An HVDC transmission link consists as a minimum of one rectifier that converts AC power to DC and one inverter that converts the DC power back to AC. The power conversion can be performed using different technologies but has since the 1950s employed electrical switches, which chops the AC voltages and currents supplied from a three-phase AC system to unidirectional DC power feeding DC circuits with one positive and one negative polarity. Depending on the conversion technology applied for the conversion, the direction of the DC power flow can be controlled either by reversing the DC voltage polarity with a unipolar DC current flow or by keeping the DC voltage polarity constant and reversing the DC current flow polarity. There are a number of options for configuring the DC sides of the converter systems described here, but they all have some common features. The configurations in its simplest form contain one rectifier connected between one positive and one negative DC poles with a power flow controlled by the rectifier feeding at least one inverter at the opposite ends of the HVDC circuit, where it converts the DC power to AC. This is referred to as a point-to-point HVDC link. HVDC systems can be configured in many different ways depending on the application, and the economics.