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In this paper, a flux density waveform controller is proposed for core loss measurements under rotating magnetic fields in the electrical steel. It is based on a vector control scheme in the synchronous dq reference frame. The proposed dq vector controller eliminates the need of commonly used harmonic compensation techniques; improving the execution and convergence times of the controller. Furthermore, it eliminates the need of filters and transformer isolators, which further reduces delays, thus improving the system's dynamic response. The performance of the controller is analyzed using simulations and validated experimentally. The proposed controller is experimentally shown to control the magnetic flux waveforms within 2% of the reference flux density signals at very high densities (up to 1.9 T) under sinusoidal conditions, and up to 1.2 T under nonsinusoidal conditions.

In this paper, an accurate two-dimensional time-stepping finite-element model of a large hydro-generator considering a nonuniform air gap for computing the magnetic-flux density and the electromagnetic forces is developed and presented. The 74-MVA industrial hydro-generator is modeled using an actual air gap obtained from a monitoring system having eight capacitive sensors installed on its stator teeth. First, the method consisted in creating geometrically the real air gap in the finite-element model. Then, the latter was experimentally validated. The study showed that the impact of the real air gap on the hydro-generator performance could be found in the electromagnetic force density and in the radial magnetic-flux density as well. Furthermore, the considered real air-gap shape contributes to induce many higher magnitude forces at rotating frequency and its multiples that can excite the natural frequencies of the generator structure. Moreover, the computed dynamic rotating net force considering poles displacement is evaluated as 29% of the average value of the static net force.

Rotational core loss data improve the accuracy of estimating electric machine core losses. The losses are measured using magnetizing yokes, which generate a rotating field. The accuracy of the measured losses is dependent on the uniform distribution of the fields for all the flux-density levels and magnetization directions in an isotropic sample in simulations. Additionally, the field distribution patterns should be the same for all the flux-density vector directions. The measurement area where the field sensors are placed should be large enough for better representation of magnetic material properties. The design should also suppress the z -component of the magnetic field and the leakage flux above the sample. A two-dimensional magnetizing yoke design with a sample measurement area of 65 mm × 65 mm on a 136 mm diameter sample is proposed to meet these requirements. The design was prototyped, and both pulsating and rotational core losses of a 0.36-mm-thick “M-19” nonoriented electrical steel were measured at a range of flux densities and frequencies.

In this paper, a two-dimensional time-stepping finite-element model of a large hydrogenerator considering field interturn short circuits (ITSCs) default is developed and presented. The study quantified the impact of the degree of rotor ITSC on the radial magnetic flux density, radial force density, unbalanced magnetic pull, and electromagnetic torque of a 74-MVA industrial large hydrogenerator with 76 poles. The proposed numerical model was validated by comparing computed and experimentally measured magnetic flux density of a chosen healthy machine; good agreement between measured and computed results is obtained. Moreover, a detection method using two search coils installed on stator teeth is used. This method allows by inspection of the peak value the quantification of the degree of ITSC. The average value of the magnetic flux density of the defective pole decreases linearly (more than 5%) with the increase of the number of ITSCs; as well, the unbalanced magnetic pull increases with a polynomial of degree 2 with the increase of the degree of ITSC.

The prognosis and health management (PHM) of hydroelectric plants are full of difficulties caused by the complexity of the hydro-generators where each machine is different and almost unique. At industrial level, several tools are used to monitor the generator condition. Among these tools, the measurement of magnetic stray flux is one which is gaining interest. This measurement is generally based on an inductive sensor and mainly mounted near the stator. The main advantages of the magnetic stray flux are the non-invasive nature and the simplicity of its implementation. In this work, the discrete wavelet transform (DWT) is used to decompose the stray flux signal. Short-Time-Wavelet-Entropy (STWE) is then applied to extract the features from the sub-bands. Finally, a variational auto-encoder (VAE) is used in an unsupervised learning process to structure the STWE signatures of more than 400 stray flux measurement collected on real hydroelectric plants. The obtained results show that the VAE has well captured the features from the wavelet entropy (WE) signatures. An analysis of the resulting latent space shows a strong correlation between a given trajectory in the reduced space and an increase of the WE.

This paper presents the analysis of the performances of an existing hydro generator of 22.5 MVA, 6.6 kV, 60 Hz, 60 poles, 396 stator slots, with 6 parallels circuits per phase. The generator is operating with 5 bypassed stator coils and 2 removed circuits. The first objective of this paper is to determine the capability of this studied generator to operate safely with rated MVA or reduced load. Analytical computations based on LABC model are carried out and the obtained results are compared to the site measured parameters.

Uprating of hydro generators requires studies to determine the potential areas of failure like hotspots in the machine core. Rotational core losses cause localized heating of the core; therefore, understanding the distribution and behavior of rotational flux in the machine is the key. This paper studies the distribution of rotational flux in a hydro generator as the core material is saturated. The percentage of rotational flux in the stator increases as the material is saturated, especially for aspect ratios above 0.6. This affects the loss distribution and total losses in the machine. Core losses at no load and full load are also computed considering the change in flux distribution, and an increase of over 50% in the total stator core losses is calculated. Moreover, experimental core loss data of nonsinusoidal flux densities in various parts of the machine at no load and full load operation are also presented.

This paper presents an insight into a digital twin concept based on finite element simulator of a large hydro generator. The digital twin is created using finite element technique to design, study, monitor and to perform tests including analysis of a large hydro generator operating in normal and both margins of the capability curve. A three-phase synchronous machine generator of 310MVA, 56 poles, 13.8kV was considered for the case study. The created model was validated according to the IEEE standard 115. The presented and discussed results are related to the air gap magnetic flux density distribution and damper bar currents for operation in both under and over excitation modes.

Core losses in an electrical machine vary depending on the type of material, frequency of operation, and flux density magnitude and waveform. Loss prediction using pulsating models is insufficient for estimating core losses in a machine, since it does not take into account rotational core losses. Moreover, superposition of pulsating losses in the radial and tangential direction overestimates losses especially at higher flux densities. The objective of this paper is to study rotational flux distribution in the stator of the generator under different operating conditions. Distribution of rotational flux in a large hydrogenerator stator is studied by plotting the aspect ratio. It was found that increasing the yoke length and reducing the air gap increases the area of the stator with rotational flux. Varying the output power showed slight changes in the rotational flux distribution. Different optimized machine designs are investigated for rotational flux distribution; moreover, rotational core losses are measured and used for a comparative study radially across the stator.

This paper proposes a flux density waveform controller based on vector control in the synchronous dq reference frame for core loss measurements under rotating magnetic fields. The proposed dq vector controller eliminates the need for realtime Fourier series based harmonic compensation caused due to the electrical steel non-linearity, improving the execution and convergence times of the controller. Furthermore, it eliminates the need for filters and transformer isolators, which further reduces the delays, thus improving the system response. The performance of the controller is analysed using simulations and validated experimentally. The proposed controller is experimentally shown to control the magnetic flux waveforms within 2% of the reference flux density signals at very high intensities (up to 1.9 T) under sinusoidal conditions, and up to 1.2 T under non-sinusoidal conditions.

This paper investigates the impact of rotor inter-turn short circuit on the radial magnetic flux density of a large hydro generator. A numerical model based on advanced 2D finite element analysis is used for computing the magnetic flux density with and without inter turn short circuit in rotor winding. At first, the simulated air gap flux density is compared to measurement in order to validate the FEA model. Then two methods based on magnetic flux density in the air gap analysis to detect this fault are presented. Simulation results performed on a 74 MVA industrial hydro generator are presented and discussed. The study showed that the magnetic flux density decreases in the defected pole with the increase of inter turn short circuits. Furthermore, the flux of the defected pole contributes less in the flux of the adjacent poles which facilitate the detection of the defected pole.

Rotational core loss data is required for core loss estimation in electrical machines. This necessitates the design of 2D magnetizers to accurately measure these losses. However, the accuracy of any 2D magnetizer is contingent on its ability to maintain a uniform field distribution in the sample, at least in the measurement area, for all flux density levels and magnetization directions. Moreover, the measurement area should be large enough to allow measurements over a wide sample area, which effectively represents the material properties. A 2D magnetizer is designed to allow a measurement area of 65 by 65 mm on a 136 mm diameter circular sample. The magnetizer was prototyped and preliminary core loss measurements presented for both pulsating and rotational core losses using M19G29 steel at 60 Hz.

In this paper, the validation of a set of parameters of a 97MVA, 50Hz, 10.3kV hydro generator obtained both by simulation from finite element analysis and by experimentation from the no load curve and the short-circuit test is investigated. As each parameter of the synchronous generator determination involves some inaccuracy due either to the methods used or to the processing of the test data, a comparison has been made with actual experimental data. Robustness of the derived d-q model is made by building a dynamic model of the synchronous generator in SimPowerSystems and compared successfully with measurement under dynamic conditions. The methodologies as well as the main hypothesis are established for the simulation. The results of this study show that finite element analysis is a good alternative to the experimental measurement for the parameter determination of synchronous generator.

Machine simulation models allow the use of embedded numerical computation techniques. The numerical model and computational accuracy is usually gauged by comparison with experimental measurements. In this paper a large hydro generator is modeled and the sensitivity of various model parameters is investigated by comparing the simulated results with experimental measurements from four units of the same design. The machine open circuit voltage was found to be very sensitive to the airgap and stator B-H curve. Moreover, for 2D simulations the effective length is also important because it should account for differences in the stator and rotor lengths, in addition to the presence of radial air ducts. A 20% variation in operation airgap between two machines of the same design induced over 18% difference in open circuit voltage. Reduction in permeability of the soft magnetic materials resulted in agreement between the numerical and measured results at saturation. Consequently for large machines, the B-H curves from the Epstein measurements are insufficient and should be adjusted accordingly.

Uprating and refurbishment of large hydro generators presents opportunities for increase in power output, efficiency and life time of the machine. Several parts of the machine can be changed depending on the need and predicted performance outcome. For the generator unit in this paper, refurbishment of the stator winding is to be done while maintaining the same stator core design, therefore a new winding pattern is proposed for the machine. This paper numerically compares the proposed new winding pattern and the old winding pattern using flux density distribution and radial electromagnetic forces in the air gap. Emphasis is given to the low frequency harmonics with the potential to excite the natural frequency of the core, causing noise and vibration. The calculated total harmonic distortion (THD) of the new winding pattern is higher than the old winding pattern by 3%. This was reflected in the increased damper bar and core losses in the machine with the new winding pattern.

Detection of lamination faults is key in the prevention of more serious generator core damage. Localized stator core faults occur frequently in rotational flux zones. Therefore, this paper studies the behavior of rotational flux in the presence of shorted laminations. Faults result in a redistribution of the magnetic field across the sample and hence the flux density distribution. Different sizes and positions of faults are simulated. Moreover a fault is imposed between two laminations and the associated rotational core losses measured. Experimental results show a distortion in the measured magnetic field with a 5% difference between the normal and faulted rotational core losses samples.

Accurate alternator's parameters are important for driving, simulating, planning and protecting power systems. These parameters are related to the geometry of the machines and depend on many factors. One of the most important is the effective air gap. In this paper, the influence of the variation of the latter on transient parameters is analysed. To this end, a sudden three-phase short circuit is simulated using a 3D FEM approach coupled to the electric circuit elements and the classical lumped parameters are determined for a variation of the air gap up to 20% of the manufacturer given value. The methodology, from modeling of the short-circuit test to the processing of the waveforms, in relation to previous works, is exposed.

Hydropower units are subject to several forces and couples, under certain conditions they can cause noise, vibration, and interaction problems with the electric network or even equipment failure. The objective is to develop numerical models of the hydroelectric generator taking into consideration the shaft interaction in order to study the behavior of the generator. An analytical method is implemented in order to study the interaction of the oscillation of the mechanical couple with the network, and its influence on the electromagnetic couple. A finite element model is developed in order to study the impact of eccentricity on the generator, which is the origin of unbalanced magnetic pulls. Depending on the degree of eccentricity, the resulting electromagnetic force can be significant. Finally, the aim of this work is to contribute to the study of the dynamic behavior of the shaft in hydropower units.

Predictions of the different magnetic core losses with acceptable accuracy are key parameters for temperaturerise calculations for considered up-rate in large hydro electrical machines. In this paper, available core loss model is used to compute the electromagnetic core losses in large hydro electrical machine during no-load and short-circuit modes. The obtained results are discussed and compared with the available test data.

The predictions of the different magnetic core losses with acceptable accuracy are key parameters for temperature-rise calculations for any considered up-rate in large hydro electrical machines. In this paper, an overview of existing magnetic core loss models is reported. A new approach for the computation of the magnetic stator and rotor core loss is proposed. Finite element electromagnetic simulations were carried out on an existing hydro electrical generator rating 122.6 MVA, 13.8 kV, 60 Hz, 60 poles with 504 slots. The magnetic core losses were computed with the new proposed approach during the post processing in the stator and in the rotor. Finally, the obtained results are compared with other models and with the available test data.

The electromagnetic modeling and simulation of existing hydro electrical generators in operation always require a large number of variables to carry out an accurate study and to be able to properly validate the different test results. Simulation can be affected by parameters, such as: the model meshed density; the magnetic materials (magnetic proprieties are usually unknown); the air gap size (the design air gap is always different from the operation air gap due to the thermal expansion) and its uniformity. The excitation current calculated during simulation to achieve the proper line current depends on the air gap and the magnetic properties at the fundamental frequency and its harmonics. This paper reports a FEM simulation of an existing hydro generator of 122.6 MVA, 13.8 kV, 60 Hz, 60 poles with 504 slots, considering the magnetic material, mesh size, air gap length and excitation current as variable inputs. The impact of the modification of these parameters on the electromagnetic finite element simulation is presented, besides, the simulation results were compared with test ones.

The electromagnetic modeling and simulation of existing hydro electrical generators in operation always requires a large number of variables to carry out an accurate study and to be able to properly validate the different test results. Simulation can be affected by many parameters, such as: The air gap size (the design air gap is always different from the operation air gap due to the thermal expansion) and its uniformity. In this paper, a sensitivity analysis taking into account the air gap length and the excitation current was performed. The sensitivity analysis based on finite element simulation results was performed on an existing large hydro generator of 122.6 MVA, 13.8 kV, 60 Hz, 60 poles with 504 slots, 120 rpm. The obtained results were compared to the certain measured parameters such as the air gap flux density, line current and magnetic core losses.

The predictions of the different magnetic core losses with acceptable accuracy are key parameters for temperature-rise calculations for any considered up-rate in large hydro electrical machines. It is common practice for hydro electrical designers to compute the core losses in different section of the machine function of the flux density given only at 60 Hz by the manufacturer of the steel lamination which probably leads to inaccurate prediction of the magnetic core losses. In this paper, certain currently available core loss models are used to compute the electromagnetic core losses in large hydro electrical machine. The obtained results are discussed and compared with the available test data.

The electromagnetic modelling of existing large hydro generator is the main focus of this paper. Electromagnetic commercial software was used where all geometrical parameters of the generator are taking into account such as: pole face geometry, air gap size, the slots geometry and pattern winding sequence. The approach used to model the machine and some simulation results are presented in this paper. The analysis of a 122.6 MVA, 60 poles, 13.8 kV generator is presented as a case study on how advanced electromagnetic numerical field simulations are employed to evaluate generator parameters and performances.

The hydroelectric generation industry faces two related challenges: increasing the output the existing hydro electrical generators but without reducing the reliability of these aging facilities. Many aspect of hydroelectric generator should be considered during any up-rate study. Magnetic losses of the stator and rotor core may change significantly with any load increase. The open and short circuit core losses are major components in the efficiency and temperature rise calculations for any considered up-rate. Advanced numerical methods are used to determine different magnetic losses in the considered hydro electrical generator before any upgrade. Magnetic core losses results will be discussed in this present paper.

Some exiting generators may have more generation capability than their nameplate rating if they hold large security margin. However, before changing the limit of a generator one must do a thorough evaluation of its behavior in different conditions to extrapolate the effect of any increase. With the help of simulation models to calculate magnetic and stray losses of a machine, it is possible to couple loss distribution with the calculated I2R losses and the measured ventilation losses in a thermal model and calculate the location and the magnitude of the hotspot. These models must be validated by in situ measurements to make sure they correspond to reality. The temperature of RTD is easily accessible, but most of the time insufficient to validate detailed models. This paper presents some of the limitation of the RTD to detect axial or radial temperature distribution. Unfortunately, the copper temperature can not be measured directly, but it can be calculated by numerical models and this was done for two generators in characteristic conditions. The results are compared with temperature and airflow measurements. The models show that up-rating should not be made without understanding the exact behavior of generator as a function of load.