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The high penetration level of Photovoltaic (PV) generation in distribution network reduces carbon emissions but also becomes problematic in terms of network management. Harmonic distortion is the main factor studied in this paper and a typical three-bus distribution network is built in MATLAB/Simulink to understand the harmonics problem. Results show that current harmonics are more vulnerable to fluctuate compared to voltage harmonics. Based on existing harmonic standards, total demand distortion of current (TDDi) is evaluated to estimate maximum PV penetration level at Point of Common Coupling (PCC), and the maximum acceptable TDDi at each bus differs according to specific loading and short-circuit levels. Meanwhile, total harmonic distortion of current (THDi) at inverter outputs represents inverter performance. Instead of assessing at standard test conditions (STC), the impact of irradiance variations is studied. Low irradiance results in an increased THDi of the inverter whilst doesn't explicitly affect TDDi at PCC. A simple and low-cost solution is proposed to dynamically vary the settings of inverter's filter elements against irradiance, and harmonic distortion at low irradiance of the inverter is successfully mitigated.