HVDC

Use of High Voltage Direct Current Transmission (HVDC) or in Indonesian terms is known as direct current power transmission (TDAS) actually started in the beginning of the first electricity was developed. Thomas Alva Edison made electricity network with a capacity of 6 x 100kW to power in 1200 using direct current light bulb in 1882. Although in its development, Edison developed dc system was 'lost' to compete with the ac system proposed by Westinghouse and Tesla, but these systems have been dc new era, the era of electricity. More than 70 years later, the dc transmission system came into use again after finding a mercury-arc tube at the end of the 1920s. Commercial HVDC project was first built in 1950 using a submarine cable to link Sweden with P. Gotland with a capacity of 20MW at 100kV voltage.
This paper briefly describes the technology, configuration, and application of direct current power transmission (HVDC).
HVDC technology started to be used again because the tube / mercury-arc already well established so that power converter ac / dc or dc / ac can be made, something that can not be done in the 1880s that resulted in the defeat of Edison's direct current alternating current system Westinghouse behind. Mercury-arc tube technology itself only lasted about 20 years until the invention of the thyristor at around 1970. This thyristor is the basis of the rapid development of HVDC technology because it can be made for the purposes of power, compared to the transistor / IGBT that with today's technology has a smaller capacity than the thyristor. The last decade, the development of IGBT technology allows for the HVDC converter is made by using IGBT (Figure 1), although its capacity is smaller than HVDC systems using thyristor converters. 

    Figure 1. Development of a static switch for HVDC

Starting from 20MW in Sweden, now has more than 100 active HVDC transmission lines in the world with a total capacity of more than 80GW (Fig. 2) scattered from North America, Scandinavia, Japan, China, India, Brazil, etc.. Starting from the voltage of 100 kV up to now reach the 500kV and 800kV are under construction. Some projects are quite famous among HVDC HVDC Gotland in Sweden in addition to the first HVDC is also an HVDC thyristor that uses the first time; HVDC Itaipu in Brazil (2 x 3150MW, + / - 500kV, 800 km) which is the biggest current HVDC system, Kii- Channel HVDC in Japan (1400MV, + / - 250kV), which uses light-triggered thyristors 8kV - 3500A. 

    Figure 2. The total capacity of HVDC

HVDC technology
There are 2 kinds of converter technology ac / dc / ac used in the HVDC system at this time. HVDC using current source converter (CSC) commutation meshes using thyristors and HVDC using Voltage source converter (VSC) that uses IGBT.
CSC-HVDC technology has been very well established for very large power converters. For the purposes of the above 1000MW this technology to be the only option at this time. HVDC Itaipu HVDC system is currently the largest commercial operations using the CSC-HVDC. CSC-biggest HVDC project that is being built now is Xiangjiaba - Shanghai HVDC 6400MW which transmits power to the 800kV as far as 2071 km.
Commutated meshes is one of the weaknesses that exist in the CSC-HVDC, HVDC result in the use of CSC required alternating current network is strong in the sending and the receiving end. Figure 3 shows that using the CSC HVDC. 

Figure 3. CSC-HVDC

VSC-HVDC is the latest development of HVDC technology. Almost since the last decade, some of VSC-HVDC project successfully built and reach the commercial stage. VSC-HVDC advantages compared to CSC-HVDC is the ability to commutation without relying condition nets, active and reactive power settings that are independent, and the ability to black-start. Excellence makes the VSC-HVDC attractive for power distribution applications to the burden of long distance that does not have a source of local nets, such as on offshore platforms, etc..
The weakness of VSC-HVDC is now IGBT technology has not been able to serve large-capacity power transmission as well as the CSC-HVDC. VSC-HVDC project is currently the largest HVDC Line Ciprivi in Namibia with a capacity of 300 MW at 350kV as far as 970 km. Figure 4 shows that using VSC HVDC. 

Figure 4. VSC-HVDC

HVDC Configurations
The system configuration is very dependent on local conditions, objectives, and economic factors. Both VSC or CSC-HVDC can use the same configuration, modifications can be made dependent local conditions respectively.
Back-to-back
This configuration is shown in Figure 5. In this configuration the converter substation located at the same location and not using long-distance direct current channel. Generally, this configuration serves as the interconnection frequency between the two alternating current systems that close together, although this configuration can also be used to interconnect two alternating current system which has the same frequency. 

 

Figure 5. Configuration of HVDC back-to-back

Monopolar
This configuration is shown in Figure 6. In this configuration two converter substations separated using a single channel direct current is far different from the configuration of back-to-back which only requires a single location. Channel direct current is used only has 1 pole voltage, can be positive or negative, so that the land is required as the return channel currents. 

 

Figure 6. Monopolar HVDC configuration

Bipolar
This configuration is shown in Figure 7. In this configuration two converter substations separated using two-channel alternating current of different voltage poles, one positive and one negative. Relative to the ground, the bipolar configuration are two different configurations monopolar voltage poles, so that each monopolar can be operated independently. In normal circumstances the current flowing through the ground will be zero due to two different poles of monopolar. The advantage of this configuration is one of the pole voltage can still operate when the voltage of the other poles are not operating due to interference or other reasons. Reliability of this configuration is better than monopolar configuration. 

 

Figure 7. HVDC bipolar configuration

MultiTerminal
This configuration is shown in Figure 8. This configuration is an extension of the bipolar configuration by placing a new converter substation in the middle of bipolar channels. The number of incoming channels in the middle of the bipolar configuration is not restricted to just one, but could be a lot depending on the requirement. 

 

Figure 8. HVDC configuration MultiTerminal 

Use of HVDC
The use of alternating current transmission system feedbacks are comprehensive indeed provide a more competitive price advantage because of the market and producers are equally well established, compared with HVDC transmission are still relatively few users. But the HVDC system will be considered more profitable than the ac system at some particular application.
Transmission distance
In large power transmission over long distances, HVDC provide an economically competitive alternative to the system of alternating current transmission Apart from the additional loss due to use of the converter compared to the alternating current system, the channel loss on HVDC transmission can be more small 30% -50% of the equivalent channel alternating current at the same distance. At a great distance, transmission systems require alternating current substation in the middle of the channel as well as reactive compensation. Compared with direct current transmission that does not require intermediate substations. A typical distance is considered the use of HVDC system will benefit economically than direct current transmission is about 500 km and above.

The use of cable
In cases where the use of cables needed, such as the transmission through the ocean, or underground transmission designed, use of HVDC is more economically beneficial than the use of alternating current cables. Another issue on the use of wires with an alternating current system is the reduction of cable capacity due to long distances due to high reactive power. This is because the characteristics of the cable that has a larger capacitance and inductance is less than the equivalent air conductor.
Interconnection frequency
Interconnection between the 2 areas of different frequencies can only be done by using the HVDC to ensure continuity of reliable operation. An example is the Shin-Shinano substation of 600 MW which connects western Japan that Japanese-frequency 60 Hz with the eastern part of the frequency of 50 Hz. Not only in cases like Shin-Shinano frequency are different between the two terminal operation, several other cases using HVDC frequency converter to connect between two different power company. In addition to managing the flow of power, it is intended to protect areas of the company one of the frequency fluctuations in neighboring companies in addition also to prevent the spread of interference resulting from neighboring companies.

Conclusion
This paper has described the technology, configuration, and application of direct current power transmission (HVDC). In certain applications HVDC transmission has advantages over alternating current transmission.