As known, nonlinear loads in power systems originate harmonic distortion and power quality issues. Converter-interfaced loads exhibit a nonlinear behaviour as well and may largely contribute to increase the harmonic pollution. The nonlinearities introduced by the PLL-synchronization and power control originate, indeed, a coupling mechanism between fundamental and harmonic frequencies. These harmonic coupling effects are not captured by traditional Norton/Thevenin equivalent converter models, leading to inaccurate harmonic power flow analyses. This paper proposes a Linear Time Periodic model of a PLL-synchronized converter to be used in Harmonic Power Flow analyses. A realistic 18-bus distribution grid hosting substantial amount of power-electronic interfaced resources is used as a case study. It is revealed that, in high grid loadability condition and with distorted grid supply voltage, the harmonics are significantly amplified by the converters and the fundamental components of the buses voltages are reduced, representing a risk for the voltage stability. This phenomenon is also influenced by the tuning of the current control loop and the PLL. The accuracy of the presented analyses is validated by comparing the harmonic power flow results with time-domain simulations.