artigo cobep 2011_bruno laurindo

8/6/2019 Artigo COBEP 2011_Bruno Laurindo

1/6

THREE-WIRE INVESTIGATION OF A HYBRID POWER FILTER WITH FOUR-

SWITCH STRUCTURE APPLIED TO THE ATTENUATION OF 5TH AND 7TH

HARMONICS WITH REDUCED INVERTER POWER RATING

B.M. Laurindo, B.W. Frana, L.G.B. Rolim, M. AredesLEMT - Laboratrio de Eletrnica de potncia e Mdia Tenso

PEE/COPPEPrograma de Engenharia EltricaUFRJUniversidade Federal do Rio de Janeiro

[brunoml|bruno|Rolim|aredes]@lemt.ufrj.br

AbstractThis paper studies a hybrid filter formed bya single tuned LC filter per phase and a small-rated

three-phase active filter, which are directly connected in

series without any matching transformer. The required

rating of the active filter is much smaller than that of a

conventional standalone active filter would be and it

comprises a four-switch H-bridge inverter connected to

the three-wire system. The analytical procedure isverified by simulation results to confirm the effectiveness

of the proposed control scheme.

Keywords Active power filter, harmonics, hybridpower filter, transformerless.

I.INTRODUCTION

In recent years, harmonic pollution has become a seriousproblem in power distribution system. Current and voltageharmonics have negative effects on the operation of theelectric power system. Thus, great attention is focused on

harmonic generation and control. Several standards haveintroduced limits on current harmonics injected into thepower system and on voltage harmonics at the bus-bars ofthe system [1].

The harmonic current has a spectrum composed harmonic,sub harmonic and inter harmonic, variables with the outputfrequency converter. The circulation of these currents by theelectric industrial system introduces a distortion, causing ahost of related problems power quality.

A conventional solution for the compensation ofnonsinusoidal currents or lagging power factor of a load is toconnected compensating devices, like capacitor banks ortuned passive filters, in parallel to the load. However, their

frequency characteristics are unfavorably influenced by avariable grid impedance and here, in reality, the danger of theexcitation of resonance exists. In addition, variations of filterparameters occur due to aging, temperature, and otheroperating conditions to assess such questions.

This paper analyzes a transformerless ac/dc powerconversion system consisting of a three-phase six-pulsediode rectifier compensated shunt hybrid filter. A single LCpassive filter per phase and a small-rated active filter using athree-phase voltage-source PWM inverter are directlyconnected in series without any matching transformer, thusforming the shunt hybrid filter. The filter absorbs mostharmonic currents produced by the rectifier, whereas the

active filter improves the filtering characteristics of the filter.The required rating of the inverter for this hybrid filter is

much smaller than that of a conventional standalone activefilter. Also, no additional switching-ripple filter is requiredfor the hybrid filter because the LC filter functions not onlyas a harmonic filter but also as a switching-ripple filter.

Hybrid power filters (HPF) consisting of dedicatedpassive and active filters connected in series or parallel havebeen reported in the literature. It has been shown that HPF

are less sensitive to parameter variations than a passive filterused alone [2]. It is also well known that HPF helps to reducethe active filter rating because it can use lower-voltageinverters and can block undesired currents.

Some research efforts have been made to develop newpower converters with reduced losses and costs. Amongthese circuits, the three-phase converters with only twoinverter legs are an alternative solution. Only four powerswitches and four clamping diodes are used in the adoptedconverter instead of six power switches and six clampingdiodes used in a conventional converter. With a reducednumber of power switches, the converter can compensateharmonics also in case of unbalanced line voltages. It is

possible to obtain near unity power factor and input currentswith nearly sinusoidal shape, strongly reducing line currentharmonics, not in focus at the moment [1].

II.SYSTEM CONFIGURATION

Fig. 1 shows the ac/dc power conversion system proposedin this paper. The shunt hybrid filter consists of a 0.4kVAactive filter with a switching frequency of 10 kHz, and apassive filter. Both filters are directly connected in serieswithout transformer. The hybrid filter is installed in parallelwith a three-phase diode rectifier rated at 4kVA, for testing.

A DC capacitor is connected to the DC bus of the powerpurpose converter to reduce its ripple; therefore, forming avoltage-source power converter. The voltage-sourceconverter is controlled by the pulse width-modulation(PWM) strategy in which a modulation signal is comparedwith a high-frequency carrier.

The voltage-source converter controlled by the PWMstrategy can generate a voltage give by (1) [2]:

FhAF VKV .*

(1)

The frequency spectrum of switching ripple voltages is

concentrated around integer multiples of the carrierfrequency. Since the switching frequency of the powerconverter is very high compared with the harmonic


2/6

frequencies existing in power system, it can be filtered outeffectively by the inductance of the passive filter.[3]

VaS

VbS

VcS

LS LAC

RL

LL

ibL

icL

iaL

rF

LF

CF

iaF ibF icF

VCCCdc

Fig. 1. A 220V transformerless ac/dc power conversion system.

A.Control Circuit1)Considerations of Harmonic Detection Methods -

Three different harmonic detection methods for the activefilter are considered and compared in view of systemstability: detecting the harmonic current flowing into thepassive filter, IFh; detecting the harmonic voltage appearingacross the passive filter, VFh; detecting the harmonic voltageappearing on the common bus voltage, VBUSh.

In the following analysis, the harmonic-extracting circuitof the active filter is assumed to be ideal without time delay,so that the transfer function of the control circuit is simplifiedas the gain k. Figure 2 shows a single-phase equivalentcircuit from which the supply harmonic voltage is removed.The total impedance of the passive filter,ZF(s) is given by

F

FFFsC

sLrsZ1

)( (2)

LSZ(s) RL

IFhVFh

VAF

CF

LF

rF

ZF

Fig. 2. Single-phase equivalent circuit when no harmonic voltageexists in the supply voltage.

The external impedance seen from the installation point of

the hybrid filter the same figure 2, andZ(s) is given by

SL sLR

sZ11

1)( (3)

At first, it is necessary to calculate a loop transfer function

of the harmonic detection method, but will be calculated onlyfor the method of detection ofVF.

)()(

)(..)(

sZsZ

sZk

V

VksG

F

F

AF

FhVFh

(4)

Moreover, the harmonic voltage appearing on thecommon bus, VPCCh is given by

FhAFFhPCCh VkVVV )1( (5)

Setting the gain to k = -1 yields an ideal condition of

VPCCh = 0. So, finding the reference voltage of the hybridfilter, Vabc INVadjusted gain k.The VF detecting method with a gain of k = -1 has the

magnitude of 0 dB and the phase angle of 0 in a frequencyrange of less than 200 Hz and of more than 400 Hz, as shownin Figure 3, so that this system is marginally stable.Therefore, the gain should be set in a range of 0> k>-1, toprovide a gain margin. Since it is detecting the 5th-harmonic,300Hz.

Fig. 3. Bode Diagram of the Harmonic-Extracting Method UnderFull-Load Conditions.

2)Feedback Control Figure 4 shows how the controlsignals VoutF are obtained in quadrature to source voltage, VS.


3/6

For such, are generated by PLL (Phase Locked Loop) sinesand cosines oft. The amplitude of RefLinkcc delivered bythe PI controller, that acts to control the DC voltage of thecapacitor. Thus, by applying the Clarke transform, signals inabc coordinates are generated in quadrature with the sourcevoltages. These VoutF signals make up the reference PWM,

controlling the active power injected or drained from thecapacitor DC link.

PLLVaS

VbS

VcS

cos

sin

X(-5)

wt

wt 5

x

x VAFa

VAFb

VAFc

Gain

K

Gain

K

Gain

K

CLARKE

TRANSFORM

PARKE

TRANSFORM

INVERSE

CLARKE

TRANSFORM

VFW a

VFW b

VFW cVref-q5

Vref-d5

w 5wt 5wt 5

IaL

IbL

IcLILq5

ILd5

ILq5-barra

ILd5-barraLPF

LPF

Reference

Voltage

FFW

I

P

RefLinkCCerro VCC

Vcc pu

VRef

Vc-INV

Vb-INV

Va-INVVoutF a

VoutF b

VoutF c

Feedback

Control

FeedforwardControl

LinkDC

Control

Fig. 4. Control system for the hybrid filter.

3)DC-Bus Voltage ControlThe DC Bus voltage of theconverter must be greater than the peak value of the inductorvoltage of the passive filter. This is necessary thecompensating current required by active filter is imposed by

LF[8].This paper discusses how this topology may allow thereduction of voltage in the inverter, and thus power rating. Inthe active filter connected in series with the passive filter, theterminals AC voltage of the active filter is effectivelyreduced.

In fact, for a given maximum harmonic current, areduction of the inductor voltage is obtained with just a smallamount of inductance. The disadvantage is that such solutionrequires a high capacitance value in order to maintain thesame of the passive filter tuning.

In the fundamental frequency, the passive filter has acapacitive behavior. This means that it injects reactive power

in the grid. A larger capacitance enhances this effect, whichcan deteriorate the resulting power factor [4].The active filter can build up and regulate the DC

capacitor voltage by itself without any external power supply[6]. When the active filter is controlled to produce afundamental voltage, being it in phase with the fundamentalleading current of the passive filter, an active power, formedby the leading current and the fundamental voltage, isdelivered to the DC capacitor. A proportional-plus-integral(PI) controller is used to do it, also seen in Figure 4.

4)Feedforward control Three load phase currents aretransformed in two orthogonal components in the

synchronous rotating reference frame, by using a unit vector(cos5t, sin5t) with the fifth harmonic angular speed 5.By using a low pass filter (LPF) of the first order with the

cut-off frequency fc= 16 Hz average d-q5 components of thefifth harmonic load currents are obtained. These componentsshould be delivered by the APF into the point commoncoupling (PCC) to prevent the system withdrawing themfrom the grid.

The reference voltage for the active filter, which should be

generated by the feedforward control loop, is then calculatedas follows [7]:

5

5

555Re .1

dqL

F

FFLdqf iC

Ljrvv (6)

The reference d-q5 voltage components v Ref d5, v Ref q5 arethen transformed back into the phase reference quantities.The final reference phase voltages for the active filter aredetermined by summing the references coming from thefeedback and feedforward loops.

5)Gain - Adjusting Control The gain-adjusting circuitcalculates a square of the extracted 5th-harmonic voltage forevery phase, and then sums all of the three, producing a vF5as follows [4]:

2

5

2

5

2

5

2

5 bFoutbFoutaFoutF vvvv (7)

As shown Figure 5, the circuit compares2

5Fv with a

square of a limit value*

5Fv . When

2

5Fv is smaller than the

square of * 5Fv , the circuit sets the gain in such a way as k =

-rF. When2

5Fv is larger, an integral feedback controller in

the circuit adjusts the gain in such a way as to make2

5Fv equal

*

5Fv . The purpose of the gain-adjusting circuit is

to prevent the passive filter and the active filter fromoverheating and overcurrent, and therefore the circuitrequires a control response as slow as 14 seconds. Thisimplies that it takes about 2 seconds to adjust the gain kfrom-2 to 0, when an overcurrent being two times as large as therated current flows into the passive filter [4].

VoutF a

VoutF b

VoutF c

X2

X2

X2

X2

1sT

-rF

Gain KVF5 col

VF5*

Fig. 5. Gain-Adjusting Circuit.

B.Filtering Characteristics of Passive Filter


4/6


5/6

current total harmonic distortion (THD) to 0,6% as figures(a) and (b) above suggest in the analysis the spectraharmonic.

The harmonic content is examined more clearly in Figure8. The harmonics are expressed as percentage of thefundamental component. The amplitude of the fundamental

is seen on the left axis limited at 1,0 and on the right axis isobserved the amplitude of order harmonics limited at 0,004.

Fig. 8. Comparison of the Amplitude of the Hamonics RelationWith Fundamental.

The load current contains a large amount of the fifth andseventh harmonic current, so that its current THD reaches27,9%. The supply current has the current THD reduced to0,6%, and it becomes an almost sinusoidal waveform with aleading power factor as good as 0,96. Where the Figures 9(a) and (b) may be to observe the waveforms of the load and

source.

(a)

(b)

Fig. 9. Waveforms of the Currents: (a) Load, (b) Source.

These results also verify that the dc voltage of 30V,through of the Figure 10, is sufficient for the active filter to

achieve satisfactory performance. Since the required dcvoltage is 100 V, no matching transformer is used to connectthe active filter with the LC filter, which means that thishybrid filter can still be practical and viable.

Fig. 10. DC Voltage of the Inverter.

From Figure 11, it is clearly seen that almost noswitching- ripple voltage appears at VabcS. This implies thattheLCfilter has an additional function of a switching-ripple

filter as expected.

Fig. 11. Waveforms of the Source of Voltage.

Fig. 12. Waveforms of the Currents of Active Filter.

IV.CONCLUSION

This paper has proposed ac/dc power conversion systemwithout any matching transformer consisting of a three-phasesix-pulse diode rectifier and a shunt hybrid filter for the220V line-to-line voltage system. The passive filter used hereconsists of only a single tuned LC filter, and no switching-ripple filter is installed in the system, thus making the systemcompact and viable. Moreover, a feedback control scheme isalso proposed to improve the performance of the active filter.Simulation results show that the hybrid filter, in which the dcvoltage of the active filter is 30V, can compensate for currentharmonics generated from the diode rectifier, thus using an


6/6

inverter with reduced power effectively. This means that nomatching transformer is required to connect the active filterin series with the LC filter. The hybrid filter results in asatisfactory supply current waveform. This allows us to put itinto practical use for low voltage systems, as there arecommercially available IGBTs compatible.

ACKNOWLEDGEMENT

The authors are grateful for the financial support providedby encouragement of sources, during the development of thisproject.

REFERENCES

[1] J. Klima, J. Tlusty, J. Skramlik and V. Valouch,

Analytical Model and Investigation of a Four-SwitchSpace-Vector Modulated Hybrid Power Filter with Six-Fold Switching Symmetry,International Conference on Renewable Energies and Power Quality (ICREPQ09),April 2009.

[2] Wu J. Jou, H. Feng, Y. Hsu, W. Huang, W. Hou, NovelCircuit Topology for Three-Phase Active Power Filter,IEEE Transactions on PD-22, No. 1, 2007, pp. 44-449.

[3] J. Klima, Analytical closed-form solution of a space-vector modulated VSI feeding an induction motor drive,IEEE Transaction on Energy Corversion, Vol. 17, No. 2,pp. 191-196, June 2002.

[4] H.Fujita, T.Yamasaki, H.Akagi, A Hybrid ActiveFilter for Damping of Harmonic resonance in IndustrialPower Systems, IEEE Transactions on PowerElectronics, VOL. 15, No. 2, pp. 215-222, March 2000

[5] H.Akagi, S.Srianthumrong, Y. Tamai Comparisons inCircuit Configuration and Filtering Performancebetween Hybrid and Pure Shunt Active Filters, IEEETransactions on Power Electronics, pp. 1195-1202,March 2003.

[6] H. Akagi, Y. Tsukamoto A. Nabae Analysis and designof an active power filter using quad-series voltage sourcePWM converters, IEEE Transactions on IndustryApplications, vol. 26No.1, pp. 93-98, 1990.

[7] J. kramlk, V. Valouch,Coupled Feedforward andFeedback Control Strategy of Parallel Hybrid Filter forHarmonic Mitigation, International Conference on Renewable Energies and Power Quality (ICREPQ05),Spain, pp. 13, March 2005.

[8] H.Akagi, E.Watanabe, M.Aredes, Instantaneous PowerTheory and Aplications to Power Conditioning, JohnWiley & Sons, New Jersey, 2007.

[9] N.Slva, A.Pomlio, E.Vendrusculo, Estratgia deControle e Anlise do Filtro Hbrido Utilizando UmInversor de Potncia Reduzida, Congresso Brasileirode Automtica (CBA-10), Mato Grosso do Sul,Setembro 2010.

artigo cobep 2011_bruno laurindo

Documents