Nowadays, research on electrification detection methods has been widely carried out at home and abroad, and various methods have been proposed. During the actual operation, it was found that most of the power cable faults were caused by deterioration of the cable insulation. There are many causes of degradation of such cables (electrical degradation, thermal degradation, chemical deterioration, mechanical deterioration, and even deterioration due to rodents), but electrical degradation is still the most important. The main deteriorating forms are: 1 Electrical discharge of partial discharge; 2 Electrical branch degradation; 3 Water branch degradation. Studies have shown that in solid insulated cables up to 33kV, the degradation of insulation is mainly caused by deterioration of the water branch. However, any kind of deterioration may cause a decrease in the insulation resistance, an increase in leakage current, and a phenomenon in which the dielectric loss tgδ becomes large. The AC loss current at the operating voltage is increased so that the DC component contained in the current flowing through the insulation increases. Therefore, it is possible to determine the degradation signal by checking the insulation of the cable online and determine whether the cable insulation can continue to operate. The deterioration signal of the cable insulation is generally extremely small. For example, the DC component current due to dendritic degradation is nA, and the largest is only μA. Therefore, on the basis of extensive research on the basic physical processes of degradation of polymer insulation materials in foreign countries, the corresponding monitoring measures have been studied and adopted for the degradation of signals. There are many methods for online monitoring of cable insulation, such as DC current method, DC voltage superposition method, AC voltage superposition method, low-frequency AC superposition method, and so on.
2.1 DC current method
Under the action of AC voltage, if the water branch is degraded, the current contains DC components, and there is a certain relationship between the length of the degraded branch and the DC component current. Therefore, DC current component monitoring method is used in this study. However, because the DC component current is extremely small (typically nA), it is susceptible to stray currents. When the leakage resistance at the cable end surface drops due to the expansion or the rain, the measurement error is large, so the end must be cleaned and measured when the weather is fine, so the use of this method is greatly limited.
2.2 DC voltage superposition method
For the relationship between the length of the water branch in the cable and the insulation resistance, the DC voltage superposition method was studied. The DC voltage superposition method causes a large measurement error due to a change in the stray current or a decrease in the leakage resistance at the end surface. The DC voltage is added to the cable through the neutral grounded voltage transformer. If the DC current is flowing through the transformer for a long time, magnetic saturation will occur and a zero-sequence voltage will be generated, which may cause the relay in the substation to malfunction.
2.3 Low-frequency AC superposition
Aiming at the relationship between the water branch length and the insulation resistance in the cable, the low-frequency AC superposition method was studied. The low-frequency AC superposition method is a better method, and the AC loss current monitored in principle becomes larger as the deterioration progresses. However, during use, the working state of the cable end should be carefully confirmed. For example, when a stress ring is installed to adjust the electric field distribution at the end, even if the cable is well insulated and the AC loss current is relatively large, then only based on the online monitoring signal, it is possible. Made an "insufficient insulation" misjudgment.
2.4 AC voltage superposition method
The measuring principle of the AC voltage superposition method is to superimpose 101 Hz (ie, 2 times power frequency + 1 Hz) AC voltage on the shielding layer of the cable and monitor the deterioration of the branches to generate a 1 Hz degraded signal. When the degradation of the branch cable overlaid processing frequency + about 1Hz voltage, the maximum degradation signal measured, this method can be used to detect the strength of the 1Hz degradation signal to determine the degree of cable degradation. The advantages of this monitoring method are: 1 The voltage can be superimposed from the ground cable of the cable. The measurement is simple and convenient. It can be used not only as on-line monitoring but also as live monitoring, and multiple cables can be monitored with one set of equipment. 2 Superimposed voltage detection The known degradation signal, that is 1Hz signal, so the detection accuracy is high, strong anti-interference ability; 3 by the armored insulation resistance and the end of the fouling and other factors less affected.
3 conclusions
Through the above analysis and comparison, we can find that in the uncharged detection method, the secondary pulse method is a better method, in the method of electrification detection, AC voltage superposition method is a better method. Although the method of electrification detection is not very mature, such as the judgment of the degree of insulation degradation, a lot of research work needs to be done, but this is a development direction of power cable detection.
Silicone Watch Strap
About silicone watch bands:
In our life,we may have many watch bands, they are made of metal,golden silver,stainless steel etc.But these materials can cause discomfort for people with sensitive skin.Is there any material for watch band that is suitable for everyone to wear? Of course! That is silicone watch bands! Silicone watch bands is made of Food-Grade silicone,it not only suitable for adult,but also suitable children,even infant! As you can imagine it is so safaty and friendly! Its texture is soft and durable, it can adjust according to your wrist. Don't worry if you dirt it accidentally , it can be clean easily and its dirty resistance is very good!
Silicone Watch Band introduction:
1.Product name:wrist watch straps,sports watch strap,watch bands for sale,silicone wrist watch band,dive watch bands,silicone watch strap
2.Place of origin:Guangdong China
3.Color:any pantone color
4.Logo:Printing,debossed,embossed
5.MOQ:500pcs.
6.Package:1 pcs/opp,customized design is available.
7.Design:Customized/stock
8.Certification:FDA,LFGB,SGS,ROHS,etc.
9.Usage:Use for watch
10.Silicone watch bands for reference.
Silicone Watch Strap,Silicon Band For Watch,Silicone Band Sport Watch,Silicone Strap Sport Watch,Watch Silicone Strap,Watch Band Rubber Silicone,Watch Silicone Band
Dongguan OK Silicone Gift Co., Ltd. , https://www.oemsiliconegift.com