How is HVDC Converted to AC?

High Voltage Direct Current (HVDC) power is converted back to alternating current (AC) primarily through the use of an inverter at the receiving terminal of an HVDC transmission system. This converter station, specifically operating as an inverter, transforms the stable DC power into a usable AC form, which can then be integrated into the existing AC electricity grid.

The Heart of HVDC Conversion: The Inverter

At the core of HVDC-to-AC conversion is the inverter, a powerful electronic device that reverses the rectification process. While a rectifier converts AC to DC at the sending end, an inverter performs the crucial task of converting DC back to AC at the receiving end. This process involves sophisticated power electronics to meticulously control the switching of the DC current, effectively synthesizing an AC waveform.

How Inverters Transform DC to AC

The fundamental principle behind an inverter is to take a constant DC voltage or current and, through rapid and controlled switching, create an oscillating output that mimics an AC waveform. This is achieved using high-power semiconductor devices such as Thyristors or Insulated Gate Bipolar Transistors (IGBTs).

The conversion process typically involves:

1. Chopping the DC: The DC voltage is rapidly switched on and off by the inverter's semiconductor devices.
2. Waveform Synthesis: By carefully controlling the timing and duration of these switching actions, the inverter creates a pulsed DC voltage that, when averaged over time, approximates a sinusoidal AC waveform.
3. Filtering: This raw, pulsed AC waveform is then passed through AC filters (composed of inductors and capacitors) to smooth out the pulses and remove unwanted harmonics, resulting in a clean, sinusoidal AC voltage and current suitable for the grid.
4. Transformation: Transformers adjust the voltage level of the converted AC power to match the requirements of the receiving AC grid.

Types of HVDC Converter Technologies

There are two primary technologies used in HVDC converter stations, both capable of operating as inverters to convert DC to AC:

Line-Commutated Converters (LCC)

• Principle: LCC technology primarily uses Thyristors, which are current-controlled semiconductor devices. These converters rely on the voltage of the connected AC grid to turn off (commutate) the Thyristors.
• Operation: At the receiving end, an LCC inverter essentially "steals" current from the DC line and shapes it into an AC waveform. The commutation process requires a strong and stable AC grid to provide the necessary voltage reversals to turn off the Thyristors.
• Characteristics:
  • Robust and mature technology.
  • Typically used for very high power applications.
  • Requires reactive power from the AC grid for operation, often compensated by large AC filters and shunt capacitors.
  • Cannot operate into a "dead" or very weak AC grid (black start capability is limited).

Voltage Source Converters (VSC)

• Principle: VSC technology utilizes Insulated Gate Bipolar Transistors (IGBTs) or similar self-commutating devices. Unlike Thyristors, IGBTs can be turned on and off independently by control signals, regardless of the AC grid voltage.
• Operation: A VSC inverter uses a pulse-width modulation (PWM) technique to create the desired AC voltage waveform. It converts the DC voltage into a controllable AC voltage by rapidly switching the DC voltage across AC phases.
• Characteristics:
  • Greater control flexibility, including independent control of active and reactive power.
  • Can operate into weak or even "dead" AC grids (black start capability).
  • Produces lower harmonics, reducing the need for extensive AC filters.
  • More compact and modular designs.
  • Suitable for multi-terminal HVDC systems and connections to offshore wind farms.

Key Components of an HVDC Inverter Station

A typical HVDC converter station configured for inversion includes several critical components working in unison:

• Converter Valves: The heart of the inverter, housing the power semiconductor devices (Thyristors for LCC, IGBTs for VSC) arranged in a bridge configuration.
• Converter Transformers: Step up or down the AC voltage to match the converter voltage levels and provide galvanic isolation.
• DC Capacitors/Reactors: Smooth the DC voltage and current, reducing ripples and aiding in commutation.
• AC Filters: Remove harmonic components generated during the DC-to-AC conversion process, ensuring a clean sinusoidal output for the AC grid.
• Reactive Power Compensation: For LCC systems, shunt capacitors or STATCOMs (Static Synchronous Compensators) may be needed to supply the reactive power consumed by the converter. VSC systems can inherently manage reactive power.
• Control and Protection Systems: Sophisticated digital systems monitor and control the entire conversion process, ensuring stable and reliable operation, and protecting against faults.

Practical Applications and Benefits

The ability to convert HVDC back to AC is fundamental to the widespread adoption of HVDC transmission for various applications, including:

• Long-distance power transmission: Efficiently transporting electricity over thousands of kilometers, minimizing losses.
• Interconnections between asynchronous AC grids: Linking grids that operate at different frequencies or phases, or that cannot be synchronized.
• Grid reinforcement: Enhancing stability and power transfer capacity within existing AC grids.
• Offshore wind farm integration: Connecting remote wind farms to the mainland grid, as AC transmission is not feasible over long subsea distances.

By effectively converting HVDC to AC, these systems ensure that power generated from diverse sources and locations can be reliably delivered and integrated into the existing electricity infrastructure, contributing to grid stability and energy security.