Search

Periodic calibrations of Energy Measurement Systems (EMS) installed in locomotives must be carried out to demonstrate the required accuracy established in the EN 50463-2 standard according to European Parliament and Council Directive 2008/57/EC on the interoperability of rail systems within the Community. As a result of the work performed in the “MyRailS” EURAMET project an AC calibration facility was developed consisting of a fictive power source was developed. This fictive power source can generate distorted sinusoidal voltages up to 25 kV-50 Hz and 15 kV-16.7 Hz as well as distorted sinusoidal currents up to 500 A with harmonic content up to 5 kHz or phase-fired current waveform stated in EN50463-2 standard. These waveforms are representative of those that appear during periods of acceleration and breaking of the train. Reference measuring systems have been designed and built consisting of high voltage and high current transducers adapted to multimeters, which function as digital recorders to acquire synchronized voltage and current signals. An approved procedure has been developed and an in-depth uncertainty analysis has been performed to achieve a set of uncertainty formulas considering the influence parameters. Different influence parameters have been analyzed to evaluate uncertainty contributions for each quantity to be measured: rms voltage, rms current, active power, apparent power and non-active power of distorted voltage and current waveforms. The resulting calculated global expanded uncertainty for the developed Energy Measuring Function calibration set up has been better than 0.5% for distorted waveforms. This paper is focused on presenting the complete set of expressions and formulas developed for the different influence parameters, necessary for uncertainty budget calculation of an Energy Measuring Function calibration.

Very fast transient overvoltages (VFTOs) due to switching operations can appear between the gas-insulated substation (GIS) enclosures and the cable sheaths and between these grounding parts and the substation Earth structures. These VFTOs achieve hundreds of kilovolts, provoking hazardous disruptive discharges for human safety. This paper includes a sensitivity analysis of the different parameters in order to establish criteria to be adopted in the grounding system design of the GIS structures and the cable connection to mitigate transient overvoltages. The use of bypass surge voltage limiters (SVLs) between the GIS enclosure and the cable sheath is also needed to avoid uncontrolled discharges. The selection of these SVL according to insulation coordination rules is presented in this paper, ensuring electrical safety without affecting the current mode of exploitation of the 220-kV GIS or the cable.

The most important advantage of voltage dividers over traditional voltage transformers is that voltage dividers do not have an iron core with non-linear hysteresis characteristics. The voltage dividers have a linear behavior with respect to over-voltages and a flat frequency response larger frequency range. The weak point of a voltage divider is the influence of external high-voltage (HV) and earth parts in its vicinity. Electrical fields arising from high voltages in neighboring phases and from ground conductors and structures are one of their main sources for systematic measurement errors. This paper describes a shielding voltage divider for a 24 kV medium voltage network insulated in SF6 composed of two resistive-capacitive dividers, one integrated within the other, achieving a flat frequency response up to 10 kHz for ratio error and up to 5 kHz for phase displacement error. The metal shielding improves its immunity against electric and magnetic fields. The characterization performed on the built-in voltage sensor shows an accuracy class of 0.2 for a frequency range from 20 Hz to 5 kHz and a class of 0.5 for 1 Hz up to 20 Hz. A low temperature effect is also achieved for operation conditions of MV power grids.

A transient analysis software for high voltage cables has been developed as an aid to partial discharge measurements carried out to assess the insulation condition of installed cables. Knowledge of the transient behavior of cable systems allow to know if a specific measuring PD system is sensitive enough to detect partial discharge pulses that can occur along a high voltage cable. Series impedances of a high voltage cable are analyzed considering its semiconducting layers using Bessel functions. The influence of semiconducting layers is also taken into account to determine the parallel impedance of a high voltage cable. A Gaussian pulse is considered as a PD pulse traveling along the cable and the inverse Laplace transform is used to determine the resulting pulse at the end of a cable length. The results presented in this paper show that attenuation and distortion of PD pulses traveling along any cable system depend largely on the resistivity and the permittivity relativity of semiconducting layers of the high voltage cables.

Partial discharges phenomenon (PD) is one of the most relevant degradation modes that affects in the gradual deterioration of the insulation elements in high voltage (HV) electrical systems, thus PD measurement is essential for their condition assessment. In the last decades on-line PD measurements have become a common technique for assessing the insulation condition of installed HV installations, although in on-line tests it is difficult to perform accurate diagnoses due to the existence of high levels of background noise and in many cases of more than one source of pulse-shaped signals. In order to overcome the difficulties mentioned, this paper proposes a method for the classification of PD signals based on a mathematical modeling. With the proposed model, representative parameters associated to the waveform of each pulse acquired are calculated so that they can be separated in different clusters. Furthermore, this mathematical model enables the reconstruction, generally with a reasonable accuracy, of the measured signals in HV installations. This approach permits to save the pulse data of a test with a low requirement of memory capacity in comparison with other approaches, that need to save all samples of each recorded pulse to visualize them and for further analysis or post-processing.