High-voltage cables usually refer to transmission lines that carry voltages above 10kV.
According to GB/T 2900.50-2008, definition 2.1, high voltage usually does not include 1000V.
Overhead Line Method
High-voltage transmission cables are usually transmitted underground by cables with insulation in cities, and often transmitted in the field using overhead lines carried by pylons.
Wire and cable are the bridge between power supply equipment and power-using equipment, playing a role in the transmission of electrical energy.
Application is widespread, so failure also often occurs, the following is a ZMScable cable editor to share the reasons for problems with high-voltage cables.
Manufacturers Manufacturing Reasons
Manufacturers manufacturing reasons according to the occurrence of different parts are divided into cable body reasons, cable joints, and cable grounding system causes in three categories.
1 Cable Body Manufacturing Reasons
Generally, the cable production process is prone to problems such as insulation eccentricity, insulation shielding thickness not uniform, insulation impurities, internal and external shielding protrusions, uneven cross-linking, cable moisture, and poor sealing of the metal sheath of the cable.
Some cases are more serious and may be completed in the test or soon after the commissioning of the failure, most the cable system in the form of defects, and the cable’s long-term safe operation of serious hidden risks.
2 Cable Joint Manufacturing Reasons
High-voltage cable joints were formerly made of winding type, die-cast type, molded type, and other types.
The workload required for on-site fabrication is large, and because of the limitations of the site conditions and the fabrication process, there will inevitably be air gaps and impurities between the insulation tape layers, so problems are prone to occur.
Cable joints are divided into cable terminal joints and cable intermediate joints.
Regardless of the form of joints, cable joint failures generally occur at the cable insulation shield fracture.
Because this is the part where the electrical stress is concentrated on the causes of cable joint failure due to manufacturing reasons including manufacturing defects in the stress cone body, insulation filler problems, oil leakage from the seal, and other reasons.
3 Cable Grounding System
The cable grounding system includes a cable grounding box, cable grounding protection box, cable cross interconnection box, sheath protector, and other parts.
Generally easy to occur mainly because the box is not well sealed into the water caused by multiple grounding, causing the metal sheath induction current to be too large.
In addition, the sheath protector parameter selection is not reasonable, and bad quality zinc oxide crystal instability also easy to triggers the sheath protector damage.
Construction Quality Reasons
There are many cases of high-voltage cable system failure due to construction quality, the main reasons are as follows.
1 Site conditions are poor, the cable and joints in the factory manufacturing environment and process requirements are very high, while the construction site temperature, humidity, and dust are not well controlled.
2 The cable construction process inevitably leaves tiny skid marks on the insulation surface, and semi-conductive particles and sand grains on the sand cloth may be embedded in the insulation.
In addition, as the insulation is exposed to the air during the construction of the joint, moisture will be absorbed into the insulation, which will leave hidden dangers for long-term safe operation.
3 Installation is not strictly by the process of construction or process regulations do not take into account the possible problems.
4 Completion acceptance using DC withstand voltage test caused the formation of the counter electric field in the joint, resulting in insulation damage.
5 Caused by poor sealing treatment.
The intermediate joints must adopt the sealing structure of metal copper shell plus PE or PVC insulation and anti-corrosion layer, and ensure the tightness of the lead seal in the field construction, which effectively ensures the sealing and waterproof performance of the joints.
Design Reasons
Cable extrusion caused by thermal expansion of the cable leads to breakdown.
When the cross-linked cable load is high, the core temperature rises and the cable expands by heat.
The table tops on the stand elevation at the turn in the tunnel, and the cable creep force is high for long-term heavy load operation.
Resulting in bracket elevation pressed through the cable outer sheath, metal sheath, squeezed into the cable insulation layer resulting in cable breakdown.
Zmscable team combined the above analysis of high-voltage cables according to the reasons for the failure of the classification is roughly divided into manufacturers’ manufacturing reasons, construction quality reasons, design unit design reasons, and external damage to the four categories.
How to distinguish low-voltage cables from high-voltage cables
Look at the cable structure
High-voltage cable layers, peel off the outermost layer inside and armor, shielding layer, insulation layer, conductor, etc.
Low-voltage cables generally set aside the outermost layer, which is the insulation layer or conductor.
Check the cross-section
The innermost part of the cable is the conductive core, generally copper or aluminum core.
In order outward: insulation layer, semiconducting layer, shielding layer, filling layer, steel Kai protective layer, rubber protective layer.
Check the thickness of the insulation layer
The high-voltage cable insulation layer is thicker, the low-voltage cable insulation layer is thinner.
The low-voltage cable insulation layer is generally within 3 mm, the high-voltage cable insulation layer is generally more than 5 mm.
Low-voltage (below 1kv) 1~3 mm thick, 10kv cable 5~8 mm, 35kv cable about 10 mm.
Low-voltage or weak cables generally on a layer of insulation and protective layer.
High-voltage cables have an insulation layer after removing the outer skin, which is wrapped in the cable core outside, white like a plastic main insulation layer, low-voltage cables do not have this main insulation layer, only the rubber protective layer.
Look at the outer layer of the cable nameplate
The outermost layer of the cable is generally printed with the relevant parameters of the cable, which include the cable type, cross-sectional area, rated voltage, length, and other parameters.
Check the voltage registration
The drawings usually have the voltage rating YJV-1KV-4*150 or YJV-10KV-4*150 and so on.
Cables are customarily classified according to different voltage levels: 1) weak cables: 450/750V and below; 2) low-voltage cables: 0.6/1kv; 3) medium-voltage cables: 3-35kv; 4) high-voltage cables: 35-110kv; 5) ultra-high-voltage cables: 110-750kv.
Look at the production process
Low-voltage cables can be produced using ordinary polyvinyl chloride and cross-linked polyethylene, low-voltage cables have two kinds of ordinary and cross-linked. Medium and high voltage cables only cross-linked cables, 6kv-35kv using three layers of co-extruded, high-density cross-linked polyethylene production.
These are some of the introductions of high-voltage cables and the causes of failure to deal with them.