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China has entered a new phase of power grid development, characterized by large-scale grids, high-capacity units, elevated 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's power infrastructure. UHV transmission is designed to meet the fundamental requirements of large-capacity, long-distance, efficient, low-loss, and cost-effective power delivery. It effectively addresses the limitations of the existing 500kV ultra-high voltage grid, such as low transmission capacity, poor economic efficiency, and instability. 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 all synchronous generators within a power system to maintain or regain synchronization after experiencing disturbances. When generators operate synchronously, the power they generate remains constant, and the phase angle differences between their electromotive force vectors, as well as the voltage phasors at each busbar, remain stable. If generators are not operating in sync, the active power output becomes unstable, leading to fluctuations in bus voltages and transmission line power. If these fluctuations do not subside after a disturbance, the system may lose stability, causing widespread disruptions that cannot be recovered by control devices, ultimately resulting in a loss of synchronization and system failure.
2. Stability of UHV Transmission Networks
The UHV grid is an integral part of the overall power network, and its stability analysis methods are similar to those used for traditional high-voltage grids. UHV transmission is designed to transport large amounts of power over long distances from generation centers to load hubs. The parameters of UHV transmission lines are determined by their transmission capacity and the characteristics of the sending and receiving systems. As a result, the transmission capacity of high-voltage lines is constrained by both voltage stability limits and power angle stability limits.
3. Stability Principles of the UHV Grid
Compared to conventional ultra-high voltage lines, UHV transmission can carry significantly more power. A sudden interruption in an UHV transmission line could lead to a major power deficit, which may severely impact the safe operation of the lower-level 500kV grid. To ensure the safe and stable operation of the entire power system—including the UHV grid—double-circuit UHV transmission lines are typically used to deliver power from generation centers to remote load areas.
During actual operations, the power delivered by UHV lines must meet the requirements of power angle stability. Based on the performance characteristics of UHV transmission, the following stability criteria must be met:
1) In the event of a serious fault or a three-phase short circuit near the transmission line, the relay protection and circuit breakers should operate correctly, isolating the faulty line and allowing the power system to maintain transient stability.
2) After the fault line is removed, the remaining line should be able to handle the original double-circuit power within the static stability limit, maintaining a sufficient margin and quickly restoring normal system operation to allow operators to readjust the power flow.
3) The remaining line should also keep the transmission power within the small interference voltage stability limit, ensuring a stable voltage margin.
4) Under large-scale operational conditions, if a large unit trips at the receiving end of the UHV transmission system, the system must have enough static stability margin and short-term active and reactive power margins to maintain voltage stability and keep the receiving end voltage within acceptable limits.
4. Economic Comparison of Transmission
When comparing the economics of EHV and UHV transmission, the primary factors considered are the costs associated with delivering 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 transmission line can carry more than four times the power of a 500kV conventional line. Moreover, the operation and maintenance costs of UHV transmission are significantly lower, with line losses being approximately one-sixth of those in 500kV lines. These advantages make UHV transmission far more cost-effective in terms of long-term operation.
5. Conclusion
Electricity is essential to China’s national economy and people’s livelihood. In the development of China’s power industry, the State Grid and other power transmission companies play a crucial role in safeguarding national energy security, optimizing resource allocation, and promoting social and economic development. By thoroughly analyzing the current state and future trends of the power industry, it is clear that accelerating the construction of a smart, strong national grid based on UHV technology is vital for achieving efficient resource distribution, enhancing energy utilization, and improving the overall efficiency of the power sector.
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