Electrical stress control is an important part in the design of medium and high voltage cable accessories. The electric stress control is to control the electric field distribution and electric field strength inside the cable accessories, that is to say, to take appropriate measures to make the electric field distribution and electric field strength in the best state, so as to improve the reliability and service life of the cable accessories.

For the cable terminal, the electric field distortion is the most serious, the biggest impact on the terminal operation reliability is the external shield cut-off of the cable, and the impact of the electric field distortion of the intermediate joint of the cable, in addition to the external shield cut-off of the cable, there are the insulation cut-off of the cable end. In order to improve the electrical stress distribution at the cut-off point of the cable insulation shield, the

a. Geometry method - stress cone is used to relieve stress concentration in electric field

b. Parameter control method -- B1. Using high dielectric constant material to relieve the stress concentration in electric field

B 2. Using non-linear resistance material to relieve the stress concentration of electric field

c. Comprehensive control method -- using capacitance cone to relieve electric field stress concentration

1.1 stress cone: the design of stress cone is a common method, and the most effective method from the electrical point of view. The stress cone extends the cut-off part of the insulation shield to form a horn like zero potential, improves the electric field distribution of the insulation shield, reduces the possibility of corona generation, reduces the damage of the insulation, and ensures the operation life of the cable.

The cable accessories designed with stress cone include wrapped terminal, prefabricated terminal and cold shrinkable terminal.

1.2 materials with high dielectric constant:

1.2.1 use of stress control layer - at the end of last century, the so-called stress control layer for medium voltage cable accessories was developed abroad. The principle is to composite the materials with appropriate electrical parameters at the end of the cable

In order to change the potential distribution of the insulation surface, the purpose of improving the electric field is achieved.

The method of applying the stress control layer is based on the analysis of various factors affecting the potential distribution. The cable insulation itself has volume resistance (RV) and volume capacitance (CV), and the insulation surface has surface resistance (RS) and surface capacitance (CS), which are all distributed parameters. In order to make the potential distribution at the end of the shield tend to be uniform, these parameters must be changed. Because there must be a section of insulation left after the shield at the end of the cable is cut off, and the volume resistance (RV) and volume capacitance (CV) of this section of insulation cannot be changed, only the surface resistance (RS) and surface capacitance (CS) can be changed. If the insulation surface resistance (RS) at the end of the cable is reduced, the potential will also be reduced, which is effective, but because the reduction of the surface resistance (RS) will increase the surface leakage current, resulting in the heating of the insulation surface of the cable, which is unfavorable. Another method is to increase the insulation surface capacitance (CS) at the shield end, so as to reduce the capacitive reactance of this part, and also make the potential drop. The reduction of capacitive reactance will increase the surface capacitance current, but will not cause heat. Because the capacitance is proportional to the dielectric constant of the material, that is to say, to increase the surface capacitance, a layer of high dielectric constant material can be added to the insulation surface at the shield end of the cable 。 At present, the products of stress control materials have heat shrinkable stress tube, cold shrinkable stress tube, stress control band and so on. Generally, the dielectric constant of these stress control materials is more than 20, and the volume resistivity is 1081012 Ω. Cm. The application of stress control materials should consider both the technical requirements of stress control and volume resistance. Although in theory, the higher the dielectric constant is, the better, but the capacitance current caused by the too high dielectric constant will also generate heat, which will promote the stress control material aging. At the same time, as a kind of polymer polyphase structure composite material, the dielectric constant and volume resistivity are a pair of contradictions in the coordination of the material itself. The higher the dielectric constant is, the corresponding volume resistivity will be reduced. And the stability of the electrical parameters of the material is often affected by various factors. When the material operates in the long-term electric field, the temperature and external environment will change Aging the stress control material, the volume resistivity of the aged stress control material will change greatly, the volume resistivity will become larger, the stress control material will become the insulating material, which will not improve the electric field, the volume resistivity will become smaller, the stress control material will become the conductive material, and the cable will fail. This is the reason why the heat shrinkable cable accessories used to improve the electric field by using stress control materials can only be used for medium voltage power cable lines and heat shrinkable cable accessories, and the cable accessories using cold shrinkable stress tube and stress control belt also have similar problems.

1.2.2 the use of non-linear resistance material - non linear resistance material (FSD) is also a new type of material developed recently. It uses the non-linear relationship between the material's own resistivity and the applied electric field to solve the problem of concentrated distribution of electric field at the cut-off of cable insulation shield. The nonlinear resistance material has the characteristic of changing resistance value to different voltage. When the voltage is very low, it shows a larger resistance performance; when the voltage is very high, it shows a smaller resistance performance. The use of non-linear resistance material can produce short stress control tube, so as to solve the problem that the cable with high dielectric constant stress control tube terminal can not be applied to small switchgear. The non-linear resistance material can also be made into a non-linear resistance piece (stress control piece), which is directly wrapped on the cut-off part of the cable insulation shield to alleviate the problem of stress concentration at this point.

Why do high voltage single core XLPE insulated power cables adopt special grounding method?

Electric power safety regulations stipulate that all non electrified metal shells of electrical equipment shall be grounded, so the aluminum package or metal shielding layer of the cable shall be grounded. Generally, cables with 35kV and below voltage level are grounded at both ends. This is because most of these cables are three core cables. In normal operation, the total current flowing through the three cores is zero, and there is basically no magnetic chain outside the aluminum clad or metal shielding layer. In this way, there is basically no induced voltage at both ends of the aluminum clad or metal shielding layer, so there will be no induction after both ends are grounded The current flows through an aluminum clad or metal shield. But when the voltage is more than 35kV, most of them use single core cable. The relationship between the core of single core cable and metal shield can be regarded as the primary winding of a transformer. When a single core cable core passes through the current, there will be an aluminum package or a metal shielding layer of the flux line cross chain, which will cause induced voltage at both ends of it. The magnitude of the induced voltage is directly proportional to the length of the cable line and the current flowing through the conductor. When the cable is very long, the induced voltage on the sheath can be superposed to the extent that it endangers the personal safety. When the line has a short-circuit fault, suffers an operating overvoltage or lightning impulse, a very high induced voltage will be formed on the shield, or even may break through the sheath insulation. At this time, if the two ends of the aluminum clad or metal shielding layer are still interconnected and grounded, there will be a large circulation in the aluminum clad or metal shielding layer, the value of which can reach 50% - 95% of the core current, forming loss and heating the aluminum clad or metal shielding layer, which not only wastes a lot of electric energy, but also reduces the current carrying capacity of the cable and accelerates the aging of the cable insulation. Therefore, the single core cable should not be two Terminal ground. [in some cases (such as short cable or light load operation), the three-phase interconnection and grounding at both ends of aluminum clad or metal shielding layer can be carried out. ] However, when one end of the aluminum clad or metal shielding layer is ungrounded, the following problems arise: when the lightning current or over-voltage wave flows along the core, the ungrounded end of the aluminum clad or metal shielding layer of the cable will have a very high impulse voltage; when the short circuit occurs in the system, when the short circuit current flows through the core, the ungrounded end of the aluminum clad or metal shielding layer of the cable will also have a high power frequency induced current Voltage, when the outer sheath insulation of the cable can not bear the over-voltage and is damaged, it will lead to multi-point grounding and form circulation. Therefore, when one end is interconnected for grounding, measures must be taken to limit the over-voltage on the sheath. During installation, special connection and grounding methods shall be adopted at a certain position of the aluminum clad or metal shielding layer according to the different conditions of the line and the principle of economic rationality. At the same time, the sheath protector shall be installed to prevent the cable sheath insulation from being broken down.

According to the requirements of gb50217-1994 code for design of electric power engineering cables, when the metal sheath of single core cable is grounded at only one point, the induced voltage at any point of the metal sheath shall not be more than 50-100v (not more than 50V if safety measures are not taken to contact the metal sheath arbitrarily; not more than 100V if effective measures are taken), And shall be insulated from the ground. If the voltage is greater than the specified voltage, the metal sheath shall be used for sectional insulation or the wiring connected * interconnected after insulation. In order to reduce the induced voltage of single core cable line to adjacent auxiliary cable and communication cable, cross * interconnection wiring shall be adopted as far as possible. In case of short cable length, single point grounding can be adopted. In order to protect the cable sheath insulation, a sheath protector shall be installed at the ungrounded end.

It can be seen that the grounding methods of high-voltage cable lines are as follows:

1. One end of the protective layer is directly grounded, and the other end is grounded through the protective layer - the method can be adopted;

2. The middle point of the protective layer is directly grounded, and the shield at both ends is grounded through the protective layer - common method;

3. Cross connection of protective layer - common method;

4. Cable transposition, metal sheath cross * interconnection - the best grounding method;

5. Grounding at both ends of sheath - not commonly used, only applicable to very short cable and small load cable line.