Horatio Rodrigo

AC Flashover Voltages Along Epoxy Surfaces in Gaseous Helium Compared to Liquid Nitrogen and Transformer Oil

Gaseous helium at high pressure is regarded as a potential coolant for superconducting cables. The dielectric aspects of cryogenic helium gas are both complex and demanding. In this experimental study, we looked at the interface between a smooth epoxy surface and high pressure helium gas in a homogeneous electric field. The ac flashover voltages of epoxy samples whose heights are 3 and 5 mm of diameters 9.5, 12.7, and 19 mm are presented. The results have been analyzed using Weibull statistics. In addition to the behavior of the epoxy in gaseous helium as a function of pressure and temperature, we also present data of the characteristics of the epoxy surface in transformer oil at 293 K and in liquid nitrogen (77 K). The breakdown characteristics of a uniform field gap in gaseous helium as a function of pressure and temperature under ac, dc, and lightning impulse voltages are also given. Field calculations have been made for one of the experimental geometries in an attempt to explain some of the anomalies in the experimental results. The results show that cold compressed helium gas is a viable dielectric but requires different insulator designs compared to insulators for liquid nitrogen.

Dielectric design validation of a helium gas cooled superconducting DC power cable

Successful validation of the dielectric design of high temperature superconducting DC cables cooled with pressurized helium gas circulation is reported. The dielectric design features were developed based on the tests of several short sample cables. The features were adapted for the dielectric design of the cable and termination components such as high voltage bushings. A high voltage DC withstand test followed by AC partial discharge measurements on a 30-m long superconducting DC cable cooled with helium gas operating at 50-60 K showed that the dielectric design is suitable for the cable system.

Dielectric Properties of Cryogenic Gas Mixtures Containing Helium, Neon, and Hydrogen

Past efforts of cooling high temperature superconducting (HTS) power cables by gaseous cryogens focused exclusively on helium. The limited dielectric strength of helium gas necessitated alternatives that could be used in the temperature range suitable for HTS power applications. This paper presents the benefits of gas mixtures containing helium with small concentrations of hydrogen or neon to mitigate the limited dielectric strength of pure helium gas. The expectation was that such gas mixtures could improve dielectric characteristics while maintaining the thermal, non-flammable and non-corrosive properties of pure helium gas. The AC breakdown voltage of helium gas mixtures containing 4 mol% neon or 4 mol% hydrogen respectively have been measured and compared to those of pure helium and pure neon. All measurements were performed at 77 K at gas pressure levels between 0.5 and 2.0 MPa. While the 4 mol% neon mixture did not result in any improvement over pure helium, the 4 mol% hydrogen mixture resulted in 80% higher breakdown strength. This is expected to enable higher operating voltages for gas cooled HTS power devices.

Use of partial discharge inception voltage measurements to design a gaseous helium cooled high temperature superconducting power cable

The occurrence of partial discharge is one of the challenges that limit the design options and voltage ratings of gaseous helium cooled high temperature superconducting (HTS) cables. The measurements of partial discharge inception voltage (PDIV) at cryogenic temperatures on several model cables were used in the optimization of the dielectric design of a 30 m long HTS power cable, which was successfully fabricated and tested at DC currents up to 6 kA and voltage of 3.5 kV RMS. Details of the PDIV measurements are described.

Enhancement of Dielectric Strength of Cryogenic Gaseous Helium by Addition of Small Mol% Hydrogen

A valuable solution to the voltage rating limitation of gaseous helium (GHe) cooled HTS power devices is demonstrated through the use of cryogenic gaseous mixtures of small mol% of gaseous hydrogen (H₂) balanced with GHe. An 80% enhancement of the dielectric strength, both in dc and ac, is observed for the mixture with 4 mol% H₂. The observed linear dependence of dielectric strength from varying the H₂ mol% suggests a significant potential for further enhancements by using mixtures with higher H₂ mol%. Partial discharge (PD) measurements were performed on a 1 m model cable to demonstrate the potential practical applications of GHe–H₂ mixtures for superconducting devices. The results of the PD measurements for model cable showed a 20–25% increase when the 4 mol% H₂ mixture was used. The demonstrated enhancements in the dielectric strength will provide additional design options to HTS power devices cooled with GHe mixtures.

Tools and dielectric requirements for the design of marine cabling systems

This paper explores various power cable challenges for notional electric ship applications including future technology trends for shipboard power cables. To meet the demands of an “all electric ship,” the cabling requirements of the design are not a trivial issue which can result in significant error in estimating final size, weight, and cost at time of construction along with costly failures and early repairs that impact lifecycle cost. This paper provides information on the development of a design tool known as a “Generic Cable Calculator” to estimate parameters such as impedances, weights, and bending radii for ship power cabling. Analysis of actual experience in designing ship cabling suggests improvements in early design tools needed to capture additional requirements in terms of grounding, shielding, and satisfying current standards for cables used in the variable frequency drive train. Also addressed are results specific to future trends of cable insulation and future standards.

Surface Flashover of Cylindrical G10 Under ac and dc Voltages at Room and Cryogenic Temperatures

The results of experimental determination of surface flashover breakdown voltage under high voltage ac and dc for cylindrical G10 are presented. The results are analysed using Weibull statistics and are compared with theoretical field calculations obtained using Ansoft, which is a software package based on finite element analysis. The results show that there is significant improvement of the parameters such as surface flashover voltage and reliability of the results at 77 k over those at 293 K.

A versatile model for estimating breakdown voltage and its application for cryogenic gas mixtures

An improved model for the estimation of the breakdown voltages of gas mixtures is developed. The application and utility of the model is demonstrated by using it in estimating the breakdown voltages of gas mixtures containing helium and hydrogen that are being developed as an alternative cryogenic insulation and cooling media for certain high temperature superconducting (HTS) power devices. The concept of the developed model, which incorporates a generalization factor m, is presented. The first step of the model performs a nonlinear least square regression on the generalized formula of the pressure normalized ionization coefficient α/p for the parameter approximation. The second step introduces the parameter m that minimizes the root mean square error (RMSE) of the nonlinear least square regression to derive the versatile model. The validity of the model is demonstrated by comparing its estimated values with the results of the experimental breakdown measurements of several gas mixtures. Compared to the conventional breakdown voltage models, the new model showed improved agreement with the experimental results of the breakdown voltages.

Determination of remaining life of rotating machines in Shipboard Power Systems by modeling of dielectric breakdown mechanisms

This paper presents a model to simulate electrical trees in dielectric materials. The model accounts for the characteristic tree patterns and the partial discharges associated with the propagation of trees. This simulation model is used as basis to develop a diagnostic tool to determine the remaining life of insulation materials by relating the fractal dimension of the tree to the supply voltage and material properties. Simulation results are presented to show the performance of the prognosis method.

Comparative Study of Cryogenic Dielectric and Mechanical Properties of Insulation Materials for Helium Gas Cooled HTS Power Devices

A comparative study of cryogenic dielectric and mechanical characteristics of electrical insulation materials is described. Breakdown strengths and tensile strengths of Cryoflex, Kapton, PPLP, and Teflon were measured at 77 K in liquid nitrogen. Breakdown strengths of Cryoflex, Kapton, PPLP were also measured in gaseous helium at 2 MPa and 77 K. The relative breakdown strengths of Cryoflex and Kapton vary between liquid nitrogen and gaseous helium. Kapton performed better than Cryoflex both for dielectric and mechanical properties. Among the materials studied, Teflon exhibited the least breakdown strength and mechanical properties.