Surface charge accumulation in the spacer modifies local electric fields, which restricts the industrialization of high voltage direct current (HVDC) gas-insulated transmission lines (GILs). In this paper, the state of art in gas/solid interface charging physics and models, covering areas of charge measurement techniques, charge transport mechanisms, charge related DC surface flashover models, and charge control methods, is reviewed and discussed. Key issues that should be considered in future studies are summarized and proposed. The purpose of this work is to provide a brief update on the most important and latest progress in this research area, and to educate readers as to the current state of the gas-solid interface charging phenomenon, which has seen great progress in the past few years.
High voltage alternating current (HVAC) gas-insulated transmission lines (GILs) can realize large-capacity power transmission in complex environments, and have been in use since 1960s. However, under high voltage direct current (HVDC), the influence of the surface charge accumulation in spacers must be considered before development of HVDC GIL. At DC voltage, charges transport along electric field lines both in the gas phase and in the spacer bulk, and accumulate on the surface of spacers inside GILs. The local electric field over spacer surface gets more disordered, and is prone to generating a higher local electric field strength as a consequence of the influence of the charge migration. Under these conditions, a surface flashover is more easily triggered.
In recent years, the urgent need for HVDC GIL, driven by offshore projects, requires more urgent breakthroughs in this field, and the problem of charge accumulation has become tremendously pronounced. As a result, an increasing number of researchers over the past few years have focused on this area, and have made significant efforts to tackle difficult problems that are still a specific challenge to us. The rapid progress of related research can be reflected both in a surging amount of research, and in special topical issues of the past 5 years.
Despite the extensive studies and output during the past few years, charge behaviors such as charge generation, transport, and relaxation in dielectrics should be studied, as these fields of research have still not been fully understood.
The Lichtenberg dust figure method can be used to reflect surface charge distribution based on the property that a charged surface can adsorb dust particles with hetero-polarities, as shown in Figure 1a . Based on the principle that dielectrics can be polarized inside electric fields, the electric field density due to charge accumulation on the surface can be depicted by polarized dielectric particles, as shown in Figure 1b. More details regarding polarized surface charge cluster formation and dust pattern phase transition can be found in the literature. In recent years, Kelvin probe methods have been more widely used. As an oscillating feedback capacitive probe, the Kelvin probe is a typical representative of an active electrostatic probe, by which the potential can be obtained from the measured point. The distribution of surface charge density can be obtained by a simulation algorithm based on the surface potential measurement result. The comparison of these two methods is shown in Table 1.
|Lichtenberg dust figure||Measurement result is not restricted by surface potential value and is not influenced by charge decay process during measurement.||Cannot quantitatively characterize charge density (or electric field strength); dust adsorbed may have a certain impact on local electric field.|
|Kelvin probe||Surface potential value can be quantitatively characterized with a high sensitivity.||Low spatial resolution and relative low voltage range; measurement takes longer.|