Harmonic issues have attracted widespread attention, and many countries and regions have formulated their own harmonic standards. my country also passed the "Provisional Regulations on Harmonic Management of Power Systems" in 1984 and the "Power Quality and Harmonics of Public Power Grids" (GB/T-14549-93) in 1993 to limit harmonic pollution from power supply systems and electrical equipment. Although the application of power electronic devices in my country is not as widespread as in developed countries, there are still instances where harmonic issues prevent the installation of frequency converters on multiple motors and the use of computer networks. Moreover, harmonic pollution is bound to become increasingly serious [1-3]. This paper will attempt to present a glimpse of the harmonic situation in my country's industrial power grid and propose a hybrid active filter solution. [b]1 Industrial Power Grid Harmonic Testing[/b] Motors are one of the most typical energy-consuming devices. Variable frequency drives (VFDs) are typical nonlinear devices and harmonic sources. Tests at different capacities and operating frequencies show that the harmonic currents caused by VFDs (groups) are quite serious. The total harmonic distortion (THDi) of the current (relative to the fundamental frequency I1) ranges from 46.5% to 157.0%, and in some cases even reaches as high as 220%, far exceeding the 5% to 20% limit specified in the IEEE-519 standard for power distribution systems. Naturally, the absolute value of the harmonic current injected into the power grid will also far exceed the national standard value. A typical inverter phase current and its frequency domain analysis waveform are shown in Figure 1. [img=569,135]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2000-5/59-1.jpg[/img] Figure 1. Phase current and harmonic waveforms of the frequency converter group [img=183,286]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2000-5/60-1.jpg[/img] Figure 2. Phase current and harmonic content of the intermediate frequency induction power supply. As can be seen from the figures, the frequency converter group in the industrial field deteriorates the phase current waveform. Although the relative harmonics tend to decrease with the increase of output frequency and frequency converter capacity, the decrease is limited. In all test samples, the current distortion rate THDi is above 46.5%, and the current distortion rate THDi_R relative to the total current is also above 42%, still far exceeding the standard value. Another typical nonlinear load—the phase current distortion rate of a medium-frequency induction heating power supply—is slightly lower than that of a variable frequency drive, but still much higher than the relevant standard values. The main harmonic parameters and typical waveforms are: current harmonic distortion rate THDi_R relative to the total current THDi_F = 30%, with the 5th harmonic content (relative to the fundamental current) HRI5 = 21%. The grid voltage harmonics at the test site were relatively small; for example, the grid voltage waveform of the inverter group is shown in Figure 3(a). Figure 3(b) shows the voltage waveform after digital filtering at a cutoff frequency fc = 1.5kHz, which is close to a sine wave. This indicates that higher-order harmonics dominate the harmonics, and their frequency domain voltage and spectrum are shown in Figures 3(c) and (d), respectively. The main parameters of phase voltage harmonics are as follows: Harmonic voltage content: UH < 8V; Total voltage distortion rate (relative to the fundamental frequency): THDu < 3.5% (the standard limit for a 0.38kV system is 5% [5]); Single harmonic content (relative to the fundamental frequency): HRUh: < 1% (HRU11 is the highest, the rest < 0.5%). The relatively small grid voltage harmonics are due to the relatively small capacity of the on-site operating load compared to the power supply system. When a large number of nonlinear loads such as frequency converters are used, it will lead to significant distortion of the grid voltage. [img=548,125]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2000-5/60-2.jpg[/img] Figure 3 Phase voltage and harmonic waveforms of the frequency converter group [b]2 Harmonic mitigation of frequency converters with integrated control of HAPF[/b] In order to solve the increasingly serious harmonic problems caused by nonlinear loads such as power electronics [1-3], many active power filter (APF) schemes have been proposed in recent years and have been gradually applied to overcome many shortcomings of passive filters (PF) [4, 7-10]. Among them, the series hybrid filter (sHAPF) proposed by F. Z. PENG [4] has attracted widespread attention for its excellent filtering performance, small APF capacity (typically 5% of the load) and low cost. However, due to the influence of grid harmonics, load changes, protection difficulties and other factors, this filter has only been reported to be used in actual power systems in the United States [7]. This paper proposes a high-performance, low-cost, easy-to-implement, and reliable integrated active power filter (HAPF) for the harmonics of a 50kVA variable frequency speed controller. It improves the structure and control method proposed by F. Z. PENG, and its principle is shown in Figure 4. In the figure, Ish is the harmonic current on the grid side; IFh is the harmonic current of the PF branch; Vsh is the harmonic voltage of the grid; Vc is the output voltage of the APF; ZF is the impedance of the filter branch; and ZS is the impedance of the grid branch. The general harmonic suppression system aims to suppress the pollution of the grid branch by the harmonic current generated by the nonlinear load and the influence of the grid harmonic voltage on the load. This can be expressed as [img=255,73]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2000-5/60-3.jpg[/img] (1), (2) where ILh is the harmonic current generated by the nonlinear load; VLh is the harmonic voltage on the load side. [align=left][img=501,135]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2000-5/61-1.jpg[/img] Lsh is the grid-side harmonic current; IFh is the branch harmonic current; Vsh is the grid harmonic voltage; Vc is the APF output voltage; ZF is the filter branch impedance; Zs is the grid branch impedance; Figure 4 A hybrid active filter and its harmonic equivalent circuit When Vc＝K×Ish (only harmonic current controlled), and K＞＞｜ZS+ZF｜, equation (1) can be written as[/align][img=276,126]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2000-5/61-2.jpg[/img] As can be seen from equations (3) and (4), when there is severe distortion in the grid voltage, a larger Vsh will worsen the harmonic suppression performance of the HAPF and increase the PF capacity. This is because system harmonics are voltage sources, and a smaller harmonic voltage will generate a larger harmonic current on the PF. Comprehensive control is adopted, namely, using ① a passive filter (PF) to reduce the impedance of the filter branch; ② an active unit of a voltage-controlled voltage source (VCVS) to make the grid branch have an infinite impedance to Vrh (open circuit) to prevent its transmission to the load side, and the controlled source output voltage VC1 = G × Vrh (G = -1); ③ a current-controlled voltage source (CCVS) unit to increase the impedance of the grid branch to Ish (finite impedance), reduce the pollution of the grid by nonlinear load harmonic current, and the controlled source output voltage VC2 = K × Ish. VCVS and CCVS can be superimposed into a controlled voltage source, that is, Vc = Vc1 + Vc2 = K(s)Ish + GVrh. When K >> |ZS+ZF| and G = -1, we have [img=205,84]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2000-5/61-3.jpg[/img] (5), (6) Comparing equations (5) and (6) with equations (3) and (4), this integrated control scheme has good harmonic suppression performance (Ish≈0) and a small capacity passive filter (|IF| is small). It should be noted that although equations (3) and (4), and equations (5) and (6) are equivalent when K >> 1, research shows that a large K value will cause an increase in APF capacity and system instability. In addition, in view of the large range of frequency converter load variation, the need to take into account reactive capacity and harmonic capacity, suppression effect and stability, a special design was made for the passive filter bank; at the same time, the control coefficient K is adaptively adjusted according to the load weight, which is also an important improvement of the author's practical application of the F. Z. Peng structure. **3. Simulation and Model Experiments** PSIM and SABER simulations were performed on the above-mentioned pollution control scheme. The main simulation parameters are as follows: Load: 50kVA/380V, three-phase uncontrolled rectifier, capacitor filter. The grid harmonic voltage is severely distorted (see Figure 5), with a total distortion rate (THDv) of 12% relative to the fundamental frequency, where V5 = 10.6% and V7 = 5.5%. The first and second from the left in Figure 5 show the load (and passive filter) input voltages before and after HAPF activation, respectively, and the second and first from the right show their corresponding spectra. It can be seen that before HAPF activation, all grid harmonic voltages are transmitted to the load end (THDv = 12%); while after activation, the total distortion rate (THDv) relative to the fundamental frequency decreases to 2.7%, with the 5th and 7th harmonic contents being V5 = 1.7% and V7 = 1.15%, respectively. Figure 6 reflects the suppression effect of HAPF on the grid pollution caused by harmonic currents generated by nonlinear loads. The first and second from the left show the grid-side input currents before and after HAPF activation, respectively, and the second and first from the right show their corresponding spectra. It is evident that before the HAPF is activated, the grid harmonic current is the same as the load current (ZS << ZF), thus containing a large number of harmonics, with a relative total distortion rate (THDi) of 85% compared to the fundamental frequency. However, after the HAPF is activated, due to the combined effect of active and passive units, its THDi decreases to less than 1.7%. If composite control is not adopted, and only current control is applied (F.Z.Peng scheme), the PF capacity should be increased, and current harmonics are difficult to suppress effectively. Figure 7 illustrates this point, with THDi ≈ 17.2%. Simultaneously, the model experiment and simulation waveforms match well, as shown below: the grid current Is and load voltage VL are close to sine waves, and the harmonic indicators comply with national standard GB/T 14549-93 and IEEE-519, indicating that this HAPF can effectively suppress the influence of grid harmonic voltage on the load end and the pollution of the grid by nonlinear load harmonic current. [img=511,239]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2000-5/62-1.jpg[/img] Figure 6. Grid current Is at HAPF startup [img=308,120]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2000-5/62-2.jpg[/img] Figure 7. Grid current Is waveform and spectrum (non-composite control HAPF, THDv = 12%) [align=left][img=488,95]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2000-5/62-3.jpg[/img] (a) Load voltage VL waveform before (upper) and after (lower) HAPE startup (b) Grid-side current IS waveform before HAPE startup (c) Grid-side current IS waveform after HAPE startup Figure 8 Harmonic suppression experimental waveforms of HAPF [b]4 Conclusion[/b] In accordance with the requirements of harmonic testing methods, instruments and data processing in relevant standards [5, 6], harmonics of widely representative industrial variable frequency speed controllers and medium frequency induction heating power supplies were tested, and the quantitative conclusion that the current harmonic THDi is above 42% was obtained. A harmonic mitigation scheme for a hybrid active power filter (HAPF) was proposed, focusing on the development of a harmonic suppression device for a 50kVA load. Simulation and experimental results show that it has good harmonic suppression performance and strong adaptability to large load variation ranges and severe grid distortion.