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MindMap Gallery Dielectric strength of liquid and solid media

Dielectric strength of liquid and solid media

Mind map of the dielectric strength of liquid and solid dielectrics: including the electrical properties of liquid and solid dielectrics, the breakdown characteristics of liquid dielectrics, the breakdown characteristics of solid dielectrics, etc.

Edited at 2022-04-18 11:45:15

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Dielectric strength of liquid and solid media

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Outline

Dielectric strength of liquid and solid media

1. Electrical Properties of Liquid and Solid Dielectrics

1. polarization of dielectric

（1） Dielectrics can be divided into ionic dielectrics and polar, weakly polar and non-polar covalent bonded dielectrics. Most solid inorganic compounds have an ionic structure

（2） Polarization: Under the action of an electric field, a dielectric changes from being non-electric to being overall electric.

（3） A physical quantity that describes the strength of polarization: relative permittivity. The larger the relative permittivity, the easier it is to polarize. The relative dielectric constant is related to the polarity of the dielectric molecules, and is also affected by temperature and the frequency of the external electric field. The gas is close to the liquid, the liquid is 2~6, the pure water is 8I, and the alcohol is 33

（4） Basic form of polarization

I. Electronic polarization: lossless elasticity and not affected by external electric field frequency

II. Ionic polarization: lossless elasticity and almost independent of the frequency of the external electric field

III. Dipole polarization (steering polarization): the type of polarization that is inelastically lossy and most affected by frequency

IV. Sandwich polarization: lossy 5. Discuss the significance of dielectric polarization

（5） Discuss the significance of dielectric polarization

I. When making a capacitor, choose a material with a relatively small dielectric constant.

2. conductance of dielectric

（1） No dielectric can be an ideal insulator. There are charged particles inside. Under the action of an external electric field, carriers with weak connections will drift directionally to form conduction current (conductance current or leakage current), which is the conductivity process of the dielectric.

（2） A physical quantity that characterizes conductance

（3） Dielectric conductivity properties

I. Ionic conductivity (ions are carriers) Electronic conductivity (metals, free electrons carry current son)

II. solid dielectric conductivity

i. Volume conductance: influencing factors include electric field strength, temperature and impurities

ii. Surface conductivity: Surface moisture and dirt are greatly affected by external factors

III. Effect of temperature: temperature rises, dielectric conductivity rises, metal conductor conductance drops

（4） Dielectric equivalent circuit

（5） Discuss the significance of dielectric conductance: high insulation resistance is not desirable in all cases

3. dielectric losses

（1） Basic form of loss

I. Conductivity loss

II. Polarization loss

III. Ionization loss

（2） Summarize the physical quantities of losses

I. DC

II. AC, dielectric loss angle tangent value or dielectric loss factor tan @ The greater the value, the greater the dielectric loss

（3） Gas dielectric loss

I. There is no loss in the polarization process, there is very little conductivity loss, and the loss increases sharply as the electric field intensity increases (ionization loss)

（4） Liquid dielectric loss

I. The loss of neutral and weakly polar liquid media (transformer oil) is mainly caused by conductance, and high temperature causes large losses.

II. Polar liquid medium (amaranth oil) conductivity loss and polarization loss

（5） Solid dielectric losses

I. Inorganic insulation materials

i. Mica conductivity loss

ii. Electrical ceramics (electrical porcelain), glass: conductivity loss and polarization loss

II. organic insulating materials

i. Non-polar: polyethylene, conductivity loss, small

ii. Polarity: Insulating paper, significant polarization loss

（6） Discuss the significance of dielectric loss

I. Select materials with small tan@ for insulation structure design

4. dielectric breakdown

2. Breakdown Characteristics of Liquid Dielectrics

1. Classification (Breakdown Mechanism)

（1） pure liquid

I. breakdown theory

i. electrical breakdown theory

ii. bubble breakdown theory

（2） Engineering liquids (impurities)

I. small bridge breakdown theory

2. Main factors affecting the breakdown voltage of liquid dielectrics

（1） Impurities

（2） temperature

（3） Voltage action time

（4） Electric field uniformity

（5） pressure

3. Methods to Improve Liquid Dielectric Breakdown Voltage

（1） Improve oil quality

（2） broken bridge

3. Breakdown Characteristics of Solid Dielectrics

1. The round body insulation is non-self-restoring insulation.

2. Breakdown theory of national dielectrics

（1） electrical breakdown theory

（2） thermal breakdown theory

（3） electrochemical breakdown theory

I. Causes of partial discharge

3. Main factors affecting the breakdown voltage of solid dielectrics

（1） Causes of partial discharge

I. The longer the voltage application time, the lower the breakdown voltage. Below 0Is - electrical breakdown; from a few minutes to tens of hours - thermal breakdown; from tens of hours to several years - electrochemical breakdown

（2） temperature

（3） Electric field uniformity and medium thickness

I. The electrical breakdown voltage increases as the thickness of the medium increases, while the thermal breakdown voltage does not increase with the thickness, and the breakdown field strength decreases.

（4） Voltage frequency

I. Breakdown voltage is inversely proportional to the square root of frequency

（5） Voltage type

I. Breakdown voltage from high to low: impact, DC, power frequency, high frequency

（6） Moisture

（7） cumulative effect

（8） mechanical load

4. Electrical strength of combined insulation

1. Combined insulation electric field distribution

（1） DC voltage: The insulation is equivalent to resistance, and the voltage shared by each layer of insulation is inversely proportional to its conductivity. Materials with high electrical strength and high conductivity are used where the electric field is strongest

（2） AC and impulse voltage: Insulation is equivalent to capacitance. The voltage shared by each layer of insulation is inversely proportional to the dielectric constant. Materials with high electrical strength and large dielectric constant are used in places with the strongest electric fields.

2. Oil-barrier insulation: power transformers, oil circuit breakers, oil-filled bushings

3. Oil-paper insulation: cables, capacitors, capacitive bushings