Harmonic current and voltage fluctuation and flicker are the last two of the four EMI projects. Next week we will start introducing EMS test methods. Welcome to follow the official account Monster Electronic Technology.
Harmonic current
1. What is harmonic current?
Strictly speaking, harmonic refers to the amount of electricity contained in the current whose frequency is an integer multiple of the fundamental wave. Generally speaking, it refers to the amount of electricity generated by the remaining current greater than the fundamental frequency after the Fourier series decomposition of the periodic non-sinusoidal electricity. In a broad sense, since the effective component of the AC power grid is a single power frequency, any component different from the power frequency can be called a harmonic.
2. Impact of harmonic currents
Due to the development of semiconductor conversion technology, people's utilization efficiency of electric energy has been greatly improved, but the use of a large number of switching power supplies and thyristors has also led to the generation of harmonic currents. Harmonic currents are extremely harmful. On the one hand, it increases the burden on the neutral line of the power grid. The harmonic currents generated by a large number of nonlinear loads will flow through the neutral line, causing the neutral line to be overloaded. In severe cases, the neutral line will be burned and cause a fire; on the other hand, it increases the burden on the high-voltage capacitors of the power grid. The transformers of power grid users are generally connected to high-voltage capacitors to filter out high-frequency interference from the power grid, and high-frequency harmonic currents flowing through the capacitors will cause the temperature to rise or even explode; in addition, harmonic currents can also cause distortion of the voltage waveform of the power grid, thereby affecting the stable operation of other electrical products.
It is precisely for the purpose of protecting the power quality of the shared power grid and ensuring the normal operation of the power grid and user equipment that IEC has proposed the harmonic current limit standard. IEC 61000-3-2 2018 Electromagnetic compatibility (EMC) - Part 3-2: Limits - Limits for harmonic current emissions (equipment input current ≤ 16A/phase) divides equipment into four categories: A, B, C and D.
3. Test standards for harmonic currents
In the category of low-voltage power supply equipment, the product standards involved are: IEC61000-3-2 (rated current less than 16A); IEC61000-3-4 (rated current greater than 16A); IEC61000-3-12 (rated current greater than 16A and less than 75A). Among the corresponding EN standards, only EN61000-3-2 and EN61000-3-12 are included in the official bulletin of the EU EMC coordination standards, so there is no corresponding coordination standard for equipment greater than 75A. The basic standard involved in the test method is IEC61000-4-7.
4. Test methods for harmonic currents
1. Equipment: harmonic analyzer and pure AC power supply
2. The test frequency range is
2 to 40 harmonics, i.e. 100Hz to 1kHz.
3. The limit value standard for harmonic current test
IEC61000-3-2 has different limit values according to the product classification Class A/B/C/D.
① Class A equipment
② Class B equipment: the test limit is not more than 1.5 times the corresponding limit of Class A equipment;
③ Class C equipment
④Class D equipment
Students who are interested in EMC testing can read the harmonic current test standard IEC 61000-3-2 in detail.
Voltage Fluctuation and Flicker
I. Voltage Fluctuation
The voltage fluctuation and flicker test mainly measures the changes in the grid voltage caused by the EUT. The interference effect caused by voltage changes depends not only on the amplitude of the voltage change, but also on the frequency of its occurrence. Voltage changes are usually evaluated by two types of indicators, namely voltage fluctuation and flicker.
Among them, voltage fluctuation mainly reflects the sudden large voltage change on the grid. Generally speaking, it has little effect on flicker measurement, but it may have a great impact on other devices in the same grid, especially electronic devices.
Flicker measurement can accurately evaluate the impact of continuous voltage fluctuations. It can reflect the unstable visual effect caused by the light stimulation that changes over time to the human eye.
Test based on standards: IEC 61000-3-3/ GB 17625.2: Limitation of voltage fluctuations and flickers generated by equipment with a rated current not exceeding 16A in a low-voltage power supply system.
IEC relevant standards stipulate: de does not exceed 3%; dd exceeds 3% for a duration of less than 200ms; dmax does not exceed 4%.
2. Voltage flicker
Flicker can be divided into short-term flicker and long-term flicker:
Short-term flicker Pst: refers to the flicker level evaluated in a short period of time (within 10 minutes), and Pst=1 is used as the threshold of flicker stimulation. Pst actually simulates the flicker perception of a 60W incandescent lamp working at 230V AC voltage in a 50Hz power grid under voltage fluctuations.
Long-term flicker PLt: refers to the flicker level evaluated over a longer period of time (within 2 hours), and the standard uses PLt=0.65 as the threshold of flicker stimulation.
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