Modeling and Power Regulation Strategy of Variable‐Speed Pumped Storage Unit With Full‐Size Converter in Pump Mode

Pumped 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.

The variable speed pumped storage units (VSPSUs) with full-size converter (FSC) have the advantages of a wide regulation range and excellent control performance, and the VSPSU with FSC is conducive to maintaining power system stability due to the fast power regulation. However, the scheme of two units sharing common pipelines is widely used in current pumped storage plants. Due to hydraulic coupling interactions of a shared pipeline, there is a hydraulic disturbance between different units. When the plants need to be regulated according to grid demand, the distribution of the power regulation between the fixed-speed and variable-speed pumped storage units (FS-VSPSUs) in the system of two units sharing common pipelines and the hydraulic disturbance law urgently need to be investigated. Therefore, a numerical simulation model of the FS-VSPSUs sharing common pipelines, incorporating VSPSU with FSC, is established based on Matlab/Simulink using the improved transfer function method (TFM); Fixed-speed and variable-speed units (FS-VSUs) are simulated to study the dynamic response characteristics of the units under power fluctuations, and the active power response rapidity of the FSU and VSU are analyzed separately; Stability indicators are selected to assess the stability of different units during power regulation. The results show that: the active power response rapidity of VSU is faster and better, with two indicators of the rapidity advantage that can reach 94% on average compared to the FSU. However, in terms of stability, the power regulation of FSU is far superior to the stability of the VSU, with five indicators of the stability showing an advantage ranging from 28% to 86%, with an average of 69%. This paper provides theoretical support for the power regulation characteristics of the FS-VSPSUs sharing common pipelines.

While dynamic regulating effects of pumped storage units have been more and more important in power systems integrated with large-scale wind power, outputs of the conventional reversible pumped storage units still cannot be dynamically adjusted in pumping mode due to the characteristics of pump-turbine. In order to expend the regulating range of reversible pumped storage unit, the variable speed solution is adopted in this paper to realize the dynamic regulating of wind power under the pumping mode. Based on the analysis of pump turbine characteristic curves, the dynamic model and corresponding control strategy of variable speed pumped storage unit are derived. And then, the IEEE 39-bus test system is used to verify wind power regulating performance of variable speed pumped storage unit in pumping mode. The results show that variable speed pumped storage unit has broadened the regulating range of wind power by dynamically adjusting pumping power and effectively reduce the frequency deviation of power system caused by the short-time wind power fluctuation.

Compared to conventional fixed-speed pumped storage units (FSPSU), the emerging variable-speed pumped storage units (VSPSU) — based on doubly fed induction machines (DFIM) — exhibit broader power regulation ranges and faster dynamic response capabilities. However, the evolution of internal flow characteristics during large-amplitude power regulation in VSPSU remains unclear. This study first obtained operational trajectories of doubly-fed VSPSU during large-amplitude power reduction regulation through one-dimensional (1D) transient simulations. Typical operating points were then extracted from these trajectories to conduct computational fluid dynamics (CFD) numerical simulations of the pump-turbine. While changing active power commands, three speed commands were considered: decrease, maintain constant, and increase. The impacts of different variable-speed control strategies on operational trajectories and unit dynamic hydraulic parameters were compared, and internal flow characteristics and pressure pulsations during large-amplitude load reduction regulation were investigated. The results show that increasing speed commands causes high-speed circulation flow to appear in the vaneless space and increases secondary flows in the runner. Different speed command control strategies significantly affect pressure pulsation amplitude in the vaneless space. Taking constant speed as the baseline: at maximum speed, decreasing speed commands reduces pressure pulsation amplitude by 3.25%, while increasing speed commands increases it by 16.7%; at minimum power, decreasing speed commands reduces pressure pulsation amplitude by 3.38%, while increasing speed commands increases it by 14.1%. The research results can provide references for safe and flexible regulation of VSPSU.

Doubly fed induction machine(DFIM) has excellent speed regulation performance, which can meet the demand of frequency and voltage regulation of power grid and solve the problem of efficiency reduction of pump-turbine resulting from the change of hydraulic head when applied to pumped storage. To omit the drainage process of runner chamber before DFIM start-up, a control method of soft-starting with slight load is proposed which ensures that the speed of DFIM increases smoothly from zero to the required for gridconnection. The field current compensator and rotor position angle compensator are designed to improve the control accuracy for grid-connection. The operation characteristic curve of pump-turbine and the output of grid frequency controller are considered comprehensively to achieve the load optimal regulative. With the methods mentioned above, an overall control strategy of doubly fed variable speed pumped storage unit in pumping mode is proposed, as well as the control flow. The control strategy is simulated in MATLAB and implemented on a towing unit platform. Results of simulation and experiment verified the effectiveness of the proposed strategy, including soft-start with light-load, grid-connection with no inrush current and speed regulation.

Doubly-fed variable speed pumped storage (VSPS) unit is an advanced hydropower system. In pumping mode, its input power can continuously be adjusted within a certain range via changing speed. Thus, it overcomes the major defect that the pumped storage unit cannot provide frequency regulation services to the power grid in pumping mode. However, for the pumping mode VSPS, how to establish its model is still a problem. In this article, an electromagnetic-electromechanical transient multi-time scale model of pumping mode VSPS is presented. This model depicts the intrinsic speed-power regulation characteristics and the extrinsic power-frequency coupling characteristics of pumping mode VSPS, which can be used for the power grid frequency regulation analysis. Firstly, the hydraulic subsystem model of VSPS is established within the electromechanical transient time scale. The model is built based on the variable speed characteristics of the reversible pump-turbine (RPT) in pumping mode and considers the hydrodynamics and additional friction effects of the wicket gate in the water conveyance system. Secondly, the electric subsystem model of VSPS is established within the electromagnetic transient time scale. The model mainly describes the operation characteristics of a doubly-fed induction machine (DFIM) and the control properties of the inner and outer loops of the back-to-back (BTB) converter system. Finally, comprehensive simulation and experimental results have fully verified the established model and theoretical analysis.

VSPS (Variable speed pumped storage units) have been introduced into pumped storage power stations in China, but FSPS (conventional fixed speed pumped storage units) still account for a large proportion. The output power regulation range of VSPS and FSPS is wide under power generation conditions. However, under pumping conditions, FSPS units can only pump or shut down at full power rate, while VSPS can only regulate a little range of power. Therefore, if you want to maintain the large power grid operating safely and stably, it is very important to study the coordinated control strategy between units suitable for the pumping condition of pumped storage units. This paper first analyzes the characteristics of VSPS and FSPS outputting power under pumping conditions, and proposes to use the fast power regulation characteristics of VSPS to track dispatching instructions. When the regulation range exceeds the capacity range of VSPS, the pumping power of the units will be smoothly converted to FSPS, Secondly, the simulation model of four terminal HVDC transmission system is established in Matlab/Simulink. Finally, the simulation analysis is carried out for VSPS and FAPS in pumping condition by adopting coordinated control strategy to decrease the outputting power fluctuation of new energy power station, The proposed control strategy can effectively absorb the fluctuation of new energy outputting power and reduce the fluctuation of active power absorbed by load side in HVDC transmission system. At the same time, the capacity ratio of VSPS and FSPS.KeywordsPumped storage unitCoordination control strategyHVDC transmission systemIntermittent energy integration

Variable speed pumped storage unit (VSPSU) is an innovative technology, and investigations regarding operating characteristics of VSPSU are significant. However, there are relatively few studies towards hydraulic disturbances of VSPSUs. The hydraulic disturbance means that pumped-storage units, which were originally in steady operation, are inevitably affected by suddenly operating changes of other units in the same waterway system. Normally, it will cause the power output of fixed-speed units to oscillate, and adversely affect the safety of generators and stability of power systems. Hence, investigations of VSPSUs’ characteristics under hydraulic disturbances are necessary. In this paper, VSPSU using doubly fed induction machine (DFIM) is adopted as a study case, and fast power control strategy is used in the controlling of VSPSU. The simulation result shows that VSPSU has good anti-interference performance, and normally operating VSPSU can still maintain stable power output and stator current even under disturbance from other units. However, despite the good anti-interference performance of VSPSU, the mechanical power of VSPSU lags far behind the electromagnetic power, and the hydraulic-mechanical transient processes of VSPSU are similar to that of a fixed-speed unit. The findings in this paper could provide understanding and guidance for practical operation of VSPSUs.

Advantage analysis of variable-speed pumped storage units in renewable energy power grid: Mechanism of avoiding S-shaped region

The stability of large doubly-fed variable speed pumped storage unit (DFVSPSU) under small disturbance was investigated. First, a model of DFVSPSU in turbine mode and pump mode is established from the two aspects of pump-turbine system and doubly-fed induction machine (DFIM) control system. Then, the effects of controller parameters, operating states, and grid intensity on the stability domain are explored. The results show that the adjustment range of integral gains are much larger than that of proportional gains in both turbine and pump modes, and the proportional gain of the current adjuster in turbine mode is the largest among the proportional gains. Furthermore, the instability of the supersynchronous operation of the unit is higher than that of the subsynchronous operation. When the controller parameters are selected appropriately, the influence of grid intensity is nonmonotonic but still in the stable domain. Finally, the correctness of the theoretical analysis in this paper is verified by time domain simulation.

ABSTRACTThis paper aims to study the frequency‐power coupling dynamic response and regulation characteristics of a variable speed pumped storage unit (VSPSU) with full‐size converter (FSC). Firstly, the mathematical model of VSPSU with FSC is established. Secondly, the dynamic response of VSPSU with FSC under power disturbance is simulated, and the effect of operating speed on dynamic response of VSPSU with FSC is analyzed. Then, the frequency‐power coupling regulation characteristics of VSPSU with FSC are revealed. Finally, the dynamic responses between VSPSU with FSC and fixed speed pumped storage unit (FSPSU) are compared. The results show that, with the same disturbance amplitude, the variation times of speed and output under positive power disturbance are longer than those under negative power disturbance. With a negative power disturbance of larger amplitude, the speed and output of VSPSU have a faster response. The regulation characteristics of VSPSU can be improved by adjusting both the parameters of governor and converter based on the operation requirements. Under both the positive and negative power disturbances, the dynamic response of active power of VSPSU is faster than that of FSPSU. Under the negative power disturbance, the dynamic response of speed of VSPSU is slightly slower than that of FSPSU.

Fault ride-through is a prerequisite for ensuring continuous operation of a variable-speed pumped storage unit with a full-size converter (FSC-VSPU) and providing support for the renewable energy and power grid. This paper proposes low-voltage ride through (LVRT) and high-voltage ride through (HVRT) strategies for FSC-VSPU to address this issue. Firstly, the structure of FSC-VSPU and its control strategy under power generation and pumping conditions are described. Subsequently, the fault characteristics of the FSC-VSPU under different operating conditions are analyzed. More stringent fault ride-through technical requirements than those for wind turbines are proposed. On this basis, the fault ride-through strategies of combining fast power drop, rotor energy storage control, power anti-regulation control, dynamic reactive current control, low power protection, and DC crowbar circuit are proposed. Simulation case studies conducted in PSCAD/EMTDC verify the correctness of the theoretical analysis and the effectiveness of the LVRT and HVRT strategies in this paper.

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.

The ultra‐low frequency oscillation and poor speed regulation performance caused by the fast power control decrease the stability and efficiency of the variable speed pumped storage unit with a full size converter (FSC‐VSPSU). This paper presents a fast frequency control strategy to solve this problem. The small‐signal model of the FSC‐VSPSU in fast frequency mode is established, and the key parameters for the system stability are studied by the eigenvalue method. Aiming at the coupling oscillation mode caused by the governor and generator side converter (GSC), the corresponding control parameters optimisation method is proposed. In order to solve the power anti‐regulation problem, the strategy of combining rotor kinetic energy release, power slope adjustment, and delay acceleration control is proposed. PSCAD and RTDS simulation results show that the proposed strategy can effectively avoid the ultra‐low frequency oscillation caused by the fast power control and significantly improves the frequency and power control performance.

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