Analysis of advantages and disadvantages of UHVDC
Advantages:
1. The transmission mode of ultra-high voltage direct current reduces the loss of the line, and the safety of the submarine transmission is higher:
UHVDC transmission systems are transmitted through direct current, resulting in lower losses during transmission. UHVDC operates at a voltage three times higher than ordinary HVAC. Due to the space charge phenomenon in the case of large current, the corona phenomenon in HVDC transmission lines is less than that in AC lines. This results in less DC wear on transmission cables and insulation. For submarine cable, due to the relatively high degree of difficulty in maintenance, it is extremely necessary to greatly reduce the maintenance of the cable. So UHVDC has increased the reliability of undersea cables, making possible the collection and transmission of massive offshore platforms, offshore wind farms, and tidal power. The development of Siemens and ABB’s miniaturization of AC to DC converters has made it possible for subsea cable DC transmission from offshore wind farms. Since most wind farms are far from shore and not connected to the mainland, stable undersea cables are needed.
2. The UHVDC system has a lower overall weight and can be transmitted underground rather than through high-voltage towers:
UHVDC cables are usually made up of several cables. High-voltage alternating current, which usually requires more cables, travels through what are known as high-voltage towers. For the same length, a DC cable can have a smaller footprint and does not need to be lighter. This allows UHVDC cables to be transmitted underground instead of needing to build high-voltage towers to occupy valuable surface area, which is beneficial to urban planning and surface environmental protection. And when the cable is turned underground, it can avoid many of the extreme weather conditions on the surface and is less affected by the weather than alternating current.
3. Lower long-distance transmission prices, smaller losses:
For power transmission, costs and losses are the core of revenue. UHVDC cable has an absolute advantage in the price per kilometer. Because fewer cables need to be used, the lighter weight not only directly reduces the cost of cable construction, but also the cost of construction and the overall cost of high-voltage tower/underground transmission. This advantage becomes more pronounced with distance — since UHVDC substations are more expensive than alternating current (more on the disadvantages). In addition, the main feature of UHVDC is lower loss. UHVDC circuits carry more power per unit area through higher voltage. The denser the current, the less corona there is, and the less electricity is wasted in transport. At the same time, because fewer cables need to be used, this reduces the active power loss generated by each cable. Taken together, long-distance losses from UHVDC can be reduced by 50 per cent compared with HVAC, making possible extra-long grids of more than 1, 000km.
4. Make it possible to use clean energy on a massive scale:
It is an advantage that occurs when the loss of long-distance transmission is reduced to an acceptable range. A typical alternating current loses more than 15% at 800-1000km. This is a huge loss of power transmission, so in a conventional high-voltage grid, fuel is preferred over electricity when distances are greater than 1,000 km. Transporting the fuel to the vicinity of the place of use, and then generating electricity, so that the place of use can use low energy consumption, if the cost of fuel transportation is less than 15% of the loss, transporting the fuel will be the optimal solution. However, as electricity generation is gradually replaced by sustainable methods, most clean electricity generation methods are limited by the site. This makes the previous generation – transport – use model of electricity needs to be changed. UHVDC is the game breaker. The lower long-distance transportation loss greatly increases the effective utilization range of clean energy, which is conducive to the promotion of clean energy and the competition with traditional energy.
5. Other advantages:
UHVDC also has some less obvious advantages because of the way it works. Unlike conventional alternating current, which requires both the transmission and the receiver to operate at the same time, dc only needs to operate on the transmission side. This more liberates the operation mode of the receiver for the existing operating system, without the need to adjust the docking in the case of different transmission voltages. This different working mode will reduce the requirement for the real-time staff of the power station.
Disadvantages:
1. Expensive converter equipment:
The basic structure and wiring of DC converter station is more complex, and it is more difficult to replace AC converter station, and the cost is also higher. A larger footprint would have exponentially higher costs in a city, not just in the converter itself. Until the conversion system is fully miniaturized, high conversion costs will limit the minimum distance of UHV DC power — this is an era of capital, and only lower overall construction prices can win orders. Therefore, as mentioned in the advantages, the advantages brought by the low cable cost at a relatively short distance are not obvious, while the high cost of the converter will become the development of short and medium distance UHV circuits.
2. It is difficult to find the problem and it is easy to cause the power failure of the whole network:
As the voltage and other parameters in the DC line are fixed and have no waveform, it is difficult to analyze the occurrence point of the problem through the fluctuation of the circuit. Therefore, if there is a problem with the line, the entire circuit needs to be cut off for inspection. Based on the state of ultra-high voltage direct current network, when a power source is cut off, the whole power grid will be affected, which is easy to cause large-scale power outages, which is not conducive to stable social development. The dual backup of the circuit reduces the advantages of low cost and light weight of the UHVDC cable.
3. Environmental influence – the effect of the grounding current:
UHVDC can run through the ground, but the nature of DC power allows occasional pole currents from the grounding wires to affect the ground. These currents can affect soil change and soil contamination and can affect small organisms and bacteria in plants or the soil.
4. Other disadvantages and conclusion:
UHVDC circuits are like toll highways, allowing large amounts of electricity to reach their destinations with much lower consumption. But because of the need to get on the highway, into and out of the tollbooths, the conversion process can be cumbersome. This gives alternating current an advantage for short distances of use in everyday life. At the same time AC better maintenance, has become a network of high voltage AC can be repaired in sections, will not affect the overall power supply. In general, in the short and medium distance, the traditional AC has absolute advantages. UHVDC has advantages over long distances. In special areas, especially the seabed and areas where overhead cables are not convenient at ground level, UHVDC may be the best and safest option.