New developments in protections and diagnostics of synchronous machines
- Autores: Pengfei Tian
- Directores de la Tesis: Carlos Antonio Platero Gaona (dir. tes.)
- Lectura: En la Universidad Politécnica de Madrid ( España ) en 2021
- Idioma: español
- Materias:
- Enlaces
- Tesis en acceso abierto en: Archivo Digital UPM
- Resumen
Synchronous generators (SG) are widely used in global power generation. SG play a very important role in power systems for frequency and voltage regulation. The frequency regulation is carried out by balancing the consumption and the generation. Therefore, the generators active power should be adapted to the consumers requirements. On the other hand, the voltage in the power system nodes should be close to the rated voltage, but a voltage range is admissible depending on the grid code.
SG should operate uninterruptedly and in a reliable way. Despite that, the various stresses acting on the machine may lead to faults. Faults should always be detected at an incipient level in order to prevent evolution of their severity and a catastrophic machine breakdown. Among all faults, the electrical ones may take place in the stator or rotor windings and lead to high and hazardous currents, consequent overheating of the machine and degradation of the insulating materials. This will lead to even higher short current and thermal stress until the eventual machine breakdown.
During this Ph.D thesis novel protection and diagnosis techniques for synchronous machines have been developed, thank to simulations and experimental tests. Most of the research works are related to the analysis stray flux of the machines. However other techniques are related on new logic combined to conventional protection functions or machine model based diagnostic methods.
This document is organized as follows. Chapter 1 present the objectives of the Doctoral Thesis. In the Chapter 2 a novel specific protection method against faulty synchronizations protection is developed. Synchronous generator paralleling to the gird is a quite common operation. The probability of a faulty synchronization of a generator is very small, but not impossible. Faulty synchronizations of synchronous generators cause overcurrent and high electromagnetic torque values that can severely damage, not only the generators their selves, but also prime movers and step-up transformers. Moreover, they produce disturbances on the power system such as power oscillations and voltage sags that can end up collapsing the system if it is not cleared quickly. Despite that, conventional synchronous generator protection systems have not a specific function against faulty synchronizations.
In Chapter 3 SM field winding inter-turn fault detection method based on a SM model is described. During operation the SM, some electrical faults that may not be reliably diagnosed online; rotor faults are among them, including the cases of ground faults or inter-turn faults in the field winding. The latter produce an unbalanced magnetic field, in which the level of unbalance is determined by the number of shorted turns as well as by the field current, which depends on the operation point. If the fault severity level is critical, it will lead to shaft oscillations and vibrations. In spite of that, typical diagnostic methods may fail to diagnose this fault at early stages. This Chapter provides some ideas, one of them is the comparison of the real-time excitation current measured and the theorical excitation current calculated. In this part of the Thesis, the three machine model excitation calculating methods are used; Behn Eschenburg, Potier and ASA.
Afterward in Chapter 4 a new method for field winding turn-to-turn fault based on stray flux analysis is presented. This chapter proves the inefficacy of the machine current signature analysis to detect this fault, in the chapter investigates the application of the stray flux monitoring using finite element analysis and extensive experimental testing. The results clearly prove that the stray flux contains fault dependent harmonics, the amplitude of which increases monotonically with the fault severity level. The proposed approach has all the desired characteristics to be applied at site while being low cost, non-intrusive and fault severity sensitive to allow for appropriate remedy actions.
Moreover, in Chapter 5 compares the analysis of different quantities under starting conditions for the detection of field winding faults in synchronous motors. More specifically, the analysis of stray fluxes under transient conditions is compared to stator and rotor currents. First, the time-frequency maps resulting from the analyses of stray fluxes under starting are much richer that those corresponding to stator or rotor currents, that is to say, the analyses of stray fluxes can provide much more information for the diagnosis of the fault, in comparison with other quantities. Secondly, it can be based on the detection of the evolutions of the multiple components amplified by the fault.
Chapter 6, mainly discuss the type of the stray flux sensor and find the better type for metering the flux. The normal practice in electrical machine monitoring is the use of air core flux sensors. After being mounted outside of the monitored machine, the induced voltage is recorded and analyzed. In this way, the stray flux of the machine is not perturbed, and there are no problems due to saturation or nonlinear behavior of the iron. However, the induced voltage may be weak, mainly due to the high reluctance between the actual stator iron core and the sensors. Hence, some new sensors have been built with standard iron lamination used for small transformers. Although the magnetic circuit of the machine is slightly modified, as is the stray flux, the faults can be easily detected. The characteristic frequencies of the rotor and stator inter-turn fault can be clearly observed in the performed tests. On the other hand, the nonlinear behavior of the iron due to the permeability, saturation, hysteresis or eddy currents does not affect the fault detection. The analysis results suggest that the use of iron core flux sensors is advantageous. The main advantage is a significantly greater amplitude of the induced voltage for the same stray flux. This feature can be crucial for the case of large machines in noisy environments.
In the Chapter 7, a method for rotating diodes of brushless synchronous machine supervision based on the stray flux of the exciter is described. Numerous tests in healthy conditions and in faulty conditions have been performed. The results show that the detection of one open diode is reliably accomplished by the harmonic analysis of the exciter’s stray flux. The reliability of the method is not affected by the flux vector component, axial or radial. Therefore no special relative spatial positioning between the sensor and the exciter is required.
Finally in Chapter 8 the conclusions, main contributions and future works of the Doctoral Thesis are exposed.
