HVDC Transmission System
Table of Contents
HVDC Transmission System
Full form of HVDC (HVDC full form) – High Voltage Direct Current Transmission
So High voltage direct current transmission is the widespread distribution of electricity in the form of DC over extensive distances using overhead or underwater transmission lines. When the cost, losses, and numerous other factors are taken into account, this type of gearbox is preferred over HVAC gearbox for very long distances. So HVDC is frequently referred to as an electrical superhighway or a power superhighway.
HVDC Transmission System
We are aware that the generating station produces AC power. This needs to be converted first to DC. A rectifier is used to perform the conversion. The overhead lines will be the conduit for the DC power. This DC needs to be converted into AC at the user’s end. An inverter is positioned at the receiving end for that reason.
As a result, the HVDC substation will have an inverter terminal at one end and a rectifier terminal at the other. The sending end’s and the receiving end’s power will always be equal (Input Power = Output Power).
So Two terminal DC systems are defined as having two converter stations at both ends of a single transmission line. Multi-terminal DC substation is the term used when there are two or more converter stations and DC transmission lines.
The components of the HVDC Transmission system and its function are explained below.
Converters: The converters are responsible for converting AC to DC and DC to AC. Transformers and valve bridges are part of it.
Smoothing Reactors: Each pole is made up of smoothing reactors, which are series-connected inductors. So It reduces harmonics, prevents discontinuity of current for loads, and prevents commutation failures in inverters.
Electrodes: They serve as the system’s connection to the earth and are actually conductors.
Harmonic filters: These are used to reduce the harmonics in the current and voltage of the converters.
DC lines: These can be overhead or cable-based.
Reactive Power Sources: The converters’ use of reactive power, which may account for more than half of the total active power transferred, is possible. In order to supply this reactive power, shunt capacitors are used.
AC Circuit Breakers: By using the circuit breakers, the transformer fault is fixed. It previously also had the DC link disconnected.
HVDC System Configurations
The classification of HVDC links are as follows:
Mono Polar Links
It only takes one conductor, and the return path can be either water or the ground. Metallic return is used if the earth’s resistivity is high.
Bi-polar Links
In each terminal, two converters with the same voltage rating are used. There is ground at the converter junctions.
Homopolar Links
It consists of multiple conductors with equal polarity, most of which are negative. The route back is on the ground.
Multi Terminal Links
It is used to connect more than two points and is rarely used.
Comparison of both HVAC and HVDC Transmission System
| HVDC Transmission System | HVAC Transmission System |
| Low losses. | Losses are high due to the skin effect and corona discharge |
| Better Voltage regulation and Control ability. | Voltage regulation and Control ability is low. |
| Transmit more power over a longer distance. | Transmit less power compared to a HVDC system. |
| Less insulation is needed. | More insulation is required. |
| Reliability is high. | Low Reliability. |
| Asynchronous interconnection is possible. | Asynchronous interconnection is not possible. |
| Reduced line cost due to fewer conductors. | Line cost is high. |
| Towers are cheaper, simple and narrow. | Towers are bigger compared to HVDC. |
Disadvantages of HVDC Transmission
- Circuit Breakers, Converters and AC filters are expensive especially for small distance transmission.
- Converters with small overload capacity are used.
- HVDC link is extremely complicated.
- No transformers for altering the voltage level.
- Uncontrollable power flow.
Application of HVDC Transmission
- AC network interconnections
- Undersea and underground cables
- Interconnecting Asynchronous system