Large Turbogenerators Research Articles

Circuit Network Method of Short Circuit Fault Between Transposed Strands for Large Turbo-Generators

The stator windings of large turbo-generators consist of some transposed strands that are connected in parallel by a joint sleeve at the end. However, the insulation damage is easily caused by the twist of strands. Long-term high voltage, high temperature and alternating electromagnetic force also increases the probability of short circuit fault between transposed strands. A circuit network method is proposed to calculate short circuit fault between transposed strands in this paper, and the calculation of self-inductance of transposed strands in the slot is also proposed by the discrete trapezoidal method. Comparing with the finite element method, repeated modeling of short circuit faults between transposed strands is convenient and the solution is time-saving using circuit network method. The detailed modeling process of circuit network method is described. The correctness of the proposed method is verified by comparing with the finite element method.

A Stator Internal Short-Circuit Fault Protection Method for Turbo-Generator Based on Instantaneous Power Oscillation Ratio

For large turbo-generators, the longitudinal differential protection can only reflect the stator internal inter-phase short-circuit fault, and cannot reflect the inter-turn short-circuit fault. The protection methods for inter-turn short-circuit faults have the problem of low sensitivity, such as the longitudinal zero-sequence voltage protection and the negative sequence component-based protection. Therefore, a stator internal short-circuit fault protection method for turbo-generator based on instantaneous power oscillation ratio (IPOR) is proposed in this paper. The instantaneous power variation of the generator before and after the fault is analyzed, and the IPOR is calculated by using the balanced power component and the oscillatory power component. Then, the method to judge whether the generator has an internal short-circuit fault based on the amplitude and phase of IPOR is proposed. The experimental results verify the effectiveness of the proposed protection method, which has a high sensitivity to the inter-turn short-circuit faults with a small turn difference. The protection is stable and does not malfunction during the system oscillation. In addition, the action time of the protection method does not exceed 1.5 power frequency cycles under various working conditions, which can meet the speed requirements of generator protection.

Fault diagnosis of inter-turn short circuit in turbogenerator rotor windings based on vibration-current signal fusion

The inter-turn short circuit of rotor winding is a common fault of turbogenerator, and fault diagnosis in the early stage is of great significance to ensure the safe and reliable operation of turbogenerator. In this paper, an inter-turn short circuit fault diagnosis model based on vibration-current fusion is proposed by using the real-time state data collected by the distributed control system (DCS) of turbogenerators. Firstly, Pearson correlation coefficient (PCC) analysis method is used to analyze the correlation characteristics between the square of generator excitation current and the shaft radial vibration when the inter-turn short circuit occurs in the rotor winding of the turbogenerator. Then, the method of cooperative gain transformation is adopted to fuse current-vibration correlation coefficient which can amplify the weak correlation between the square of the excitation current and the vibration signal during the early defect, and the residual between the measured and predicted excitation current is combined to further calculate the cooperative gain residual, and an early warning signal can be issued when cooperative gain residual exceeds threshold. Finally, the model is verified by the historical DCS data of a 600 MW large turbogenerator. The results show that the proposed model can sensitively and accurately diagnose the early defects of inter-turn short circuit in turbogenerator rotor windings and ensure the safe operation of the generator.

Forecasting the eddy current loss of a large turbo generator using hybrid ensemble Gaussian process regression

For the issue that the sample space of wedge winding eddy current losses of large generator does not obey Gaussian distribution, a hybrid ensemble Gaussian process regression (HEGPR) model is proposed in this paper. The HEGPR contains three layers. First, four tree regression models (XGBoost, CatBoost, LGBM and NGBoost) are built. Then, the output of the first layer is taken as the input of multiple Gaussian regression models, so that the input samples of the second layer obey Gaussian distribution, which can effectively improve the generalization ability of Gaussian process regression. The results show that the root mean squared error (RMSE) is 0.0282 and the goodness of fit (R2) is 0.9973. The model has good prediction performance for the eddy current loss of large turbo generator. Compared with kinds of Gaussian process models and traditional ensemble learning models, the prediction accuracy of this model is higher, and it is more suitable for forecasting eddy current loss of the large generator. HEGPR model can effectively solve the problem of insufficient regression accuracy of Gaussian process when sample space does not obey Gaussian distribution.

Onshore and offshore turbo generators preventive maintenance program

The aim of this article is to suggest a common preventive maintenance program for large turbogenerators used on onshore or offshore, the objective is to be generic, under the intention to allow owners of different turbo generators brands a level of standardization. It motivation derives from lack of literature under this approach, either publication from the own manufacturers or old literature were found, in the time where industry 4.0 productivity and availability needs to work together, and the suggest preventive maintenance program works to collaborate on this direction

A new four‐row integral transposition method of stator windings and circulating current loss reduction in large turbo generators

Transposition of stator windings for large turbo generators is a key technology. At present, the stator windings with four-row strands for large turbo generators are usually transposed on both sides of the slot centreline and connected at the end by the joint sleeve. The different transposition methods of strands have an important effect on strand currents, circulating currents, and circulating current losses. A new four-row integral transposition method is proposed in this study. Several transposition methods based on the four-row integral transposition method are compared with the double Roebel transposition methods using 3-D time-harmonic field. The 3-D model without the end and with the end are both calculated. The results show that the four-row integral transposition method has an obvious advantage in reducing circulating current loss. Finally, a 2-D finite element model of transient magnetic field is calculated and its calculation results further support for the effectiveness of the proposed transposition method. The proposed transposition method has a great application value, which can reduce additional loss, avoid partial overheating and short circuit fault of stator windings in large turbo generators.

Grounding fault location of large turbo generator based on stator winding distribution

Stator grounding is the most common fault of a large generator. If the accurate coil of the fault can be located quickly and effectively, the maintenance progress of the generator can be greatly sped up, and the fault generator can be put back into use as soon as possible. For this purpose, a stator grounding fault location method for a large steam generator is proposed, which takes the distribution of winding potential into account. Firstly, the fault characteristics and the influence of winding potential distribution are analyzed when a single-phase ground fault occurs in a large generator, and the amplitude equation and phase equation of zero sequence voltage are constructed. On this basis, the potential distribution of stator winding can be analytically expressed, so the fault location can be directly calculated by the numerical solution method. Simulation analysis verifies the effectiveness of the proposed method.

Real-time monitoring system for rotor temperature of a large turbogenerator based on SmartMesh IP wireless network communication technology

In this study, a real-time rotor temperature monitoring system for large turbogenerators using SmartMesh IP wireless network communication technology was designed and tested. The system is capable of providing comprehensive, accurate, continuous, and reliable real-time temperature monitoring for turbogenerators. Additionally, it has demonstrated satisfactory results in a real-time monitoring test of the rotor temperature of various famous large-scale turbogenerators and giant nuclear power half-speed turbogenerators designed and manufactured in China. The development and application of this wireless temperature measurement system would aid in improving the intelligent operation quality, safety, and stability of China's large turbine generators and even the entire power system.

An Efficient Modeling of Air Gap and Nonlinear Analysis of Magnetic Equivalent Circuit for Large Turbogenerators Under No-Load Condition

Fast and accurate modeling of large turbogenerators is of great importance for optimal design and commercial value. Therefore, a 2-D nonlinear magnetic equivalent circuit (MEC) model considering saturation and rotor motion is proposed in this article. When the node-based MEC adopts relaxation iteration for nonlinear calculation, it is not easy to converge by using traditional interpolation function and piecewise fitting of the core <i>B&#x2013;H</i> or <inline-formula> <tex-math notation="LaTeX">$u_{r} (B)$ </tex-math></inline-formula> curve. The main contribution of this article is to find that using continuous fitting function is of great significance for the convergence of nonlinear iteration. At the same time, a simple and effective MEC of air gap is also provided. The flux density of air gap and stator tooth obtained by the proposed method is in good agreement with results of the finite element. No-load characteristics were also verified by the finite element and experiment. The calculation time of a cycle takes about 700 s by the finite element, and however, MEC only takes about 2 s.

Robust equilibrium optimization of structural dynamic characteristics considering different working conditions

In the design of complex products such as turbo-generators and compressors, it is very important to optimize the dynamic characteristics of the key components by moving their natural frequencies away from the excitation ones as well as their multiples to avoid resonances and resultant malfunctions. At the same time, the uncertainties such as the fluctuations of material properties inherent in the manufacture of key components will inevitably cause the variation of their structural dynamic characteristics and thus need to be fully considered in the optimization process. Considering that the robustness requirement of structures with uncertain material properties (specifically, uncertain elastic modulus and density) usually varies under different working conditions, the robust equilibrium optimization of structural dynamic characteristics considering different working conditions is investigated in this paper for the first time. Firstly, the robust optimization model for the dynamic characteristics of uncertain structures is constructed with the uncertainties described as interval variables. Subsequently, a novel robust equilibrium optimization algorithm is proposed to solve the constructed robust optimization model. Specifically, a new concept of overlap coefficient between interval boundary angles (OCBA) is proposed with the introduction of an overlap sensitivity factor, based on which the constraint robustness considering the requirements of different working conditions can be flexibly assessed. Then, the robust equilibrium strategy for all structural performance indices are proposed for the direct ranking of various design vectors, based on which the robust equilibrium optimization algorithm is developed. The validity and effectiveness of the proposed robust equilibrium optimization approach are demonstrated by realistic engineering examples including the cone ring fixture of a large turbo-generator and the compressor shell of a refrigerator.

Complex Frequency Domain Model Reduction: A Comparison of Techniques

A mathematical model of the plant to be controlled must first be identified in order to design a controller to improve the process’ performance and stability. This mathematical model usually is in the form of complex frequency domain state-space realization or continuous/ discrete time domain regression algorithm. This paper works on the complex frequency domain state-space realization. Reducing the order of the state-space model of the physical plant is usually desirable since it reduces computational time and hardware implementation for the resulting controller designed based on the model. For a frequency domain state space realization of a linear time-invariant and non-causal plant, there are numerous methods available to reduce its order. These include Balancing Model Reduction, Frequency-Weighted Balancing, Low-Frequency Approximation and Balanced Truncation with Reduced Error Bound. Besides background information and a summary, the model-reduction methodology for each of the above-said techniques is also presented. Using a two-input two-output six-state model for a large turbo-generator and a large industrial boiler with 8 states, 2 inputs and 2 outputs model, this paper provides a comparison between three of the four methodologies mentioned above in terms of the spectral norms of the model reduction error.

Investigation on the shaft voltage generation of large synchronous turboalternators

PurposeThis paper aims to go deeper on the analysis of the shaft voltage of large turbogenerators. The main interest of this study is the investigation process developed.Design/methodology/approachThe analysis of the shaft voltage because of several defects is based on a two-dimensional (2D) finite element modeling. This 2D finite element model is used to determine the shaft voltage because of eccentricities or rotor short-circuit.FindingsDynamic eccentricities and rotor short circuit do not have an inherent impact on the shaft voltage. Circulating currents in the stator winding because of defects impact the shaft voltage.Originality/valueThe original value of this paper is the investigation process developed. This study proposes to quantify the impact of a smooth stator and then to explore the contribution of the real stator winding on the shaft voltage.

Comparative analysis of electromagnetic parameters and power interactions in a large turbogenerator in asymmetric load mode

Purpose. Presentation of the results of a rotating magnetic field numerical calculation of a turbogenerator in an asymmetric load mode within its standard limitations. Methodology. The calculation of the rotating magnetic field at full rotation of the rotor is carried out by the finite element method according to the FEMM program. Calculations are automated by controlling the program using Lua script. Results. Calculations are carried out for a 325 MW turbogenerator. The temporal functions of magnetic flux linkage and the corresponding EMF of the rotor winding and phase stator windings were obtained, the power processes in the ferromagnetic core of the stator of the turbogenerator were analyzed, namely the force actions on the teeth and on the core as a whole, the pulsations of the electromagnetic moment and the time functions of its variable component. For a more detailed illustration of the results, similar calculations are performed for the symmetric load of the turbogenerator. Practical value. Analysis of the operation of the TG with an unbalanced load showed problems of power and electromagnetic nature. It is shown that the time functions of the forces acting on the teeth of the stator core are almost the same as those with a symmetrical load, but the compressive forces acting on the core have increased. It is determined that the pulsations of the electromagnetic moment reach 17% of its average value.

Evaluation of the usability of the Canay’s equivalent circuit diagrams for the calculation of subsynchronous resonances

PurposeIn the shaft and end windings of large turbogenerators, unacceptably high mechanical stresses can occur as a result of subsynchronous resonances (SSRs) in the system network-generator-shaft. These stresses can cause severe damages. Subsynchronous resonances are characterized by the occurrence of currents and electromagnetic torques in the air gap of the generator with frequencies that are significantly below the synchronous frequency. When simulating the balancing processes in multi-machine networks, the generators are represented by Canay’s equivalent circuit diagrams. The parameters used here are determined from geometric dimensions of the generator, taking into account material properties, and verified by means of surge short-circuit tests in which the 50 and 100 Hz components are dominant. This paper aims to examine whether the parameters of the equivalent circuit diagram determined in this way reproduce correctly the dynamic behavior of a synchronous machine, even if the SSR occur.Design/methodology/approachThe simulation program NETOMAC is used to simulate the SSRs for different parameters. The results of these simulations are then compared with those obtained by the finite difference (FD) method calculations.FindingsThe comparison of the waveforms calculated with NETOMAC and FELMEC for an SSR shows that the original equivalent circuit diagram parameters provide satisfactory results. An extension of Canay’s equivalent circuit diagram is not necessary. Optimization of the discussed parameters leads to a significant improvement in comparison to the calculation with the parameters from the generator data sheet.Originality/valueThe unresolved doubt has been proven, that the Parka generator model with the manufacturer’s parameters can also be used for subsynchronous studies of electromechanical resonances of systems. However, it was advisable to improve the simulation results by optimizing the generator parameters used in the calculations. By optimizing the parameters for the SSRs, the calculation of the occurring torques has been significantly improved.

Practical model of the axial magnetic field in the end region of large turbo-generators

PurposeThe axial magnetic field occurs in the end-region of large turbo-generators is known to induce hot points or voltages between laminations, that may cause insulation breakdown and thus stator faults.Design/methodology/approachIt is important to dispose of simple methods for estimating the axial flux rapidly with regard to the operating point of the machine.FindingsThe authors provide a practical model of the axial magnetic field based on a simplified vector diagram. The parameters required to build the vector composition of the flux densities are assessed with a limited number of finite element method simulations of the whole end-region of the machine. These simulations were validated by an experimental test on a real turbo-generator. Then the axial flux density was simply estimated for various operating points.Originality/valueThe originality of the paper concerns the practical model of the axial magnetic field based on a simplified vector diagram.

Subsynchronous resonance oscillations mitigation via fuzzy controlled novel braking resistor model

Subsynchronous resonance (SSR) torsional torque oscillations is a problem of a great concern in the power engineering community. SSR causes torsional torque oscillations with ever-increasing magnitudes occurring in the machine shaft sections causing a premature fatigue life expenditure of the shaft metal. In this paper, dynamic braking switching strategy designed through fuzzy logic control theory and implemented via novel braking resistor model, namely chopper rectifier controlled braking resistor for tempering SSR torsional torque oscillations of a large turbo-generator. The proposed mitigation scheme has been tested on the IEEE second benchmark model for SSR studies. Comparative simulation study via MATLAB/Simulink-based modeling and simulation environment of the test model with and without the suggested mitigation regime should demonstrate its effectuality for mitigation of SSR torsional torque oscillations.

Study of shaft voltage of a simplified synchronous generator

Shaft voltage is a well-known variable in the case of large turbo-generators for its potential destructive effect. However, as it is due to magnetic asymmetries, its use as a diagnosis indicator can be very interesting. Previous works were carried out to investigate the harmonic contents of this vo ltage in the case of some usual defects such as eccentricities or rotor short circuit. However, in our knowledge, no analysis has been conducted to determine the inherent effects of each of these defects solely while avoiding any interaction with the different other parameters of the machine. To this aim, the present paper deals with the study of two simplified synchronous generators, of 2 and 4 poles, with a smooth slotless stator using a numerical model in order to quantify the inherent effect of static and dynamic eccentricities on the shaft voltage. The obtained results for both machines with each defect are presented and discussed.

Comparison of Electrical Quantity Characteristics with or without Neural Point on Load Side of 1400MVA Nuclear Power Turbo-generator

At the time being, the discussion on comparison of the current and voltage characteristics between with and without neural point on load side about the large nuclear turbo-generator is not sufficient. To investigate the current and voltage characteristics in large nuclear power turbo-generator with alteration of load impendence coefficient with or without neural point on load side, in this paper a 1400MVA nuclear power turbo-generator is selected as a research object and its two-dimensional finite element model is established. By no-load and short-circuit characteristics, the comparison of the armature current and voltage between the two-dimensional finite element method and the experimental value shows preferable consistency and the accuracy of the simulation model is verified. With or without neutral point on load side the finite element simulation model is developed to achieve three-phase current and voltage characteristics with alteration of load impendence coefficient respectively. Then on this basis, the symmetrical method and Fourier transform are harnessed to process the simulation date. Moreover, the positive sequence voltage characteristics, as well as the negative sequence voltage characteristics, is presented. The concrete conclusion in this paper has precious theoretical and practical meanings for over- and light-voltage protection in nuclear power turbo-generators as well.

Dynamic response of large turbogenerator stator end winding under fault conditions

The electromagnetic fields of a 600MW turbogenerator stator end region under three-phase short circuit and two-phase short circuit are calculated by numerical simulation base on a three-dimensional electromagnetic model. The variation of electromagnetic force density can also be obtained in the same time. Then the dynamic responses consist of deformation and stress distributions of end winding under three-phase short circuit are numerically analyzed by the fine finite element model which is set up in ABAQUS software. The presented numerical models and method can in turn be used to study the dynamic responses and safety of the end structure under various operation conditions.

Analysis of complex fluid flow and temperature field in the turbogenerator end region with magnetic shield structure under underexcitation operation

The large electromagnetic losses and high temperature rise under underexcitation operation are the main issues of large turbogenerator design. In order to study the eddy current losses, fluid velocity, and temperature distribution in the turbogenerator end region with magnetic shield structure under underexcitation operation, three-dimensional transient electromagnetic field of a 1250 MW turbogenerator end region is investigated. The losses of end parts and stator end cores (heat source) are obtained under underexcitation operation. Velocity values of different inlets and pressure value of fan outlet (boundary condition) are determined by flow network calculation. Meanwhile, three-dimensional fluid and thermal fields are calculated. The distribution of fluid flow between magnetic shields is obtained. The temperature distribution of end parts and stator end cores in the end region of large turbogenerator is studied under underexcitation operation with 0.9 pf lead. Calculation results are compared with experiment data, which provides a reliable basis for the design and operation of larger turbogenerator.