Study On Loss Of Field Process In Extra High Voltage Generator

An innovational structure of the EHV (extra high voltage) generator, that XLPE (cross-linked polyethylene) cable is used as stator windings, enables the generator to generate extra high voltage and to be connected to the power grid directly. So, it is necessary to investigate the operation of EHV generator systematically. For EHV generator, when the loss of field failure occurs, the stator current increases, temperature rise of stator windings gets higher, rotor over voltage occurs, asynchronous torque becomes larger, temperature rise of rotor surface and damping winding will also become too high to asynchronous operation. All of these reasons make asynchronous operation of EHV generators impossible. This paper studies the performance of EHV generator under loss of field, including characteristics of electrical quantities, utmost temperature and electromagnetic field.Varying parameter model of EHV generator, considering mutual leakage reactance between field winding and direct-axis damping windings, is established. After calculation of the mutual leakage reactance with the varying parameter model, it can be derived that this mutual leakage reactance cannot be ignored in transient analysis of EHV generator. According to empirical formula, eddy current of solid rotor is related with rotor slip. The slip varies with time during the process of loss of field. So this paper adopts a second-order model considering the mutual leakage reactance between field winding and direct-axis damping winding and eddy current of rotor.The loss of field of EHV generator can affect system static stability. In this paper, power characteristic and static stability limiting angle is calculated by numerical method. It is proved that stability margin of EHV generator is large. Under loss of field,circuit lump parameter of EHV generator must be different from that of rated operation. So terminal impedance is studied here and it can provide basic date for impedance relay for the loss of field protection of EHV generator.A loss of field model with varying parameters is established, and the dynamic simulation method is used to analyze the process of EHV generator loss of filed. By comparison with the experimental units, the model and simulation method are proved to be correct. And the impact on the near and the far units is analyzed after the loss of field faults of EHV generator.A two-dimension physical model of EHV generator is established, and adaptive mesh generation method is adopted to mesh the solution region, which is smarter in meshing so as to reduce the number of finite elements without sacrificing calculation accuracy. For the model consists moving part and stationary part, which will be separated by a circular arc on the middle position of the air gap, by coupling node equations on the nodes locating on the moving interface between the moving and the stationary part, finite element model considering motion can be set up. By analyzing and comparing the field distributions during synchronous operation and steady asynchronous operation, it can be concluded that the amplitudes of fundamental flux densities are the same, but the amplitudes of the harmonic flux densities are much different. Besides, it can also be concluded that the less load before the loss of field, the less harmonic flux density there is, and the air gap flux density is closer to sinusoid.The model of a two-dimensional magnetic field EHV generator is established. Magnetic distribution of the stator core and air-gap of EHV generator, during steady-state asynchronous operation, is quantitatively analyzed. And it can provide basic data for solutions to the temperature field. Based on these analyses, a three-dimensional temperature field model of EHV generator is created, which is used for analyzing temperature distribution of EHV generator under synchronous operation. By comparing the iron losses calculated in two different methods, stator segment flux density and average flux density, it can be found that difference between the two methods for iron losses calculation is small. The three-dimensional temperature model is proved to be accurate by comparing the calculated temperature rise with experimental data. The model is used to calculate the utmost temperature of stator and rotor of EHV generator under loss of field.