25-08-2017, 09:32 PM

A Series Active Power Filter Based on a Sinusoidal Current-Controlled Voltage-Source Inverter

A Series Active Power Filter Based on a Sinusoidal.pdf (Size: 256.25 KB / Downloads: 76)

Abstract

A series active power filter working as a sinusoidal

current source, in phase with the mains voltage, has been developed

and tested. The amplitude of the fundamental current in

the series filter is controlled through the error signal generated

between the load voltage and a preestablished reference. The

control allows an effective correction of power factor, harmonic

distortion, and load voltage regulation. Compared with previous

methods of control developed for series active filters, this method

is simpler to implement, because it is only required to generate a

sinusoidal current, in phase with the mains voltage, the amplitude

of which is controlled through the error in the load voltage. The

proposed system has been studied analytically and tested using

computer simulations and experiments. In the experiments, it

has been verified that the filter keeps the line current almost

sinusoidal and in phase with the line voltage supply. It also

responds very fast under sudden changes in the load conditions,

reaching its steady state in about two cycles of the fundamental.

Index Terms—Active filters, current control, power electronics,

power filters, pulsewidth-modulated power converters.

I. INTRODUCTION

HARMONIC contamination, due to the increment of nonlinear

loads, such as large thyristor power converters,

rectifiers, and arc furnaces, has become a serious problem

in power systems. These problems are partially solved with

the help of LC passive filters. However, this kind of filter

cannot solve random variations in the load current waveform.

They also can produce series and parallel resonance with

source impedance. To solve these problems, shunt active

power filters have been developed [1], [2], which are widely

investigated today. These filters work as current sources,

connected in parallel with the nonlinear load, generating the

harmonic currents the load requires. In this form, the mains

only need to supply the fundamental, avoiding contamination

problems along the transmission lines. With an appropriated

control strategy, it is also possible to correct power factor and

unbalanced loads [3] .

However, the cost of shunt active filters is high, and they

are difficult to implement in large scale. Additionally, they also

present lower efficiency than shunt passive filters. For these

Manuscript received April 15, 1996; revised April 7, 1997. This work was

supported by Conicyt under Proyecto Fondecyt 1940997 and 1960572.

J. W. Dixon is with the Department of Electrical Engineering, Pontificia

Universidad Cat´olica de Chile, Santiago, Chile (e-mail: jdixon[at]ing.puc.cl).

G. Venegas was with the Department of Electrical Engineering, Pontificia

Universidad Cat´olica de Chile, Santiago, Chile. He is now with Pangue S.A.,

Santiago, Chile.

L. A. Mor´an is with the Department of Electrical Engineering, Universidad

de Concepci´on, Concepci´on, Chile (e-mail: lmoran[at]renoir.die.udec.cl).

Publisher Item Identifier S 0278-0046(97)06534-9.

reasons, different solutions are being proposed to improve the

practical utilization of active filters. One of them is the use of

a combined system of shunt passive filters and series active

filters. This solution allows one to design the active filter for

only a fraction of the total load power, reducing costs and

increasing overall system efficiency [4].

Series active filters work as isolators, instead of generators

of harmonics and, hence, they use different control strategies.

Until now, series active filters working as controllable voltage

sources have been proposed [5]. With this approach, the

evaluation of the reference voltage for the series filter is

required. This is normally quite complicated, because the

reference voltage is basically composed by harmonics, and

it then has to be evaluated through precise measurements of

voltages and/or current waveforms. Another way to get the

reference voltage for the series filter is through the “ –

theory” [6]. However, this solution has the drawback of

requiring a very complicated control circuit (several analog

multipliers, dividers, and operational amplifiers).

To simplify the control strategy for series active filters, a

different approach is presented in this paper, i.e., the series

filter is controlled as a sinusoidal current source, instead of a

harmonic voltage source. This approach presents the following

advantages.

1) The control system is simpler, because only a sinusoidal

waveform has to be generated.

2) This sinusoidal waveform to control the current can be

generated in phase with the main supply, allowing unity

power-factor operation.

3) It controls the voltage at the load node, allowing excellent

regulation characteristics.

II. GENERAL DESCRIPTION OF THE SYSTEM

The circuits of Fig. 1(a) and (b) show the block diagram and

the main components, respectively, of the proposed system: the

shunt passive filter, the series active filter, the current transformers

(CT’s), a low-power pulsewidth modulation (PWM)

converter, and the control block to generate the sinusoidal

template for the series active filter. The shunt passive

filter, connected in parallel with the load, is tuned to eliminate

the fifth and seventh harmonics and presents a low-impedance

path for the other load current harmonics. It also helps to

partially correct the power factor. The series active filter,

working as a sinusoidal current source in phase with the line

voltage supply , keeps “unity power factor,” and presents a

very high impedance for current harmonics. The CT’s allow

0278–0046/97$10.00 ã 1997 IEEE

DIXON et al.: SERIES ACTIVE POWER FILTER BASED ON VOLTAGE-SOURCE INVERTER 613

(a)

(b)

Fig. 1. Main components of the series active filter. (a) Block diagram. (b)

Components diagram.

for the isolation of the series filter from the mains and the

matching of the voltage and current rating of the filter with

that of the power system. In Fig. 1, represents the load

current,, the current passing through the shunt passive filter,

and the source current. The source current is forced to

be sinusoidal because of the PWM of the series active filter,

which is controlled by . The sinusoidal waveform of

comes from the line voltage , which is filtered and kept in

phase with the help of the PLL block [Fig. 1(b)].

By keeping the load voltage constant, and with the

same magnitude of the nominal line voltage , a “zeroregulation”

characteristic at the load node is obtained. This

is accomplished by controlling the magnitude of through

the error signal between the load voltage and a reference

voltage . This error signal goes through a PI controller,

represented by the block . is adjusted to be equal

to the nominal line voltage .

The two aforementioned characteristics of operation (“unity

power factor” and “zero regulation”), produce an automatic

phase shift between and , without changing their magnitudes.