Surface Charge Accumulation on Insulators in DC GILs: History
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Contributor:
Fangwei Liang
,
Hanhua Luo
,
Xianhao Fan
,
Xuetong Li
,
Xu Wang

Gas-insulated power transmission lines (GILs) can replace cables and overhead transmission lines, playing an important role in DC transmission systems. However, the influence of surface charge accumulation on insulation reliability cannot be ignored as the operational voltage of the DC GIL increases. 

  • gas-insulated power transmission line (GIL)
  • surface potential measurement
  • surface charge accumulation mechanism

1. Introduction

Primary energy is mainly distributed in the northwest of China, while most electric energy is consumed in the central and eastern regions. Therefore, it is necessary to implement high-capacity and long-distance power transmission to meet the increasing demand for electricity [1][2][3]. Compared with AC, high-voltage DC transmission has advantages such as low losses, a large transmission capacity, and convenience for grid interconnection. DC transmission has been widely used in the long-distance transmission of electric energy [4][5][6]. Ultra-high voltage (UHV) and high-voltage direct current (HVDC) power transmission systems have rapidly developed since the 1950s [7][8]. The DC system is of great significance for China to optimize its energy allocation [9][10]. However, the HVDC transmission lines are facing new challenges, such as high altitudes, large drops, harsh meteorological conditions, low electromagnetic radiation, and strict environmental protection rules [11][12], and the overhead transmission lines and power cables cannot meet these requirements. Under such power supply requirements, gas-insulated transmission lines (GILs) with a SF6 or SF6/N2 gas mixture as the insulation medium have many advantages, such as a large transmission capacity, small footprint, high operational stability, and environmental friendliness [13][14][15]. GILs can be directly buried underground or installed in tunnels. Therefore, GILs are an excellent choice for large-scale power transmission in addition to overhead transmission lines and power cables [16].
DC GILs will play an important role with the development of DC transmission [17][18][19]. However, the GILs in operation are basically a AC systems. There are only a few DC GILs due to the difference between AC and DC insulation systems [20][21]. This is because of the different electric field distributions caused by the surface charge accumulation on the insulator [22]. For AC systems, the distribution of the electric field depends on the permittivity. However, the DC system is mainly determined by the resistivity of the material [23][24]. The electric field distribution gradually transitions from the initial capacitive electric field to a steady resistive electric field due to surface charge accumulation [25][26]. Furthermore, the internal insulation system of a DC GIL consists of insulators and compressed gas (e.g., SF6 or a SF6/N2 gas mixture) [16][27]. However, the SF6 in GILs is one of the greenhouse gases limited by the Kyoto Protocol. Its greenhouse effect is 23,500 times greater than that of CO2 [28]. The arc extinguishing capability of insulation gas is not required. This is because there are no switchgears in GILs [16]. Furthermore, the SF6/N2 gas mixture has the advantages of good insulation strength, stable chemical properties, and lasting mixing characteristics. The SF6/N2 gas mixture has been widely used in second-generation GILs to reduce the use of SF6 [29]. Moreover, the cost can be further reduced due to the low price of N2. For example, a 20% SF6/80% N2 gas mixture has 69% of the insulation strength of pure SF6 under the same gas pressure. The pressure of the gas mixture needs to be increased by 45% in order to achieve the same insulation strength. However, the use of SF6 is reduced by 71% [16]. Therefore, the impact of greenhouse gases has been reduced. As a result, the insulator has become the weakest component in DC GILs [30][31].
The insulator is the core component of a GIL [32]. GIL insulators are mainly divided into basin insulators and tri-post insulators. High-voltage conductors are supported by insulators. The conductor is mounted on the axis of the GIL. Additionally, high-voltage conductors are kept insulated from the ground by the insulator [33][34][35][36]. The gas seal of the pipeline is achieved by a basin insulator with a sealing ring. The installation position of the tri-post insulator can be easily adjusted. This is because the grounded electrode of the tri-post insulator has a wheel. GIL insulators are mainly composed of epoxy resins with high resistivity [37][38][39]. As a result, the accumulated surface charge cannot dissipate for a long time. Therefore, the operation of the GIL is threatened due to the flashover triggered by the accumulated surface charges [40][41][42][43].

2. Surface Potential Measurement

The study of surface charges should be based on the accurate measurement of the surface potential distribution on the GIL insulator [44]. The measurement of surface potential is different from that of the voltage for a high-voltage conductor. This is because a certain insulation gap must be maintained between the probe and the measured surface to prevent the leakage of the induced charge onto the probe. At present, the main measurement methods for insulator surface potential include dust maps, the Pockels effect, and electrostatic probes. [45][46].

2.1. Dust Map

Dust maps, also known as Lichtenberg figures, were proposed by Lichtenberg in 1777 [44]. Firstly, the red lead oxide and yellow sulfur are selected. The powder is uniformly suspended in gas. The charged powder with different polarities is absorbed on the insulator surface due to the electrostatic attraction of the surface charges. As a result, the visual analysis of the charge distribution is realized. However, the surface charge distribution cannot be quantitatively measured by a dust map [47]. Since the dust particles are adsorbed on the measured surface, it is not possible to achieve repeated measurements through a dust map [48]. The movement of charged dust particles is also affected by gravity and buoyancy [49]. Furthermore, the original surface charge distribution may be changed. This means that the accuracy of dust maps needs to be further improved. 

2.2. Pockels Effect

The Pockels effect method was proposed by Takada in 1991. The principle was based on the electro-optical effect of Pockels crystals, such as Bi12SiO20 and Bi4GeO12 [12][50]. The refractivity of Pockels crystals varies linearly with the electric field [51]. Once the electric field inside the crystals is changed due to the accumulated surface charge, the phase of the incident laser is delayed. The phase delay of light is transformed into a difference in light intensity. Then, the electric field distribution inside the crystal is obtained. As a result, the surface charge density is calculated [52]. The real-time and quantitative measurement of surface charges on certain thin and transparent materials can be realized by the Pockels effect [18]. Furthermore, the method is only applicable to the measurement of insulator surface charges under an AC voltage or impulse voltage due to the electrostatic relaxation of Pockels crystals [12].

2.3. Electrostatic Probe

The electrostatic probe method is a non-contact measurement technique [53][54]. There are two commonly used electrostatic probes: capacitive electrostatic probes based on electrostatic induction and Kelvin electrostatic probes designed according to the principle of electric field compensation [55][56]. The capacitive electrostatic probe was proposed by Davies in 1967 [57]. The quantitative measurement of surface charges can be realized by a voltage-following circuit. The data acquisition unit of the probe is composed of a high-input-impedance operational amplifier [58]. In contrast to capacitive probes, Kelvin probes are active probes. Furthermore, Kelvin probes are not affected by the surrounding gas. Kelvin probes are widely used due to their higher accuracy.
The inductive electrode in the probe is kept vibrating by an oscillator. As a result, the capacitance between the electrostatic probe and the measured surface is changed. If the voltage on the probe is not equal to the measured surface potential, a displacement current is generated on the induction electrode. The voltage on the probe is adjusted until the induced current becomes zero. At this moment, the voltage on the probe is equal to the measured surface potential [59]. Since there is no potential difference between the probe and the measured surface, the partial discharge triggered by the electrostatic probe during the measurement is avoided [49]. This method is an ideal measurement method for surface charges. Typical products include the Trek-341B [60], Trek-347 [61][62], and Monroe 244A electrostatic voltmeter [63][64].

3. Surface Charge Accumulation Mechanism

The surface charge accumulation of a DC GIL insulator is affected by the applied voltage, insulation gas, insulator shape, temperature, etc.

3.1. Applied Voltage

Under a DC voltage, the surface charge density increases with the applied time. The surface charge distribution is similar under different voltage levels [39]. Furthermore, the polarity of surface charges is reversed due to the increase in the DC voltage when only natural radiation is considered for the generation of charged particles in gas [25]. Since the conductivity of the insulation gas and insulator is a function of the electric field [65], the surface charge accumulation is also affected by the electric field.

3.2. Insulation Gas

In order to reduce the use of SF6, the insulation gas of second-generation GILs is a SF6/N2 gas mixture [16]. Therefore, the surface charge distribution of insulators in 0.5 MPa pure SF6 and a 20% SF6/80% N2 gas mixture were compared by Wang et al. [66]. The results showed that the amount of surface charge was larger in the 20% SF6/80% N2 gas mixture under −30 kV. This was mainly because the partial discharge in the gas increased with the decrease in the SF6 content. Furthermore, the accumulation of surface charges on insulators in the pure SF6 had a significant polarity effect. That is to say, the surface charges under a negative voltage were obviously larger than those under a positive voltage. However, no significant polarity effect was observed in the 20% SF6/80% N2 gas mixture. This was because the pure SF6 was more sensitive to metal particles. The ionization coefficient of the gas sharply increased due to the enhancement in the local electric fields. As a result, the sources of the surface charges increased [67][68]. For the 20% SF6/80% N2 gas mixture, a polarity effect was not observed. This was mainly because the sensitivity of the partial discharge to the electric field distortion was reduced by the N2.

3.3. Insulator Shape

The accumulation of surface charges is inevitably affected by the geometry of the insulator. Early on, cylindrical insulators were used as simplified models for GIL insulators. Then, cylindrical insulators with shielded electrodes were adopted by Wang et al. [53]. In order to truly reflect the operational conditions of GIL insulators, the circular insulator was proposed by Li et al. [61]. Furthermore, proportionally reduced conical insulators were adopted by Deng et al. [29]. Bowl-shaped insulators were designed to achieve the self-adaptation of surface charges based on nonlinear materials [69]. In addition, an actual GIL basin-type insulator was researched by Qi et al. [70].
Based on a surface charge transport model considering the microscopic parameters of the gas, the influence of the insulator shape was analyzed by Ma et al. The initial capacitive electric field distribution was compared with the steady-state resistive electric field distribution [71]. The results showed that the surface charge distribution was similar to the normal component of the initial capacitive electric field. The amount of surface charge accumulated on the conical insulator was the largest.

3.4. Temperature

Due to the Joule heating of the operational current, the temperature of the internal conductor in a GIL may rise to 90 °C [72]. Furthermore, the volume conductivity of epoxy insulators is a function of the temperature. The partial discharge in a gas is also affected by the temperature [73]. Therefore, the influence of the temperature on the surface charge accumulation cannot be ignored. The temperature distribution inside a GIL was calculated by Zhou et al. based on thermal convection, radiation, and conduction [74]. The results showed that the temperature distribution inside the GIL gradually decreased from the center conductor to the grounded shell. The temperature gradient was 30 K under a current of 3450 A. The peak of surface charge density was 7.68 nC/cm2, which increased by 60%. Additionally, high-temperature oil circulation was used to heat the internal conductor of the GIL.
Once the temperature gradient was 70 °C, the average value of the surface potential increased from 278 V to 1670 V under a positive voltage. Under a negative voltage, the average surface potential increased nearly twofold. Tang et al. pointed out that the mean free path of electrons increased with the temperature. Therefore, the initial voltage of the corona decreased. As a result, the positive charge density increased under the temperature gradient [41].
The above factors affecting insulator surface charges are mainly due to the differences in the charge accumulation pathway. There are three pathways to accumulating charge on an insulator surface: through the insulator volume, insulation gas, and along the insulator surface [75]. For the ideal case of ignoring the insulation defects inside the GIL, the surface resistivity of the insulator is large enough. The current along the insulator surface is small enough to be ignored. Therefore, the charge accumulated through the insulator volume is uniformly distributed [18]. The conduction current is determined by the volume conductivity of the insulator. For conical insulators, charges of the same polarity accumulate on the insulator surface near the inner conductor [36]. Since space charges are easily generated due to the uneven distribution of volume conductivity, the space charge migrates to the insulator surface, resulting in surface charge accumulation. That is to say, it follows the volume conductivity model proposed by Cooke et al. [12].
The conduction current through the gas is affected by the generation rate, recombination coefficient, mobility, diffusion coefficient, and other factors [7]. Insulation defects are inevitable within a DC GIL, such as protrusions on the electrode, metal particles on the insulator surface, and the gas-electrode-insulator tri-junction, since the corona discharge of insulation defects is the main cause of surface charge accumulation. This means that the insulator surface charge distribution is uneven [18]. Furthermore, a normal electric field model was proposed by Knecht et al. [76]. It showed that the normal electric field on the insulator surface is the cause of charge accumulation.
The conduction current along the surface is closely related to the surface conductivity of the material [77]. For insulators with surface modification, the surface charge distribution depends on the conduction current along the surface. The tangential electric field model was proposed by Nakanishi et al. [78]. This model points out that there is an exponential relationship between surface conductivity and tangential electric field, resulting in uneven material surface. Therefore, the discontinuity of conducting current along the surface is the main cause of surface charge accumulation.
The three accumulation pathways of surface charges exist simultaneously. However, the proportion of each part varies significantly under different tests [28]. For the vacuum, corona discharges from the tri-junction of the cathode are the main source of insulator surface charges [79]. Yu et al. pointed out that the surface charge density is closely related to the secondary electron energy emission curve [80]. When the partial discharge in the gas is ignored, the conduction current through the insulator volume is the main accumulation pathway for surface charges [71]. For high-resistivity insulation systems, the accumulation of surface charges is determined by the conduction current in the gas side. The main accumulation pathway gradually changes from the gas to the insulator when the volume resistivity is less than 5 × 1017 Ω ·cm [22]. Furthermore, the generation rate of ion pairs in the gas is one of the key factors. The accumulation pathway changes to the conduction current in the gas with the increase in the ion pair generation rate [81][82].
For basin insulators, a unified surface charge accumulation mechanism was proposed by Li et al. The electric field is divided into three levels based on the leakage current [83][84][85]. For the low electric field, there are many factors, such as residual static charges on the surface, bulk charges inside the insulator, and charged particles in the gas. Each factor may become the core. Thus, charge accumulation under low electric fields can occur in a variety of ways. Under a medium electric field, charges of the same polarity injected through the insulator are the main source. Since the ionization in the gas side is enhanced due to insulation defects under a high electric field, the influence of charged particles in the gas on the surface charge accumulation increases. Therefore, the source of the surface charge is electric-field-dependent.

This entry is adapted from the peer-reviewed paper 10.3390/en16166027

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