How does the tertiary winding in a three-winding transformer contribute to harmonic mitigation in a distribution substation?

A three-winding transformer has three sets of windings, usually a primary, a secondary, and a tertiary winding. The tertiary winding plays a crucial role in harmonic mitigation, especially the third harmonic, which is a common problem in power systems. The key contribution lies in the tertiary winding's ability to provide a low-impedance path for third harmonic currents to circulate. In a delta-connected tertiary winding, the third harmonic currents generated by nonlinear loads or transformer magnetizing currents circulate within the delta loop. This circulating current minimizes the flow of third harmonic currents into the main distribution network. Without the delta-connected tertiary winding, these third harmonic currents would flow into the system impedance, causing voltage distortion and potentially overloading equipment. Additionally, the tertiary winding can be used to connect harmonic filters, such as tuned LC filters, specifically designed to absorb certain harmonic frequencies. By connecting these filters to the tertiary winding, the substation can actively reduce harmonic distortion in the broader distribution system.