People's living standard is gradually improving with the continuous improvement of the social economy, and the demand for electricity is also gradually becoming higher, which brings huge pressure to our national power supply system. Although some new energy sources are being discovered, the uncontrollable nature of new energy generation will have a great impact on the power quality in the power grid, so it is necessary to further study the power quality problems caused by new energy access to ensure the stable operation and reliability of new energy generation systems. Therefore, the relevant departments and related staff should pay attention to this aspect of the work of research.
The impact of new energy generation on the grid
On the basis of new technologies, the systematic use, and development of renewable energy sources, such as nuclear energy, wind energy, solar energy, ocean energy, hydrogen energy, biomass energy, geothermal energy, and so on. Unconventional energy sources are often referred to as new energy sources other than traditional energy sources. Since the 21st century, the massive use of fossil energy has brought environmental pollution, energy depletion, and climate change to society, and the development and progress of human society have been seriously challenged. However, renewable energy and new energy sources have entered an era of rapid development and will definitely become the strategic direction of social development and the driving force of the world energy system in the future. One of the priorities of China's future development is to master new energy technologies.
Because of the late development of China's technical research on new energy generation and its limitations, China lacks stability in the use of new energy generation, mainly in terms of strong intermittency, volatility and randomness, and the control technology of new energy generation is very difficult. At this stage, the power grid's capacity is still insufficient, and it is difficult to accept the power generated by new energy on a large scale, mainly in the following aspects.
(1) The impact of fluctuating and intermittent power generation
For wind power generation, the output power of wind turbines is related to the size of wind speed. Since the energy of wind is proportional to the cube of wind speed, and the wind speed in nature is extremely unstable, it is difficult to control the wind size artificially, resulting in a great variation of the output power of wind turbines. For photovoltaic power generation, the output power of photovoltaic arrays is related to the intensity of light, so it is more influenced by weather and temperature, and as easy to fluctuate as wind power generation. A large amount of fluctuating wind power and PV power will be integrated into the traditional power grid, resulting in huge current shocks to the grid, causing frequency deviation, voltage fluctuation, and flicker problems. For example, when the voltage of a wind farm drops, if the wind turbine does not have the low-voltage ride-through capability, it is easy to have instantaneous failure. Therefore, in order to avoid these situations, first of all, the new energy equipment connected to the grid should have the required technical performance, and secondly, the grid should have sufficient peaking capacity and power acceptance capability. One of the most indispensable new energy generation system devices is the active power adjustment and dynamic reactive power adjustment function. In addition, wind farms and photovoltaic power stations are prone to reactive power loss, so the new energy generation system configuration application also needs to be equipped with a reactive power compensation function.
(2) Impact of harmonics
In new energy generation system, there are two main ways that may bring harmonics to the power grid. Take wind power generation system as an example, one is the power electronic device equipped with wind turbine itself will bring harmonic problems. For a constant-speed wind turbine directly connected to the grid, it is connected to the grid through power electronics during the soft-start phase, which generates certain harmonics. For variable speed wind turbines, which are connected to the grid system through rectifiers and inverters, a very serious harmonic problem can arise if the switching frequency of the power electronics happens to be in the range where harmonics are generated; another is that the shunt compensation capacitors of the wind turbine may resonate with the line reactance. In order to balance the reactive power in situ, the wind power system needs to be equipped with shunt compensation devices, but because the harmonics are constantly changing, coupled with the complexity of the grid, they may cause harmonic amplification and even system resonance under certain conditions.
Therefore, in wind farms, a single variable-speed or constant speed wind turbine should be connected centrally to avoid as much as possible, which will cause excessive harmonic voltage locally, and a mixed configuration of different types of wind turbines should be used. The second is that the power system should be properly configured for filtering devices to reduce the harmonic content of the grid, and to reasonably use dynamic or static reactive power compensation devices while suppressing harmonics.
(3) The impact of isolated network
When the grid loses voltage, the grid-connected wind power and photovoltaic power generation will continue to supply power to a certain part of the grid and form a connection with the local load to reach a new supply-demand balance, forming an isolated network. At this time, the voltage and frequency in the isolated network are not controlled by the grid and are not supported by the grid. When the load on the load side changes or the output power on the generation side changes, it will lead to an imbalance between supply and demand in the isolated network, making the voltage and frequency of the isolated network prone to fluctuations, which will seriously affect the safety of the equipment if the fluctuation range exceeds the allowed safety range. When supply is greater than demand, the voltage and frequency will exceed the allowable range, which may cause damage to the user's equipment. When the supply is less than the demand, it will overload the inverter and may burn up the inverter. Therefore, before new energy sources are connected to the grid, power dispatch must carry out strict gate-keeping and auditing. For wind power generation systems and photovoltaic power generation systems, having a power prediction function is one of the necessary entry conditions.
(4) Impact of non-uniform grid connection standards
The distribution of new energy is wide, but not evenly distributed, so it is difficult for our country to realize a relatively unified standard for grid connection of new energy generation for the time being. According to China's current technical development status, it can only unify the technical standards of those large grid-connected systems, but not the unified detection technology, and there are certain defects in the system detection, so we should continue to further improve the system certification.
At this stage, China lacks the professional knowledge to analyze the main reasons for destabilizing the power system and reducing the power quality of medium and large new energy sources on the grid, and even the factors affecting the grid dispatch and grid operation are lacking in a more reasonable determination, and the problems in the grid acceptance technology are still under continuous research and exploration, and no unified conclusion has really been formed for the access system. To carry out perfect testing of our current technical capabilities is still very difficult to complete. For example, the detection of inverters and controllers we still need to continue to improve. In addition, although we are constantly developing and exploring the testing technology of transmission and distribution equipment and bi-directional metering equipment, there is still a big gap between our technology and that of some western developed countries.
Measures to be taken in the future
From the perspective of the impact of new energy grid connection on the power quality of the power grid, China's grid connection technology in the future in the four aspects of DC transmission technology, grid connection mode, intelligent control technology and scheduling technology research and development needs to make greater efforts and strengthen, in the grid connection mode to play a pillar role of renewable energy, and to carry out effective operation and control in the technical aspects. With the increasingly widespread application of information technology continues to develop, technology continues to progress, human control will certainly be replaced by intelligent control someday in the future. In order to effectively enhance the technical level of safety and security of the power grid system, it is necessary to use intelligent control technology; in transmission technology, the research and development of DC transmission technology and large-scale energy storage technology should be continuously strengthened and improved, and then the reliability and stability of the power grid operation system through DC transmission technology, so that it will reduce the damage to the grid through large-scale energy storage technology. At the same time, the dispatching technology is widely used while working on the mutual dispatching work in the relatively new energy-poor areas or areas with more new energy, and the corresponding technical level will be improved.
Conclusion
Due to the characteristics of new energy and the low level of technology in China at this stage, the new energy grid connection at this stage has produced some negative effects on the application of electric energy in China's power grid, threatening the safe and stable operation of the power system to a larger extent. (Note: According to the experience of developed European countries, when photovoltaic power generation and wind power generation exceed 6% of the total grid power, it will affect the stable operation of the grid and must be installed with energy storage devices) Therefore, in the face of this situation, the relevant personnel and relevant government departments to improve the relevant technology and standards, to ensure that in the elimination of new energy grid in China's grid power to bring adverse effects at the same time, effectively Promote the development of China's power system, and eventually provide people with safe and stable power resources, to ensure that people's normal production and living needs, to improve and enhance the quality of life.
