Tokio Yamagiwa

Solid-state current limiter for power distribution system

To prevent voltage decrease of distribution systems, the principle and fundamental characteristics of a solid-state current limiter using GTO thyristors were investigated. Basic components of the apparatus were a fast solid-state switch and a current limiting impedance of low resistance in parallel with the switch. Experimental results of the test current limiter showed the fault current was limited successfully, regardless of DC component size. The time from detection of fault occurrence to interruption of the fault current by the solid-state switch was 40 mu s. This time was very short in comparison with that before the fault current reached a large value. Thermal rise of the solid-state switch for conduction was solved by a self-cooling apparatus using a noncombustible cooling liquid. The results indicated that the solid-state current limiter was a valuable protecting device for high fault current distribution systems. >

Principle and characteristics of a fault current limiter with series compensation

A power system fault current limiter with series compensation, which is composed of a compensation capacitor and a limiting reactor in series, is proposed. A solid-state switch connected in parallel with the capacitor controls either the ordinal series compensation or fault current limitation. A feasibility study of the current limiter by simulation analysis is presented and the effectiveness of the current limiter is evaluated from the viewpoints of transient stability improvement and device capacity. The current limiter is a useful protection device for large, high power transmission systems.

Development of Insulation Structure and Enhancement of Insulation Reliability of 500 KV dc GIS

The paper describes the development of insulation structure and enhancement of insulation reliability of 500 kV DC GIS (gas insulated switchgear). Considering problems of metallic particles and charge accumulation on spacers, the developed insulation structure of GIS consisted of three regions, i.e. the spacer, particle scavenging and nonlevitating regions. Particles could be efficiently scavenged, and the developed GIS withstood all insulation specifications with sufficient margins under the clean and particle-contaminated condition. Now, a long-term site test for one year of the 500 kV DC GIS has been continued since October in 1995.

Detection of harmful metallic particles inside gas insulated switchgear using UHF sensor

Gas insulated switchgears (GIS) are widely used as major substation equipment because of their excellent insulation and interruption performance. However, while a GIS is highly reliable, it is a concealed device, meaning diagnostic technology using various sensors is required to detect any insulation abnormality. Among various potential defects inside a GIS, the most problematic is caused by the existence of fine metallic particles, and thus the ability to detect harmful particles in tests at a factory and on site as well as in operation is becoming an important subject. This paper examines, on an experimental basis, the UHF sensors characteristics of detecting partial discharge (PD) signals generated from particles inside the GIS with those placed freely on the bottom surface of the tank, having adhered to the insulating spacers surface, and fixed on the center conductor, respectively. Based on previous reports, breakdown characteristics with the existence of particles were arranged in terms of the particle length, and the harmful particle length was estimated. Both results were compared with each other to evaluate the possibility of detecting harmful particles in each location with the UHF sensor. Consequently, it emerged that free particles were detected comparatively easily. However, while those having adhered to the spacers surface and fixed on the center conductor could be detected in insulation tests at 2 pu at a factory, detection was difficult in many cases in the field.

Development and field test evaluation of optical current and voltage transformers for gas insulated switchgear

An optical current transformer (CT) based on the Faraday effect and an optical potential divider (PD) based on the Pockels effect, for application to electric power systems, are described. This optical CT is suitable for highly reliable gas insulated switchgear (GIS), and has free space light transmission parts among the optical fiber signal transmission lines. The developed optical CT and PD were built into 77/ square root 3 kV 1200 A single-phase prototype GIS. Their basic characteristics of current and voltage measurement conformed well with the standards of the JEC (Japan Electrotechnical Commission) 1201 for instrument transformers. A long-term field test of the prototype GIS was carried out at a substation for 20 months, during which time no significant variation of current and voltage measuring characteristics was observed. >

Thyristor controlled ground fault current limiting system for ungrounded power distribution systems

A thyristor controlled ground fault current limiting system (TGCL) was proposed to prevent one-line ground fault current rises due to increased capacitance to ground. Basic components of the TGCL are a main ground fault current limiter, which rapidly adjusts a compensating reactor level for the capacitance to ground, and the TGCLs controller. Control is ensured by an in-phase control method for zero-phase sequence voltage and current. The method determines the direction of ground faults and the compensating reactor level. The fast control which can be realized shows the TGCL is a valuable protecting system for high ground fault current distribution systems.

Development of preventive maintenance system for highly reliable gas insulated switchgear

The concept of gas-insulated switchgear (GIS) preventive maintenance system construction is presented along with application examples and new sensing technologies. High-sensitivity partial discharge, abnormal current conduction, and methods of detecting very-small-particles are described. >

Preventive maintenance system with a different gas injecting facility for GIS

A preventive maintenance system for gas insulated switchgear (GIS) is developed. It detects signs of trouble and prevents breakdowns in service. The system constantly monitors UHF signals, which are generated by partial discharges (PDs), and propagate in the GIS, by using couplers built into the apparatus. The PDs are detected at high sensitivity (5 pC) and located according to the attenuation of the signals. Then the system injects a different gas into the section where PDs are occurring and improves the dielectric strength. This prevents faults in service and allows remedial actions to be taken with less accuracy. By injecting a small quantity (5-10%) of c-C/sub 4/F/sub 8/ into the GIS, dielectric strength is raised more than 20%. A preventive maintenance system with a facility to inject a different gas is constructed for a full-scale GIS model. The system detects and locates PDs, and automatically injects the different gas to improve the dielectric strength. >

Experience of GIS condition diagnosis using partial discharge monitoring by UHF method

Partial discharge (PD) monitoring system by UHF method is suitable for internal condition diagnosis of gas-insulated switchgear (GIS) due to its high sensitivity.We have developed the UHF method diagnostic system, which uses the neural network. Through the experience of using PDM system, we have built confidence in its performance. The GIS condition diagnosis using PDM by UHF method is quite useful on site. This paper describes outline of our PDM system and also introduces some examples of our experiences during both factory test and on-site test with which we have ensured advance diagnoses for insulation reliability of GIS.

Insulation Characteristics of Gis for Non-Standard Lightning Surge Waveforms

Evaluation of lightning surge waveform, which actually enter into substations, is important when investigating the test voltage of equipment. The waveform of the standard lightning impulse waveform (1.2/50 µs) is used for testing; however, the lightning surge waveforms in actual fields are complex waveforms in which various different oscillations are superimposed1,2). Investigation of the insulation characteristics of the equipment against the complex waveforms and standard one has significant importance. The insulation characteristics of Gas Insulated Switchgear (GIS) were investigated for these waveforms.