Effects of temperature and humidity on ground total electric field under HVDC lines

Highlights

• The coupling change of temperature and humidity in natural environment is simulated in an artificial climate chamber.

• Under the condition of coupling change of humidity and temperature, with the increase of relative humidity and the synchronous decrease of temperature, the ground total electric field keeps increasing under positive corona, while it increases at first and then decreases under negative corona.

• The effects of humidity and temperature on total electric field under positive and negative corona show an obvious difference in polarity.

Abstract

The characteristics of ground total electric field of HVDC transmission lines impacted by temperature and humidity are of significance in line design and environmental protection assessment. In this paper, an experimental setup encompassing wire-plate electrodes is built, and the effects of temperature and humidity on the ground total electric field under HVDC lines are studied under three conditions: constant temperature with variable humidity, constant humidity with variable temperature, and coupling change of temperature and humidity. The experimental results show that when the temperature and humidity increase separately, the ground total electric field increases, and it is of polarity dependence. When the coupling change of temperature and humidity in natural environment is simulated, that is, with the increase of relative humidity and decrease of temperature, the ground total electric field increases when the HVDC line is in positive corona, and it increases first and then decreases when the line is in negative corona. An important inference from the results is that the characteristics of ground total electric field obtained under the conditions that temperature and humidity change separately are not applicable for the analysis of ground total electric field in natural environment where temperature and humidity vary simultaneously.

Introduction

HVDC transmission line has many advantages over HVAC transmission line [1,2], such as large transmission capacity, long transmission distance, low power loss and so on. Moreover, the corridor can be fully utilized and the short-circuit current of the system can be limited in HVDC transmission lines. Additionally, for HVDC transmission lines, there is no need for the synchronization of AC systems on both sides and it is convenient to carry out phased construction and capacity expansion construction. Accordingly, in countries where the energy resources centers are far away from the power load centers, such as China and Brazil, HVDC transmission lines are being vigorously developed to improve the transmission efficiency.

For HVDC transmission lines, the ground total electric field is a very important electromagnetic environmental parameter in engineering design and environmental impact assessment [3]. It should be noted the conductors of HVDC transmission line are usually in corona, and corona will generate space charge. As a result, the electric field around HVDC transmission line is composed of the electrostatic field stimulated by the charge on the conductor and the ion flow field generated by the space charge. The term total electric field is used to describe the electric field around the conductor when it is in corona state, to distinguish it from the electrostatic field.

The meteorological environment along HVDC transmission lines is complex and diverse [4], [5], [6]. The temperature and humidity vary greatly in different regions at the same time and change a lot in different seasons at the same place, which results in the ground total electric field along the transmission lines being of regional and seasonal characteristics. In the past, in order to consider the influence of humidity on the ground total electric field in the line design of HVDC transmission lines, it is generally assumed that the conductor is in two extreme states of dry conductor and wet conductor. Using the prediction results of the ground total electric field under these two extreme states, the minimum height and the corridor width of the entire line are determined [7]. This simplified design method is widely adopted in China, but it ignores the regional and seasonal characteristics of ground total electric field, making line design too conservative. If the regional and seasonal characteristics of the total electric field can be taken into account in the line design, the cost of construction of transmission lines can be reduced. Driven by this idea, it is necessary to carry out researches on the effects of temperature and humidity on the ground total electric field.

Several institutions have studied the effects of temperature and humidity on ground total electric field of the actual HVDC transmission line or the full-scale test line in outdoor natural environment. The measurement results of EPRI on the electromagnetic environment of actual HVDC transmission lines show that no significant correlation is found between the ground total electric field and ambient temperature or absolute humidity, but it is greatly affected by relative humidity [8]. The total electric field increases with increasing relative humidity in fair weather. The electromagnetic environment tests conducted by CEPRI under the full-scale test line at the State Grid UHVDC Test Base in Beijing show that the ground total electric field of the HVDC line varies greatly with relative humidity [9]. In summer, the electric field beneath the positive conductor is round 24.2 kV/m at about 20% relative humidity; while it is as high as 39.6 kV/m when the relative humidity is above 80%. By statistical analysis of the long-term measurement data of total electric field under the actual HVDC transmission line by BPA [6], it is found that in dry weather, the electric field under the negative conductor increases as the ambient temperature increases, but the total electric field under positive conductor is almost independent of the ambient temperature. The existing research results carried out outdoor in natural environment aimed at verifying whether the values of ground total electric field under HVDC transmission line is below the limit under different temperature and humidity conditions, and the characteristics of ground total electric field varying with temperature and humidity are not systematically studied.

Since the experiments in natural environment are easily interfered by many other factors, many scholars have studied the influences of temperature and humidity on total electric field in indoor artificial climate chambers [10], [11], [12]. In these researches, the mono-polar HVDC transmission line can be simplified as a wire-plate electrode model, and the surface total electric field of plate electrode corresponds to the ground total electric field of HVDC transmission lines. The experiment carried out by Wang et al. shows that when the temperature is fixed, as the relative humidity increases in the range of 25%−80%, the electric field around the negative electrode increases, and at the same time the value of the electric field is larger at higher temperature [10]. The experiments conducted by Zhang [11] using a positive wire-plate electrode indicate that at a constant temperature, with the relative humidity increasing from 45% to 85%, the corona discharge intensity increases and the corona inception voltage decreases, resulting in an increase of the ground total electric field. In these researches, temperature and humidity change separately, and there is no report on the characteristics of ground total electric field when temperature and humidity varies simultaneously. However, in natural environment, the temperature and humidity mostly change in coupling patterns in which the temperature rising with the humidity decreasing or the temperature decreasing with the humidity increasing [13]. According to the statistical analysis of the measured weather data in the State Grid UHVDC Test Base from 2015 to 2019, it is found that in fair days, the days when the temperature and relative humidity are significantly correlated account for 91.98%, and in 99.3% of the fair days, the correlation coefficient are negative. It indicates that in natural environment the relative humidity and temperature are two significantly negatively correlated variables. The researches on corona characteristics by Geldenhuys [14] and Zebboudj [15] show that the influence of humidity on corona is different at different temperatures. Therefore, the characteristics of ground total electric field in natural environment is not necessarily the same as that obtained when the humidity changes under the constant temperature. Consequently, research results in the latter case are less helpful in predicting that in the former case.

In addition, some researchers have studied the effects of temperature and humidity on the corona inception electric field and the corona current [16], [17], [18], [19], [20], [21], [22]. Since the ground total electric field is significantly affected by the corona inception electric field, the characteristics of the ground total electric field can be estimated by the that of corona inception electric field. Their conclusions on the impacts of temperature on corona characteristics are consistent, that is, with the increase of temperature the corona inception electric field decreases, and the corona intensity enhances [16], [17], [18]. In terms of the effect of humidity on corona characteristics, Peek advocates that humidity does not affect the corona inception voltage when there is no condensation on the surface of the wire [19]. Many scholars, represented by Khaled [20], pointed out that as the humidity increases, the corona inception electric field rises accordingly. However, experiments conducted by Allen et al. [21] in rod-plane gap indicate that the corona inception voltage decreases as the humidity increases. In fact, the influence of humidity on corona inception voltage depends on the size and structure of the electrodes [22], which needs to be analyzed in combination with the parameters of the electrodes. Hence, the characteristics of the ground total electric field of HVDC transmission line varying with humidity cannot be directly predicted on the basis of these research results.

It is the aim of this paper to study the impacts of temperature and humidity, more importantly the impact of the coupling change of the two on the ground total electric field under HVDC lines. By coupling change, it is meant in this paper that temperature and humidity vary simultaneously in a same way as they do in natural environment. To this end, a temperature and humidity controllable indoor experimental setup based on wire-plate electrodes is built, of which the detail introduction is in Section 2. The setup is used to study the effects of temperature and humidity on the ground total electric field under three different conditions: constant temperature with variable humidity, constant humidity with variable temperature, and coupling change of temperature and humidity. The experimental results and discussion are given in Section 3. In Section 4, the experimental results are analyzed theoretically.