Effect of Grid-Following VSC on Terminal Frequency

Grid-following voltage source converter (GFL-VSC) does not establish frequency like synchronous generator (SG), but follows the frequency of the terminal bus using phase-looked loop (PLL). This letter demonstrates that GFL-VSC also affects the terminal frequency, rather than simply follows it, due to the dynamics of the PLL and outer control loops. The relationship between PLL frequency, d-axis current, and terminal frequency is derived. Both the PLL frequency and the d-axis current affect the terminal frequency in the form of their derivatives, which is completely different from conventional SG and the corresponding frequency divider formula (FDF). Time-domain simulation results validate the derived relationship.