Power Angle–Frequency Droop Control to Enhance Transient Stability of Grid-Forming Inverters Under Voltage Dips
Joseba Erdocia, Andoni Urtasun, Luis Marroyo
Abstract
Due to the replacement of synchronous generators (SGs), grid operators are currently demanding to control grid-connected inverters in grid-forming mode to make them participate in the maintenance of the grid. To carry this out, the traditional droop controls based on the active and reactive powers are usually adopted, achieving a satisfactory performance in normal operation. Nevertheless, the power–frequency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P-\omega$ </tex-math></inline-formula> ) droop may become transiently unstable under voltage dips. This is because of the modification of the active power response caused by the inverter current limitation together with the voltage reduction. To enhance this, the power angle–frequency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\delta _{\mathrm {inv}}-\omega$ </tex-math></inline-formula> ) droop is proposed, consisting in employing an estimation of the inverter power angle as input to obtain the inverter frequency. The proposed method provides the inverter with the same performance as the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P-\omega $ </tex-math></inline-formula> droop in normal operation while enhancing considerably the transient stability margins under current limitation. This is due to the higher variation of the inverter power angle with the phase difference between the inverter and the grid. Simulation results show the transient stability problems of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P-\omega $ </tex-math></inline-formula> droop as well as the superior performance of the proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\delta _{\mathrm {inv}}-\omega $ </tex-math></inline-formula> droop control, which has also been verified by means of HIL results.