A novel modular approach to active power-line harmonic filtering in distribution systems

Abstract

Recently, AC distribution systems have experienced high harmonic pollution due to the wide use of power electronic loads. These non-linear loads generate harmonics which degrade the distribution systems and may affect the communication and control systems. Harmonic filters, in general, are designed to reduce the effects of harmonic penetration in power systems and they should be installed when it has been determined that the recommended harmonic content has been exceeded.

Two approaches have been proposed to reduce the effect of the harmonic distortion, namely active filtering approach and passive filtering approach. Passive filters have the demerits of large size, resonance and fixed compensation. In the active filtering approach, the harmonic currents produced by the nonlinear loads are extracted, and their opposites are generated and injected into the power line using a power converter. Several active filtering approaches based on different circuit topologies and control theories have been proposed. Most of these active filter systems consist mainly of a single PWM power converter with a high rating which takes care of all the harmonic components in the distorted signal. The combination of high power and high switching frequency results in excessive amounts of power losses. Furthermore, the reliability of the existing active filters is a major concern, as the failure of converter results in no compensation at all.

Active power line filtering can be performed in the time domain or in frequency domain. A distinct advantage of the frequency-domain techniques is the possibility of selective harmonic elimination, thanks to the availability of information on individual harmonic components.

The objective of this research is to develop an efficient and reliable modular active harmonic filter system to realize a cost-effective solution to the harmonic problem. The proposed filter system consists of a number CSC modules, each dedicated to filter a specific harmonic of choice(Frequency-Splitting Approach). The power rating of the modules will decrease and their switching frequency will increase as the order of the harmonic to be filtered is increased. The overall switching losses are minimized due to the selected harmonic elimination and balanced a "power rating"-"switching frequency" product.

Two ADALINEs are proposed as a part of the filter controller for processing the signals obtained from the power-line. One ADALINE (the Current ADALINE) extracts the fundamental and harmonic components of the distorted current. The other ADALINE (the Voltage ADALINE) estimates the line voltage. The outputs of both ADALINEs are used to construct the modulating signals of the filter modules. The proposed controller decides which CSC filter module(s) is connected to the electric grid. The automated connection of the corresponding filter module(s) is based on decision-making rules in such a way that the IEEE 519-1992 limits are not violated. The information available on the magnitude of each harmonic component allows us to select the active filter bandwidth (i.e., the highest harmonic to be suppressed). This will result in more efficiency and higher performance. The proposed controller adjusts the Idc in each CSC module according to the present magnitude of the corresponding harmonic current. This results in optimum dc-side current value and minimal converter losses.

The comparison of the proposed modular active filter scheme and the conventional one converter scheme on practical use in industry is presented. This comparison shows that the proposed solution is more economical, reliable and flexible compared to conventional one. High speed and accuracy of ADALINE, self-synchronizing harmonic tracking, intelligence and robustness of the controller, optimum Idc value, minimal converter losses, and high speed and low dc energy requirement of the CSC, are the main features of the proposed active filter system.

Simulation results using the EMTDC simulation package are presented to validate the effectiveness of the proposed modular active filter system.