Measurement of the impact of electrical equipment on power supply quality of the power grid

    Abstract: This article describes in detail the measurement methods of harmonics, voltage fluctuations and flicker of electrical equipment and the key points of related technical standards.

    Keywords: Standard Harmonics Voltage Fluctuation Flicker Measurement

1 Introduction

    The quality of power supply is a hot topic that people pay attention to. The large-scale adoption of switching power supplies, thyristor devices, etc., on the one hand, improves people's utilization efficiency of electric energy. On the other hand, a large amount of harmonic currents are formed in the power grid, which has the consequence that other parts of the same power grid are affected. When electronic products are interfered with, harmonic currents can also cause overloading of the grid neutral current, affecting the grid's ability to transmit power. In addition, the phase control of the AC power supply will also cause changes in the effective value of the current on the power grid, resulting in significant fluctuations in the effective value of the voltage. This voltage fluctuation may cause the lighting device to flicker.

    In order to ensure the quality of grid power supply and safeguard human health, the International Electrotechnical Commission (IEC) TC77 Committee has compiled two standards suitable for evaluating the impact of low-voltage equipment below 16A on grid power supply quality, namely:

    (1) IEC61000-3-2 (1995) "Electromagnetic compatibility, Part 3: Limits, Chapter 2: Limits of harmonic current transmission caused by low-voltage electrical equipment with a rated current of less than or equal to 16A per phase."

    (2) IEC61000-3-3 (1994) "Electromagnetic compatibility, Part 3: Limits, Chapter 3: Voltage fluctuation and flicker limits for equipment with a rated current less than or equal to 16A in low-voltage power supply systems."

    Among them, the IEC61000-3-2 standard has been equivalently transformed into my country's national standard, GB1765.1-1998 "Harmonic current limits emitted by low-voltage electrical and electronic equipment (equipment input current ≤16A per phase)", and has been officially published.

    From an international perspective, the IEC61000-3-2 and IEC61000-3-3 standards were published to replace the IEC555 series of standards published by IEC in the early 1980s:

    IEC555-1 (1982) "Interference caused by household appliances and similar electrical equipment in power supply systems, Part 1: Definition";

    IEC555-2 (1982) "Interference caused by household appliances and similar electrical equipment in power supply systems, Part 2: Harmonics";

    IEC555-3 (1982) "Interference caused by household appliances and similar electrical equipment in power supply systems, Part 3: Voltage fluctuations".

    Comparing the names of the IEC61000-3 series and the IEC555 series standards, it is not difficult to see that the new standard has expanded the scope of application of the standard, from the earlier household appliances and similar electrical equipment to all electrical equipment connected to the low-voltage power grid. Its significance major. Complying with the requirements of the new standards can ensure that all equipment on the low-voltage power grid is protected from harmonics and voltage fluctuations, and protect the health of power users.

    In Europe, the European Community transformed IEC61000-3-2 and IEC61000-3-3 into European standards almost at the same time as the international standards were published. The standard numbers are EN61000-3-2 and EN61000-3-3 respectively. . The European Community also stipulates that these two standards will be enforced from July 1, 1998. This move shows that, like the enforcement of other electromagnetic compatibility standards, the impact of equipment on the quality of power grid power will also become an important weight in international trade and no one should take it lightly.

2 Test of harmonic current

    According to IEC61000-3-2 and GB17625.1 standards.

2.1 Mathematical basis of harmonic measurement

    According to mathematical analysis, any non-sinusoidal periodic waveform can be expressed by Fourier series:

    where F0 is the DC component. If it is further assumed

    that f(t) can be rewritten as

    Therefore, the measurement of non-sinusoidal periodic waveforms can be transformed into the measurement of its DC component (if there is a DC component) and the amplitude and phase angle of each harmonic.

2.2 Classification of equipment

    According to standard requirements, different electrical equipment can be classified into four categories. Different categories of equipment have different harmonic current limits.

    Category A refers to three-phase balanced equipment (the difference in rated current of each line is not more than 20%), and other equipment that does not belong to the following three categories of equipment.

    Category B refers to portable tools (especially handheld short-duration electrical tools), but symmetrically controlled, short-duration household appliances (such as hair dryers, etc.) are still tested as Class A equipment.

    Category C refers to lighting devices including dimming equipment. Class D refers to equipment whose input current has a special waveform (for example, there are rectifiers and capacitors in the input circuit so that the input current falls within the centered area for at least 95% of each half-cycle), and the input The active power is less than or equal to 600W (for equipment greater than 600W, it is still assessed according to the limit value of Class A equipment).

    The harmonic current limits caused by Class A, Class C and Class D equipment are shown in Table 1, Table 2 and Table 3 respectively. For Class B equipment, the standard stipulates that it should be 1.5 times that of Class A equipment.

Table 1 Harmonic current limits for Class A equipment

Table 2 Harmonic current limits for Class C equipment

The maximum allowable value of harmonic current expressed as a proportion of the fundamental input current

Note: λ is the power factor of the circuit

　　For incandescent lighting devices, if phase control is used, the firing angle should not exceed 145°.

Table 3D harmonic current limits for equipment

Note: The "mA/W" limit applies to devices with active power greater than 75W, and may be reduced to 50W in the future.

2.3 Harmonic current measurement

    Harmonic current measurement is divided into steady state and transient state:

    (1) Steady-state harmonic current measurement

    If the relationship between the harmonic components and the fundamental wave has the characteristics of periodic changes, the steady-state harmonic current can be measured. The limits mentioned in the GB17625.1-1998 standard are suitable for the requirements of the steady-state harmonic current.

    (2) Transient harmonic current measurement

    If there is no periodic relationship between the harmonic components and the fundamental wave, the transient harmonic current must be measured, and the following regulations apply:

    ●If this situation is caused by the equipment input or withdrawal process, and the duration is less than 10s, the harmonic current change of this transient process can not be considered.

    ●If the maximum duration of each harmonic does not exceed 10% of the observation period limited to 2.5 minutes, the limit of each harmonic current can be relaxed to 1.5 times the steady-state limit specified in the standard.

2.4 Test line

    The circuit in Figure 1 is suitable for testing single-phase and three-phase equipment respectively.

    Each part of the line has different requirements.

    (1) Test power supply S

    The performance that the test power supply must have is:

　　●The internal resistance must be small enough;

    ●The output voltage should be adjustable within a certain range to adapt to the power supply voltage rating requirements of various countries and regions;

    ●Voltage stability is ±2%;

　　●Frequency stability is within ±0.5% of the rated frequency;

　　●For three-phase power supply, the phase accuracy of the inter-phase fundamental wave is 120°±1.5°;

　　●When the test product is running, the voltage harmonic content of the test power supply should be small (such as the 3rd, 5th, 7th, 9th and 11th harmonic content should be lower than 0.9%, 0.4%, 0.3 respectively of the rated output voltage %, 0.2%, 0.1%; the 2nd to 10th even harmonic content should be lower than 0.2% of the output rated voltage).

    (2) Test instrument M

    Various waveform analysis instruments can be used to measure harmonic currents, such as frequency-selective amplifiers, heterodyne analyzers, multi-channel passive filters, and spectrum analyzers; time domain analysis such as digital filters and discrete Fourier transformers can also be used. instrument. However, from a global perspective, among the harmonic current measuring instruments provided by major manufacturers, discrete Fourier transform time domain measuring instruments are being considered as reference measuring instruments.

2.5 Some regulations in the test

    (1) There are no regulations on harmonic current limits for lighting equipment with active power less than or equal to 25W, and the test may not be conducted temporarily.

    (2) Generally, equipment is not allowed to use asymmetric power supply control methods (meaning that the positive and negative half-cycle current waveforms are not the same, or the number of positive and negative half-cycles in each conduction time is different).

    (3) For those that directly use half-wave rectification of the power supply, the maximum power should be less than 100W, otherwise it will be regarded as exceeding the standard limit of the equipment.

    (4) Equipment that directly uses the half-wave rectification of the power supply can also include short-time working portable equipment (such as hair dryers, etc.) powered by two-core cables.

    (5) For heating elements, when the input power is ≤200W, or the operation does not exceed the limit of Class D equipment, the power supply can be allowed to use symmetrical control method to control the power (as long as the positive and negative half cycles of the current waveform are the same; Or the number of positive and negative half cycles in each conduction time is equal).

    (6) In the measurement, for harmonics greater than the 19th order, if the spectrum components decrease monotonically as the harmonic order increases, the harmonics above the 19th order can be ignored.

    (7) During the measurement, if the harmonic current is less than 0.6% of the input current or less than 5mA, no matter which condition is met, the test is deemed to have passed.

3 Test of voltage fluctuation and flicker

    According to ICE61000-3-3 standard.

3.1 Assessment content and limits

    As can be seen from the title of this section, two aspects need to be assessed, namely voltage fluctuation and flicker. Voltage fluctuations mainly reflect sudden large voltage changes on the power grid. Generally speaking, it has little impact on flicker measurement, but it may have a large impact on other equipment in the same power grid, especially electronic equipment.

    As flicker measurement, the impact of continuous voltage fluctuations can be accurately assessed, which can reflect the unstable visual effects on the human eye caused by time-varying light stimulation.

    (1) Voltage fluctuation

    There are three indicators to describe voltage fluctuations. (See Figure 2)

    Relative steady-state voltage change characteristics dc: refers to the percentage value of the difference between two adjacent steady-state voltages separated by at least one voltage change and the rated voltage. The standard stipulates that it shall not be greater than 3%.

    Relative voltage change characteristic d(t): refers to the time function of the voltage effective value during each cycle relative to the voltage change when the voltage is in a steady-state condition of at least 1s. The standard stipulates that the relative steady-state voltage change shall not be greater than 3% within a measurement time exceeding 200ms (conversely, if the relative steady-state voltage change is greater than 3%, the duration must be less than 200ms).

    Maximum relative voltage change characteristic dmax: refers to the percentage of the difference between the maximum and minimum effective values ​​of the voltage change characteristic and the rated voltage. The standard stipulates that it shall not be greater than 4%.

    (2) Flashing

    There are two types of flickering: short-term flickering and long-term flickering:

    ●Short-term flicker Pst: It is the degree of flicker evaluated in a short period of time (within 10 minutes). Use Pst=1 as the threshold for flicker stimulation. Pst actually simulates the degree of flicker experienced by a person under voltage fluctuations of a 60W incandescent lamp operating in a 50Hz power grid under 230V AC voltage, as shown in Figure 3.

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

3.2 Measurement methods

    Regarding voltage fluctuation and flicker, there are three alternative evaluation methods mentioned in the standard, namely direct measurement method, simulation method, and analysis method. Among them, the direct measurement method using a scintillation measuring instrument is a benchmark measurement method.

3.3 Measuring line

    The measurement circuit of the direct measurement method is shown in Figure 4.

    (1) AC test power supply

    The open circuit voltage of the test power supply should be equal to the rated voltage of the equipment, and the voltage stability should be maintained within ±2%; the frequency stability should be ±0.5% of the rated frequency (50Hz), and the voltage harmonic distortion of the power supply should be less than 3%. In addition, the maximum short-time flicker Pst of the test power supply itself must be less than 0.4. And it is required to perform verification before and after the test.

    (2) Reference impedance

    Since voltage fluctuations and flickers are determined by monitoring voltage changes at the load end, an appropriate line impedance must be selected as a unified reference impedance during the test. The total impedance of the power supply line (including the internal impedance of the power supply and the reference impedance) should be suitable to ensure an overall accuracy of ±8% throughout the entire evaluation measurement.

    When the power supply impedance value is unknown, a reference impedance composed of resistance and inductance must be added between the power supply and the device under test. The measurement should be carried out at the power supply end and the device under test end of the reference impedance, and the measured value from the power supply end must be specified. The maximum relative voltage change dmax1 should be 20% smaller than the maximum value dmax2 measured at the equipment under test.

    (3) Measuring instruments and measurement accuracy

    Although the standard lists several methods for evaluating voltage fluctuations and flicker of the device under test, the direct measurement method is the benchmark test method specified by the standard. The standard requirements for measuring instruments are: the measurement accuracy of current amplitude should be within ±1%. If the real part and imaginary part of the current are not measured, but its phase angle is measured, the phase measurement error shall not exceed ±2°, and the measurement accuracy of the relative voltage change shall be better than ±8%.

3.4 Some regulations in testing

    (1) Limit value testing is only applicable to the power terminals of the equipment under test.

    (2) In the case of emergency switches and emergency interruptions, the limits do not apply.

    (3) For equipment that operates for more than 30 minutes at a time, a PLt evaluation is generally required.

    (4) When voltage changes are caused by artificial switching of power supplies, or the occurrence rate is less than once per hour, Pst and PLt do not need to be considered; the three indicators of voltage changes can also be relaxed to 1.33 times the limit.

    (5) When evaluating Pst and PLt, ensure that the most severe voltage change process is included in the working cycle of the test product.

    (6) In the evaluation of Pst, the work cycle should be repeated continuously. If the operating cycle of the equipment is lower than the observation period and the test sample automatically shuts down at the end of operation, the minimum time required to restart the equipment should be included in the observation period.

    (7) In the evaluation of PLt, when the operation cycle of the equipment is less than 2 hours and the test sample does not often work continuously, the working cycle can no longer be repeated.

    (8) When the device under test has multiple discrete control circuits, if each control circuit can work independently and does not perform switching control at the same time, then each circuit can be regarded as a separate device for testing (vice versa) , if the control of these separate circuits is carried out simultaneously, then the combination is still tested as a single device).

    (9) For electric motors, the locked-rotor method can be used to measure to determine the maximum value dmax of the voltage change that occurs during the starting phase of the electric motor.

    (10) For three-phase balanced equipment, the voltage change of one phase neutral line in the phase line can be measured. However, for three-phase unbalanced equipment, the voltage change of each phase neutral line should be tested.

    (11) The test sample should be tested under the test conditions provided by the manufacturer. The motor should be pre-driven before testing to ensure that the results obtained correspond to normal use.

4 Introduction to practical test systems

    The instruments and test methods for measuring harmonics, voltage fluctuations and flicker provided by major instrument suppliers in the world today all use standard test voltage sources and multi-functional tests based on discrete Fourier transformers with time domain analysis. The combination of instruments is just that some companies combine two functional parts into one instrument. Some companies use two independent instruments to form a system. When separated, they can have their own application fields. When combined, they can complete the test requirements of IEC61000-3-2 and IEC61000-3-3 standards.

    Shanghai Sanji provides the latter test system, which consists of a 6630 multi-function analyzer and a 6530 AC power generator.

    In addition to the AC rms voltage, peak voltage, AC rms current, peak current, active power, power factor, form factor and frequency measurements of traditional power supply analysis instruments, the 6630 multi-function analyzer is also capable of adapting to the diversified applications of power electronic products. In addition to the requirements for the implementation of corresponding international standards, there are also measurements of voltage harmonics, current harmonics, voltage fluctuations, and voltage flicker. The results are automatically recorded and the waveforms are analyzed.

    The 6530 programmable AC power generator is a high-performance and cost-effective power supply product designed using advanced PWM technology. It provides pure AC power and can be used with the 6630 multi-function analyzer to complete the measurement of harmonics, voltage fluctuations and flicker. .

    In fact, in addition to the above uses, the 6530 programmable AC power supply can also be used as a multi-functional AC power supply. For example, the effective output voltage it provides is (0~300) V; the frequency is (15~2000) Hz. , so in addition to meeting the commercial requirements of (47~63) Hz, it is also suitable for naval and aviation occasions that require frequencies above 400 Hz. Its low-frequency output can also be used for 20 Hz frequency testing of motors or air-conditioning compressors. The output of the 6530 is a very pure sine wave or square wave (waveform distortion is less than 0.5%).

    In addition, since the 6530 programmable AC power supply uses digital frequency synthesis technology, the 6530 programmable AC power supply can also simulate various power interference, surges, noise injection angles, voltage and frequency ramp-up and ramp-down. This feature of 6530 can meet the requirements of IEC61000-4-11 (1994) (the equivalent Chinese national standard is GB/T17626.11-1998), making electronic equipment resistant to instantaneous drops, short interruptions and voltage gradients in the power supply voltage. Disturbance test.

    The above-mentioned 6630 and 6530 instruments are equipped with RS-232 and IEEE488 interfaces. Through these interfaces, they can be combined with a computer to form a fully automatic test system.