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Chen, G.; Liu, C.; Fan, C.; Han, X.; Shi, H.; Wang, G.; Ai, D. Ultra-Low-Frequency Oscillation. Encyclopedia. Available online: https://encyclopedia.pub/entry/2088 (accessed on 28 March 2024).
Chen G, Liu C, Fan C, Han X, Shi H, Wang G, et al. Ultra-Low-Frequency Oscillation. Encyclopedia. Available at: https://encyclopedia.pub/entry/2088. Accessed March 28, 2024.
Chen, Gang, Chang Liu, Chengwei Fan, Xiaoyan Han, Huabo Shi, Guanhong Wang, Dongping Ai. "Ultra-Low-Frequency Oscillation" Encyclopedia, https://encyclopedia.pub/entry/2088 (accessed March 28, 2024).
Chen, G., Liu, C., Fan, C., Han, X., Shi, H., Wang, G., & Ai, D. (2020, September 21). Ultra-Low-Frequency Oscillation. In Encyclopedia. https://encyclopedia.pub/entry/2088
Chen, Gang, et al. "Ultra-Low-Frequency Oscillation." Encyclopedia. Web. 21 September, 2020.
Ultra-Low-Frequency Oscillation
Edit

One of the prominent problems faced with hydro-dominant power system is ultra-low-frequency oscillation (ULFO) with a frequency deviation of 0.1 Hz lower, which is caused by the negative damping of hydro generators. ULFO might lead to an oscillation of all generators with the same oscillation frequency, restricting consumption of hydropower renewable energy.

Ultra-Low-Frequency Oscillation (ULFO)

1. Introduction

The ULFO phenomenon has been observed in power systems all over the world. In a Turkish power system, oscillations with frequency deviation of about 0.05 Hz were observed [1]. In 2008, the ULFO occurred in Colombia power grid when the proportion of hydropower was increased [2], and the oscillation lasts about 20 s. In 2012, there was an abnormal frequency fluctuation in JinSu DC Island test in China, with a period of about 14 s and an amplitude of ±0.26 Hz [3]. On 23 January 2015, there was an ULFO with an oscillation period of 13 s and an oscillation frequency of about 0.08 Hz, lasting for about 4 min. After the trip of the Tibetan Wooden unit, the frequency of the Tibetan middle power grid exceeded the low-frequency load-shedding limit [4]. In the 2016 asynchronous networking test of Yunnan power grid (YNPG), long-term and large-amplitude ULFO also occurred with an oscillation period of about 20 s and an amplitude of ±0.1 Hz lasting for nearly half an hour. It posed a major threat to the safe and stable operation of YNPG after asynchronization [5]. The asynchronous operation of the Southwest China Power Grid (SCPG) faced with an obvious risk of ULFO according to the simulation. The governor parameters optimization [6] and the DC frequency limit control (FLC) are adopted to suppress it. The asynchronous operation test of SCPG verifies the effectiveness of the strategy.

In fact, the ULFO is not a new problem in the power system. In the early stage of the power system development, there are cases of ULFO in the local power grid with isolated load or dominated by hydropower. P Kundur discovered this problem as early as the 1970s and 1980s and analyzed it. It was pointed out that an over-fast adjustment of the speed control system will produce negative damping within the ultra-low-frequency band, so it is necessary to carefully adjust the governor parameters [7][8]. However, with the development of thermal power and the synchronous interconnection of power grid, the ULFO has not been the focus of academic and industrial circles for a long time. No research or technical reserve has been built. When the ULFO occured in the asynchronous operation test of YNPG, the dispatchers had no clear means to quell the oscillation, so they could only try to find the effective control method by trial and error [9].

With the continuous occurrence of ULFO events, the mechanism, characteristics and control strategy of ULFO have become a research hotspot. In terms of mechanism and characteristics, the relevant research mainly analyzes the causes and main influencing factors from the aspects of eigenvalue analysis [10], parameter sensitivity [11][12] and damping torque coefficient [13][14]. In terms of control strategy, the research mainly focuses on the means of governor parameter optimization and adjustment [3][15][16], introduction of frequency limit control (FLC) of HVDC [17][18], and the means of optimizing PSS parameters or using governor additional control [19][20] to suppress ULFO, but it has not yet been applied.

For the hydropower grid that SCPG and YNPG represent, there is no clear standard or requirement for the level to which the ULFO damping needs to be raised when formulating the ULFO suppression strategy. As the ULFO period is distinctly longer than the low-frequency oscillation, the existing low-frequency oscillation dynamic damping control index can not be directly adopted as the guide of the ULFO control. The lack of guiding foundation for the oscillation analysis in the operation of a practical power system requires for a specific index, based on which the control strategy and the damping control are designed. Simulation analysis has shown that the range and strategy of governor parameter optimization and adjustment are significantly different according to different damping control indexs.

2. Mechanism and Control Strategy of ULFO in Hydropower Dominant Power Grid

2.1. The Inevitability of the ULFO in Hydropower Dominant Power Grid

The open-loop system model of a governor and a turbine can be expressed in Figure 1. The model of the most commonly used governor and turbine are detailed in the Appendix A.

Figure 1. Open-loop system for a governor and a turbine. Symbols: ω, rotating speed; Ggov, governor transfer function; Gh, turbine transfer function; Pm, mechanical power.

The open-loop transfer function of the governor and turbine system in Figure 1 can be expressed as:

Decomposing (3) in the Δδ–Δω coordinate system, mechanical power  deviations of a governor can be expressed in terms of its speed ω and angle δ deviations, known as damping and synchronizing torque, respectively [25].

where  (p.u./rad/sec) and  (p.u.) are damping and synchronizing coefficients, respectively.

With the open-loop transfer function of hydro turbine governor, the damping coefficient  at frequency f is obtained by substituting the Lagrange operator  into (6). The physical meaning of  is that it is the projection of mechanical torque on the  axis, indicating the magnitude of the damping torque. When  > 0, the turbine governing system provides positive damping to the system.

With the typical parameters, the governing system of the hydropower unit provides negative damping in the ultra-low-frequency range, as shown in Figure 2.

Figure 2. Damping coefficient of the hydraulic turbine regulating system with different Tw.

Figure 2 illustrates that as the water starting time Tw increases, the damping coefficient is of large negative values in the ultra-low-frequency band of [0–0.1] Hz. The water starting time Tw usually varies with different load, penstock and hydraulic head. However, regardless of Tw values, the damping coefficients are always negative, which means the damping of whole system increases with the primary frequency regulation system being taken out [21].

2.2. Analysis of Influencing Factors

The influence of key parameters such as time constant of water hammer effect and PID parameter of governor on ULFO has been analyzed in detail by root locus, parameter sensitivity and other methods [5][6][7][8][9]. The simulation text further shows that the damping and amplitude of the ULFO are closely related to the disturbance form, unit parameters and operation mode of the power grid. This section takes an actual power system of sending plant as an example to build a single power plant with load model, as shown in Figure 3. The generator G1–G4 adopts a typical 8-type governor model and parameters according to field tests. The N-1 fault at transmission line can excites ULFO. This section analyzes the generator inertia time constant, the governor frequency input limit in the governor model and the output limit of water turbine to further clarify their influence on amplitude of ULFO.

References

  1. Cebeci, M.E.; Karaağaç, U.; Tör, O.B. The Effects of Hydro Power Plants’ Governor Settings on the Stability of Turkish Power System Frequency. Available online: http://www.emo.org.tr/ekler/ad6635f33710af6_ek.pdf (accessed on 31 May 2016).
  2. Villegas, H.N. Electromechanical Oscillations in Hydro-Dominant Power Systems: An Application to the Colombian Power System; Iowa State University: Ames, IA, USA, 2011; p. 10116.
  3. He, J.B.; Zhang, J.Y.; Li, M.J.; Li, W.F. Frequency Domain Analysis and Control for Governor Stability Problem in Islanded HVDC Sending Systems. Proc. CSEE 2013, 33, 137–143.
  4. Gong, T.R.; Wang, G.H.; Li, T. Analysis and control on ultra low frequency oscillation at seeding end of UHVDC power system. In Proceedings of the 2014 International Conference on Power System Technology, Chengdu, China, 20–22 October 2014; pp. 832–837.
  5. Liu, C.X.; Zhang, J.F.; Chen, Y.P. Mechanism Analysis and Simulation on Ultra-Low Frequency Oscillation of Yunnan Power Grid in Asynchronous Interconnection Mode. South. Power Syst. Technol. 2016, 10, 29–34.
  6. Chen, G.; Tang, F.; Shi, H.B.; Yu, R.; Wang, G.H.; Ding, L.J.; Liu, B.S.; Lu, X.N. Optimization Strategy of Hydrogovernors for Eliminating Ultralow-Frequency Oscillations in Hydrodominant Power Systems. IEEE J. Emerg. Sel. Top. Power Electron. 2018, 6, 1086–1094.
  7. Kundur, P.; Lee, D.; Bayne, J.; Dandeno, P. Impact of turbine generator overspeed controls on unit performance under system disturbance conditions. IEEE Trans. Power App. Syst. 1985, PAS-104, 1262–1269.
  8. Dandeno, P.; Kundur, P.; Bayne, J. Hydraulic unit dynamic performance under normal and islanding conditions—Analysis and validation. IEEE Trans. Power App. Syst. 1978, PAS-97, 2134–2143.
  9. Chen, L.; Lu, X.M.; Chen, Y.P.; Min, Y.; Mo, W.K.; Liu, Y.S. Online Analysis and Emergency Control of Ultra-low-frequency Oscillations Using Transient Energy Flow. Autom. Electr. Power Syst. 2017, 41, 9–14.
  10. Wang, G.H.; Yu, Z.; Zhang, Y. Troubleshooting and Analysis of Ultra-Low Frequency Oscillation Mode in Power System. Power Syst. Technol. 2016, 40, 2324–2329.
  11. Wang, S.; Wu, X.Y.; Chen, G.; Xu, Y. Small-Signal Stability Analysis of Photovoltaic-Hydro Integrated Systems on Ultra-Low Frequency Oscillation. Energies 2020, 13, 1012.
  12. Yue, L.; Xue, A.C.; Li, Z.Q. Effects on Extra-Low Frequency Oscillation Caused by Hydro Generator Governor System, Model Suitability Analysis. Proc. CSEE 2019, 39, 227–235.
  13. Pico, H.V.; McCalley, J.D.; Angel, A.; Leon, R.; Castrillon, N.J. Analysis of very low frequency oscillations in hydro-dominant power systems using multi-unit modeling. IEEE Trans. Power Syst. 2012, 27, 1906–1915.
  14. Gao, J.R.; Li, G.J.; Wang, K.Y. Damping Torque and Synchronous Torque Analysis of Power System Ultra-Low Frequency Oscillation. Power Syst. Technol. 2020, 44, 1001–1007.
  15. Zhang, J.X.; Liu, C.X.; Chen, Y.P. Countermeasures and Experiments on Ultra-Low Frequency Oscillation of Yunnan Power Grid in Asynchronous Interconnection Mode. South. Power Syst. Technol. 2016, 10, 35–39.
  16. Huang, W.; Duan, R.H.; Jiang, C.X.; Zhou, J.H.; Gan, D.Q. Stability Analysis of Ultra-low Frequency Oscillation and Governor Parameter Optimization for Multi-machine System. Autom. Electr. Power Syst. 2018, 42, 185–191.
  17. Li, W.; Xiao, X.N.; Guo, Q. Ultra-low-frequency Oscillation and Countermeasures in Yunnan-Guangdong UHVDC Sending-end System in Islanded Operating Mode. Autom. Electr. Power Syst. 2018, 42, 161–166.
  18. Liu, C.Z.; Wang, Y.H.; Wang, B. Multi-HVDC Modulations Coordination Based on DC Sensitivity Sequence and System Transient Energy Function. Proc. CSEE 2018, 38, 6295–6304.
  19. Liu, S.B.; Wang, D.L.; Ma, N.N. Study on Characteristics and Suppressing Countermeasures of Ultra-low Frequency Oscillation Caused by Hydropower Units. Proc. CSEE 2019, 39, 5354–5362.
  20. Zhang, J.X.; Chen, G.; Zhou, J. Suppression of Very Low Frequency Oscillation in Asynchronous Yunnan Grid Based on Hydroelectric Governor Additional Damping Control. South. Power Syst. Technol. 2018, 12, 38–43.
  21. Shi, H.B.; Chen, G.; Ding, L.J. PID Parameter Optimization of Hydro Turbine Governor Considering the Primary Frequency Regulation Performance and Ultra-Low Frequency Oscillation Suppression. Power Syst. Technol. 2019, 43, 221–226.
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