Comparative Study between Shunt Active Filter and Hybrid

مجلة الجامعة الأسمرية للعلوم الأساسية والتطبيقية

م7132، الجزء الثاني ، ديسمبر (13)العدد

65

Comparative Study between Shunt Active Filter and Hybrid Filter

MOHAMED Muftah Saleem

Department of Electrical and Computer Engineering, Almergib University, Alkhoms, Libya

[email protected]

Abstract—This paper presents an comparative study between shunt active filter and shunt

hybrid filter based on configuration system and harmonic isolation. The harmonic isolator is

based on Self Tuning Filters (STF) (it can be tuned at any frequency). The study of the active

filter control is divided in two parts. The first part deals with the harmonic isolator which

generates the harmonic reference currents. The second part focuses on the generation of the

switching pattern of the IGBTs of the inverter by the modulated hysteresis current controller.

The studied hybrid active filter consists of an active filter and three phase tuned LC filters for

7th

harmonic frequency. The nonlinear load is a diode rectifier feeding a (R, C) parallel load.

The use of STF's simplifies the control scheme by reducing the number of extraction filters.

The use of STFs instead of classical extraction filters allows extracting directly the voltage

and current fundamental components. The effectiveness of this study has been verified by

computer simulation and comparing the results.

Index Terms—shunt active filter, hybrid active filter, harmonics, self tuning filter (SFT).

I. Introduction

The power system must be free from harmonics and which will lead to numbers of

benefits. A clean network has less strain on appliances and their lifespans are lengthened.

Nowadays with the advancement of technology, the demand for electric power is increasing at

an exponential rate. The performance of the end user equipment is heavily dependent on the

quality of power supplied to it. But the quality of power delivered to the end user is affected

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66

by various external and internal factors. They are like voltage and frequency variations, faults,

outages etc.

The main affect caused by these problems is the presence of harmonics. This leads to the

overheating of the equipment, insulation failure and over speeding of induction motors etc

[1,2]. The solution to overcome these problems is to filter out these harmonics.

There are many filters topologies present in the literature. Filters can be classified into three

types (passive, active, hybrid) [3,4]. This study presents and compare two types of filters and

the control strategy to control the filter in such a way that the harmonics are reduced.

The hybrid filter which is a combination of active filter and shunt passive filter is studied.

While the passive filter mitigates load produced harmonics the active filter helps to enhance

filtering properties of passive filter. This ensures a great diminution of the rating of the active

filter guiding to an economical practical system.

II. Active filter and control strategy

A. System configuration

The harmonic currents are the source of adverse effects for many types of equipment's

such as heating in distribution transformer, perturbation of sensitive control equipment's and

resonances with the grid. Many solutions have been studied in the literature to mitigate the

harmonic problems, such as filtering (passive, active, and hybrid) with various topologies

(shunt, series or both). These solutions have been proposed to improve the power quality of

the AC mains.

The passive filter which is the simple way to eliminate the harmonic currents, has many

drawbacks such as the series or parallel resonance with the system impedance and it cannot

completely eliminate all of the harmonic currents [5].

The active filter which is developed and widely used to overcome to the drawbacks of the

passive filters and improve power quality. As known, the performances of the active filter



67

system mostly depend on the accuracy of the harmonic isolation and on the current control

technique used to generate the switching patterns for the inverter [6]. Hysteresis and PMW

techniques are usually used.

The hysteresis strategy allows faster dynamic response and better robustness according to the

variation of the non-linear load. Nevertheless, with this strategy, the switching frequency is

not constant. In this paper, to fix the switching frequency, we studied a three-phase non-linear

current controller resulting from the hybridization between a PWM current controller and a

hysteresis current controller, so called “modulated hysteresis controller”. More, a focus is

made on the improved harmonic isolator based on Self Tuning Filter (STF) [5,7].

This filter (STF) is used instead of classical harmonics extraction based on High Pass filters or

Low Pass Filters. The effectiveness of the proposed method is verified by computer

simulation.

Fig.1. Active filter system.

Fig.1 presents the schematic diagram of the three-phase active power filter and the

associated control strategy for harmonic cancellation. The filter is connected in parallel with

Rd

Ld

Modulated

hysteresis

Current controller

Active Filter A/D

D/A

Harmonic

Isolator

DSPACE

(DS1104)

i*

fa, i*fb, i

*fc

ifa, ifb

Vdc

iLa iLb

6 control signals

Si SL

FL Diode Rectifier

Fi

Vd

c

vs

Li

vsb vsa



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the ac three-phase three-wire system by three inductors Lf. The switching patterns of each

inverter leg are complementary to each other to avoid any short circuit.

The non-linear load is a three-phase diode rectifier feeding a RL load. This load will generate

harmonics currents in the supply system.

The control method is divided in two parts. The first one consists in the harmonic isolator

which generates the harmonic reference currents and is implemented into a DSPACE DS1104

development board. The second part is the generation of the switching pattern for the inverter

by using an analogue modulated hysteresis current controller.Simulation results

Fig.2 shows the simulation results for the system depicted in the Fig.1 under sinusoidal

voltage conditions. The simulation parameters are defined in the Table I.

The total harmonic distortion (THD) of the load current is 28.08%. The THD of the supply

currents is reduced to 2.3% after compensation. A difference can be noticed in the figure 2

between the fundamental components values of the load current and the supply current.

It is justified by the fundamental component of the filter current in phase with the supply

voltage to regulate the dc bus voltage. The harmonic isolator is implemented by using the

DSPACE system associated with a DS1104 development board. It generates the harmonic

current references.

TABLE I. System parameters

System frequency 50 Hz

System voltage 130 Vmax

Inductor : LF 3 mH

Inductor: LC 0.8 mH

DC bus voltage 400 V

Capacitor: Cd 1100 µF

Resistor: Rd 48.6 Ω

Inductor: Ld 40 mH



69

0.1 0.12 0.14 0.16 0.18 0.2

-5

0

5

Time (s)

i L (A)

0 5 10 15 20 25 30 35 400

1

2

3

4

5

6

Harmonic order

Mag

(A)

THD = 28.08 %

-a-

0.1 0.12 0.14 0.16 0.18 0.2

-5

0

5

Time (s)

i L (A)

0 5 10 15 20 25 30 35 400

1

2

3

4

5

6

Harmonic order

Mag

(A)

THD = 2.3 %

-b- 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

100

200

300

400

500

600

Time (s)

Vdc (

V)

-c-



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Fig. 2. Simulation results for: (a) load current, (b) supply current after compensation, (c) DC-

Bus voltage Vdc (v).

III. HYBRID FILTER

B. System configuration

The cost of active filters in industrial applications could be very high. The power rating of

power converter for active filters is very large. For these reasons, the application of active

filters for power system is cost limited [8]. In the last few years, many different topologies of

hybrid active filters with various control strategies have been proposed in the literature as

lower cost alternatives to active filtering for harmonic compensation. Nowadays, hybrid active

filters are considered as one of the best solutions for improving power quality [9]. Fig.3 shows

the hybrid active filter topology studied in this paper.

The hybrid filter consists in a three-phase LC filter tuned for the 7th harmonic frequency,

connected in series with an active filter without any transformer. The passive filter absorbs

harmonic currents generated by the load whereas the active filter improves filtering

performances of the passive filter. The associated control scheme combines a feedback and

feedforwad loop [10,11].

Fig. 3. Parallel hybrid filter configuration.

SL

Diode Rectifier

Active Filter

FL

FC

Fi

Si Li

dcC

Vvdc 105

dC

dR



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Fig. 4. Control scheme of the hybrid active filter.

IV. SIMULATION RESULTS

The efficiency of the proposed control scheme has been examined by computer simulation

using MATLAB and associated toolboxes “Simulink” and “Power System Blockset”. The

parameters of the system are given in Table II.

The simulated hybrid filter consists in a three-phase LC filter tuned at the 7th

harmonic

frequency, connected in series with a three-phase active filter based on MOSFETs power

semiconductors. The nonlinear load is a diode rectifier feeding a RC parallel load. Table III

shows THD values for different choices of the gain value K for the feedback loop of the active

filter. The optimal value is equal to 20 which provides better filtering characteristics.

Fig. 5 show simulation results that present the capability of compensating the fifth harmonic

frequency successfully.

sin (1)& cos(1 ) abcv

PLL

Shi K

i~

i~ di

~

qi~

*

dcv

dcv

*

AFv Self

tuning

filter

*

1qi

PI

Sabci

Feedforward Loop

Feedback Loop Loop

dq

dq abc

abc

i

i

*

5v 5i

5i *

5v

Labci Self

tuning

filter

abc

abc

Calculation

of

V*

α5 &V*β5



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Fig. 5. Simulation results for the : Source current iS (A), Load current iL (A) and DC-Bus

voltage Vdc (v).

The major aims of this paper were to compare the efficiency of the shunt active filter and

hybrid filter. As presented in Table III, the THD of the non-linear load Li is equal to 27.8%

Si

)(st 0.7 0.705 0.71 0.715 0.72 0.725 0.73 0.735 0.74 0.745 0.75

-100

-80

-60

-40

-20

0

20

40

60

80

100

Li

)(st 0.7 0.705 0.71 0.715 0.72 0.725 0.73 0.735 0.74 0.745 0.75

-100

-80

-60

-40

-20

0

20

40

60

80

100

dcv

)(st 0.7 0.705 0.71 0.715 0.72 0.725 0.73 0.735 0.74 0.745 0.750

50

100

150



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because of the large amount of the 5th

harmonic current while it is equal to 2.9% for the source

current Si . The results we obtained demonstrated an improvement of the THD values. The LC

filter is tuned at the 7th

-harmonic frequency and absorbs the voltage of the network at the

fundamental frequency. Consequently, the dc voltage of the inverter dcv can be reduced as low

as 105V. This enables the hybrid filter to use low-voltage MOSFETs which are less

expensive.

Table III presents load current and source current THD as the harmonic-to fundamental

current ratio in (%). It demonstrates the effectiveness of the control scheme by using STFs in

the feedback and feedforward loops.

TABLE II. Simulation parameters

Capacitor : CF 57.6 F

Inductor : LF 2.5 mH

Inductor: LS 0.15 mH

Quality factor: Q 22

DC bus voltage 105 V

Capacitor: Cd 1500 F

Resistor: Rd 21

Capacitor: Cdc 1500 F

System

frequency 50 Hz

System voltage 480 V

TABLE III. Load current and source current THD and harmonic-to fundamental current ratio

(%)



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5th

7th

11th

13th

17th

19th

THD

Li

26

7.5

4.7

3.2

1.5

1.4

27.8

Si

1

0.4

1.6

1.3

0.9

0.9

2.9

V. CONCLUSION

This paper has discussed the configuration system, control and performances of a shunt

active power filter and hybrid filter. The control of the active filter was divided in two parts, a

digital one realized by the DSPACE system to generate the reference currents, , the modulated

hysteresis current controller used for the switching pattern generation. STF used instead of

classical extraction filters for both grid voltages and load currents. The proposed hybrid filter,

which is composed of an active filter and three phase tuned LC filters. The simulation results

have demonstrated and conforted the major advantages of using HSFs in the filter control.

This paper describes a comparative study between shunt active power filter and shunt

hybrid power filter. Simulation results proved that performance of the hybrid filter is much

better than the active filter. The DC link voltage of hybrid filter is twice more than that of

active filter. The combined system of passive and an active filter has some features as (Source

impedance no longer governs the filtering characteristics, the active filter has the ability to

dump the parallel and series resonance between the source and the passive filter, the required

rating of hybrid filter is much less than a conventional active filter used alone). Table IV

summarize some major advantages of hybrid filter over the active filter.

TABLE IV. Comparison between shunt active filter and hybrid filter.

Active filter

Hybrid filter

Type of switch

IGBT MOSFET



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Voltage (Vdc) at the terminals of the

capacitor

400 V 105 V

Capacity value (Cdc) DC side

8 mF 1,5 mF

Energy stored in the storage element 1,96×103 J 8,26 J

Total cost of the system high Reduced

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Comparative Study between Shunt Active Filter and Hybrid