What Is a Casing String?

Bushing is an insulation device that introduces a live conductor into electrical equipment or through a wall. The former is called electrical bushing and the latter is called wall bushing. Casing is usually used in the basement of the building. It is used to protect the pipeline or to facilitate the installation of pipelines. The classification of casings includes rigid casings, flexible waterproof casings, steel casings and iron casings.

Bushing is an insulation device that introduces a live conductor into electrical equipment or through a wall. The former is called electrical bushing and the latter is called wall bushing. The bushing structure generally consists of three parts: conductor (guide rod), insulator and metal flange. The conductor passes along the axis of the cylindrical insulator, and a metal ring flange is installed outside the insulator and used to ground. The bushing belongs to an insulating structure with a strong vertical electric field component. The electric field strength at the metal flange is very large, and it is easy to generate corona discharge and flash discharge along the dielectric surface. The radial electric field strength between the flange and the guide rod is also very high, and breakdown of the insulating medium is prone to occur. Except for bushings of 35kV and below, which use a single solid insulation material, multiple insulation materials or electric fields are often used.

The design of the casing usually meets:

No partial discharge or thermal breakdown occurs under long-term working voltage;

No flicker discharge occurs during the power frequency withstand voltage test;

No breakdown or flashover occurs during power frequency and impulse voltage tests.

There are many types of bushings, which can be divided into single insulating material bushings (including pure ceramic bushings and resin bushings) and composite insulating bushings (including oil-filled bushings and inflatable bushings) according to different structural characteristics and main insulation media. ) And capacitor bushings (including oil-paper capacitor bushings and adhesive paper capacitor bushings) are three types of pure porcelain bushings, which are insulated with electric porcelain and air. Due to its simple structure, it is widely used in wall bushings of 35kV and below and electrical bushings of 10kV and below. To ensure sufficient creepage distance, the outdoor part of the porcelain sleeve is also provided with ribs. In order to prevent the air in the porcelain sleeve from corona, the surface of the porcelain sleeve is often sprayed with a metal coating and the diameter of the guide rod is enlarged to improve the electric field distribution.

The main insulation sleeve is the insulating oil filled with the inner cavity of the porcelain sleeve and the insulation barrier. Only the porcelain bushing is filled with insulating oil, that is, porcelain-oil bushings are mostly used for voltage levels of 110kV and below. For higher voltage levels, use conductive rods to wrap insulation paper or rubber sleeves, or add multiple concentric cylindrical insulation barriers in the oil gap to increase the breakdown voltage. Sometimes a metal plate is also laid on the insulating barrier to distribute the electric field uniformly.

The inner cavity of the porcelain sleeve is filled with a sulfur hexafluoride gas insulated sleeve. It is usually used as the outlet sleeve of gas-insulated metal-enclosed switchgear (GIS). Capacitive bushing uses oil paper or adhesive paper as the main insulation, and uses a capacitor screen to uniformly distribute the radial and axial electric fields. The electrical bushings and wall bushings of modern high-voltage electrical equipment are mostly capacitive bushings. The core of the capacitive bushing is the capacitor core, which is a dense insulator composed of multiple layers of oil paper or adhesive paper. Metal foil electrodes are sandwiched between the insulation layers to form a plurality of concentric cylindrical capacitors. The diameter of the concentric cylindrical capacitor electrode increases from the inside to the outside, while its length decreases in sequence. The diameter and length of the electrode are selected according to a certain law, so that the radial and axial electric field distribution becomes uniform, so that the size of the sleeve is minimized on the premise of meeting the electrical performance requirements. In order to improve the electric field distribution at the edges of metal foil electrodes, semiconductor foils or metal foils with semiconductor piping are sometimes used ^[1]

Dimension tolerance:

Type of steel pipe	Outside diameter (D)	Wall thickness (S)
Cold drawn tube	Outer diameter of steel pipe (mm)	Allowable deviation (mm)
<114.3	± 0.79	-12.5%
114.3	0.5% + 1%

Mechanical properties:

standard	Grade	Tensile strength (MPa)	Yield strength (MPa)	Elongation(%)	hardness
API SPEC 5CT	J55	517	379 552	Look up table
K55	517	655
N80	689	552 758
L80 (13Cr)	655	552 655	241HB
P110	862	758 965

chemical composition:

standard	Grade	chemical composition(%)
C	Si	Mn	P	S	Cr	Ni	Cu	Mo	V	Als
API SPEC 5CT	J55K55 (37Mn5)	0.34 0.39	0.20 0.35	1.25 1.50	0.020	0.015	0.15	0.20	0.20			0.020
N80 (36 Mn2V)	0.34 0.38	0.20 0.35	1.45 1.70	0.020	0.015	0.15				0.11 0.16	0.020
L80 (13Cr)	0.15 0.22	1.00	0.25 1.00	0.020	0.010	12.0 14.0	0.20	0.20			0.020
P110 (30CrMo)	0.26 0.35	0.17 0.37	0.40 0.70	0.020	0.010	0.80 1.10	0.20	0.20	0.15 0.25	0.08	0.020

A carbon steel straight pipe short section is sleeved on the pipe and the pipe and the threaded pipe to protect the interface from being hurt by welding. The purpose is to protect the wires and cables.