The detection of ground faults in the rotor of brushless synchronous machines is usually a difficult task due to the absence of available measurements. This paper presents a new on-line rotor ground fault location method for brushless synchronous machines. The proposed method is able to detect and locate a ground fault in the excitation system without removing the rotor for testing. The method is based on the measurement of the exciter field current and on the analysis of a voltage. This voltage is measured at a high ohmic grounding resistor installed between the neutral of the exciter armature and ground through a slip ring. The method has been validated by computer simulations and by experimental tests using a special laboratory setup, obtaining excellent results.

There are several techniques for current measurement. Most of them are capable of measuring both alternating and direct current (AC/DC) components. However, they have severe drawbacks for rotating applications (large size, sensitivity to external fields, and low signal amplitude). In addition to these weaknesses, measured signals should be transmitted to a stationary part. In order to contribute solving these difficulties, this paper presents a sensor that can measure AC/DC simultaneously based on the electromagnetic coupling of two coils. To this aim, the measured waveform is analysed. In this paper, the design of such a sensor is presented. This design is validated through computer simulations and a prototype is built. The performance of this sensor prototype is analysed through experimental tests.

Brushless synchronous machines (BSMs) are replacing conventional synchronous machines with static excitation in generation facilities due to the absence of sparking and lower maintenance. However, this excitation system makes measuring electric parameters in the rotor challenging. It is highly difficult to detect ground faults, which are the most common type of electrical fault in electric machines. In this paper, a ground fault detection method for BSMs is proposed. It is based on an inductive AC/DC rotating current sensor installed in the shaft. In the case of a ground fault in the rotating parts of the BSM, a fault current will flow through the rotor’s sensor, inducing voltage in its stator. By analyzing the frequency components of the induced voltage, the detection of a ground fault in the rotating elements is possible. The ground faults detection method proposed covers the whole rotor and discerns between DC and AC sides. This method does not need any additional power source, slip ring, or brush, which is an important advantage in comparison with the existing methods. To corroborate the detection method, experimental tests have been performed using a prototype of this sensor connected to laboratory synchronous machines, achieving satisfactory results.

Brushless Synchronous Machines (BSM) are replacing conventional synchronous machines with static excitation in generation facilities due to the absence of sparking and lower maintenance. However, this excitation system makes challenging measuring electric parameters in the rotor. This fact highly difficult the detection of ground faults, which are the most common type of electrical fault in electric machines. In this paper, a ground fault detection method for BSM is proposed. It is based on an inductive AC/DC rotating current sensor installed in the shaft. In case of a ground fault in the rotating parts of the BSM, a fault current will flow through the rotor’s sensor, inducing voltage in its stator. Analyzing the frequency components of the induced voltage the detection of a ground fault in the rotating elements is possible. The ground faults detection method proposed covers the whole rotor and discern between DC and AC sides. To corroborate the detection method, experimental tests have been performed using a prototype of this sensor connected to laboratory synchronous machines achieving satisfactory results.

The most commonly used electrical machine for the generation of electrical energy is the synchronous machine. This machine, in order to generate electrical energy, requires that its rotor, which is rotating, feeded with direct current. To supply the rotor of a synchronous machine with direct current, a configuration widely used in the 1910s and following years, was to couple to the main shaft of the machine a direct current generator (exciter) with sufficient power to supply the rotor of the main generator. In this configuration, the DC current generated in the exciter field exits through a mechanical commutator and brushes installed on its rotor and is connected via slip rings and brushes to the rotor of the main generator. Therefore, two machines are coupled by their shafts rotating at synchronous speed, each using brushes and slip rings to connect to each other. The dust on these brushes (graphite) when worn, together with oil vapours from the bearings, generates a conductive environment which must be removed periodically to avoid untimely damage to the machines. This cleaning means hours of machine downtime and maintenance costs. To reduce the above, another configuration, also widely used in the electricity generators park, is to supply the rotor of the main generator with direct current through slip rings and brushes, taking energy from the generation busbar (alternating current), previously transforming it into direct current by means of thyristor bridges (static excitation). If it is really desired to eliminate the brushes in a synchronous machine arrangement, there is a configuration called brushless, in which a specially manufactured auxiliary synchronous machine (the armature is in the rotor), called an exciter, coupled to the same shaft, is used. Protection for brushed machines is simple and consists mainly of detecting the current flow between the rotor and earth. In brushless machines, where there is no access point to the rotor, it is more complex. Current protection systems for this type of excitation use an auxiliary ring with its brushes to access the rotor and inject an electric current to detect leakage current to ground. As it is a brush system for measurement only, not many brushes are necessary and they can be motorized, being connected only when a measurement is required (2 or 3 times a day). In this thesis is a novel system for detecting earth faults without the need to use a brush system is presented. The system is based on a rotating sensor placed on the shaft of the machine. This sensor will allow continuous monitoring of the rotor. The second objective of this thesis is to develop a location ground fault method, once it occurs. In synchronous machines, the number of poles is a function of the rotation speed. In hydraulic machines, whose rotational speed is usually low, the generator has a large number of poles. Discerning whether the fault is occurring on the AC or DC side, and if it is on the rotor, identifying the damaged pole, can mean a very considerable saving in the unavailability of the unit during repair. The use of the rotary sensor will make it possible to detect earth faults without the need for brushes. It also allows knowing if the fault happens on the AC side or on the DC side. If it happens on the DC side, it is possible to identify whether it is on one half or the other. To locate the damaged pole more precisely, a brush should be used to access the neutral point of the brushless armature. RESUMEN La máquina eléctrica más utilizada para la generación de energía eléctrica es la máquina síncrona. Esta máquina, para generar energía eléctrica, requiere que su rotor, que está en rotación, sea alimentado en corriente continua. Para alimentar en corriente continua el rotor de una máquina síncrona una configuración muy usada en los años 1910 y siguientes, fue acoplar al eje principal de la máquina, un generador de corriente continua (excitatriz), con potencia suficiente para alimentar el rotor del generador principal. En esta configuración, la corriente continua generada en la excitatriz se conecta mediante anillos rozantes y escobillas al rotor del generador principal. Por tanto, se dispone de dos máquinas acopladas por sus ejes girando a la velocidad de sincronismo y utilizando cada una de las escobillas y anillos rozantes para conectarse entre sí. El polvo de estas escobillas (grafito) al desgastarse, unido a vapores de aceite de los cojinetes, genera un ambiente conductor que debe de ser eliminado periódicamente si no se quiere ocasionar daños intempestivos en las máquinas. Esta limpieza son horas de indisponibilidad de la máquina y costes de mantenimiento. Para reducir lo anteriormente descrito, otra configuración, también muy utilizada en el parque de generación eléctrica, es alimentar en corriente continua el rotor del generador principal a través de anillos rozantes y escobillas, tomando energía de la barra de generación (alterna), transformándola previamente en corriente continua mediante puentes de tiristores (excitación estática). Si realmente se desea eliminar las escobillas en la disposición de la máquina síncrona, existe una configuración denominada brushless en la que se emplea una máquina síncrona auxiliar de fabricación especial (el inducido está en el rotor), llamada excitatriz, acoplada al mismo eje. Dicha excitatriz genera energía eléctrica para la alimentación del rotor de la máquina principal que, mediante diodos rotativos fijados en el eje, transforman la corriente alterna generada en el devanado inducido de la excitatriz en corriente continua. La protección para máquinas con escobillas es simple y consiste principalmente en la detección de la circulación de corriente entre...