"The Pulse Width Modulated Voltage Source Converter a close to ideal component in the transmission network. From a system point of view it acts as a motor or generator without mass that can control active and reactive power almost instantaneously."
Conventional HVDC converter technology is based on the use of line-commutated or phase-commutated converters (PCC). With the appearance of high switching frequency components, such as IGBTs (Insulated Gate Bipolar Transistor) it becomes advantageous to build VSC (Voltage Source Converters) using
PWM (Pulse Width Modulation) Technology .
The key part of the HVDC Light converter consists of an IGBT valve bridge. No special converter transformers are necessary between the valve bridge and the AC-grid. A converter reactor can separate the fundamental frequency from the raw PWM waveform. If the desired DC voltage does not match the AC system voltage, a normal AC transformer may be used in addition to the reactor. A small shunt AC-filter is placed on the AC-side of the reactor. On the DC-side there is a DC capacitor that serves as a DC filter too.
Active and reactive power control
The fundamental frequency voltage across the converter reactor defines the power flow between the AC and DC sides. Changing the phase angle between the fundamental frequency voltage generated by the converter (Ug) and the voltage on the AC bus controls the active power flow between the converter and the network. The reactive power flow is determined by the amplitude of Ug, which is controlled by the width of the pulses from the converter bridge.
The control is performed by the MACH2™ system developed by ABB. All functions for control, supervision and protection of the stations are implemented in software running in a family of microprocessor circuit boards.
Station Design
The majority of equipment in a HVDC Light is delivered in enclosures and tested at factory before shipment. For example the IGBT valves, the control equipment, the valve cooling equipment and the station service are all delivered in enclosures. This simplifies the civil works and also makes the installation and commissioning faster than for a traditional converter.
The HVDC Light concept lends itself to a modular standardised design with a high degree of factory testing.