# A Review of Harmonic Detection, Suppression, Aggregation, and Estimation Techniques
## 3 Harmonic Suppression Methods
### Active Power Filter  
Active power filters (APF) are designed to overcome the performance defects of passive power filters. Most active filters try to suppress dynamic harmonics and track the varying harmonic signals. Active filters can compensate for reactive power, suppress dynamic harmonics, avoid overload, and avoid over-resonance in case of a change in power signal frequency [31]. In addition, APF has considerable safety and working stability. The suppression method using APF is a mainstream research topic in harmonic control now. This APF method of suppression consists of mainly two portions; the first is the detection of harmonic signals, and the second is the compensation of harmonics [32]. In [33], APF techniques first detect, compensate the harmonic, and then transmit it to the grid system through a control strategy so that the original harmonic signal can be canceled out. Doing so results in high performance in suppression and accurate harmonics detection, as harmonic detection is the critical link in the suppression process. Figure 3: Harmonic suppression methods. Research on APF is further divided into different methods: topology, DC voltage control, harmonic detection technique, control of current tracking, and PWM methods. Active power filters have various classifications; depending on the application, they are divided into AC and DC [34; 35], as presented in Figure 4. APF inverter circuits can be classified as voltage-type (part a) or current-type (part b) filter devices depending on the energy sting element used [36]. Active power filters can be connected to the grid in various arrangements, in series, parallel, or hybrid schemes. The above Figure 5 shows the possible schematics of the APF with the grid used in the literature [37; 38; 39]. Figures 5a and 5b shows the in-parallel and in-series connections to the grid, respectively. As passive power filters are low-cost, they can be used along with active power filters to create hybrid schemes. The hybrid APF schemes slightly affect the device's capacity but improve the economics [36]. Figure 5c shows the schematic to improve the filtering characteristics of a passive filter when introducing in series a small capacity APF. Figure 5e is referred to as a parallel hybrid active power filter, where the device is in series with a passive filter; this schematic has good performance and a reasonable cost, so it is most common. Figure 5f represents the uniform power quality regulator [39], which combines a passive filter and a series active power filter; this can eliminate power distortion by non-linear loads through adjusting the load terminal voltage and suppressing the harmonic system distortion. Figure 5g,h are LC series and parallel resonant circuits, respectively; this structure can compensate for harmonic currents as it prohibits the oscillation of harmonics between the grid and loads. A harmonic suppression method based on fractional lower order statistics (FLOC) has been proposed in [40] to deal with the impulse noise in the power system, as the impulsive noise seriously degrades the harmonic suppression performance of the power system. It is an Active Power Filter (APF) method based on the principle of self-sensing actuators, illustrated in Figure 6. The key advantage of this model is that it has a tracking ability for changing harmonics while suppressing impulsive noise activity. Another method to suppress harmonics is using a \"Dynamic Voltage Restorer (DVR)\" [41; 42]. It is a device that can be used to mitigate voltage sags, swell, and harmonic distortion in power systems. The DVR works by injecting a voltage that is in phase with the system voltage, but with a higher or lower amplitude, depending on the situation. Table 2 presents the comparative analysis of harmonic suppression methods. In summary, Passive filters, often using inductors and capacitors, offer a cost-effective solution for static harmonic reduction, but they may face performance issues with resonance.