Sunwoele VFD (Variable-frequency Drive) harmonic hazard solutions

VFD (Variable-frequency Drive) is a widely used device in the field of industrial speed transmission. Due to the switching characteristics of the VFD (Variable-frequency Drive) inverter circuit, a typical non-linear load is formed on its power supply. VFDs (Variable-frequency Drives) are often run simultaneously with other devices in the field, such as computers and sensors. These devices are often installed close together, which may cause mutual influence. Therefore, power electronic devices represented by VFD (Variable-frequency Drive) are one of the most important sources of harmonics in the public power grid. The harmonic pollution generated by power electronic devices has become a major obstacle to the development of power electronics technology itself.

1 Related definitions

1.1 What are harmonics

The fundamental cause of harmonic generation is due to nonlinear loads. When the current flows through the load, it does not have a linear relationship with the applied voltage, forming a non-sinusoidal current, thus generating harmonics. Harmonic frequency is an integral multiple of the fundamental frequency. According to the analysis principle of French mathematician M. Fourier, any repeated waveform can be decomposed into a sine wave containing the fundamental frequency and a series of harmonics that are multiples of the fundamental wave. Portion. Harmonics are sine waves, each harmonic has a different frequency, amplitude and phase angle. Harmonics can be divided into even-order and odd-order harmonics. The 3rd, 5th, and 7th harmonics are odd-order harmonics, while the 2nd, 4th, 6th, and 8th harmonics are even-order harmonics. For example, when the fundamental wave is 50Hz, the 2nd The harmonic is l00Hz, and the third harmonic is 150Hz. Generally speaking, odd harmonics cause more harm than even harmonics. In a balanced three-phase system, due to the symmetry, even harmonics have been eliminated and only odd harmonics exist. For a three-phase rectified load, the harmonic current that appears is 6n±1 harmonics, such as 5th, 7th, 11th, 13th, 17th, 19th, etc. VFD (Variable-frequency Drive) mainly generates 5th and 7th harmonics. The schematic diagram of harmonic definition is shown in Figure 1.

1.2 Relevant standards for harmonic control

VFD (Variable-frequency Drive) harmonic control should pay attention to the following standards:

• Anti-interference standards: EN50082-1, -2, EN61800-3;

• Radiation standards: EN5008l-1, -2, EN61800-3.

Especially IEC10003, IEC1800-3 (EN61800-3), IEC555 (EN60555) and IEEE519-1992. Common anti-interference standards EN50081 and EN50082 and standard EN61800 (1ECl800-3) for VFD (Variable-frequency Drive) define the radiation and anti-interference levels of equipment when operating in different environments. The above standards define acceptable radiation levels under different environmental conditions: Level L, no radiation restrictions. Suitable for users who use VFD (Variable-frequency Drive) in an environment without interference and users who deal with radiation restrictions by themselves. Class H, limits determined according to EN61800-3, first environment: restricted distribution, and second environment. As an optional RFI filter, configuring the RFI filter can make the VFD (Variable-frequency Drive) reach commercial grade and is usually used in non-industrial environments.

2. Harmonic control measures

To control harmonic problems and suppress radiation interference and power supply system interference, technical means such as shielding, isolation, grounding and filtering can be adopted

① Use passive filter or active filter;

② Increase the capacity of the transformer, reduce the impedance of the loop and cut off the transmission line;

③Use green VFD (Variable-frequency Drive) without harmonic pollution.

2.1 Use passive filter or active filter

The use of passive filters mainly changes the impedance of the power supply at special frequencies, and is suitable for stable and unchanging systems. The use of active filters is mainly used to compensate for non-linear loads.

The traditional method mostly uses passive filters. Passive filters appeared earliest. Because of their simple structure, low investment, high operational reliability and low operating costs, they are still the main means of harmonic suppression. The LC filter is a traditional passive harmonic suppression device, which consists of filter capacitors,