Hydro-dominated Power System Research Articles

Integrating Hydrological and Machine Learning Models for Enhanced Streamflow Forecasting via Bayesian Model Averaging in a Hydro-Dominant Power System

Streamflow forecasting plays a crucial role in the operational planning of hydro-dominant power systems, providing valuable insights into future water inflows to reservoirs and hydropower plants. It relies on complex mathematical models, which, despite their sophistication, face various uncertainties affecting their performance. These uncertainties can significantly influence both short-term and long-term operational planning in hydropower systems. To mitigate these effects, this study introduces a novel Bayesian model averaging (BMA) framework to improve the accuracy of streamflow forecasts in real hydro-dominant power systems. Designed to serve as an operational tool, the proposed framework incorporates predictive uncertainty into the forecasting process, enhancing the robustness and reliability of predictions. BMA statistically combines multiple models based on their posterior probability distributions, producing forecasts from the weighted averages of predictions. This approach updates weights periodically using recent historical data of forecasted and measured streamflows. Tested on inflows to 139 reservoirs and hydropower plants in Brazil, the proposed BMA framework proved to be more skillful than individual models, showing improvements in forecasting accuracy, especially in the South and Southeast regions of Brazil. This method offers a more reliable tool for streamflow prediction, enhancing decision making in hydropower system operations.

Suppression method of ultralow-frequency oscillation under ambient excitation in hydro-dominant power systems

Ultralow-frequency oscillations (ULFOs) have frequently occurred in hydro-dominant power systems in recent years. These phenomena seriously threaten the safe and stable operation of power systems. Therefore, measures that can effectively suppress ULFO need to be taken. Currently, the hydropower unit governor can enhance the dynamic stability and dampening oscillations of a system through adjusting the proportional–integral–derivative parameters. However, modifying these parameters can affect the primary frequency regulation performance, which compromises the assurance of grid frequency quality. This study proposes a switching type of hydro generator governor based on the local damping contribution rate to address the aforementioned problems. The approach incorporates the energy structure into the classical governor model, calculates the local damping contribution rate, and enables automatic switching of the hydro generator governor parameters based on positive and negative local damping contributions. This way allows for segmented control with different parameter settings, which enables the system to effectively consider the damping characteristics of ULFO while maintaining primary frequency regulation performance. It can also accurately and efficiently suppress the ULFO. The proposed method is verified through arithmetic analysis conducted on a 16-machine 5-zone system and an actual grid system. Simulation results demonstrate the effectiveness of the method.

Analysis of Ultra-Low Frequency Oscillation in Hydro-Dominant Power System and Suppression Strategy by GPSS

After asynchronous operation, the Southwest Power Grid in China has the risk of ultra-low frequency oscillation, which seriously threatens the stable operation of the power grid. Based on the hydropower single-unit with load system, the extend Philips-Heffron model is built considering PSS/GPSS (Governor Power System Stabilizer). Then, the damping torque analysis is carried out in both prime mover and excitation side. The negative damping in the prime mover side is prominent in the ultra-low frequency band and the effect of damping ratio increase by PSS and GPSS is compared. After that, the small signal analysis of the whole system is conducted, the participation factors of the ULFO mode shows the high participation and controllability by GPSS control. At last, the proper GPSS parameters configured by small signal analysis are equipped in the hydro unit of asynchronous four-machine two-area system which reveals an obvious ULFO suppression effect. Furthermore, the practical ULFO identification algorithm based on feature points extraction and analysis which crossing the PFC (Primary Frequency Control) dead zone (49.95 - 50.05 Hz) is proposed, the GPSS control cubicle which embedded the practical ULFO identification algorithm is developed and the semi-physical simulation is conducted to verify the real ULFO suppression effect by GPSS control.

Hydropower Aggregation by Spatial Decomposition—An SDDP Approach

The balance between detailed technical description, representation of uncertainty and computational complexity is central in long-term scheduling models applied to hydro-dominated power system. The aggregation of complex hydropower systems into equivalent energy representations (EER) is a commonly used technique to reduce dimensionality and computation time in scheduling models. This work presents a method for coordinating the EERs with their detailed hydropower system representation within a model based on stochastic dual dynamic programming (SDDP). SDDP is applied to an EER representation of the hydropower system, where feasibility cuts derived from optimization of the detailed hydropower are used to constrain the flexibility of the EERs. These cuts can be computed either before or during the execution of the SDDP algorithm and allow system details to be captured within the SDDP strategies without compromising the convergence rate and state-space dimensionality. Results in terms of computational performance and system operation are reported from a test system comprising realistic hydropower watercourses.

Hydrothermal Unit-Commitment Problem of a Large-Scale System with Representation of Forbidden Zones

As we move towards electrical networks with a growing presence of renewable generation, the representation of the electrical components becomes more important. In hydro-dominated power systems, modelling the forbidden zones of hydro plants becomes increasingly challenging as the number of plants increases. Such zones are ranges of generation that either should be avoided or are altogether unreachable. However, because representing the forbidden zones introduces a substantial computational burden, hydrothermal unit-commitment problems (HTUC) for large systems are usually formulated ignoring the forbidden zones. Nonetheless, this simplification may demand adjustments to the solution of the HTUC, because the generation of the hydro stations may fall in forbidden zones. In practice, the adjustments are usually performed based on the experience of system operators and, then, can be far from an optimal correction. In this paper, we study the impact of explicitly representing the hydro-generation forbidden zones in a large-scale system with more than 7000 buses, 10,000 lines, and 700 hydro units. Our findings show that the simplified model that is current used can deviate significantly from the model with forbidden zones, both in terms of the generation of hydro plants, as well as the generation of thermal plants and the system marginal costs.

Equivalent circuit modelling of large hydropower plants with complex tailrace system for ultra-low frequency oscillation analysis

Focused on the hydraulic factor-induced ultra-low frequency oscillation phenomenon occurring in a large hydropower plant, this paper investigates the precise modelling of hydraulic transient characteristics in its complex tailrace system and reveals the feasible oscillation suppression schemes from the source side. Combining the telegraphist's equations with the Saint-Venant equations, the mathematical expression of equivalent circuit model (ECM) for open channels is deduced, thus to expand the utilization of ECM to hydraulic transients description in pressurized pipe and open channel combination systems and to establish the high-order equivalent circuit topology of the overall tailrace system in a real hydropower plant. Based on the mathematical model, the feasibility and effectiveness of two different oscillation suppression approaches involving increasing the hydraulic losses on the branch tailrace channels or the main tailrace channel, respectively, are discussed in detail via numerical simulation. Furthermore, the superiority of the oscillation suppression scheme involving increasing hydraulic losses in branch channels in reducing the extra power generation loss is demonstrated. The application of equivalent circuit theory in complex hydraulic systems can contribute to the mechanism-related research on ultra-low frequency oscillations in hydro-dominant power systems.

Impact of Governor Non-Linearities Representation on Power System Ultra-Low Frequency Oscillations

Power system dynamic models are essential for the stability analysis of a perturbed electrical system. In recent years, hydro-dominant power systems have experienced the occurrence of ultra-low frequency oscillations (ULFO). While power system dynamic simulation stands as an essential tool for the research community to analyze and understand this phenomenon, such systems involve non-linearities in their dynamic model affecting the ULFO analysis’s validity. This present work investigates the impact of governor non-linearities representation on the validity of the ULFO analysis. Sinusoidal-input describing function (SIDF) is borrowed from control theory to appreciate existing governor non-linearities models’ capacity to respond accurately to ULFOs. Through rigorous test-cases, results prove the imperativeness of prior accurate measurement of those governor non-linearities before their numerical implementation to ensure the power system dynamic simulation model’s validity for ULFO analysis.

Generation Scheduling of Hydro-dominated Provincial Power Grid: Problems and Solutions

Hydro-dominated power grids such as Sichuan and Yunnan in China face many new problems in power generation scheduling that have not been encountered in the small- and medium -sized hydropower systems. This brings great challenges to the safety and efficiency of power system operations and large-scale absorption of clean energy such as hydropower. Sichuan Power Grid whose hydropower capacity ranks first in the country is taken as the background. With its hydropower status and actual operation requirements, this study analyzes several main problems of hydro-dominated power system operations, including curse of dimensionality of large-scale hydropower plants, generation safety control of giant hydropower units, hydropower transmission and absorption in flood season, and coordinated operations of inter-provincial hydropower plants on main streams. With the theory of hydropower optimization and practical experiences, this study also puts forward appropriate suggestions and strategies.

Improvement of regulation quality for hydro-dominated power system: quantifying oscillation characteristic and multi-objective optimization

With the construction of cascaded hydropower stations and high-voltage direct current transmission systems, hydro-dominated power systems are being developed all over the world. Regulation stability of sending terminals is an important guarantee for utilizing clean energy. However, the potential hydropower-induced frequency oscillations in these systems might limit the hydropower delivery quality. There is a need for comprehensive understanding and systematic tools to support decision-making processes in frequency regulation. This paper studies this issue from three aspects: modelling adaptability, quantitative analysis and coordinated optimization. Firstly, four models of hydropower governor system in two regulation modes are conducted to analyze the effect of model simplification on stability. The refined eighth-order model of hydraulic damping is derived. Secondly, the oscillation characteristics are quantified by applying a new method of stability margin. The operating boundary is proposed to guide the high-quality operation. Finally, a novel multi-objective optimization strategy is proposed to coordinate the stability-tracking conflict. The reliability and effectiveness are demonstrated in the isolated operation mode and the islanded operation mode. This work can be applied by power producers and grid operators to a better understanding of oscillation features, and supply insights and method on how to improve the regulation quality of hydro-dominated system.

Research on Damping Control Index of Ultra-Low-Frequency Oscillation in Hydro-Dominant Power Systems

In recent years, ultra-low-frequency oscillation (ULFO) is a prominent problem in power systems of dominant hydropower. In order to suppress ULFO, the most effective strategy is the governor parameters optimization to improve the system damping within the ultra-low-frequency band. However, no explicit standard has been established to stipulate the level of system damping for ULFO suppression. In this paper, the key factors that affect the amplitude and damping of ULFO are firstly analyzed. Then, the design principle of the damping control index of ULFO is put forward. According to it, a damping control index is proposed. After that, a case of ULFO in an actual power system proves the rationality of the proposed index. Finally, the application of the index in the Southwest China Power Grid illustrates its effectiveness and feasibility.

A control method of an additional damper for hydro turbine speed governor system in hydro-dominant power system

With the rapid development of the national inter-connected power grid, the fast automatic excitation regulators and fast turbine governor have been widely used. In the current system, some power plants adjust the response of turbine governor system to a very sensitive level to meet the primary frequency regulation assessment index, which, however, brings the negative damping intensively. Under the circumstances, the ultralow frequency oscillation (ULFO) phenomenon emerged in hydro-dominant power system. In order to solve the problem of ULFO, the hydro-turbine Governor Additional Damper (GAD) based on the phase compensation which adapt to the ULFO is developed. It is improved on the basis of traditional PSS and the design of the control system is relatively simple. As phase compensation based on the field testing and modeling of turbine governor, GAD can provide the power system with the positive damping, which contributes to improving the stability and security of the system. According to the impact of the mechanism of turbine governor on the dynamic stability of the power system and the research of modeling and parametric test, the proposed method not only supports positive damping, but also eliminates the negative damping of the speed governing system, which can significantly suppress the ultralow frequency oscillation and make it feasible to configure the GAD parameters accoraing to field test.

Techno-economic benefits of grid-scale energy storage in future energy systems

The increasing penetration of renewable energy sources in the power sector represents an important challenge for energy systems due to their elevated intermittency. Energy storage constitutes a key component for its ability to add flexibility to the system allowing further integration of these renewable sources. Therefore, the aim of this study is to analyse the impact of grid-scale energy storage in a hydro dominated power system with increasing renewable generation shares. The Colombian energy system is used as a case study. The model used in this work is built using the EnergyPLAN tool and validated against actual data. Successively, the techno-economic effects of large-scale energy storage technologies are assessed on three different future scenarios for the year 2030. The results evidence that increasing levels of storage could allow significant reductions in both the curtailed energy and the total fuel consumption of the country. The best-case scenario shows an estimated 67% reduction in the emission intensity of the power sector by 2030 compared to the baseline scenario.

Probabilistic Reliability Enhancement Strategies of Hydro Dominant Power Systems under Energy Uncertainty

Climatic hydrological changes cause considerable seasonal and yearly energy variation in hydro dominant electric power systems. Extreme weather events are becoming more frequent in recent years causing dramatic impacts on energy availability in such systems. A significant amount of energy is often wasted due to reservoir overflow during wet seasons. By contrast, the scarcity of water in dry seasons results in inadequate power generation to meet the system demand, and therefore degrades overall system reliability. The high risks associated with an extreme dry hydrological condition should not be ignored in long term system adequacy planning of hydro dominant utilities. This paper presents a probabilistic method to incorporate diurnal, seasonal and yearly energy management strategies in run-of-river and storage type hydropower plant planning and operation in order to minimize the adverse impact of energy uncertainty and maintain long-term system adequacy. The impacts of reservoir capacity and demand side management on water utilization and system reliability are investigated with case studies illustrated using the IEEE Reliability Test System modified to create a hydro dominant system. The achieved benefits of reliability enhancement strategies are analyzed and compared in this paper.

Risk-averse stochastic dual dynamic programming approach for the operation of a hydro-dominated power system in the presence of wind uncertainty

Operation planning models for hydro-dominated power systems usually use low temporal resolutions due to the excessive computational burden, thus ignoring short-term characteristics of such systems. As a result, in systems coupled with wind energy, such models may fail to accurately capture wind variability, and may not appropriately take into account potential consequences of uncertainty on the system operation. This paper addresses this drawback by (i) “controlling” the cost associated with the operation of a hydro-dominated power system equipped with wind power and batteries via a risk-measure and (ii) formulating the short-term load balance as probabilistic constraints in order to hedge against potential extreme wind power scenarios. The risk-averse scheme is embedded in the stochastic dual dynamic programming framework. Simulation results for a case study on a real industrial setting show that hedging the system against the short-term volatility of wind power contributes to mitigating the risk of excessive operations costs or load curtailments, and that the consideration of the decision maker risk profile contributes to decreasing the variability of the solutions. In addition, the results of the application also illustrate the potential of the scheme to assess the energy situation of a country or a region under the penetration of wind energy and batteries deployment.

Climate change impacts and adaptation strategies for a hydro-dominated power system via stochastic optimization

As outlined in the Paris Agreement on climate change, efforts to mitigate and adapt to climate change will require new modes of development of the energy sector including the transformation and expansion of power systems to low-carbon and more resilient designs. However, there is a need for more systematic tools to support decision-making processes in the context of climate change impacts and adaptation strategies for the energy and power sectors. For instance, quantitative approaches should be developed and implemented for the assessment of the impacts and hedging strategies associated with the uncertainties inherent to energy and power planning problems. This study addresses the development and implementation of an integrated model-based system analysis, which uses general circulation models, global sensitivity analysis, and stochastic optimization techniques, for the optimal design and planning of the Colombian power system in view of submitted climate pledges and climate change adaptation. It was found that during the 2015 to 2029 time frame, climate change will likely reduce the capacity factor of hydropower generation by 5.5–17.1%. Additionally, it was established that the independent effects of three key uncertain parameters, i.e., capacity factor of hydropower generation, gas prices, and emission reduction target, account for ∼96% of the variance in the total cost for the required expansion and operation of the power system. Furthermore, when uncertainty is taken into account, the optimal expansion strategy consists of rescheduling of investments in hydropower plants and investing more in carbon management technologies and renewable power plants to compensate for the uncertainty in hydropower generation, climate policy, and gas prices.

Optimization Strategy of Hydrogovernors for Eliminating Ultralow-Frequency Oscillations in Hydrodominant Power Systems

Southwest China power grid (SCPG) and central China power grid are planned to be unsynchronized by using back-to-back voltage source converter based high voltage direct current transmission between Chongqing and Hubei power grids. After asynchronous interconnection, the proportion of hydro in installed capacity is about 70%. In other words, the SCPG is a typical hydrodominant power system. Due to the negative damping effect of governors of hydrogenerators, there is a serious risk of ultralow-frequency oscillation (ULFO) in the range of 0.05 Hz. In this paper, optimization strategies of governor for hydropower unit are presented to eliminate ULFOs. First, the control mode is selected based on comparing damping characteristic of different control modes of governor. Then, a relevant coefficient index is proposed to select sensitive generators of ULFO. Afterward, PID parameters of governor are optimized based on a new performance criteria considering the damping characteristic of governor. Finally, optimization strategies are presented and applied to SCPG. Simulation results show effectiveness and practicability of the proposed measures, which have been adopted and will be implemented before the asynchronous interconnection.

Hydraulic damping mechanism of low frequency oscillations in power systems: Quantitative analysis using a nonlinear model of hydropower plants

As power systems grow reliant on an increasing mix of intermittent renewables, hydropower units are being called upon to provide more aggressive power-frequency control. This dynamic is enlarging the significance of interaction between hydraulic and electrical subsystems in hydropower plants (HPPs). The influence from HPPs on power system stability is of great importance, especially for hydro-dominant power systems like the Nordic power system and the China Southern Power Grid. This paper aims to quantify and reveal the influencing mechanism of the hydraulic damping of low frequency oscillations in power systems. An equivalent hydraulic turbine damping coefficient is introduced, and a nonlinear HPP model that combines electrical subsystems with a refined hydraulic-mechanical subsystem is established and verified. A novel quantifying methodology is proposed through simulations by two different models based on case studies on a Swedish HPP. Then, the quantification results of the damping coefficient are presented and the influencing mechanism behind is revealed, by studying three representative factors from the hydraulic-mechanical system: the delay in turbine governor systems, governor parameter and penstock length. Observations and discussions of on-site measurements are included to support the analysis. The results show that the damping effect from hydraulic turbines can be considerable. Based on the limited cases in the HPP, the damping coefficient can vary from + 3.0 to −2.3, while previously the contribution has been unclear and normally assumed to be positive. The phase shift in the mechanical power response with respect to the rotational speed deviation is an important reason for the different damping performance. Furthermore, the effect and significance of implementing the damping coefficient on cases with power system stabilizer (PSS) are demonstrated.

Bayesian and Robust Nash Equilibria in Hydrodominated Systems Under Uncertainty

In this paper, we model strategic interaction of multiple producers in hydrodominated power systems under uncertainty as an equilibrium problem with equilibrium constraints (EPEC), reformulated as a stochastic mixed-integer linear program with disjunctive constraints. We model strategic hydropower producers who can affect the market price by submitting strategic bids in quantity, price, and ramp rate. The bids are submitted to the system operator who minimizes the dispatch cost. We take into account the hydrospecific constraints and uncertainty in the system. Solving the problem results in finding Nash equilibria. We discuss two types of Nash equilibria under uncertainty: Bayesian and robust Nash equilibria. Large EPEC instances can be solved using a decomposition method—Modified Benders Decomposition Approach. This method eliminates the problem of tuning the disjunctive parameter and reduces the memory requirements, resulting in improved computation time.

Comparison of 2L-VSC and MMC-based HVDC Converters: Grid Frequency Support Considering Reduced Wind Power Plants Models

Several technologies for wind power plants (WPPs) have been developed during the last years, increasing the number of such plants connected to the electrical power systems. Therefore, studies for the integration of these technologies into the grid are required, including their contribution to the power system stability. This paper compares two technologies of high-voltage direct-current (HVDC) transmission system during frequency support provided by WPPs. Considering the grid side converter, modular multilevel converters (MMCs) and two-level voltage-source converters are compared in terms of dynamic behavior, and disturbance rejection capability. A frequency domain analysis for frequency support studies using MMC-based HVDC is proposed. For study cases, a large WPP connected in a hydro-dominated power system, based on IEEE 12-bus system is simulated using Matlab/Simulink tool. In frequency and time domain analysis, similar dynamic performances were found out in normal operation conditions, but during frequency support conditions, a slight improved stability performance was observed in MMC topology. This fact is important in HVDC systems, since a good DC-link regulation, with more disturbance rejection capability, can improve the system reliability.

Climate change impacts in the energy supply of the Brazilian hydro-dominant power system

Over the past few years, there has been a growing global consensus related to the importance of renewable energy to minimize the emission of greenhouse gases. The solution is an increase in the number of renewable power plants but unfortunately, this leads to a high dependence on climate variables which are already affected by climate change. Brazil is one of the largest producers of electricity by renewables through its hydro-dominant power generation system. However, hydro-generation depends on water inflows that are directly affected by climate change that consequently affect the electricity production. Therefore, these changes need to be considered in the operation and planning of a hydro-dominant power system. In this paper, we present the effects of different climate scenarios in the water inflows produced by the regional Eta climate model. Normally, studies use an optimization model to make decisions in case of a hydro-thermal scheduling problem and use the assured energy to evaluate the hydro-production. In this analysis, water inflows used in the optimization process consider different trends according to its associated climate scenario. Our paper shows that climate change may drastically impact the system assured energy and consequently, the system's capability to supply load.