Uwe Schichler

A sensitive method for on-site partial discharge detection on XLPE cable joints

Many problems of XLPE-insulated cable systems are caused by internal defects of the joints which have to be mounted on-site. Faults arise from wrong assembly of slip-on accessories, impurities or voids. A suitable solution for a monitoring of cable joints during the after-laying test and service is a sensitive on-site partial discharge (PD) detection. This can be realized in an advantageous way with two rogowski coils, which are placed on both sides of the joint. The addition of the two wide-band measured PD signals will suppress the PD-like noise from outside the joint and only detect the PD caused within the joint.

Partial discharges at DC voltage - measurement and pattern recognition

Knowledge of the current status of the components of an electrical power supply is essential for power grids to operate safely and economically. This means that the partial discharge diagnosis is of great importance for AC grids. It also plays an important role in connection with the increase in HVDC technology; the PD diagnosis is important for the condition assessment of the involved components. The AC PD measurement, the measurement procedure, and especially the interpretation of results cannot fully be adapted to DC voltage. In this paper, the previous findings of investigations about the PD behavior of some typical defects under different artificial environmental conditions and different measuring procedures are summarized. To analyze the numerous measured PD sequences, a software tool called “PDDC Analyzer” was developed. With this tool, well-known DC PD graphs can be shown and new methods for creating a DC PD pattern for defect identification and risk assessment can be investigated. The quality of the improved NoDi* Pattern is shown and the measurement procedures are discussed. At the end, new methods and aspects for PD identification at DC are introduced and a short overview of further research is given.

GIS lifetime analysis based on failure frequency

The major and minor failure frequency of gas-insulated switchgear is the basis for utilities and asset managers buying GIS and organizing maintenance as also for manufacturers improving their GIS quality and reliability. The time-dependent failure frequency of GIS populations can be investigated with respect to ageing, e. g. the typical bath-tube curve of lifetime. An increase of failure frequency over time indicates GIS end-of-life and measures such as modified maintenance or refurbishment can result in life extension. The correct analysis of available failure data is very important for achieving accurate conclusions.

Risk assessment of electrical equipment under the influence of GIC

Geomagnetically induced currents are observed disturbances in power grids caused by variations in the earths magnetic field. These magnetic occurrences are the result of solar activity. For power transmission equipment GIC imposes extra loading, resulting in additional harmonics and higher temperatures. If critical hotspot levels are exceeded during GIC events the service life of transmission equipment may be reduced. This paper therefore discusses suitable methods to evaluate the risk exposure of different power grid apparatus during GIC events. The introduction briefly explains the relation between GIC and DC effects in electrical power equipment. One of the effects of GIC is the heating of structural steel parts. Simulation results and measurements presented in this paper identify potential locations for hotspots in large power transformers and shunt reactors. Thereafter the paper focuses on an online risk assessment tool. The approach of a simple hotspot calculation method for specific steel parts in power transformers is discussed and the essential input parameters are demonstrated. In order to show the usability of the calculation method for online monitoring, the transient hotspot behavior of a DC test is simulated and a comparison with the measurement results is carried out.

Partial discharge patterns on cross-linked polyethylene DC power cables

Since electrical energy supply is crucial and HVAC applications play a significant role in that field, work and discussion about PD are mostly related to AC voltage. The situation is different recently because HVDC applications have started to take a part on the electrical energy supply. HVDC cable systems nowadays have been replaced the use of HVAC cable systems for transporting energy over long distances because some advantages it has. Cross-linked polyethylene (XLPE) cables are increasingly taking the place of mass-impregnated (MI) and oil-filled (OF) cables in power transmission because they are easier to maintain and are less likely to cause oil leakage. The number of new installed high voltage XLPE power cable increases because of the increase of the use of XLPE cables. In order to investigate partial discharge (PD) phenomena on a cable, XLPE cables with artificial defects are created and tested under DC voltage. This paper describes the DC voltage partial discharge characteristics for different sources such as surface discharge and void discharge. By comparing new representations of DC PD named “NoDi* Pattern” with conventional “PD magnitude as a function of time”, DC PD patterns can be shown and discussed.

The influence of surface modification on the electrical properties of silicon carbide flakes

Field grading materials with nonlinear behaviour are used in many high voltage applications to avoid any stress concentration that can deteriorate the material performance. In the present work, the current-voltage characteristics of surface functionalized silicon carbide (SiC) flakes were studied and compared to untreated particles. The nonlinear behavior of SiC flakes can be described by transport mechanisms at the grain contact, which can be modeled by Schottky-like barriers. The influence of the organic surface layer on the conduction mechanisms and the corresponding electrical properties were evaluated. In addition, physico-chemical analysis, including X-ray photoelectron spectroscopy (XPS), optical microscopy and zeta-potential measurements were accomplished in order to analyze the surface properties of the SiC flakes, prior to and after surface modification.

High voltage test on 245 kV post insulators with different materials

Post insulators are exposed to pollution which can influence the performance of the insulation. To overcome this problem, post insulators were designed and made with different type and materials. One of the methods to observe the insulation characteristic of the post insulators is by performing withstands voltage test and measuring the leakage current. This test objective is to perform withstand voltage test and leakage current measurement under alternating voltage and direct voltage on 245 kV post insulators of different materials (ceramic, silicon-coated and hybrid) and analyze the behaviors and performance of the 245 kV post insulators under electrical and environmental stress.