Method for protecting DC line impedance phase based on protection and control coordination

A method for protecting DC line impedance phase based on protection and control coordination, an application scenario of the method for protecting being a three-terminal flexible DC transmission network, comprising steps as follows:

• S1, after a line fails, a fault point generating a fault traveling wave, the traveling wave propagating at a speed of light to both ends of the line;
• S2, upon sensing a sharp change in voltage by measuring apparatuses at both the ends of the line, it is considered that a DC system has a fault, and a discriminant formula is as follows:
• |duBus2/dt|>ΔUset (3)
• wherein, uBus2 is voltage at a point UBus2, and ΔUset, is a threshold value set for detecting a fault;
• S3, when the fault is identified, further using a formula (4) to determine a direction of the fault, if a voltage change rate on a line side of a current-limiting reactor is greater than a voltage change rate on a converter side of the current-limiting reactor, the fault is considered as a forward fault, sending a converter active injection control start signal, or if a voltage change rate on a line side of a current-limiting reactor is less than a voltage change rate on a converter side of the current-limiting reactor, active injection control is not being activated;
• |duBus2/dt|>|duCon2/dt| (4)
• wherein, uCon2 is voltage at a point UCon2;
• S4, when the formulas (3) and (4) are satisfied at the same time, switching a MMC2 converter to active injection control, and activating active injection control, a frequency of a generated characteristic signal being 600 Hz and lasting for at least 2 cycles;
• and
• S5, extracting a characteristic voltage and a current signal at an injection frequency, and calculating a phase angle difference between the voltage and the current;
• and constructing a fault identification criterion:
• ∠Ūin−∠Īin=θ>Δin=0° (5)
• wherein, Uin is a characteristic voltage vector at the injection frequency at an input terminal, and in is a characteristic current vector at the injection frequency at the input terminal;
• and
• if a calculated 0 is greater than 0, it is considered that an internal fault occurs, or if a calculated 0 is less than 0, it is considered that an external fault occurs.