Correlation Characteristics Comparison of SF₆ Decomposition versus Gas Pressure under Negative DC Partial Discharge Initiated by Two Typical Defects

Abstract

Aimed to clarify the correlation characteristics between the internal partial discharge (PD) in negative direct current (DC) gas insulated system (GIS) and gas pressure initiated by two typical defects (i.e., free-metal particles and metal protrusion), this study on PD decomposition of sulfur hexafluoride (SF₆) was investigated under different pressures on the basis of constructing a SF₆ decomposition experimental platform with DC PD. Free-metal particles and metal protrusion in a GIS were simulated using a spherical-bowl electrode and a needle-plate electrode, respectively. Trends and differences in the performance of SF₆ decomposition components SOF₂, SO₂F₂, CO_2, and SO₂ at different pressures were compared and analyzed by experiments under different defects. Based on gas microscopic ionization theory, the relationship between the decomposition component and gas pressure was deduced and verified. The concentrations of different decomposition components were found to vary with the change in gas pressure under different defects, whereas the characteristic ratios of decomposition components versus gas pressure showed a similar trend.

1. Introduction

With the large-scale renewable energy access and the application of flexible direct current (DC) transmission technology [1], DC gas insulated system (GIS) equipment has been widely investigated aiming at improving the reliability of the power system and reducing the space occupied by the equipment [2,3]. However, GIS inevitably encounter different levels of insulation defects in design, manufacturing, transportation, installation, operation, and maintenance. In time, these insulation defects may escalate into insulation failure during operation, thus posing a risk to the safe and reliable operation of the equipment and the entire power grid [4]. Extensive studies [5,6] have reported that the SF₆ molecules crack to form low fluoride (SF_x, x = 1, ..., 5) under partial discharge (PD) [5,6]. After long-term operation, different levels of trace moisture, oxygen, and other impurities may be present inside of GIS. SF_x will react with impurities to form new components, which can result in the deterioration of the insulation. The generated components are closely related to the internal fault factors. The decomposed components analysis (DCA) method, which is based on the generation of and variation in these characteristic components, can be used in condition monitoring and fault diagnosis of SF₆ gas insulation equipment [7].

The physical and chemical processes of SF₆ decomposition under PD are very complex, because many factors influence the decomposition, such as discharge energy, electrode material, trace oxygen, trace water, and gas pressure. Most of the previous studies have focused on the influence of discharge energy, electrode material, trace oxygen, and trace water [8,9,10]. Only a few studies have conducted a qualitative research and description on the effect of pressure [11,12]. Moreover, the aforementioned research primarily focused on the characteristics of alternating current (AC) GIS under PD decomposition initiated by metal protrusion, and does not extend to the DC field and other typical defects. According to statistics, free metal particles and metal protrusion defects are encountered at the forefront of insulation defects from the point of triggering GIS failure rates [4]. Gas pressure in GIS varies with the room temperature, and the gas pressure of SF₆ gas insulation equipment at different voltage levels varies as well. Therefore, investigating the relationship between gas pressure and the SF₆ decomposition component under free metal particles and metal protrusion defects can elucidate the SF₆ gas decomposition mechanism, and provide a theoretical basis for gas pressure effect modification when diagnosing the insulation fault by DCA.

In the present work, defects caused by free-metal particles and metal protrusion were simulated by spherical-bowl electrode and needle-plate electrode, respectively, based on the SF₆ DC PD decomposition experiment platform. Formation data of SOF₂, SO₂F₂, CO₂, and SO₂ were obtained by experiments under 0.2 MPa, 0.25 MPa, 0.3 MPa, 0.35 MPa, and 0.4 MPa, respectively. This paper systematically studied the variation mechanism of SF₆ decomposition component versus gas pressure under different defects by experimentation and theory. Our results provided a theoretical and experimental basis for using SF₆ decomposition products to identify gas insulation equipment failure and to assess its state of insulation.

2. SF₆ DC PD Decomposition Experiment

2.1. Experimental Research Platform

The SF₆ DC PD decomposition experimental device is shown in Figure 1. The regulator (T₁: 0–380 V) and the experimental transformer (T₂: 50 kVA/100 kV) provided the AC high voltage to the circuit. The half-wave rectifier circuit composed by the high voltage silicon stack (D: 100 kV/5 A) and the filter capacitor (C₁: 0.2 µF) converted AC voltage into a negative DC voltage, which was applied on the insulation defect. Protection resistors (R₁: 10 kΩ) and (R₂: 20 kΩ) limited the current at breakdown to protect the entire experimental device. C_v and R_v were used as a capacitive voltage divider and resistance divider to measure the AC and DC voltage, respectively. The above devices were made by Hdbl Company, Wuhan, China. Coupling capacitance (C_k: 500 pF/100 kV), non-inductance resistor (Z_m: 50 Ω), and digital storage oscilloscope (DSO) were used to measure discharge magnitude. The characteristic components of SF₆ were separated and quantitatively detected using Shimadzu QP-2010Ultra gas chromatography (GC/MS) with special capillary column (CP-Sil5CB). The specific experimental steps are described in references [10,12].

2.2 Experimental Content

2.2.1. Defect Model

The experiment had to overcome the problem of experimental data dispersion caused by the decrease of particles during test, as well as to simulate an axisymmetric and mostly uniform field between the real GIS equipment shell and the high voltage (HV) conductor. Free-metal particles defect in GIS devices was were simulated by 20 six-series aluminum alloy balls with a diameter of 3 mm [13], which are located in a spherical bowl electrode, as shown in Figure 2a. The bowl was cut with a hollow stainless steel sphere (Φ = 100 mm). The cut diameter was 90 mm, which enabled the jumping aluminum balls to fall back into the bowl electrode effectively and thus prevented the decrease in number of aluminum balls during the experiment and stabilize the experimental data.

The metal protrusion defect was simulated by a stainless steel needle–plate electrode. The distance between the needle and plate was 10 mm. The diameter of the plate electrode was 100 mm, and the curvature radius of the needle electrode tip was 0.3 mm. The physical model of the metal protrusion defect is shown in Figure 2b.

2.2.2. Experiment Conditions

The operation pressure of SF₆ DC GIS (such as in medium- and low-pressure wall bushing, and switchgear cabinet) is generally less than 0.4 MPa. The pressure of laboratory equipment is 0.1–0.4 MPa. Considering the above information and the current experimental conditions, five SF₆ absolute pressure were applied, namely, 0.2, 0.25, 0.3, 0.35, and 0.4 MPa. To ensure experimental accuracy, all experiments were conducted in room temperature of 20 °C, and a relative humidity of 40%. The concentrations of trace water and trace oxygen in every experiment were maintained at about 40 μmol/L and 70 μmol/L to ensure that the experimental conditions are consistent. During the experiment, the applied voltage were 60 kV to the needle-plate electrode, and 48 kV to spherical-bowl. PD decomposition experiments were carried out for 96 h in each group. Samples were collected at 12 h intervals and quantitatively analyzed by GC/MS. The SF₆ was quantitatively analyzed at 0 h before every experiment to obtain the concentration impurities that may remain in the chamber during the previous experiment. The concentrations of SF₆ decomposition components in this paper are net production which has been subtracted from the 0 h background of impurities. To verify the reproducibility of the experiment, all experiments were carried out twice under the same experimental conditions in this paper. The experimental data were averaged for two of the experimental data. And the maximum relative mean deviation RSD_max of each group of data at same discharge time is used to illustrate the reproducibility of the experimental data.

$$RS{D}_{max}=\max \left[\frac{\left|x_i^1-{\overline{x}}_i\right|+\left|x_i^2-{\overline{x}}_i\right|}{\overline{x_i}}\right]\hspace{1em}i=0.2\mathrm{Mpa},0.25\mathrm{Mpa},....0.4\mathrm{Mpa}$$

(1)

where, $x_i^1$ is the first measured data under the i pressure; $x_i^2$ is the second measured data under the i pressure; ${\overline{x}}_i$ is averaged for two measured data under the i pressure. RSD_max is the maximum value of the five groups of pressure at the same discharge time.

3. Change Trend in SF₆ Decomposition Components versus Gas Pressure

Considering constant volume, an increasing pressure of the reaction chamber was increased to change the molar concentration of the gas in the chamber, which in turn affected the progress of the chemical reaction. However, GC/MS detects the volume fraction of the gas (μL/L), which cannot directly reflect the change in the absolute generation of decomposition components in the gas chamber. Therefore, the volume fraction of gas was converted into molar concentration prior to analysis. Given that the experimental chamber was about 60 L, only a small amount of gas was collected to quantitatively detect decomposition products every 12 h. In addition, the SF₆ decomposition ratio was very small, and thus, the change in gas pressure can be neglected. The conversion formula between volume fraction of decomposition components and the concentration is shown in Equation (2).

$$c_i=\frac{p_i}{RT}{v}_i$$

(2)

where, c_i is the concentration of component (μmol/L); p_i is the gas pressure of the experiment chamber; R is the gas constant; T is the room temperature; and v_i is the volume fraction of component.

The types and amount of decomposition components under the two typical defects at 96 h were shown in Figure 3. SF₆ were decomposed into CO₂, SO₂F₂, SOF₂, and SO₂ in five groups of different gas pressure experiment under both metal protrusion defect and free-metal particles defect. The yield relationship of metal protrusion defect is SOF₂ > SO₂F₂ > CO₂ > SO₂ under different gas pressures. The concentrations of SO₂F₂ and SOF₂ were much higher than the others, which is the main decomposition product under the metal protrusion defect. The yield relationship of free-metal particles defect under different pressures presented a different relationship. When 0.2 MPa ≤ p ≤ 0.25 MPa, SOF₂ > SO₂F₂ > CO₂ > SO₂; when 0.3 MPa ≤ p ≤ 0.35 MPa, SOF₂ > CO₂ > SO₂F₂ > SO₂; when p = 0.4 MPa, SOF₂ > CO₂ > SO₂ > SO₂F. The concentrations of SOF₂ are much higher than others. The average single discharge energy of metal protrusion defect at different pressures were 1.53 × 10⁻⁵ J, 1.49 × 10⁻⁵ J, 1.49 × 10⁻⁵ J, 1.38 × 10⁻⁵ J, 1.34 × 10⁻⁵ J, respectively. The average single discharge energy of free metal particles defect were 9.99 × 10⁻⁵ J, 1 × 10⁻⁴ J, 8.67 × 10⁻⁵ J, 8.2 × 10⁻⁵ J, 7.54 × 10⁻⁵ J.

3.1. Change Trend in SF₆ Decomposition Component Content versus Gas Pressure

3.1.1. The Characteristics of the Changes in SOF₂ Concentration versus Gas Pressure

The relationship of SOF₂ concentration versus pressure is shown in Figure 4. The generation concentration of SOF₂ exhibits an inverse “S” curve with increasing gas pressure within the same PD time initiated by the metal protrusion defect. The change curve of SOF₂ concentration versus gas pressure can be divided into three periods: Saturated area, Exponent area, and Flat area. When the gas pressure is at 0.2 MPa ≤ p ≤ 0.25 MPa, the energy obtained by the particle in the electric field decreases as gas pressure increases. Furthermore, the probability of the excitation or ionization of the gas molecules caused by collision decreased; the formation of SOF₂ gradually declines. When gas pressure is at 0.2 MPa, the longer mean free path can lead to further particle collision reaction. However, the concentrations of H₂O and O₂ in the chamber inhibited SOF₂ formation. With the increase in gas pressure, the collision ionization is gradually reduced. The inhibition of trace amounts of H₂O and O₂ was also gradually weakened. Consequently, the slope between the generation concentration of SOF₂ and gas pressure gradually increased. With decreased gas pressure to nearly 0.25 MPa, the inhibitory effect of trace amounts of H₂O and O₂ on SOF₂ formation became negligible. Hence, the 0.2 MPa ≤ p ≤0.25 MPa stage can be called the Saturated Area. In addition, when the pressure further increases to 0.3 MPa, the generation of SOF₂ is primarily related to the amount of SF₆ ionization decomposition. At this stage, the reduction of the electron mean free path leads to a significant reduction of the particle energy and drastically reduces and the possibility of SF₆ collision ionization is sharply reduced. Therefore, the SOF₂ generation concentration is rapidly reduced with increased gas pressure. This stage can be called the Exponent Area. When the gas pressure is in within the range of 0.3 MPa ≤ p ≤ 0.4 MPa, the mean free path was very small compared with the Exponent Area. At this stage, the probability that the electron free path was greater than the required path of the collision ionization slowly reduced [14]. The formation concentration of SOF₂ was also gradually reduced, and its slope with the gas pressure is less than the Exponent Area. This stage can be called the Flat Area.

With the free-metal particles defect, SOF₂ generation concentration almost did not change with the increase in gas pressure, and remained in the Flat Area. SOF₂ is usually derived from the hydrolysis of SF₄. SO₂F₂ mainly comes from the reaction of SF₂ and O₂ [15]. The energy required for SF₄ generation is lower than that required for SF₂, which indicates that the effect of increased pressure on lower energy discharge is weak in the free-metal particles defect. This result can be explained as follows: the metal protrusion defect can maintain a stable electric field distortion for a long time. The electrons need to be accelerated by a long distance to get enough energy and achieve SF₆ collision ionization. The gas pressure affects the electron mean free path, therefore affecting its PD process. Hence, the formation of components under the defect is significantly affected by the pressure. In the free-metal particles defect, free particles that have induced enough charge to overcome its own gravity take off and collide with the ball electrode. The process of generating the induced charge is weakly affected by the gas pressure. In addition, the electric field exhibits serious distortion and PD occurs when the particles are very close to the electrode. The discharge time is very short and the discharge is violent. The average single discharge energy in the free metal particles defect at 0.4 Mpa was much greater than the metal protrusion, which indicates that the electrons can obtain a certain amount of energy even at higher pressure under free metal particles defect. Therefore, under this defect, the change in gas pressure has less effect on the lower energy discharge.

3.1.2. The Change Characteristic of SO₂ Concentration versus Gas Pressure

The relationship of SOF₂ concentration versus gas pressure under the two typical defects is presented in Figure 5. SO₂ is mainly derived from the hydrolysis of SOF₂ [15]. The relationship of its concentration versus gas pressure is similar to that of SOF₂. This relationship can be divided into three periods as well, namely, Saturated Area, Exponent Area, and Flat Area under the metal protrusion defect. In the free-metal particles, the concentrations observed all fall under the Flat Area.

3.1.3. The Change Characteristic of CO₂ Concentration versus Gas Pressure

The relationship between CO₂ concentration and gas pressure in the two typical defects is presented in Figure 6. The generation concentration of CO₂ exhibits an inverse “S” curve with increased gas pressure within the same PD time initiated by metal protrusion defect. The change curve of SOF₂ concentration versus gas pressure can be divided into three periods: Saturated Area, Exponent Area, and Flat Area. When the gas pressure is within the range of 0.2 MPa ≤ p ≤ 0.25 MPa, CO₂ formation is inhibited by the concentrations of H₂O and O₂ and thus the formation of CO₂ does not increase rapidly with the decrease in gas pressure. When the gas pressure increases and approaches 0.25 MPa, the limits of H₂O and O₂ are no longer significant. At this stage, the collision ionization and the concentration of CO₂ decreases rapidly with the increase in gas pressure. As the gas pressure further increases, the inflection point of the curve appears near 0.3 MPa. Similar to the change curve of concentration of SOF₂ in the Flat Area, the number of excited C particles slowly declines as gas pressure increases, compared with Exponent area and the increase in gas pressure. This stage is called the Flat Area.

In the free-metal particle defects, the concentration of CO₂ is almost constant with increased gas pressure, falling in the Flat Area. This phenomenon can be compared to the reason for the formation of SOF₂ in free-metal particles, which suggests that the energy required for CO₂ formation is also low.

3.1.4. The Change Characteristic of SO₂F₂ Concentration versus Gas Pressure

The relationship between SOF₂ concentration and gas pressure in the two typical defects is presented in Figure 7. The concentration of SO₂F₂ is less than that of SOF₂ for the energy of SO₂F₂ formation is larger than of SOF₂ [15].

In the same PD time caused by the metal protrusion, the concentration of SO₂F₂ versus increased gas pressure shows an inverted “J” type curve relationship, which is different from the concentration of SOF₂ with increased gas pressure, which exhibits an inverse “S” curve relationship. That is, the change curve of SOF₂ generation concentration versus the pressure can be divided into two periods: Exponent Area and Flat Area. As shown in Figure 4a and Figure 7a, both the Flat and Exponent Areas appear in the correlation curves of SOF₂ and SO₂F₂ versus the pressure whose cause is similar, but the curve of the SO₂F₂ concentration versus the gas pressure does not appear in the Saturated Area. This result is explained as follows: the concentration of H₂O and O₂ in the gas chamber limits the formation of SOF₂ and CO₂ and enters the Saturated Area at 0.2 MPa ≤ p ≤ 0.25 MPa. However, SF₂ has a much stronger reducibility than SF₄ and C. Consequently, O atoms are easily obtained for SF₂, which makes SO₂F₂ formation at this stage remaining in the Exponent Area.

Under the free-metal particles defect, the concentration of SO₂F₂ decreases rapidly with increased gas pressure, which all falls under the Exponent Area. From the previous analysis of the free-metal particles defect, electrons obtain some energy even under high gas pressure. The change in gas pressure has little effect on the low-energy discharge in the PD caused by the defect. However, the formation of SF₂ requires SF₆ to break four S-F bonds at the same time, which requires considerable energy. Electrons need to go through enough free paths to obtain enough energy and cause the decomposition of SF₆. Thus, the formation of SO₂F₂ was considerably affected by gas pressure. Its concentration deceases rapidly along with an increase in gas pressure. The change in gas pressure significantly affected on the higher energy discharge under metal free particle defects.

3.2. Change Characteristics of the Characteristic Components Ratios versus Gas Pressure

The ratio method has been widely used in the power equipment fault diagnosis as this method can prevent the influence of volume effect on the fault diagnosis results. In this paper, the effect of gas pressure on the ratio of two decomposition components, namely, c (SO₂F₂)/c (SOF₂ + SO₂) and c (CF₄ + CO₂)/c (SO₂F₂ + SOF₂ + SO₂), is studied under negative DC. The effective characteristic ratio (CR_RMS) is used to study SF₆ decomposition characteristics under different gas pressures to achieve significant statistical results [8]. CR_RMS is defined as follows:

$$C{R}_{RMS}=\sqrt{\frac{1}{n}{\displaystyle \sum _{m=1}^n{(\frac{C_{im}}{C_{jm}})}^2}}$$

(3)

where C_im and C_jm are the concentrations of components i and j, respectively, measured at mth time. n is the number of times after the characteristic ratio reaches the steady state.

As shown in Figure 8, the effective characteristic ratios of c (SO₂F₂)/c (SOF₂ + SO₂) under two typical defects decrease mostly linearly along with increased gas pressure. Given that the formation of SO₂F₂ consumes more energy than SOF₂, this reaction inhibits the formation of SO₂F₂, with increased gas pressure, which reduces the energy acquired by electrons. Therefore, c (SO₂F₂)/c (SOF₂ + SO₂) continues to decrease. Moreover, the effective concentration ratio c (CO₂)/c (SO₂F₂ + SOF₂ + SO₂) gradually increases along with gas pressure, and remains stable after reaching 0.3 MPa. Therefore, this conclusion shows that there is no significant change of c (CO₂)/c (SO₂F₂ + SOF₂ + SO₂) in the GIS whose gas pressure ranges from 0.3 to 0.4 MPa.

4. Quantitative Analysis of Gas Pressure on SF₆ Decomposition

Chen C.L. [16] reported that given the relatively low PD energy caused by insulation defects in an SF₆ gas insulation equipment, the difference in temperature change in the relevant area is not obvious. Therefore, the main reason for the decomposition of SF₆ gas is the strong distortion electrical field. Considering the role of electrons as a medium for energy transfer, the electron temperature and the room temperature are not equal. Therefore, the coefficient k_d of the electron collision reaction under different gas pressures is no longer constant. Hence, the relationship between rate coefficient k_d and collision ionization coefficient α is given by:

$$k_d=\frac{\alpha w_e}{N}$$

(4)

where N is the particle concentration and w_e is the electron drift velocity.

$$w_e=\mu E$$

(5)

where u is the electron mobility that is inversely proportional to pressure p. The relationship among electron drift velocity w_e, electric field E, and air pressure p is as follows [14]:

$$w_e=a\frac{E}{p}$$

(6)

The relationship between the collision ionization coefficient α and the gas pressure p is given by [14]:

$$\alpha =A_{1}p{e}^{(-B_{1}p/E)}$$

(7)

where A₁ and B₁ are the constants determined by the nature of gas itself. Particle concentration N is proportional to the pressure p under a certain volume and temperature. Substituting (7) and (6) into (4) results in the following equation:

$$k_d=\frac{A_2{e}^{-B_{1}p/E}E}{p}$$

(8)

where A₂ is a constant.

For the primary reaction e + SF₆→SF_6-x + xF + e (x ≤ 6), the pure SF₆ concentration is larger than the concentration of primary products such as SF₂ and SF₄. Therefore, the primary reaction can be regarded as a “zero order reaction” [17] (i.e., the reaction rate is independent from the reactant concentration). The reaction rate is given by:

$$\frac{d{c}_d}{dt}=k_d$$

(9)

In the following equation, c_d is the concentration of the reaction product SF_x, and its relationship with time t is:

$$c_d=k_{d}t$$

(10)

SF₂ and SF₄ react with H₂O and O₂ molecules to produce SO₂F₂ and SOF₂ in the main gas zone of the regional decomposition model proposed by Van Brunt RJ [18]. According to chemical kinetics analysis, this reaction process is a chemical second-order reaction [19]. Its reaction rate is proportional to the product of the reactant concentrations [20], as follows:

$$\gamma =-\frac{d[{\mathrm{SF}}_x]}{dt}=k_0[{\mathrm{H}}_2\mathrm{O}/{\mathrm{O}}_2][{\mathrm{SF}}_x]$$

(11)

where γ is the reaction rate; [H₂O/O₂], [SF_x] denote the reactant concentration; k₀ is the secondary reaction rate that is a pressure-independent and temperature-dependent coefficient according to the Arrhenius formula. The temperature of ions and molecules is equal to room temperature in the PD process [21]. Hence, k₀ can be considered as a constant in this experiment. Regardless of the chemical reaction to reach saturation, when the amount of SF_x generation is higher than that of consumption (i.e., when t > Y/k_d), given that [H₂O/O₂] remains substantially unchanged for X during this period and the consumed concentration of SF_x is Y at time t, H₂O/O₂ and SF_x concentrations are [H₂O/O₂] = X and [SF_x] = k_dt − Y, respectively. The reaction rate is as follows:

$$\gamma =\frac{d(k_{d}t-Y)}{dt}=k_{0}X(k_{d}t-Y)$$

(12)

Equation (12) can be further derived as follows:

$$Y=k_{d}t-e^{-k_{0}Xt}$$

(13)

When t is sufficiently large, e^−k₀Xt→0, Equation (14) can be further derived as follows:

$$Y=k_{d}t$$

(14)

The concentration C of SO₂F₂ or SOF₂ is proportional to the consumed concentration Y of reactant SF_x. The voltage and electrode were consistent in all five experiments and thus, electric field E did not change. Therefore, the concentration of SO₂F₂ and SOF₂ can be expressed as follows:

$$C=\frac{A{e}^{-Bp}}{p}t$$

(15)

where A and B are constants.

SOF₂, SO_2, and CO₂ of the free-metal particles defect were not discussed since they are almost free from the influence of gas pressure. The CO₂ formation process can also be explained by the collision theory, given that C atoms in metal protrusion defect are released by the collision between high energy particles. SO₂ is mainly derived from the hydrolysis of SOF₂ whose concentration versus the pressure’s relationship curve is similar to that of SOF₂. Therefore, the relationship between the concentrations of CO₂, SO₂F₂, SOF₂, and SO₂ at 96 h in the metal protrusion defect, SO₂F₂ concentration in free-metal particles defect, and the pressure was approximately fitted by Equation (15). The fitting curves and expressions are presented in Figure 9 and

Table 1. Fitting formula.

Typical Defects	96 h Component Fitting Formula
Metal Protrusion Defect	$C_{96h}({\mathrm{SOF}}_2)=\frac{40.34e^{-6.0p}}{p}$; $C_{96h}({\mathrm{SO}}_2{\mathrm{F}}_2)=\frac{28.11e^{-7.7p}}{p}$; $C_{96h}({\mathrm{CO}}_2)=\frac{3.77e^{-3.54p}}{p}$; $C_{96h}({\mathrm{SO}}_2)=\frac{0.57e^{-5.47p}}{p}$;
Free metal Particles Defect	$C_{96h}({\mathrm{SO}}_2{\mathrm{F}}_2)=\frac{0.17e^{-2.7p}}{p}$

, respectively.

Figure 9 shows that the fitting degree R² of the gas pressure of SOF₂, CO₂, and SO₂ concentrations at 96 h in metal protrusion defect, whose correlation curves of component concentration versus gas pressure present a Saturated Area, are lower than that of SO₂F₂, whose curve does not fall under the Saturated Area. This result is because the derivation process does not consider the saturation of the chemical reaction. However, the fitting degrees (R²) of all decomposition components concentrations versus gas pressure are high, which indicates that the fitting curve fits well with the measured values. These results show that the derived formula can explain the relationship between the concentrations and the gas pressures of SO₂F₂, SOF₂, CO₂, and SO₂.

5. Conclusions

To investigate the correlation between PD and gas pressure in the negative DC gas insulation equipment under two typical defects, SF₆ PD decomposition under different gas pressures were systematically implemented based on constructing the SF₆ decomposition experimental platform under DC PD. The following conclusions are drawn:

(1)

SF₆ was decomposed into CO₂, SO₂F₂, SOF₂, and SO₂ in five groups of different gas pressures experiment under metal protrusion and free-metal particles defects. The yield relationship of metal protrusion defect is SOF₂ > SO₂F₂ > CO₂ > SO₂ under different gas pressures. The concentrations of SO₂F₂ and SOF₂ are much higher than others. The yield relationships of the free-metal particles defect under different pressures present a different relationship. The concentrations of SOF₂ are much higher than other concentrations.

(2)

Under metal protrusion defect, the concentration of each decomposition component gradually decreases with increasing gas pressure. The change curves of SOF₂, CO₂, and SO₂ versus gas pressure can be divided into three stages, namely, Saturated, Exponent, and Flat Areas. Moreover, the change curves of SO₂F₂ can be broadly divided into two stages: Exponent and Flat Areas. The change curves of SOF₂, CO₂, and SO₂ versus gas pressure all fall under the Flat Area in metal free particles defect, whereas SO₂F₂ falls in the Exponent Area.

(3)

Under the two typical defects, the effective characteristic ratio of c (SO₂F₂)/c (SOF₂ + SO₂) decreases as gas pressure increases. The pressure effect should be modified when GIS is used for fault diagnosis. The effective concentration ratio c (CO₂)/c (SO₂F₂ + SOF₂ + SO₂) gradually increases with the gas pressure and remains stable after 0.3 MPa. Therefore, the effect of gas pressure on the effective characteristic ratio of c (CF₄ + CO₂)/c (SO₂F₂ + SOF₂ + SO₂) can be neglected when GIS is used for fault diagnosis whose gas pressure falls within the range from 0.3 to 0.4 MPa.

(4)

The relationship between the concentrations of CO₂, SO₂F₂, SOF₂, and SO₂ in the metal protrusion defect and the SO₂F₂ concentration in free-metal particles defect with the pressure can be expressed as $C=\frac{A{e}^{-Bp}}{p}t$.