Simple Air Gaps Research Articles

Impulse behavior of dielectric-covered rod-plane air gaps

The impulse behavior of composite short rod-plane air gaps with a dielectric-covered rod under positive lightning and switching impulse voltages is investigated, by monitoring the electric field strength at the earthed plane, by still photographs of corona discharges and through electric field computations. Experiments were performed in normal laboratory air at pressure around 0.1 MPa, temperatures in the range 19 to 28°C and absolute and relative humidity varied naturally between 11-21 g/m3 and 60-90%, respectively. Breakdown probability distributions were obtained for composite gaps with a dielectric cover made of either PTFE or epoxy resin and for simple air gaps with a bare rod. Also, basic characteristic parameters of coronas preceding breakdown in composite gaps, namely initial corona in the vicinity of the covering tip, surface coronas emerging along the dielectric cover and developing mainly in air, and secondary corona in the vicinity of the cover upper end, were measured. Surface corona development is greatly affected by initial corona growth and facilitates secondary corona inception. Breakdown, occurring at higher voltages for composite gaps, is closely related with the secondary corona inception; a similar dependence upon impulse waveshape and cover material was seen to exist. At breakdown, the spark channel bridges the gap that includes the dielectric cover length. The spark channel develops either along the cover surface or partly or solely in free air, being affected by surface corona development.

Lightning impulse behaviour of short rod–plane gaps with a dielectric-covered rod

The breakdown mechanism of dielectric–covered rod/plane air gaps under positive lightning impulse voltages is investigated. Several gap lengths, varying between 2.5 and 15 cm, were employed and the simple air gaps were regarded as reference. The basic characteristics of coronas, namely inception time and voltage, were measured at applied voltages just sufficient for their inception up to voltages causing breakdown. Multiple level tests were conducted; thus, corona inception and breakdown probability distributions were obtained and time to breakdown was measured at several voltage levels. Interpretation of the results was made based on oscillograms of the electric field strength at the earthed plane, which was monitored through a capacitive probe, geometric field calculations and still photographs of the discharge at breakdown. In the dielectric-covered rod/plane gaps, breakdown occurs at higher applied voltages as a result of increase in both corona inception voltages and discharge path. The development of the discharge depends on the electric field distribution as modified by both space and surface charges associated with coronas preceding breakdown. Surface charge accumulation may affect the discharge path at breakdown.

Air-Gap Flux Position Estimation of Inaccessible Neutral Induction Machines by Zero-Sequence Voltage

This paper describes a new and straightforward approach to estimate the air-gap flux position in induction machines. The proposed scheme is based on the injection of a high-frequency oscillating stator voltage instead of a three-phase high-frequency voltage, thus allowing a reduction in audible noises and torque ripple. The main feature of the proposed approach is that a simple air-gap flux tracking algorithm is developed using the amplitude variation of the high-frequency component of the zero-sequence voltage. Moreover, the zero-sequence voltage is sensed at the output of the inverter rather than at motor terminals. The estimated position is not influenced by load and parameter variation and can be performed at any frequency, including low and zero frequency. An experimental prototype has been implemented showing the performance of the proposed approach.

Amperometric air gap cell for the measurement of free cyanide

A simple air gap cell is described for the determination of the free cyanide content of sample solutions. The cell contains a micro-sized silver disc working electrode. It employs amperometric detection of the cyanide at 0.0 V vs. a 0.1 M silver/silver chloride reference chloride reference electrode. The three-electrode assembly necessary for detection is located on the flat surface of a probe that is coated by a basis measuring film. The cyanide content of the sample is liberated by the addition of tartaric acid. The hydrogen cyanide gas diffusing through the air gap is accumulated in the measuring film. Owing to the working principle, cyanide concentrations as low as a few ν1 −1 can be measured. The effect of interfering species was also studied.

A brief history of AC surge arresters

Since the beginning of AC transmission, approximately 100 years ago, lightning protection of transmission equipment has been provided by gaps and by nonlinear resistors, alone or in various combinations. Protection during the early part of the period (1892-1908) was provided by simple air gaps from line to ground. During the period 1908 to 1930, nonlinear resistors based on puncturing and reforming of films came in to use. Further developments were oxide film arresters (1920-1930), silicon carbide nonlinear resistors with nonactive gaps (1930-1954), and silicon carbide nonlinear valve elements with active gaps (1954-1976). Current limitations of the types of arresters and the requirements imposed by 500 and 800 kV systems are discussed. The zinc oxide arresters that have been used from 1976 to the present are then considered. An approximate table of sparkovers, discharge voltages, and arrester height is given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>