A wider spread adoption of power electronic converters interfaced renewable energy systems has brought more attention to harmonic issues to the electrical grid, and means are taken to improve it in the control. More advanced closed-loop harmonic controllers are thus demanded to enhance the renewable energy integration in order to be grid-friendly. However, usually being treated as a constant factor in the design of harmonic controllers, the grid frequency varies with the generation-load imbalance, and thus may lead to deterioration of the power quality. This paper explores the frequency sensitivity of the most popular harmonic controllers for grid-interfaced converters. The frequency adaptability of these harmonic controllers is evaluated in the presence of a variable grid frequency within a specified reasonable range, e.g., +/- 1% of the nominal grid frequency (50 Hz). Solutions to the improvement of the frequency variation immunity of the discussed harmonic controllers are also emphasised. Case studies of a single-phase grid-connected photovoltaic system are provided to verify the analysis.