Variable Speed Pumped Storage Plant Research Articles

Reduction of Pressure Pulsation for Pump-Turbine by Variable Speed Operation

Abstract Variable speed operation has emerged as a key direction in the development of pumped storage technology. Maintaining pressure pulsation within the control range is particularly critical for ensuring operational safety of variable-speed pumped storage plants (VSPSPs). However, there is limited research on the relationship between pressure pulsation for pump-turbine and variable speed operation. This paper presents amplitude distribution diagrams of pressure pulsation, obtained from processing model test results of a real VSPSP. Different conditions of variable speed operation are simulated by a numerical model to analyze the influence of operating trajectory on pressure pulsation, and the intensity of pressure pulsation is quantitatively evaluated. According to the results, when the initial speed or speed command increases, the trajectory passes through more regions with high-amplitude pressure pulsation and gradually moves toward the S-shaped region, leading to pressure oscillations. When speed command reduces, maximum pressure pulsation at the volute inlet and in the draft tube can be reduced by 82.18% and 63.24% at most, and the evaluation score can be increased by 28.77%. The findings of this study can offer theoretical guidance for operating VSPSPs.

Performance Assessment for Primary Frequency Regulation of Variable-Speed Pumped Storage Plant in Isolated Power Systems

Primary frequency regulation (PFR) is a crucial operating condition for PSPs to realise frequency modulation, and the effectiveness of PFR is significant to the stability of power system frequency. Several challenges and risks have been presented in the PFR process for conventional PSPs, especially for those which run in the isolated grid, such as water inertia, negative damping of speed governor and ultra-low frequency oscillation (ULFO). Variable-speed pumped storage plants (VSPSPs) have the potential to overcome the negative impacts on regulation performance caused by hydraulic factors, due to the advantages of rapid power regulation and independent active power control from turbine output. In this paper, the primary task is to conduct a comprehensive assessment for PFR performance of VSPSPs in isolated power systems. Initially, the hydraulic–mechanical–electrical numerical models are established. Secondly, the rotational speed stability of the pump-turbine is quantified and the advantages of VSUs in suppressing ULFO are assessed. Relevant results reveal that the performance of VSUs is better than that of FSUs in the regulation process. Finally, assessments of frequency regulation performance under various scenarios are conducted with four indicators (standard deviation of power differences, power regulation time delay, settling time and overshoot).

Active power fluctuations and control in hydraulic disturbance of variable speed pumped storage plants

Variable-speed pumped-storage unit (VSPSU) is state-of-the-art technology and has appeared as a new orientation of the world pumped-storage industry in recent years, but the dynamic performance of VSPSU under hydraulic disturbance has yet to be developed. The hydraulic disturbance means that normally operating pumped-storage units are inevitably affected by sudden operation changes of other units in the same waterway system. It is an important working condition in transient simulations of pumped-storage plants. In this study, a nonlinear model of a VSPSP containing multiple units is established based in MATLAB/Simulink, integrating hydraulic, mechanical, and electrical subsystems. The model is validated through comparison with on-site experiments. The fast power control strategy is adopted, enabling the unit to follow the power command rapidly and stably. The simulations show that VSPSU outperforms fixed-speed pumped-storage unit (FSPSU) in mitigating active power fluctuations under hydraulic disturbance. But VSPSU still faces active power fluctuations. This phenomenon is induced by the effect of the speed pulling-back control, which is used to keep the unit speed within the allowable speed variation range. Factors that influence the active power fluctuations are analyzed, and a useful countermeasure is proposed to solve active power fluctuations. The findings in this paper could provide understanding and guidance for the practical operation of VSPSUs.

Variation law of pressure pulsation during variable speed operation of pumped storage units

Variable speed operation technology has become an emerging development direction in hydropower industry. Variable-speed pumped storage plants (VSPSPs) constantly undergo various transient processes, and ensuring the amplitude of pressure pulsation within the control range is critical to the safe operation of variable-speed pumped storage units (VSPSUs). However, its extreme value is usually determined by rule of thumb in design stage, and few researches focus on the relationship between pressure pulsation and variable speed operation of VSPSUs. In this paper, characteristic curves of pressure pulsation are plotted by processing model test results with Gridfit function, taking a real VSPSP as an example. The total pressure during transient processes is predicted by simulation of a numerical model. Load rejection conditions (change initial speed) and large load regulation conditions (change speed command and power command) are simulated, and the influence of operating trajectory on pressure pulsation during variable speed operation is analyzed. The results show that with the increase of initial speed or speed command, the value of pressure pulsation increases; the trajectory passes through more high-amplitude pressure pulsation region and gradually shifts to the S-shaped region, causing the pressure oscillation. This study could provide theoretical reference for the operation of VSPSUs.

Dynamic analysis of variable-speed pumped storage plants for mitigating effects of excess wind power generation

The integration of hydropower and variable energy sources emerges as a functional method for handling power variability. Due to the advantage of adjustable power input of the pumped storage operation, variable-speed pumped storage plants (VSPSPs) have been proposed as a potential alternative to traditional fixed-speed pumped storage plants (FSPSPs) for dealing with the variable storage demand. The performance evaluation of VSPSPs in coping with changes in loads and variations in renewable energy sources during the energy storage process is crucial to ensure adequate flexibility and reliability of the grid. In this paper, the regulation performance assessment of VSPSP in mitigating wind power variations is presented based on IEEE 14-bus test system. The goal is to draw up the regulation reliability of the VSPSP under pumping mode for balancing the dynamic characteristics of wind power. Specifically, a numerical model of VSPSPs with doubly-fed induction machines is described and developed which is then validated by the measured data in the form of a case study. Moreover, by considering ten wind scenarios having differences in frequency, gradient, guts intensity, and standard deviation; simulations of VSPSP performing power regulation in the pump mode are conducted and the dynamic regulation effects are assessed. Finally, the power regulation quality comparison is provided between the power generation and storage modes to validate the regulation reliability of the VSPSP under the pumped storage operation. The regulation time delay (RTD) ratios of VSUs to FSUs are found in the range of 4.67–9.16%, demonstrating the rapidity of VSPSPs in the power regulation.

A coordination controller in variable speed pumped storage plant

With the integration of increased variable renewable energy generation, much attention has been devoted on the development of pumped storage. The variable speed pumped storage technology has become a research focus due to its wide operating range, high production efficiency and strong power regulation ability. Moreover, the coordination controller is the key component of variable speed pumped storage plant (VSPSP), it can coordinate the joint operation of governor and converter. The coordinated controller is studied in this work. Aiming at the coordinated control between governor and converter, the novel adaptive strategy of maximum efficiency point tracking is proposed. First, the optimal efficiency curves are described based on model experiment; and then the optimal operating curves of variable speed unit in the operation region is presented based on the BP neural networks; the convergence analysis of mean square error and regression analysis are applied to verify the reliability of results. Aiming at the application of adaptive strategy in the coordination controller, and ensuring the real-time performance, a new effective digital interpolation method is put forward, using programmable logic controller. The signal flows of the control system in variable speed unit are clarified. The proposed adaptive strategy could be easily applied in engineering, providing a technical means for the efficient and flexible operation of VSPSP.

Nonlinear simulation of speed variation of variable-speed unit under large disturbance by Simulink

The variable-speed pumped storage plant (VSPSP) has become an innovative technology and new orientation in recent years. Investigation of dynamic processes in operating and control of VSPSP is of great importance for safety, stability and efficiency. In this paper, a nonlinear integrated model for dynamic processes of VSPSP with doubly fed induction machines (DFIMs) is built based on Simulink. From the comparison between simulation results and on-site measurement data of a fixed-speed pumped storage plant (FSPSP), the model under load rejection is validated. Meanwhile, maximum rotational speed during transient processes is a key indicator for the safety of pumped storage units. Hence, the transient process and performance of VSPSP under load rejection are simulated and analyzed by using the model, and the speed variation of the variable-speed unit is specifically investigated. The speed rising value and relative speed rising value will decrease when the initial value increases because of the momentum conversion mechanism and the torque characteristic of pump-turbine. The result demonstrates that, if the initial speed increases in the operating range of variable-speed unit under load rejection, the maximum speed will not rise significantly, and there is a certain safety margin from the maximum allowable speed.

Variable Speed Pump Storage for the Mitigation of SSR in Power System with Wind Generation

Subsynchronous resonance (SSR) can bring significant negative effects on the grid system like stability, security, and even generator shaft damage. This work presents a new method of using a doubly-fed induction machine (DFIM) based system of variable speed pumped storage plant (VSPS) to mitigate SSR in the power system with high penetration of wind generation. The mitigation is reached based on the principle of balanced shaft mechanical input power and electromagnetic power. The fundamental concepts and phenomena of SSR with wind farm are discussed in brief in this work. For the sake of analysis and verification of performance of the proposed system, the network model including wind farm, VSPS, HVDC, and synchronous machine is built based on Ningxia grid for the SSR study using PSCAD platform. The result shows that VSPS brings significant effect to dampen the SSR in the power system with high penetration of wind generation.

Boost multi‐level NPC‐fed VS large rated asynchronous pumped storage hydro‐generating unit

The authors intent using a boost multi-level converter for the doubly fed induction machine (DFIM) used in variable-speed pumped storage plants (VS-PSP). Voltage-source converters connected on the rotor side of the machine control the active and reactive powers of the unit. The proposed boost neutral point clamped (NPC) converter topology provides a voltage output two times larger than a conventional three-level NPC (3L-NPC) with similar DC-link voltage and equal number of switches. Hence, it increases the speed variation of the unit, which improves the efficiency during generation and pumping modes. Moreover, it reduces the starting period of the unit at the pumping mode, which is significant during mode changeover time. Furthermore, it reduces switching and conduction losses in the converter. It also reduces the total harmonic distortion in the output current, as it provides five output voltage levels. These improvements show that the boost NPC converter topology is better among VS-PSP project authorities. In addition, the reliability of the proposed topology is investigated, where converter redundancy is a challenging issue in asynchronous VS-PSP units. The proposed boost NPC was compared with the conventional 3L-NPC system by examining a 250 MW DFIM hydro-generating unit.

Evaluating fast power response of variable speed pumped storage plants to balance wind power variations

Abstract Currently, the balancing act for variable renewable energy (VRE) is crucial for power systems all around the world, and how to handle the variability of the power generation from VRE on different time scales is a key problem. The application of variable-speed pumped storage plants (VSPSPs) has become a novel solution and new orientation. How to quantitatively evaluate the advantages of VSPSPs is an important and practical topic. In this work, the focus is on the regulation rapidity that is a key characteristic of VSPSPs in terms of the contribution to renewable energy systems. First, a model of VSPSPs integrating hydraulic-mechanical-electrical subsystems with doubly fed induction machine is built based on MATLAB/Simulink, and a Chinese VSPSP in feasibility study stage is adopted as a study case. Then, performance of regular fixed speed pumped storage plants (FSPSPs) and VSPSPs on regulating real wind power variations is evaluated and compared, by applying numerical simulations of hydro-wind hybrid power variation based on small time scale. The fast power response as a key advantage of VSPSPs is quantitatively demonstrated.

Advantage of variable-speed pumped storage plants for mitigating wind power variations: Integrated modelling and performance assessment

Developing the joint operation of hydro and variable renewable energy has emerged as a research trend, for handling the power variability. In recent years, variable-speed pumped storage plants (VSPSPs) have been proposed as an alternative to fixed-speed pumped storage plants, but VSPSPs require a higher investment cost for equipment. Hence, evaluating the advantages and demonstrating the value of VSPSPs are meaningful topics that have seldom been studied by quantitative analysis with a small timescale. In this paper, a performance assessment of VSPSPs in terms of power regulation for mitigating wind power variations is undertaken based on a timescale of seconds, and the assessment combines the analysis of physical features and the economic indicators regarding ancillary service markets. First, a numerical model integrating hydraulic-mechanical-electrical subsystems of VSPSPs with doubly fed induction machines is built with MATLAB/Simulink, and it is validated by on-site measurements of a Japanese VSPSP. Then, based on a Chinese VSPSP, a quantitative comparison between variable-speed units (VSUs) and fixed-speed units (FSUs) is conducted through four indicators based on ancillary service compensation in electricity markets in China and the USA. Twelve scenarios are investigated, including case studies based on measured wind power variations. The results show that the VSU outperforms the FSU by one order of magnitude in the aspect of power regulation performance: the maximum ratios between the VSU and the FSU of the four indicators (average of power difference, standard deviation of power difference, penalty energy, and power delay) are 3.92%, 7.85%, 3.92%, and 5.56%. VSUs not only stand out for contributing to power system stability but can also obtain a significantly higher assessment in the ancillary service of the electricity market from an economic perspective. These results could be an important source of support for the investment and development of variable-speed pumped storage technology.

Wind power fluctuation compensation by variable speed pumped storage plant in a grid integrated system: Frequency spectrum analysis

An electrical power generation from the wind technology is the most rapidly growing technology due to its ample characteristics. Nevertheless, because of its stochastic feature, it has unnecessary impacts on the operation s and stability of the power grid system. The fluctuation of the grid frequency problem, for example, is more pronounced. The fluctuation of the frequency in turn impacts even the collapse of the power system. To minimize such problem, a droop-vector control strategy applied on a doubly-fed induction machine based (DFIM) variable speed pumped storage (VSPS) system is proposed in this paper. This method is supposed to be used as a wind power fluctuation compensation solution in the wind farm-grid integration system. The system model is made on the basis of the technique called a phasor model. The frequency spectrum analysis approach is used in the VSPS plant for determining the dynamic performances of the grid in case of contingencies including wind power fluctuation compensation. The software platform MATLAB/Simulink is used for verifying the performance of the proposed system. The results show that the method of frequency spectrum analysis technique is effective for determining the wind power fluctuation and stability requirement in the large power network. The control strategy proposed on this paper implementing the VSC-DFIM based VSPS plant integrated with the power gird and wind farm network achieves well-controlled power flow and stable grid frequency with the deviations in acceptable ranges.

A Review of Power Electronic Converters for Variable Speed Pumped Storage Plants: Configurations, Operational Challenges, and Future Scopes

Pumped storage power plant has gained a high level of attention in recent years, mainly because of its ability to act as a large-scale energy storage option and to improve power system flexibility. Doubly fed asynchronous machine with the partially rated power electronic converter is adopted in pumped storage plants to provide variable speed operation and improve energy efficiency. This paper summarizes the power converter topologies in large rated variable speed pumped storage plant (VSPSP), also covers the suitability of each converter topology, modulation techniques, and parallel converter schemes. Also, it presents operational issues of the parallel converter system in VSPSP such as the shutdown of the plant due to insufficient converter redundancy and lack of fault-tolerant control schemes, a power outage due to the inadequacy of converter protection system, and unbalanced power sharing due to circulating current. Furthermore, the reliable mitigating/additional techniques are discussed for future research. It concludes that the refinement of these aforementioned issues will provide continuous operation of the plant.

Nonlinear Control of a Fully-Fed Variable Speed Pumped Storage Plant

This paper presents the nonlinear control of a Fully-Fed Synchronous Machine based Variable Speed Pumped Storage Plant (FFSM PSP). First, a model is developed and the control objective is established followed by the control design. The control algorithm is designed using Feedback Linearization robustified by a Second Order Sliding Mode Control (SOSMC). Using both techniques allows a fast response of the system while relaxing the conditions on the Sliding Mode Control and avoiding the known chattering problem. The control is compared with the industrial standard PI controller from the dynamic response point of view and also during short-circuits.

Performances achieved to the Grid by a Full Power Converter Used in a Variable Speed Pumped Storage Plant

The growth of renewable energies likes wind and solar requires pumped-storage plants to increase their performances to stabilize grid frequency and voltage. The introduction of a full-power converter constitutes the ultimate step forward to meet the requirement in a safe, reliable and sustainable manner. This article quickly introduces the converter topology and technology before describing the performances it aims to deliver to the grid. Finally, converter bypass is discussed.