Asymmetric low-voltage ride-through scheme and dynamic voltage regulation in distributed generation units

Most grid codes mainly focus on the low-voltage ride-through (LVRT) requirements under balanced grid faults, and simply provide the LVRT curves which only apply for the positive-sequence voltage value. Under short-term asymmetric faults, this brings some shortcomings: i) disconnecting the large units under temporary unbalanced faults worsens the situation most of the time, and may cause cascaded outages; ii) a reconnection process is required after the fault is cleared; iii) it is not an economical option, and the power may be wasted. Therefore, a new regulation scheme, called asymmetric low-voltage ride-through (ALVRT), is proposed in this paper. The ALVRT scheme is intended to provide the allowable margins for each phase voltage magnitude rather than for just positive-sequence voltage. This aids the large converter-interfaced distributed generation units not only ride through the asymmetrical grid faults, but also support the grid with a seamless transition over the fault and enhance the power system reliability. A new voltage regulation method is also proposed to address the ALVRT specifications. The successful results of the proposed regulation scheme and voltage support method are verified using simulation test cases.