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Variable Speed Drives (VSDs) are key pieces in most of the industrial processes and thus, a secure operation must be ensured against possible harmful events such as ground faults. Fault location is a complex task that has been aggravated by the increasing presence of power converters. The switching commutations as long with the high frequency leakage currents can cause the malfunction of protection relays. This paper proposes a ground fault location method in the AC drive side of VSDs. The method is mainly based on a grounding resistor connected to the midpoint of the DC bus. Measuring the phase-to-neutral voltages of the AC drive side and the grounding resistor voltage, the method can determine the faulty phase. Moreover, it can estimate the fault position and fault resistance applying phasors equations with the fundamental and the switching frequencies. The method has been validated by means of computer simulations and experimental tests using a 140 kW power converter showing promising results for both fault position and fault severity estimations.

The increasing presence of electric drives demands the development of new detection and diagnosis methods of electrical faults. In particular, ground faults are the most common type of faults in electrical systems. Locating these faults is not trivial, as they depend not only on the position, but also on its fault resistance. This paper proposes a new method to detect and locate ground faults in the AC side of an electric drive. The method is based on the ground connection of a high value resistance between the midpoint of the DC bus and ground. By means of the study of the relations between the DC voltage on this resistance and the phase-to-neutral voltages in the AC side of the drive, the location of the fault can be determined. This method has several advantages, as it does not require to know the phase impedance or to estimate an initial ground resistance. The method has been validated by means of computer simulations and in an experimental test bench using a 140-kW power converter obtaining satisfactory results.

The use of Double Fed Induction Machines (DFIM) is widely implemented in wind farms and pumping stations due to their capacity to regulate voltage and frequency. Typically, they are connected to low-power converters to vary the machine's operational point by changing the frequency of the rotor, which is, in fact, one of the most expensive components. Nevertheless, the injection of pulsed voltages into the rotor can cause high insulation stresses, leading to the deterioration of these machines. A fault in the insulation can provoke a ground fault, potentially damaging the machine. In this context, this paper proposes a new method for detecting ground faults in power-converter fed DFIMs. The method involves placing a grounding resistor at the midpoint of the DC bus, where the voltage is then measured. The frequency response of this measurement allows detecting ground faults, not only in the DFIM's rotor, but also in its stator or even in the converter's own DC bus. To validate this method, numerous simulations were conducted in MATLAB-Simulink, complemented by experimental tests on a 7 kW Double Fed Induction Generators (DFIG) test bench. These tests yielded positive results.

With the increasing demand of renewable energy and recent research in smart Direct Current (DC) microgrids, electric systems with high presence of power electronics are gaining momentum. One piece of evidence is the emergence of ungrounded low voltage DC (LVDC) microgrids and their energy management, which are crucial for achieving high efficiency systems. However, they are difficult to protect against electrical faults, especially against Ground Faults (GF), due to the commutation frequencies and the duality of AC and DC currents that cause a lack of selectivity in conventional protection relays. In this paper a GF detection method is proposed for addressing this issue. The method discerns between the affected zone (AC or DC) by sequentially switching a grounding resistor among the different neutrals and DC midpoints of the system. The method is based on the frequency domain analysis of the voltage waveform measured on this grounding resistor. Afterwards, in the case of AC fault, the phase-to-neutral voltages in the affected zone are measured and compared with the voltage across the grounding resistor to identify the faulty phase. In the case of DC faults, the polarity of this last voltage also allows the identification of the faulty pole. To validate the method, numerous simulations and experimental tests have been performed obtaining satisfactory results.

Although frequency response analysis (FRA) is a renowned diagnostic technique in the field of power transformers, it has not gained much traction yet in the domain of rotating electrical machinery. This article presents a novel diagnostics technique that applies FRA to the detection of ground faults in the wound field poles of synchronous machines. To this end, numerous experimental tests were carried out on a testbench comprising a pole of a 40-MVA hydro-generator. The data obtained in the experimental tests was used to build a novel surface fitting-based methodology that estimates the fault position, providing the faulty turn, and that estimates the fault severity, providing the fault resistance. The outputs of the proposed fault location method are valuable to reduce the corrective maintenance times, with the aim of ensuring a proper operation of the power system at the same time that costs are optimized.