Kenichi Nojima

A unique sensitivity verification combined with real-time partial-discharge identification method

The sensitivity verification with the ultrahigh-frequency (UHF) method will be discussed in this paper. The sensitivity line with the UHF method based on the conventional (IEC 60270) one is different in the radial position of the defects inside gas-insulated substation (GIS), because transverse-electromagentic mode can be more weakened while transverse electric (TE)/transverse magnetic (TM) modes can be more enhanced as the defect comes close to the enclosure than the conductor. The result makes it difficult to detect the free particle with IEC 60270. A unique sensitivity verification, which is combined with real-time PD identification method, first enables us to identify the defects and thereafter to calibrate the apparent charge of each specified defect according to the sensitivity line.

Real-time PD identification in diagnosis of GIS using symmetric and asymmetric UHF sensors

A new practical method for real-time insulation diagnosis of GIS is introduced: a conical coupler is used as an asymmetric UHF sensor while a loop electrode embedded in barrier insulator as a symmetric one. This method can immediately identify the kinds of defects in GIS and discriminate inside partial discharge (PD) from noises by taking single PD pulse. This is because the output ratio of those UHF sensors shows the instinct difference in the kinds of defects due to dissimilar electromagnetic field distributions in GIS. The method will lead to sequential risk assessment based on each identified PD pulse.

Electromagnetic interference with electronic apparatus by switching surges in GIS-cable system

Thyristor malignitions and protector relay failures caused by gas circuit breakers closing in the gas insulated switchgear (GIS) of a power cable system could not be controlled by reducing the ground impedance or adding surge absorbing capacitors. As a result, a study was carried out on this problem with a 66 kV mock-up apparatus installed in a factory. It was found that malignitions and failures such as those mentioned above could be controlled by preventing 10 MHz ultra high-frequency surges emitted by the GIS from leaking out into the grounding system. >

Resistance characteristics and electrification characteristics of GIS epoxy insulators under DC voltage

Now that gas insulated switchgear (GIS) for ac systems are becoming increasingly compact as specifications are rationalized, more consideration of their insulation characteristics for residual dc voltage is required. Furthermore, with dc power transmission technology drawing more and more global attention, clarifying the insulation characteristics of GIS for dc voltage is increasingly important. In this paper, to understand the insulation characteristics of epoxy resin, which is widely used for GIS insulating spacers, factors determining the resistivity of the epoxy insulator surface layer under dc voltage were initially investigated on an experimental basis. Consequently, it emerged that the bulk resistance was more dominant than the surface resistance for the dc resistance of epoxy resin due to the dependency of the test sample resistance value on their radius. Since the electric field might be concentrated if some part of this insulator surface layer showed non-uniform resistivity, the influence of the curing agent, one of the potential causes of this non-uniformity, was subsequently investigated with its content as a parameter. As a result, the volume resistivity in the long-term region was likely to decline or vary for epoxy resin containing less curing agent due to the presence of numerous polarized components unreacted with curing agent. In addition, the presence of micro protrusions or similar, if any, on the insulator surface or electrode is considered to cause electrification due to the concentration of electric field on the surface layer. Accordingly, the relationship between their surface roughness and electrification level was investigated using gaps between insulators or an electrode and an insulator facing each other, respectively. Consequently, where the surface roughness of the insulator or electrode was high, a current component with a large damping time constant, considered attributable to electrical charges moving across the gap, appeared after the charging current components and an electrification condition was observed.

Grounding System Transient Characteristics of Underground GIS Substations

The transient grounding impedance of a gas-insulated switchgear (GIS) substation was investigated through measurements on a commercial 300-kV underground substation. Based on the results, the macroscopic transient impedance of the grounding grid was clarified to be the inductance component and estimated at approximately 0.2 μ H. In the low-frequency region where the frequency is almost zero or so, the impedance is shown to be lower than the general concrete resistance value by one digit due to the complexity of grounding systems specific to underground substations. Hence, the simple model of the grounding grid can be expressed mainly by inductance. The detailed model was expressed by using a π-type equivalent circuit, where each circuit constant was calculated using the specific low resistivity of concrete. In addition, the validity of both models was confirmed by comparing the measurement results. The proposal models are applicable to the Electromagnetic Transients Program analysis on the surge phenomena within the GIS grounding system.

Insulation characteristics of GIS epoxy insulators with non-uniform surface resistance under DC voltage

Now that gas-insulated switchgear (GIS) for ac systems are becoming increasingly compact as specifications are rationalized, more consideration of their insulation characteristics for residual dc voltage is required. Furthermore, with dc power transmission technology drawing more and more global attention, clarifying the insulation characteristics of GIS for dc voltage is increasingly important. For GIS insulating epoxy spacer, the present paper experimentally and analytically studied the influence of spacer surface condition on dc withstand voltage performance. In specific terms, initially, an insulating spacer model with an area of non-uniform resistivity present in the surface layer was created and the dc voltage breakdown characteristics were obtained. As a result, it was clarified that the time delay to breakdown increased with lowering the applied voltage, or in other words, even if a low voltage was applied, breakdown may occur after an extended period. Subsequently, using the same non-uniform resistivity model, the change in the electric field distribution over time under dc voltage was investigated through transient electric field analysis. Consequently, it was found that the electric field distribution varied from a capacitive to a resistive field and the maximum electric field was generated in the boundary between the high- and low-resistivity areas. It was further clarified, based on these breakdown characteristics and electric field analytical results, that the presence of an area of nonuniform resistivity on the insulating spacer creepage surface caused electric field concentration over time and breakdown occurred if the electric field in the creepage surface direction exceeded 40 kV/mm or so. Moreover, the breakdown characteristics were investigated where a lightning impulse voltage was superimposed over a dc voltage. Consequently, the creepage breakdown electric field where a dc voltage was applied alone was almost identical to that where the superimposed voltage was applied. It means that the application of the superimposed voltage may potentially allow the insulating spacer withstand voltage performance to be evaluated by a shorter time test for a dc voltage alone or the dc voltage with a superimposed VFTO.

Phenomena of leaking electromagnetic waves and high frequency surges from isolated joint of gas insulated switchgear

High frequency surges leaking through the insulated joint of a GIS were measured, using a model GIS, and the phenomena, characteristics and mechanism were clarified based on the experiment and the circuit theory. High frequency surges decrease the impedance of the insulated joint, producing an almost short circuit state, and the coefficient for refraction from the inside of the GIS tank to the grounding system becomes significantly small. In a multi-phase GIS, surges leaking through the insulated joint form the tank-to-tank mode voltage for the different phase tank, generating the voltage equal to the potential of the voltage application tank to ground. On the other hand, no potential to ground is directly generated to the adjoining bus. The amplitude and the frequency of surges leaking through the insulated joint of a GIS bus are almost identical to those calculated from the distributed constant circuit theory. The phenomena can be, therefore, reproduced using the EMTP through the simulation of the line part by the multi-phase distributed constant and the simulation of the insulated joint by the capacitance.

Forces affecting metallic particle motion in GIS

The bounce height of a metallic particle in Gas Insulated Switchgear (GIS) is one of the key factors which affect GIS reliability. Simulation of particle motion can evaluate bounce height if the relevant forces acting on the particle are taken into account. This paper presents an approach to simulating particle trajectories including dipolar force and drag force from SF6 viscosity. The influence of various forces is evaluated through the comparison between the simulation results and particle trajectories determined from high speed camera observation of particle motion.

Circuit models of gas insulated switchgear elements for electromagnetic wave leakage analysis

The leakage of electromagnetic waves and high frequency surges caused by the partial discharge inside gas insulated switchgear (GIS) to the outside of its tank may become a signal for the partial discharge detection, and, on the other hand, possibly cause electromagnetic interference (EMI) to electronic equipment via the grounded system. It is, therefore, important to investigate and understand the leakage phenomenon of electromagnetic waves and high frequency surges to the grounded system of the GIS. In the present study, experiments of the characteristics of leaking electromagnetic waves and high frequency surges were conducted using test models for the cable connections, the disconnecting switch (DS), the earth switch (ES), the capacity type potential divider (PD) inside the GIS, and the grounding wire to clarify the characteristics of the respective elements. These results could be approximated using distributed parameter circuits, and models for the leakage surge analysis were proposed.

Computation of particle motion in gas-filled systems

The force on a particle in a gas-insulated system is considered in two contexts, viz., (i) the force caused by a field gradient at a conductor surface and (ii) the forces (torques) that drive “firefly” motion at a conductor. These two problems have a degree of commonality, as demonstrated in the mathematics.