HVDC

High voltage direct current (HVDC):

High voltage direct current (HVDC) is used to convert energy from AC to DC, to transport the energy as DC, and to convert it back from DC to AC. To meet the ever growing demand for bulk power transmission over long distance, one solution is to build HVDC transmission lines.

This is a simple system with two converters C1 and C2, and one DC line.Based on the polarity of DC lines, HVDC systems are classified as monopolar, bipolar, and homopolar systems. a monopolar system, has only one DC line that normally has negative polarity and uses the ground as the current return path. The monopolar system is mainly implemented to reduce the cost of line construction. Bipolar systems uses two DC lines, one positive and the other negative. All lines have the same polarity in the homopolar connection.

The HVDC converter bridge is a three-phase converter bridge circuits as shown in Fig;

Each converter bridge has six branches of valves. The DC terminals of a converter connect to DC lines and the AC terminals to AC lines. The converter transformer is a conventional transformer with on-load tap changers. The turns-ratio of the converter transformer can then vary to manage the converter operation. The ‘‘DC side’’ of the converter transformer is usually delta or Y connected with ungrounded neutral, so that the DC line can have an independent voltage reference relative to the AC network. Harmonic voltages and currents arise during converter operation. The inductance reduces the harmonic voltages and currents on the DC lines, to prevent commutation failure of inverters, to maintain continuous current under light loading, and to curtail short-circuit current in converters during faults.