Discussion on Electromagnetic Compatibility of Variable Frequency Speed Regulating Electric Drive System_Century Wisdom (Suzhou) Testing Technology Co., Ltd

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Discussion on Electromagnetic Compatibility of Variable Frequency Speed Regulating Electric Drive System

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2022-05-05 00:00

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　　1 Introduction

　　All kinds of electrical equipment used in industrial sites, such as PLC, numerical control equipment, frequency converters, low-voltage electrical appliances, industrial control instruments, etc., have their own electromagnetic compatibility standards. GB12668.3 proposes electromagnetic compatibility requirements and specific test methods for speed-adjustable electric drive systems. The frequency conversion speed regulation transmission system is widely used in electric transmission regulation due to its simple circuit, high power factor, small output harmonics, stable start, wide speed regulation range, perfect protection, good control performance, strong overload capacity, and convenient use and maintenance. In various industrial production fields of speed control. Most of the frequency converters operate in harsh electromagnetic environments. At the same time, as power electronic equipment, they are composed of electronic components, micro-processing chips, etc., and will be subject to external electromagnetic interference; in addition, the voltage and current of the input and output sides of the frequency converter It is rich in high-order harmonics and is also a source of interference, which will interfere with other electronic equipment.

　　2 Frequency conversion speed regulation drive system

　　For the AC-DC-AC inverter, according to its main circuit mode, it can be divided into two categories: current type and voltage type. Current-type inverters are suitable for single-machine driving and need to be frequently reversed or frequently accelerated and decelerated; voltage-type inverters are mainly suitable for occasions that supply power to multiple motors, irreversible drag, work at a stable speed, and do not require high speed . There are two control modes of frequency converters: voltage control and current control, which are applicable to both current-type and voltage-type frequency converters. General-purpose inverters are suitable for voltage-type voltage control, which controls the output voltage in proportion to the output frequency. For occasions that require fast response, current control mode can be used.

　　Figure 1 is a structural block diagram of a voltage source AC-DC-AC inverter. It can be seen from the figure that the main circuit of the inverter is composed of a rectifier, a smoothing circuit, and an inverter. The rectifier converts the incoming alternating current to direct current through the power diodes. The smoothing circuit is composed of large capacitors. In addition to filtering and buffering the reactive power of the load, this link also keeps the DC voltage of the main circuit stable. The function of the inverter is to convert the direct current processed by the smoothing circuit into a rectangular wave voltage with adjustable frequency and variable width under the control of the PWM controller.

Figure 1

　　3 The main source of electromagnetic interference of frequency conversion speed regulation drive system

　　3.1 The interference of the external power grid to the variable frequency drive system

　　Harmonic interference in the power grid mainly interferes with the inverter through the power supply of the inverter. There are a large number of harmonic sources in the power grid, such as various rectification equipment, AC and self-current switching equipment, electronic voltage adjustment equipment, nonlinear loads and lighting equipment. These loads cause voltage and current waveform distortion in the grid, thereby causing interference to other equipment in the grid. If the power supply of the frequency converter is disturbed by the polluted AC power grid without treatment, the noise of the power grid will interfere with the frequency converter through the power supply circuit of the power grid. The interference of the power supply to the inverter is mainly manifested in overvoltage, undervoltage, voltage drop, three-phase voltage imbalance, surge, drop, peak voltage pulse, radio frequency interference, etc.

　　3.2 Common mode interference of variable frequency speed regulation drive system

　　The common mode interference caused by common ground impedance or electromagnetic field can form a loop interference through the control signal line of the frequency converter, which will also interfere with the normal operation of the frequency converter.

　　3.3 Harmonic interference of variable frequency speed regulation drive system

　　The input part of the frequency converter is a rectifier circuit, which has nonlinear characteristics. Therefore, the high-order harmonics generated will distort the input voltage waveform and current waveform. Figure 2 is the rectification circuit of the frequency converter. After the three-phase alternating current is rectified by full wave, it is filtered by the capacitor. Figure 3 shows the input voltage and current waveforms taking phase A as an example. It can be seen that the waveform of the input current is distorted.

Figure 2 Figure 3

　　After analyzing the frequency spectrum of the input current, it can be seen that the 5th and 7th harmonic components are very large, not much smaller than the fundamental component. As shown in Figure 4, the ordinate is the amplitude of the harmonic component and the fundamental The ratio of the amplitudes of the wave components. These higher harmonics will interfere with the input power supply system.

Figure four

　　At the same time, the output current and output voltage of the frequency converter have higher harmonics. Figure 5 is the structure diagram of the inverter part of the frequency converter. For PWM-controlled voltage source inverters, the output voltage waveform is a rectangular wave. Among them, the level of harmonic frequency is related to the modulation frequency of the inverter. Figure 6 shows the output voltage and current waveforms of the inverter. The Fourier analysis of the voltage and current waveforms can be used to obtain the content of each harmonic. The high-order harmonic current directly interferes with the load; in addition, the high-order harmonic current also radiates to the space through the cable and interferes with adjacent electrical equipment.

Figure 5 Figure 6

　　4 Electromagnetic Compatibility Test Requirements for Frequency Converters in Variable Frequency Drive Systems

　　According to the requirements of GB12668.3-2012 (Speed Control Electric Drive System Part 3: Electromagnetic Compatibility Requirements and Specific Test Methods), EMC related tests are required. Among them, the first type of environment in the standard refers to the environment of civil houses, including the application environment directly connected to the low-voltage power supply network that supplies power to civil buildings without intermediate transformers. The second environment refers to all environments except the application environment directly connected to the low-voltage power supply network supplying buildings used for domestic purposes. Category C1: Refers to equipment with a rated voltage lower than 1000V and used in the first environment. Category C2: Refers to equipment with a rated voltage below 1000V, which is neither plug-in equipment nor mobile equipment. When used in the first environment, it can only be installed and adjusted by professionals. Category C3: Refers to equipment with a rated voltage lower than 1000V and used in the second environment. Category C4: Refers to equipment with a rated voltage equal to or higher than 1000V or a rated current greater than or equal to 400A, or whose intended use is in complex systems in the second environment.

　　Figure 7 shows three acceptance (performance) criteria of A, B, and C for the speed-regulating electric drive system and its sub-components according to the impact of a given disturbance.

Figure seven

　　4.1 Basic immunity requirements: low frequency disturbance

　　General principle: When designing the immunity of variable-speed electrical drive systems against low-frequency disturbances, the manufacturer may verify compliance with the requirements by means of tests, calculations or simulations.

　　4.1.1 Harmonics and commutation notches/voltage distortion

　　4.1.1.1 Low-voltage adjustable-speed electrical drive system (voltage distortion)

　　Figure 8 shows the minimum immunity requirements for harmonics and commutation gap/voltage distortion of low-voltage (rated voltage ≤ 1000V) adjustable-speed electric drive system.

Figure eight

　　4.1.1.2 Speed-adjustable electric drive system with rated voltage higher than 1000V (voltage distortion)

　　4.1.1.2.1 Main Power Port

　　Figure 9 shows the minimum immunity requirements for harmonics and commutation notches/voltage distortions at the main power port of variable speed electric drive systems rated above 1000V.

Figure 9

　　4.1.1.2.2 Auxiliary Power Port

　　Figure 10 shows the minimum immunity requirements for harmonics and commutation notches/voltage distortions on the auxiliary low voltage power supply port of variable speed electrical drive systems rated above 1000V.

Figure ten

　　4.1.2 Low voltage deviations (changes, variations, fluctuations), voltage dips and short interruptions

　　4.1.2.1 Low-voltage speed-regulating electric drive system (voltage deviation)

　　Figure 11 shows the minimum immunity requirements for voltage deviation, voltage dips and short interruptions on the power port of the low-voltage speed-adjustable electric drive system.

Figure Eleven

　　4.1.2.2 Speed-regulating electric drive system with rated voltage higher than 1000V (voltage deviation)

　　4.1.2.2.1 Main Power Port

　　Figure 12 shows the minimum immunity requirements for voltage deviation, voltage dips and short interruptions on the main power port of the variable speed electric drive system with a rated voltage higher than 1000V.

Figure 12

　　4.1.2.2.2 Auxiliary Power Port

　　Figure 13 shows the minimum immunity requirements for voltage deviation, voltage dips and short-term interruptions on the auxiliary low-voltage power supply port of the adjustable-speed electric drive system with a rated voltage higher than 1000V.

Figure 13

　　4.1.3 Voltage unbalance and frequency variation

　　4.1.3.1 Low-voltage adjustable-speed electrical drive system

　　Figure 14 shows the minimum immunity requirements for voltage unbalance and frequency variation on the power port of a low-voltage variable speed electric drive system.

Figure Fourteen

　　4.1.3.2 Speed-regulating electric drive system with rated voltage higher than 1000V

　　4.1.3.2.1 Main Power Port

　　Figure 15 shows the minimum immunity requirements for voltage unbalance and frequency variations on the main power port of variable speed electrical drive systems with rated voltages above 1000V.

Figure 15

　　4.1.3.2.1 Auxiliary Power Port

　　Figure 16 shows the minimum immunity requirements for voltage unbalance and frequency variation on the auxiliary low-voltage power supply ports of adjustable-speed electrical drive systems with rated voltages higher than 1000V.

Figure sixteen

　　4.1.4 Power frequency magnetic field

　　The test that meets the requirements of GB/T17626.8 is aimed at products that use components that are sensitive to magnetic fields. However, variable speed electrical drive systems often employ Hall effect current sensors, which are designed to operate in the presence of high levels of magnetic fields (in close proximity to power conductors). The amplitude of its magnetic field is much higher than the test level specified in GB/T 17626.8. Therefore, the power frequency magnetic field immunity test is not required for the variable speed electric drive system.

　　4.2 Basic immunity requirements: high frequency disturbance

　　Figure 17 shows the minimum immunity requirements for the first environment, and Figure 18 shows the minimum immunity requirements for the second environment.

Figure seventeen

Figure 18

　　4.3 Basic Emission Limits: High Frequency Emissions

　　4.3.1 Category C1 and C2 equipment

　　4.3.1.1 The power port disturbance voltage limit is shown in Figure 19.

Figure 19

　　4.3.1.2 Process Measurement and Control Ports

　　If the intended use of a process measurement and control port is to connect to a fieldbus, the port shall comply with the conducted emission requirements of the relevant standard for that fieldbus.

　　If the intended use of a process measurement and control port is connection to a public telecommunication network, the port shall be considered a telecommunication port and the Class B conducted emission requirements in CISPR 22 apply to the port.

　　4.3.1.3 Radiated emission limits, see Figure 20

Figure 20

　　4.3.1.4 Power interface limits, see Figure 21

Figure 21

　　4.3.2 C3 equipment

　　4.3.2.1 The power port disturbance voltage limit is shown in Figure 22.

Figure 22

　　4.3.2.2 Process measurement and control port

　　If the intended use of a process measurement and control port is to connect to a fieldbus, the port shall comply with the conducted emission requirements of the relevant standard for that fieldbus.

　　If the intended use of a process measurement and control port is to connect to a public telecommunication network, the port shall be considered a telecommunication port and the Class A conducted emission requirements in CISPR 22 apply to the port.

　　4.3.2.3 Radiated emission limits, see Figure 23

Figure 23

　　4.3.2 C4 equipment

　　In these applications of equipment of category C4, the user and the manufacturer shall agree on an EMC plan to meet the EMC requirements for the intended use. In this case, the EMC characteristics of the environment (including the overall equipment and its surroundings) are defined by the user.

　　5 Commonly used anti-interference measures

　　Combined with the industrial site and work debugging experience, the following measures to minimize the EMC impact of the frequency converter are summarized:

　　1) To ensure that all the equipment in the electrical drive cabinet is well grounded, it is required to use short and thick grounding wires to connect to the common grounding point or the grounding busbar. It is especially important that any control equipment (such as a PLC) connected to the inverter must share the ground with it, and short and thick wires should also be used for grounding. At the same time, the ground wire of the motor cable should be connected directly to the ground terminal (PE) of the corresponding inverter.

　　2) In order to effectively suppress the radiation and conduction of electromagnetic waves, the motor cables driven by frequency converters must use shielded cables.

　　3) It is better to use shielded cables for control cables. The transmission line of the analog signal should use a double-shielded twisted pair. Different analog signal lines should be routed independently and have their own shielding layers to reduce coupling between lines. Transmission cables for analog and digital signals should be shielded and routed separately.

　　4) The motor cable should be routed independently of other cables. At the same time, avoid long-distance parallel routing of the motor cable and other cables to reduce the electromagnetic interference caused by the rapid change of the output voltage of the inverter.

　　5) The line reactor is used to reduce the harmonics generated by the frequency converter, and can also be used to increase the impedance of the power supply, and help absorb the surge voltage and the voltage spike of the main power supply generated when nearby equipment is put into operation. The line reactor is connected in series between the power supply and the power input terminal of the inverter. If an RFI filter is also used, the RFI filter should be connected in series between the line reactor and the frequency converter. When the situation of the main power grid is unknown, it is best to add a line reactor.

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