Alain Girodet

Partial discharge analysis of gas insulated systems at high voltage AC and DC

Partial discharge (PD) measurements are commonly used for monitoring and diagnostics of AC gas insulated systems (GIS). Because of the growing importance of DC GIS, a research project was started focused on the measurement and interpretation of PD signals obtained under DC stress. Results are presented using IEC60270 and unconventional methods through UHF antenna and high frequency current transformer (HFCT) on three different PD generating defects in a GIS chamber. The results of the PD analysis of the defects are compared for AC and DC stress. Phase resolved partial discharge patterns (PRPD) are used for the HVAC analysis, whereas time resolved partial discharge patterns (TRPD) are used for HVDC analysis. Finally, relevant characteristics of PD patterns in HVAC and HVDC are presented and related with the physical phenomena of the discharges.

Analysis of the sensitivity of the UHF PD measuring technique

Over the last years the UHF PD measuring technique has proven to be a very sensitive measuring tool for gas insulated systems. However, the sensitivity of the method strongly depends on the type of discharge and the number and type of GIS components between the discharge and the sensor. In this paper investigations regarding UHF signal propagation through GIS are analysed, in particular UHF signals originating from real discharges and artificial pulses. Based on measurements in a laboratory set-up it is shown that it is not possible to determine one value for the signal reduction of different GIS components. Moreover, a linear dependence of the signal reduction of a GIS component on the magnitude of the UHF signal propagating through this component has been observed.

Risk assessment of free moving particles in GIS using spectral and partial discharge analysis

Moving particles occurring in SF/sub 6/ gas-insulated switchgear can result in breakdown of the gaseous insulation. Therefore, it is important to trace such defects in time and to determine their risk for a possible breakdown. For this risk analysis process, the following four steps have to be taken into account: detection, location, identification and risk assessment of the defect. In this contribution, the UHF method has been used to detect insulation defects. Time-of-flight measurements can be used to locate the defect. The identification is an essential part for risk analysis of defects. Different methods can be applied based on spectral analysis or phase-resolved PD pattern analysis.

Estimation of UHF signal propagation through GIS for sensitive PD detection

Defects in SF/sub 6/ gas-insulated switchgear can result in breakdown of the gaseous insulation. Therefore, it is important to detect defects, such as protrusions and particles, in time and to determine their risk for a possible breakdown. For on-line PD detection, the UHF method has proven to be a sensitive method. Unfortunately, the propagation path of the electromagnetic waves from the location of the defect to the UHF coupler influences the magnitude of the UHF signals: the UHF signals attenuate, mainly due to reflections at the different components. As a result, several UHF couplers are required to reach enough sensitivity. In this contribution a transmission line model has been used to estimate the signal reduction of each GIS component. This has been used to determine coupler locations to reach certain sensitivity, which has been verified using the CIGRE sensitivity check.

Partial discharge analysis and monitoring in HVDC gas insulated substations

Partial discharge (PD) detection is a widely used tool for monitoring and diagnostics of high voltage gas insulated substations (GIS). Despite the fast growing demand of high voltage direct current (HVDC) interconnections, little research has been dedicated to PD monitoring systems for HVDC GIS. In this paper, conventional and unconventional PD detection systems are employed for the detection of three different defects common in GIS, and relevant PD analysis methods are presented. Finally, the analysis methods and detection systems are compared to point out their potential and limitations in monitoring PD in HVDC GIS.

INFLUENCE OF THE POWER FREQUENCY ON PARTIAL DISCHARGE PHENOMENA

For commissioning testing of GIS, a voltage level around 0.8 times the power frequency withstand level (PFWL) is applied according to IEC 60517. This voltage level is applied at least for 1 minute and longer in case of pre-conditioning when partial discharge tests are combined with power frequency tests. Otherwise, partial discharge measurements are performed at different voltage steps until the maximum test voltage is reached. In any case leads the test procedure to a voltage level much higher than the maximum operating voltage of voltage measurement transformers (VT’s). As a result, the core will saturate during these test over-voltages, giving rise to high currents flowing in the windings of the VT. If the time such a current flows is too long a thermal failure of the voltage transformer can occur. Therefore, voltage measurement transformers are disconnected during commissioning testing.

Breakdown Strength of N2-SF6 Gas Mixtures Containing 10% and 5% SF6 Compared with pure SF6

SF6 as insulating gas has a very good dielectric strength. Therefore, SF6 is used in many applications. Nowadays, it is investigated again whether SF6-N2 mixtures can replace pure SF6 in for example gas-insulated substations and lines. For pure SF6 the optimal gas pressure varies between 4 and 6 bar. In order to find a suitable SF6-N2 gas mixture to substitute SF6, the gas pressure has been varied in such a way that the similar lightning impulse breakdown voltage levels were obtained. In further experiments, we compared gases of corresponding dielectric strengths.

Risk Assessment of Free Particles in GIS/GIL Based on Spectral Analysis

Conducting particles, such as small aluminium splinters which can freely move in GIS, often result in dangerous situations for the operation of the GIS [1]. Such free particle can start to move under influence of the applied electrical field. In particular when a particle approaches the high-voltage conductor, breakdown can occur [2]. In this contribution, we analyse UHF measurements obtained from free particles to assess their risk for the insulation condition.

Comparison of PD phenomena in pure SF/sub 6/ and N/sub 2/-SF/sub 6/ mixtures

Recently the research in breakdown and PD phenomena in pure SF/sub 6/ and gas mixtures is intensified due to a growing interest in the use of gas mixtures. In particular gas mixtures can be a very interesting substitute for pure SF/sub 6/ in large gas insulated systems, such as gas insulated lines. In this paper investigations on PD phenomena and breakdown voltages, performed in SF/sub 6/-N/sub 2/ mixtures containing 5% and 10% and pure SF/sub 6/, are presented. To make this comparison the influence of the test voltage and gas pressure was studied for different types of PD sources, such as protrusions and free moving particles. It is shown that differences can be observed in phase-resolved PD patterns, PD magnitude, UHF spectra and breakdown voltage.