Analysis of factors affecting discharge voltage between rod and plate

Author： GOZ Electric Time：2024-10-23 09:38:33 Read：18

The rod-plate long air gap positive polarity operation impulse discharge test was carried out at the outdoor test site of the State Grid UHV DC test base, which is located in Changping District, Beijing, at an altitude of 55m. Rod-plate gap discharge is the result of the coupling of multiple physical fields and is affected by many factors such as gap structure, voltage waveform, voltage polarity, and meteorological conditions. This paper analyzes the influence mechanism of these factors on the discharge voltage and extracts key features as input variables to train the intelligent model.

1.1 Gap structure

The structural characteristics that affect the discharge voltage of the rod-plate gap mainly include the rod-plate gap distance, rod electrode diameter, and end shape. Among them, the gap distance is one of the most important factors and is positively correlated with the discharge voltage. When the gap distance increases to a certain extent, the discharge voltage will gradually tend to saturation. According to the classical gas discharge theory, the uneven distribution of the gap electric field will affect the gap discharge voltage. The difference in the diameter and end shape of the rod electrode leads to differences in the unevenness of the electric field, which affects the discharge voltage.

1.2 Voltage waveform

When the voltage waveform applied to the rod electrode is different, the discharge characteristics of the rod-plate gap will also change. Under DC voltage, the discharge voltage is approximately proportional to the gap distance. Under power frequency voltage, when the gap distance is large, the discharge voltage will gradually decrease as the gap distance increases. Under impulse voltage, due to the formation of space charge, the dispersion of the discharge voltage is large, and it will gradually tend to saturation as the gap distance increases. Under operating impulse voltage, the voltage wavefront time will also affect the rod-plate gap discharge. When the voltage wavefront time is short, after the streamer starts, due to the randomness of the position of the effective electrons and the fast voltage rise rate, some filamentary streamer channels may be quickly formed and the spatial electric field distribution will be changed, thereby inhibiting the development of nearby streamer channels, and the entire streamer discharge area will be relatively small. If the wavefront time is long, due to the slow voltage rise rate, the streamer corona discharge is relatively uniform, and each discharge channel can be fully developed. In this case, the streamer discharge area formed will be larger. The relevant literature gives the rod-plate gap test curve obtained by the impulse voltage test with different wavefront times.

1.3 Voltage polarity

Due to the different discharge processes of positive and negative rod-plate gaps, the voltage polarity will affect the magnitude of the discharge voltage. When the rod electrode is positive, the electrons in the rod-plate gap begin to move toward the rod, and after entering the strong electric field area, electron avalanches begin to occur. When the electrons gathered at the head of the electron avalanche reach a certain value, photoionization will occur, accelerating the development of the electron avalanche and transitioning to streamers, thereby forming a leader. In the case of negative polarity, the electrons in the negative corona move in the direction of decreasing the electric field, while the positive corona moves in the opposite direction. Therefore, there are positive streamers and negative streamers developing in opposite directions in space, and their leader development is hierarchical. In both cases, a large number of positive ions gather near the rod electrode. When the rod electrode is positive, these positive ions strengthen the electric field in the external space, which is conducive to the development of the streamer toward the plate electrode; in the case of negative polarity, the opposite is true. At this time, the discharge voltage is higher than that of the positive polarity, and the time required to complete the breakdown process is also longer.

1.4 Meteorological conditions

Among meteorological factors, air pressure has a relatively large impact on the discharge voltage. The higher the air pressure, the smaller the mean free path of the particles, the smaller the kinetic energy accumulated by the charged particles between two collisions, the more difficult it is to cause ionization, and the higher the discharge voltage of the air gap. The temperature is negatively correlated with the discharge voltage of the air gap. The higher the ambient temperature, the faster the molecules in the air move, the greater the average kinetic energy of the air molecules, the greater the kinetic energy accumulated by the charged particles before collision ionization, the easier it is to produce collision ionization, and the lower the discharge voltage of the air gap. Generally speaking, the higher the absolute humidity in the atmosphere, the higher the discharge voltage of the air gap. This is because water vapor molecules in the atmosphere can capture free electrons and form negative ions, which leads to the suppression of the discharge process in the gas.

There is a complex coupling relationship between meteorological factors, and it is impossible to control other variables to remain unchanged. The quantitative effect of a single meteorological factor on the breakdown voltage of the air gap is often difficult to determine through experiments. Generally speaking, the temperature of the atmosphere is affected by factors such as sunshine, altitude, landform type, and climate type. As the temperature rises, the limit of water molecules in the air increases, which leads to changes in the air's ability to accommodate water vapor. The absolute humidity of the air mainly depends on factors such as the source of water vapor, transportation and the ability of air to retain water vapor. Changes in temperature will affect the absolute humidity of the atmosphere. At the same time, rising temperatures will also cause air expansion, and the number density of air molecules will decrease, resulting in a decrease in air pressure. Since the molecular weight of water vapor is smaller than that of air, when the absolute humidity increases, the air density decreases, resulting in a decrease in air pressure.

Usually, when conducting gap discharge tests, environmental conditions such as air pressure, temperature, humidity and weather during the test are recorded. However, since most parts of my country are in high and low temperatures for a relatively short period of time in a year, test data under high and low temperatures are still relatively scarce. In addition to the above three meteorological factors, conditions such as wind speed, solar radiation illumination, and rain will also affect air gap discharge. Due to insufficient relevant test data, this article does not discuss the role of other meteorological factors.