Introduction to Self-Commutating in Voltage Source Converter HVDC Systems

In the realm of modern electricity infrastructure, the Voltage Source Converter (VSC) High Voltage Direct Current (HVDC) transmission technology stands out. A crucial element of VSC HVDC systems is self-commutation, which refers to the ability of the converter to control the switching of its power electronic devices without the need for an external commutation source. This feature distinguishes VSC technology from traditional Line Commutated Converters (LCC).

Key Points about Self-Commutating in VSC HVDC

| Active Control: In self-commutating converters, the switching of the devices is actively controlled by the converter's control system. This allows for precise modulation of the output voltage and current, ensuring optimal performance.

| Bidirectional Power Flow: Self-commutating converters enable bidirectional power flow, meaning they can both transmit and receive power. This is advantageous for applications such as connecting renewable energy sources and facilitating grid interconnections.

| Flexible Operation: VSCs can operate independently of the AC system's voltage level, allowing for more flexible operation in various grid conditions. This flexibility supports functionalities such as voltage support, reactive power compensation, and black start capabilities.

| Modulation Techniques: Self-commutation is often achieved through advanced modulation techniques such as Pulse Width Modulation (PWM). PWM allows for smooth control of the output waveform and reduced harmonics, contributing to more efficient power transmission.

| Applications: Self-commutating VSC HVDC systems are particularly useful for connecting offshore wind farms, integrating distributed generation, and enhancing the stability and reliability of power systems. These systems offer a robust solution for modern electricity grids, enabling more efficient and sustainable energy management.

Explanation of Acronyms

VSC - Voltage Source Converter: A type of power electronic converter that can control the switching of its power electronic devices without the need for an external commutation source.

HVDC - High Voltage Direct Current: A method of transmitting electrical power that uses direct current (DC) voltage at high voltage levels.

Conventional HVDC systems use Current Source Converters (CSC) and primarily employ thyristors (Silicon Controlled Rectifiers - SCR) as power switches. These devices are line commutated, meaning they are turned off by reversing the polarity of the AC voltage. In contrast, VSC converters use transistors like Insulated Gate Bipolar Transistors (IGBTs) which are self-commutated, meaning they are turned off by the voltage on their gate terminal.

Differences Between VSC and LCC Converters

LCC Converters rely on the AC voltage from the grid to turn off the thyristors. This process is known as line commutation. The thyristors are turned on by injecting current into their gates but can only be turned off if their current goes to zero or they are polarized reversely, meaning the anode-cathode voltage becomes negative. The usual way is reversing the polarity and it is achieved by turning on the next thyristor which imposes an instantaneous AC voltage with reverse polarization to the former thyristor, turning it off.

VSC Converters use IGBTs, which are turned on and off by imposing a voltage to its gate related to its emitter terminal. This is the self-commutation mode. For VSCs to function optimally, the DC part of the circuit path of the IGBT must have very low series inductance to avoid overvoltage during the turning off process. When the IGBT is turned off, there must be a current path available, usually the diode in parallel with the IGBT, not the IGBT itself, which is turning off but a neighboring IGBT. The Voltage Source part of the name relates to this low inductance path needed on the DC side of the converter.

Applications of Self-Commutating VSC HVDC Systems

Self-commutating VSC HVDC systems are particularly useful for connecting offshore wind farms, integrating distributed generation, and enhancing the stability and reliability of power systems. These systems offer a robust solution for modern electricity grids, enabling more efficient and sustainable energy management.

VSCs for HVDC applications do not operate with Pulse Width Modulation (PWM) due to losses, but they switch with lower frequencies (tens of Hz) and are connected in a fashion known as MMC (Modular Multilevel Converter). MMCs are modular and consist of multiple modules, each of which can be independently controlled to achieve more complex voltage waveforms and better harmonic suppression.

Conclusion

In summary, self-commuting in VSC HVDC systems allows for flexible, controllable, and efficient power transmission. This technology is a crucial component of modern electricity grids, providing support in areas such as renewable integration, grid stability, and distributed energy deployment. By leveraging advanced switching techniques and flexible operational modes, VSC HVDC systems are setting the standard for future energy infrastructures.