Abstract: With the "double-high" development of power systems, the capacity of nonlinear loads or equipment is becoming increasingly larger, and the demand for their connection to the HV-EHV power systems is getting increasingly higher. However, the connection of the nonlinear loads will inject harmonic currents into the power systems, causing voltage waveform distortion. In order to assess whether the connection of nonlinear loads to power systems will cause unacceptable harmonic voltage distortion, the IEC/TR 61000-3-6: 2008 technical standard and the UK G5/5 Engineering Guidelines in 2020 have provided corresponding assessment methods. This paper firstly introduces the assessment methods for the connection of nonlinear loads to HV-EHV power systems provided in these two standards, and then makes a comparative analysis, which can provide a different vision and idea for the revision of the national standards on harmonics.