Variable Speed Operation Research Articles

Modeling and numerical simulation of a double stator induction machine for a wind energy conversion system

In this paper, modeling and numerical simulation of a dual stator induction machine for wind energy conversion system are presented. Among the many benefits of this sort of machine, which is an emerging electric multiport machine, are power segmentation, high reliability and fault tolerance. The doubly fed induction machine is a type of wind generator that has been the subject of several studies in recent years. There are many reasons for using this type of machines for a variable-speed wind turbine, such as producing high power in variable-speed operation while reducing the size of the power converters, reducing noise and making it possible to control active and reactive powers. The presence of brush-ring systems limits the operating power and requires regular maintenance, thus the idea of a cascade of doubly fed induction machine or a dual stator induction machine was developed. In this paper the twin stator induction generator will be described and then its mathematical model will be established in (d, q) reference frame. The next step involves using the model of this machine to develop a simulation program using the Matlab/Simulink environment. We will present and analyse the simulation results obtained under load change situations that describe the three operating modes of this machine (no-load, generator and motor).

Effect of Speed Linear Decrease on Internal Flow Characteristics and Pressure Pulsations of Variable-Speed Pump Turbine in Turbine Mode

Pumped storage units often deviate from the optimal operating conditions in the process of regulating new energy fluctuations. To effectively improve the performance of the units, the variable speed of the units is one of the more feasible means at present. This paper focuses on the part-load condition of turbine operation, with an emphasis on the internal flow characteristics and pressure pulsation characteristics of the pump turbine during the linear reduction of the rated speed. It is found that the streamlines in the runner become turbulent in the process of speed reduction, forming a vortex at the inlet of the runner, and the vortex scale gradually increases with the speed reduction. The vortex rope in the draft tube undergoes three types of changes during the speed reduction: helical eccentric vortex rope, vanishing vortex rope, and columnar vortex rope. Before the speed change, the low-frequency components with high amplitude exist in each flow-passing part, but gradually disappear with the speed reduction. Except for the runner, the frequency affected by rotor–stator interference of each flow-passing part increases with the decrease of speed, and the growth is most obvious in the vaneless region. The findings of this research can serve as a valuable reference for the variable speed operation of pumped storage units.

A Bayesian-interference approach to quantify degradation parameters in a water-cooled variable speed screw compressor chiller

Abstract Variable-speed screw chillers can provide effective capacity modulations in commercial buildings to reduce energy consumption, match the load requirements, and contribute to lower equivalent carbon emissions. However, variable-speed operation can also yield to degradation phenomena inside the compressor. Assessing the real-time health status of chillers and detecting anomalies are important aspects to ensure long-term operation and improve reliability. To this end, an automated accelerated life test (ALT) procedure was developed and applied to a 513 kW (145.9 RT) water-cooled variable-speed screw chiller in a laboratory test facility to experimental assess performance degradation over time. After every 1,000 operating hours of ALT, steady-state performance tests at 30%, 50%, 75% and 100% load were conducted according to AHRI Standard 550/590. Additional operating conditions were included while conducting the tests to provide a more complete assessment of the degradation trends compared to the initially measured baseline. The experimental data set was then used to quantify the performance degradation based on the impact on the screw compressor operation. A Bayesian-inference identification approach has been developed to identify changes of sensitive parameters that statistically provide evidence for the likelihood of a performance degradation of the compressor. The information was used to implement the parameters in a dynamic model which was used to perform studies to predict the degradations of a chiller. The outcomes of this study help to improve the maintenance schedule of a compressor and even the whole chiller so that the system can be operated more economically and unplanned downtime can be minimized.

Numerical study of pressure fluctuation in vaneless space of variable speed reversible pump-turbine in turbine mode at maximum head

Abstract Efficiency and grid stability of pumped storage plants with significant head variations can be enhanced by the implementation of variable speed operation. The reversible pump-turbine (RPT) operating in turbine mode is particularly prone to fatigue and vibrations when operating at maximum head conditions, primarily due to hydraulic instabilities caused by pressure fluctuations in vaneless space and thus, the performance of the RPT becomes increasingly critical under such operating conditions. It is, therefore, important to examine the pressure fluctuations in vaneless space of RPTs operating at higher heads. The findings indicate that the main source of pressure fluctuations across all operating conditions is rotor-stator interaction (RSI), where the dominant frequency is blade passing frequency (9fn ) and its harmonics.

Experimental investigation of a FSFC variable speed pump-turbine prototype – Part2: runner fatigue reduction

Abstract The demonstration of flexibility technologies to enhance ancillary services of Hydropower Plants (HPPs) is showcased by the XFLEX HYDRO H2020 European Project. The potential of Full Size Frequency Converters (FSFC) is numerically and experimentally assessed by a case study on the Z’Mutt HPP. The investigated unit is a 5 MW variable speed reversible Francis pump-turbine fed by a FSFC that provides full speed control from −100% to +100%, allowing for fast start-up and mitigation of damage caused to different components. In this context, several turbine start-up sequences with controlled guide vane opening and rotational speed are developed, implemented, and tested on prototype scale. The definition of the different sequences, their impact on penstock fatigue and further FSFC capabilities like fast active power control in pump mode and fast power reversion are presented in part 1 of this contribution. Part 2 deals with fatigue damage investigations of the runner with respect to variable speed operation in generating mode using coupled numerical and experimental techniques. Strain signals from the runner blades are extrapolated to the critical areas to assess the relative fatigue damage induced by each turbine start-up. The scaled normal stress approach is applied, which is a critical plane method, adapted for multiaxial stress states. The results reveal that runner damage is mitigated by more than two orders performing a variable speed start-up compared to a classical one with fixed speed technology. Simultaneously, the delay for active power provision is significantly shortened. Thus, the study approves that fast and frequent turbine start-ups are realizable without affecting the runner’s service life thanks to the FSFC.

Analysis of variable speed operating range of centrifugal pump with large flow and wide head variation

Abstract The advantages of variable speed centrifugal pump include its broad operating range, wide operating efficiency zone, high operational stability, and regulative characteristics of input power. When pump station has characteristics such as large flow rate changes, large pump head changes, the conventional fixed speed centrifugal pump cannot meet requirements of safe and stable operation under characteristic pump conditions. In order to ensure the safe and stable operation of the pump station within the operation range, the variable speed operation strategy of centrifugal pumps is adopted to enhance its wide amplitude adjustment capability and efficient operation. This paper uses an 8MW centrifugal pump with wide head range in China to analyze the factors that influence the operating range of pump unit in detail. The results indicate that the variable speed pump unit’s safe and effective operating range is determined by the cavitation margin, hump margin, speed range, and pump maximum input power. The hydraulic design can improve the hump safety margin and cavitation characteristics of the operating zone, and then expand the operating range of the pump unit. Meanwhile, increasing the static suction head, reducing the hump margin, controlling pump head amplitude, and appropriately increasing the maximum input power limit are the best ways to extend centrifugal pump’s operating range and improve its adjustable capability. The results can provide reference for parameter selection and hydraulic design of large variable centrifugal pump in the future.

Operating modal parameter identification of railway vehicle bogie based on k-value optimization variational mode decomposition and second-order blind identification method

Due to the variable operating conditions and strong background noise, the accuracy of modal parameter identification of the railway vehicle bogie is low. This paper proposes a method for identifying operational modal parameters by combining the K-value optimization variational mode decomposition (KVMD) and second-order blind identification (SOBI) method (KVMD–SOBI method). Firstly, the variational mode decomposition (VMD) algorithm is used to adaptively decompose a single-channel vibration signal into intrinsic mode function (IMF) components with different scale characteristics. The feature IMF components are selected through the K-value optimization method based on energy ratio and stability graph. Then, the SOBI algorithm is used to separate the matrix composed of the feature IMF components. And the mode shape and single-mode signal are obtained. Finally, the natural frequency and damping ratio of each order mode are extracted by the single-mode parameter recognition method. Through simulation and experimental verification, it is shown that the proposed method has good noise robustness and can effectively identify the first three modal parameters of railway vehicle bogies in variable-speed operation. At the same time, the feature IMF components extracted by KVMD algorithm eliminate the sensitivity of sensor installation positions and operating conditions to detection results. Therefore, this method is an excellent operational mode identification method based on a single-channel signal with good noise and working condition robustness.

A hybrid fuzzy logic-based MPPT algorithm for PMSG-based variable speed wind energy conversion system on a smart grid

Recently, wind power has gained popularity as a sustainable energy source. Wind energy conversion systems (WECSs) can accept fixed speed and variable speed (VS) operations. VS-WECSs are preferable to conventional WECSs because of their higher electricity collection capacity. Maximum power point tracking (MPPT) systems are essential for maximizing the efficiency of wind energy generation in wind turbine (WT) installations linked to power grids. This study introduces a hybrid fuzzy logic controller-based MPPT (FLC-MPPT) for WTs connected to permanent magnet synchronous generators (PMSGs) to accurately determine the maximum power output of WTs. This study employs a three-phase back-to-back converter to link a PMSG to a utility grid. The reference signals for pulse width modulation controllers comprise two-phase system currents. It constructs a converter that can transfer electrical energy in both directions using insulated-gate bipolar transistor technology and is powered by a battery. The machine-side converter uses model predictive control for the present control loop. Given the generator’s susceptibility to changes in wind conditions, this factor is of the utmost importance. A WT simulation was conducted using MATLAB/Simulink and an FLC methodology was employed. The model used a PMSG. Measurements of rotor speed, power, induced voltage, and current were taken in relation to variations in wind speed. The simulation results show that the FLC-MPPT can maximize the output power over a wide range of wind speeds with a higher efficiency of 92.6% and performance of 95.7%.

Nonlinear optimal frequency control for dynamic vibration absorber and its application

A principal limitation of passive dynamic vibration absorber (DVA) lies in its intrinsic design constraint, as it is tuned to a specific resonant frequency, rendering it less effective at mitigating vibrations over a variable frequency range. To ensure that the DVA retains its vibration reduction capability under loads with time-varying frequencies, this paper proposes a method for designing the optimal suspension frequency based on the external load frequency. The approach involves altering the stiffness of the DVA through semi-active or active control to achieve the optimal suspension frequency and the best vibration reduction performance. In this study, the ratio R between the optimal suspension frequency and the load frequency is defined, and the nonlinear characteristic of R, based on the lowest vibration transmission rate, is studied using a two degree-of-freedom DVA model. Furthermore, an optimal suspension frequency variation control strategy based on load frequency identification is presented. Combined with the typical load with time-varying frequency encountered in metro vehicles, a multi-body dynamics theoretical model and a rigid–flexible coupling dynamics model are established to verify the effectiveness of the nonlinear optimal frequency control. Finally, the full-size railway vehicle roller rig is used to verify the correctness of the principle of optimal frequency curve. The research results show that when the mass ratio is 0.1, compared with rigid suspension, the nonlinear optimal frequency control can reduce the root mean square (RMS) value of the carbody’s entire time period acceleration during variable speed operation by 62.2%, and the maximum RMS value is reduced by 83.6%. In nonlinear control, unlike passive DVA, the vibration is never higher than that of rigid suspension within a specific velocity range. Compared with linear control strategy, nonlinearity can further reduce the elastic vibration. The test results from roller rig indicate that the semi-active DVA can effectively reduce the elastic vibrations of the carbody, while also validating the correctness of the nonlinear optimal suspension frequency principle. Therefore, the nonlinear frequency control in this study can provide reference for structural vibration control under loads with time-varying frequencies.

Performance improvement of a DPC-FPID strategy with matrix converter using CDFIG in wind power system

This study focuses on optimizing energy extraction from Wind Power Generation Systems (WPGS) using a cascaded Doubly Fed Induction Generator (CDFIG) amidst variable wind speed operation. The aim of this study is to provide a new robust nonlinear control technique. The proposed method is founded on a fractional-order Proportional–Integral–Derivative controller (FPID). To well control the injection of both active and reactive power into the electrical grid and respond to the needs of consumers despite the changes in environmental conditions, a robust Direct Power Control (DPC) is employed with FPID (DPC-FPID) associated with Matrix Converter (MC). To extract the greatest aerodynamic power Maximum Power Point Tracking (MPPT), is employed. We compare the suggested method’s outcomes with traditional PID results in order to confirm and validate the findings. It can be argued that the DPC-FPID method exhibits superior performance compared to the DPC-PI method. In terms of Total Harmonic Distortion (THD), the outcomes indicate that the proposed technique utilizing FPID (1.20%) outperforms the conventional technique relying on PID (1.87%). The DPC-FPID approach gives a reduction in ripple values in comparison to the DPC-PI strategy in terms of ratios for the torque (6.25%), active power (14.28%), and reactive power (14.705%).

Effect of conversion time on the stability of mixed-flow pump in rotating speed conversion

The rotating speed conversion strategy has a significant impact on the stability of the variable speed operation of the mixed flow pump. Taking a mixed-flow pump as the research object, this paper collects the shaft vibration, pressure pulsation, and external characteristic parameters under different conversion times based on the synchronous acquisition system, and analyzes the influence of conversion time on these parameters. Finally, based on the improved TOPSIS evaluation method, the influence of different conversion time on the stability of the mixed-flow pump is evaluated. The results indicate that the conversion time is positively correlated with the amplitude of shaft vibration and pressure pulsation in the pump during the transition of rotating speed reduction. The local rubbing of the shaft can induce the vibration component corresponding to the high harmonics frequencies such as 2 f n, 3 f n, and 4 f n. The high correlation frequency band of the impeller outlet pressure pulsation and shaft vibration is located between 64 Hz and 120 Hz. And this frequency band is not affected by the conversion time. According to the TOPSIS evaluation results, in the transition process of speed reduction, the longer the rotating speed conversion time, the worse the stability of the mixed-flow pump during the conversion process.

Enhancing wind power with permanent magnet synchronous generator control strategies

AbstractDue to the substantial rise in wind power generation, the direct-drive permanent magnet synchronous generator has emerged as a leading technology for efficient variable speed operation, meeting grid demands effectively. This paper presents a comparative analysis of control strategies for permanent magnet synchronous generator based wind turbine using real variable wind speed data from a 2 MW of Tetouan wind farm in Morocco. The proposed approach is based on evaluating two primary control strategies: the adaptive fuzzy-proportional-integral controller and the conventional proportional-integral controller aimed at enhancing the wind turbine's output power. The simulation performed on MATLAB-Simulink indicates that pitch control mechanisms play a crucial role in optimizing power generation, also demonstrating its ability to achieve satisfactory performance.

Numerical investigation of the vorticity transportation and energy dissipation in a variable speed pump-turbine

Variable speed technology is utilized in pump turbines to enhance their operational efficiency and stability. The unsteady internal flows of a variable speed pump turbine operating at different rotation speeds are numerically investigated with SST k-ω turbulence model. Firstly, the ideal rotating speed is selected for each output. Amongst three researched output levels, the lowest entropy production consistently occurs at minimum rotation speed,1350 rpm. Under similar outputs, higher rotation speeds result in higher entropy production. Difference between various rotation speed is up to 40 %. Secondly, Liutex method is used to structure the vortex while vorticity transport equation is applied to analyze its evolution. Results show that variable speed operation can effectively reduce pressure fluctuation amplitude with approximately 20 %–40 % amplitude drop. Subsequently, the relationship between vortex rope and entropy production in the draft tube is deeply explored as it reveals that entropy production in the draft tube correlates with vortex rope evolution. Moreover, direct dissipation's entropy production and turbulence dissipation's entropy production are two primary components leading to irreversible energy loss. It is clearly revealed that the entropy production reduces with rotation speed. The relationship between entropy production and vortex rope in the draft tube is confirmed.

Review of Fault Diagnosis Methods for Induction Machines in Railway Traction Applications

Induction motors make up approximately 80% of the electric motors in the railway sector due to their robustness, high efficiency, and low maintenance cost. Nevertheless, these motors are subject to failures which can lead to costly downtime and service interruptions. In recent years, there has been a growing interest in developing fault diagnosis systems for railway traction motors using advanced non-invasive detection and data analysis techniques. Implementing these methods in railway applications can prove challenging due to variable speed and low-load operating conditions, as well as the use of inverter-fed motor drives. This comprehensive review paper summarizes general methods of fault diagnosis for induction machines. It details the faults seen in induction motors, the most relevant signals measured for fault detection, the signal processing techniques for fault extraction as well as some classification algorithms for diagnosis purposes. By giving the advantages and drawbacks of each technique, it helps select the appropriate method that could address the challenges of railway applications.

Numerical study of rotor-stator interaction in variable speed reversible pump-turbine in turbine mode at low head

Abstract Pumped storage hydropower (PSH) offers the cost-effective storage of large quantities of energy with high efficiency. Reversible pump-turbines (RPT) are used in PSPs having large head variations and where variable speed operation can enhance its efficiency and grid stability. The RPT may experience higher fatigue and vibrations due to hydraulic instabilities caused by pressure fluctuations in the vaneless space between runner blades and guide vane interaction, known as rotor-stator interaction (RSI). Because of this instability, the local components of the powerhouse vibrate severely. Therefore, there is a need to study the pressure fluctuations caused by RSI in the variable speed RPTs. The present numerical study focuses on the pressure fluctuations due to RSI in turbine mode conditions having low available head. The present study used a reduced scale high-head variable speed RPT model, and the SST k-ω turbulence model was used to carry out numerical calculations. The analysis was performed at the best efficiency point and low head operating conditions having optimized rotational speed. The analysis showed that the main source of pressure fluctuations in the RPT at all operating conditions is the RSI, where the dominant frequency is BPF and its harmonics.

Study of axial hydraulic thrust and radial force characteristics of a variable speed pump-turbine unit

Abstract When the pump turbine does variable speed operation, it can improve the operation efficiency in the turbine mode, and it can improve the automatic frequency adjustment efficiency in the pump mode. However, the hydraulic thrust will also change when the speed changes, and the magnitude of the hydraulic thrust is crucial for the safe and stable operation of the unit. The work of this paper is to analyze the changes of the axial hydraulic force and radial force with the change of the speed by numerical simulation method. It is found that the fluctuation of axial hydraulic thrust can reach about 34 t at 398.57rpm speed, 37 t at 412.16rpm speed and 21 t at 428.6rpm speed. the fluctuation of radial force can reach about 13.78 t, 14.4 t and 12.4 t. The purpose of this paper is to use the simulation results to understand the hydraulic thrust characteristics and ensure the stability of the unit to select the appropriate speed range, provide the basis for stable operation of the unit under design 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.

Vortices Evolution Characteristics and Pressure Pulsations of Variable-speed Francis Turbine

Abstract The stability of Francis turbine under low load conditions has become the focus in the power grid dominated by variable renewable energy. In this paper, the variable-speed approach is used to explore the operating characteristics of the Francis turbine, the RANS/LES hybrid turbulent model is adopted to investigate the fluid structure evolution process, and using the Q-criterion depicts the vortex cores. The results show that the numerical simulation agrees well with the experimental data acquired from the model test. It is found that the variable-speed operation can effectively eliminate the vortex ropes in the draft tube. Unexpectedly, the vortices in the runner will evolve from the separated vortices at the runner outlet for the fixed-speed Francis turbine to the inter-blade vortices at the runner inlet for the variable-speed Francis turbine in the case of small guide vane opening. It is concluded that the optimum operating range of a variable-speed Francis turbine (VSFT) should be within ±30% rated speed and 60% to 100% opening, and the pressure pulsation coefficient in the draft tube can be reduced from 7% to 1% at minimum water head, the output and efficiency of the turbine are increased by about 30% and 10% respectively.

Tacho-less spur gear condition monitoring at variable speed operation using the adaptive application of variational mode extraction

Effective condition monitoring of machine components contributes to a safer working environment for operators and assists in averting critical machinery shutdowns. In real-world industrial scenarios, fault detection must remain simplified, user-friendly and robust despite speed variations. The operational demands of machinery frequently result in speed fluctuations, leading to spectral smearing, thereby compromising the efficiency of the analysis methodology. Furthermore, the intricacies of parameter initialisation often increase the complexity of the analysis methods. This study introduces a fault diagnosis algorithm that requires minimal initialisation and operates independently of tacho pulses. The algorithm proposed in the study incorporates variational mode extraction with the maxima tracking algorithm for instantaneous frequency estimation. Hankel matrix-based selective spectral fusion is proposed to mitigate the impact of frequency tracking errors caused by transient noise. The results of spectral side-band-based fault severity analysis, conducted on an in-house spur gearbox test-bed with seeded tooth chips, underscore the superior performance of the proposed algorithm when compared to contemporary non-stationary analysis methods.

Synchronous averaging with sliding narrowband filtering for low-speed bearing fault diagnosis

The health condition of low-speed rolling bearing, such as the main bearing in wind turbines which bears the heavy dead weight and operates under variable speeds, has a big impact on the safe operation of the machinery. Therefore, damage detection of low-speed bearings plays a key role in the health management of large-scale rotating machinery. However, in popular vibration based bearing damage detection algorithms, due to the fact that the additional vibration features incurred by the low-speed operated bearing damage are typically weak in amplitude and low in frequency contents, the additional responses caused by damage are difficult to be isolated by conventional algorithms. Especially, in the cases of variable speed operations, the smearing effect caused by Fourier transform makes it more difficult to extract the damage features by spectrum analysis based methods. To deal with these issues, we developed a damage detection procedure specially designed for bearings operated at low and variable speeds. According to the dynamic properties of the vibration signals incurred by a low-speed bearing with damage, an envelope analysis method based on synchronous averaging with sliding narrow band-pass filters is designed and developed for extracting damage features in the low frequency range. The fundamental theory used in the method is derived first. Then, a damage detection signal processing procedure is constructed based on the elaborated theory. The feasibility and advantages of the proposed methodology are validated by numerical simulations as well as the measured data from a wind turbine field example.