Deoartamento De Ingeniería Eléctrica

This paper presents a new selective and non-directional protection method to detect ground faults in neutral isolated power systems. The new proposed method is based on the comparison of the rms value of the residual current of all the lines connected to a bus, and it is able to determine the line with ground defect. Additionally, this method can be used for the protection of secondary substation. This protection method avoids the unwanted trips produced by wrong settings or wiring errors, which sometimes occur in the existing directional ground fault protections. This new method has been validated through computer simulations and experimental laboratory tests.

Ground faults in electrical power systems represent more than 90% of total faults. Their detection, location, and elimination are essential and must be carried out in a precise way to allow multiterminal high-voltage direct current (HVDC) cable networks to operate in stable conditions by removing only the faulty cable from service. This paper presents a new differential protection method based on the measurement of currents at both ends of the shields of power cables. This new method is cheaper and easier to set in operation compared to other protection methods that measure currents circulating in the active conductors. The values of such intensities and their polarities were evaluated to know which cable had a ground fault in a multiterminal HVDC cable network. The method was successfully validated by computer simulations, and experimental results were successfully obtained.

The location of ground faults in railway electric lines in 2 × 5 kV railway power supply systems is a difficult task. In both 1 × 25 kV and transmission power systems it is common practice to use distance protection relays to clear ground faults and localize their positions. However, in the particular case of this 2 × 25 kV system, due to the widespread use of autotransformers, the relation between the distance and the impedance seen by the distance protection relays is not linear and therefore the location is not accurate enough. This paper presents a simple and economical method to identify the subsection between autotransformers and the conductor (catenary or feeder) where the ground fault is happening. This method is based on the comparison of the angle between the current and the voltage of the positive terminal in each autotransformer. Consequently, after the identification of the subsection and the conductor with the ground defect, only the subsection where the ground fault is present will be quickly removed from service, with the minimum effect on rail traffic. This method has been validated through computer simulations and laboratory tests with positive results.

Analysis of the types of defects and the degradation modes in different insulation materials of highvoltage (HV) electrical systems has shown that the presence of partial discharges (PD) is a very common characteristic in all of them. Once PD activity is detected, the identification and localization of the associated type of defect is very important to evaluate whether the discharges are harmful or not. During the last two decades, different approaches have been developed and applied to PD measurement and processing techniques to deal with the most common problems arising in on-site and on-line measurements, such as: high levels of background noise, appearance of interferences, simultaneous presence of different PD sources and the difficulty of locating the site of the defects and to identify them. This paper proposes a measuring method together with the implementation of three signal processing tools in order to overcome all these difficulties that appear in on-site and on-line tests. The measuring method has been implemented to evaluate in an on-site practical experience the insulation condition of the power equipment in a GIS substation. After the application of the method, important insulation defects were detected, identified and localized, on time to avoid uncontrolled failures in the electrical system of the substation under test. I.

This MSc Thesis’s main objective is to determine the impact of transformer inrush currents in the grid and develop a tool that helps E.ON evaluate transformer sizing. The motivation of E.ON is to have a more precise way to estimate the energization consequences of increasing the transformer size in the grid since currently is being used as a rule of thumb that relates the transformer rated power to the short-circuit capacity of the bus at which the transformer is connected. This rule of thumb is established because they have the concern that large power transformers can cause serious inrush currents.

An adequate study of the causes and consequences of the inrush current phenomenon is conducted to determine the duration and magnitude of undesired disturbances. Python is used to design simplified transformer models based on banks of three single-phase units due to the substantial limitations of more advanced alternatives. PSCAD and Matlab Simscape Electrical, two superior transient analysis software, are used to verify the results of test simulations. The nonlinearity of the differential system of equations
required numerical methods and the utilization of the Trapezoidal Rule of Integration and the Newton-Raphson Method.

Then, different strategies are considered as potential alternatives to estimate the voltage dip at the surrounding grid, which is the primary concern for E.ON. For the grid analysis part, PSS/E Xplorer 35.3 (which is a free limited version) is utilized because PSS/E is the preferred tool at E.ON for steady-state and short-circuit calculations. All the strategies are compared and evaluated for a hypothetical energization of a distribution transformer in a reduced network correspondent to the Oland island. Some results coincide with historical data from a similar distribution network, but on-site measurements or dynamic grid studies must validate the results. The in-built Python API1 of PSS/E allows automating the process and incorporating it with the rest of the program.

As a result of this MSc Thesis, E.ON engineers will be able to use a friendly interactive tool that simulates with high accuracy the energization of a simplified three-phase transformer model and estimates the possible effect on the surrounding grid. However, they must be aware of the limitations and assumptions to make suitable sizing decisions.