The Harmonic Balance Method (HBM) for modelling and analysis of DC-biased power transformers in power and distributed renewable energy (DRE) systems will be introduced in this paper. The HBM can be used to calculate all forms of harmonics generated from nonlinear inductive loads, saturated transformers and power electronics; therefore it can provide accurate prediction of harmonic distribution in electric power and DRE systems. This paper intends to presents a detailed concept of HBM and its application in calculating the harmonics caused by non-linear inductive loads, half-cycle saturation of DC-biased power transformers in HVDC power systems and HV power systems during GeoMagnetic Disturbance and geomagnetically induced currents (GDM/GIC) events, and power electronic devices in electric power and DRE systems. Several case studies including numerical model of DC-biased power transformers in HVDV power systems and HV power systems during GDM/GIC events, and power electronic devices used in electric power and DRE systems will be presented.