Active Power Filter Based On Cascaded Transformer Multilevel Inverter

Active Power Filter Based On Cascaded Transformer Multilevel Inverter

The widespread increase of non-linear loads nowadays, significant amounts of harmonic currents are being injected into power systems. Active power filters (APF) have proved to be an interesting and effective solution to compensate current harmonics and reactive power in power distribution systems. In this paper, operation of APF based on asymmetric cascaded multilevel converter to compensate current harmonics, unbalance currents and reactive power in power distribution systems is discussed. Power quality improvement with proposed power electronic transformer has been verified by the simulation results with MATLAB/SIMULINK.

The widespread increase of non-linear loads nowadays, significant amounts of harmonic currents are being injected into power systems. Active power filters (APF) have proven to be an interesting and effective solution to compensate current harmonics and reactive power in power distribution systems. In this paper, operation of APF based on the asymmetric cascaded multilevel converter to compensate current harmonics, unbalance currents and reactive power in power distribution systems is discussed. Power quality improvement with the proposed power electronic transformer has been verified by the simulation results with MATLAB/SIMULINK.

The widespread increase of non-linear loads nowadays, significant amounts of harmonic currents are being injected into power systems. Harmonic currents flow through the power system impedance, causing voltage distortion at the harmonic currents’ frequencies. The distorted voltage waveform causes harmonic currents to be drawn by other loads connected at the point of common coupling (PCC). The existence of current and voltage harmonics in power systems increases losses in the lines, decreases the power factor and can cause timing errors in sensitive electronic equipments. The use of grid connected power electronic converters to improve power quality in power distribution systems represents the best solution, in terms of performance and stability, for the elimination of harmonic distortion, power factor correction, balancing of loads, and voltage regulation. The most common example of this type of equipment is the active power filter (APF) which has two main configurations: the shunt connected active power filter is placed in parallel with a non-linear load (NLL) and controlled to cancel the current harmonics created by it; its dual, the series active power filter, is employed for voltage correction and is connected in line with the NLL.

The traditional method of current harmonics reduction involves passive LC filters, which are its simplicity and low cost. However, passive filters have several drawbacks such as large size, tuning and risk of resonance problems. On the contrary, the 4-leg APF can solve problems of current harmonics, reactive power, load current balancing and excessive neutral current simultaneously, and can be a much better solution than conventional approaches. The three-phase four-wire system is now being widely used in different areas including industry, office and civil buildings and power supplies for cities and factories. This configuration results in problems with harmonics in addition to the potential unbalance of the three phases. Active power filters may be used to effectively compensate the harmonic and reactive power on a three-phase four-wire grid. In the former the fourth leg is used to compensate the neutral wire current directly.

One of the APF has been designed based on the asymmetrical cascaded multilevel converter in recent years. This type presents an active power filter implemented with multiple single-phase cells connected in series. Each cell is composed of a DC capacitor and a full–bridge single–phase PWM voltage–source inverter. The cascaded topology requires a separate DC-link capacitor for each cell, requiring a complex control strategy to regulate the voltage across each capacitor. Recently cascaded transformer multilevel topology is proposed. Cascaded transformer multilevel has the advantage of having single storage capacitor for all its cells. Therefore, the DC voltage across each cell is equal. This topology has much significance for higher rated converters used for high or medium voltage distribution system, as they require transformers to increase the inverter output voltage at the distribution level.