Petri Hyvönen

Application of dielectric response measurement on power cable systems

Water treeing is one of the factors leading to failure of medium voltage XLPE cables in long-term service. Increased moisture content inside oil-paper insulated cable is not desirable. To identify water tree degraded XLPE cables or oil-paper cables with high moisture content, diagnostic tests based on dielectric response (DR) measurement in time and frequency domain are used. Review of individual DR measurement techniques in the time and frequency domains indicates that measurement of one parameter in either domain may not be sufficient to reveal the status of the cable insulation. But a combination of several DR parameters can improve diagnostic results with respect to water trees present in XLPE cables or increased moisture content in oil-paper cables. DR measurement is a very useful tool that reveals average condition of cable systems. However, it is unlikely that DR measurement will detect few, but long water trees. In addition, DR cannot locate the defect or water tree site within the cable system. Combination of DR and partial discharge (PD) measurements can improve diagnostic results with respect to global and local defects. However, it is doubtful whether PD test can identify the presence of water trees inside a cable in a nondestructive manner. Further research is needed for more detailed conclusions regarding the status of a particular insulation and for predicting the remaining life of the insulation system.

Selectivity of damped AC (DAC) and VLF voltages in after-laying tests of extruded MV cable systems

The purpose of HV after-laying tests on cable systems on-site is to check the quality of installation. The test on extruded MV cable systems is usually a voltage test. However, in order to enhance the quality of after installation many researchers have proposed performance of diagnosis tests such as detection, location and identification of partial discharges (PD) and tan /spl delta/ measurements. Damped AC voltage (DAC) also called oscillating voltage waves (OVW) is used for PD measurement in after-laying tests of new cables and in diagnostic test of old cables. Continuous AC voltage of very low frequency (VLF) is used for withstand voltage tests as well as for diagnostic tests with PD and tan /spl delta/ measurements. Review on the DAC and VLF tests to detect defects during on-site after-laying tests of extruded MV cable systems is presented. Selectivity of DAC and VLF voltages in after-laying testing depends on different test parameters. PD process depends on type and frequency of the test voltage and hence, the breakdown voltage is different. The withstand voltage of XLPE cable insulation decreases linearly with increasing frequency in log scale. Experimental studies with artificial XLPE cable model indicate that detection of defects with DAC or VLF voltage can be done at a lower voltage than with DC. DAC voltage is sensitive in detecting defects that cause a breakdown due to void discharge, while VLF is sensitive in detecting defects that cause breakdown directly led by inception of electrical trees.

Probabilistic model for MV spark-gap characteristics with lightning induced overvoltage superimposed on AC voltage

In this paper, the breakdown probability of MV spark-gaps is modeled using the Gaussian distribution function under an impulse voltage test in accordance with the IEC 60060-1 standard. The model is presented in the form of the well-known Gaussian tail probability. Accordingly, a modified probabilistic model is introduced to study the effect of impulse voltage superimposed on the AC voltage on the breakdown probability of MV spark-gaps. The modified model is verified using experimental data, where the experimental setup is arranged to generate a range of impulse voltages superimposed on the ac voltages. The results show evidence of the efficacy of the proposed probabilistic model. Furthermore, the proposed model is used to evaluate single-phase, two-phase and three-phase spark-gap breakdown probabilities in the case of lightning induced overvoltages. Finally, these breakdown probabilities are used along with the simplified Rusck expression to evaluate the performance of MV overhead lines above a perfectly conducting ground under lightning-induced voltages using a statistical approach.

Integration of online proactive diagnostic scheme for partial discharge in distribution networks

Partial discharge (PD) diagnostic is considered as the main concern while making condition assessment plan for medium voltage assets. PD detection and localization in multi-section (straight and branched power lines) MV network is difficult by using the conventional Time Domain Reflectometry and Time Difference of Arrival methods. It is due to interconnected cables of different lengths and properties which make the interpretation of PD signals quite complex as compared to single cable routes. This paper presents an online technique of PD localization in MV cable network. PD activity emits current pulses propagating away from the PD site. Polarity of the detected PD pulses with reference to polarity of the supply voltage determines the direction of arrival of these pulses. Comparison of the polarity of PD pulses identifies the faulty section of the overhead covered conductor line as well as the cable network. A detailed discussion has been presented to address the practical issues of sensor implementation and polarity comparison in order to ensure the accuracy of polarity based direction of arrival (DOA) assessment during field application. Evaluation of DOA is based on directionally calibrated sensors. Allocated induction sensors are employed along the feeder for integrated implementation of proposed technique in accordance with the distributed agent for improving the network reliability.

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.

Test setup for measuring medium voltage power cable and joint temperature in high current tests using thermocouples

Thermal aspects of power cable performance are an important field of research as power grids depend increasingly on the reliability of underground cable systems. Overheating is a possible cause of cable faults and it should be ensured that no part of the cable exceeds its rated operating temperature. A test setup was established in Aalto University to study the changes of temperature across different power cable layers in various cable operating modes. The setup also includes three different types of joints to identify differences in behavior during temperature change. Temperature measurement was achieved using thermocouples inserted into the cable and each of the joints: heat shrink joint, cold shrink joint and hybrid shrink joint. This paper discusses the structure of the test setup and the capacity of different tests available with the given setup.

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.

Influence of rainfall on lightning flashovers from trees to medium voltage bare and covered conductors

In this paper, a laboratory experiment was conducted to compare the performance of lightning discharge characteristics of bare conductor (BC) and covered conductor (CC), resulting from very-close lightning strokes to trees. Special attention is paid to the influence of rainfall on the lightning arc characteristics. All the experiments are performed using full-scale configurations. Other factor investigated is the effects of tree-to-line clearance. The studies reveal a substantial amount of new information which is required to fully understand the true behavior of very-close lightning interaction with power conductors in rainfall, which is often the true lightning situation.