Variable Speed Pumped Storage Units Research Articles

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.

Modeling and Transient Response Analysis of Doubly-Fed Variable Speed Pumped Storage Unit in Pumping Mode

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.

Improved start-up strategy for a doubly fed induction machine fed large rated variable speed pumped storage unit in pumping mode operation

Improved start-up strategy for a doubly fed induction machine fed large rated variable speed pumped storage unit in pumping mode operation

Analysis of Pressure Fluctuation of Pumped Storage Units

In pump turbines, the observation in the engineering field shows that pressure fluctuation has become one of the most serious hydraulic instability phenomena. The pressure pulsation will cause vibration and fatigue failure of the unit, even vibration of the plant, which greatly affects the safe operation of the power station. In this paper, the research on pressure fluctuation by domestic and foreign researchers is introduced and discussed. It includes the characteristics of pressure pulsation in various regions of pump turbines under various operating conditions, existing research methods, and the possibility of improving pressure pulsation problems with variable-speed pumped storage units, which will serve as the basis for further research and improvement of pressure pulsation.

Rotor short-circuited start-up strategy for a doubly fed induction machine-fed large-rated variable-speed pumped storage unit operating in pumping mode

Rotor short-circuited start-up strategy for a doubly fed induction machine-fed large-rated variable-speed pumped storage unit operating in pumping mode

Equilibrium analysis of doubly fed-based variable-speed pumped storage unit under remote grid fault

As the flexible-controlled power source in the power system, the doubly fed-based variable-speed pumped storage unit (DF-VSPSU) has a significant role in the power rebalance and the reactive power support. However, as with the voltage sag in the remote grid side, the DF-VSPSU with the conventional low-voltage ride-through control may lose the synchronization and result in instability due to the absence of an equilibrium point. Thus, taking a DF-VSPSU infinite system as a case, the generator-side, and the grid-side model is well established. The large disturbance instability mechanism of doubly fed-based variable-speed pumped storage unit under remote grid faults is explained in detail, which is caused by the absence of the equilibrium point. Together with the key influencing factors, including the voltage depth and the short circuit ratio, the available gain of the reactive current control is presented. Finally, the availability of the theoretical analysis is demonstrated based on MATLAB/Simulink.

Multi-time scale model reduction strategy of variable-speed pumped storage unit grid-connected system for small-signal oscillation stability analysis

This paper studies the time-scale characteristics, model reduction, and oscillation stability of the grid-connected variable speed pumped storage unit (VSPSU). Firstly, a method based on the modal analysis and time-scale separation ratio indicator is established to analyze the time-scale characteristics of VSPSU. The results indicate that the state variables of VSPSU can be approximately separated on four-time scales. On this basis, the singular perturbation method (SPM) is applied to obtain the reduced-order model (ROM) of VSPSU under different conditions. The results show that the accuracy of ROMs can be affected by changes in operating conditions. To balance the complexity and accuracy of ROM, a model reduction strategy combining SPM and reduction margin is proposed. The results indicate that the proposed method enables the order minimization of ROM while the selected oscillation mode reaches a given accuracy condition. Finally, the feasibility of ROM for oscillation stability analysis is verified. The ROMs obtained with the proposed strategy are more accurate than those obtained with SPM for oscillation stability analysis of VSPSU. The influence of grid strength, turbine speed, and torque conditions on system oscillation stability is also revealed. This study provides new insights for model order reduction and stability analysis of VSPSU.

Suppression strategy of coupled oscillation and power anti‐regulation of variable speed pumped storage unit with full size converter

AbstractThe 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.

Reactive Power Circulation Based Fault Tolerance Schemes for Multi-Megawatt 3L-NPC Paralleled Converters in Variable Speed Hydro Applications

Parallel connected multi-MW three-level neutral point clamped power converters (3L-NPC) are widely preferred in large-rated variable speed pumped storage unit as it improves operating efficiency and reliability. During open circuit fault conditions/gate drive malfunction, the entire unit must be shut down to ensure the safety of other healthier components. This article proposes reactive power circulation-based fault tolerance control strategies for grid side converter (GSC) during inner and outer switch fault conditions. In case of an open-circuit fault in outer switches, the normal sinusoidal currents are achieved in GSC by circulating reactive power among the healthy to faulty power converters. In case of an open-circuit fault in inner switches, the reactive power is circulated between healthy converters and a doubly fed induction machine (DFIM). In addition, neutral-point (NP) voltage oscillations are suppressed by injecting a zero-sequence signal into the reference voltage. The feasibility study and performance analysis of the proposed fault tolerance control strategies are carried out in the MATLAB/Simulink 2014a environment for the 250 MW DFIM unit. Furthermore, experimental validation is carried out in 3HP DFIM scale-down laboratory prototype with three parallel connected converters.

Dynamic modeling and favorable speed command of variable-speed pumped-storage unit during power regulation

Variable-speed pumped-storage unit (VSPSU) has advantages in flexibility, efficiency, and reliability. It has become a state-of-the-art technology in pumped-storage industry. However, there is little research about the transient process of VSPSU working at a favorable speed command during power regulation. To study the issue, a refined model considering hydraulic–mechanical and electrical subsystems is built in this work. All submodels are created by the authors as individual modules on Simulink platform, and these modules can be easily combined to model different VSPSUs. For the VSPSU that uses a doubly fed induction machine and adopts the fast power control strategy, there are several speed constraints. To satisfy the speed constraints during transient process, the speed command is adjusted to avoid exceeding the allowable speed variation range and eliminate the oscillation caused by the S-shaped region, while being guided by the optimal speed to achieve higher efficiency. The faster the speed command changes, the more rapid the power regulation could be. But fast-changing speed command also leads to larger pressure fluctuations in penstocks. Hence, different schemes of the speed command are compared, and suitable schemes are proposed by comprehensively considering the balance between rapidity and safety. The dynamic model and analysis of favorable speed command can provide support and advice for engineering practice.

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.

Self-starting control strategy of variable speed pumped storage unit based on adaptive full order state observer

When the variable speed pumped storage unit operates in the electrical state, the self-starting control strategy is generally used to start the process from zero speed to grid connection, which saves the starting device of the traditional constant speed unit and saves the construction cost of the unit. For the self-starting control process, the traditional open-loop flux calculation method will produce cumulative errors in the amplitude and angle of the flux when the motor parameters change. To solve this problem, an adaptive full order state observer is designed to observe the motor stator flux, which has stronger parameter robustness. Based on the flux observation, the speed adaptive law is designed to estimate the rotor speed and position angle, and the speed sensorless vector control of variable speed pumped storage unit is accomplished. The simulation results show that compared with the open-loop flux calculation method, the full order state observer can greatly reduce the flux estimation error and accurately estimate the rotor speed and position angle.

Flexibility of variable-speed pumped-storage unit during primary frequency control and corresponding assessment method

As a new type of large-scale energy storage facility, variable-speed pumped-storage unit (VSPSU) has unknown potential in providing flexibility for power system to help solve the frequency control problem caused by variable renewable energy. To assess VSPSU’s flexibility during primary frequency control (PFC), in this paper, VSPSU’s complicated dynamic model is simplified first with internal currents retained for operating constraint evaluation, based on which a concise assessment model is then established by combining the necessary dynamics and the PFC metric. For the assessment model, the solution algorithm is discussed and a pattern-search-based algorithm is proposed for the parameter optimization of VSPSU’s PFC strategy. Finally, a case study based on IEEE 24-bus and 57-bus systems is conducted to verify the accuracy and adequacy of the proposed assessment method under various scenarios. The results show that the average assessment error is 2.92% and the bottleneck constraints of VSPSU are as close to be violated as expected. In the case study, VSPSU is able to provide 33.97% more flexibility than traditional fixed-speed pumped-storage unit on average, while the variability of VSPSU’s flexibility is also revealed.

Excitation control of variable speed pumped storage unit for electromechanical transient modeling

In this paper, a new excitation control of variable speed pumped storage unit (VSPS) for electromechanical transient modeling is presented. Compared with the traditional VSPS excitation control based on current source, the inner loop control of rotor current is retained. The new VSPS excitation control model considers the rotor flux dynamics and represents it as a second-order system. The effectiveness of the proposed excitation control model is verified by the comparison between electromagnetic transient simulation and electromechanical transient simulation. The electromechanical transient simulation of VSPS based on current source and VSPS based on voltage source is compared to verify the necessity of considering rotor flux dynamics. The proposed VSPS excitation control model reflects the characteristics of controllable voltage source and can provide more accurate reactive power and voltage simulation results in transient stability analysis.