Role of Power Electronics in HVDC

High Voltage Direct Current (HVDC) technology, which is powered by power electronics, is important in electric power transmission. Traditionally, electricity was transmitted via AC systems; however, HVDC transmission has arisen as a viable and, in some circumstances, more beneficial alternative for specialized applications.

Power electronics have been critical in making HVDC technology feasible and efficient, allowing for its widespread use. Converters are the principal power electronics devices used in HVDC transmission systems. In HVDC systems, two types of converters are used: line commutated converters (LCC) and voltage source converters (VSC).

Line Commutated Converters (LCC): LCCs, also known as thyristor-based converters, are the classic converters used in HVDC systems. Because of their resilience and dependability, these converters have been used in numerous HVDC systems across the world. They operate on naturally commutated thyristor valves, with the current zero-crossing of the AC side triggering the converter's shutdown. Their downside is that they require a powerful AC system to work well and create harmonic currents, which necessitate the use of filters to mitigate these effects.

Voltage Source Converters (VSC): VSC technology is used in more contemporary HVDC systems. Self-commutated devices such as IGBTs (Insulated Gate Bipolar Transistors) are used in VSC-based HVDC systems. They can regulate both active and reactive power separately and do not rely on a powerful AC system for commutation, giving them more operational flexibility than LCC-based HVDC systems. Furthermore, they have a smaller footprint and generate less harmonics, which eliminates the need for huge filtering equipment.

At the transmitting end, these power electronic converters convert AC to DC (rectification) and DC to AC (inversion) at the receiving end. They enable HVDC systems to transport electricity over long distances with low losses and to manage power flows in the network, improving grid stability.

Furthermore, power electronics contribute to the development of complex control and protection schemes in HVDC systems, which improves system dependability, efficiency, and safety. Overall, power electronics are critical to the operation of HVDC systems, since they enable efficient, controlled, and dependable long-distance power transmission.

Control and Protection in HVDC Systems

A High Voltage Direct Current (HVDC) transmission system relies heavily on control and safety. Both of these factors contribute to the overall stability, efficiency, and safety of HVDC systems, assuring power transmission dependability.

Control in HVDC Systems

In HVDC systems, the key control objectives are to maintain system stability and to assure the appropriate power flow between the converter stations. This is accomplished by two forms of control: direct current control and converter control.

Direct Current Control: Typically, this is done to regulate the active power delivered between two converter stations. By changing the pulse width modulation (in Voltage Source Converters) or the firing angle (in Line Commutated Converters), the DC voltage and current, and therefore the active power, may be adjusted .

Converter Control: Converter control manages the reactive power exchange between the converter and the AC network. This can have an effect on the voltage at the point of common coupling, making it an important feature of HVDC control.

HVDC control systems also incorporate telecommunication elements for coordination between the two ends of the transmission line, as well as advanced monitoring systems for the health and status of system components.

Protection in HVDC Systems

Protection in HVDC systems is a difficult problem due to the absence of natural current zero-crossings, which are present in AC systems and are commonly used for preventive measures. The protection system's job is to detect and isolate defects quickly and reliably in order to prevent equipment damage and assure the system's safety.

Converter Protection: The converters in an HVDC system are the most sophisticated and expensive components, necessitating strong protection. Overvoltage, overcurrent, and temperature rise protection are all included into converters. In LCC converters, special care is taken to prevent commutation failures.

DC Line Protection: An HVDC system's DC line is susceptible to both temporary and enduring problems. Ionization decay causes transient faults to frequently self-extinguish, whereas chronic faults call for intervention. Fast-acting breakers and grounding switches are examples of preventative measures.

Substation Protection: This entails safeguarding the converter station's auxiliary equipment, such as the smoothing reactors and transformers.

Case Studies of HVDC Systems

In numerous operating systems around the world, High Voltage Direct Current (HVDC) transmission systems have proven their effectiveness. Let's look at two prominent case studies that emphasize the function of power electronics in these systems.

The Itaipu HVDC Transmission System

The Itaipu HVDC transmission system in Brazil is one of the greatest HVDC systems in the world. It was built to transfer power from the Itaipu hydroelectric facility on the Paraná River to load centers near Sao Paulo, about 800 kilometers distant.

The system is bipolar, which allows for simultaneous positive and negative pole functioning, decreasing the impact of a single-pole fault. It has a rated voltage of ±600kV and a total transmission capacity of 6300MW, demonstrating the high power handling capabilities of HVDC systems.

For power conversion, this HVDC system employs thyristor-based line-commutated converters (LCCs). The control and protection techniques of the converters are largely designed to handle variations in transmitted power caused by changes in the production of the hydroelectric plant or demand variations at the load center.

The Pacific Intertie

Another notable HVDC use is the Pacific Intertie in the United States. The system transports power over a distance of approximately 1360km between Oregon and Los Angeles, allowing the Pacific Northwest and Southern California to share power resources.

Originally commissioned in 1970 with a capacity of 1440MW, the system was later extended to 3100MW in the 1980s. The major goal of the Pacific Intertie is to take advantage of the time difference between the two regions, allowing for effective power exchange. For example, extra power generated during off-peak hours in the Pacific Northwest is sent to Southern California, where peak demand occurs later in the day.

The Pacific Intertie uses line-commutated converters (LCCs) and innovative control algorithms to optimize power flow and system stability. It's also a great example of how HVDC cables may connect AC grids of varying frequencies.