In the actual insulation structures encountered in power engineering, the electric field is mostly uneven, and usually the gap distance is very large, and the electric field is extremely uneven. Inhomogeneous electric fields can be divided into two categories: slightly inhomogeneous electric fields and very inhomogeneous electric fields. The characteristics of discharge in a slightly inhomogeneous electric field are similar to those in a uniform electric field, that is, when the discharge reaches self-sustaining, the gap will break down. However, the discharge of gas in a very inhomogeneous electric field has many new characteristics, such as corona discharge before breakdown, and polarity effect when the electric field is asymmetrical. This section focuses on the breakdown process in a very inhomogeneous electric field.
Model: ZJC-50KV
Test voltage: AC 0--50 kV, DC 0--50 kV;
Appliance capacity: 3 kva;
High voltage classification: 50kV (full range without binning);
Breakdown voltage: 0-50kv;
Boost rate: 01kv/s、0.2 kv/s、0.5 kv/s、1.0 kv/s、2.0 kv/s；
Voltage measurement accuracy (10%-100%fs): 2 ;
Boost mode: constant speed boost, withstand voltage test;
Overcurrent protection device: 0 when the specimen breaks downCut off the power supply within 1s;
Test electrode: 25 two, 75 one, R3 rounded;
Software control: concise man-machine operation interface, simple and easy to learn; It is easy to set the test voltage and select the boost rate.
Dimensions: 1100 800 1500mm;
Power supply: AC220V 50 60Hz 16A.
Extremely uneven electric field air gap, due to the large gap distance,Breakdown voltageIt is mainly determined by the gap distance, and has little to do with the shape of the electrode, so the rod-plate electrode or the rod-to-rod electrode is often used as a typical electrode to study the discharge characteristics of inhomogeneous electric field. The former represents an asymmetrical inhomogeneous electric field, and the latter represents a symmetrical extremely inhomogeneous electric field.
1. Corona discharge
When the electric field pole is uneven, with the increase of the voltage applied to the gap, near the electrode with a small radius of curvature, the electric field strength will first reach the value of causing the free process, and the gap forms a self-sustaining discharge in this local area, called corona discharge, and a thin luminous layer can be seen around the electrode in the dark, which is caused by the discharge process of the free zone. The free and recombination in the free zone, from the excitation state to the normal state, and other processes may produce a large amount of light blessing, and the free layer around the corona electrode is called the corona layer. The electric field outside the corona layer is weak, and when a corona discharge occurs, a sizzling sound can be heard, and the ozone smell can be smelled, and the current in the loop increases significantly (but the absolute value is still small), and the energy loss can be measured.
Corona discharge is a form of self-sustaining discharge, when the external voltage is very low, the discharge process in the gap depends on the external free factors, the discharge is non-self-sustaining, the discharge current is extremely small, and it is difficult to measure with general instruments. After the occurrence of corona, the discharge current suddenly increases to a perceptible value, and the discharge process is not maintained by external free factors, so corona discharge is a form of self-sustaining discharge unique to extremely inhomogeneous electric places. The voltage at the beginning of corona is called the corona onset voltage, and the electric field strength on the electrode surface is called the corona onset electric field strength.
Corona discharge has its own characteristics, when corona occurs, the free zone is confined to the vicinity of the corona electrode, and the discharge current is limited by the area where the free process does not occur. This is due to the extremely uneven distribution of the electric field, and space charge also plays an important role. For example, when the electrode with a high curvature is positive, positive ions accumulate in the corona layer, which are also positive ions in the weak electric field. When the electrode with large curvature is of negative polarity, positive ions accumulate in the corona layer, and electrons move towards the weak electric field region (towards the other pole), forming negative ions. That is, the ions outside the free zone are the same number as the corona electrode, which reduces the field strength in the corona layer and stabilizes the discharge. As the voltage increases, the dissociation strengthens and the current increases. However, the space charge density in the outer layer also increases, and the electric field in it is further strengthened, which limits the voltage drop across the corona layer, so that after the corona layer is slightly expanded, the discharge process is balanced again. Because the outer layer is not free and the conductivity is not large, the discharge current is very small, and the gap has not been completely broken.
Extremely inhomogeneous electric fields are often encountered in engineering, such as overhead power lines. Corona sizzles can often be heard near high-voltage power lines during bad weather such as rain and snow, and purple glow can be seen around the power lines at night. Corona can also occur on some high-voltage equipment. Corona discharge can have many adverse effects. The effects of light, sound, heat and chemical reactions during gas discharge can cause energy loss. In the process of corona discharge, due to the pulse phenomenon of discharge, high-frequency electromagnetic waves will be formed, causing interference. Electrical discharge can also cause chemical reactions in the air, resulting in products such as ozone and nitrogen oxide, causing corrosive effects. Corona discharges should be avoided or limited. In the construction of ultra-high voltage transmission lines, the energy loss and electromagnetic interference caused by corona of the conductor are important issues that must be considered.
2. Polar effects
In a very inhomogeneous electric field, the voltage applied to the gap is not enough to cause a breakdown, and the dissociation phenomenon may occur in the vicinity of the electrode with large curvature, because the electric field is strongest, and the accumulation of space charge in the extremely inhomogeneous electric field will have an effect on the discharge process. The rod (tip) – plate electrode is a typical extremely inhomogeneous electric field region, and the discharge process in the rod (tip) plate gap is discussed below.
In the gap between the rod and the plate, the beginning of the free process is always near the rod electrode, and the effect of space charge is different when the polarity of the rod is different.
1.The rod is extremely negative
As shown in Figure 1-5(a), after being free near the rod electrode, the positive empty case charge gradually moves towards the rod pole and disappears from the rod pole, but due to its slow motion, a more concentrated positive space charge appears near the rod electrode, while the free electrons leave the strong electric field region and move towards the anode at a slower and slower speed. The negative space charge has little effect on the external electric field due to the small concentration, while the positive space charge will distort the electric field, as shown in Figure 1-5(b) (curve 1 is the distribution of the external electric field, and curve 2 is the electric field distribution of the space charge distortion). The electric field near the rod pole is enhanced, so the self-sustaining condition is easy to meet, and it is easy to form corona discharge. In the positive space, the electric field will be weakened in the direction of the charge towards the plate pole, and it will be difficult to develop electron collapse and form gap breakdown.
2.The rod is extremely positive
As shown in Figure 1-6(a), when there is free near the rod pole, the electrons move towards the rod pole and enter the strong electric field region, which begins to cause free electron collapse and self-sustaining discharge with the increase of the applied voltage. Prior to Corona, this electron collapse had formed quite a bit in the gap. When the electrons collapse to the bar, the electrons enter the bar, while the positive ions remain in space, and they move slowly towards the plate pole. As a result, a positive space charge accumulates near the rod pole, which reduces the electric field close to the rod pole and strengthens the electric field in the outer space, as shown in Figure 1-6(b) (the meaning of EEX, ESP, E, and X in Figure 1-5 is the same). In this way, the free near the rod pole is weakened, making it difficult to achieve self-sustaining discharge or corona formation, but the space charge strengthens the electric field towards the plate pole range, which is beneficial to the development of electron collapse, so that the discharge voltage is lower when the rod pole is positive than when the rod is negative.
In the case of extremely uneven electric fields, rods (tips)-plates and rods (tips)-rods (tips) can be used as typical electrodes. When encountering a very uneven electric field in engineering, it can be based on these typical electrodesBreakdown voltagedata to estimate the insulation distance. If the electric field distribution is asymmetrical, you can refer to the data of the rod (tip) plate electrode; If the electric field distribution is symmetrical, refer to the data of the rod (tip) and rod (tip) electrodes.
Direct current, power frequency and impact in extremely inhomogeneous electric fieldsBreakdown voltageThe difference is obvious, the dispersion is also larger, and the polarity effect is significant.
Figure 1-7 shows the DC of the tip plate and the tip of the air gapBreakdown voltageThe relationship between UB and the gap distance. As can be seen from the figure, for the gap between the pointed plates with a very asymmetrical distribution of the electric field, itBreakdown voltageIt has a lot to do with the polarity of the tip electrode. This is the polarity effect mentioned above. When the tip electrode is positiveBreakdown voltageIt is much lower than the negative polarity, and as can be seen from the figure, between the tip and the tip of the electrodeBreakdown voltageIt is also understandable that there is a positive polarity tip in the pointed electrode device with different polarities, and the effective electricity is easy to develop from itBreakdown voltageIt is lower than that of the negative tip and the positive plate. However, the tip of the electrode has two tips, that is, there are two strong electric field regions, and after the increase of the strong electric field area at the same gap distance, the uniformity of the electric field will usually increase, so the maximum field strength between the tip and the tip of the electrode should be lower than that between the tip and the plate electrodeBreakdown voltageIt is higher than the positive tip and the negative plate.
3. Under the power frequency voltageBreakdown voltage
Figure 1-8 shows the power frequency of the air gap between rod and rod and plateBreakdown voltageThe relationship curve with the gap distance, the gap distance is up to 250cm, and when the processing frequency voltage is applied between the electrodes of the rod and the plate, the breakdown always occurs when the polarity of the rod is positive and the voltage reaches the amplitudeBreakdown voltage(amplitude) and DC voltage under the positive rod and negative plateBreakdown voltageSimilar. As can be seen from Figure 1-8, except for the starting part,Breakdown voltageIt is approximately in a straight line with the distance, and the average of the gap between rods and rodsBreakdown field strengthAbout 38kv cm (RMS) or 536kV cm (amplitude), the gap between the rod and the plate is slightly lower, about 335kv cm (RMS) or 48kV cm (amplitude).