Variable Speed Pumped Storage Units Research Articles

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.

Optimal operation strategies of pumped storage hydropower plant considering the integrated AC grids and new energy utilization

The large-scale development and utilization of new energy resource extremely promotes the construction and application of the flexible DC power grid especially in China. This paper focuses on the operation stability and new energy transmission of an actual regional power grid in North China, including new energy plants, the flexible DC power grid, a pumped storage hydropower plant (PSHP) with variable speed pumped storage unit (VSPSU) and regional AC power grids. According to the operation rules and goals, the economic operation model of the flexible DC-PSHP system is established by the mixed-integer second-order cone programming. Considering the transmission and consumption of the new energy generation, two operation strategies and three simulation scenarios are proposed. The day-ahead optimal operation schemes of PSHP in different scenarios are given by the case simulation. The results show that the proposed strategies can satisfy the operation demands of the regional power grid both in new energy generation transmission and operation economic benefits.

Anti-interference characteristics of variable speed pumped storage units for load cases of hydraulic disturbances

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.

Reduced-Order System Frequency Response Modeling for the Power Grid Integrated With the Type-II Doubly-Fed Variable Speed Pumped Storage Units

The traditional system frequency response (SFR) model is not applicable to the study of the frequency response of contemporary power grid due to the response of the large capacity (single unit up to 500 MW) type-II doubly-fed variable speed pumped storage (VSPS) unit is not considered. This article proposes an improved reduced-order SFR model for the power grid integrated with the large-scale type-II VSPS. proposed models Compared with the traditional SFR model, this model takes into account the active power coupling effect of this type of VSPS, and the computational burden increase of the model is small due to the model of VSPS has been reduced to the greatest extent. Therefore, the SFR model can be used by the system operators to rapidly calculate and evaluate the frequency response of contemporary power grid. Firstly, a reduced-order frequency response transfer function model of the Type-II VSPS is established by using the state-space model and small-signal incremental method. Compared with the higher-order nonlinear model of the Type-II VSPS, the state variables and computation burden of the established model is dramatically decreased. Secondly, an improved SFR model for the power grid integrated with the type-II VSPS is presented, and then the reduced-order analytical solution for the frequency response of the grid is put forward based on the parameter weighted aggregation method. The improved SFR model is more realistic and can better express the frequency response characteristics of the contemporary power grid. The proposed SFR model have demonstrated advantages in solving speed and efficiency when contrast it with the higher-order nonlinear model of the grid. Finally, the simulation and experimental results are respectively compared with the analytical solution of the reduced-order models, and the results have demonstrated the effectiveness of the.

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.

Short circuit fault identification and diagnosis analysis of rotor winding for variable speed pumped storage unit

Rotor winding short circuit is a frequent fault type of generator unit. Compared with the traditional fixed-speed unit using DC excitation, the three-phase AC excitation structure of the variable speed pumped storage unit is more complicated, and the fault diagnosis of the rotor winding short circuit is more difficult. Therefore, this paper proposes an offline diagnosis method for variable speed pumped storage units based on the principle of Repetitive Surge Oscillograph (RSO). Firstly, the application scheme of the repeated pulse method in the variable speed pumped storage unit is designed. By injecting low-voltage pulses into three phases simultaneously and detecting the response curve to determine whether the unit has a short-circuit fault. Then, based on the response characteristics of different rotor winding short-circuit fault types, an identification scheme of rotor winding short-circuit fault types and faulty phases is proposed. Subsequently, the variational mode decomposition (VMD)-teager energy operator (TEO) method is used to estimate the injected pulse and characteristic curve’s initial sudden change position and then the fault position is determined by the time difference and the measured wave velocity. The simulation results show that the fault identification and fault location method proposed in this paper can be effectively applied to various rotor winding short-circuit faults of variable speed pumped storage units.

A Rotor Winding Internal Short-Circuit Fault Protection Method for Variable-Speed Pumped Storage Units

In electrical machinery, the rotor windings’ internal short-circuit faults are addressed by the instantaneous over-current protection of the power electronic excitation device, which has low sensitivity and has difficulty meeting the safety requirements. In this paper, a rotor windings’ internal short-circuit fault protection method is proposed based on the harmonic characteristics of the circulating current between stator branches. The magnetomotive force distribution of the short-circuit coils in the rotor windings is theoretically deduced, and the characteristic frequencies of the circulating current between stator branches are analyzed. On this basis, the protection criterion of the rotor windings’ internal short-circuit fault is constructed by using the harmonic component of the circulating current. Then, an analytic model of the variable-speed pumped storage unit is established based on the multi-loop method, and the finite element method is used to verify the correctness of the proposed modeling method. An actual large variable-speed pumped storage unit is taken as an example, and the possible faults under different slip ratios are simulated. In the simulation results, the stator branch circulation has the obvious characteristic frequency harmonic components, which is consistent with the theoretical analysis. It verifies the effectiveness of the proposed protection method. Finally, it is analyzed and verified that the proposed protection has a strong maloperation prevention ability under other kinds of faults.

Hopf Bifurcation and Parameter Sensitivity Analysis of a Doubly-Fed Variable-Speed Pumped Storage Unit

The doubly-fed variable speed pumped storage unit is a storage system suitable for joint operation with renewable energy sources to smooth the imbalance between renewable energy supply and electricity demand. However, its working principle and operation control are more complex than those of constant speed pumped storage. In this study, a nonlinear model of doubly-fed variable speed pumped storage units (VSPSUs) considering nonlinear characteristics of the head loss is established. The study finds that a supercritical Hopf bifurcation occurs in the system, and the area enclosed by the lower side of the bifurcation line and the coordinate axis is the stability domain of the system. The active power step perturbation from −0.3 to 0.3 will gradually reduce the area of the stability domain and narrow the adjustable range of the control parameters. In addition, the sensitivity of the model full state variables and the primary and secondary relationships to the changes of subsystem parameters is analyzed systematically using the trajectory sensitivity. It is found that there is a large difference in the sensitivity of different state variables to the parameters. The state variables are much more sensitive to the transfer coefficient of hydraulic turbine torque to guide vane opening, the unit inertia time constant, and the controller proportional gain change than other parameters, which are defined as highly sensitive parameters. The receiver response time constant and the turbine flow-to-head transfer coefficient are the corresponding low-sensitivity parameters.

Research on control strategy of f ull‐ s ize converter‐based variable speed pumped storage unit in power generation condition

Research on control strategy of f <scp> ull‐ <b>s</b> ize </scp> converter‐based variable speed pumped storage unit in power generation condition

Analysis of Control Characteristics and Design of Control System Based on Internal Parameters in Doubly Fed Variable-Speed Pumped Storage Unit

In this paper, the control strategies and their characteristics when applied to the doubly-fed variable-speed pumped storage unit in generating mode and pump mode are discussed. The composition of doubly fed variable-speed pumped storage unit is introduced, and the mathematical model of every component is proposed. Two control strategies of the unit are introduced. One is the power priority control strategy and the other is the speed priority control strategy. Then, the control block diagrams of the two control strategies in the generating mode and the pump mode are established, and the parameters of the unit in each control strategy are designed. The two control strategies are simulated in power generation and pumping conditions. Finally, the conclusion that power priority strategy has a better effect on power control is obtained.

An experimental platform of variable speed pumped storage unit under wave disturbance: introduction and preliminary progress

The importance of pumped storage plants (PSPs) is continuously increasing in power systems with more and more variable renewable energies. For the research and development of PSPs, variable speed technology and sea-water PSP are two important new orientations. In this paper, an experimental platform of variable speed pumped storage unit under wave disturbance is introduced. The purpose of the platform is to study the design and operation mechanism of variable speed PSP based on various common technical problems, especially for the four aspects: transient processes, variable speed unit, coordination control, wave disturbance. The platform adopts a Chinese sea-water PSP in feasibility stage as the prototype, and a model scale of 1:4 is applied. It consists of nine sub-systems, including waterway system, variable speed reversible model unit, circulating water system, speed governor system, AC excitation and electric protection, coordination control, monitoring system, intelligent AC load and measurement system. The model unit contains a reversible pump turbine, a doubly fed induction machine, etc. The speed variation range is ±8%, and the rated power is 78.13 kW. The model runner meets the IEC standard with a reference diameter of 284.3 mm. The upstream water tank is a closed pressure tank that can adjust the pressure and discharge. The downstream water tank is an open water tank that can simulate wave level fluctuation. Regarding the control system, it has four core parts: local control unit, speed governor, converter and coordination controller. The experimental study content contains three key aspects: (1) performance validation for variable speed unit; (2) operating stability under complex disturbances; (3) capability of variable speed PSP during transient processes. The experimental study based on the platform aims for laying a theoretical and technical foundation for the follow-up construction and operation of real sea-water PSPs with variable speed units.

Coordination Control Between Excitation and Hydraulic System During Mode Conversion of Variable Speed Pumped Storage Unit

Variable speed pumped storage (VSPS) unit has various operation modes and it is often required by the dispatcher to convert rapidly and frequently between these modes. Unlike conventional units, however, VSPS is controlled by both excitation and hydraulic system, and control of them is independent of each other. The mismatch between the rapid electromagnetic response of the former and the relatively slow mechanical response of the latter could result in unfavorable transients or even conversion failure. Hence, coordination controls between excitation and hydraulic system during mode conversion is a meaningful topic but has seldom been investigated. To this end, the electromechanical transient model of VSPS considering the excitation dynamics of the converter is first established. Then, a multistage soft coordination control strategy is proposed in this article. By applying this strategy, the soft and smooth mode conversion of VSPS is achieved with favorable dynamic performances during the complete conversion process. And finally, simulation and experimental results verified the effectiveness of the strategy.

Modeling and Simulation of Mechanical and Electrical Transient of Variable-Speed Pumped Storage Units

In order to study the operation characteristics of large capacity variable-speed pumped storage units connected to the system, the electromechanical transient model is established. Firstly, based on the basic equations of the doubly-fed machine, the generator model represented by an equivalent voltage source is built. Secondly, on the premise of coupling term cancellation, the frequency conversion controller, frequency converter and generator connection module are normalized. Then, the generator model and variable frequency controller model are integrated to form the electromechanical transient model of the unit, which is not only the fitting of external characteristics but also retains the dynamic process of the motor flux linkage and the dynamic characteristics of the controller’s inner loop. Finally, the electromechanical transient model is simulated on the RTDS platform. The simulation results show that the model is correct and effective, which is of great significance to the study of transient problems of large-capacity doubly-fed units connected to the grid.

Control of the Variable-Speed Pumped Storage Unit-Wind Integrated System

The integration of variable-speed pumped storage unit (VS-PSU) guarantees an efficient peak regulation and frequency modulation of the power grid. The present research analyzes the active power flow of the VS-PSU under synchronous, subsynchronous, and supersynchronous speed in both generation and pumping modes. The control strategy of the VS-PSU is realized by the dq-axis vector control method. Furthermore, the control of the VS-PSU integrated with wind power has been conducted using DIgSILENT platform with the VS-PSU of 300 kW capacity. Results show that the fluctuation of the power output of the wind power plant can be reduced effectively. The safe, economical, and stable operation of power system integrated with wind power can be achieved by the significant contribution of the VS-PSU.

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

Pumped storage units (PSU) as the most efficient energy storage device (ESD) play an important role in absorbing renewable energy (RE) sources. Nevertheless, pump-turbines have inherent S-shaped characteristics caused by their reversible design which seriously threaten the operation stability of PSU. In recent years, the variable-speed pumped storage units (VSU) have been proposed as an alternative to the fixed-speed pumped storage units (FSU). Therefore, it is a meaningful subject to evaluate the operation advantage of VSU, which has not been adequately quantified. This paper aims to analyse the advantages of VSU in operation stability by revealing the mechanism of avoiding trapping in the S-shaped region. Firstly, a precise coupling model for VSU with nonlinear pump-turbine characteristics is established and the operation areas in turbine mode are analysed. Then, the mechanism of how the trap of S-shaped region could be avoided is revealed and the regulation performance of the control system is quantified by comparing with FSU. Based on a real Chinese VSU, three scenarios are studied, including turbine start-up process, power regulation and turbine closing process. The results indicate that the S-shaped region could be escaped effectively by operating the VSU. The periodic oscillations times and setting time index of rotational speed in turbine start-up process are 0.5 and 20.74 s respectively, which are much smaller than the values of FSU. VSU outperforms the FSU in the aspect of power regulation performance: much smaller average and standard deviation of power difference for wind and load disturbance. Moreover, VSU can also obtain a significantly higher assessment in closure process: the maximum water pressure deviation is only 40% of FSU.

Effective Utilization of Parallel-Connected Megawatt Three-Level Back-to-Back Power Converters in Variable Speed Pumped Storage Units

Power electronic converter plays a significant role in the hydropower generating systems of both fixed and variable speed types. These converters are used in the form of a simple rectifier for the static dc excitation in the fixed speed system and in the form of the ac excitation (ac–ac or ac–dc–ac) in the variable speed system. The parallel-connected multiple three-level neutral-point-clamped (3L-NPC) megawatt back-to-back (BTB) converters have been adopted in recently commissioned variable speed pumped storage units (VSPSU). For such configurations, reliability and efficiency are considered to be prominent. This article proposes a methodology for effective utilization of a parallel-connected BTB power converter for VSPSU of large capacity by performing the load-dependent activation /deactivation of converters. A vector control strategy for the active and reactive power deliveries of VSPSU is formulated. This article also focusses on the losses and thermal behavior of converters at different speeds of generator in accordance with water head variations. The proposed control system is designed in such a way to decrease overall losses in the power converter system. The effectiveness of the proposed method is explained on a real scale of 250-MW doubly fed induction machine with 5 × 5 MW power converters using MATLAB/PLECS software, and experimental validation is performed through a downscaled 2.2-kW laboratory prototype.

Self-adaption Dynamic Virtual Inertia Control Strategy of Doubly fed Variable Speed Pumped Storage Unit

The power decoupling control characteristic of doubly fed variable speed pumped storage unit (VSPS) makes the rotor of VSPS unable to respond to the change of grid frequency and reduces the inertia of power system. In this paper, a self-adaption dynamic virtual inertia control strategy for VSPS is proposed. The strategy introduces correction factor to change the coefficient of control by detecting the change of dΔf/dt and automatically control the active power output of VSPS to provide dynamic frequency support for power grid. Simulation results demonstrate that by dynamically compensating the reference power of doubly fed converter, the strategy can effectively increase the frequency stability of power grid.

Research on an Output Power Model of a Doubly-Fed Variable-Speed Pumped Storage Unit with Switching Process

The doubly-fed variable-speed pumped storage (DFVSPS) is an effective method to balance the fluctuation of renewable energy generation and is an important means of frequency and voltage regulation of a power grid. Firstly, this paper introduces the structure and mathematical model of the DFVSPS unit. Secondly, the control methods of each switching stage in generating mode and pump mode are proposed, and the simulation study of each stage of DFVSPS switching process is carried out by MATLAB/Simulink. Thirdly, when studying the regulating effect of DFVSPS unit in the power system, due to the high switching frequency of converter in the electromagnetic transient model, the simulation speed is very slow and the data volume is large, so the model of DFVSPS unit needs to be simplified. By analyzing the dynamic behavior of the pumped storage power station, the mathematical model of output power of the DFVSPS unit is established, which includes start-up stage, load ramping stage, stable operation stage, load rejection stage and shutdown stage of generating mode and pump mode. Finally, the simplified model of DFVSPS unit is applied to a simple power system to verify its regulating effect on grid power.

Modelling and Simulation of Doubly-Fed Variable-Speed Pumped Storage Unit

On the basis of the mathematical model of doubly-fed variable speed pumped storage unit, the paper proposes a coordinated control strategy between the AC excitation system and governor. Based on the operation curve of pump-turbine, the optimal speed and gate opening of doubly-fed variable speed pumped storage unit are determined by active power command and real time water head. Simultaneously, the method taking into account of active power command and frequency fluctuation will make doubly-fed variable speed pumped storage unit to automatic regulate the frequency rapidly. Finally, the doubly-fed variable speed pumped storage unit is simulated by Real Time Digital Simulator (RTDS) software under the generating and pumping mode to validate and analyse the operational characteristics and performance.