Book excerpt: The development of large-scale renewable generation and load electrification call for highly efficient and flexible electric power integration, transmission and interconnection. High Voltage DC (HVDC) transmission technology has been recognized as the key technology for this scenario. HVDC transmissions, including both the line commutated converter (LCC) HVDC and voltage source converter (VSC) HVDC have played an important role in the modern electric power system. However, with the inclusion of power electronic devices, HVDC introduces the characteristics of nonlinearity and different timescales into the traditional electromechanical system and thus careful modeling and simulation of HVDC transmission are essential for power system design, commissioning, operation and maintenance.