What is HVDC Transmission? - Types, Advantages, Disadvantages & Applications

Both electrical science and the practical applications of electricity began with direct currents and its first practical application was DC Telegraphy powered by electrochemical batteries. Electrical lighting also began with dc power using dynamos. The first electrical central station was built by Thomas Alva Edison at pearl street in new york and it began in the year 1882 with operating dc voltage at 110 volts.

Nowadays, modern civilization mainly depends on the consumption of electrical energy for domestic, industrial, commercial, and agricultural applications. For this purpose, an efficient transmission system is very necessary since the generating stations are located in remote areas. An efficient transmission system has to meet the following requirements.

• Bulk power transmission over long distances,
• Low transmission losses.
• Less voltage fluctuations.
• Possibility of power transfer through submarine cables.
• System of interconnection.

Up to the 1980s, ultra high voltage ac (UHV-AC) transmission lines above 765 kV were used for bulk power transmission, and due to the development of accurate control in thyristor, the HVDC (high voltage direct current) transmission lines are using which are having a distinct superiority over UHV-AC transmission lines.

What is HVDC Transmission System?

The High Voltage Direct Current (HVDC) transmission system uses direct current for the transmission of power over long distances. The HVDC transmission system provides efficient and economic transmission of power even to very long distances that meet the requirements of growing load demands. Due to its simple constructional feature and less complexity, research and development authority discovered its usage in modern power transmission.

By comparing ac and dc transmission, it is clear that for transmission of power over long distances ac is not much suitable, and for generation and utilization of power, dc is not favorable compared to ac.

Thus, for HVDC transmission it requires terminal equipment for convening ac into dc at the sending end, and terminal equipment is again required at the receiving end to invert this dc supply obtained into ac.

Principle of HVDC Transmission System :

The HVDC transmission system mainly consists of converter stations where conversions from ac to dc (rectifier station) are performed at sending end and at the receiving end the dc power is inverted into ac power using an inverter station. Hence, the converter stations are the major component of the HVDC transmission system.

Also, by changing the role of the rectifier to inverter and inverter to rectifier the power transfer can be reversed which can be achieved by suitable converter control. The below shows the schematic diagram of the HVDC transmission system.

The ac substations at both ends of the HVDC line consist of ac switchgear, bus bars, current transformers, voltage transformers, etc. The converter transformers are connected between converter values and ac bus valves which transfers power from ac to dc or vice-versa.

Smoothing reactors are necessary for converter operation, and for smoothing the dc current by reducing ripples obtained on the dc line. The electrode line connects the midpoint of converters with a distant earth electrode.

Types of HVDC Transmission Systems :

The HVDC transmission systems are mainly classified into the following types on the basis of arrangement of the pole (line) and earth return. They are,

• Monopolar HVDC System - An HVDC system having only one pole and earth return.
• Bipolar HVDC System - An HVDC system with two poles of opposite polarity.
• Homopolar HVDC System - It has two poles of the same polarity and earth return.
• Back to Back HVDC Coupling System - It has no dc transmission line. The rectification and inversion are taken place at the same substation by a back-to-back converter.
• Multi-Terminal HVDC Systems - It has three or more terminal substations.

Advantages of HVDC Transmission System :

Nowadays HVDC systems are preferred over HVAC systems because of the following advantages,

• The HVDC transmission requires narrow towers, whereas ac systems require lattice shape towers, this makes the construction simple and reduces cost.
• The ground can be used as the return conductor.
• No charging current, since dc operates at unity power factor.
• Due to less corona and radio interference, it results in an economic choice of the conductor.
• Since there is no skin effect in dc transmission the power losses are reduced considerably.
• Large or bulk power can be transmitted over long distances.
• Synchronous operation is not required.
• Low short-circuit current on dc line.
• Tie-line power can be easily controlled.
• Power transmission can be also possible between unsynchronised ac distribution systems (interconnection of ac systems of different frequencies).
• Cables can be worked at a high voltage gradient, which makes them more suitable for undersea cables.
• Power flow through the HVDC line can be quickly controlled.

Disadvantages of HVDC Transmission System :

• It is very difficult to break the dc currents hence it requires a high cost of dc circuit breakers.
• It is not possible to use transformers to change the voltage levels.
• Due to the generation of harmonics in converters, it requires ac & dc filters, hence the cost of converting station is increased.
• It requires continuous firing or triggering thyristor valves hence is it is complex.
• Converters have little overload capability.
• HVDC system is not economical for primary transmission, sub-transmission, and distribution hence it is not used.
• HVDC substations have an additional loss at converter transformers and valves. These losses are continuous.

Applications of HVDC Transmission System :

Since HVDC transmission systems have various technical and economic superiority features as compared to the EHV-AC transmission systems. Hence, in these modern days, HVDC transmission systems are mainly using in the following applications.

• Long-distance bulk power HVDC transmission by overhead lines.
• Underground or underwater cables.
• Interconnection of ac systems operating at different frequencies.
• Back-to-back HVDC coupling stations.
• MTDC asynchronous interconnection between 3 or more ac networks.
• Control and stabilization of power flow in ac interconnection of large interconnected systems.