Explain the Principles of Link Control in a VSC HVDC system? 5 marks

In a Voltage Source Converter High Voltage Direct Current (VSC HVDC) system, link control is crucial for ensuring efficient and stable power transmission. Here are the principles of link control in a VSC HVDC system:

1. Power Flow Control: VSC HVDC systems allow for precise control of power flow between the connected AC systems. This is achieved by adjusting the phase angle and magnitude of the AC voltage at the converter terminals. By controlling these parameters, the active and reactive power can be independently managed, enabling efficient power transfer and system stability.

2. Voltage Regulation: The VSC technology provides the ability to regulate the voltage at the point of common coupling (PCC). This is essential for maintaining voltage stability in the connected AC networks. The converters can inject or absorb reactive power, thereby supporting the voltage levels and improving the overall power quality.

3. Frequency Support: VSC HVDC systems can contribute to frequency regulation in the connected AC grids. By adjusting the power transfer, the system can help balance supply and demand, thus supporting the frequency stability of the grid. This is particularly beneficial in integrating renewable energy sources, which can cause fluctuations in power generation.

4. Black Start Capability: VSC HVDC systems have the capability to perform a black start, which means they can energize a de-energized grid without the need for an external power supply. This is possible due to the self-commutated nature of the converters, which can generate a stable AC voltage waveform independently.

5. Fault Ride-Through: VSC HVDC systems are designed to handle faults effectively. They can continue to operate during AC system faults by rapidly adjusting their control strategies. This includes reducing power transfer or switching to a different control mode to maintain system stability and prevent cascading failures.

These principles of link control in VSC HVDC systems ensure reliable and flexible operation, making them suitable for modern power grids with increasing integration of renewable energy sources and complex interconnections.