- HVDC Transmission Definition: HVDC transmission is the method of transmitting electricity in DC form over long distances using either submarine cables or overhead lines.
- Conversion and Components: The hvdc transmission system uses rectifiers and inverters for converting AC to DC and vice versa, with components like smoothing reactors and harmonic filters to ensure stability and reduce interference.
- Link Types: HVDC links can be mono-polar, bipolar, or homopolar, with configurations chosen based on the specific transmission needs and geographical conditions.
- Efficiency Advantages: HVDC is favored over HVAC for its lower losses, better voltage regulation, and higher reliability, especially for long-distance transmission.
- Practical Applications: HVDC technology is ideal for undersea and underground cables, allowing for effective interconnections between different power systems and overcoming geographical challenges.
High-voltage direct current (HVDC) transmission moves bulk electrical power as direct current through overhead lines or submarine and underground cables. Converter stations connect the DC link to alternating-current networks. HVDC is often economical for long routes and cable links because it can reduce line losses and avoid the distance limits that cable capacitance places on AC transmission.
HVDC Transmission System
A typical point-to-point link converts AC to DC at the sending station, carries DC over the transmission route and converts it back to AC at the receiving station. The sending converter operates as a rectifier. The receiving converter operates as an inverter. Modern converter controls regulate the direction and amount of transmitted power.
A two-terminal HVDC link therefore has one converter station at each end. Power received is lower than power sent because the converters, conductors and other equipment dissipate energy. The loss depends on the link design, loading and route length.
A two-terminal system has two converter stations connected by one DC transmission line or cable route. A multi-terminal system connects three or more converter stations through a DC network.
The main HVDC Transmission components perform these functions.
Converters: Semiconductor valve bridges change AC to DC or DC to AC. Converter transformers provide voltage matching and electrical isolation in designs that require them.
Smoothing Reactors: Series inductors reduce DC-current ripple and limit rapid current changes. Their role in commutation and harmonics depends on the converter technology.
Electrodes: Ground-return schemes use buried or submerged conductors to transfer current between the DC circuit and earth while controlling local voltage gradients.
Harmonic Filters: Filters limit converter-generated harmonic voltage and current on the connected networks.
DC Lines: The transmission path may use overhead conductors, underground cables or submarine cables.
Reactive Power Supplies: Line-commutated converters draw reactive power from the AC system. Sources such as synchronous condensers, filters or shunt capacitors support the required AC voltage.
AC Circuit Breakers: The fault in the transformer or AC network is isolated by circuit breakers. Opening the AC breakers also de-energises the converter station, although dedicated DC switching may be needed for faults within a DC network.
HVDC System Configurations
HVDC links are classified by their conductor arrangement, return path and number of terminals.
Mono Polar Links
A monopolar link uses one high-voltage conductor. Its return path may be earth or a dedicated metallic conductor where ground return is unsuitable or restricted.
Bipolar Links
A bipolar link has two poles at opposite DC polarities, usually with equal voltage magnitude. The midpoint may be grounded. If one pole is unavailable, some schemes can continue at reduced capacity using a permitted return path.
Homopolar Links
A homopolar link uses two or more conductors at the same polarity, historically negative, with earth as the return path. Restrictions on continuous ground current make this arrangement uncommon.
Multi Terminal Links
A multi-terminal link connects three or more converter stations. It supports power exchange among several locations but needs coordinated protection and control.
Comparison of both HVAC and HVDC Transmission System
| HVDC Transmission System | HVAC Transmission System |
| Line losses can be lower on long routes. | AC conductors have skin effect; both systems can have corona discharge. |
| Converters directly control transmitted active power. | Power flow depends on network voltage, impedance and phase angle unless separate control equipment is installed. |
| Often economical for long-distance bulk transfer and long cable links. | Often economical for shorter overhead routes because terminal equipment is simpler. |
| DC insulation is designed for the chosen pole-to-ground voltage and polarity stresses. | AC insulation is designed for RMS voltage, peak stress and switching or lightning overvoltages. |
| Reliability depends on the link configuration, converter design and network support. | Reliability also depends on circuit design, protection and redundancy. |
| Can interconnect asynchronous AC networks. | A direct AC interconnection requires synchronized frequency and phase. |
| An overhead link may use fewer high-voltage conductors for the same transfer duty. | Three-phase transmission normally uses at least three phase conductors. |
| Towers may be narrower for an equivalent power-transfer requirement. | Tower size depends on phase clearances, conductor arrangement and route conditions. |
Disadvantages of HVDC Transmission
- Converter stations add capital cost and electrical losses at both ends of the link.
- Converters, filters and specialised switching equipment can make short links less economical than AC alternatives.
- A DC voltage cannot be changed by a conventional AC transformer without electronic conversion.
- Control, protection and insulation coordination are more specialised than for a simple AC line.
- Multi-terminal DC fault detection and interruption require dedicated protection.
Application of HVDC Transmission
- Long undersea and underground cable connections
- Long-distance bulk power transfer between AC networks
- Controlled interconnection of asynchronous AC systems
