Capacitive voltage divider test system and basic principles

作者： GOZ Electric 时间：2014-04-25 09:30:30 阅读：17

    The principle of a capacitive voltage divider for measuring impulse voltage. The capacitance value of the high-voltage arm C0 of the voltage divider should be small, and it can withstand most high voltages and have small losses; the low-voltage arm C1 has a low voltage, so it The capacitance needs to be large.

    Each part of the capacitive voltage divider has stray capacitance to ground, forming an accommodating branch, which distorts the output waveform and also affects the voltage dividing ratio to a certain extent. In order to minimize the influence of stray capacitance, for unshielded capacitive voltage dividers, the capacitance value of the high-voltage arm should be appropriately increased.

         Equivalent circuit of capacitor

The general structure of a capacitor usually consists of three parts:

(1) Capacitor core: It is mainly composed of dielectric and plate. It is the core part of the capacitor and is usually represented by Z1 in the equivalent circuit.

(2) Protection structure: Its function is to protect the capacitor core so that it can work reliably for a long time. This part of the impedance is usually represented by Z2.

(3) Lead or insulator terminal: This part of the impedance is usually represented by Z3.

         In the Z1 part, R1 and C1 represent the equivalent parallel resistance and capacitance of the capacitor core medium; r1 and L1 represent the resistance and inductance of the plate. In the Z2 part, R2 and C2 represent the parallel resistance and capacitance of the protective layer medium; r2 and L2 represent the resistance and inductance of the leads in the protective layer. In the equivalent circuit of the protective layer structure, the lead-out piece in the protective layer is very short and the area is small, so the values of r2, L2, C2, and R2 are smaller than the corresponding parameters of the capacitor core dielectric. Generally speaking, this part has little impact on the entire capacitor. For the convenience of analysis and calculation, the equivalent circuit of the above capacitor can be further simplified.

         inductance in capacitor

         When current continues to flow through a conductor, magnetic flux will be generated around the conductor and a magnetic field will be formed. The size and direction of the magnetic flux depend on the size and direction of the current in the conductor. Therefore, there is a proportional relationship between the magnetic flux and the current. This ratio or the ratio between its increments is called the inductance of the conductor. Obviously, capacitors are also metal conductors, so capacitors also have inductance. When alternating current or changing current passes through conductive parts such as the capacitor's lead wires, plates, and metal shells, inductance will be generated. In fact, the capacitance and inductance of the capacitor coexist at the same time. The size of the inductance depends on the geometric size of the conductor through which the current flows and the magnetic permeability of the conductor, and has nothing to do with the size of the current passing through it.

         Although the specific sizes and structures of various capacitors are different, and their inductances are different, the inductance of a general capacitor is composed of three parts: the core inductor (mainly refers to the plate part, but also includes the lead-out inductor for contact and multiple cores connection part between), lead inductance and case inductance (when the case of the capacitor is metal and connected to one end of the capacitor). When making a capacitor voltage divider, in order to make the capacitor have a smaller inductance, the following three aspects can be considered:

         (1) The current-carrying part of the capacitor should be configured so that the magnetic fluxes cancel each other out as much as possible. Therefore, conductors with opposite current directions should be kept close together and conductors with the same current direction should be kept away from each other.

         (2) Minimize the length of the current-carrying conductor, and special considerations should be given to the shape and structure of the conductor (increase the width and thickness of the current flow).

         (3) The material for the metal part of the capacitor should be non-magnetic.

         In the capacitor core, if the direction of the current flowing through the upper and lower plates can be reversed, and the distribution on the plates is assumed to be uniform, then theoretically, this situation will present the smallest inductance to the outside world. As shown in Figure 2-6, the current direction on both sides of the capacitor plate is opposite, so the inductance can be considered to be zero. As for Figure 2-7, the lead wires between the two plates are staggered from each other. There is an area between the plates where the current direction is the same, and a magnetic field is formed in this area to increase the value of the inductance.

         Capacitance temperature characteristics of capacitors

         In addition to being affected by frequency, the capacitance of a capacitor is also affected by temperature. The change of capacitance with temperature is first determined by the dielectric material. Electrolytes with various neutral media and ionic structures with electron and ionization displacement polarization belong to this category; the second category is materials that show a strong non-linear relationship with temperature and have Type II ceramic material with obvious Curie point. According to the change of the dielectric constant of the medium with temperature, it can also be divided into two categories: the first category has a linear relationship with temperature, and the second category has no linear relationship with temperature.

         In addition, the structure and process of the capacitor will also affect the temperature characteristics of the capacitor.