Large Hydro-generator Research Articles

Standstill Frequency Response Test and Validation of a Large Hydrogenerator

This paper intends to contribute to the revision process of the IEEE Standard 115 by demonstrating the applicability of the standstill frequency response (SSFR) test on large salient pole hydrogenerators. The presented SSFR tests are carried out on a 55.6-MVA salient pole machine with laminated rotor, non-continuous damper windings, and a nonintegral slot number. The IEEE-115 SSFR test procedure is applied with special care to rotor positioning as well as accurate data acquisition in the low-frequency range. The maximum likelihood estimation method is utilized for machine parameter identification from the SSFR tests. Obtained parameters are compared with design values in addition to the ones obtained using traditional “sudden no-load three-phase short-circuit,” Dalton–Cameron and “open stator d-axis transient time constant” methods. The accuracy of parameters is also confirmed by comparing the measured three-phase short-circuit current waveforms with the ones obtained by simulating the SSFR-based machine models in an electromagnetic transient -type software. Unlike previous SSFR test cases on large salient pole hydrogenerators, accurate results are obtained.

Calculation and Analysis of Rotor Thermal Static Field for Inter-Turn Short Circuit of Large Hydro-Generator Excitation Winding

Rotor winding inter-turn short circuit a common fault in hydro-generators. This fault would change the temperature, stress, and other thermal fields of a rotor and threaten the safe operation of the generator. In this paper, the Three Gorges hydro-generator is taken as an example. Mathematical models of three-dimensional temperature field and thermal stress field of rotor magnetic poles are established based on heat transfer theory and solved by finite element method. The temperature field, thermal deformation, and thermal stress distribution of magnetic poles in rotor winding inter-turn short circuit are calculated. On the basis of the calculation, the effects of the different turn numbers and positions of short circuit on the temperature, thermal deformation, and thermal stress of rotor magnetic poles are further studied. It is concluded that the thermal stress of the winding adjacent to the shorted turn would decrease, the thermal stress of the winding farther away from the shorted winding would increase, and so on. The results of this paper can provide references for inter-turn short circuit fault diagnosis and lay a foundation for the further studies of related faults.

3D Electromagnetic-Temperature Field Close-Coupling Calculation of Losses and Heat in the Damper Winding of a Large Tubular Hydro-Generator

To obtain more precise and rational calculation details of the loss and heat of a damper winding in a tubular hydro-generator, this study develops a three-dimensional, finite-element, electromagnetic-temperature field close-coupling model of the damper winding. On the basis of multi-physical field coupled theory, the model fully considers the temperature effects of the damper winding resistivities and heat conductivities, and the eddy current loss in the end region of the damper winding. The model was verified by direct tests of the damper winding temperature. Unlike the conventional weak-coupling model, the proposed close-coupling model fully captures the interaction between the electromagnetic and temperature fields. Therefore, of the model more accurately and reasonably calculates the loss and heat of the damper winding than the conventional model. The proposed calculation model can properly assess the loss and heat of damper windings in large hydro-generators, which is helpful for improving the design standards of hydro-generators.

Modeling and Optimization for Connection Design of the Asymmetric-Paths Winding by Novel Combinatorial Approach

This paper proposes a novel combinatorial approach for the optimization design of the asymmetric-paths winding, to solve the design contradiction between the asymmetric degree and the tidiness of end connections. In the proposed approach, the combinatorial models for the arrangement and the connection are built. In addition, some grouping strategies are applied into the combinatorial models, and some matching strategies are provided to deal with the combination of coils among groups for the optimized arrangement. At last, the winding design of a large hydro-generator combing with the electromagnetic analysis is provide for the validation. The results show that the optimized design presents a low asymmetric degree between parallel paths as well as a tidy connection of end windings simultaneously, which are better than the existing designs. This approach provides a novel idea for the modeling and optimization design for the ac windings, especially for the asymmetric-paths windings.

Analysis on Eddy Current Losses in Stator Windings of Large Hydro-Generator Considering Transposed Structure Based on Analytical Calculation Method

Aimed at the problem that the existing calculation method of eddy current losses in stator windings of large hydro-generators cannot consider the transposed structure and circulating current, the equivalent circuit model considering the actual transposed structure and circulating current is established, and the analytical calculation method for eddy current losses of stator winding is proposed in this paper, which is suitable for different transposed structures. Then, eddy current losses of stator transposed winding in an 180MW hydro-generator is analyzed by the proposed method and validated by the finite element method. Finally, the influence of the transposed structure on eddy current losses of stator windings is analyzed quantitatively and compared with the existing calculation method. The results show that the value and distribution trend of eddy current losses in strands change a lot when considering the influence of circulating current and the actual transposed structure, and the influence is different for different transposed structures, which must be considered.

Impact of Real Air-Gap Nonuniformity on the Electromagnetic Forces of a Large Hydro-Generator

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.

Optimization of the Electric Field Distribution at the End of the Stator in a Large Generator

The electric field distribution at the end of a large hydro-generator is highly nonuniform and prone to corona discharge, which damages the main insulation and significantly reduces the service life of the hydro-generator. In order to reduce the thickness of the main insulation and the physical size of a large hydro-generator, it is necessary to understand the distribution of the electric field at the end of its stator bar. In this paper, the stator bar at the end of a large generator is simulated using the finite element method to determine the distribution of the potential, electric field, and loss at the rated voltage, as well as to elucidate the differences between the linear corona protection, two-segment nonlinear corona protection, and three-segment nonlinear corona protection structures. The influences of the arc angle, length of each corona protection layer, intrinsic resistivity of the corona protection material, and nonlinear coefficient are also analyzed. The results manifest that the angle of the stator bar should be 22.5°, the difference in resistivity between the two adjacent corona protection coatings should not exceed two orders of magnitude, and the resistivity of the medium resistivity layer should be nearly 106 Ω·m or 107 Ω·m, for an optimal design of the corona protection structure.

Fault Tolerant Operation of Parallel-Connected 3L-Neutral-Point Clamped Back-to-Back Converters Serving to Large Hydro-Generating Units

In recent years, doubly fed induction machine (DFIM) is increasingly being preferred in variable speed pumped storage power plants due to its merits like partial-rated excitation system (cost factor), unity power factor, real and reactive power controls. In large-rated DFIM (250 MW), parallel-connected three-level neutral-point clamped (3L-NPC) back-to-back converter systems are employed as rotor excitation circuit to handle slip power requirements (–10% to +8.33% in Tehri PSPP, India). The power semiconductor device failure in the rotor excitation circuit may cause serious concerns in grid stability and system reliability. To ensure the continuous operation of a generating unit, this paper aims to design a redundant parallel leg configuration with six antiparallel IGBTs for a 3L-NPC back-to-back converter structure to acquire fault tolerant operation under power semiconductor device failure. Furthermore, an efficient active current sharing control strategy with fault tolerant operation is implemented for each converter to obtain better performance of the drive system. In fault tolerant operation, fault identification is computed based on a normalized phase currents average method with less computational effort and timely identification of open switch faults in both grid side converter and rotor side converter. The circuit reconfiguration under postfault operation is performed by antiparallel IGBTs with the help of fault tolerant control scheme. The effectiveness of a designed fault tolerant control scheme is validated through MATLAB/Simulink environment for a 250-MW DFIM system with 25 MW (5-channel) power converters. To ensure the trustworthiness of the simulation, an experimental demonstration is performed through a 2.2-kW DFIM laboratory prototype with a 2-channel 3L-NPC back-to-back converter structure.

Rotor Interturn Short Circuit Impact on Large Hydrogenerator Magnetic Quantities

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.

A Comprehensive Study on Shaft Voltages and Bearing Currents in Rotating Machines

This paper puts into perspective the state-of-the art knowledge on bearing currents and diagnosis tools, which rely on shaft voltages and bearing currents phenomena. Today, shaft signals measurements are progressively included in predictive health monitoring of large turbo and hydro generators. In addition, bearing fault diagnosis through shaft signals have recently gained some attention to improve reliability and estimation of the remaining useful life in electrical drives. The main objective of this paper is to provide diagnosticians with a broad overview of the current trends in fault diagnosis through shaft signals. Therefore, the study presents a critical review of the measurements methods, advantages, and disadvantages of fault diagnosis methods, and suggests future possible enhancements for bearing health monitoring through shaft signals.

Energy efficiency test and analysis of large hydro-generators based on economic evaluation

A new evaluation method was proposed for the determination of economic operation zone of large hydrogenerators. Firstly, a large number of test data were obtained by carrying out efficiency tests of the turbine at different heads. Secondly, through data analysis, the value method of K0 and the water consumption rate were constructed to determine the economic operation zone and the optimal economic operation zone, separately. Thirdly, the validity of the proposed method was verified by applying to an actual hydrogenerator. Compared with traditional methods, this method can be used to guide the economic operation of hydropower plants.

Challenges in Modeling of Large Synchronous Machines

Machine simulation models allow the use of embedded numerical computation techniques. The finite element model accuracy is usually gauged by comparing its performance with experimental measurements. In this paper, a large hydrogenerator is modeled and the sensitivity of various model parameters is investigated by comparing the simulated results with experimental measurements. Four hydrogenerator units with the same design drawings are considered in the analysis. The effect of the model parameters was studied by considering the response of the open-circuit voltage in comparison with the measured open-circuit voltage, of the different units. Parameters considered included the effective model depth for a two-dimensional simulation, effective air gap, and effective material permeability of the B–H curve. A 20% variation in operational air gap between two machines of the same design resulted in over 18% difference in open-circuit voltage, particularly beyond the knee point. Reduction in permeability of the soft magnetic materials resulted in agreement between the simulation and measured results at saturation. Consequently, for large machines, the B–H curves from the Epstein measurements are insufficient and should be adjusted accordingly.

Effect of Damper Winding and Stator Slot Skewing Structure on No-Load Voltage Waveform Distortion and Damper Bar Heat in Large Tubular Hydro Generator

We study the influence of the damper bar pitch, the stator slot skewing degree, and the damper bar number per pole on the no-load voltage waveform harmonic distortion factor (HDF) and the damper winding heat of a tubular hydro generator. We implemented 288 different design schemes using a 36-MW fractional-slot tubular hydro generator. The following influence factors were investigated: continuous variation, variation range, variation step, and higher-order harmonics of no-load voltage. Thus, the relation among these structure parameters and the HDF, the losses, and the highest temperature of the damper bars is revealed. In addition, the calculated models and results are validated directly by test data, and measures are proposed for their improvement. The present research not only corrected the errors in the related papers we published before, but also provides guidelines for optimizing the no-load voltage waveform and decreases the damper bar losses and heat, as well as a more comprehensive, accurate, and effective reference for improving tubular hydro generator design and manufacture.

Comprehensive electrical and thermal analysis of the stress grading system of a large hydro generator

In the slot portion of a generator core the installed stator bars are covered with a semi-conductive coating to prevent partial discharge (PD) activity between the main insulation surface and the grounded laminated sheets. Typically, this outer corona protection (OCP) layer ends a few centimeters outside the slot portion. To suppress PD-activity at the end of the OCP, a field grading is mandatory for machines with a rated voltage greater than approximately 6 kV. Figure 1 shows the effect of a stress grading layer on stator bars energized to high voltage. It suppresses potential creepage discharges at the OCP ends caused by field enhancement.

Electromagnetic Vibration Simulation of a 250-MW Large Hydropower Generator with Rotor Eccentricity and Rotor Deformation

The electromagnetic vibration caused by electromagnetic force on the stator has threatened large hydro generators operating safely and stably. At the Zhexi hydropower station, the hydro generator was beset by electromagnetic vibration for a long time. Therefore, the paper provided a new method to help to find the vibration source and detect the hydro generator fault, through the combination of simulation and experiments. In this paper, the 3D stator pack structure model and the 2D hydro generator electromagnetic models under rotor eccentricity and rotor ellipse deformation conditions were built. Then, electromagnetism simulations were conducted to study the characteristics of the electromagnetic flux and electromagnetic force under different conditions by using the finite element method (FEM). Lastly, the vibration testing experiments and harmonic response simulations of stator frame were performed to present the characteristics of vibration distribution in frequency conditions. The simulation results were compared with the generator measured data to try to find out the main vibration source and guide the overhaul.

Damper Currents Simulation of Large Hydro-Generator Using the Combination of FEM and Coupled Circuits Models

A high-order model is required for an accurate simulation of the synchronous alternator spatial harmonic content. However, some hydro-generators have fractional slot per pole per phase windings, and carrying transient simulations of these machines with finite-element software often leads to unacceptable computation time. This paper details a large alternator modeling method that provides an excellent compromise between accuracy and speed. It is a combination of magnetostatic finite element method and coupled circuits model. The paper details the construction of this model and explains the procedures to identify several model parameters from experimental data as the standstill frequency response and open and short circuit tests. The effectiveness of this modeling method is validated by the study of a large 109-MVA hydrogenerator having an instrumented rotor pole. We compare transient and steady-state simulation waveforms of the damper bar currents during sudden two- and three-phase short-circuit to the experimental measures.

Influence of Rotor Radial Ventilation Ducts Number on Temperature Distribution of Rotor Excitation Winding and Fluid Flow State Between Two Poles of a Fully Air-Cooled Hydro-Generator

With the increase of the capacity of a fully air-cooled hydro-generator, the electromagnetic and thermal designs of such machine face more and more pressure. To ensure the reliability of a large hydro-generator, the thermal issue in rotor should be emphasized. In this paper, the temperature distribution in exciting windings and the fluid flowing between rotor poles are highlighted and investigated. Taking a 250 MW fully air-cooled large hydro-generator as an example, a three-dimensional solid–fluid heat transfer coupling model of the fluid and temperature was established, which includes half-axial rotors, two poles in circumferential direction, and the yoke with four radial ventilation ducts. Via investigating on the loss distribution and the heat transfer condition in the rotor, the fluid and temperature fields of half-axial rotors are calculated by using the conjugated heat transfer method. The obtained results are verified by the measured values. The influences of the velocity and temperature of fluid on its effectiveness of cooling excitation windings, and the relationship between the fluid flow velocity and the heat transferred from outer surface of rotor windings are investigated as well. Similarly, the variations of fluid temperature distribution, the flowing velocity, and the heat transfer coefficient distribution in rotors with two or three ventilation ducts are investigated. The effects of the corresponding velocity and temperature of fluid on temperature distribution within both the windward and leeward side excitation windings are also studied.

Experimental Estimation of Journal Bearing Stiffness for Damage Detection in Large Hydrogenerators

Based on experimental pieces of evidence collected in a set of twenty healthy large hydrogenerators, this article shows that the operating conditions of the tilting pad journal bearings of these machines may have unpredictable and significant changes. This behavior prevents the theoretical determination of bearing stiffness and damping coefficients with an adequate accuracy and makes damage detection difficult. Considering that dynamic coefficients have similar sensitivity to damage and considering that it is easier to monitor bearing stiffness than bearing damping, this article discusses a method to estimate experimentally the effective stiffness coefficients of hydrogenerators journal bearings, using only the usually monitored vibrations, with damage detection purposes. Validated using vibration signals synthesized by a simplified mathematical model that simulates the dynamic behavior of large hydrogenerators, the method was applied to a journal bearing of a 700 MW hydrogenerator, using two different excitations, the generator rotor unbalance and the vortices formed in the turbine rotor when this machine operates at partial loads. The experimental bearing stiffnesses obtained using both excitations were similar, but they were also much lower than the theoretical predictions. The article briefly discusses the causes of these discrepancies, the method’s uncertainties, and the possible improvements in its application.

Experimental Aspects in the Vibration-Based Condition Monitoring of Large Hydrogenerators

Based on experimental observations on a set of twenty 700 MW hydrogenerators, compiled from several technical reports issued over the last three decades and collected from the reprocessing of the vibration signals recorded during the last commissioning tests, this paper shows that the accurate determination of the journal bearings operating conditions may be a difficult task. It shows that the outsize bearing brackets of large hydrogenerators are subject to substantial dimensional changes caused by external agents, like the generator electromagnetic field and the bearing cooling water temperature. It also shows that the shaft eccentricity of a journal bearing of a healthy large hydrogenerator, operating in steady-state condition, may experience unpredictable, sudden, and significant changes without apparent reasons. Some of these phenomena are reproduced in ordinary commissioning tests or may be noticed even during normal operation, while others are rarely observed or are only detected through special tests. These phenomena modify journal bearings stiffness and damping, changing the hydrogenerator dynamics, creating discrepancies between theoretical predictions and experimental measurements, and making damage detection and diagnostics difficult. Therefore, these phenomena must be analyzed and considered in the application of vibration-based condition monitoring to these rotating machines.

Using Simplified Models to Assist Fault Detection and Diagnosis in Large Hydrogenerators

Based on experimental evidence collected in a set of twenty 700 MW hydrogenerators, this article shows that the operating conditions of large hydrogenerators journal bearings may have unpredictable and significant changes, without apparent reasons. These changes prevent the accurate determination of bearing dynamic coefficients and make the prediction of these machines dynamic behavior unfeasible, even using refined models. This makes it difficult to differentiate the normal changes in hydrogenerators dynamics from the changes created by a fault event. To overcome such difficulty, this article proposes a back-to-basics step, the using of simplified mathematical models to assist hydrogenerators vibration monitoring and exemplifies this proposal by modeling a 700 MW hydrogenerator. A first model estimates the influence of changes in bearing operating conditions in the bearing stiffnesses, considering only the hydrodynamic effects of an isoviscous oil film with linear thickness distribution. A second model simulates hydrogenerators dynamics using only 10 degrees of freedom, giving the monitored vibrations as outputs, under normal operating conditions or in the presence of a fault. This article shows that simplified models may give satisfactory results when bearing operating conditions are properly determined, results comparable to those obtained by more refined models or by measurements in the modeled hydrogenerator.