Nonlinear Field Grading Materials Research Articles

Controlling electrical stress of polymeric insulators using zinc oxide microvaristor

Non-linear field grading materials (FGMs) based on zinc oxide (ZnO) microvaristors have been proposed to control the electric stress and prevent premature aging and degradation of high-voltage insulators near the end fittings. The current work reports on the effectiveness of using a laboratory fabricated ZnO microvaristor in controlling the electric stress on an 11 kV polymeric outdoor insulator. The proposed design involved applying a microvaristor-based compound sheet of varying dimensions onto the insulator core under the silicone rubber housing. Experimental investigations showed that the proposed FGMs improved the performance of the polymeric insulator by 38.9%.

New measurement method of displacement susceptibility based on time‐domain current and potential combined spectra

AbstractDisplacement susceptibility is a very important parameter to characterise the displacement polarisation of dielectrics. Two methods based on the time‐domain current and potential combined spectra are proposed to measure the displacement susceptibility. One is based on the time‐domain polarisation current and decay potential combined spectra, and the other is based on the time‐domain depolarisation current and recovery potential combined spectra. The principle of the proposed methods are verified by simulation. In addition, a measurement system for the two proposed methods is built and is used to measure the displacement susceptibility of transparent organic glass and opaque nonlinear field grading materials. The experimental results are compared with those of the present optical method and the frequency‐domain method. Simulation and experimental results show that the proposed methods for the measurement of displacement susceptibility are feasible and practical, which lays a foundation for the research on the polarisation characterisation of dielectrics.

One-Dimensional Model for the Nonlinear Resistive Electric Field Control in Medium-Voltage Rotating Electrical Machines

This study provides an insight in the analytical estimation of the electric field along the surface of a stress grading system (SGS) in the end-winding region of an electrical machine. An analytical approach is derived for the field grading materials (FGMs) having power-law electric-field-dependent conductivity. Despite some limitations, the introduced model can be used for the preliminary analysis of SGS with specific geometric and dielectric parameters, both during sinus and pulsed voltage condition. The analytical considerations are verified with finite element simulations for 15 markedly different nonlinear FGMs, which were previously reported in the applications related to the medium voltage rotating electrical machines.

PD measurements, failure analysis, and control in high‐power IGBT modules

Increased voltage blocking capability and the development of packaging technology for IGBTs can enhance the local electric field that may become large enough to increase partial discharges (PDs) within the module. The study presents a survey on (i) simulation the electric field within an IGBT module; (ii) current standards for evaluation of the insulation systems of IGBTs; (iii) PD detection and localisation methods as well as other diagnostic and quality control test methods about IGBTs; and (iv) various methods for PD control in an IGBT module. The survey shows remarkable technical gaps in all four areas. More sophisticated numerical and theoretical techniques are needed to model complicated geometries, e.g. extremely sharp edges of the copper metallisation and protrusions in the substrate, and composite non-linear field grading materials. There is no model to take into account defects in the gel and on the ceramic substrate. IEC 61287-1 cannot sufficiently assess the behaviour of PDs on IGBT module under the actual operating conditions exposing fast rise pulse-width modulation-like voltages. There is no agreement on the exact origin and location of PDs in the module with relying on measured phase-resolved PD patterns. PD control methods using non-linear grading materials are not mature enough.

Direct current conduction in SiC powders

Silicon carbide (SiC) powder is used in nonlinear field grading materials. The composite material, consisting of an insulating polymer matrix filled with the SiC-grains, is usually a percolated system with established conducting paths. In order to explain the properties, the electrical characteristic and conduction mechanisms of the SiC powder itself are of interest. SiC powders have been studied by current–voltage measurements and the influences of grain size and doping have been investigated. The macroscopic current characteristics of green and black SiC powders can be described by the transport mechanisms at the grain contacts, which can be modeled by Schottky-like barriers. The SiC is heavily doped and tunneling by field emission is the dominating conduction mechanism over the major part of the nonlinear voltage range. It is suggested that preavalanche multiplication influences the current at the highest voltages, especially for p-type black SiC.

Alternate current characteristics of SiC powders

Silicon carbide (SiC) powder is used in nonlinear field grading materials. The composite material, consisting of an insulating polymer matrix filled with the SiC grains, is usually a percolated system with established conducting paths. In order to explain the properties, the electrical characteristic of the SiC powder itself is of interest. The ac characteristics of SiC powders have been studied by dielectric response, capacitance–voltage, and ac-pulse measurements. The frequency, electric field, and pressure dependencies have been analyzed for green and black SiC, which have different doping. The ac characteristics of green and black SiC powders are governed by both the barrier regions at the SiC-grain contacts and the surrounding matrix. The nonlinear loss is determined by the conduction current at the contacts. Depending on the doping level of the SiC grains, the capacitance may be controlled by either the nonlinear capacitance of the barrier region or the linear capacitance of the surrounding matrix. Each contact zone may be modeled by a nonlinear resistance in parallel with both a nonlinear and a linear capacitance. The components are considered to be frequency independent. However, in order to explain the macroscopic frequency and field dependencies of the SiC powders, the use of a network of unique contact zones with dissimilar properties is suggested.

One-dimensional model for nonlinear stress control in cable terminations

A one-dimensional model for field control in cable terminations is analyzed mathematically both for linear and nonlinear field grading materials. Far an applied ac voltage, the concept of the penetration length allows for a useful characterization of the voltage profile, even for materials where both the conductivity and the permittivity are field dependent. For lightning impulse events an analytical solution for purely resistive field control is presented, allowing for an arbitrarily strong nonlinearity of the conductivity. The model results yield a useful input for more detailed finite element calculations. The results show that conclusions for nonlinear field control based on linear calculations in general are misleading.