: For harmonic and reactive power adjustment in two-wire (single phase), three-wire (three phase without neutral), and four-wire (three phase with neutral) ac power networks with nonlinear loads, active filtering of electric power has already reached maturity.
This article provides a thorough analysis of active filter (AF) designs, control methods, component choices, additional relevant technical and economic factors, and their selection for certain applications. It aims to give researchers and application developers working with AF technology a comprehensive perspective on the state of the technology.
Power engineers encounter issues with power quality when nonlinear harmonic generating loads are used in the distribution system. Because of developments in semiconductor technology, power electronics devices are being used by end users much more frequently. Problems like harmonic production, poor power factor, reactive power disturbance, low system efficiency, disruption to other consumers, device heating, etc. are brought on using power electronics devices. It is crucial to minimise this issue because it may grow significantly in the coming year.
APFs are frequently employed in power systems to reduce harmonic distortion. They introduce harmonic components into the electrical network via power electronics converters that counteract the harmonics introduced by non-linear loads in the source currents.
Lower order harmonics of the line current (5th, 7th, 11th, etc.) have been eliminated using passive LC filters, which have also been employed to control the flow of harmonic currents in the distribution system. However, these passive second order filters have a few drawbacks, including series and parallel resonances, tuning issues, and power system complexity, especially when there are more harmonic elements that must be suppressed.
In the past thirty years, active filters have been created, enhanced, and commercialised. They can be used to correct current-based distortions such neutral current, reactive power, and current harmonics. They are also used to correct voltage-based distortions such voltage harmonics, flickering, sags, and imbalances.
There are two types of active filters: single-phase and three-phase. Single phase active filters are used to compensate for power quality problems caused by single-phase loads such as DC power supplies. Three-phase active filters can have or not have a neutral connection. Three-phase active filters are used for high-power nonlinear loads like adjustable speed drives (ASDs) and AC-to-DC converters , .
There are two types of active filters based on topologies: current source active filters and voltage source active filters. As shown in Fig. 1, current source active filters (CSAF) use an inductor as the DC energy storage device. As shown in Fig. 2, a capacitor serves as the storage element in a voltage source active filter (VSAF). In comparison to CSAF, VSAF are less expensive, lighter, and easier to control , . Active filters can be connected in a variety of ways, including shunt active filters, series active filters, parallel active filters, and hybrid active filters .
A shunt active filter can remove harmonics from commercial and industrial power supplies. Simon Round and colleagues use a new technique based on sinusoidal subtraction to create an inverter that is more responsive to harmonics .
III. COMPENSATION METHODOLOGY
The idea of using this active design, as shown in Fig. 1, is to account for reactive power and reduce harmonic components. The active filter can be used as a controlled source of current to generate a current wave that is as near to the current reference as feasible. A balance between instantaneous power supplied by the source and the active filter and drained by the load must be determined to generate the current reference. If ps and qs are the main's real and imaginary instantaneous powers, and pf and qf are the active power filter's real and imaginary instantaneous powers, then the main ought to provide ps=pl and qs=0 to offset reactive energy and minimize harmonic currents. To accomplish power compensation, the APF must serve the oscillatory component of pl, whereas ql must be completely supplied by the APF. Because the oscillating component of pl is due to harmonic components, when it is sent to the load by the active filter, the source current remains sinusoidal while the load continues to receive the same amount of harmonic and fundamental current. Power balance results in:
A shunt APF based on MLI is researched for harmonic compensation of high-power high voltage nonlinear loads. MLI eliminates the requirement for a high-rating transformer in a high-voltage system. To generate APF reference currents, the instantaneous reactive power theory is applied. The fact that real and reactive powers associated with fundamental components are DC quantities is a benefit of p-q theory.
These values can be retrieved using a low pass filter. Gating signals are generated using carrier phase shifted PWM. Despite the converter\'s high frequency output, the CPS-PWM approach minimises individual device switching frequency.
Extensive simulation results are achieved to validate the APF\'s performance during transient and steady-state circumstances for various loading scenarios. The suggested approach successfully adjusts current harmonics of nonlinear loads in high voltage systems. After correction, the source current becomes sinusoidal and in phase with the source voltages.
The corrected source current\'s THD falls substantially below 5%, which is the maximum limit specified by the IEEE 519-1992 standard.
Nonlinear loads, such as diode rectifiers with R-C components and phase regulated converters, draw current with strong rising and falling edges, making APF\'s job more difficult. To facilitate APF operation and achieve effective compensation, a passive filter is connected in front of the nonlinear load.
The p-q theory is appropriate when supply voltages are optimal, however mains voltages in typical industrial power systems are frequently distorted. In this scenario, the p-q theory produces mistakes in reference currents and limits compensation. Even when the supply voltage is distorted and there are unbalanced loads, the average power approach produces accurate results.
In addition, the performance of a seven-level MLI-based APF is explored. When compared to five-level MLI, the DC side capacitor voltage is reduced. It demonstrates that for higher voltages, higher-level MLI can be employed to avoid the usage of a costly and bulky transformer.
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