrids based on gradient of branch admittance difference

Fast and reliable short fault detection is one of the key technologies in the development of DC multi-microgrids (MMG). In order to improve the speed and reliability of DC relay protection, especially to solve the problem of protection failure in high impedance short-circuit, a fault criterion based on the gradient of the branch admittance differential is proposed. The branch admittance is the quotient of current and voltage. It makes effective use of the characteristics of the fault current and voltage, so it has higher sensitivity. By detecting the current and voltage at the two sides of the branch, the admittance differential gradient of the branch is calculated and used as the fault criterion. In case of normal operation or fault outside the protection zone, the differential admittance gradient of the branch is zero, and in case of fault inside the protection zone, the admittance differential gradient of the branch is greater than zero regardless of high impedance or low impedance short-circuit. Meanwhile, the admittance difference gradient is the largest at the beginning of the fault. Therefore, the detection time is reduced. Finally, the experimental results confirm the effectiveness of the proposed strategy.