Masakuni Chiba

Potential distribution measurement of surface discharge by Pockels sensing technique

A new technique for measuring the transient potential distribution of electrical discharge is proposed. The measuring system consists of a Bi4Ge3O12 Pockels crystal, an Ar ion laser, a beam expander, a streak camera and a charge-coupled device camera. The potential profile of the discharge along a linear path of 50 mm in length is measured in the range of 0∼±36 kV. The temporal and spatial resolutions of this system are estimated to reach 2 ns and 0.1 mm, respectively. The potential distribution of a surface discharge creeping on a dielectric material is measured to clarify its developing mechanism. The tip of the propagating surface discharge has a high potential close to an applied potential. The maximum value of electric fields of a negative surface streamer exceeds 16 kV/cm in the direction of the streamer propagation.

Pockels surface potential probe and surface charge density measurement

Abstract A surface potential probe based on Pockels sensing technique is developed and applied to surface potential distribution measurement on insulating material. The probe consists of a super luminescent diode, a Pockels crystal, a polarized beam splitter, a 1 8 wave plate and optical fibers. As the detecting part of the probe is isolated from a grounded electrode, this probe can be placed closer to the measured object. The resolving power is 2 mm which is equivalent to the twice size of the detecting part. The modulation technique is introduced to this probe and the minimum sensitivity reaches 10 V . This probe is applied to the measurement of the potential distribution on an insulating material just after occurrence of a surface discharge. From this measured potential profile, the distribution of surface charge density is computed through a surface charge method.

Kerr effect in gas and its application to noncontact measurement of electric field

A noncontact measurement of electric field is proposed based on the Kerr effect in gases. With the Kerr sensing technique, the electric field can be measured without causing any field disturbance. Kerr constants of gases are, however, very small compared with those of liquids, and this ideal technique has not been applied to electric field measurement in gas. In this study, a system is developed for a highly sensitive measurement of the small phase retardation caused by the Kerr electro-optic effect in gases. An optical phase modulation technique is effectively adopted to improve the sensitivity of the system. With this optical system, (i) dc stressed uniform electric fields in sulfur hexafluoride, nitrogen, and carbon dioxide gases at the pressure of 0.2–0.5 MPa and (ii) ac stressed uniform electric fields in atmospheric air are measured.

Repetitive operation of three-phase superconducting fault current limiter in a model power system

Superconducting fault current limiters (SCFCLs) are expected to improve reliability and stability of power systems. To introduce an SCFCL to power systems, the repetitive operation at the severe continuous faults is one of the important function. The recovery time of the proposed 3-phase FCL was confirmed experimentally to be sufficiently short for the repetitive operation. This paper shows the experimental results and discussion on the repetitive current limiting operation of the 3-phase SCFCL, which was demonstrated in a model power system. The test condition was that the fault line is open at the fault within 100 ms and is re-closed within 930 ms, but the fault is not cleared during the interval while the fault line is open. It was confirmed that the SCFCL started to limit the fault current immediate after the fault and recover to the stand-by mode during the interval, and limited the fault current again at the re-closure without any degradation of the trigger current level and the limiting impedance.

Inception voltage of positive streamer and its length on PMMA in air

A positive surface discharge on a solid insulator in air consists of many branched streamers. The length of the streamers is controlled by the applied voltage at the instant when the streamers occur. The relationship between the inception voltage and the streamer length depends on the thickness of the solid insulator and the number of streamer branches. At a low voltage, the length of a streamer decreases with increasing the thickness of the insulator, but at a high voltage, it increases with thickness. The length of a streamer with a large number of branches is shorter than that with a few branches. This phenomenon can be explained as follows: the electrons generated in each branch flow into a positive high-potential electrode through the stem of the streamer, and the potential drop in the stem is enhanced due to this electron flow.

Over-voltage suppression in a fault current limiter by a ZnO varistor

A superconducting fault current limiter (SCFCL) of the transformer type with a ZnO varistor (metal oxide varistor) in parallel was investigated to determine the effects of the ZnO varistor as an over-voltage suppressor for the SCFCL. An SCFCL of the transformer type, which has an adjustable trigger current level, has been studied. A small model of this type of SCFCL was designed and built with NbTi superconductors. Since a fault current is reduced by its inductive component, a large over-voltage is observed at the beginning of the current limiting event. It is important to suppress the over-voltage to avoid any damages to the power system apparatus. Experimental results on the fault current limiting operation of the SCFCL with ZnO varistor in parallel are shown. It was confirmed that the surge voltage that appears at the terminal of the SCFCL can be successfully suppressed by ZnO varistor. Current limiting and recovery characteristics of the SCFCL with a ZnO varistor are investigated and discussed. The trigger current level of the SCFCL is not affected by the ZnO varistor. The recovery time is a little longer with the ZnO varistor than that without it. Energy dissipation in the ZnO varistor and the SCFCL is discussed.

Two-dimensional potential distribution measurement of surface discharge with subnanosecond resolution

The Pockels sensing system has been developed for directly measuring the potential and electric field distribution on a dielectric material. The system consists of an Ar ion laser, a Nd:YAG laser, a streak camera, and charge coupled device cameras. With this system, both the transient change in the potential distribution along a linear path of 50 mm in maximum length and the instantaneous two-dimensional potential distribution on a 25 mm square area can be measured simultaneously. The minimum resolution of this system reaches up to 0.2 ns. This article describes the transient change in potential distribution with a surface discharge in atmospheric air. The transient change of potential distribution with a propagating negative surface discharge in atmospheric air is measured with this system. The propagating direction component of the electric field Ex near the tip of the propagating streamer exceeds 3 MV/m.

Fundamental Characteristics of Y123 thin film for switching surge current to design SCFCL

Superconducting Fault Current Limiters (SC-FCLs) have been expected not only to reduce fault current but also to improve power system stability. For introducing them, various kinds of characteristics, such as trigger current level, must be examined. They should be designed not to be triggered by switching surge that comes from out of the target section. In this paper, fundamental characteristics of SCFCL for surge current have been studied. Y123 thin films with Au protection layer on sapphire substrate have been prepared, and the voltage-current characteristics have been measured. The measurements have been carried out by use of the impulse and sinusoidal currents to evaluate the characteristics of Y123 thin film for switching surge. The relation between operational current levels and their operational times and the relation between operational current levels and the wave front times are shown in the paper. These characteristics are useful for designing the SCFCL which will not to be triggered by switching surge.

Characteristic of AC surface spark voltage in LN/sub 2/ and LHe

The AC surface spark-over voltages on solid insulators with a back electrode in liquid N/sub 2/ (LN/sub 2/) and liquid He (LHe) were measured and discussed. The larger gap length did not give a higher spark-over voltage. The spark-over voltage in LHe was about two-thirds of that in LN/sub 2/. Experimental studies were carried out on how bubbles in liquid influence surface spark-over voltage. The result shows that it does not influence the voltage.

Experimental study on characteristics of superconducting fault current limiters connected in series

Superconducting fault current limiters (SCFCLs) can reduce fault current; thus SCFCLs allow the capacity of circuit breakers to be reduced and lend flexibility to power system network designs. Furthermore, SCFCLs are expected to improve power system stability, as has been confirmed by some experiments. Therefore, various kinds of SCFCLs were proposed and have been studied. For introducing SCFCLs in power systems, there should be some specifications, such as the impedance, the trigger current level, and the recovery time. Fault analyses pointed out that accuracy of the trigger current level is required. A transformer type SCFCL with an adjustable trigger current level was proposed, designed, made, and tested. Then, limiting tests on three-phase SCFCLs by use of an artificial transmission line with a small generator were carried out. The test results indicated that at phase-to-phase faults, the SCFCLs for the fault phases do not always turn into the limiting mode. From two points of view, it is important to study characteristics of the series connection for SCFCLs: one is as mentioned above, and the other is that a series connection of some SCFCLs of small impedance may be installed in place of an SCFCL of larger impedance. This paper describes the results of limiting tests on two SCFCLs connected in series. The tests were carried out under some different conditions of fault currents, fault phases, and trigger current levels of SCFCLs. Some discussion on characteristics of series connection of SCFCLs appears also.