Ultra-low frequency oscillations (ULFOs) have frequently been detected within small-scale, isolated networks that are predominantly powered by hydropower such as Zangzhong Grid, as well as the the sending systems of Yunguang and Jinsu HVDC transmission project in China. ULFOs have been frequently noted within asynchronous interconnected systems through DC interconnection. The imbalance between generator power and system load power is the underlying factor behind frequency oscillations found in systems where hydropower is a dominant component. The limitation was revealed that the systems who rely heavily on hydropower are susceptible to generating ULFOs when subjected to changes in local load or delivered power. Mitigating this issue based on conventional output feedback design approach would necessitate handling intricate bilinear matrix inequalities. As an alternative, the differential evolution algorithm can handle the problem, which can be used to design the high performance coordinating supplementary controller of multiple DC circuits. The approach was seen to significantly enhance system damping and effectively suppress ULFOs.
The multiple DC coordinated suppression method is a control technique used to dampen ultra-low frequency oscillations in power systems. These oscillations can be caused by a variety of factors, including changes in system load or generation, and can lead to instability or even blackouts. The basic idea behind this method is to introduce a series of controlled power injections into the system in a coordinated manner, with the goal of cancelling out the oscillations. The injections are typically made using power electronic devices, such as high-voltage direct current (HVDC) converters, and are controlled using feedback signals from sensors that measure the oscillations. By injecting DC power into the system at specific locations and at specific times, the method can effectively counteract the oscillations and stabilize the system. The key to its success lies in the careful coordination of the injections, which must be timed and located in such a way as to maximize their impact.
In a new study published in the peer-reviewed Sustainable Energy Technologies and Assessments, Dr. Wencheng Wu and colleagues at Southwest Jiaotong University, Southwest Electric Power Design Institute, and China Electric Power Research Institute, revealed the mechanism of ULFOs and proposed the method of restraining it by multi-DC coordinated control. They proposed that unsuitable governor parameters and the water hammer effect of hydroelectric plants are the primary reasons for the system’s ULFO. This creates a 90-degree phase difference between the turbine’s power regulation and the system frequency oscillations, with almost half of the turbine’s oscillation period in the anti-regulation state, causing hydroelectric unit output to increase with an increase in system frequency, further worsening the system frequency oscillations. Small proportionality constants of the governor, large integration constants, or water hammer effect time constants can easily cause ULFO. So, in actual systems, optimizing governor parameters alone may not solve the frequency stability problem due to the damping characteristics and regulating performance limitations of the speed control system, hydraulic system, turbine parameters, and primary frequency modulation capability of hydro units.
The authors proposed, a controller-design approach that utilizes a differential evolution algorithm. As a result, the use of the supplementary control function of HVDC is important to enhance damping that represents a promising measure for addressing the occurrence of ULFOs in asynchronous interconnected grids through HVDC. The authors presented a practical application of a unified framework for handling the coordinated control of multiple DC systems across three distinct structures, namely centralized, decentralized, and lead-lag link-based decentralized controllers in engineering practices.
Initially, the proposed methodology involves centralizing or decentralizing the structure of multiple DC coordinated controllers of a specific order, followed by the derivation of corresponding state space equations of the closed-loop system. Next, a random method is utilized to generate the initial controllers, and the minimum damping ratios of the closed-loop system are subsequently computed. Finally, the minimum damping ratios of the closed-loop system are optimized utilizing the differential evolution algorithm.
According to the authors, the designed controllers of all three structures exhibited a substantial improvement in the damping of the system and eliminated the risk of ultra-low frequency oscillations during system operation. This outcome highlights the efficacy of the proposed methodology in facilitating the coordinated control of multiple DC systems. Additionally, the approach’s practicality was demonstrated through its successful application to DC supplementary controllers based on the lead-lag module which is widely used in engineering.
In conclusion, Dr. Wencheng Wu and colleagues provided evidence of the proposed methodology’s potential to suppress ULFO that addressed the coordinated control of multiple DC systems. They addressed this challenge by initially generating a random solution and subsequently employing differential evolution algorithms to optimize the damping ratio of the closed-loop system. The results of calculations demonstrate that the designed controllers with all three structures significantly enhance the system damping and eliminate the risk of ULFO during system operation. Furthermore, the method’s practicality is strengthened by its applicability to DC supplementary controllers based on the lead-lag link. The practicality, adaptability, and scalability, as well as its ability to improve the damping of the system and eliminate ultra-low frequency oscillation risks, demonstrate the method’s significant contribution to various engineering practices. The multiple DC coordinated suppression method has been shown to be effective in a number of studies and has been successfully implemented in some power systems around the world. However, it is still a relatively new technique and further research is needed to fully understand its potential and limitations.
(Where Ak, Bk, Ck, Dk are the relative matrix in the state space equations of the Multi-DC coordinated controller, Ac, Bc, Cc, Dc are the relative matrix in the state space equations of the closed-loop system, ρ is the damping ratio of each mode of the closed-loop system matrix Ac, ρset is the specified damping ratio, Gdc is the controller space, and P is a positive definite matrix with the same dimension as Ac)
The paper proposed a controller-design approach based on a differential evolution (DE) algorithm to suppress ULFOs in power system. Initially, the proposed methodology involves centralizing or decentralizing the structure of multiple DC coordinated controllers of a specific order, followed by the derivation of corresponding state space equations of the closed-loop system. Next, a random method is utilized to generate the initial controllers, and the minimum damping ratios of the closed-loop system are subsequently computed. Finally, the minimum damping ratios of the closed-loop system are optimized utilizing the differential evolution algorithm.
Wu, W., Wang, X., & Xiao, X. (2022). Multiple DC coordinated suppression method for ultra-low frequency oscillations. Volume 53, Part A, 102301 Sustainable Energy Technologies and Assessments, 53, 102301.