Variable Speed Pumped Storage Units Research Articles

Flexibility Assessment of Ternary Pumped Storage in Day-Ahead Optimization Scheduling of Power Systems

To explore the advantages in the flexibility of ternary pumped storage units (T-PSUs) compared with fixed-speed pumped storage units (FS-PSUs) and variable-speed pumped storage units (VS-PSUs) in the day-ahead optimal scheduling of power systems, this paper firstly establishes the mathematical model of T-PSUs which is suitable for the target application, and establishes a new hybrid pumped storage model combining both FS-PSUs and T-PSUs for the purpose of improving existing FS-PSUs. Secondly, a day-ahead optimal scheduling model used for a wind-thermal-pumped storage bundled transmission system with the goal of obtaining the lowest operating cost is established. Additionally, various indicators are proposed to comprehensively evaluate the flexibility of different pumped storage unit types on day-ahead optimal scheduling. Finally, a sensitivity analysis is conducted from three aspects: covering the wind power capacity, load rate, and pumped storage capacity. The results indicate that T-PSUs are superior to FS-PSUs and VS-PSUs in maintaining the smooth, economic, and robust operation of thermal power units, as well as in promoting wind power consumption. This provides a significant reference for evaluating the technical and economic benefits of the different types of pumped storage units in applications of future power grids.

A Data-Driven Predictive Control Method for Modeling Doubly-Fed Variable-Speed Pumped Storage Units

In this study, a data-driven model predictive control (MPC) method is proposed for the optimal control of a doubly-fed variable-speed pumped storage unit. This method combines modern control theory with the dynamic characteristics of the pumped storage unit to establish an accurate dynamic model based on actual operating data. In each control cycle, the MPC uses the system model to predict future system behavior and determines the optimal control input sequence by solving the constrained optimization problem, thereby effectively dealing with the nonlinearity, time-varying characteristics, and multivariable coupling problems of the system. When compared with a traditional PID control, this method significantly improves control accuracy, response speed, and system stability. The simulation results show that the proposed MPC method exhibits better steady-state error, overshoot, adjustment time, and control energy under various operating conditions, demonstrating its advantages in complex multivariable systems. This study provides an innovative solution for the efficient control of doubly-fed variable-speed pumped storage units and lays a solid foundation for the efficient utilization of new energy sources.

Fault Ride-Through Control Strategy for Variable Speed Pumped Storage Unit with Full-Size Converter

Fault Ride-Through Control Strategy for Variable Speed Pumped Storage Unit with Full-Size Converter

Research on grid-related performance laboratory testing for variable speed pumped storage unit

Abstract Variable speed pumped storage unit is an energy storage device with excellent operation and regulation performance. For the grid-related performance laboratory testing of the AC excitation controller of variable speed pumped storage unit integrated into the power grid, this paper studied the methods of grid-related testing of the AC excitation controller based on actual pumped storage power station data, established semi-physical hardware in the loop simulation platform based on RTDS and its function boards, and proposed the key laboratory testing items for the grid-related test. The simulation testing results verified the functions of the platform.

Study on phase-locked out-of-step fault characteristic analysis and protection of VSPSU

Abstract The variable-speed pumped storage unit (VSPSU) adopts flexible three-phase AC excitation, and the control dominant characteristics of the equipment are significant, which deeply affects the out-of-step (OOS) fault law and stability mechanism of VSPSU. Aiming at the OOS fault of VSPSU, this paper first defines the power angle of VSPSU and analyzes the influence of a phase-locked loop on the power angle stability of the unit. Then, the fault characteristics of the power angle instability of VSPSU are further studied. It is found that the traditional lens impedance protection scheme based on measuring the number of sliding poles has the problem that the phase-locked OOS oscillation cannot be responded to or the response is not timely. Combined with the research on the fault characteristics of power angle instability of VSPSU, an OOS protection method based on a bilateral annular blocker is proposed. The simulation results show that the proposed protection method can quickly and accurately reflect the phase-locked OOS fault of VSPSU, and meet the requirements of VSPSU for the reliability of OOS protection.

Rotating speed pulling-back control and adaptive strategy of doubly-fed variable speed pumped storage unit

When the doubly-fed variable speed pumped storage unit (DFVSPSU) adopts the power priority control, the rotating speed may change excessively during the transient process, potentially causing the rotor side power to exceed the converter capacity, which may damage the converter. Therefore, this study aims to analyze the mechanism underlying rotor side power overlimit and conventional rotating speed pulling-back control (RSPBC), proposing an adaptive rotating speed pulling-back control (ARSPBC). First, the unit's speed operating range and the converter capacity were determined based on the characteristic curve of the pump-turbine. Second, the optimal speed and speed limit were compared, indicating that the unit's operation is closer to the lower speed limit, which increases the likelihood of power overlimit occurrences. Third, the mechanism of rotor side power overlimit and conventional RSPBC were analyzed. It was noted that although conventional RSPBC can prevent rotor side power overlimit, it can result in significant generator power fluctuations. Finally, an ARSPBC incorporating speed change rate feedback was proposed. The numerical simulation showed that the ARSPBC can not only prevent the rotor side power overlimit, but also reduce the generator power fluctuations. Compared to the conventional RSPBC, the power fluctuation was reduced from 70 % to 62.5 %–4 %.

A Nonlinear Hybrid Modeling Method for Pump Turbines by Integrating Delaunay Triangulation Interpolation and an Improved BP Neural Network

In order to solve the problem that the data processing accuracy of the characteristic curves is not high, which affects the accuracy of the simulation of variable-speed pumped storage units, this paper proposes a nonlinear hybrid modeling method for pump turbines by integrating Delaunay triangulation interpolation and an improved back-propagation neural network. Firstly, the improved Sutter transform is used to preprocess the original curve, and the convex hull of the transformed curve is calculated. With the unknown point inside the convex hull, Delaunay triangulation interpolation is used to model the transformed curve. With the unknown point outside the convex hull, the back-propagation neural network is first established based on the input and output, and then the initial weights and thresholds of the network are determined using the mind evolution algorithm to complete the modeling and simulation of the curve. Simulation results show that the proposed method fully integrates the high-precision features of interpolation and the powerful nonlinear fitting ability of the neural network, which greatly improves the efficiency and accuracy of pump turbine modeling.

A rotor open-phase imbalance protection for variable speed pumped storage unit based on rotation transformation fault component ratio

Doubly-fed variable speed pumped storage unit (VSPSU) adopts slip ring injection AC excitation mode, and its rotor side structure is complex and prone to open-phase faults. It is very important to configure specific open-phase imbalance protection to ensure its safe operation. Traditional doubly-fed motors only stay in the state monitoring stage for rotor electrical imbalance, which cannot meet the requirements of large-capacity VSPSU for fast, real-time, and high-reliability protection. Therefore, this paper proposes a VSPSU rotor open-phase imbalance protection method based on the rotation transformation fault component ratio (FCR). The fundamental frequency component and fault characteristic component of the stator current space vector are extracted by rotation transformation and FCR is calculated, and then different protection action modes are divided according to the amplitude of FCR. To realize the engineering application, a microcomputer protection algorithm suitable for the proposed protection method is further proposed, and then a complete microcomputer protection scheme is formed. Simulation and experiment results show that the proposed protection scheme has high sensitivity and fast action speed. At the same time, the proposed algorithm can be directly embedded in the existing microcomputer protection device to meet the needs of engineering applications.

Quantifying power regulation characteristics in pump mode of variable‐speed pumped storage unit with DFIM

AbstractPumped storage plants (PSPs) play an important role in renewable energy consumption in power systems. Variable‐speed technology is a new and critical direction for the development of PSPs. In pump mode, variable‐speed pumped storage units (VSPSUs) have wider power regulation ranges and more flexible power responses than fixed‐speed pumped storage units (FSPSUs); however, the corresponding quantification study of VSPSUs is rare. Therefore, this study aims to quantify the static and dynamic power regulation characteristics of VSPSU in pump mode. First, the modeling of the fast speed control strategy of VSPSU in pump mode is improved. Then, the power regulation range of the VSPSU is explored. The factors (i.e., power command, governor, and converter) affecting the power response rapidity are quantified through the engineering case of a variable‐speed PSP adopting the doubly fed induction machine. Accordingly, suggestions on the settings of the parameters are provided. Quantification results show that parameter tuning based on this study effectively solves the problem of low regulation rate caused by great power fluctuations under typical power regulation cases in pump mode. This VSPSU has the fastest regulation rate of 13.75%/s when adjusting 40% power. This study could provide technical support to the practical operation of VSPSU.

Analysis of power system frequency and active power based on wind-storage combined frequency modulation system

Abstract To solve the problem of frequency and active power regulation of power systems, this paper proposes a wind-storage combined frequency modulation strategy. The mathematical models of doubly-fed induction machines, pump turbines, and wind turbines are established. Based on wind power frequency modulation, the doubly-fed variable-speed pumped storage unit is added to establish a combined frequency modulation system, and a simulation model on Simulink to analyze the participation of different parts of the combined system in frequency modulation and the active power output of the unit when the load changes. The simulation results show that when the load increases or decreases, the addition of a doubly-fed variable speed pumped storage unit can effectively improve the frequency modulation performance of the system, reduce the fluctuation numbers and amplitude, and reduce the deviation after stabilization. By changing its active power output, a new balance of the system power can be reached, which is conducive to the secure and steady functioning of the power system.

The method for identifying the potential of the fast response capability of a variable-speed pumped storage turbine in pump mode

Abstract Variable-speed pumped storage technology was being increasingly adopted by new power system to improve the operating and regulating characteristics of the hydropower unit and to stabilize the grid fluctuations. By connecting to a frequency converter, the variable-speed pumped storage unit can respond to grid fluctuations at a speed of hundreds of milliseconds. The fast response capability of a variable-speed pumped storage turbine should be considered in the design stage, be identified during the model test and be used as prerequisite during the commissioning stage of the unit. This article described the method for identifying the fast response capability and methods for improving the power regulation potential of a variable-speed pumped turbine in pump mode. The pipeline hydraulic loss, the guide vanes opening, the energy, cavitation and hump characteristics of the pump are the key parameters and should be carefully considered during the selection of the variable-speed operating zone.

Research on Simulation and Power Regulation Control Strategy of Fengning Variable-Speed Pumped Storage Unit

Abstract Variable-speed pumped storage unit demonstrates rapid power response and flexible adjustment of unit speed, significantly enhancing unit operational efficiency and the power system’s stability. This paper focuses on simulating the control strategy of the Fengning variable-speed pumped storage unit. First, the water pump/turbine and motor models are established based on the relevant parameters of the Fengning variable-speed pumped storage unit. Second, according to the distinct responsibilities of the governor and the excitation system, the control strategies of the unit in the power priority mode and speed priority mode are designed. Finally, we compare and analyze the power response characteristics of a single-machine infinite bus system under various operating conditions and control strategies. We also summarize the applicable scope and existing problems under different operating conditions.

Vibration of Variable Speed Pump-Turbine in Turbine Operating Range: Simulation by CFD+FEM

Abstract Variable speed pumped-storage unit can operate efficiently and stably in wide range of power and head through the coordinated control of the rotational speed and guide vanes. The regulation in rotational speed brings about frequency and amplitude changes of pressure pulsations, which further influence the avoidance margin of vibration and the dynamic response of the runner. The vibration characteristics of the variable-speed pump-turbine need to be studied when it runs in the wider operating region. Selecting an actual pumped-storage unit as the subject and using the CFD+FEM coupled simulation method as a tool, we conducted three-dimensional pump-turbine vibration simulations and analyse the flow patterns, pressure pulsations, and runner vibration characteristics of several typical operating points in turbine mode. The study focused on the variation laws of pressure pulsations and dynamic stresses when rotational speed is regulated. Comparison with the results for the constant speed unit under the corresponding working conditions was conducted. It is shown that the variable speed operation can effectively avoid the severe vibration conditions that the constant speed unit encounters; the amplitudes of pressure pulsations and dynamic stresses of the runner are small at low rotational speed; in the condition of high speed, low head, and small load, the vibration is the most obvious. This study can provide a reference for the design and operation of variable speed pump-turbine.

Quantifying the performance and compensation of secondary frequency regulation of pumped storage plants considering variable-speed operation

Aiming at the “net-zero carbon” target, a higher proportion of variable renewable energies (VREs) has been integrated into power grids, and pumped storage plants (PSPs) are crucial for guaranteeing the safe and stable operation of hybrid energy systems. As secondary frequency regulation (SFR) is related to the economic operation and the quality of auxiliary services provided by PSPs, it is critical to clarify its performance and compensation. Therefore, the corresponding quantitative evaluations of the SFR of pumped storage units (PSUs) are carried out in this paper. First, the performance of SFR is quantified based on the fuzzy analytic hierarchy process (FAHP). Considering the current situation in China’s electricity market, the average compensation of the PSP with each MW is determined to be 9.336 CNY per day, and a method for calculating the compensation of SFR is constructed. Then, taking a real Chinese PSP as an example, based on different wind power deviation signals, the quantitative evaluation of the compensation of SFR is conducted in accordance with the fixed-speed pumped storage units (FSPSUs) and the variable-speed pumped storage units (VSPSUs) in power priority and speed priority modes. The results show that the compensation of FSPSUs and VSPSUs (power priority mode and speed priority mode) is 10,900 CNY per day; 54,400 CNY per day; and 17,300 CNY per day. This paper could contribute to clarifying the compensation of SFR and providing technical support for the development of the auxiliary service market.

Multi-timescale analysis and quantification of dynamic regulation characteristics of DFIM-based variable-speed pumped storage units in alleviating wind power fluctuations

To comprehensively clarify the regulating role of variable-speed pumped storage system (VSPS) in alleviating wind power fluctuations, this paper quantitatively evaluates the multi-timescale frequency regulation performance of VSPS in a typical wind-hydro-pumped storage hybrid system (WHPHS). Firstly, the flexible equivalent frequency response models of VSPS and WHPHS are established. Then, the effectiveness of fixed-speed pumped storage system (FSPS) and VSPS in enhancing the stability of dominant hydropower system (HS) in WHPHS is compared, and the dynamic regulation behaviors between VSPS and FSPS are quantitatively evaluated. Based on the operational characteristics, the differences in the allocation of regulation responsibilities between VSPS/FSPS and HS for frequency regulation are clarified. The results indicate that VSPS contributes to an exhaustive improvement in HS stability, reduces the power fluctuation and frequency deviation of WHPSHS. Specifically, in the established simulation scenarios, WHPSHS with VSPS outperforms an equivalent FSPS in reducing frequency deviation frms by 20 %, power fluctuation Is by 40 %, and complementarity Ic by 50 %. Particularly effective in suppressing power fluctuations in range of 0.01 Hz to 0.8 Hz, which decreases the resonant peak of frequency deviation by 61.3 % compared to FSPS. It further highlights the regulatory advantages of VSPS in responding to low-amplitude high-frequency power variations. These findings can provide novel insights into the role of VSPS in mitigating RES fluctuations and energy management strategies in multi-type hybrid systems.

The impact of hump characteristics on variable speed pumped storage units under pump mode and improvement measures

The input power of variable speed pumped storage units (VSPSUs) under pump mode is adjustable, which effectively increases the units' frequency regulation capacity and ability during low demand periods in the power system, providing strong support for the stable operation of the grid. However, the operation of the VSPSU under pump mode is affected by the hump characteristics, which limits the regulation capability of the unit. Therefore, this paper analyzes the mechanism of the impact of the hump characteristics on VSPSUs and proposes an effective improvement measure. Firstly, on the basis of the pump on-cam curves, a simple and effective efficiency optimization method of the VSPSU under pump mode is proposed. Then, based on the characteristics of the hump region, the relationship between the optimal efficiency points and the hump characteristics is obtained, revealing that the hump characteristics affect the operating conditions of the variable speed units under any head. Subsequently, a novel nonlinear model of the pump-turbine is proposed based on the basic characteristic curves of various openings under pump mode to solve the multi-value phenomenon, and the influence mechanism of hump characteristics on pump mode of VSPSU from a control perspective is obtained. Finally, an improvement measure is proposed to modify the optimal efficiency points by setting operating safety margins, and the sub-optimal efficiency point that is not affected by the hump characteristic at the same input power is used to replace the optimal efficiency point that is affected by the hump characteristic to avoid the impact of the hump characteristic. Compared to the method of setting hump characteristic limit, the adjustment range of the VSPSU under pump mode is greatly increased.

Modeling and Operating Characteristics of Excitation System for Variable Speed Pumped Storage Unit with Full-Size Converter

The variable speed pumped storage unit with a full-size converter (FSC-VSPSU) can provide fast and flexible regulation capacity for the power grid, assisting the rapid development of the new energy-dominated power systems, and its application is gradually becoming widespread. The excitation system of FSC-VSPSU is crucial for maintaining the motor-port voltage and further supports the stable operation of the entire unit. However, there is currently no research on excitation system modeling, and their essential operating characteristics have not been discussed in detail, which needs further study. With this aim, this paper firstly establishes a detailed transfer function model for FSC-VSPSU, which is then used to study the influence rules of FSC-VSPSU parameters on the operating characteristics. In addition, based on the summarized operating characteristics, the excitation system can be optimized, and its complexity is greatly reduced while ensuring the original performance of the unit. Finally, the results of the simulation can verify the correctness of the established model and the explored operating characteristics.

Calculation method of external fault short-circuit current for variable-speed pumped storage units based on time-domain dynamic circuit

The calculation method of the external fault short-circuit current of large units with clear mechanisms and practicality is of great value in engineering practice. Due to the special AC excitation structure and control mode, the external fault short-circuit characteristics of variable-speed pumped storage units (VSPSUs) are very different from those of conventional units. Therefore, this paper proposes a calculation method for external fault short-circuit current based on a time-domain dynamic circuit. According to the excitation and control characteristics of VSPSUs under external fault, the VSPSU under external fault is divided into two cases: continuous excitation and jumper action. Based on space vector analysis, the time-domain dynamic circuit models are established, respectively. On this basis, the analytical formula of the external fault short-circuit current of VSPSUs is derived by combining the transition boundary conditions of the two cases and the dynamic characteristics of the stator flux linkage. Finally, the comparison with PSCAD/EMTDC simulation results shows that the proposed VSPSUs’ external fault short-circuit current calculation method has high accuracy and practical value, which can meet the needs of primary equipment selection and relay protection of variable-speed pumped storage power plants.

Simulation of Different Control Strategies in Pumping Mode of Variable-speed Pumped Storage Unit

Because of the complex structure, the variable-speed pumped storage (VSPS) unit would face more stability problems in operation compared with conventional generators in the power grid. To overcome the shortcomings of the PI controller, the fractional order PI (FOPI) controller is applied for VSPS in pumping mode. To compare the quality of the two controllers, the WSCC 9-bus system with VSPS is studied under different disturbances. The dynamic performance is evaluated by calculating the integral of time multiplied absolute error criterion (ITAE) of rotor speed and active power of the stator. The result shows that the FOPI control strategy can effectively improve the dynamic performance of VSPS.

Stability characteristics analysis and ultra-low frequency oscillation suppression strategy for FSC-VSPSU

The research on control strategies of the variable speed pumped storage unit with full size converter (FSC-VSPSU), mainly focuses on the rapidity of power regulation. But the speed oscillation and slow speed regulation performance caused by fast power regulation are ignored, which is not conducive to the safe and efficient operation of the unit. This article investigates the dynamic stability characteristics of the FSC-VSPSU in detail. In turn, the stability risks are clarified and serve for the design of its control strategy and the improvement of operational efficiency. The small-signal model of FSC-VSPSU considering the variation of water head and load in fast power mode is established. The dominant instability control link and key parameters are revealed based on the eigenvalue analysis. Oscillation mode analysis shows that the unit has a wide-band oscillation characteristic. The dominant oscillation mode is the ultra-low frequency oscillation (ULFO) caused by the governor control system. To suppress the ULFO and improve the optimal speed tracking performance of the unit to the greatest extent, a coordinated control strategy of governor variable parameter control and governor power system stabilizer (GPSS) is proposed. The electromagnetic transient simulation based on PSCAD verifies the effectiveness of the proposed measures.