1 Introduction

Power electronic products are widely used in the field of industrial control, and users have higher and higher requirements for power quality, among which the most prominent are voltage quality and harmonics. Therefore, how to improve voltage quality and control harmonics has become one of the most urgent issues in power transmission and distribution technology. Low-cost passive filter PF (Passive Filter) is currently a commonly used compensation method, but its filtering effect is closely related to the system operating parameters. Under certain circumstances, the passive filter may also resonate with the system. Since the 1980s, the research on active power filter APF (Active Power Filter) using power switches has attracted more and more attention. APF is a new type of power electronic device used for dynamic harmonic suppression and reactive power compensation. However, since the power supply voltage is directly added to the inverter bridge, it has high requirements for the voltage level of the switching device; when the load harmonic current is large, the capacity of the active filter device is also relatively large; harmonics higher than the switching frequency of the active filter cannot be filtered out by the active filter, so it is difficult to have a large compensation capacity and a wide compensation frequency band at the same time.

Combining APF with PF and reasonably sharing the compensation demand can reduce the capacity of APF. The basic principle of the hybrid compensation scheme is to remove the fundamental voltage on the conventional APF so that the active device only bears the harmonic voltage, thereby significantly reducing the capacity of the active device and giving full play to the high withstand voltage, large capacity, easy implementation and other characteristics of PF as well as the wide harmonic suppression range and automatic tracking of APF.

2 Passive filter

 The traditional method for harmonic control is to connect a parallel passive LC filter, select the parameters of R, L, and C, so that the filter network resonates at a certain harmonic signal frequency, thereby achieving the purpose of suppressing harmonics. Passive filters can be mainly divided into two categories: tuned filters and high-pass filters. The most commonly used tuned filter is the single-tuned filter, which is composed of the series resonance principle of inductors and capacitors.

3 Active filter

The basic principle of the active filter is to detect the harmonic current from the compensation object, and the compensation device generates a compensation current equal to the harmonic current but opposite in polarity, so that the grid contains only the fundamental component, achieving the purpose of real-time compensation current. If the active filter is required to compensate for the reactive power of the load while compensating for harmonics, it is sufficient to add a component with the opposite polarity to the reactive component of the load current to the command signal of the compensation current. This filter can quickly and dynamically track and compensate for harmonics whose frequency and size change with time and reactive power.

(1) Harmonic detection Harmonic measurement methods include the use of analog bandpass (or bandstop) filters, harmonic detection analysis based on Fourier transform, and harmonic detection based on instantaneous reactive power. The most commonly used harmonic detection method in real-time detection of harmonics and reactive current is the harmonic detection method based on instantaneous reactive power. Here, the pq algorithm based on instantaneous reactive theory is used to detect harmonics in real time. The principle block diagram of the pg algorithm is shown in Figure 1.

In Figure 1, C23 = C32T. This method transforms the instantaneous values ​​of the voltage and current of each phase of the three-phase circuit into the α-β two-phase orthogonal coordinate system. According to the definition, the instantaneous real power p and the instantaneous imaginary power q are calculated, and the DC components of p and q are obtained through the low-pass filter LPF. When the grid voltage waveform is not distorted, p is generated by the action of the fundamental active current and voltage, and q is generated by the action of the fundamental reactive current and voltage. Therefore, the fundamental components iaf, ibf, and icf of the detected current ia, ib, and ic can be calculated from p and q. Subtracting iaf, ibf, and icf from ia, ib, and ic can obtain the harmonic components iah, ibh, and ich.

(2) Establishment of current tracking control module Since the compensation current generated by the active power filter should follow the changes of its command current signal in real time, that is, it has good dynamic response characteristics, its control mostly adopts tracking control technology. The desired output waveform is used as the command signal, and the actual waveform is used as the feedback signal. By comparing the instantaneous values ​​of the two, the on and off of each device in the inverter circuit is determined, so that the actual output tracks the change of the command signal. At present, the instantaneous value comparison method (hysteresis control) of the hysteresis comparator and the triangular wave linear comparison method (tri-angle wave linear control) are mainly used. The latter is used here.

4 Establishment of the hybrid active power filter circuit

 The circuit structure of the hybrid active power filter is shown in Figure 2.

There are two basic connection modes for the hybrid filter composed of passive and active filters: series type (SHAPF) and parallel type (PHAPF), and the latter is adopted here. Among them, the specific harmonics are mainly compensated by passive filters, which are composed of multiple single-tuned filters. The tuning frequency of the single-tuned filter is determined according to the harmonic components of the compensated object. The passive filter can be composed of 5th, 7th and 11th single-tuned filters. The active filter adopts a voltage-type inverter to output the superposition of each harmonic voltage to filter out some harmonics of the power grid and suppress the resonance between the power grid impedance and the passive filter. The output filter adopts an LC low-pass filter to filter out the high-frequency burrs generated by the switching devices of the voltage-type inverter.

5 Simulation analysis

The active power filter is simulated and calculated using PSIM, and its parameters are set as follows: the grid voltage of the active power filter is 380 V line voltage and the frequency is 50 Hz. The harmonic source is a common thyristor three-phase bridge rectifier circuit with inductive load, and the main harmonic content is 5th and 7th harmonics. The simulation results under the action of the active filter are shown in Figure 3, and Figure 4 is the Fourier analysis of the simulation results.

It can be seen from the simulation results that the load current harmonics are very large before the active power filter is put into use. The active power filter can offset the severely distorted grid harmonic voltage, reducing the load harmonic current relative to the fundamental wave total distortion rate THD from 30.44% to 5.3% (of which the 5th, 7th and 11th harmonics are reduced by 25.12%, 5.56% and 4.74% respectively), and it also has good filtering characteristics for high-order harmonics. The waveform of the grid current is close to a sine wave after compensation, and the active power filter has a good filtering effect. However, only under the control of the active filter, the compensation current is about 70 A, and the compensation capacity borne by the active filter is relatively large. Therefore, the task of compensating low-order harmonics with a large content can be undertaken by the passive filtering network. The hybrid active power filter shown in Figure 2 was simulated and the results are shown in Figure 5.

The Fourier analysis is performed on the compensation currents output by the passive filter and the active filter in the hybrid active power filter, and the results are shown in FIG6 .

As can be seen from Figure 6a and b, the 5th and 7th harmonics with large contents are mainly filtered out by the passive filter; the output grid current basically does not contain the 5th and 7th harmonic components. The active filter has a certain compensation effect on the 5th and 7th harmonics, but the compensation value is greatly reduced, as shown in Figure 6c. Therefore, its capacity is greatly reduced compared with the case of using active power filter alone.

6 Conclusion

 Theoretical analysis and simulation results show that the circuit structure of the hybrid APF gives full play to the respective advantages of passive filter and active filter, reduces the capacity of active filter, improves the performance of passive filter, and achieves the designed goal. It is particularly suitable for comprehensive control of harmonics and reactive power in high-voltage and large-capacity occasions.