Electrohydrodynamic Analysis of Dielectric Guide Flow Due to Surface Charge Density Effects in Breakdown Region

Abstract

A fully coupled finite element analysis (FEA) technique was developed for analyzing the discharge phenomena and dielectric liquid flow while considering surface charge density effects in dielectric flow guidance. In addition, the simulated speed of surface charge propagation was compared and verified with the experimental results shown in the literature. Recently, electrohydrodynamics (EHD) techniques have been widely applied to enhance the cooling performance of electromagnetic systems by utilizing gaseous or liquid media. The main advantage of EHD techniques is the non-contact and low-noise nature of smart control using an electric field. In some cases, flow can be achieved using only a main electric field source. The driving sources in EHD flow are ionization in the breakdown region and ionic dissociation in the sub-breakdown region. Dielectric guidance can be used to enhance the speed of discharge propagation and fluidic flow along the direction of the electric field. To analyze this EHD phenomenon, in this study, the fully coupled FEA was composed of Poisson's equation for an electric field, charge continuity equations in the form of the Nernst-Planck equation for ions, and the Navier-Stokes equation for an incompressible fluidic flow. To develop a generalized numerical technique for various EHD phenomena that considers fluidic flow effects including dielectric flow guidance, we examined the surface charge accumulation on a dielectric surface and ionization, dissociation, and recombination effects.

1. Introduction

Electrical insulation is the most important consideration in electrical power devices such as power transformers, vacuum interrupters, circuit breakers, and transmission lines [1-8]. In power electrical systems, most failures of the insulation systems are associated with the breakdown of solid insulators [9]. One of the key issues in the analysis of solid insulating materials is electrical breakdown prediction, which considers electric discharge processes including the surface tracking stages. To analyze surface tracking accurately, the discharge mechanism should be well understood, including electric field dependent charge dynamics and surface charge density.

In addition, to analyze fluidic flow effects such as flow electrification and pumping, engineers should examine the distributions of flow vectors considering solid insulating materials. For these types of problems, the surface charge density on the dielectric surface plays an important role as it affects flow behavior and velocity. To analyze this electrohydrodynamics (EHD) phenomenon, in this study, the fully coupled finite element analysis (FEA) was composed of Poisson’s equation for an electric field, charge continuity equations (in the form of Nernst–Planck equations for electrons and ions), and the Navier–Stokes equation for an incompressible fluidic flow.

It is well known that there are four basic phenomena in EHD: flow, electric field dynamics, energy variation, and charge dynamics [10]; these four phenomena are considered in this paper. Fig. 1 shows the schematic diagram of the multiphysics analysis method for EHD phenomena coupled with electric discharge.

Fig. 1.Schematic diagram of multiphysics analysis method for EHD phenomena coupled with electric discharge.

 

2. Fully Coupled Governing Equation for Generalized EHD Formulation

2.1 Governing equations in liquid region

The generalized hydrodynamic drift-diffusion equations combined with Poisson’s equation have been widely employed for analyzing discharge analysis in dielectric liquids as follows [6-10]:

where the subscripts +, −, and e indicate the positive ions, negative ions, and electrons, respectively, ε is the dielectric permittivity, V is the electric scalar potential, ρ is the charge density, t is the time, GI(| E |) is the electric-fielddependent molecular ionization source term, GD(| E |, T) is the electric field and the temperature dependent ionic dissociation source term, e is the electron charge, Rxy is the recombination rate of x and y carriers, v is the velocity of fluidic medium, τa is the electron attachment time constant, T is the temperature, ρl and cv are the mass density and the specific heat capacity, respectively, kT is the thermal conductivity, E⋅ J represents the electrical power dissipation term in the fluidic medium, η is the dynamic viscosity, p is the pressure, and F is the body force density including the electric force density. The formulation of GI(| E |) and GD(| E |, T) was developed in [9], in this study, we adopt the same setup for discharge analysis.

2.2 Governing equations in solid region

The governing equation in solid insulation is Gauss’s law with zero space charge, as follows:

where the current density J SD is zero because the solid insulator has zero conductivity. ρSD is the mass density, cSD is the specific heat capacity, and kSD is the thermal conductivity in the solid insulator region. COMSOL Multiphysics software implementing a detailed numerical setup was used to solve these coupled equations.

2.3 Electric and buoyant body force density

To examine body forces, we included the Coulomb force for net charge, the Kelvin force for the electric field gradient, and the Boussinesq buoyancy force for the temperature gradient as [10, 11]

where P is the polarization vector, β is the thermal expansion coefficient, Tref is the buoyancy reference temperature, and g is the acceleration due to gravity.

2.4 Calculation of surface charge accumulation at dielectric liquid-solid interface

The surface charge density σs at the dielectric liquid–solid interface can be calculated as

where Ji = ρiμiE and n is the outward normal unit vector from solid to liquid. To determine the surface charge density in Poisson’s equation, the surface charge density can be calculated using Gauss’s law as

 

3. Space Charge Propagation with Liquid-Solid Interfaces by the Applied Step Voltage

Tip-plate electrodes with two different dielectric liquid–solid interfaces are shown in Fig. 2. Case I has a disk-shaped dielectric solid located on the cathode, which forms a vertical liquid-solid interface, and Case II has a parallel interfaces representing the direction of the liquid-solid tube-shaped dielectric solid between the anode and the cathode, which forms a parallel liquid-solid interface. The vertical and parallel interfaces are defined with respect to the direction of the electric field. The order of the initial electric field intensity around the tip was approximately 108 V/m, which is sufficient for the initiation of streamers [9].

Fig. 2.Tip-plate electrode model with two different liquid–solid interfaces. Case I has a disk-shaped vertical dielectric solid and Case II has a tube-shaped parallel dielectric solid (with respect to the direction of the electric field).

Figs. 3 and 4 show the electric field distributions from 100 ns to 2.15 μs in Case I and from 100 ns to 325 ns in Case II, respectively. In Case I, there are three sections for the discharge propagation of the electric field wave: Fig. 3(a)-(c) for the pure oil region with a propagation speed of 6.7 km/s; Fig. 3(c)-(e) for the top surface with a propagation speed of 1.6 km/s; and Fig. 3(e)-(f) for the side surface with a propagation speed of 0.23 km/s. In Fig. 3(e)-(f), the velocity significantly decreases due to the small electric field aligned to the surface slowing down the charge development. In Case II, the propagation speed of the electric field wave was 11.1 km/s, which is much faster than that in Case I.

Fig. 3.Electric field distributions from 100 ns to 2.15 μs with the vertically aligned insulator in Case I. The propagation velocity of the electric field wave into the space in (a)-(c): 6.7 km/s, (c)-(e): 1.6 km/s, and (e)-(f): 0.23 km/s.

Fig. 4.Electric field distributions from 100 ns to 325 ns with the parallel insulator in Case II. The propagation velocity of the electric field wave into space in (a)-(f) is 11.1 km/s.

Fig. 5 shows the temporal dynamics of the electric field and surface charge density with a step voltage of 20 kV and 100 ns rising time for Case I and Case II. With the tube-shaped interface, the propagating speed was approximately 11.1 km/s. In the literature [12, 13], the experimental propagating speed is approximately 12 km/s. Our numerical result has good agreement with that of the experimental results. The high accuracy of our numerical setup produced the velocity profile depicted in Fig. 6. The velocity was almost equal to the propagation speed in the breakdown region.

Fig. 5.Temporal dynamics of the electric field and surface charge density with a step voltage of 20 kV and 100 ns rising time for Case I and Case II.

Fig. 6.Distributions of normalized vector fluidic flow (as arrows) and temporal electric field (as surface plots) for different time steps.

Table 1.Comparisons of breakdown velocity with tube-shaped interface

 

4. Conclusion

A fully coupled FEA was successfully developed for electrohydrodynamics (EHD) analysis of dielectric liquid flow, including the surface charge density effects on dielectric flow guidance. The simulated speed of surface charge propagation was compared through experimental results shown in the literature. To analyze this complicated multiphysics problem, the generalized hydrodynamic drift-diffusion equations were composed of Poisson’s equation, the Nernst-Plank equation, the Navier-Stokes equation, continuity equations, and the energy balance equation. With the parallel guidance with respect to the direction of the electric field, the speed of discharge propagation was significantly enhanced and had good agreement with that of experiments.