HVDC has many advantages, such as low cost of paper, low line loss, no reactive power, easy power connection, and easy control and adjustment. Especially in long-distance transmission, DC power systems have been widely used. In the 1980s, China built a 500kV DC transmission line from Gezhou Dam to Shanghai, and recently constructed a 500kV DC transmission line in Sanxia, ​​Changzhou. The DC power cable has the following advantages: the strength of the insulated working electric field is small, the insulation thickness is thin, the cable diameter is small, the weight is light, the flexibility is good, and the manufacturing and installation are easy; the dielectric loss and the conductor loss are low, the current carrying capacity is large; there is no AC magnetic field, there is Environmental advantages. Compared with AC power systems, the development of DC power cables is lagging behind. For example, in the development of 1,000kV power equipment in Japan, 500kV XLPE power cables were also successfully developed and quickly put into operation, but 500kV DC The power cable has not yet been researched successfully. w. There are still some difficulties in the development of compacted DC cables. With the difficult Hi space charge problem, only high-voltage DC power cables can be successfully designed by understanding the space charge.
1 DC transmission capacity DC high voltage power transmission began in the 50's. With the advancement of helium-pressure thyristor technology, the commutation technology of helium pressurizing and high-current has become simpler, and helium pressurization and direct current transmission has become a competitor of AC transmission in advanced countries. In the world, the rolling capacity of DC transmission increases with the year as shown.
2 Factors influencing the dielectric strength of DC insulation It is well known that the electric field distribution in composite insulation in AC electric field is determined by the dielectric constant. In DC electric field, the electric field is based on the resistivity of a voltage transformer oil laminate and oil composite insulation. The distribution of the electric field distribution in the electric field, the power frequency electric field changes so fast, the positive and negative charge in the material can not keep up with the change of the electric field, so the space charge will not be generated in the insulation, but in the DC electric field, the opposite will form the space. The charge affects the distribution of the electric field. As (a) shows the electric field distribution of composite insulation of laminates and oils in DC and AC electric fields. From (b), where the equipotentials are laminated parallel to the alternating current and the power lines are perpendicular to the laminate, the dielectric strength of the composite insulation is very low, but it can also be seen from (a) that in the DC electric field, the two layers meet each other. In the oil gap of the cover, the equipotential lines are almost perpendicular to the laminate. The electric field intensity is distributed along the surface of the laminate and the dielectric strength is very low. If the influence of the space charge on the DC electric field is further considered, the electric field distribution is more uneven and the dielectric strength of the composite insulation is lower.
3 Space charge effect When the polyethylene is insulated, the polymer has a large number of local states and the space charge effect is particularly serious. Using polyethylene as a sample, the sample is subjected to DC preloading during the pulse breakdown test. The effect of DC preload on the intensity of pulsed breakdown electric field is shown.
It can be seen that when the DC voltage and the pulse breakdown electric field strength are the same, the intensity of the pulse breakdown electric field also slightly increases with the increase of the DC voltage amplitude. When the two polarities are opposite, the pulse breakdown voltage decreases linearly with the increase of the DC voltage. When the DC preload voltage is removed and then the pulse voltage is added, the gap time has a great influence on the intensity of the pulse breakdown field, and the gap time increases. Long, the higher the pulse breakdown electric field strength.
the above.
The relationship between E/E. and the pressurization time can be seen. The space charge increases the electric field intensity multiple times with the increase of the pressurization time in either the inner shield of the cable or the medium near the outer shield. For the sake of clarity, the theoretical and measured values ​​of the electric field strength in the insulation after 48 hours of pressurization are as follows: The insulation strength of the cable near the inner shield is almost 8 times of the theoretical value after the cable is insulated for 48 hours. The outer shield layer Nearby has also increased by 6 times. The breakdown of the cable insulation after 60 hours of pressurization fully illustrates the danger of space charge in DC plastic insulation.
5 Research and development of DC plastic cable insulation The key to DC plastic cable is to eliminate the space charge in the insulation material. Japan added two kinds of fillers in the XLPE: + polarized inorganic filler (XQ); 2. XLPE + conductive inorganic filler (XLA ). The inorganic filler oriented by the dipole polarization suppresses space charge, or the conductive inorganic filler adsorbs carriers and reduces space charge. The above two kinds of insulation material manufacturing model cable, the measured cable insulation breakdown voltage and insulation thickness relationship. From the figure we can see that ordinary XLPE has the lowest dc breakdown voltage and tends to saturate as the insulation thickness increases. The addition of polarized and conductive inorganic fillers in the XLPE has almost the same effect and breakdown in a fairly large range. The voltage and thickness have a linear relationship. Under equal thickness, the breakdown voltage of XQ and XL*A is 80% higher than that of ordinary XLPE. The breakdown voltage and thickness of different DC cable insulation impose different voltages on the insulation of the model cable. The relationship between the measured insulation resistance and the electric field strength is shown. It can be seen from the figure that the general XLPE has the lowest resistivity, followed by XLPE with conductive inorganic fillers, and the highest is XLPE with polarized inorganic fillers. Adding a small amount of inorganic filler (1%) not only reduces the electrical P-rate of XLPE, but also increases the resistivity. This phenomenon just shows that the inorganic filler has the effect of adsorbing carriers.
The relationship between electrical resistivity and electric field strength of model cable insulation In 1976, when Hitachi, Japan, first reported the development of DC cables, space charge distribution measurement technology in insulation was still not used. The space charge distribution in cable insulation can only be measured by powder method. For qualitative analysis, the mixed powder of red lead and sulfur is scattered on the cross-section of the cross-linked polyethylene cable, and the existence of positive and negative charges is distinguished based on the distribution of the red and yellow rings. Or use TSC curve to calculate the distribution of charge in insulation.
Shows the relationship between breakdown electric field strength and cable temperature of ordinary cross-linked polyethylene and cross-linked polyethylene cable insulation (insulation thickness 6 mm) containing inorganic fillers.
It can be seen that the DC breakdown electric field strength of ordinary XLPE insulation decreases linearly with increasing temperature. Under 9 (TC), ordinary cross-linked polyethylene and DC insulation containing cross-linked polyethylene cables with inorganic fillers The reverse polarity breakdown test results in the lowest reverse polarity breakdown electric field strength. When the crosslinked polyethylene contains inorganic fillers, the DC breakdown electric field strength is almost independent of the temperature, and at 9CTC, the reverse polarity breakdown electric field strength is lower. high.
6 Structure of 250kV XLPE cable in Japan A 250kV XLPE submarine cable was developed with XQ and XL*A insulation materials. The cross-sectional structure of the cable is shown in Chinese. Medium cable conductor cross-section area is 800mm2, insulation thickness is 20mm, working temperature, designed field strength is 50kV/mm, pulse design field strength is 55kV/mm. In order to manufacture long cables, it is necessary to connect segmented cables in the factory. The structure of the cable connector used in the factory is shown as 0. As can be seen from the figure, the insulating material of the connector is the same as the insulating material of the cable body. The outer diameter of the connector is equal to the diameter of the cable. After the connector is manufactured, the entire cable is wrapped around the armored cable to complete the submarine cable.
Conductor (coffee mm \ steel) in the semiconductor layer insulation outside the semiconductor layer anti-hit sheath polyethylene anti-corrosion liner molybdenum wire sheathed (batch mmx39) polypropylene rope rope outer diameter of the largest! The space charge conductor in the insulation of the 24mm XLPE cable is the semiconductor screen fis outside the semi-antelope screen plating layer (Pressure S measurement conditions are: (1) the wire and air temperature are 5C, plus *500kV 3 hours, then plus + 500kV for 3 hours; (2) Wire temperature 85. and air temperature 7*C plus 500kV for 3 hours, then +500kV for 3 hours. At different temperatures, the space charge distribution in the DCXLPE cable insulation was measured as shown in FIG.
2 The outer electrode of the cable is located at 0 and the inner electrode is located at 20mm. At room temperature, when the cable is applied with a voltage of 500kV immediately or after a pressure of 3 hours, a small amount of different polarity charges appear near the insulation. (2 (A) (a) (b)), the negative charge near the outer electrode is distributed in a narrow range of l2mm, and then a small positive charge peak appears after the negative charge, and is distributed over a relatively wide range. Over time, these peaks increased slightly. After a short circuit (2 (A) (c)), there was a small positive charge peak at the outer electrode.
Under voltage, the space charge peak slightly increases, the distribution range is wider, and more short-circuits remain. Visible from (d) (e) in (2), at +500kV, compared with room temperature, The peak of positive charge at the outer electrode does not increase and the short circuit does not completely release the space charge charge in the space 2DCXLPE cable insulation.
The electric field distribution in the 8250kVDC XLPE cable insulation is based on the space charge distribution in the cable insulation. The Poisson equation can be used to solve the distribution of the electric field strength in the insulation. 3 shows the electric field near the inner and outer electrodes, the insulation and the semi-conductive interface at different temperatures. The intensity with the pressurization time.
If there is no space charge in the insulation, the electric field strengths at the semiconductors inside and outside the cable can be calculated to be 36kV/mm and 18kV/mm, respectively, from the curve at 3(a) low temperature. When the conductor is negative, the external electrodes are The electric field intensity is almost independent of the pressurization time, and the electric field intensity at the inner electrode slowly rises as the pressurization time increases. When the wire is positively charged for 240 minutes, the electric field intensity at the inner and outer electrodes reaches the lowest value due to space charge accumulation.
It can be seen from the curve at 3 (b) high temperature that when the wire is negative, the electric field strength at the outer electrode slowly rises with the increase in pressurization, while the electric field strength at the inner electrode always fluctuates in a small range when the wire is positive At the time of sex, the electric field intensity at the inner electrode slowly decreases with the increase of the pressurization time, while the outer electrode is at the opposite and slowly rises.
The presence of space charge only increases the electric field intensity at the inner electrode by 10% to 40%. Compared with the pure XLPE mentioned in Section 3, the space charge can make the increase of the electric field intensity much smaller. The addition of inorganic filler almost completely eliminates the space charge. Impact.
9 Dynamics of DC plastic cable insulation The dynamic development of high-voltage DC cables is the key to materials, although Japan has time (minutes) (4) low temperature time (minutes) (h) temperature 3 maximum electric field strength in the vicinity of semi-conductive electrodes in and around insulation The relationship between time has led to the successful development of the world's first 250kV DC XLPE cable, but people are constantly striving for better DC cable insulation. Khalil et al. measured the thermal excitation current and studied the effect of BaTiO 3 and electrode materials on space charge formation in polyethylene. In order to measure the space charge distribution in this sample, we also used 100-element BaTi03 in 11 (TC in the polyethylene blender and pressed into a 0.5mm plate last year. The electro-acoustic pulse method was used to measure the Space charge distribution M. It is well known that the use of inorganic fillers can eliminate the space charge effect in polyethylene, the inorganic filler ratio is significant, and the weight of the cable must be increased. The coupling agent must be used to increase the interface strength and the process is complicated.
Japanese studies have found that adding 1 % of polar groups in polyethylene can greatly reduce the space charge; South Korea grafts and blends with polar groups to reduce the space charge of polyethylene; space charge effect of polyethylene Closely related to its own morphological structure, blending high-density polyethylene with low-density polyethylene can reduce the space charge and is expected to be used on DC cables; our recent research shows that adding a little chlorinated polyethylene to polyethylene, Can also greatly reduce the space charge effect, chlorinated polyethylene and polyethylene with good compatibility, with the prospect of industrial applications; with effective nucleating agent dispersed in polyethylene, improve the morphology can also reduce the space charge.
10 Conclusion In the past decade, the measurement of space charge distribution in solid media and the study of the formation and inhibition mechanism of space charge are the most active topics in the field of insulation. Researching this topic not only has great industrial application prospects, but also changes the dielectric medium. The basic theory of electric strength, modern research shows that the trap of space charge, the generation of high-energy particles is the root cause of polymer molecular chain fracture, space charge effect is also a way to develop new sensors.
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