What Are Voltage-source Converters?

Voltage-source converters are power electronic converters that maintain a stiff DC bus voltage, typically held constant by a large shunt capacitor, and use this as the energy source from which an AC output voltage is synthesized by switching semiconductor devices at high frequency. The term distinguishes this topology from current-source converters, which maintain a stiff DC current using a series inductor on the DC side and require the AC network to commutate the current. Voltage-source converters can operate with a variety of AC loads and sources, can supply reactive power independently of active power, and can operate into weak or passive networks, making them the dominant technology for modern high-power AC-DC and DC-AC conversion.

The technology emerged from advances in self-commutated semiconductor switches: thyristors required external commutation circuits, but insulated-gate bipolar transistors (IGBTs), which became commercially viable at power converter ratings during the 1990s, can be turned on and off by gate drive alone. The first commercial VSC-based high-voltage direct current (HVDC) link, the Hellsjön project in Sweden, entered service in 1997 and demonstrated that voltage-source converters could be scaled to high-power transmission applications. Research on VSC HVDC technology, including its benefits for integrating remote renewable resources, is covered in the IEEE Xplore publication on new VSC technologies for HVDC transmission.

Topology and Switching

A basic voltage-source converter for three-phase applications uses six IGBT switches arranged in three half-bridge legs, identical in structure to the voltage source inverter. Each leg connects the positive and negative DC rails to one AC output terminal. By controlling the duty cycle of each switch using sinusoidal pulse-width modulation or space vector modulation, the converter synthesizes the desired AC voltage waveform at the output terminals. A filter inductor on the AC side limits the current ripple produced by the switching action. Modular multilevel converters (MMCs), introduced commercially around 2010, replace each switch with a chain of submodule cells, each containing a capacitor and two switches. This architecture distributes the voltage stress across many low-voltage components, eliminates the need for large output filters, and reduces switching losses, enabling practical VSC-HVDC schemes at ratings exceeding 1 GW.

HVDC Transmission

VSC-HVDC systems connect AC networks that operate at different frequencies or that cannot be synchronized, transmit bulk power over long submarine or underground cables where AC charging current would limit the usable length, and deliver offshore wind farm output to shore. Unlike the earlier line-commutated converter (LCC) HVDC technology based on thyristors, VSC-HVDC maintains a constant DC voltage polarity and reverses power flow by reversing the direction of current rather than voltage, which makes it compatible with multi-terminal DC grid configurations. VSC converters also provide reactive power support to the AC network independently of their active power dispatch, functioning as static synchronous compensators when required. The IEEE Power and Energy Society technical report on voltage control in emerging transmission systems addresses the role of VSC-based resources in providing reactive power and voltage support in modern grids.

AC-DC Conversion and Back-to-Back Schemes

Back-to-back VSC schemes, where two converters share a common DC bus without any transmission line between them, are used as frequency converters and asynchronous AC bus couplers. They allow two AC networks to exchange power at independently controlled rates without requiring synchronization, which is valuable at borders between different grid operators or between systems running at 50 Hz and 60 Hz. Drive systems for large motors also use voltage-source converter AC-DC-AC configurations: a rectifier front-end converts the supply AC to DC, and an inverter stage converts the DC to variable-frequency AC for motor control. The IEA PVPS report on reactive power management with distributed energy resources discusses how grid-connected VSC systems can provide ancillary services including reactive compensation and fast frequency response.

Applications

Voltage-source converters have applications across a wide range of high-power electrical systems, including:

• Offshore wind farm collection and export systems, where VSC-HVDC cables bring generation to shore over distances beyond the AC cable limit
• High-speed rail power supplies, where back-to-back converters interface the traction network to the utility grid
• Battery energy storage systems, where four-quadrant VSC operation enables both charging and discharging with reactive power support
• Flexible AC transmission systems (FACTS), where VSC-based STATCOMs and unified power flow controllers manage power flow on congested corridors
• Industrial variable-speed drives for compressors, fans, and pumps in refineries and chemical plants