Capacitive voltage transformersIt is composed of a capacitive voltage divider and an electromagnetic unit, which can be divided into a single-column (or integral) structure or a split structure according to its composition.
The structure of a capacitor voltage divider.
The capacitor divider is composed of one or more capacitor units, and for the capacitive voltage transformer of 110kV and below voltage level, the capacitor divider basically has only one capacitor unit;220kV equipment generally contains 2 capacitor cells, 330kV equipment generally contains 3 capacitor cells, 500kV equipment generally contains 3-4 capacitor cells, 750kV equipment generally contains 4 capacitor cells, and 1000kV equipment generally contains 4-5 capacitor cells.
1.1 Capacitor unit.
The capacitor unit is basically an oil-immersed hermetically sealed structure. The housing, in the form of a porcelain or composite sleeve, contains a core consisting of several capacitive elements connected in series, an insulating oil medium, and a metal expander for temperature compensation. The metal expander is used to compensate for the volume change caused by the change of the oil medium inside the capacitor with the change of operating temperature.
Metal expanders can be divided into two types: external oil type and internal oil type. The external oil expander is installed inside the capacitor, which is a sealed gas inside and the oil medium of the capacitor on the outside. The internal oil expander is installed on the outside of the capacitor, and the inner cavity is connected with the inside of the capacitor, filled with oil medium, and the outside is atmospheric.
1.2. Liquid oil medium.
The oil medium of the capacitive voltage transformer and the capacitance voltage divider is basically dodecylbenzene (DDB oil for short) in the early morning. Around 2000, phenylxylene ethane (also known as diaryl ethane, referred to as PXE or S oil) or phenyl ethylphenyl ethane (referred to as PEPE or low-temperature S oil) began to be used. PXE and PEPE have better electrical and physical properties than dodecylbenzene and compatibility with polypropylene films, which can significantly improve the electrical performance of capacitors. The electrical and physical properties of PXE and PEPE are similar to benzene, but PEPE has excellent low-temperature properties and can be used to operate under environmental conditions in cold regions. There are also some capacitors that use benzyltoluene (abbreviated as M DBT or C101 oil), which has better low temperature performance and is suitable for operation at ambient temperatures in cold areas.
1.3 capacitor hearts.
The capacitor unit has one or several cores inside, which are stacked on top of each other and electrically connected in series. Each core is generally composed of several capacitive elements connected in series. The series connection between the capacitive components of the early products generally adopts the lead welding structure, and since the mid-90s of the 20th century, the lead crimping structure has been the main one.
The component is the most basic capacitive unit of a capacitor. The capacitive components of capacitive voltage transformers are generally made of two solid dielectrics, polypropylene film and capacitor paper, and the plates are aluminum foil. Polypropylene film has the characteristics of high electrical resistance and low dielectric loss, and is the most commonly used dielectric material in power capacitors. Capacitor paper is the traditional dielectric material of capacitors, which contains a large number of overlapping flat fibers and voids, as well as a certain amount of hemifibers and lignin. The density of the paper is large, the content of cellulose in the paper is more, the dielectric constant and dielectric loss tangent value of the device are high, the dielectric constant of capacitor paper is high, the impregnation performance and arc resistance ability are better, but its electrical resistance is low, and the dielectric loss tangent value is large. Because the capacitive voltage transformer is affected by the temperature change will cause the temperature additional error, it is necessary for the capacitance voltage divider to have a small capacitance temperature coefficient, and the capacitance change is within the required range when the temperature changes, so as to ensure the accuracy requirements. Polypropylene film has a negative capacitance temperature coefficient, that is, when the temperature increases, it will cause the capacitance to decrease, while capacitor paper has a positive capacitance temperature coefficient, that is, when the temperature increases, it will cause the reactance to increase. Therefore, the use of polypropylene film and capacitor paper can make the capacitance temperature coefficient as small as possible to reduce the error change caused by temperature change.
2.1 electromagnetic unit.
At present, electromagnetic units are basically oil-immersed fully sealed structures. The electromagnetic unit contains intermediate transformers, compensating reactors, dampers and other devices, assembled with a metal oil tank, and the oil tank is impregnated with transformer oil. The secondary winding, the low-voltage terminal of the medium-voltage circuit and the low-voltage terminal of the capacitor divider are led from the terminal plate on the side of the fuel tank to the secondary outlet box.
The following facilities are generally required on the electromagnetic unit fuel tank:
1) Oil level inspection window. It is used to observe the oil level in the electromagnetic unit.
2) Oil injection holes. It is used to replenish or change the oil of the electromagnetic unit on site.
3) Sampling valve. It is used to take oil samples or drain oil from electromagnetic units on site.
2.2. Intermediate transformer.
There are two kinds of cores commonly used in intermediate transformers: laminated core core C-shaped core, and the laminated core is generally a single-phase three-column structure. The C-shaped iron core is a strip silicon steel sheet wound into a rectangular coiled iron core, and then cut on the long column, and the winding is sleeved through the open long column, and its cross-section can be square or its four corners in a stepped shape.
2.3. Compensation reactor
The core of the compensating reactor is basically a C-shaped core, and the cross-section is rectangular, and the inductive reactance of the compensating reactor is determined according to the capacitive reactance of the equivalent capacitance of the capacitance divider.
2.4-speed saturated damping reactor.
The core of the fast saturation damping reactor is made of annular core, and the core materials used are permalloy, amorphous alloy and silicon steel sheet.
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Zhejiang Yishuo Electric Co., Ltd. is a high-tech company specializing in R&D, design, production, sales and service of low-voltage power capacitors, smart capacitors, reactors, reactive power compensation controllers, and reactive power compensation devices.
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