Four basic types of hybrid active power filters

Over the past half century, power electronics technology, as an emerging technology, has continued to develop and has covered almost all fields of the electrical energy industry. However, the extensive use of active power filters has resulted in a large amount of harmonics, and the harm caused by harmonics is very significant. Therefore, the management of harmonics becomes extremely important.

Active Power Filter (APF) has been used for power harmonic suppression in the past decade and is regarded as an effective method. It usually calculates the harmonics of the power grid and/or the load in real time, and then uses different control schemes to amplify the harmonic power through high-frequency inverters, thereby suppressing the harmonic voltages and currents at different compensation purpose points to a sufficiently low level.

From an industrial perspective, metallurgy, cement, and large manufacturing enterprises were the first to be affected. Harmonics made it difficult for the previously installed reactive power compensation devices to operate normally, resulting in a decrease in power factor, overheating of power supply transformers, and disruption of normal continuous production, which seriously affected the economic benefits of these enterprises.

From the perspective of the civilian market, in recent years, inverter appliances have begun to be accepted by the general public. Taking the typical inverter air conditioner as an example, if an active power filter scheme is adopted to limit the harmonics current entering the grid, considering factors such as size and price, after the input current exceeds 11A, it is difficult to meet the requirements of GB17625.1 "Harmonic Current Limiting of Low-Voltage Electrical and Electronic Equipment". If the common Boost circuit method of PFC technology is used, it not only brings the problem of voltage boost but also reduces the conversion efficiency.

Although the structures of APF are diverse, the basic types commonly used for hybrid active power filters at present can be roughly classified into the following four categories.

1) The HAPF (Hybrid Active-Passive Filter) which is a combination of a series APF and a parallel PPF is a type that has received considerable attention in current research. Since most of the harmonics are filtered out by relatively inexpensive passive filters, the capacity of the active part is very small, typically 5% to 10% of the compensation power. Therefore, the cost is relatively low. Moreover, since the series part of the harmonic current can be equivalently regarded as a "harmonic resistor", this resistor prevents possible resonances between the grid and the PPF, providing a great potential for modification of the current passive filter solutions in industrial applications. However, its disadvantage is that the series insertion of the APF requires the addition of a coupling transformer, which makes the connection and maintenance inconvenient, and introduces distortion in the load voltage.

2) The HAPF of the APF2 type, which is connected in series with the parallel PPF, is a hybrid filter that combines the functions of the series APF and the parallel APF. The series APF isolates the power supply and the load, preventing the power supply's harmonic voltage from entering the load end and the load's harmonic current from flowing into the power grid. The parallel APF provides a zero-impedance harmonic path to absorb the harmonic current in the load. This solution simultaneously purifies both voltage and current between the power grid and the common connection point. However, due to the use of two APFs, the cost is very high and it is difficult to be widely applied.

3) Parallel APF and parallel PPF type HAPF has the advantage of convenient circuit connection. The APF can compensate for reactive power. However, its disadvantage is that the APF has a large capacity, there is a harmonic channel between the power grid and the APF, and there is a possibility of resonance. Moreover, this device is sensitive to the harmonic voltage at the power grid end and is only suitable for current-source type harmonic loads.

4) The advantages of the parallel PPF and its series-connected APF type HAPF are that the voltage drop of the active part's fundamental voltage is significantly reduced. At the same time, using the current control voltage source (CCVS) control method can effectively prevent possible resonances between the PPF and the power grid or the load. Moreover, since the injection coupling transformer can be connected to the neutral point of the Y-connected PPF, it is convenient for protection and isolation, and is more suitable for high-voltage system applications. However, its disadvantage remains that it is sensitive to the harmonic voltage at the power grid end and is only suitable for current-source type harmonic loads.

Given the contradiction between the limited controllable energy level of current power electronic devices and the huge control energy requirements of power systems, the hybrid active power filter (HAPF) has become the main focus of APF research and application at present. HAPF combines APF and PPF. It not only has the advantages of PPF such as simple structure, low cost, and mature technology, but also has the advantages of APF such as good dynamic tracking effect and wide harmonic suppression spectrum. Therefore, HAPF has a very promising application prospect in the industrial field.