Analysis of EMC Harmonic Current, Voltage Fluctuation and Flicker Test_Century Wisdom (Suzhou) Testing Technology Co., Ltd

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Analysis of EMC Harmonic Current, Voltage Fluctuation and Flicker Test

Release time:

2022-07-31 00:00

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　　1. Harmonic current

　　1.1 What is harmonic current?

　　Strictly speaking, harmonics refer to the electricity contained in the current whose frequency is an integer multiple of the fundamental wave, generally refers to the Fourier series decomposition of the periodic non-sinusoidal electricity, and the remaining currents greater than the fundamental frequency are generated. electricity. In a broad sense, since the effective component of the AC power grid is a single frequency of the power frequency, any component different from the power frequency can be called a harmonic.

　　1.2 Influence of harmonic current

　　Due to the development of semiconductor converter 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 current is very harmful. On the one hand, it increases the burden on the neutral line of the power grid. The harmonic current generated by a large number of nonlinear loads will flow through the neutral line and cause the neutral line to be overloaded. In severe cases, it will burn the neutral line and cause a fire; on the other hand It also increases the burden on the high-voltage capacitors of the power grid. The transformers of power grid users are generally connected with high-voltage capacitors to filter out high-frequency interference from the power grid, and the high-frequency harmonic current flowing through the capacitor will cause the temperature to rise or even explode; in addition, harmonics Current can also cause grid voltage waveform distortion, thereby affecting the stable operation of other electrical products.

　　Harmonic current, the input voltage of the equipment is a sine wave (50Hz or 60Hz), when the input load of this voltage is a nonlinear circuit (such as: ballast, switching power supply, electronic fluorescent lamp, etc.), it will cause distortion of the input current , that is, the input current is not a sine wave. According to the Fourier transform, the non-sinusoidal signal will have harmonics in the frequency domain. These harmonic currents will reduce the efficiency of the power supply of the equipment, and will flow back to the power grid, causing pollution to the power grid. Harmonic current is a very important EMI project in the field of electromagnetic compatibility. However, with the continuous update of IEC harmonic standards in the past two years, most of the lighting products exempted from the old standard can no longer meet the harmonic requirements, and the products need to follow the standards. Replacement.

　　It is precisely to protect the power quality of the shared power grid and ensure the normal operation of the power grid and user equipment that the IEC has proposed a harmonic current limit standard. IEC 61000-3-2 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 .

　　Class A : Equipment not designated as Class B, C or D shall be considered as Class A equipment, examples of Class A equipment are:

　　Three-phase balanced equipment; household appliances, excluding appliances specified as belonging to classes B, C or D; vacuum cleaners; high-pressure washers; tools, excluding hand tools; independent phase control dimmers; audio equipment; stage lighting and work Professional lighting for the room.

　　Class B : Portable tools; arc welding equipment.

　　Class C : lighting equipment

　　Class D : Equipment with power less than or equal to 600w, personal computers and personal computer monitors; television receivers.

　　1.3 Harmonic current exemption requirements:

　　1) Electrical and electronic equipment not connected to the public low-voltage power distribution system;

　　2) Electronic and electrical equipment whose nominal voltage is powered by DC;

　　3) Electrical and electronic equipment with a nominal voltage less than AC 220 V;

　　4) Lighting equipment with a rated power of less than 5w; electrical and electronic equipment with a rated power of 75w (except lighting equipment);

　　5) Professional equipment with a total rated power greater than 1kw.

　　1.4 Test standard for harmonic current

　　In the category of low-voltage power supply equipment, the product standards involved are:

　　Ø IEC61000-3-2 (rated current less than 16A);

　　Ø IEC61000-3-4 (rated current greater than 16A);

　　Ø 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 harmonized standard, so there is no corresponding harmonized standard for equipment greater than 75A. The basic standard related to the test method is IEC61000-4-7.

　　1.5 Test method of harmonic current

　　1. Equipment: harmonic analyzer and pure AC power supply

　　2. Test range: 2~40 harmonics.

　　3. The limit standard of harmonic current test: In IEC61000-3-2, there are different limit values according to the product classification Class A/B/C/D.

　　Class A equipment :

　　Class B equipment: the test limit is not greater than 1.5 times the corresponding limit of Class A equipment;

　　Class C equipment

　　Note: 5W≤electronic and electrical product input power≤25W test requirements

　　Lighting equipment with a rated power greater than or equal to 5w and less than or equal to 25w must meet one of the following three sets of requirements.

　　Ø Clause 1: The harmonic current shall not exceed the limit value related to power in column 2 of Table 3.

　　Ø Clause 2: The third harmonic current, expressed as a percentage of the fundamental current, should not exceed 86%, and the fifth harmonic current should not exceed 61%. In addition, the waveform of the input current should reach 5% of the current threshold before or at 60°, peak before 65°, and cannot drop below the current threshold before 90°, and the current threshold is equal to 5% of the highest peak value occurring during the measurement cycle %.

　　Ø Clause 3: THD shall not exceed 70%. Expressed as a percentage of the fundamental current, the third harmonic current should not exceed 35%, the fifth current should not exceed 25%, the seventh current should not exceed 30%, and the ninth and eleventh currents should not exceed 20%. The second current does not exceed 5%.

　　Class D equipment:

　　1.6 Precautions for Harmonic Current Test

　　Test duration:

Equipment operation type	observation period
quasi-steady state	T obs are of sufficient duration to meet the requirement for repeatability in 6.2.3.1
Short cycle ( T cycle ≤ 2.5min)	T obs ≥ 10 cycles ( reference method ) or T obs has sufficient duration or synchronization a to meet the repeatability in 6.2.3.1 requirements
random	T obs has sufficient repeat time to meet the repeatability requirement in 6.2.3.1
Long cycle ( T cycle >2.5min)	Complete device program cycle ( reference method ) or a typical 2.5min operating cycle that the manufacturer believes will produce maximum THC

　　The basic method to solve the problem of excessive harmonic current emission is to add a power factor correction (PFC) circuit to the original power supply circuit, or to change the existing PFC circuit; ), in normal operation, the peak time of the current waveform may lag behind or lead the peak value of the voltage waveform, causing the power factor of the product to drop. The most commonly used method for this type of equipment is the corresponding capacitive or inductive compensation. The peaks of the waveforms appear time-synchronized.

　　2. Voltage fluctuation and flicker

　　The voltage fluctuation and flicker test mainly measures the change of grid voltage caused by EUT. Due to the low power factor of the fluctuating load, the large amount of reactive power fluctuation, and the block of the power change process, the fluctuating load EUT is the main cause of the voltage fluctuation.

　　Fluctuating EUTs such as electric arc furnaces will cause continuous voltage fluctuations in the power supply, and they are irregular random voltage fluctuations; while the frequent start-up of the motors of EUTs such as rolling mills and winches and the intermittent power on and off of loads such as welding machines will also cause Voltage fluctuations, but this fluctuation is a regular voltage fluctuation.

　　The disturbing effect of a voltage change depends not only on the magnitude of the voltage change, but also on how often it occurs. Voltage changes are usually evaluated by two types of indicators, namely voltage fluctuation and flicker.

　　Hazards of voltage fluctuation and flicker:

　　Ø Lighting lights flicker, affecting people's vision;

　　Ø Unstable picture on TV/monitor;

　　Ø Electronic instruments and equipment in the same power grid are not working properly or even damaged;

　　2.1 Voltage fluctuation

　　Among them, the voltage fluctuation is mainly reflected in the sudden large voltage change on the grid. Generally speaking, it has little impact on flicker measurement, but it may have a great impact on other equipment in the same grid, especially electronic equipment.

　　Flicker measurement can accurately assess the impact of continuous voltage fluctuations, which can reflect the unstable visual effects caused by the human naked eye to produce time-varying light stimuli.

　　The test is based on the standard: IEC 61000-3-3/ GB 17625.2: Limitation of voltage fluctuation and flicker generated by equipment with a rated current not greater than 16A in a low-voltage power supply system.

voltage fluctuation	limit value
Relative voltage change characteristics d(t)	When do>3%, its duration ≤200ms
Maximum desired voltage change dmax	≤4%
Relative steady-state voltage change do	≤3%

　　2.2. Voltage flickering

　　Voltage fluctuations can cause other electrical equipment in the same grid to malfunction. Flicker is a result of voltage fluctuations causing lighting products to flicker.

　　Flicker detection rate F(%):

　　According to the test conditions recommended by IEC, use different waveforms, frequencies, and amplitudes of AM waves with power frequency voltages of 230V and 60W to power incandescent lamps for lighting, and make statistics on the observed flicker visual perception tests, and obtain obvious observation and The proportion of the number of unacceptable, that is,

　　Note: A is the number of people who are not aware; B is the number of people who are slightly aware; C is the number of people who are obviously aware; D is the number of people who are unbearable

　　F greater than 50% is the voltage flicker limit.

　　Instantaneous flicker sensitivity S(t)

　　Indicates the instantaneous subjective visual response of people to illumination fluctuations, when the flicker detection rate F=50%, S=1

　　Flicker is divided into short-term flicker and long-term flicker:

　　Short-term flickering Pst : It is the degree of flickering evaluated in a short time (within 10 minutes), and Pst=1 is used as the threshold of flickering stimulation. Pst is actually simulating the flickering feeling produced by human beings to the 60W incandescent lamp working under 230V AC voltage in the 50Hz power grid under the condition of voltage fluctuation.

　　Long-term flicker PLt : refers to the degree of flicker evaluated within a long period of time (within 2H). The standard uses PLt=0.65 as the threshold of flicker stimulation.

　　Flicker limit

flashing	limit value
Short-term flashing Pst	1.0
long flashing Plt	0.65

　　3. Harmonic current, voltage fluctuation and flicker test

　　The voltage fluctuation and flicker test mainly measures the change of grid voltage caused by EUT. In the voltage fluctuation and flicker circuit, the composition of the reference impedance Z (at 50Hz) of the power supply S: RA=0.24Ω, jXN=0.15Ω; RN=0.16Ω, jXN=0.10Ω.

　　System connection diagram - single phase 16A

　　Typical configuration - single phase 16A

Harmonic current /voltage flicker test system
serial number	product	illustrate
1	DPA 500N	Single-phase harmonic flicker analyzer, including 16A flicker impedance and control software dpa.control
2	ACS500N6	6KVA pure power supply, the impact current can reach 200A
3	CE-TAB	CE test environment, including solid wood test table, insulation support, etc.

　　System connection diagram - three-phase 32A

　　Typical configuration - three-phase 32A

Harmonic current /voltage flicker test system
serial number	product	illustrate
1	DPA 503N	Three-phase harmonic flicker analyzer control software dpa.control
2	AIF 503N	Three-phase flicker impedance
3	Netwave 30	Three-phase Multifunctional Programmable AC / DC Power Quality Immunity Simulator
4	CE-TAB	CE test environment, including solid wood test table, insulation support, etc.

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