Kunihiko Hidaka

A new method of electric field measurements in corona discharge using Pockels device

The investigations of the dc electric field distorted by space charges have been made by means of the optical method using a Pockels device. The Pockels device was rotated to eliminate field disturbances caused by surface charges accumulated on it. The present method has the advantage of directly measuring the electric field in a dc corona discharge. The method was successfully applied to the measurement of the electric‐field distribution in a needle‐to‐plane gap of 10 cm with a positive dc corona. The field distortion was clearly observed in a wide range of applied voltages between 30 and 98% of the breakdown voltage 52 kV. It was found that the electric field inside the corona streamers was 3.5 kV/cm, while 14 kV/cm near the tip of the streamers. Furthermore, the experimental results were theoretically confirmed at the relatively low voltage.

Directly High-Voltage Measuring System Based on Pockels Effect

To measure high voltage up to ± 450 kV in the dc to GHz range, a new optical high-voltage measuring system using a Pockels crystal in a longitudinal modulation arrangement is developed. The maximum measurable voltage for conventional Pockels sensors has been limited by the half-wavelength voltage Vπ. In the new system, a two-wavelength dual laser system is introduced to expand measurable voltage up to the least common multiple of Vπ for each light. The measured results for dc, ac, lightning impulse, and step voltages by the system are presented and compared with the predicted values.

Residual charge distribution of positive surface streamer

The measuring system of charges accumulated on an insulating plate is developed with an electrostatic probe whose sensing electrode is of 0.5 mm diameter. In the inverse calculation from the probe outputs to the charge distribution, Tikhonovs regularization technique is effectively used to suppress the excessive amplification of the noise. In the case of measuring a 2 mm thick PMMA plate, the spatial resolution of the measuring system reaches 1.0 mm. With this system, the residual surface charge distribution on an insulating plate just after the occurrence of a positive surface discharge at 25 kPa (190 Torr) air is measured. On the head of a positive surface streamer, there remains 10−11 C surface charge, which satisfies the criterion of an electron avalanche-to-streamer transformation. The charge density across a streamer takes a minimum on its centre and a maximum on its sheath. This fact suggests that a quasineutral channel exists in the centre of a streamer, and positive ions remain on the sheath of it.

Nanosecond surface discharge development using the computer simulation method

The authors have analytically studied the experimental results of the nanosecond surface discharge by using a simulation model. The results used were obtained using the dust figure technique with a nanosecond square pulse generator under the following conditions: an applied voltage of 5–20 kV, a pulse width of 3, 5, 8,..., 920 ns, and a gas pressure of 50, 100, 150,..., 760 Torr. Three kinds of simulation models were used: the first was based on the continuity equations together with the Poisson’s equation, without any photoionization effects; the second examined the gas photoionization effect on the first model; and the third considered further the photoelectric effect on the insulation surface. From a series of computations, it was found that by using the third simulation model, the experimental results such as the developing length of the corona streamer and its velocity were roughly confirmed, and the field intensity at the streamer tip was also obtained. Finally, the photoelectric emission coefficien...

Simultaneous measurement of two orthogonal components of electric field using a Pockels device

A new optical method using a Pockels device is developed to simultaneously measure two directional components of an electric field. In this method, an output light beam from a Pockels device is split into two beams and then each of the beams is transmitted through a polarizing plate whose polarization direction is different, depending on the measured field component. The relation between two measured field components and the required polarization direction of the polarizing plate is theoretically derived for representative materials of the Pockels device and is also examined experimentally. A field sensor is made on the basis of this new idea and is used to investigate a nonuniform electric field. An electric field changing spatially by 20% per centimeter can be measured with an accuracy of 5% using the Pockels sensor. The sensor is successfully applied to measure an impulse electric field between a sphere‐to‐sphere gap.

Resolution and signal processing technique of surface charge density measurement with electrostatic probe

When an electrostatic probe is used to measure the surface charge on an insulating plate of constant thickness, the measuring system is regarded a shift-invariant system and the relation between the surface charge density and the probe output can be treated in the spatial frequency domain. The distribution of the surface charge density on an insulating plate just after occurrence of a surface discharge is measured by a Pockels probe, which is regarded as a kind of electrostatic probe without the guard electrode, and restored by Wiener inverse filter. The performance of a Pockels probe and a conventional electrostatic probe are compared quantitatively in terms of the spatial resolution. In the case that the measured object is 3 mm thickness PMMA plate and is charged up to 10 nC/cm/sup 2/ in atmospheric air, it is estimated that the spatial resolution of the Pockels probe with 0.2 mm gap is 1.5 mm and that of the conventional electrostatic probe with the grounded guard electrode with 3 mm gap is 2.2 mm.

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.

3.0-ns surface discharge development

The nanosecond time‐scale developing mechanisms of positive and negative corona streamers on the insulation surface are investigated using a square pulse generator and the Lichtenberg figure technique. The developing length of the corona streamers in air is measured at a pulse duration of 3 ns, an applied voltage of 7–20 kV, and a gas pressure of 150–760 Torr. The results are compared with those obtained in the time range of 5–920 ns. The ratio of the developing length obtained at a certain pressure and that obtained at atmospheric pressure becomes a constant value irrespective of the magnitude of the applied voltage, and it increases as the gas pressure is reduced. The time lag of corona streamer inception increases with decreasing pressure. The time lag for the negative corona streamer obtained at a voltage of 20 kV is 0.1 ns at atmospheric pressure and 0.35 ns at a pressure of 150 Torr, while that for the positive corona streamer obtained at the same voltage, is 0.55 ns at the atmospheric pressure and ...

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.

Assessment of corona models based on the Deutsch approximation

Corona simulation by the Popkov/Sarma models involve the Deutsch approximation of the invariance of the electric field direction. The paper implements the models to obtain current density and field profiles at the collector electrode for positive corona in wire-plane, paraxial-cylinder and point-plane gaps in air. The simulations are compared with experimental data and are found to give satisfactory agreement. Consideration is given to reasons why the Deutsch approximation yields low errors