In an electrical power system, transmission of bulk electrical energy from generating stations to load centers is one of the most important functions. For this purpose, power can be transmitted using either High Voltage Alternating Current (HVAC) or High Voltage Direct Current (HVDC) systems. The choice between HVDC and HVAC depends on factors such as transmission distance, cost, losses, controllability, and application requirements.
This article explains HVDC and HVAC transmission systems in detail, including their types, advantages, disadvantages, and a complete comparison.
What is HVDC Transmission?
- HVDC (High Voltage Direct Current) transmission is a system in which electrical power is transmitted in the form of direct current at very high voltage levels. Since most electrical power is generated in alternating current form, conversion is required at both ends of the HVDC transmission line.
- At the sending end, AC power is converted into DC using a rectifier station. The DC power is transmitted over the transmission line, and at the receiving end, an inverter station converts DC back into AC suitable for distribution or utilization.
- HVDC transmission is preferred for long-distance bulk power transmission because it offers lower losses, better voltage regulation, and higher controllability compared to HVAC systems.
Types of HVDC Transmission
Monopolar HVDC System
- In a monopolar HVDC system, a single conductor is used to transmit power, and the return path is provided through earth or sea.
- This system has a lower initial cost and simpler structure. However, earth return current may cause corrosion of buried metallic structures and interference with communication systems.
Bipolar HVDC System
- The bipolar HVDC system uses two conductors, one operating at positive polarity and the other at negative polarity with respect to ground.
- Under normal operating conditions, earth current is zero. If one pole fails, the system can continue operation in monopolar mode, making it highly reliable. This is the most widely used HVDC configuration.
Homopolar HVDC System
- In a homopolar HVDC system, two or more conductors having the same polarity (usually negative) are used, and the earth provides the return path. Although insulation requirements are lower, continuous earth current causes serious environmental and technical problems. Hence, this system is rarely used in practice.
What is HVAC Transmission?
- HVAC (High Voltage Alternating Current) transmission is the conventional and most widely adopted method of power transmission. In this system, electrical power is transmitted in alternating current form at high voltage levels such as 132 kV, 220 kV, 400 kV, 765 kV, and above.
- The main advantage of HVAC transmission is the ease of voltage transformation using transformers. This makes HVAC systems highly suitable for interconnected power networks and distribution systems.
Types of HVAC Transmission
Single-Phase HVAC Transmission
- Single-phase HVAC transmission is used only for special applications or low-power loads. It is not suitable for bulk power transmission due to poor efficiency and unbalanced operation.
Three-Phase HVAC Transmission
- Three-phase HVAC transmission is the standard method used worldwide for power transmission. It provides constant power, better efficiency, and economical utilization of conductors.
Overhead HVAC Transmission
- In overhead HVAC transmission, conductors are supported on towers or poles. This system is economical, easy to maintain, and suitable for long-distance transmission.
Underground HVAC Transmission
- Underground HVAC transmission is used in urban and environmentally sensitive areas. However, it is expensive and limited in length due to high charging current and insulation constraints.
Advantages of HVDC over HVAC
- HVDC transmission offers several technical and economic advantages over HVAC transmission. Since DC power does not involve reactive power, there is no reactive power loss in HVDC systems. Effects such as skin effect and proximity effect are absent, resulting in better conductor utilization.
- HVDC transmission has no charging current, making it ideal for long underground and submarine cables. Transmission losses are lower for long distances, and fewer conductors are required compared to three-phase HVAC systems. HVDC also allows precise and fast control of power flow and enables interconnection of asynchronous power systems.
Advantages of HVAC over HVDC
- HVAC transmission systems have simpler and cheaper terminal equipment since converter stations are not required. Voltage transformation is easy using transformers, and protection techniques are well developed and widely understood.
- HVAC transmission is more economical for short and medium distances and is already integrated into existing power grids across the world.
Comparison Between HVDC and HVAC Transmission
| Parameter | HVDC Transmission | HVAC Transmission |
|---|---|---|
| Nature of Current | Direct Current | Alternating Current |
| Reactive Power | Absent | Present |
| Skin Effect | Absent | Present |
| Charging Current | Absent | Present |
| Transmission Losses | Lower for long distances | Higher for long distances |
| Number of Conductors | Two (Bipolar system) | Three |
| Terminal Equipment Cost | High | Low |
| Break-even Distance | 600–800 km (overhead), 50–100 km (cable) | Suitable below break-even distance |
| Submarine Transmission | Highly suitable | Not suitable for long lengths |
| Grid Interconnection | Possible for asynchronous grids | Not possible |
In conclusion, HVAC transmission is preferred for conventional short and medium-distance power networks, while HVDC transmission is chosen for long-distance bulk power transfer, submarine cables, and interconnection of different power systems. Both systems play an important role in modern power transmission.