Cross-linked insulation has dominated power cables,replacing oil- paper insulation,and replacing PVC plastic insulation gradually.Although cross-linked insulation is various,they are divided into two categories:physical and chemical cross-linking,their insulation quality is completely consistent,so in the national standard GB12706-91,it does not distinguish which cross-linking method to use.
Physical cross-linking: physical cross-linking is also called irradiation cross-linking,has the best insulation quality,high degree cross-linking,and good weather resistance,it’s a ideal process method to various soft wires, equipment wires and high temperature resistance (105c or above) and flame resistance cables. The main disadvantages are uneven irradiation,repeated irradiation in production,too many cable bending times, and easy injection of space charge in insulation,which is not suitable for power cable production.Some enterprises use radiation lines to produce some small-section aluminium core cables and 1-10kv aluminum core aerial cables,problems are not too big.
Chemical cross-linking: chemical cross-linking have high temperature and low temperature two cross-linking methods,high temperature cross-linking includes steam and dry cross-linking.Steam cross-linking due to insulation in water content 2000x10-6, insulation quality is not good,and has completely eliminated; Dry cross-linked insulation moisture content 1OO—200)×10-6,and has been widely used in 10-500kV cables.Low-temperature cross-linking, scientific name silane cross-linking,cross-linked cable in 70cC—90c warm water,can also be cross-linked in the humidity in the air, so it also called warm water cross-linking.Cross-linking agent (silane) absorbs water to allow linear structure of the PE insulation to react into a network-like cross-linked structure.Absorbed moisture has become a part of insulating molecule,so moisture content is small, but also (100—200)x10-6,insulation quality is completely same as dry cross-linking,insulation performance is also higher than dry cross-linking.Warm water cross-linking is not easy to infiltrate into thick PE insulation,generally suitable for 10kv or below cables.
Cross-linked cables procurement and selection
Cross-linked cables have high insulation quality,its power frequency breakdown strength up to 50KV/mm,tan 6 only 5 x10-4,dielectric constant 2.3, is the most ideal cable insulation,and working temperature of cross-linked cable is up to 90c, good heat and weather resistance,and have high mechanical properties and corrosion resistance and other chemical properties,in the low-middle cable products are basically all cross-linked cables.Although cross-linked cables have different process,insulation quality is exactly the same.Now we will introduce the selection of different types of commonly used cables as follows:
(1)1kV three cores aluminium core 70mm’XLPE insulated PVC sheathed steel pinned cable, type YJLV22—0．6／1、3 x70:the type of cross-linked insulation(YJ) can be any one cross-linking process,but warm water cross-linking is the best, if taking dry cross-linking method,investment for equipment is huge,production speed is slow,such as cross-section smaller,cables will be pulled to be thin or break down in huge production line,if taking irradiation cross-linking,as cable bending times are too many,it’ll damage cable compacted conductor,compacted copper conductor is easier to be damaged.Consideration should be taken when purchasing: don’t use dry cross-linking method when deals with aluminium core cables with small cross-sections; don’t use irradiation cross-linking for cables with large cross-sections or copper cores, warm water cross-linking will have greater flexibility.
(2)10kV three cores copper core 120mm’XLPE insulated PVC sheathed steel taped cables,type YJV22—8．7／10、3×120:10kV grade cable cross-section generally larger,especially copper core cable which can’t take irradiation cross-linking process,except cable is easily damaged due to too many bending times,and space charge is accumulated in insulation to degrade insulation quality.10kV grade cable is currently the most suitable for dry cross-linking production process,stable product quality,reasonable price.However,if the number of cable is small,and there are many changes in specification,the rate of dry cross-linking crapes is extremely high,and it’s easy to break cable with small cross-sections,this should be done with warm water cross-linking.Warm water cross-linking is in the category of low-temperature cross-linking, no insulation thermal stress,insulation quality is good,eliminate the misunderstanding of warm water cross-linked “certain water”,as the developing of warm water cross-linked new material,in the field of medium and low voltage cable,it has a very good development prospect.
(3)10KV aluminium core 70mm’aerial insulated cable, type JKLYJ—6／lO、1x70: this kind of cable is also produced in three processes,the company adopts irradiation cross-linking process,to eliminate space charge,the cable conductor is grounded for a long time,afterwards,cables can be safely used on overhead cable lines of semi-insulated systems.Comparison of dry cross-linking or warm water cross-linking is consistent with the production of the above 10kV cable.