How HVDC Works

Siemens Video: How HVDC Provides Remote Power to the Market

Historically, the transfer of electricity between regions of the United States has been over high voltage alternating current (AC) transmission lines, which means that both the voltage and the current on these lines move in a wave-like pattern along the lines and are continually changing direction.  In North America, this change in direction occurs 60 times per second (defined as 60 hertz [Hz]).  The electric power transmitted over AC transmission lines is exactly the same as the power we use every day from AC outlets, but at a much higher voltage.

Unlike an AC transmission line, the voltage and current on a direct current (DC) transmission line are not time varying, meaning they do not change direction as energy is transmitted.  DC electricity is the constant, zero-frequency movement of electrons from an area of negative (-) charge to an area of positive (+) charge.

The first commercial electric power system built by Thomas Edison in the late nineteenth century carried DC electricity, but given some early advantages, AC power eventually became the primary power system in the United States.  Some of these advantages are no longer applicable (e.g., technology has advanced to allow efficient conversion from AC to DC), and DC transmission is the preferred solution for moving large amounts of renewable power over long distances.

Clean Line’s projects will deliver power from new, renewable energy resources.  These resources will be AC generators, as is normally the case, and their energy will be transmitted along collector lines.  These collector lines will then be connected to a substation where the power will be collected and the voltage will be transformed from the voltage of the collector lines to a common voltage (such as 345,000 volts).  The power will then be converted to DC, a process known as rectification, using power electronic switches called thyristors.  The power will then be transmitted several hundred miles along a set of conductors called a transmission line before getting converted back to AC, a process known as inversion, again using thyristors as the switching devices.  After the DC power is converted back to AC it is transformed to the common voltage of the grid to which it is being connected (e.g. 500,000 volts or 765,000 volts, in the case of Clean Line’s projects).  This power is then distributed via the interconnected grid by the local utilities to homes and businesses.  See below for an illustration of this process.

High Voltage DC Transmission Systems (HVDC)

Connecting the Grid

Clean Line Energy has made progress in the interconnection processes at both ends of the line. At the eastern end of the project, we submitted an interconnection request with the regional transmission operator PJM, the first step in the process of studying the electrical impacts of connecting the Rock Island Clean Line to the grid in Illinois. In addition, in the summer of 2011, Clean Line Energy acquired 2007-vintage interconnection queue positions that have advanced steadily through the regional merchant transmission study process. These queue positions will greatly accelerate the interconnection process for the Rock Island Clean Line. Engineering studies are currently underway pursuant to all of these interconnection requests. At the western end of the project, Clean Line Energy is in discussions with the Midwest Independent Transmission System Operator (MISO) to ensure that electric reliability is maintained. Clean Line Energy has also engaged leading environmental and technical firms to conduct the environmental, technical and routing studies necessary for the development of the Rock Island Clean Line.