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Electromagnetic interference refers to the unwanted or unintentional coupling of electromagnetic energy from one device or system to another device or system by a source of interference. It can be seen that in the power system of pure electric vehicles, the electromagnetic interference generated by the potential electromagnetic interference source must pass through a certain medium and pass through a certain coupling path before it may be coupled to the disturbed equipment in the system. Therefore, when studying the electromagnetic environment of pure electric vehicle power system, it is first necessary to analyze the propagation and coupling characteristics of electromagnetic interference. Electromagnetic energy has many different propagation coupling paths depending on the propagation medium. If the electromagnetic interference source in the electromagnetic environment of the pure electric vehicle power system is regarded as a transmitter, and the electromagnetic interference device in the pure electric vehicle power system is regarded as a receiver, then in the electromagnetic environment of the pure electric vehicle power system, there are roughly the following five coupling paths for the electromagnetic interference generated by the electromagnetic interference source to propagate to the disturbed device:
① Direct radiation from the transmitter to the receiver;
②The cable harness radiated from the transmitter to the power system, and then transmitted to the receiver by the harness;
③ Directly radiated from the cable harness of the transmitter to the receiver;
④ Radiated from the cable harness of the transmitter to the cable harness in the power system, and then conducted to the receiver by the harness;
⑤ Conducted directly from the transmitter to the receiver through the common conductor.
The above-mentioned coupling modes of electromagnetic interference propagation can be divided into two categories: radiation coupling and conduction coupling.
1. Radiation coupling
Radiation coupling means that the electromagnetic energy generated by the electromagnetic interference source is coupled to the disturbed device in the form of radiation through a certain medium. The radiation coupling interference received by the power system can be manifested as the electromagnetic energy generated by the electromagnetic interference source in its electromagnetic environment being directly radiated to the disturbed equipment in the system, or the electromagnetic energy can be first coupled to the wiring harness in the power system, and then conducted to the victim device. The radiation coupling of electromagnetic energy is transmitted to the dynamic system in the form of electromagnetic waves, and the propagation of electromagnetic waves will form an electromagnetic field, which can be divided into far field and near field according to different properties. Among them, the electromagnetic field strength in the near-field region is much larger than that in the far-field region. Therefore, the radiated coupled electromagnetic interference received by the electric vehicle power system has a great relationship with the field area where it is located. The nature of the electromagnetic field in the near-field region has a great relationship with the characteristics of the electromagnetic radiation interference source. When the source of electromagnetic radiation interference is a low-voltage, high-current source, the near-field is dominated by the magnetic field. At this time, the magnetic field energy mainly propagates to the disturbed equipment in the power system through the coupling between the inductors; when the electromagnetic radiation interference source is a high-voltage small current source, the near field is dominated by the electric field. At this time, the electric field energy is transmitted to the disturbed equipment in the power system through the coupling between the capacitors.
In the electromagnetic environment of the electric vehicle power system, there are both electromagnetic interference sources from inside the vehicle and electromagnetic interference sources from outside the vehicle. Therefore, the power system of an electric vehicle must not only bear the radiation coupling interference generated by itself, but also bear the electromagnetic radiation coupling interference generated by the electromagnetic interference source outside the system. External broadcast signals, TV signals, nearby power lines and wireless communication equipment used by drivers and passengers may radiate electromagnetic interference to the power system. However, these external electromagnetic interference sources are usually far away from the power system, so the electromagnetic interference to the power system is generally not very serious. Relatively speaking, the main source of radiated coupling electromagnetic interference in the power system still comes from the inside of the system.
The radiation coupling interference in the electric vehicle powertrains mainly includes electromagnetic transmission signals, radiation coupling interference generated by the wire harness in the power system as the antenna, radiation coupling interference caused by power electronic pulse width modulation, and radiation coupling interference caused by radio frequency equipment in the system.
2. Conductive coupling
Conducted coupled interference refers to the electromagnetic interference formed by electromagnetic energy directly passing through the conductive path between the electromagnetic interference source and the disturbed device. The conducted coupling interference of the pure electric vehicle power system can be expressed as the electromagnetic energy generated by the electromagnetic interference source in the electromagnetic environment is directly conducted to the disturbed equipment in the system through the common conductor, and can also be manifested as the electromagnetic energy coupled to the wiring harness conducted through the wiring harness to the disturbed equipment. Conducted coupling interference can be divided into two types according to the different propagation directions: external interference introduced by the wiring harness of the power system and electromagnetic interference generated by the components or equipment in the power system and conducted by the wiring harness. Conducted coupled electromagnetic interference can also be divided into common mode interference and differential mode interference according to the direction of current transmission in the wire. The differential mode interference is mainly caused by the voltage on the energy storage capacitor, while the common mode current is mainly caused by the loop formed by the stray capacitance of the cable harness to the ground. In addition, when two or more devices (or components) share power lines and ground lines, there will also be common impedance interference between different devices or components. Differential mode interference and common mode interference usually exist at the same time, and the two can sometimes transform into each other.
The propagation forms of conduction coupled EMI mainly include direct conduction coupled EMI, transfer impedance coupled EMI and common impedance coupled EMI. The conducted coupling interference suffered by the electric vehicle power system mainly refers to the electromagnetic interference energy generated by the electromagnetic interference source in the electromagnetic environment, which is coupled to the disturbed parts or components in the system in the form of voltage or current through the metal wires or lumped elements in the system. equipment. After the electromagnetic energy caused by the transient process generated in the circuit by the switching on and off of the switching device, the switching of the relay and the commutation of the motor, etc. enters the cable harness and act on other connected electrical equipment or systems, or act on other circuits in the form of capacitive coupling and inductive coupling through transmission wires. Since the electric vehicle powertrain does not have cables connected to the outside of the vehicle during operation, conducted-coupled disturbances are limited to the interior of the vehicle.