HVDC transmission has many advantages, such as line cost paper, low line loss, no reactive power, convenient power connection, easy control and regulation, especially in long-distance transmission. In the 1980s, China has built a 500kV DC transmission line from Gezhouba to Shanghai, and recently built a 500kV DC transmission line 4 in Changzhou, the Three Gorges. The DC power cable has the following advantages: the working electric field strength of the insulation is 篼, the insulation thickness is thin, the outer diameter of the cable 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 alternating magnetic field, Environmental advantages. Compared with AC power systems, the development of DC rolling power cables is lagging behind. For example, while developing 1000kV power equipment in Japan, the 500kV XLPE power cable was successfully developed and put into operation very quickly. However, 500kV DC Power cables have not been successfully researched so far. W. The development of rolling DC cables has some difficulties to overcome. The difficult Hi space charge problem can only be successfully designed for high-voltage DC power cables.
1 DC transmission capacity DC high-voltage transmission began in the 1950s. With the advancement of the technology of rolling thyristor, the commutation technology of high-voltage rolling has become simpler. The rolling direct current transmission has become a competitor of AC transmission in advanced countries. Worldwide, the increase in rolling DC transmission capacity with the year is shown.
2 Factors Affecting Dielectric Insulation of DC Insulation It is well known that the electric field distribution in composite insulation in an alternating electric field is determined by the dielectric constant. In a DC electric field, the electric field is based on the resistivity of a crossover oil laminate and oil composite insulation. The distribution of the electric field in the electric field, the power frequency electric field changes so fast, the migration of positive and negative charges in the material can not keep up with the change of the electric field, so there is no space charge in the insulation, but in the DC electric field, the space will be formed. The charge affects the electric field distribution. As shown in (a), the electric field distribution of the composite insulation of the laminate and the oil in the direct current and alternating electric fields is shown. By (b), in the case of alternating current, the parallel lines of the equipotential lines, 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 are matched. In the oil gap of the cover, the equipotential lines are almost perpendicular to the laminate, and the electric field strength is distributed along the surface of the laminate, and the dielectric strength is low. If we consider the influence of space charge on the DC electric field, the electric field distribution is more uneven, and the dielectric strength of the composite insulation is lower.
3 Space charge effect When polyethylene is insulated, the polymer has a large number of local states, and the space charge effect is particularly serious. When polyethylene is used as a sample, when the pulse breakdown test is performed, the sample is first subjected to DC preloading. The effect of DC preload on the pulse breakdown electric field strength.
It can be seen that when the DC voltage is the same as the pulse breakdown electric field strength, the pulse breakdown electric field intensity also increases slightly as the DC voltage amplitude increases. When the polarities of the two are opposite, the pulse breakdown voltage decreases linearly with the increase of DC voltage. When the DC preload voltage is removed, the pulse voltage is added. The gap time has a great influence on the pulse breakdown electric field strength. The clearance time is more Long, the higher the pulse breakdown electric field strength.
the above.
The relationship between E/E. and pressurization time can be seen, whether in the inner shield of the cable or in the medium near the outer shield layer, the space charge increases the multiple of the electric field strength as the pressurization time increases. 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 electric field strength near the shield layer after the cable insulation is pressed for 48 hours is almost 8 times of the theoretical value, and the outer shield layer It has also increased by 6 times. The cable insulation breaks down after 60 hours of pressurization, which fully demonstrates the danger of space charge in DC plastic insulation.
5 DC plastic cable insulation research and development The key to DC plastic cable is to eliminate the space charge in the insulation material. Japan added two fillers in XLPE is + polarization inorganic filler (XQ); 2. XLPE + conductive inorganic filler (XLA) ). The space charge is suppressed by the dipole-polarized inorganic filler, or the conductive inorganic filler adsorbs carriers, reducing the space charge. The above two kinds of insulating materials are used to manufacture the model cable, and the measured relationship between the cable insulation breakdown voltage and the insulation thickness is as follows. It can be seen from the figure that the ordinary XLPE has the lowest DC breakdown voltage, and tends to become saturated as the thickness of the insulation increases. The polarized and conductive inorganic filler is added thereto, and the effects of the two are almost equal, and the breakdown is in a considerable range. The voltage is linear with thickness. Under the same thickness, the breakdown voltage of XQ and XL*A is 80% higher than that of ordinary XLPE. The relationship between breakdown voltage and thickness of different DC cable insulation exerts different voltage on the insulation of the model cable. The relationship between the measured insulation resistivity and the electric field strength is as shown. As can be seen from the figure, ordinary XLPE has the lowest resistivity, followed by the addition of conductive inorganic filler in XLPE, and the highest is the addition of polarized inorganic filler in XLPE. The addition of a small amount of inorganic filler (1%) not only reduces the electrical P rate of XLPE, but also increases the resistivity. This phenomenon just indicates that the inorganic filler has the effect of adsorbing carriers.
Relationship between resistivity and electric field strength of model cable insulation In 1976, when Hitachi Company first reported the development of DC cable, the space charge distribution measurement technique in insulation was not enough. The space charge distribution in cable insulation can only be achieved by powder image 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 according to the distribution of the red and yellow rings. Or use the TSC curve to estimate the distribution of charge in the insulation.
The relationship between the breakdown electric field strength of the ordinary crosslinked polyethylene and the inorganic filler crosslinked polyethylene cable insulation (insulation thickness 6 mm) and the cable temperature is shown.
It can be seen that the DC breakdown electric field strength of ordinary XLPE insulation decreases linearly with the increase of temperature. Under 9 (TC), common cross-linked polyethylene and DC-bonded with cross-linked polyethylene cable with inorganic filler The reverse polarity breakdown test results have the lowest reverse polarity breakdown electric field strength. When the crosslinked polyethylene contains inorganic filler, the DC breakdown electric field intensity is almost independent of temperature. Under 9CTC, the reverse polarity breakdown electric field strength is higher. high.
6 Structure of Japan 250kV XLPE Cable Japan has developed a 250kV XLPE submarine cable with XQ and XL*A insulation materials. The cross-sectional structure of the cable is shown in the middle. The medium cable has a cross-sectional area of ​​800mm2 and an insulation thickness of 20mm. At the working temperature, the designed field strength is 50kV/mm, and the pulse design field strength is 55kV/mm. In order to manufacture long cables, the segmented cables must be connected in the factory. The structure of the cable connector used in the factory is shown as 0. It can be seen from the figure that 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 armor cable to complete the submarine cable.
Conductor (coffee mm\steel) inner semiconducting layer insulation outer semiconducting layer anti-mite sheath polyethylene anti-corrosion liner layer molybdenum wire armor (batch mmx39) polypropylene strand outer diameter largest! 24mm heavy XLPE cable insulation space charge conductor semiconductor screen fis outer half body body screen layer (pressure S piece measurement conditions are: (1) wire and air temperature are 5C, plus * 500kV 3 hours, then add + 500kV for 3 hours; (2) wire temperature 85. and air temperature 7*C plus 500kV for 3 hours, then add +500kV for 3 hours. At different temperatures, the space charge distribution in DCXLPE cable insulation is measured as shown in 2.
The outer electrode of the cable is located at 0, and the inner electrode is located at 20mm. At room temperature, when the voltage is applied to the cable at 500kV or after 3 hours of pressurization, a small amount of heteropolar charge appears near the electrode. (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, distributed over a wide range, pressurized Over time, these peaks increase slightly, after a short circuit (2 (A) (c)), leaving a small positive charge peak at the outer electrode.
Under voltage, the peak of space charge increases slightly, and the distribution range is wider. After the short circuit, more is left. It can be seen from (d)(e) of 2, at +500kV, compared with room temperature. The peak value of the 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 insulation of 8250kVDCXLPE cable is based on the space charge distribution in the cable insulation. The Poisson equation can be used to obtain 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 strength varies with the pressurization time.
If there is no space charge in the insulation, the electric field strength at the semiconductor inside and outside the cable can be calculated to be 36kV/mm and 18kV/mm, respectively. It can be seen from the curve at 3(a) low temperature, when the conductor is negative, the outer electrode The electric field strength is almost independent of the pressurization time, and the electric field strength at the inner electrode gradually rises as the pressurization time increases. When the wire is positive for 24 minutes, the electric field strength at the inner and outer electrodes reaches a minimum due to the accumulation of space charge.
It can be seen from the curve of 3(b) high temperature that when the wire is negative, the electric field strength at the outer electrode increases slowly with the increase of the pressure, and the electric field strength at the inner electrode always fluctuates within a small range. At the time of sex, the electric field strength at the inner electrode slowly decreases with the increase of the pressurization time, while the outer electrode is opposite, and slowly rises.
The presence of space charge only increases the electric field strength at the internal electrode by 10% to 40%. Compared with the pure XLPE mentioned in Section 3, the space charge can increase the electric field strength much less. The addition of inorganic filler almost completely eliminates the space charge. Impact.
9 DC plastic cable insulation material development dynamic development of high-voltage DC cable, the key is the material, although Japan has time (minutes) (4) low temperature time (minutes) (h) still temperature 3 inside and outside the semi-conductive electrode near the maximum electric field strength with pressure The relationship between time has successfully developed the world's first 250kV DC XLPE cable, but people are constantly striving to find better DC cable insulation. The measurement of thermal excitation current by Khalil et al. studied the effect of BaTi03 and electrode materials on the formation of space charge in polyethylene. In order to measure the space charge distribution in this sample, we also used 100 mesh BaTi03 in 11 (TC hot melt machine last year, mixed into polyethylene, pressed into a 0.5 mm plate, and measured in the sample by electroacoustic pulse method. 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 large, and the weight of the cable is 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 to polyethylene can greatly reduce space charge; Korea uses polar groups to graft and blend polyethylene, which reduces the space charge of polyethylene; the space charge effect of polyethylene Closely related to its morphological structure, blending high-density polyethylene and low-density polyethylene can reduce space charge and is expected to be used on DC cables; our recent research has shown that adding a little chlorinated polyethylene to polyethylene, It can also greatly reduce the space charge effect. The chlorinated polyethylene has good compatibility with polyethylene and has the prospect of industrial application. The effective nucleating agent is dispersed in the polyethylene, and the shape can also reduce the space charge.
10 Conclusions In the past decade, the measurement of space charge distribution in solid media and the study of the formation and suppression mechanism of space charge are the most active topics in the field of insulation. Studying 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 and the high energy particles produced by the composite are the root causes of polymer molecular chain breakage. The space charge effect is also a way to develop new sensors.
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