Due to the extensive use of non-linear load equipment in commercial and industrial fields, the power supply system of enterprises exists heavy harmonic pollution. To improve the power quality of the grid and ensure the grid’s safe operation, companies usually use active harmonic filters or passive harmonic filter devices to reduce the harmonic. So, what are the differences between active harmonic filters and passive harmonic filters?

1. Configuration and Working Principle Is Different

Active Harmonic Filter (AHF), also called Active Power Filter (APF), is a new type of power electronic equipment which are developed based on the high-speed DSP devices and adopts the modern power electronics technology and digital signal processing technology to realize dynamically suppressing harmonics and compensating reactive power.
Active Harmonic Filters system is mainly made of three parts: harmonic detection module, control module and inverter bridge module. When the harmonics in the grid are detected by the harmonic detection module, the control module will calculate the current value which need to be compensated and send the command to the inverter bridge module, then the inverter bridge module generates an equal but opposite current to offset the harmonic current. We can take the active filter as a current source, which can achieve the effect of harmonic filtering by actively generating reverse harmonic current.
Passive Harmonic Filter (PHF) is a mechanical filtering device which made by combining power capacitors, reactors, and resistors. Based on the inherent impedance characteristics of the capacitor and resistor, the passive power filter presents a low impedance to the harmonic of a specific frequency and provides a bypass channel for the harmonic current at this frequency, allowing the harmonic current to flow into the filter instead of the power grid, thereby eliminating harmonics. The passive filter is like an additional load that absorbs harmonics. It achieves the effect of mitigating harmonics by passively absorbing harmonic contents.

2. Ability of Harmonic Filtering and Reactive Power Compensation is Different.

APF can actively respond within a certain range, and dynamically track and compensate the changing harmonic current in the power grid. It can compensate all the harmonics or selectively compensate the harmonic orders between the 2nd and 50th order. The harmonic oscillations effect can reach more than 95%. While filtering out harmonics, the active filter can also dynamically compensate the reactive power of the fundamental wave and improve the power factor. Moreover, the AHF adopts a real-time dynamic linear method for compensation, which can reduce the fluctuating harmonics and reactive power that vary in both frequency and size at the same time, and the reactive power correction is stepless and without switching thresholds.
Most passive harmonic filter are designed for harmonics of specific frequencies. Every order of harmonic requires a separate single-resonant filter. It can only eliminate the harmonics of fixed orders (3,5,7) in a certain frequency range. Because the structure is simple, the real-time dynamic compensation cannot be realized. Therefore, harmonic mitigation effect can only reach about 65%. And because the reactive power filtering needs to occupy the capacity, the PHF are rarely used for power factor correction at the same time. Moreover, passive filters use a non-linear compensation method in dealing with reactive power optimization. Because it is stepwise input, there is a switching threshold for compensation.

3. Respond Time and Influence by Frequency Variation are Different.
The active harmonic filter adopts the IGBT control system, which has high controllability and fast response, and the compensation performance is not affected by the grid frequency. The advanced algorithm enables the device to calculate the output of the next switching frequency within 10ms. Therefore, the response speed does not exceed 10ms. It can realize fast and stable compensation for fast-changing harmonics.
Passive harmonic filtering reactor uses contactor switching. The response speed is slow, and the frequency change of the passive filters will cause the resonance point to be shifted, which will decrease the harmonic filtering effect.

4. Influence by System Impedance and Load Variations are Different.
Active Filters are not influenced by system impedance and load variations. It can eliminate the risk of resonance with the system impedance. When the component of harmonics is greater than the AHF compensation capability, only compensation effect reduces, and the equipment will not be damaged.
Passive filters are seriously affected by grid impedance and there are risks of harmonic amplification and harmonic resonance. The PHF compensation effects vary with load changes. PHF can be damaged or even burned due to overloading.

5. Capacity Expansion Ability is Different
Active Harmonic Filter diagram is modular design with high power density and light structure. It is easy to install and expand in the future. When the space in the cabinet is sufficient, the capacity can be expanded by inserting new AHF modules. What’s more, in addition to the rack type active harmonic filter, the AHF also has wall-mounted modules. It can be installed even without the cabinet.
Passive Harmonic filters have a large amount of configuration of power capacitors banks. The device is large and need to occupy more space. They must be assembled into a cabinet and then can be used. Once the installation is finished, the performance parameters are difficult to change. If the PHF expansion is required, it needs to re-add the filter circuit and re-arrange the space of the whole cabinet. It is very likely to add a new power distribution cabinet, which will increase the cost. What’s more, the filtering characteristics of passive filters are greatly affected by system parameters. When the PHF system parameters change, the filtering device may fail or even cause resonance.

6. Production and Maintenance Cost is Different
Harmonic Filters Active use high frequency switching of advanced electric component to realize harmonic cancellation, so the production cost is relatively high. But although the initial investment cost is high, the better compensation effect, low loss and low maintenance cost make it more economical from a long-term perspective to consider.
Harmonic Filters Passive is using contactor switching. The initial investment cost of PHF is low, but due to its disadvantages such as large loss and the device itself has harmonic content etc., the future maintenance cost is relatively high.

7. Application Sites are Different.
Active Power filter is mainly used for the sites where the harmonic component and size change dynamically, and the harmonic component is rich.
Passive power filters are mainly suitable for sites where the harmonic components and size are relatively stable, and the harmonic components are relatively simple.

All in All, AHF VS PHF, which one is better to improve the power quality?

It depends on your facility profile the load configurations. Active Harmonic Filters are the modern series of Passive Harmonic Filter. Generally, If the budget is enough, Active Power Filter is the first choice. If the budget is limited, the hybrid combination compensation of Active Harmonic Filter and conventional capacitors banks are also acceptable. Of course, for the high voltage sites, the passive filters will be the best choice.

Inomax is a professional harmonic filtering expert. We not only produce harmonic filter equipment, but also offer harmonic filtering solution. Our engineers will help you to analyze and give the exact solution to improve PF and reduce THDi for free.

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