China has entered a new phase of power grid development, characterized by large-scale grids, high-capacity units, high voltage levels, and advanced automation. As the economy continues to grow rapidly, so does the demand for electricity, prompting the gradual construction of Ultra High Voltage (UHV) transmission systems in China. UHV transmission is designed to meet the essential requirements of large-capacity, long-distance, efficient, low-loss, and cost-effective power delivery. It effectively addresses the limitations of the current 500kV ultra-high voltage grid, such as low transmission capacity, poor economic efficiency, and unstable operation. Therefore, the development of an UHV power grid has become an inevitable trend in China's energy sector.
**1. Overview**
Power system stability refers to the ability of synchronous generators within the grid to maintain or regain synchronized operation after experiencing disturbances. When generators operate synchronously, the power generated remains constant, and the phase angle differences between generator electromotive forces, generator voltages, and busbar voltages remain stable. If generators lose synchronization, the active power output fluctuates, causing instability in busbar voltages and transmission line power. If these fluctuations do not dampen over time, the system may eventually lose stability, leading to cascading failures that can disrupt the entire grid.
**2. Stability of UHV Transmission Networks**
The UHV grid is an integral part of the national power system, and its stability analysis methods are largely similar to those used for conventional high-voltage grids. UHV transmission involves transporting large amounts of power from generation centers to load hubs over long distances. The performance of UHV lines depends on their transmission capacity and the parameters of the sending and receiving systems. Consequently, the transmission capacity of high-voltage lines is constrained by both voltage stability and power angle stability limits.
**3. Stability Principles of UHV Grids**
Compared to traditional ultra-high voltage lines, UHV transmission can carry significantly more power. A sudden interruption in an UHV line can lead to a major power deficit, potentially compromising the safe operation of the next-level 500kV grid. To ensure the overall stability of the power system, including the UHV grid, double-circuit UHV transmission lines are typically employed to deliver power from generation centers to remote load areas.
During actual operation, the power delivered by UHV lines must meet the requirements of power angle stability. Based on the characteristics of UHV transmission, the following stability criteria are essential:
1) In the event of a severe fault or three-phase short circuit near a transmission line, relay protection and circuit breakers should function correctly, isolating the faulty line and maintaining transient stability in the power system.
2) After the fault line is disconnected, the remaining line should be able to maintain the original double-circuit transmission power within the static stability limit, ensuring a sufficient margin for quick recovery and allowing operators to reconfigure the system efficiently.
3) The remaining line should also keep the transmission power within the small disturbance voltage stability limit, with an adequate margin to prevent voltage collapse.
4) During large-scale operations, if a large unit trips at the receiving end of the UHV transmission, the system must still maintain a short-term static stability margin and sufficient active and reactive power margins to ensure the receiving-end voltage remains within acceptable limits.
**4. Economic Comparison of Transmission**
When comparing the economics of EHV and UHV transmission, it is common to evaluate the costs associated with delivering the same amount of power over the same distance at different voltage levels. This includes comparing initial investment costs based on reliability indicators and analyzing lifecycle costs. Studies show that a 1100kV UHV transmission line can carry more than four times the power of a 500kV conventional line. Additionally, the operational and maintenance costs of UHV lines are significantly lower, with power losses being about one-sixth of those in 500kV lines. These advantages make UHV transmission a more economically viable option in the long run.
**5. Conclusion**
Electricity plays a vital role in China’s national economy and people’s daily lives. State Grid and other power companies play a crucial role in safeguarding national energy security, optimizing resource allocation, and promoting sustainable economic growth. By thoroughly analyzing the development trends of China’s power industry, it becomes clear that accelerating the construction of a smart, strong national grid based on UHV technology is essential for achieving efficient resource distribution, enhancing energy utilization, and improving the overall efficiency of the power sector.
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