The relationship between smart grid and direct AC UHV is briefly analyzed

UHV is divided into DC UHV and AC UHV, the technical characteristics of the two and their role in the power grid are very different, should not be confused
.
DC UHV is used for long-distance, large-capacity power transmission and has obvious economic and technical advantages
.
China's hydropower, wind power and solar energy resources are concentrated in the western region, there are long-distance transmission requirements, so DC UHV as an advanced transmission technology is an indispensable means
to build a smart grid and realize the greening of China's power grid.
This conclusion is clear and has long been agreed
upon.

What is controversial is AC UHV
.
The voltage level of the power grid continues to increase from low to high with the expansion of the scale of the power grid, which is indeed a common path
taken by the development of power grids around the world for more than 100 years.
Looking back at history, it is not difficult to see that the technical route of the traditional power grid is to solve the problems in the development of the power grid by improving the voltage level, so as to meet the demand for electricity in the economy and society
.
But the development of things is not always a straight line, there are laws that only play a role in one stage
.
The route of developing the power grid by increasing the voltage level is formed under the condition that energy development tends to be more and more concentrated based on fossil energy, which can be said to be the product
of the second industrial revolution.
When the new energy revolution, the third industrial revolution, rose around the world, the development of the power grid has actually departed from this route and embarked on a new, that is, the road of building a smart grid
.

American futurist Jeremy & dot; In his book The Third Industrial Revolution, Rifkin presents the insights
of experts from the American Electric Power Research Institute on this issue.
U.
S.
experts believe that the development of distributed energy production in smart grids may take a very similar route to the development of the computer industry: mainframes give way to miniaturized, geographically dispersed desktops and laptops, which are interconnected and fully integrated into a highly resilient network
。 In the power industry, centralized power plants will still play an important role, but there is a greater need for small, clean, decentralized power plants, and energy storage technologies will support their development; In order to control and deal with the massive information and energy circulation problems caused by the complex interconnected system formed by this, advanced electronic control technology is absolutely indispensable
.

It can be said that relying on advanced information, control, energy storage, superconductivity and other new technologies, focusing on the development of clean and renewable energy in a small-scale decentralized way, focusing on the use of new management concepts and technical means to improve energy utilization and achieve sustainable energy development, this is the development route
of modern power grid (that is, smart grid) shown by the third industrial revolution 。 Since the distribution of renewable energy is relatively uniform, the result of decentralized development will make the layout of power sources more reasonable; Small power supplies and microgrids can be directly connected to the distribution network to supply power to users at lower voltages; In particular, the continuous progress of superconducting transmission technology and its increasing application in engineering show that the voltage level of modern power grids will not only not be increased on the basis of ultra-high voltage, but even gradually decreased
.

The voltage level of the power grid developed from low to high during the second industrial revolution, and no longer rose after entering the third industrial revolution, and showed a trend of development from high to low, from the perspective of the entire development process, it follows the law of "negation of negation" of the development of things
.
The idea that the power grid will always develop according to the law of "larger and higher and higher voltage levels" is a metaphysical view
that lacks dialectical thinking.

Modern power grids no longer develop to AC UHV power grids, but also because AC UHV itself has the following fatal shortcomings:

The environmental cost is high
.
Since the construction of AC UHV is superimposed on the existing 500 kV power grid and then a 1000 kV power grid, after the power to the receiving end, it must first be reduced to 500 kV and then distributed to the 220 kV power grid, so it not only can not replace the 500 kV substation capacity and the corresponding 500 kV transmission line, but also increase the voltage level of China's power grid from the existing 5 layers to 6 layers (that is, become 0.
4/10/110/220/500/1000 kV), Become the power grid
with the most voltage levels in the world.
Moreover, the noise of AC UHV and electromagnetic pollution to the environment are far stronger than UHV, and its power transmission and transformation equipment are all behemoths, and the floor space required is also larger (the tower double-loop AC UHV line tower is about 80 meters high, and the width of the corridor is about 70 meters; An AC UHV substation covers an area of 9-20 hectares).

Therefore, the construction of AC UHV power grids means that while the complexity of the grid increases, it also has to pay an unbearable environmental cost for adding an additional layer of power grid (especially in the densely populated "Sanhua" area).

Poor
return on investment.
From a technical point of view, long-distance transmission is not as good as DC UHV, and short-distance transmission is not as good as 500 kV voltage, but the investment is very large, which is a very low "cost-effective" solution
.
China Southern Power Grid compared the use of AC UHV scheme with the DC UHV plus AC 500 kV power grid scheme, and the result was that the investment of the former was 60-100 billion yuan more than that of the latter, about 1.
3-1.
5 times higher [5].

The plan of building an AC UHV synchronous power grid in the "Sanhua" area will cost at least 300 billion yuan more investment
than the asynchronous interconnection scheme that only builds a DC UHV plus AC 500 kV power grid.
In addition to driving up electricity prices, it also means consuming more steel, aluminium and cement, and the production of these energy-intensive products will undoubtedly increase environmental pollution
.

Two-stage boost, which increases energy loss and reduces transmission capacity
.
AC UHV transmission, power supply generally has to be boosted to a 500 kV substation, and then boosted to the AC UHV grid
.
Since there is basically no direct connection to the generator unit equipped with it, the AC UHV grid is actually a passive "empty shelf", which has never occurred in the development of other voltage levels
.
The highest level of the power grid is very important to ensure the stable operation and power balance of the entire power grid, and the generator capacity directly connected to it will not be less than 35%
after it is formed.
For example, by the end of 2012, the capacity of China Southern Power Grid connected to the 500 kV grid was 36.
6% in Guangdong, 43.
7% in Guangxi, 58.
8% in Yunnan, and 50.
6% in Guizhou.
At the end of 2013, Sichuan Power Grid had a capacity of about 47%
connected to the 500 kV grid.
The generator is connected to the power grid through two-stage boosting, which is a taboo practice in the planning and design of the power system, because this will inevitably increase the impedance of the system, which not only leads to an increase in power loss, but also limits the improvement
of transmission capacity.
The emergence of this phenomenon shows from one side that along the traditional route of increasing the voltage level, when it comes to UHV, this road is no longer possible
.

Due to the above problems, coupled with the advancement of DC transmission technology to achieve large-capacity long-distance transmission of electricity, developed countries such as Europe, the United States, Russia, Japan and other developed countries have since abandoned AC UHV after the research and experimental boom of AC UHV
technology in the seventies and eighties of the last century.
Although the European interconnected power grid is very large, the maximum voltage is only 400 kV (100 kV lower than the 500 kV commonly used in China's power grid), and it will certainly not develop AC UHV
in the future.

But even so, the proportion of renewable energy in their grid is much higher than in China
.
Germany plans to share 35% renewable energy by 2020, 50% by 2030 and 80%
by 2050.
It shows that the realization of grid greening is not related
to the level of grid voltage.
The main ways to increase the proportion of renewable energy in the power grid: First, vigorously develop various energy storage technologies and continuously expand the scale of energy storage in the power grid; The second is to develop, enrich and optimize the means of peak shaving in the power grid, including the reserve and mobilization of demand response resources; The third is to maximize the complementary operation of various renewable energy sources
.

As for the problem of controlling smog, the construction of AC UHV power grids consumes more steel, aluminum and cement that could not be consumed, which is undoubtedly counterproductive
.
The "electricity from afar" strategy mainly relies on DC UHV, but this strategy alone is not enough, but also to implement the "electricity from the side" strategy, that is, to develop all the clean energy
that can be developed and utilized "around" in a decentralized way.
Only by combining the two can we achieve the purpose of
effective smog control.

The development direction of the world's power grid shown by the third industrial revolution determines that the modern power grid can no longer follow the old road of improving the voltage level of the power grid, and the higher the voltage level, the stronger the power grid
.
In fact, various innovative and developing renewable energy development and utilization technologies, distributed power microgrid technologies, energy storage technologies, superconducting transmission technologies, multi-terminal DC technologies, and other new energy technologies including shale gas development and nuclear fusion are all denials
of AC UHV.

History has proved and will continue to prove that AC UHV is a product of the traditional technical route, and it bears the imprint
of the second industrial revolution that cannot be erased.
Although it has reached the technological pinnacle of the traditional power grid, it is only
a more eye-catching "new landmark" for the traditional power grid.
Today people can "create the conditions" for it to continue to play an "important role" in the third industrial revolution, or they can "follow the trend" and withdraw it from the stage of history
.
"The wise hear the silent, the wise see the formless"
.
What is the road to innovation in China's power grid? Chinese should have the wisdom to make the right choice
.