Transient and steady-state currents in epoxy resin

Abstract

Charging and discharging currents have been measured in a diglycidyl ether of bisphenol-A epoxy resin with and without silica fillers, below and above its glass transition temperature Tg = 65 °C. Both transient and steady-state current densities have been analysed. The average applied fields ranged from 3 to 35 kV mm−1 with a sample thickness of 0.5 mm. Above Tg, transient currents suggested a phenomenon of charge injection forming trapped space charges even at low fields. Steady-state currents confirmed that the behaviour was not Ohmic and suggested Schottky-type injection. Below Tg, the current is not controlled by the metal–dielectric interface but by the conduction in the volume: the current is Ohmic at low fields and both transient and steady-state currents suggest a phenomenon of space-charge limited currents at high fields. The field threshold is similar in the filler-free and the filled resin. Values in the range 12–17 kV mm−1 have been measured.

Conduction processes in gas-insulated HVDC equipment: from saturated ion currents to micro-discharges

“…An example of the calculated current through the solid insulation is shown in Figure 7b. The close agreement with the measured currents in high electric fields confirms the assumption of an ohmic conduction regime of the solid as measured by other researchers [10][11][12][13][14]. By applying the derived volume conductivity of the solid to Equation 4, the IP generation rates were calculated by subtracting the conduction current carried through the solid sample from the changes in the surface charges presented in Figure 5b, and by dividing the remaining current by the active gas volume and elementary charge.…”

“…A number of studies have analyzed the volume and surface conductivities of such epoxy resins as functions of temperature, the electric field, and humidity [10][11][12][13][14]. Their measurements suggest that, for temperatures less than and greater than the glass transition temperature, the volume conductivity increases by approximately one and two decades per 40°C, respectively [10][11][12][13][14], in accordance with the properties of other polymers [15]. Therefore, an inhomogeneous temperature distribution is expected to strongly influence the insulation characteristic of gas-insulated devices [16].…”

“…Moreover, some authors have observed the presence of an ohmic conduction regime in electric fields of up to 3 kV/mm, followed by characteristics that may pertain to space-charge-limited currents [10,11,14]. In contrast, other authors have observed an ohmic conduction regime in fields of up to 20 kV/mm [12,13].…”

Investigation Into Surface Potential Decay of Polyimide by Unipolar Charge Transport Model

“…The trap filling time constant will be, however, dominated by the trapping property when n free0 ν th S C >> υ ATE exp (−E t /k B T ). Experimental results of space charge evolution properties [29], [65]- [67] and time-dependent current properties [68], [69] of polymeric insulating materials have shown that the trap filling time constant of deep traps would be very long.…”

Highly accurate current response measurements of insulation materials for HVDC applications

“…residual currents after relaxation of transient phenomena, are an integral part of insulation characterization. Studies on commonly used solid insulations such as filled/unfilled epoxy resins [7], reinforced thermoplastics, or resin impregnated papers/synthetics have shown transition times to steady-state currents in the order of hours up to months. A large number of the materials response with very slow time-varying currents following power-laws t -n with small values of n (0<n<1) [8].…”