Influence of moisture on the insulation characteristics of nitrogen

Abstract

Using N₂ or dry air to replace SF₆ as high-voltage switchgear insulation media is a hot topic in the development of environmental friendly switching equipment. In order to make switchgear equipment with N₂ has a much compact structure in space, engineers should fully understand the insulation characteristics of N₂. In this paper, the breakdown characteristics of N₂ at power frequency and with lightning pulse in the sphere-plate electrodes has been investigated and the effect of humidity on breakdown voltage analyzed. From the test results, it was revealed that the AC and impulse breakdown voltages of N₂ were not seriously affected by different moisture content ranging from 0 to 1000 μL/L in N₂. And the breakdown voltage was decreased 8~15% with increasing moisture contents from 1500 to 2000 μL/L.

Keywords:

• Insulation characteristics
• nitrogen
• moisture
• environmentally friendly gas

1. Introduction

SF₆ gas has an excellent insulation and interruption performance, so that it has been extensively used as an insulating medium in high-voltage electric devices [1]. However, in 1997, by Kyoto Protocol, SF₆ gas was classified as a greenhouse gas because it has 23,900 times higher GWP (global warming potential) than that of CO₂ [2,3]. Therefore, it is needed to reduce the total usage of SF₆ in electric power industries [4–7].

In order to decrease and substitute SF₆ gas, many research works have been done, and one of the candidates which could replace SF₆ gas in the medium voltage-level is the application of N₂ as an insulating medium. N₂ has been received noticeable attraction due to its environment-friendly characteristics. Recently, many manufactures announced that 12~24 kV C-GIS has been successfully developed using N₂ as a main insulating medium [8–10].

Although the history of the insulation performance of N₂ is very long, there is a grave shortage of the complete data for the disruptive discharge voltage of N₂. In order to expand the usage of N₂ for extra high voltage level, detailed knowledge and experimental works on the breakdown characteristics of N₂ should be proceeded. Up to now, however, only a few fundamental research works of N₂ have been reported compared to those of SF₆. And it was generally accepted that the control of moisture content in N₂ is the critical factor to determine the performance of N₂ but, there are only a few fundamental research works regarding the insulation characteristics affected by the ratio of moisture content in N₂ [11–14]. There are only some papers dealing with humidity effects regarding the atmospheric conditions (see Table 1).

Table 1. Comparison of main characteristic of N₂ and SF₆.

Physical and chemical characters	N2	SF6
Molecular weight	28.02	146.07
Liquefaction temperature	−198	−51
Toxicology	–	–
Gas density	1.16	6.05
GWP	0	23,900
ODP	0	0

In this paper, the author study the relation between moisture content in N₂ and breakdown characteristics of N₂. The humidity of N₂ has been controlled by mixing the dry-N₂ with the N₂ which has a certain moisture content (2000 μL/L). And then AC withstand tests and lightning impulse breakdown tests were performed using sphere to plane electrode N₂ gap configurations.

2. Experimental setup

The experimental setup is illustrated in Figure 1. Tests were made between a stainless steel electrode and a horizontal plate electrode in a cylindrical closed chamber made of stainless steel. The diameter of the sphere electrode was 100 mm and the diameter of the plate diameter was 400 mm. The discharge gap can be changed from 0 to 100 mm. The calculated inhomogeneity factors for the gas-gaps used in the test are shown in Table 2. The chamber could withstand the pressure varied from 0.0 to 1.0 MPa. And the observation window was set up to watch to watch the discharge path during breakdown tests. A 300 kV AC transformer was used in the AC withstand test. The rising rate of AC voltage was 1 kV/s, and it was increased continuously until the breakdown took place. AC withstand voltage was obtained from the average of 10 measurements by the voltage increasing method. Lightning impulse breakdown test was carried out by using a 400 kV impulse voltage generator. The waveform was ±1.2/50 μs. The breakdown voltage characteristics were determined by the ‘Up & Down’method with 5 kV steps.

Figure 1. Schematic diagram of the test rig.

Table 2. Non-uniform coefficient for electric field at different gap distance.

Sphere-Plate electrode
Gap distance (mm)	Inhomogeneity factor
5	1.067
10	1.137
15	1.208
20	1.282
25	1.357
30	1.434
40	1.591

2.1. Control of gas moisture

The artificial moisture content in N₂ was controlled by mixing the specific humidity N₂ (2000 μL/L) with dry-N₂. Four kinds of N₂ with different moisture content were obtained (see Table 3). Humidity readings before and after each set of breakdown measurements were obtained using a water analyzer. The accuracy of the instrument used was 10 μL/L.

Table 3. Moisture content of N₂.

No.	Calculated value (μL/L)	Measurements (μL/L)
I	0	23
II	500	532
III	1000	1042
IV	1500	1553
V	2000	2057

The saturation vapor pressure is the level at which air is saturated with water vapor and condensation occurs for further increases in water vapor content. And saturated vapor pressure is only determined by temperature. Absolute humidity in the air is the amount of water (by mass) present in the volume divided by the N₂ volume. In order to use the N₂ as electrical insulation medium in higher voltage level, pressurization is inevitable. It is possible that the liquefaction of water occurs when highly pressurized condition with high humidity. Thus, the water content would not increase with the pressure. In this paper, we experiments were carried out under 0.1 MPa which is the 12 kV C-GIS working pressure (see Figure 2).

Figure 2. H₂O saturated steam pressure curve [15].

3. Result and discussion

The effect of humidity on the lightning impulse breakdown characteristics of N₂ was shown in Figures 3 and 4.The positive and negative lightning impulse have shown the same characteristics. There were no significant changes of breakdown voltage of N₂ at the prepared humidity range from 0 to 1000 μL/L. The breakdown voltage increases slightly when the moisture contents change from 0 to 500 μL/L. When the moisture content increased to 1500 μL/L, the breakdown voltage reduces about 5% compared with the dry-N₂. And the value reduces about 15% when the moisture content increased to 2000 μL/L.

Figure 3. Breakdown characteristics of N₂ in the condition of different moisture content under lightning impulse (Li−).

Figure 4. Breakdown characteristics of N₂ in the condition of different moisture content under lightning impulse (Li+).

Figure 5 shows AC withstand characteristics of N₂ for different humidity respectively. The breakdown voltage was reduced linearly with increasing the moisture content in N₂. The decline of the result is the same with that of the lightning impulse breakdown result. The breakdown voltage was decreased 8~15% with increasing moisture contents from 0 to 2000 μL/L.

Figure 5. Breakdown voltage of N₂ with different moisture content.

This is separate to the trend for short spere/plane and uniform gaps presented in previous papers. Short non-uniform field gaps exhibit increasing breakdown voltage with increasing humidity. Reinforcement of breakdown strength due to the humidity can be observed when the relative humidity is higher than 60~90%. However, in this work, breakdown strength decreased slightly due to the humidity. From this result, it could be deduced that the rather small water vapor up to approximately 1000 μL/L could not affect the breakdown strength in dry-N₂.

4. Conclusions

The relation between moisture content in N₂ and breakdown characteristics of N₂ was analyzed.

1. The positive and negative lightning impulse have shown the same characteristics. There were no significant changes of breakdown voltage of N₂ at the prepared humidity range from 0 to 1000 μL/L. When the moisture content increased to 2000 μL/L, the breakdown voltage reduces about 10~15% compared with the dry-N₂.

2. The breakdown voltage was reduced linearly with increasing the moisture content in N₂. The breakdown voltage was decreased 8~15% with increasing moisture contents from 0 to 2000 μL/L.

Funding

This work was supported by Science and Technology Project of SGCC (Research on SF₆ Alternative Gas for Insulation and Arc Quenching Application, 524606160173).

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes on contributors

Fanyi Cai was born in Changzhou City, Jiangsu Province, China in 1987. He received the Ph.D. degree in natural science chemistry from the University of Science and Technology of China in 2014. He is with the Smart Electrification Equipment Bussiness Unit of Nari Group Corporation as a R&D Engineer for the benign solid insulated materials and SF6 alternatives gas in high voltage equipment.

Baijie Zhou was born in Jiangsu provice, China. He received the Ph.D. degree from NanJing University, currently he works as engineer in NARI GROOUP CORPORATION. He is working on the development of solid insulated and green-gas insulated switchgears, and failure analysis of electrical insulation of high voltage equipment.

Jian Xue was born in LiaoNing provice, China in 1986. He received the Ph.D. degree from NanJing University, China in 2014, currently he works as engineer in NARI GROOUP CORPORATION. He is working on the development of solid insulated and green-gas insulated switchgears, and failure analysis of electrical insulation of high voltage equipment.