Grounding system for power supply units

Power supply device Due to its own structural characteristics and working characteristics, it works in a complex and diverse electromagnetic environment and is easily affected by various interference sources, which may disrupt signal transmission or distort the signal, causing the system powered by the power supply device to not work properly. The use of grounding technology is an extremely important measure to ensure the reliable operation of the power supply device, and is also an important means to ensure the safe and stable operation of the power supply.

1. Classification of grounding of power supply devices

There are many types of grounding for various power supply devices currently used in China, which can be summarized into the following categories:
(1) The working ground of the neutral point of the power supply to the power supply device: refers to the grounding of the neutral point potential of the stable power supply system;
(2) Lightning protection grounding of the power supply device: refers to the protective grounding to prevent lightning overvoltage during the thunderstorm season;
(3) Safety protection ground of the power supply device: refers to the grounding of the metal shell of the microelectronic device set up to prevent contact voltage and step voltage from endangering the safety of people and equipment;
(4) The DC system ground of the power supply device? Also known as the logic ground, working ground?, it provides a stable reference potential (usually the zero point) for each part and each link of the microelectronic device. This ground can be connected to the earth or just a common point. If the system ground is not connected to the earth, that is, the system ground is in a suspended working state, it is called a floating ground;
(5) Shielding ground of the power supply device: set up to suppress various interference signals. There are many types of shielding, but they all require reliable grounding.

Shielded ground is the grounding of the shielded network. Although the power supply devices in actual applications are produced by different companies, and the types of grounding and the resistance values ​​of grounding resistors of each company's products are different, the system grounding requirements of the power supply device are much stricter than the other grounding requirements, and there is a trend of increasing requirements. In order to avoid mutual interference between the "grounds", the above-mentioned "grounds" should be set up with their own independent grounding networks. The grounding wire must be an insulated copper wire connected to a unified grounding point to form a common potential point.

2. Grounding principles for microelectronic devices

In the grounding design of the automation control system, the principle of one point grounding must be followed. Because the system is composed of multiple automation equipment, the entire system must be grounded at one point. However, the system grounding wire and grounding resistance cannot be zero, especially in high frequency or transient conditions; in addition, when a large current is injected into the earth from the grounding electrode, the earth potential of the grounding electrode and its vicinity rises. If there are multiple grounding points, there will be a potential difference between the grounding points, forming interference. Even the system grounding wire in the same device should follow the principle of one point grounding, otherwise a grounding loop will be formed, and the grounding potential difference between each point will form interference and be introduced into other circuits. In order to study the relationship between the above-mentioned various grounding systems, analyze the many factors of the grounding network system and the effective ways and specific methods to reduce the grounding resistance, in recent years, "automation device grounding engineering" as a new discipline has been valued in the field of automation control, and has also laid a theoretical foundation for the research and practice of the grounding system of automation devices.

The types of grounding and technical indicators of various power supply devices vary depending on the manufacturer and equipment function. Grounding plays a vital role in the safe and reliable operation of power supply devices. Different grounds have different processing technologies. The following introduces the grounding processing principles that power supply devices should follow.

1. Application of single-point grounding and multi-point grounding principles

As far as electronic technology common sense is concerned, in low-frequency electronic circuits, the inductance between wiring and components does not appear to be serious. In order to avoid ground loops caused by ground wires, it is recommended to adopt the single-point grounding principle.

For high-frequency electronic circuits, the influence of inductance will be more prominent, because the increased impedance of the ground wire will lead to inductive coupling between the wires. Generally speaking, when the frequency is less than 1MHz, one-point grounding can be used, while when it is higher than 10MHz, multi-point grounding should be used. When the frequency is between 1MHz and 10MHz, if one-point grounding is used, the length of the ground wire should not exceed 1/20 of the wavelength, otherwise, multi-point grounding should be used.

2. Comparison between floating grounding and true grounding

Floating grounding means that each ground terminal of the system is not connected to the earth. This grounding method is simple, but it has high requirements for the insulation resistance with the ground, generally greater than 50MΩ, otherwise interference will occur due to insulation degradation. In addition, floating grounding is prone to electrostatic interference.

True grounding means that the grounding end of the system is directly connected to the earth. As long as the grounding is good, this method has a strong anti-interference ability. However, the grounding process is complicated, and once the grounding is poor, it will cause unnecessary interference.

3. The composition of grounding resistance

Any manufacturer of power supply devices has strict requirements on the grounding resistance of its products. When designing the grounding, the grounding resistance of the power supply device should be used as the technical basis for designing its grounding. Therefore, the composition of the grounding resistance is analyzed here so that corresponding measures can be taken in the main links in the design to reduce the grounding resistance.
(1) Grounding lead resistance refers to the resistance of the lead itself from the grounding body to the grounding busbar of the power supply device. Its resistance is related to the geometric size and material of the lead;

(2) The resistance of the grounding body (horizontal grounding body, vertical grounding body) itself, whose resistance value is related to the material and geometric dimensions of the grounding body;
(3) The contact resistance between the grounding body surface and the soil, whose resistance value is related to the nature, particles, water content of the soil, and the contact surface and tightness of the soil and the grounding body;
(4) The soil resistance of the path through which the current diffuses from the grounding body to a distance (20m), that is, the stray current resistance. The main factor determining the stray current resistance is the water content of the soil.

4. Main measures to reduce grounding resistance

Although the grounding resistance consists of four parts, the first two parts account for a smaller proportion of the grounding resistance, and the contact resistance and the scattered current resistance play a decisive role. Therefore, the work of reducing the grounding resistance value should be carried out from these two parts, and the methods of reducing the contact resistance and the scattered current resistance are discussed from the aspects of the best burial depth of the grounding body, the technology of unequal length grounding bodies and chemical resistance reducing agents.

1. Optimal burial depth of vertical grounding electrode

The best burial depth of the vertical grounding body refers to the burial depth that can make the stray resistance as small as possible and is easy to reach. The factors of the three-dimensional grounding network should be taken into consideration when determining the best depth of the vertical grounding body. The so-called three-dimensional grounding network refers to a grounding network in which the burial depth of the grounding body and the equivalent radius of the grounding network are in the same order of magnitude (that is, the ratio of the burial depth to the equivalent radius is greater than 1/10). The burial depth should be taken as much as possible within the possible range, but the deeper the burial depth, the better. If the vertical grounding body is approximated as a hemispherical grounding body, its resistance is:
R = ρ / 2πr = ρ / 2πL
In the formula: ρ - soil resistivity;
L - burial depth of the vertical grounding body.
It can be seen from the formula that R is inversely proportional to L. In order to reduce R, the larger L, the better. However, the partial differential of the above formula:
aR / aL = -ρ / 2πL2
shows that as L increases, the resistance reduction rate aR / aL decreases inversely proportional to L2. When L is increased to a certain extent, it is basically saturated and the resistance reduction rate has approached zero. The optimal burial depth of the vertical grounding electrode is not fixed. It should be determined according to the equivalent radius of the grounding grid and the geological conditions in the area during design. Generally, it is appropriate to take a depth between 1.5m and 2.5m.

2. Unequal length grounding technology

Since the spacing between the buried single grounding bodies in the grounding grid is generally only about twice the length of each single grounding body, when the current flows into each single grounding body, it is restrained by each other and prevents the current from dissipating, which is equivalent to increasing the dissipation resistance of each single grounding body. This phenomenon that affects the current dissipation is called shielding. As shown in the figure. Due to the shielding effect, the dissipation resistance of the grounding body is not equal to the parallel value of the dissipation resistance of each single grounding body. At this time, the dissipation resistance of the grounding body group is:

Ra=RL/nη
, where:
RL is the stray current resistance of a single grounding body;
n is the number of grounding body groups connected in parallel with a single grounding body;
η is the utilization coefficient of the grounding body, which is related to the shape and position of the grounding body.

Theoretically, the distance of 20m from the grounding body is the electrical "ground", that is, when the distance between two grounding bodies is greater than 40m, the utilization coefficient η of the grounding body can be considered to be 1. However, in the layout of the grounding body of the grounding grid, it is difficult to make two single grounding bodies 40m apart. In order to solve the contradiction between design practice and theoretical analysis, the unequal length grounding body technology is adopted to achieve good results. The unequal length grounding body technology means that the lengths of each vertical grounding body are not equal. In the layout of the grounding body, the vertical grounding bodies are arranged as two long and one short or one long and two short to minimize the shielding effect between the grounding body groups. The unequal length grounding body technology, from theory to practice, has better solved the problem of shielding effect between multiple single grounding bodies, improved the utilization coefficient of each single grounding body, and reduced the stray resistance of the grounding body group.

3. Application of chemical drag reducing agent

The resistance reduction mechanism of chemical resistance reducing agent is that it seeps from the grounding body to the outer soil in liquid state, and after solidifying for several minutes, it plays a role in increasing the contact area of ​​the diffused current electrode. Because the resistance reducing agent itself is a good conductor, it is used between the grounding body and the soil. On the one hand, it can be in close contact with the metal grounding body, reduce the contact resistance between the grounding body and the soil, and form a sufficiently large current flow cross-section; on the other hand, it can penetrate into the surrounding soil, reduce the resistivity of the soil, and form a variable low-resistance area around the grounding body. Thereby significantly expanding the equivalent diameter and effective length of the grounding body, it has a significant effect on reducing the contact resistance and diffused current resistance. For example, the service life of JZG-02 long-term anti-corrosion resistance reducing agent can reach more than 20 years. Its performance is stable during its life cycle, and it does not require maintenance. It can still have good electrolyte performance and water absorption, and maintain its good physical and chemical mechanism.

V. Conclusion

The design of the grounding of the power supply device should be based on the technical requirements of the power supply device and the geographical and geological conditions of the area where it is located. Different measures should be taken to achieve the highest performance-price ratio and try to use new technologies and new materials for design. Because "grounding engineering" is a multidisciplinary marginal subject, it involves geology, electromagnetic field theory, electrical measurement, applied chemistry, drilling technology, construction technology and other disciplines. Therefore, it is still necessary to study it in future work and continue to explore in practice to ensure the safe and reliable operation of the power supply device.