Large Hydro-generator Research Articles

Loss Modeling of Large Hydrogenerators for Cost Estimation of Reactive Power Services and Identification of Optimal Operation

As a result of the worldwide energy transition, reactive power generation has started to become a more scarce resource in the power grid. Until recently, reactive power has been an auxiliary grid service that classical power generation facilities have provided without necessarily allocating any cost for this valuable service. In this paper, a new approach for predicting the additional costs of reactive power services delivered by large hydrogenerators is proposed. We derive the optimal reactive power (ORP) with minimal losses as a function of the active power level within the generator's capability diagram. This pathway can then be used to calculate additional losses from operational regimes deviating from the ORP. To back up the analysis, a dedicated example study was handpicked consisting of four real-world generators scaled in terms of power rating, i.e., 15MVA, 47MVA, 103MVA, and 160MVA. The objective was to identify how the ORP scale from smaller to larger MVAsized generators. Moreover, a sensitivity analysis of the machine characteristics is conducted. We find the ratio between the rotor and stator losses as the determining factor. Finally, we show how our framework could justify profit for reactive power services, which are projected to increase in the future.

Non-Invasive Detection of Rotor Inter-Turn Short Circuit in Large Hydrogenerators by Using Stray Flux Measurement Combined With Convolutional Variational Autoencoder Analysis (CVAE)

Non-Invasive Detection of Rotor Inter-Turn Short Circuit in Large Hydrogenerators by Using Stray Flux Measurement Combined With Convolutional Variational Autoencoder Analysis (CVAE)

A simple and accurate method for estimating the stator winding real-time temperature of air-cooled hydrogenerator

In this paper, a novel and simple method is proposed for estimating the stator winding real-time temperature of the air-cooled hydrogenerator. Firstly, the structure and temperature characteristics of the hydrogenerator are analyzed. Then, a data-driven method for estimating the stator winding real-time temperature is proposed, and the implementation steps are illustrated in detail. Finally, the real-time temperature of the stator winding is estimated by the proposed method with different days, which is validated by the test. The result shows that the proposed method can well predict the temperature of the stator winding, which provides a new idea for the temperature measurement of the large hydro-generator.

Non-invasive Detection of Rotor Inter-turn Short Circuit of a Hydrogenerator Using AI-Based Variational Autoencoder

This paper presents a non-intrusive technique for detecting the Rotor Inter-Turn Short Circuit (RITSC) of a hydrogenerator using an Artificial Intelligence (AI) based Vari- ational AutoEncoder (VAE). The technique is applied to a large hydrogenerator of 74 MVA and 76 poles, to test its health monitoring and classification potential. The model is trained and validated based on the acquisition of real vibratory data collected in situ from a healthy machine. The frequency pattern of the fault in the vibration signal is obtained based on Finite Element Methods (FEM). Then, to test the sensitivity of the model in early fault detection, the signature is injected into another set of real healthy vibration signals, and the results are compared to those obtained using the traditional vibration monitoring technique. Furthermore, clustering in the latent space of the model is explored. The obtained results prove the ability of this technique and its potential in detecting anomalies at earlier stages as well as its capacity to cluster different degrees of severity of the fault in a 3D user-friendly space.

A Stator Slot Wedge Loosening Offline Detection System Based on an Intelligent Maintenance Robot of a Large Hydro Generator

Hydropower is at the backbone of low-carbon clean energy, while large hydroelectric generators play a key role in hydropower systems. In this paper, the tightness detection of stator trough wedges in the offline state of large hydroelectric generators is mainly studied. The offline state means that the generator stops running but does not need to move the stator and rotor. The traditional detection of generator stator wedges has problems, such as long maintenance intervals and low work efficiency. According to the structural characteristics of the generator, a generator maintenance robot device based on the track mechanism is designed. The device can simultaneously visualize the internal of the generator and detect the tightness of the stator slot wedge, effectively improving the maintenance efficiency. According to the electromagnetic magnitude of the stator rod in the alternating magnetic field, different stator slot wedge models are built for tightening, slightly tightening and loosening. For the conventional slot wedge loosen detection, there is a problem that the characteristic parameter is single, and the state of the slot wedge cannot be fully fed back. In this paper, a method for extracting the Linear Prediction Cepstrum Coefficient (LPCC) and Mel Frequency Cepstrum Coefficient (MFCC) of percussion sound signal is proposed, and the tightness recognition of the stator slot wedge is realized by combining the BP neural network algorithm. Experimental results show that the proposed method can effectively identify stator slot wedges in different states.

Injected current regulation based stator ground fault arc suppression method for large hydro-generators

Generator stator ground faults are common, and a reliable ground fault arc suppression method is required to ensure generator safety and avoid unnecessary impact on the power system. A fault arc suppression method is proposed in this paper by regulating the neutral injected current and controlling the fault point voltage to a low level. Considering the required injected current is different for different fault positions on the stator winding, a fault location method and a fault winding potential calculation method are proposed. In addition, a fault type discrimination method based on the neutral fundamental voltage variation is proposed to distinguish temporary and permanent faults. In this paper, the grounding transformer impedance and irregular distribution of the stator grounding capacitance are considered. The PSCAD/EMTDC simulation results verify that the proposed method can effectively realize arc suppression at different fault scenes and the ground fault type can be distinguished effectively.

Research on Control of Levitation Force and Torque of a Maglev Device for Water-Turbine Generator Set

Hydropower generation is clean, pollution-free, and renewable, and has good social and economic benefits, so it is given priority for development throughout the world. The capacity of hydropower stations is increasing to 1000 MW from 700 MW. As the p value on the bearing reaches a new height, coupled with the original risk of easy damage, the thrust bearing faces new technical challenges. Maglev technology is studied and applied to a large vertical-shaft hydro-generator set to solve the bearing problem. The maglev device is designed, and the working principle is expounded, using active-control repulsive-suspension technology. The levitation-force addition and the torque cancellation are realized by controlling the frequency of the excitation power supply. The dynamic mathematical models of levitation force and torque are derived. Combined with the design and theoretical analysis, the vector-control strategy is developed and the simulation analysis is completed. According to the results, the controller is improved to enhance the response performance. Finally, a control experiment is carried out on the prototype, and the results verify the effectiveness of the design and control strategy.

Study on Dynamic Characteristics of Single-Phase Grounding Fault of 1000 MW Hydro Generator under Different Grounding Modes

The continuous operation of 1000 MW large hydro generators in China is of great value to the implementation of “double carbon strategy” and power grid safety. However, with the continuous increase of single unit capacity of the hydro generator, the problems of unit fault grounding current and neutral point voltage drift become more and more prominent, and the traditional analysis methods cannot accurately obtain the fault dynamic data. Therefore, starting from the neutral point grounding mode of the large hydro generator, taking a 1000 MW hydro generator as the research object, this paper establishes the single-phase grounding fault model under the condition of three-phase parameter asymmetry of stator winding and analyzes the dynamic characteristics of fault current, transient overvoltage, and neutral point voltage drift under different grounding modes by using transient analysis method; the influence of frequency offset on neutral voltage drift and the dynamic characteristics of single-phase grounding fault of the large hydro generator under different grounding modes are also obtained. Finally, according to the analysis results, the selection and optimization method of neutral grounding parameters are proposed, which provides a reference for the selection of neutral grounding mode and the research of protection strategy of the large hydro generator.

Discussion of Third Harmonic Voltage Characteristics of Large Hydro Generator Stator Winding and Improvement of Ground Protection

Large hydro-generators require a 100% protected area for stator ground protection. Because the fundamental voltage criterion has low sensitivity near the neutral point, a high sensitivity third harmonic voltage criterion is needed to ensure that the generator can accurately respond to stator ground fault. This paper analyzes the third harmonic voltage characteristics of two large hydro-generators, calculates the sensitivity of the existing third harmonic criterion, and points out that the impedance parameters of the grounding transformer should be considered. In view of the lack of sensitivity of the third harmonic criterion for large hydro-generators, this paper proposes a stator ground protection criterion based on the third harmonic voltage variation, which has zero action quantity under the generator non-fault conditions and zero braking quantity after a ground fault occurs, and thus has high sensitivity and can effectively respond to high-resistance ground faults. PSCAD/EMTDC simulation results show that the proposed method has high sensitivity at different fault locations and transition resistances, which can provide theoretical support for the improvement of stator grounding protection in practical engineering.

Influence of Complex Fluid Flow on Temperature Distribution in the Rotor Region of Large Hydrogenerator Under the Rotor Rotation

Ventilation cooling design is one of the key technologies during the design of large hydrogenerator. With the increase of hydrogenerator capacity, the overheating problem of rotor region become more and more serious. In this paper, a 250 MW hydrogenerator is analyzed. The transient electromagnetic field of the hydrogenerator is calculated. The losses (heat sources) of rotor components in the rotor region of the hydrogenerator are determined. Three-dimensional fluid and thermal coupled mathematic model of the hydrogenerator rotor region is established. The rotor rotation of the hydrogenerator is considered. The distribution of complex fluid velocity in the rotor region is calculated using the finite volume method. The influence of fluid velocity in the different directions on the temperature of the rotor excitation winding is studied under the different flow rates in the rotor region. The surface heat-transfer coefficient distribution of the rotor components is determined. The temperature distribution of the rotor excitation winding, rotor pole body, rotor press plate, rotor damping bar, and rotor end ring is obtained. The calculated temperature results match well with test values. These provide an important reference for the rotor structural design and optimization of larger hydrogenerator.

The Energy Transition's Impact on the Accumulated Average Efficiency of Large Hydrogenerators

The energy transition is aimed to take advantage of the operational flexibility of hydropower to extend the integration of intermittent renewable sources. Consequently, the hydrogenerators will have to operate in regimes far away from their designed best-point operation. In order to accurately assess the impact, this paper presents a useful approach to determine the overall operating efficiency of synchronous generators under intermittent operation. An accumulated average efficiency (AAE) model is proposed and compared against an alternative approach; the weighted average efficiency (WAE) model. It is found that the WAE approach produces unrealistic low efficiencies when the generator operates in synchronous condenser mode (SCM) for long periods. In general, the AAE supersedes the WAE for all the different load distributions that were investigated. This was further illustrated by a worked example and by constructing more complex load distributions. A load distribution dominated by SCM yields a difference as high as 33.18 %, while an even distribution deviates 1.43 % in their respective efficiencies. Finally, a yearly on-site measurement of our studied 103 MVA generator's concentrated load distribution revealed a discrepancy of 0.67 %, which is a significant deviation considering what the operating regime would mean in terms of economic implications.

Book Reviews [7 books reviewed

The following seven books are reviewed: Handbook of Large Hydro Generators: Operation and Maintenance (Mottershead, G., et al; 2021); Heat Transfer—Principles and Applications (Forsberg, C.H.; 2021); Renewable Energy in Power Systems (Infield, D. and Freris, L.; 2020); Noise in Radio-Frequency Electronics and Its Measurement (Fouquet, F.; 2020); Polymers in Organic Electronics (Khalifeh, S.; 2020); Transient Analysis of Power Systems (Martinez-Velasco, J.A., ed.; 2020); and Lens Design Basics—Optical Design Problem-Solving in Theory and Practice (Gerhard, C.; 2020).

Efficiency Mapping and Weighted Average Efficiency for Large Hydrogenerators

This paper presents the concept of efficiency mapping for large synchronous hydrogenerators, which are useful for evaluating hydropower generation performance <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">vis-à-vis</i> interconnected intermittent generation to machine efficiency-oriented design. Tests for loss segregation at rated conditions are needed, and models for loss variation along all operational areas are presented. The weighted average efficiency is obtained as the summation of all operating load point efficiencies times related weights, defined according to criteria or from a 2D-histogram of machine operation. The proposed methodology is applied and compared for two machine designs and to an existent machine under operation.

Simulation and experimental illustration of vibration at load rejection in a continuously overloaded large hydrogenerator

Large hydro generators are usually overloaded at various situations such as the period when excess water is available and when excess inflow is to be discharged. The period may have extended to many weeks under exigent conditions e.g. six weeks in case of a 1000 MW hydropower plant (HPP) located at northern region, India at 20% overload. All the performance related parameters including vibrations were observed during commissioning except runaway speed test at load rejection. At overloads, if there is a sudden load rejection the vibration transients will be even more severe due to drop in current and internal impedance. Hence this paper aims to analyze the vibration signature of large synchronous generator (SG) when operated at overloads and subjected to sudden load rejection. The substantiation of the factual model of 250 MW hydro generating units (HGU) replicating the case plant has been simulated using SIMSEN software. Apart from case report and simulative validation an experimental verification is also conducted with a prototype model of 2.2 kW SG. This paper provides the feasibility to operate the SG's 20% beyond its rated load as designed with proper maintenance and safety carried out by plant operators.

Analysis and Treatment of an Overheated Ablated Fault on the Pole Shoe Surface of a Large Tubular Hydro-Generator

Aimed at a new kind of overheated ablated fault on the pole shoe surface of a large tubular hydro-generator, based on the real site characteristics of the fault with combination of a physical field analysis, this paper reveals the occurrence mechanism of such faults from operational conditions, generator structure, material characteristics, and so on. On the basis of the results, the fault treatment and the generator improvement strategy are proposed. In addition, the treatment method has been applied to a faulty generator, and it successfully eliminating the fault. The research is of great significance for restraining the loss and heat of poles in large tubular hydro-generators as well as improving the safety and reliability of the operation of generators and power grids.

Analysis on Thermal Behavior of Large Hydrogenerators Operating With Continuous Overloads

Large capacity hydrogenerators are operated at overloads during the period of excess water flow in the river or frequency support to the grid at peak hours. The extensive conditions may extend to six weeks with 20% overload for a typical hydropower plant located in the northern region of India. High-power, low-speed, direct-driven hydroturbine synchronous generators (HTSGs) are having resistance in rising temperature and self-cooling techniques to withstand the multiple loading points. This article presents a 250-MW CB 870/300-28 HTSG machine modeling along with electromagnetic design and simulations through a three-dimensional finite-element analysis software. The temperature distribution of the machine, operates at 10% and 20% continuous overloads, are explored via a coupled thermal and fluid-dynamical analysis. The simulated thermal results at continuous overloads are presented and are validated with a case plant's test report. A parallel study when the load goes unbalanced at overloads is done through simulation and a prototype validation of a 2.2-kW machine is substantiated in addition. The strength and flexibility of the operation of this machine assist the generalized results to operate at 20% overload continuously with the utility of thermal distribution.

Analysis of Orbital Eccentricity and UMP in Large Salient Pole Synchronous Machines

The problem with unbalanced magnetic pull (UMP), often analyzed for induction motors, is also a huge issue in large hydro generators. This paper focuses on large vertical shaft synchronous machines describing the UMP as a function of eccentricity, irrespective of its origin on the rotor, stator, or both. The static and the dynamic eccentricity impact on the air-gap length is described through an original mathematical model in which the magnetic flux density spatial distribution is amplitude modulated and the UMP is calculated applying Maxwell's tensor. This process allows including the machine saturation when calculating the no-load rated voltage. Hence, with constant rotor excitation current, corresponding to the rated generator voltage at no-load, the magnetic field amplitude variation results in a linear function of the air-gap length variation. Seven case studies are presented comparing traditional methodology with the proposed one. Two of them are also calculated by applying the finite element method (FEM). The proposed methodology gets more realistic answers if FEM results are taken as a reference. Finally, UMP triggered by a dynamic eccentricity is shown, and the resulting force that followed the orbital eccentricity is depicted clearly.

3D eddy current and temperature field analysis of large hydro-generators in leading phase operations

As a common practice, a large hydro-generator will operate in leading phase conditions to absorb the reactive power of the power grid. However, the accurate and precise prediction of the leading phase operation capacity of a large hydro-generator has always been a formidable challenge to engineers and academicians because it is extremely hard to compute the eddy currents and losses as well as the local overheating in the pressure plate and finger. To address this problem, a full three dimensional (3D) finite element model and method of the coupled eddy current and temperature fields in the end region of a large hydro-generator are developed. The equivalent medium parameters used in the computations are comprehensively discussed. Moreover, some numerically based solution methodologies for accurate computation of the field and armature currents under different leading phase conditions are proposed. Numerical results on the coupled eddy current and temperature fields in the end regions of a 250 MW hydro-generator confirm positively the feasibility of the present work.

Study on Parameter Calculation of 1000MW Hydro-generator based on Field-circuit Coupled Finite Element Method

This paper mainly presents a field-circuit coupled finite element model (FEM) of a large hydro-generator for the study of generator parameters. In the model, the two-dimensional (2D) moving electromagnetic field finite element model is coupled with the external circuit model, and some influencing factors are taken into account, such as saturation of iron-core, rotation of rotor, distribution of stator windings and end rings of damper bars, as well as end-impedance of stator and rotor windings. Based on this model, the parameters of the 1000MW hydro-generator are calculated, and the steady-state, transient, and sub-transient reactances are obtained. Furthermore, the calculation results are compared with the designed values, and some meaningful conclusions are reached. Finally, the influences of different loads and power factors on saturated synchronous reactance are discussed.

Thermal Performance Prediction in the Air Gap of a Rotor-Stator Configuration: Effects of Numerical Models

This study is dedicated to a numerical investigation of convective heat transfer on the rotor surfaces of a rotor-stator configuration that is typically found in large hydro-generators. The computational fluid dynamics calculations with two turbulence modelling approaches are used to predict the flow structure and heat transfer in the air gap of the rotor-stator configuration. The steady state mixing plane approach is employed at the interface to couple the rotor and stator components. Results show that the location of mixing plane interface in the air gap plays an important role in the prediction of heat transfer on the pole face. Also, it is indicated that the prediction of temperature distribution on the pole face is greatly affected by the turbulence models used. Furthermore, through a comparison between the pure convective and conjugate heat transfer methodologies, it is shown that the inclusion of solid domain into the numerical model significantly improves the thermal prediction of the solid components of the machine.