Grounding technology is indispensable in EMC testing

Introduction

In electromagnetic compatibility, grounding technology was first applied to strong current systems (power systems, power transmission and transformation equipment, electrical equipment). For equipment and personal safety, the ground wire is directly connected to the ground. Due to the very large capacitance of the ground, the potential of the ground can usually be considered as zero potential. Later, grounding technology was extended to weak current systems. For power electronic equipment, when current flows through a reference potential, connect the ground wire directly to the ground or to a conductor that serves as the reference potential. However, improper grounding can cause electromagnetic interference, such as common ground wire interference, ground ring interference, and lead to abnormal operation of power electronic equipment. It can be seen that grounding technology is one of the important aspects of electromagnetic compatibility technology for power electronic equipment, and it is necessary to discuss grounding technology in detail.

Type and purpose of grounding:

Power electronic equipment is generally grounded for the following purposes:

◆ Safety grounding

The safety ground is about to be grounded. One is to prevent the accumulation of electric charges in the housing, resulting in electrostatic discharge, which endangers the safety of equipment and personnel; The second is to promote power supply protection, cut off the power supply, and protect personnel safety when the equipment insulation is damaged and the housing is charged.

◆ Working grounding

Working ground is the reference potential provided for the normal operation of a circuit. The reference potential can be set to a point, segment, or block in the circuit system. When the reference potential is not connected to the ground, it is considered as a relative zero potential. This relative zero potential can vary with changes in external electromagnetic fields, resulting in unstable operation of the circuit system. When the reference potential is connected to the ground, the reference potential is considered as the zero potential of the ground and does not change with changes in external electromagnetic fields. However, incorrect working grounding can increase interference. Such as ground wire interference, ground ring interference, etc. In order to prevent various circuits from interfering with each other during operation and enable them to work in a mutually compatible manner. According to the nature of the circuit, there are different types of working grounding, such as AC, digital, analog, signal, power, and power supply. The above different grounding shall be set separately.

1. Signal ground

The signal ground is a common reference ground for the zero potential of various physical quantities of sensors and signal sources. Due to the generally weak signal and easy interference, the requirements for signal ground are high.

2.Simulated

The analog ground is the common reference ground wire for the zero potential of the analog circuit. Because analog circuits not only undertake the amplification of small signals, but also undertake the power amplification of large signals; Low frequency amplification and high frequency amplification; Therefore, analog circuits are easy to accept interference and interference. Therefore, the selection of simulation sites and the laying of ground wires should be fully considered.

3. Digitally

Digital ground is the common reference ground wire for the zero potential of digital circuits. Because digital circuits operate in pulsed conditions, especially when the front and rear edges of the pulse are steep or the frequency is high, it is easy to interfere with analog circuits. Therefore, full consideration should also be given to the selection of grounding points for digital ground and the laying of grounding wires.

4. Power supply ground

The power ground is the common reference ground wire for the zero potential of the power supply. Since power supplies typically supply power to each unit in the system simultaneously, the nature and parameters of the power supply required by each unit may be very different. Therefore, it is necessary to ensure not only the stable and reliable operation of the power supply, but also the stable and reliable operation of other units.

5. Power ground

The power ground is a common reference ground wire for the zero potential of the load circuit or power drive circuit. Due to strong current in the load circuit or power drive circuit, high voltage, and large interference from the power ground wire. Therefore, it is necessary to set the power source with other weak current sources to ensure the stable and reliable operation of the entire system.

◆ Lightning protection and grounding

When power electronic equipment encounters a lightning strike, whether it is a direct lightning strike or an inductive lightning strike, power electronic equipment will be greatly damaged. Install lightning rods to prevent lightning strikes from endangering equipment and personal safety.

The above two types of grounding are mainly for safety purposes and should be directly connected to the ground.

◆ Shield grounding

Shielding and grounding should be used together to achieve shielding effects.

For example, electrostatic shielding. When a positively charged conductor is surrounded by a complete metal shield, the same negative charge as the charged conductor will be induced inside the shield, and the same positive charge as the charged conductor will appear outside, so there is still an external electric field. If the metal shield is grounded, external positive charges will flow into the earth, and there will be no external electric field, that is, the electric field with a positive conductor will be shielded by the metal shield.

Another example is alternating electric field shielding. In order to reduce the coupling interference voltage of the alternating electric field to the sensitive circuit, a metal shield with good conductivity can be installed between the interference source and the sensitive circuit, and the metal shield can be grounded. As long as the metal shield is well grounded, the coupling interference voltage of the alternating electric field to the sensitive circuit is very small.

The above two types of grounding mainly consider electromagnetic compatibility.

3. Grounding method:

The following grounding methods are used according to the operating frequency:

1、 Single point grounding:

Low operating frequency (1MHz) adopts single point grounding, which means that a structural point in the entire circuit system is regarded as a grounding reference point, and all grounding connections are connected to this point, and safety grounding bolts are provided. This prevents circuit coupling where two points of grounding generate a common ground impedance. The single point grounding method for multiple circuits is divided into series and parallel. Due to circuit coupling where series grounding creates a common ground impedance, it is best to use parallel single point grounding for low-frequency circuits.

To prevent interference with the signal ground wire by operating frequency and other stray currents, the signal ground wire should be insulated from the power ground wire and the housing ground wire. Connect only to power ground, enclosure ground, and ground safety ground bolts (except for floating ground).

The relationship between ground wire length and cross-section is as follows:

S>0.83L(1)

Type: L-ground wire length, m;

S-ground wire section, mm2.

2、 Multipoint grounding:

High operating frequency (>30mHz) uses multi-point grounding (that is, in a circuit system, a grounding plate is used to replace the grounding circuit of various parts of the circuit). As the induced impedance of the ground lead is proportional to frequency and length, when the operating frequency is high, it will increase the common ground impedance, thereby increasing the electromagnetic interference generated by the common ground impedance. Therefore, the length of the ground lead should be as short as possible. When using multi-point grounding, try to find the closest low resistance grounding.

3、 Hybrid grounding:

The circuit with a working frequency of 1~30mHz adopts a hybrid grounding type. When the length of the ground wire is less than 1/20 of the working signal wavelength, the single point grounding type is used, otherwise, the multi-point grounding type is used.

4、 Floating ground

Floating type refers to the non-conductive connection between the ground of the circuit and the ground. The advantage is that the circuit is not affected by ground electrical performance; The disadvantage is that the circuit is susceptible to the influence of parasitic capacitors, which changes the ground potential and increases the induced interference of analog circuits; Due to the lack of conductor connection between the circuit and the ground, electrostatic discharge is prone to electrostatic accumulation, which may lead to electrostatic breakdown or strong interference. Therefore, the effect of floating ground depends not only on the size of the insulation resistance, but also on the size of parasitic capacitors and the frequency of the signal.

4.2 Methods for reducing ground resistance:

The grounding resistance consists of grounding resistance, contact resistance, and grounding resistance. There are three ways to reduce ground resistance:

Reduce the resistance of the ground wire and select a multi-stranded thin wire with a large overall cross-section and short length.

Reduce the contact resistance, make the grounding wire tightly and firmly connected to the grounding bolt, and increase the contact area and tightness between the grounding electrode and the soil.

Reduce the ground resistance, increase the surface area of the ground electrode, and increase the conductivity of the soil (such as injecting salt water into the soil).

5. Shielding ground:

"The shield of the circuit should be grounded, and the shield should be installed in circuits that are susceptible to electromagnetic radiation interference, such as various signal sources and amplifiers.". Due to the presence of parasitic capacitance between the signal circuit and the shield, the ground wire end of the signal circuit should be connected to the shield cover to eliminate the impact of parasitic capacitance, and the shield cover should be grounded to eliminate common mode interference.

The shielding layer of low-frequency circuit cables should adopt point grounding, and the grounding point of the shielding layer should be consistent with the grounding point of the circuit. For multilayer shielded cables, each shield layer should be grounded at a point, and each shield layer should be insulated from each other.

The shielding layer of high-frequency circuit cables should be grounded at multiple points. When the cable length is greater than 0.15 times the working signal wavelength, an interval multipoint grounding type of 0.15 times the working signal wavelength is used. If this is not possible, at least ground both ends of the shielding layer.

4. When the entire system needs to resist external electromagnetic interference or prevent the system from generating electromagnetic interference to the external world, the entire system should be shielded, and the shield should be connected to the system ground.

6. Equipment location

In order to meet design requirements, equipment usually includes multiple circuits, such as low-level signal circuits (such as high-frequency circuits, digital circuits, analog circuits, etc.), and high-level power supply circuits (such as power supply circuits, relay circuits, etc.). In order to install circuit boards and other components, the equipment needs to have a certain mechanical strength and shielding efficiency to resist external electromagnetic interference. The grounding situation of typical equipment is shown in Figure 1.

图1.设备接地

The following points should be noted when grounding equipment:

-The 50Hz power supply zero line should be connected to a safety grounding bolt. For independent equipment, a safety grounding bolt should be installed on the metal shell of the equipment and have a good electrical connection;

-To prevent the casing from being electrified and endangering personal safety, it is not allowed to replace the casing ground wire with the power supply zero line;

-To prevent high voltage, large current, and high-power circuits (such as power supply circuits, relay circuits) from interfering with low-level circuits (such as high-frequency circuits, digital circuits, analog circuits, etc.), separate their grounding. The former refers to power grounding (strong current grounding), and the latter refers to signal grounding (weak current grounding). Signal grounding is divided into digital grounding and analog grounding. The signal grounding wire should be insulated from the power grounding wire and the housing grounding wire;

-For the signal ground wire, another signal ground bolt (insulated from the equipment housing) can be installed. There are three ways to connect the signal grounding bolt and the safety grounding bolt (depending on the grounding effect): one is to become floating without connecting; The second is direct connection, becoming a single point grounding type; Third, through 3 μ The F capacitor is connected to form a DC floating ground type and an AC grounded type. Other grounds eventually converge on the safety ground bolt (this point should be located at the incoming line of the AC power supply), and then the ground electrode is buried in the soil through the ground wire.

7. Systematically

When a system consists of multiple devices, the grounding of the system is shown in Figure 2.

图2.接地系统

The following points should be noted for system grounding:

-Refer to equipment grounding precautions;

-The equipment shell is connected to the cabinet shell using the equipment shell ground wire;

-The cabinet shell is connected to the cabinet shell ground wire and the system shell;

-For the system, a safety grounding bolt is installed on the metal shell of the system, and there is good electrical connection;

-When there are too many cabinets and equipment in the system, it can lead to too many digital ground wires, analog ground wires, power ground wires, and cabinet housing ground wires. In this regard, it may be considered to lay two semi circular grounding buses, which are parallel to each other and insulated from the system enclosure. One is the signal grounding bus, the other is the shield grounding bus and the cabinet enclosure grounding bus; Each signal in the system is close to the signal grounding bus, and each shield grounding and cabinet shell in the system are close to the shield grounding and cabinet shell grounding bus; The middle of the two semi circular grounding buses is close to the safety grounding bolt, and the shield grounding and cabinet shell grounding buses are connected to the safety grounding bolt; Signal grounding bus to signal grounding bolt;

-When the system uses three-phase power supply, due to the difference in power consumption and power consumption time of each load, it inevitably leads to three-phase imbalance, resulting in a potential shift at the center point of the three-phase power supply. Therefore, the power supply zero line is connected to the safety grounding bolt, forcing the three-phase power supply center point potential to maintain zero potential, preventing interference caused by the three-phase power supply center point potential offset;

-Galvanized steel pipes are used for grounding electrodes, with an outer diameter of not less than 50mm and a length of not less than 2.0m; During embedding, the grounding electrode is driven to a certain depth on the ground and saline water is poured into it. Generally, the grounding resistance is required to be less than 4 Ω, and for mobile devices, the grounding resistance is less than 10 Ω.

8. Conclusion

During electromagnetic compatibility testing, it is necessary to study grounding technology to ensure the safety of equipment and personnel, as well as the normal and reliable operation of power electronic equipment. The ground can be connected directly to the ground or to a conductor that serves as a reference potential. Unreasonable grounding can introduce electromagnetic interference, leading to abnormal operation of power electronic equipment. Therefore, grounding technology is one of the important technologies of electromagnetic compatibility, and full attention should be paid to the research of grounding technology.