New Development of Harmonic Problems in Power Grid——Talking about Super Harmonics
The study of 2 to 150 kHz superharmonics is a new field of rapid development, especially for grid inverters and switching power supplies for renewable energy. A large number of such harmonics have been introduced into modern low-voltage power grids, causing many new problems in power quality. At present, there are several working groups in the world who are engaged in research in this area. Based on the recent foreign literature, the article mainly introduces the generation, influence, main features and current research trends of superharmonics.
introduction
The problem of harmonics at higher frequencies (referring to power frequency 40 or more) is first seen in the literature [1-2] and listed as an informative annex to the two IEC international standards in 2002. In [1], more than 50 harmonic voltages and interharmonic voltages are included in the category of "unwanted voltage". Reference [2] specifically discusses the measurement of harmonic frequencies from 50 to 9 kHz. It should be noted that the IEC defines the demarcation point of low frequency and high frequency as 9 kHz, so the so-called "higher frequency harmonic" refers to the harmonic of about 2 kHz (corresponding to the 40th harmonic commonly used in Europe) to 9 kHz, which is still low frequency conduction. The range of interference. Since the response level of the ripple control receiver in the power grid is as low as 0.3% of the nominal voltage, in order to avoid interference, it is recommended in Appendix B of the literature [1] more than 50 times, and the single harmonic limit below 9 kHz is = 0.2%; For harmonics of any 200 Hz bandwidth in this range, if the center frequency is set, the recommended limits are:
Where: U 1 is the effective value of the fundamental voltage; U f is the effective value of the voltage component of frequency f; F is the center frequency of the frequency band (the frequency band higher than the 50th harmonic).
It is pointed out in Appendix B of the literature [1] that there have been some examples of interference caused by the above-mentioned levels, but the current impact on these frequency components in the power grid is poorly understood and is not sufficient to determine the accepted level of compatibility.
Since 2000, for "higher frequency" harmonics, international organizations such as the International Electrotechnical Commission (IEC), the European Electrotechnical Standardization Committee (CENELEC), the International Conference on Large Power Grids (CIGRE), the International Conference on Power Supply (CIRED), and IEEE Research is carried out in the middle, and the frequency is extended to 2 kHz to 150 kHz according to the spectral coverage of interference sources (such as inverters, switching power supplies) and sensitive devices (such as power line carrier communication), and is defined as "super harmonics" ( Supraharmonics).
For super-harmonics, China has not carried out relevant research, but some effects have been detected. Based on recent foreign literatures, this paper briefly introduces the generation, impact, main features and current research trends of superharmonics, and provides reference for relevant professionals, in order to carry out research in this area in China.
1, the generation of super harmonics
Today, power electronics technology is still developing rapidly, and its application range is almost infiltrated into various fields. One of the important signs of the development of this technology is that the switching speed of the thyristor is greatly improved. For example, the switching frequency of the inverter (DC/AC converter) has been increased from the early tens of Hertz, several kilohertz to several tens of kilohertz or even several Hundred kilohertz. With the vigorous development of renewable energy, especially the investment of a large number of solar photovoltaic inverters (ie PV inverters), and the application of various switching power supplies, the power grid (mainly in the low-voltage power grid) is super in the range of 2 kHz to 150 kHz. Harmonics are rapidly increasing and the number of harmful effects is on the rise.
In addition, the utility grid is generally used for signal transmission, and three types of signal systems are considered in the literature [3]:
1) The power company's pulsation control system, the frequency range is 100Hz ~ 3kHz (generally less than 500Hz), under normal circumstances within 5% N, when there is resonance, up to 9%N.
2) The power company's power carrier, with a frequency range of 3 kHz to 95 kHz, allows a signal level of 5% N, and these signals are quickly attenuated (greater than 40 dB) when transmitted in the grid.
3) The signal system of the end user (residential area or industrial user), such as Europe (ITU area 1), has a frequency range of 95 kHz to 148.5 kHz, and the allowable signal levels are 0.6% N or 5% N, respectively. In some countries, the upper frequency limit is 500 kHz and the signal level is allowed to be 2 mV to 0.6 mV. The frequency of these signals is quite in the super-harmonic range, so the super-harmonic source in the power grid is generated by various electronic devices and by some used communication devices.
2, the characteristics of super harmonics
Studies have shown that the transmission dispersion of harmonics from 2 kHz to 150 kHz is different from ordinary harmonic emission, which is a new phenomenon of power quality. One of the features is the so-called primary emission and seco ndary emission. The primary emission refers to the emission caused by the disturbance source (device) alone; the secondary emission refers to the emission of the disturbance source (device) after the other device transmits it. The two are significantly different. The main factors affecting the primary emission are: 1 topology of the device; 2 impedance of the connection point; 3 resonance. In a low-voltage network, the connection point impedance of the device consists of a transformer, a cable (referred to as the "impedance of the network"), and the internal wiring of the building, along with other connected device impedances, which can be referred to as the impedance of the facility, between 2 kHz and 150 kHz. In the frequency range, the network impedance is mainly inductive, which is relatively higher than the facility impedance (often based on the capacitance of the connected device). The main factors affecting the secondary emission are: 1 the emission intensity of the neighboring device; 2 the relationship between the device connection point impedance and the network and the facility impedance.
Figure 1 shows the measured waveforms of the primary and secondary emissions of a TV (TV) to a PV inverter [4]. The main harmonic frequency generated by the PV inverter is 16 kHz. When the TV is connected, the secondary emission of the PV inverter is significantly amplified at 16 kHz; in addition, the 55 kHz line appears, and the latter does not appear in the native emission. TV generated. The test also found that the higher the superharmonic frequency, the closer the transmission distance: the 16 kHz component level produced by the PV inverter remains fairly stable at the connection point in the test house, slightly lower at the common connection point (PCC), The voltage or current component of 55 kHz to 150 kHz generated by other devices in the house is not detected at the PCC, which means that these frequency components are only transmitted between the indoor appliances and do not enter the power grid. This is another feature of super harmonics.
Figure 1 Effect of a TV on the primary and secondary emissions of a PV inverter
Therefore, it is necessary to know the currents at the terminals of electronic devices (such as PV inverters, TVs, LED lights, portable computers, etc.) operating in facilities (such as a laboratory, a home), and it is necessary to distinguish between primary and secondary emissions. The secondary emission of a device is greatly affected by the capacity and performance of the neighboring device, which requires detailed study when considering the electromagnetic compatibility level of the electronic device. Taking a facility as a whole, the measurement results cannot give a correct indication of the emission level inside the facility; conversely, the emission measurement at the terminals of each device cannot represent the emission of the overall facility to the grid . The results of the experiments in the laboratory also do not reflect the actual use, which brings great complexity to the research of superharmonic impact research and standards.
3. The influence of super harmonics
The literature [5] pointed out that the emission level of the frequency range from 2 kHz to 150 kHz in the power grid continues to rise, and the interference events caused by it are also increasing, such as equipment malfunction, meter indication distortion, and increased noise of electronic ballasts. Studies have shown that high-frequency (HF) voltage components are generally not widely transmitted in the power grid because other electronic devices installed near the HF source often provide much lower impedance than the network itself. This low-impedance path is mainly From the shunt capacitor (which may be part of the EMC filter) or the DC link of the rectifier, the key is the dielectric capacitor, which generates additional heat when passing through the HF current, reducing the lifetime and causing electronic equipment failure.
[6] The communication between the power line and the narrowband user terminal equipment in the laboratory and the field made five different types of interaction tests, end-user devices that cause a low impedance path is the most common cause of communication failure occurs; as The shunt capacitance of the end user equipment may also cause carrier communication to damage equipment connected to the grid.
4, the simulation model
In order to study the interference effects of super harmonics in the low-voltage distribution network, the literature [7] based on Matlab/Simul ink software and its SimPowerSystems library to establish a simulation model, which is introduced as follows:
1) Inverter model. See Figure 2. Using the most versatile PWM control switch, this method makes the low frequency harmonics less, but the high frequency harmonics are larger. Only one direct current (DC) voltage source (=360V) is used on the DC side because the loop has little effect on the AC side HF interference; the f filter is selected (f = 12.8mH), mainly for harmonics below 2kHz The wave meets the standard requirements and can reduce harmonics from 2 kHz to 150 kHz (no standard yet).
Figure 2 inverter model
2) LV distribution network model. The model is divided into two parts, the first is the LV feeder, and for the sake of simplicity, a three-phase segmented π line model is used, see Figure 3. Compared with the low frequency (LF) model, there are two main differences: first, the capacitance cannot be ignored in the case of high frequency (HF), which complicates the analysis calculation; in addition, the resistance is increased due to the skin effect under HF, and the different frequencies should be Different resistance values ​​are taken, but in the integrated component model (for example, using the model in the SimPowerSystem library) the resistance value must be fixed. The model parameters are derived from the positive and negative sequence parameters of Table 1. In addition to the LV feeder, the MV/LV transformer is the master device in the network. The parameters of the transformer are obtained by special tests. They are pre-contained in the Thevenin equivalent model of the medium voltage (MV) network. The above MV power supply uses the SimPowerSystem program. The three-phase voltage of the library indicates that its short-circuit power is 150 MVA and / is 4/7, which are typical values.
Figure 3 three-phase segmentation π line
Figure 4 Studying the simulation circuit diagram of the interaction between an inverter and a rectified load
Based on the above model, various simulation circuit diagrams are built. For example, Figure 4 is a simulation block diagram for studying the interaction between an inverter and a rectified load.
5. Research trends
Due to the development and rapid popularization of power electronics technology, in the past 10 years, the level of “super harmonics†in the power grid has been increasing, and related abnormalities and accidents have emerged. It is urgent to conduct in-depth research and standardization. At present, a number of work groups have carried out the work of this topic. CIGRE/CIRED Joint Working Group C4.24: “Power Quality and EMC Issues Related to Future Electrical Networks†Research on superharmonics in the frequency band from 2 kHz to 150 kHz is an important part of current activities; superharmonics are also CIGREC4/C6.29 Working Group: One of the themes of the “Solar Power Quality†discussion; CIGRE C4.31 on the power line communication 9 kHz to 150 kHz band, as potential interference issues are discussed. Within the IEEE, the IEEE P1250 (Institute of Electric Power and Energy) has studied superharmonics and is an important area of ​​the IEEE EMC Society TC7. Several frequency groups are covered by several subject groups and working groups within IEC TC 77A. Although the distinction between TC 77A (low frequency phenomenon) and TC 77B (high frequency phenomenon) is defined at 9 kHz, this limit should now be considered historical.
In Europe, the standardization organization CENELEC, especially the working group responsible for the European voltage characteristic standard EN 50160, the need for the standardization of superharmonics in the 2 to 150 kHz band is also mentioned in the application guidelines, and the super harmonic problem is also included in the recent Developed in IEC TS 62749. The interference information observed by the superharmonic emissions is summarized by CENELEC TC 210, where potential interference with power line communication becomes a major factor. The quantification of this frequency range emission is part of the newly drafted IEC 61000-4-30 (ie Power Quality Measurement Method) standard. The European standard for power line communication (EN 50065) and its corresponding IEC standard (IEC61000-3-8) have a clear frequency range of 9 to 148.5 kHz.
6, the conclusion
The study of super-harmonics from 2 to 150 kHz is a rapidly evolving field, especially for grid inverters and switching power supplies for renewable energy. Such harmonics are introduced in large quantities into modern low-voltage networks. It has triggered a lot of new problems in power quality, and it is urgent to explore it in depth. At present, there are many working groups in the world who are engaged in research in this area, and the research work should be actively carried out in China. Based on recent foreign literatures, this paper briefly introduces the generation, impact, main features and current research of superharmonics for reference by relevant professionals.
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