What is HVDC?

HVDC stands for high voltage direct current in the electric power transmission system. The high voltage direct current system uses direct current (DC) for the transmission of electrical power through transmission lines in contrast with the alternating current that is the AC system. The HVDC is used to transmit bulk power point-to-point through long distances transmission lines. Power is rapidly transferred because the efficiency of transmission lines is greatly increased over long distances due to HVDC lines.

Voltages between 100 kV and 800 kV are used by most of the HVDC links. Between AC transmission systems that are not synchronized, the HVDC system allows the power transmission among them. The power since can be controlled independently using an HVDC link of the phase angle between the load and the source so due to rapid changes in power it can stabilize a network against disturbances. The transfer of power between grid systems that are running at different frequencies is also allowed by the HVDC systems such as 50 Hz and 60 Hz. By allowing the exchange of power between incompatible networks, it improves the stability and economy of each grid. The technology which was developed extensively in Sweden and Germany in the 1930s is used by the HVDC grids transmission systems.

History

In 1882 at Miesbach-Munich Power transmission the first long distance transmission of electrical power was demonstrated by using direct current but only 1.5 kW was transmitted line-to-line. The Swiss engineer Rene Thury developed an early method of HVDC transmission and his method was put into practice in Italy in 1889. To increase the voltage this system used series connected motor-generator. With up to 5000 volts across each machines the transmission lines were operated at "constant current" mode. Over a distance of 120 km this system transmitted 630 kW at 14 kV DC. During the period of 1920 to 1940 the HVDC grids controlled mercury arc valves became available for power transmission. To force the current to zero and thus to turn of the valve, these valves require an external circuit. The new HVDC systems have used only solid state devices such as thyristors since 1977. Similar to the mercury valves the thyristor type valves also require connection to an external AC circuit in HVDC systems applications. The line-commutated converters has certain disadvantages in their use of HVDC systems. These disadvantages can be compensated by the use of the capacitor-commutated converter or CCC used in small number of HVDC systems. For bulk power transmission when two AC systems need to be interconnected a back-to-back station HVDC arrangement is used. In a back-to-back station of HVDC has there are no cables or overhead lines which helps in separating the inverter or rectifier. But because of the advent of Voltage source converters or VSC-based converters the CCC has remained only a niche application which completely helps in the elimination of the turn-off time. VSC converters were used for black-start applications. HVDC systems are now emerging as the bedrock upon which the new energy system which is based upon renewable energy and is being implemented and developed. The renewable energy systems such as wind power projects and solar projects are often highly volatile and are located in remote areas.

High Voltage transmission

To reduce the energy lost in the resistance of the wires the high-voltage is used for electrical power transmission over long distances. Doubling the voltage will give us the same power at only half of the value of the current for a given quantity of the power transmitted. The power which is lost as heat in the wires is proportional to the square of the current so doubling the voltage reduces the line loss by a factor of 4. During power transmission, the power loss can also be reduced by increasing the size of the conductor and larger conductors are heavier and are more costly. The transmission level voltages must be reduced for end-use equipment because high-voltage cannot be easily used for lighting or motors. In endpoint-to-endpoint alternating current transmission circuits, the transformers are used to change the voltage levels. Voltage changes were practically made by the transformers only and the AC generators were more efficient in use than the DC generators. Around the turn of the 20th century, these advantages led to the early low voltage DC transmission systems being supplanted by the AC systems. With the development of power electronic devices, the practical conversion of power between AC and DC became possible with time.

Comparison with AC transmission

Advantages

• AC transmission scheme is a long-distance point-to-point HVDC transmission scheme that generally has a lesser overall cost and also has lower losses.
• HVDC systems require a fewer amount of conductors than the AC lines as there is no skin effect and no need to support three phases.
• At the same voltage, the losses for HVDC systems are 50% less than that of the AC transmission systems this is only because direct current deals with only the active power and incurs lower losses in comparison to AC which transfers both active and reactive powers.
• Power between separate AC networks can also be transferred using an HVDC transmission system.

Disadvantages

• The limitations of HVDC transmission systems are in switching, conversion, control, availability, and maintenance.
• Then alternating current system the HVDC systems are less reliable and has lower availability mainly due to extra conversion equipment.
• The converter station of HVDC is a specialized type of substation which forms the equipment of the terminal for an HVDC line.
• The converter stations which are required have limited overload capacity and are expensive.
• Than AC systems the HVDC systems are less standardized and technology changes more quickly.
• Then AC systems the realization of multi-terminal systems are more complex as is expanding the external available schemes to multi-terminal systems. Because of arching the HVDC circuit breakers are difficult to build but in the case of AC the voltage inverts and so crosses zero-volts a dozen times.

Context and Applications

This topic is significant in the professional exam for undergraduate, Graduate, and Post-graduate courses.

• Bachelors of Science in Electrical Engineering
• Masters of Science in Electrical Engineering

Practice Problems

Q1. What is the full form of HVDC?

1. High voltage direct current
2. High voltage distance current
3. Higher value direct current
4. Higher value distance current

Answer: Option a

Explanation: HVDC stands for high voltage direct current in the electric power transmission system.

Q2. Which type of current does the HVDC system uses for transmission?

1. DC
2. AC
3. Both 1 and 2
4. None

Answer: Option a

Explanation: The high voltage direct current system uses direct current (DC) for the transmission of electrical power through transmission lines in contrast with the alternating current that is the AC system.

Q3. How the power loss can be reduced during power transmission?

1. By reducing the size of the conductor
2. By increasing the size of the conductor
3. By increasing the length of the conductor
4. By reducing the length of the conductor

Answer: Option b

Explanation: During power transmission, the power loss can also be reduced by increasing the size of the conductor as larger conductors are heavier and are more costly.

Q4. Which of the following is preferred in the transmission of bulk power over long distances?

1. High voltage direct current
2. High voltage high current
3. High voltage low current
4. Low voltage alternating current

Answer: Option a

Explanation: The high voltage direct current is used to transmit bulk power point-to-point through long distances transmission lines.

5. Which of the following is true in the HVDC transmission system?

1. High radio interference
2. Expensive converters
3. High corona loss
4. Right-of-way

Answer: Option b

Explanation: At both the sending and receiving ends the converter stations are required which increases the overall cost.

Related Concepts

• DC-to-DC converters
• Flexible AC transmission system
• High-voltage cable
• Transmission tower
• Valve hall
• Insulated-gate
• Concept of six-pulse and twelve-pulse in HVDC