Joni Klüss

The Smart Solution for the Prediction of Slowly Developing Electrical Faults in MV Switchgear Using Partial Discharge Measurements

An electrical fault in switchgear results in interruption of power supply, damage to equipment, and poses a hazard to personnel. This paper focuses on the detection of slowly developing faults leading to internal arc, using online monitoring technologies in medium-voltage switchgear. Unconventional radio-frequency (RF) techniques for discharge measurement are highly attractive but have not been widely applied in the industry due to their ineligibility to quantify actual discharge. On the basis of various benefits, a new application of a differential electric field ( D-dot) sensor for partial-discharge (PD) measurements has been introduced in this paper. The reliability of the sensor has been confirmed through comparison with a commercial high- frequency current transformer. An attempt has been made to quantify the apparent charge of online PD measurements. The energy of signal captured by the D-dot sensor has been compared with the apparent charge quantity calculated from current pulse measured by the conventional method. A second degree polynomial relation exists between the cumulative energy and apparent charge. It has been shown that when apparent charge is plotted against the cumulative energy of the RF signal for a number of pulses, defects can be separated on the basis of cluster positions within the scatter plot.

Preventing Bird Streamer Outages Using Alternative Tower Configurations

This paper presents methods for mitigating the destructive impact of bird excrement contamination on 110 kV power lines. In addition to the typical retrofitting techniques (cover-ups and perch management), focus is mainly on reconfiguration and alternative tower designs aimed at eliminating streamer outages. Advantages and disadvantages of existing designs are assessed and novel ideas are presented by the authors.

Impulse characteristics of 24 kV overhead line pin insulators

One of the main characteristics of the distribution grid is the reliability of medium voltage overhead lines which can be improved by appropriate insulation coordination. This paper assesses common insulator types in Estonia. Old glass pin insulators and newly installed porcelain pin insulators were tested against different lightning overvoltages (impulse flashovers) in order to determine their volt-time characteristics. Test results show convincingly that the new porcelain pin insulators can be easily physically broken when stressed by lightning impulses. This can lead to the numeral increase of outages in medium voltage overhead lines in the distribution grid.

Electric field distribution in glass and porcelain pin insulators

Distribution grid reliability depends heavily on insulation coordination of medium voltage overhead lines (OHL). Adequate knowledge of OHL insulators is needed to improve distribution grid performance. This paper discusses the distribution of electrical fields in 24 kV pin-type porcelain and glass insulators. To visualize the electric fields affecting the insulators, computer simulations were conducted. The simulations results were compared to previously conducted lightning impulse puncture withstand tests. The comparison showed that the electric stress in the insulators was not sufficient to cause puncturing, but in the laboratory tests, puncture did occur in some insulators.

Practical e-field sensors for EMP testing

The suitability of different electric field sensors compiled from commercially available standard coaxial RF connectors are compared by measuring nanosecond high voltage pulses generated by an EMP simulator. The EMP simulator is a parallel-plate transmission line connected to a high voltage pulse generator capable of producing 2-5 ns risetimes and voltages up to 100 kV. The propagating electric fields produced in this research are approximately 50 kV/m along the parallel portion of the transmission line. The electric field sensors are essentially very short monopole antennas. The output of the sensors (D-dot probes) is the first derivative of the incident electric field and requires integration to yield the final waveform. The investigated sensors include 12 different variations of ground plate mounted SMA, BNC, UHF and N type coaxial connectors with bandwidths ranging from 0.2 - 18 GHz. The reliability of the constructed electric field sensors is verified using magnetic field sensors. Despite the largely diverse characteristics of the sensors, their outputs are rather consistent indicating that all of the selected sensors are equally applicable for measuring ns-range pulses.

AC-voltage performance of field aged glass insulator strings

This paper presents the AC-voltage withstand performance of field aged 110kV transmission line glass insulator strings investigated under different environmental conditions. Insulator strings were removed from two 110 kV lines, one located near a coastal area and steel mill industry and the other located inland next to a dump. AC-voltage withstand levels were tested and leakage currents across insulator strings were measured in dry, mist and wet conditions. Results suggest that the polluted insulators from the steel mill should be replaced whereas protective measures need to be applied to the insulators at the dump to mitigate bird-related problems.

Feasibility of reduced insulation thickness in medium voltage power cable designs

Rural and suburban areas in Finland mainly implement overhead lines for distribution at the 20 kV level. Large scale outages resulting from severe storms have added pressure to develop weather-proof distribution networks. Replacement of existing overhead line systems with traditional cables and traditional installation techniques is far too expensive an operation to undertake. As such, the development of novel cable installation methods are required along with new types of cables. A new cable design with reduced insulation intended for both underground and overhead installation was investigated as part of a sustainable community research program. Although the thickness of the new cable insulation differs from IEC standard nominal values, the new cable design successfully passed large scale electrical testing according to international and national standards. Cable accessories including terminations for indoor and outdoor installations and cable joints also passed the testing routines. From the tests it was concluded that the quality of the installation work is crucial - the observed issues are not related to the insulation system but to the quality of installation and assembly. Thus, this crucial point requires particular attention in real installations.