E. Rebollo

Evaluation of the applicability of FRA for inter-turn fault detection in stator windings

In this paper, the applicability of the FRA technique is discussed as a method for detecting inter-turn faults in stator windings. Firstly, this method is tested in an individual medium-voltage stator coil with satisfactory results. Secondly, the tests are extended to a medium-voltage induction motor stator winding, in which inter-turn faults are performed in every coil end of one phase. Results of the frequency response in case of inter-turn faults are evaluated in both cases for different fault resistance values. The experimental setup is also described for each experiment. The results of the application of this technique to the detection of inter-turn faults justify further research in optimizing this technique for preventive maintenance.

Novel Rotor Ground-Fault Detection Algorithm for Synchronous Machines With Static Excitation Based on Third-Harmonic Voltage-Phasor Comparison

This paper presents a new algorithm for detecting ground faults in rotor windings. This location method is suitable for synchronous generators with static excitation, whose excitation field winding is fed by controlled rectifiers through an excitation transformer. This new algorithm is an improvement of an online ground-fault location method presented previously, in which the effect of the rotor capacitance has a high influence in the correct detection of the fault. The detection of the ground fault is performed by the supervision of the phase angle between two phasor voltages. The first one is the third-harmonic phasor voltage obtained from the measurement of the field voltage, which is used as a reference. The second one is the third-harmonic voltage phasor obtained by the measurement in a grounding resistance placed in the neutral of the excitation transformer. It allows not only detecting the ground fault along the field winding but also distinguishing the cases of ground fault in the rotor winding, from the cases of high influence of the rotor capacitance in healthy condition. This new algorithm has been tested with satisfactory results in a 106-MVA hydro generating unit.

Energy, Materials and Person-hour Savings through a New Decentralized Power Supply for HVAC in Large Buildings. A Case Study: A Shopping Center in Spain.

Abstract Currently, heating and cooling systems used for achieving thermal comfort in commercial buildings represent a significant energy consumption. Traditionally, power supply installations of these Heating, Ventilation and Air Conditioning Systems (HVAC) present low efficiency. In these installations the power is supplied from the substation to the HVAC system by low voltage cables. Due to the long distances of the power distribution, the system suffers considerable energy losses. This paper proposes a decentralized power supply installation based on a power distribution at medium voltage, which achieves not only significant energy savings, but also important savings in material and person-hours. These improvements entail substantial savings in cost and CO2 emissions.

New On-Line Excitation-System Ground Fault Location Method Tested in a 106 MVA Synchronous Generator

In this paper, a novel excitation-system ground-fault location method is described and tested in a 106 MVA synchronous machine. In this unit, numerous rotor ground-fault trips took place always about an hour after the synchronization to the network. However, when the field winding insulation was checked after the trips, there was no failure. The data indicated that the faults in the rotor were caused by centrifugal forces and temperature. Unexpectedly, by applying this new method, the failure was located in a cable between the excitation transformer and the automatic voltage regulator. In addition, several intentional ground faults were performed along the field winding with different fault resistance values, in order to test the accuracy of this method to locate defects in rotor windings of large generators. Therefore, this new on-line rotor ground-fault detection algorithm is tested in high-power synchronous generators with satisfactory results.

Design and construction of a laboratory bench system for the teaching and training of engineers on diagnostics of permanent magnet motors

Permanent magnet synchronous motors PMSM are widely used in industry due to their higher torque and higher power to volume ratio. Moreover they have a better dynamic performance compared to the motors with electromagnetic excitation. Despite its robustness electrical, mechanical and magnetic faults has been described. It has led to the development of specific diagnostic systems for such machines. These systems are constantly evolving according to the literature. In this situation of increasing use of PMSM it is necessary to teach and train professionals in diagnostic techniques. This paper describes the design process of an experimental bench on which can be studied all types of faults associated with the PMSM operation. The design of a PMSM prototype, which is the most important equipment of this bench, is described in more detail.

Enhancement of a high speed de-excitation system for brushless synchronous machines by large blocking voltage semiconductors

One of the main disadvantage of the synchronous machines with brushless excitation is that the field winding is not accessible for the de-excitation. The de-excitation process is very slow as the field current flows through the rotating diodes which operate in freewheel mode. Therefore in case of an internal fault, despite the correct operation of the protection relays, the machine could have severe damages. A high-speed de-excitation system for these machines was developed. The de-excitation is achieved by inserting a resistance in the field circuit by a static switch semiconductor based, obtaining a dynamic response similar to that achieved in machines with static excitation. This paper presents the improvement in the de-excitation system in the second commercial 20 MVA hydro generator in operation. In this case, high blocking voltage semiconductor was used, making the dynamic response even better, than in the first hydro generator.

Validation study of the use of MATLAB/Simulink synchronous-machine block for accurate power-plant stability studies

This paper presents results of the validity study of the use of MATLAB/Simulink synchronous-machine block for power-system stability studies. Firstly, the waveforms of the theoretical synchronous-generator short-circuit currents are described. Thereafter, the comparison between the currents obtained through the simulation model in the sudden short-circuit test, are compared to the theoretical ones. Finally, the factory tests of two commercial generating units are compared to the response of the synchronous generator simulation block during sudden short-circuit, set with the same real data, with satisfactory results. This results show the validity of the use of this generator block for power plant simulation.

New selective earth faults only current directional method for isolated neutral systems

In order to achieve total selectivity at electrical distribution networks it is of great importance to analyze the defect currents at ungrounded power systems. This information will help to grant selectivity at electrical distribution networks ensuring that only the defect line or feeder is removed from service. In the present work a new selective and directional protection method for ungrounded power systems is evaluated. The new method measures only defect currents to detect earth faults and works with a directional criterion to determine the line under faulty conditions. The main contribution of this new technique is that it can detect earth faults in outgoing lines at any type of substation avoiding the possible mismatch of traditional directional earth fault relays. This detection technique is based on the comparison of the direction of a reference current to the direction of all earth fault capacitive currents at all the feeders connected to the same bus bars. This new method has been validated through computer simulations. The results for the different cases studied are remarkable, proving total validity and usefulness of the new method.

Novel adaptive 100% stator ground fault protection based on the third harmonic measurement

The conventional 100% stator ground fault protections based on the third harmonics produce unwanted trips, mainly caused by the lack of third harmonic produced by the generators in some operations conditions. This paper presents an adaptive 100 % stator-ground-fault detection technique for synchronous machines based on the measurement of the third harmonic generated by the synchronous machine and the real-time calculation of the 3rd harmonic voltage at the machine neutral under normal conditions. The main contribution of this technique is that the sensitivity of the protection keeps constant: its operation do not depend on load, operation point or design of the generator. This technique (self-adjusting) has been validated through computer simulations under faulty conditions and 106 MVA generator measurements under normal operation.

Simplified model of brushless synchronous generator for real time simulation

This paper presents a simplified model of brushless synchronous machine for saving hardware resources in a real time simulation system. Firstly, a brushless excitation system model is described. Thereafter, the simplified transfer function of an AC exciter and rotating diodes of the brushless excitation system is estimated. Finally, the complete system is simulated, comparing the main generators voltage with both detailed and simplified excitation systems in several scenarios. These results show the accuracy of the simplified model against the detailed simulation model, resulting on an important hardware resources savings.