China has entered a new phase of power grid development characterized by large-scale grids, high-capacity units, high voltage levels, and advanced automation. With the rapid growth of the economy, electricity demand is increasing significantly, prompting the gradual implementation of Ultra High Voltage (UHV) transmission in China’s power infrastructure. UHV transmission offers substantial advantages such as high capacity, long-distance delivery, efficiency, low losses, and cost-effectiveness. It effectively addresses the limitations of the existing 500kV ultra-high voltage grid, including low transmission capacity, poor economic efficiency, and instability. As a result, the construction of an UHV power grid has become an essential trend in China's power system development.
**1. Overview**
Power system stability refers to the ability of all synchronous generators within the grid to maintain or regain synchronized operation following disturbances. When generators operate in sync, the generated power remains stable, and the phase angle differences between electromotive force phasors, generator voltages, and busbar voltages remain constant. If synchronization is lost, the active power output from the generator fluctuates, leading to unstable bus voltages and power flows across the network. If these fluctuations do not diminish after a disturbance, the system may lose stability, making it impossible for control systems to restore synchronization and normal voltage levels.
**2. Stability of UHV Transmission Networks**
As part of the broader power grid, the stability analysis of UHV transmission networks shares similarities with that of traditional high-voltage systems. However, UHV lines are designed to transport large amounts of power over long distances, which makes their parameter characteristics highly dependent on the transmission capacity and the parameters of both the sending and receiving systems. The transmission capacity of high-voltage lines is constrained by voltage stability limits and power angle stability limits.
**3. Stability Principles of UHV Grids**
Compared to ultra-high voltage lines, UHV transmission can carry more power. A sudden interruption in an UHV line can cause significant power shortages, affecting the safe operation of the next-level 500kV grid. To ensure the overall stability of the power system, double-circuit UHV lines are often used to transmit power from generation centers to remote load areas.
During actual operations, UHV transmission must meet the requirements of power angle stability. Based on the performance of UHV transmission, the following stability criteria apply:
1) In the event of a serious fault or a three-phase short circuit near the line, the relay protection and circuit breakers should function properly, isolating the faulty section while maintaining transient stability.
2) After the fault is cleared, the remaining line should be able to maintain the original double-circuit power within the static stability limit, ensuring a sufficient margin for quick recovery and allowing operators to adjust the system flow accordingly.
3) The remaining line should also keep the power within the small interference voltage stability limit, leaving enough room for voltage stability.
4) Under large-scale operating conditions, if a large unit trips at the receiving end, the UHV line must still have enough static stability margin and short-term active and reactive power margins to maintain voltage within acceptable ranges.
**4. Economic Comparison of Transmission**
The economic comparison between EHV and UHV transmission is typically based on the costs required to deliver the same amount of power over the same distance at different voltage levels. This includes initial investment costs based on reliability indicators and life cycle costs. Analysis shows that a 1100kV UHV line can transmit more than four times the power of a 500kV conventional line. Additionally, UHV transmission has significantly lower operational and maintenance costs, with line losses being approximately one-sixth of those in 500kV lines. These factors make UHV transmission far more cost-effective in the long run.
**5. Conclusion**
Electricity plays a crucial role in China’s national economy and people’s livelihood. The State Grid and other power companies are vital in ensuring energy security, optimizing resource allocation, and supporting economic and social development. By analyzing the current state and future 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 efficient resource allocation and promoting sustainable energy use. This initiative holds great significance for improving the overall efficiency of China’s power sector.
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