Control Strategy for Hybrid Power Filter to Compensate Unbalanced and Non-linear, Three-phase Loads-2009

7/25/2019 Control Strategy for Hybrid Power Filter to Compensate Unbalanced and Non-linear, Three-phase Loads-2009

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Control strategy for hybrid power filter to compensate unbalanced and

non-linear, three-phase loads

S. P. Litrn, P. Salmern, J. R. Vzquez, R. S. Herrera, A. Prez

ESCUELA POLITCNICA SUPERIOR. UNIVERSITY OF HUELVAElectrical Engineering Department, Ctra. de Palos de la Frontera s/nPalos de la Frontera, Huelva, Spain

Tel.: +034 959 21 75 85Fax: +034 959 21 73 04

E-Mail: [email protected]; [email protected]; [email protected]; reyes.sanchez@[email protected];[email protected]

URL: http://www.uhu.es

Acknowledgements

This work is part of the projects "A new technique to reduce the harmonic distortion in electricalsystems by means of equipment of active compensation", ref. DPI2004-03501, sponsored by theComisin Interministerial de Ciencia y Tecnologa, CICYT, del Ministerio de Ciencia y Tecnologaof Spain, and Design and implementation of a new equipment of active compensation with seriesconnection for the improvement of the electrical waveform quality, ref. P06-TEP-02354, sponsoredby the Consejera de Innovacin, Ciencia y Empresa de la Junta de Andaluca, Spain.

Keywords

Active power filters, harmonics, instantaneous reactive power, power quality, hybrid filters

Abstract

A control algorithm is proposed for a three-phase hybrid power filter constituted by a series activefilter and a shunt passive filter. The control strategy is based on the dual formulation of the vectorialtheory of electrical power, so that the voltage waveform injected by the active filter is able tocompensate the reactive power, to eliminate harmonics of the load current and to balanceasymmetrical loads. An experimental prototype was developed and experimental results presented.

Introduction

Many social and economic activities depend on electrical energy quality and its efficiency. Bothindustrial and commercial users are interested in guaranteeing the signal quality which feed theirelectric systems, therefore, the mitigation of the current harmonics or voltage harmonics generate bythe load is necessary.

Traditionally, a passive LC power filter is used to eliminate current harmonics when it is connected inparallel with the load [1]. This compensation equipment has some drawbacks [2-4], because of which,the passive filter cannot provide a complete solution. Shunt active power filter, series active powerfilter and combined systems with passive filter and active filter have been proposed to improve thebehavior of passive filters [5-16]. A configuration with an active filter connected in series to thesource and passive filter in parallel to the load (Fig. 1) is analyzed in this work [11-14]. In thistopology, the shunt passive filter suppresses the prevalent harmonics currents produced by the load,while the active filter connected in series with the source acts as a harmonic isolator between thesource and the load, besides it compensates the reactive power and balances asymmetrical loads. Theused control strategy is based on the instantaneous power theory [17-20]. The active power filter

designed consists of a three-phase PWM (pulse With Modulation) voltage source inverter (VSI),which is connected in series with the ac source impedance and load, through three single phase

CONTROL STRATEGY FOR HYBRID POWER FILTER TO COMPENSATE

UNBALANCED AND NON-LINEAR THREE-PHASE LOADS

LITRAN Salvador P.

EPE 2009 - Barcelona ISBN: 9789075815009 P.1


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Fig. 1. Scheme of series active filter combined with shunt passive filter

iS iL

iC

+ vC -LS

LR L5 L7

C5 C7

Vdc -Vdc

CR

Ideal behaviorvPC

transformers (Fig. 1). A small rate passive filter to suppress switching ripples is connected between thetransformers and the inverter.

The control is verified through an experimental prototype, and measured are carry out to corroboratethe theoretical analysis.

Control strategy

Electrical companies try to generate electrical power with sinusoidal and balanced voltages. It hasbeen obtained as a reference condition in the supply. Due to this fact the compensation target is basedon an ideal reference load which must be resistive, balanced and linear. It means that the sourcecurrents are collinear to the supply voltages and the system will have unity power factor. Therefore atthe point of common coupling the following expression is satisfied

iv eR= (1)

Reis the equivalent resistance,vthe voltage vector at the connection point andithe load currentvector. According to Fig. 2 both vectors can be defined as

[ ] [ ]TcbaT

cba iiivvv == iv (2)

When the currents are unbalanced and non-sinusoidal, a balanced resistive load can be considered theideal reference load. So, the active power supplied by the source will be

eS RIP2

1+= (3)

Here,I1+2is the norm of the positive sequence fundamental component of the current vector. This

norm is defined by

Load

a

b

c

0

ia

ib

i0ic

va

vbvc

Fig. 2. Three-phase four-wire system



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( ) +++ =

TT

dtI'0

21 T

111ii (4)

Where i+1is the positive sequence fundamental component of the instantaneous current vector.

Compensator instantaneous power is the difference between the total real instantaneous powerrequired by the load and the instantaneous power supplied by the source. It is,

SLC ppp = (5)

When the average values are calculated in this equation and taking into account that the active powerexchanged by the compensator has to be null, the following expression is satisfied

eL RIdtpT

21

10 += (6)

Therefore, the equivalent resistance can be calculated by

21

21

1

++ ==

I

P

I

dtpTR L

L

e (7)

Here,PLis the load average power. It can be obtained by means of

== dtTdtpTPTLLL iv

11 (8)

Being vTLthe transpose voltage vector at the load side.

The aim is that the compensation equipment and load have ideal behavior from the point of commoncoupling (PCC). The voltage at the connection point of the active filter can be calculated as follows

iv2

1+

=I

PLPCC (9)

The reference signal for the output voltage of the active filter is

LLCC

*

Cvivvv ==

+21I

PLP (10)

When the active filter supplies this compensation voltage, the set load and compensation equipmentwill behave as a resistor with aRevalue.

Control scheme

The control scheme to determine the reference signal is shown in Fig. 3. The input signals are thevoltage vector, measured at the load side and the current vector measured at the source side, before thecoupling transformer. Both vectors are multiplied to obtain the instantaneous power. A low pass filterlet the load active power to be obtained. This power will be divided by the norm of the positivesequence fundamental component of the current vector. Thus, the direct sequence component of the

current vector has to be calculated. For it, the following expression is implemented in a calculationblock



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Fig. 3. Reference voltage calculation

LPF

DirectSequencecomponen

Fund.Harm.

InverseTransf.

Instant.norm

+

vL

i

pL(t) PL Re

i+ i1

+ I1

+2

vPCC

vC*

1i

Fig. 4. Fundamental component calculation

LPF

LPF

+

+ i

+ i1

+

sin

cost

( )cba iaiaii 23

1++=+ (11)

Here, aoperatoris defined as 3/2jea= , what supposes a 120 phase shift. This operator is

implemented by an all pass filter.

The fundamental harmonic of direct sequence component is determined with a block which uses thescheme shown in Fig 4. Each component of the source current vector is multiplied bysintand costwhere is the fundamental frequency in rad/s. The average values of the results are obtained usingtwo low pass filters. They are multiplied bysintand costagain and then by 2. This allows thefundamental harmonic of current direct sequence component to be obtained.

The Fortescue inverse transformation allows current vector of direct sequence fundamental componentto be obtained. It is calculated by means of the expression

[ ] [ ]TaaaTcba iaiaiiii +++++++ == 11211111i

(12)

Where, i1a+, i1b

+and i1c+are the fundamental components of the direct sequence current vector.

To calculate the norm of the +1i

vector is possible to use the equation

21

21

21

21

++++ ++= cba iiiI (12)

Instead of (4). It is due to +1i

vector is a balanced three-phase vector with sinusoidal components.

The compensation voltage has to be generated by a voltage source inverter (VSI), which uses powertransistors, therefore, a PWM (pulse With Modulation) generator is necessary, Fig. 5. So, a hysteresis



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Fig. 5. Control DC-link and PWM generator scheme

++ PI

++ +

DQ

Vdc+

Vdc

2VRef

vC*

vinv

PWM

Samplingclock

1i

Hysteresiscomparator

Table I. Passive element values

Source Ls= 2.8 mH; Rs=1.8

Passive filterL5= 13.5 mH C5= 30 FL7=6.75 mH C7= 30 F

Ripple filter Lrf= 13.5 mH Crf= 50 F

band control was developed. The gating signals are generated comparing the reference signal with theinverter output voltage (vinv) considering a hysteresis band. In this method the transistors switch whenthe error exceeds a fixed magnitude: the hysteresis band. This technique has a variable switchingfrequency, so, the IGBTs maximum switching frequency could be reached. The sampling clock signallimits the switching frequency.

Fig. 5 includes a control loop with PI controller to ensure the capacitor voltages to remain constant atthe inverter DC side. Here, VRefis the reference voltage to remain in the capacitors; Vdc+and Vdc-arethe measured voltage. The difference between the reference value of the DC-link voltage and themonitored DC-link voltages serves as an input signal to the PI controller. The output controller ismultiplied by direct sequence fundamental component vector which is calculated with the voltage vC

*.It corrects the reference voltage calculated by means of (10).

Experimental prototype

The experimental prototype scheme is shown in Fig. 6. The power circuit is a three-phase system

supplied by a sinusoidal balanced three-phase 100 V source and 50 Hz frequency with a sourceinductance of 2.8 mH and a source resistance of 1.8 . The inverter consists of an IGBT (InsulatedGate Bipolar Transistor) bridge. It is a Semikron SKM50GB123-type IGBT bridge. Two 2200 Fcapacitors are connected at the DC side. The reference voltage at the capacitors is 100 V. An LC filterhas been included to eliminate the high frequency components at the output of the inverter. Theselection criteria for ripple filter have been the following: for low frequency components the inverteroutput voltage must be almost equal to voltage across Crf. However, for high frequency componentsthe dropped voltage in Lrfmust be higher than in Crfcapacitor. Furthermore, Lrfand Crfvalues must beselected to not exceed the transformer burden. Table I includes their values. This set is matched to thepower system by means of three single-phase transformers with a turn ratio of 1:1 to ensure galvanicisolation.The passive filter is constituted by two LC branches tuned to the 5th and 7th harmonics. Element

values are included in Table I.

The control strategy was implemented in a control and general application data acquisition cardscompatible with Matlab-Simulink and developed by dSPACE. Real-Time Interface (RTI) from

dSPACE together with Real Time Workshop (RTW) from Mathworks automatically generate realtime code. It allows the processor board to be programmed and I/O boards to be selected. It is based



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Fi . 6. Series active ower and assive filter

va

vb

vc

Ls

Ls

C7 C7

Crf Crf Crf

Lrf Lrf Lrf

Non-linear

Load

C5 C5 C5 C7

L5 L5 L5 L7 L7 L7

Ls

T1

T2

T3

iSa

iSb

iSc

vLa

vLb

vLc

Vdc+

Vdc-

on the DS 1005 PPC placed in a dSPACE expansion box. The input board was the dSPACE DS 2004A/D and the output board the DS 51001 DWO. The control board has a PowerPC 750GX processorrunning at 1 GHz. For developed experimental prototype the sampling rate is limited to 50 s in orderto avoid overrun errors.

The non-linear unbalanced load consists of three single-phase uncontrolled rectifiers with an inductorand a resistor connected in series at the DC side. The inductor is the same for the three phases, 55 mH.However, the resistor was different for each phase; it was 8.3 for a phase, 12.5 for b phase and16.6 forcphase. Fig. 7 shows the three source currents without compensation equipment. A three-

phase power quality meters, Fluke 434, was used to measure the THD, harmonics and powers. Themeasured rms values for a, bandcphase are 8.7 A, 6.1 A and 5.0 A and the THD 28.5%, 27.6 % and25.6 %. The power factors are 0.91, 0.91 and 0.92 for each phase.

The neutral current is shown in Fig. 8. The rms value is 5.4 A. The fundamental harmonic rms value is3.3 A and the third harmonic is 4.1 A. They are most significant harmonics.

Fig. 9 shows the source current in phase a, b and c when the passive filter is connected. The rmsvalues are similar to previous case, they are of 8.6 A, 6.1 A and 5.0 A. However the THD values haverisen to 34.9 %, 32.4 % and 28.9 %. Although, the 5thand 7thharmonic is lower than previous

Fig 7. Source currents, system without compensate. 4 A/div. 5ms/div

Phase aPhase b

Phase c



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Fig. 8. Neutral current without compensating. 10 A/div. 5ms/div

Fig. 10. Neutral current with passive filter. 10 A/div. 5ms/div

situation, the 3rdharmonics has gone up. It is due to the presence of this harmonic in the voltage at thepoint connection common, what do to flow this component through the passive filter.

With the passive filter the neutral current has 6.7 A rms value, being the fundamental component of3.3 A, the same value that without passive filter, however, the 3rdharmonic rises from 4.1 to 5.8 A. Fig10 shows the neutral current waveform.

When the active filter is connected the source currents waveforms are sinusoidal and balanced, Fig.11, which is the aim of the control strategy. The power factor measured is the unit for the three phases.

Fig. 9. Source currents, system with passive filter. 4 A/div. 5ms/div

Phase a

Phase b

Phase c



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Fig. 11. Source currents, with passive and active filter. 4 A/div. 5ms/div

Phase a Phase b Phase c

Fig. 12. Neutral current with passive and active filter. 4 A/div. 5ms/div

The rms values are 5.7 A, 5.6 A and 5.7 A. The unbalance is due to outliers in the measured sensorsand modulation technique. The current THD are reduced to 2.6 %, 2.6 % and 2.9%.

The neutral current is shown in Fig 12. When active and passive filter are connected, the rms value isreduced to 0.5 A. The fundamental component is reduced to 0.3 A and the 3rdharmonic to 0.4 A.

Fig. 13 shows the voltage and current of the aphase. They are practically sinusoidal and in phase. Itdemonstrates the resistive behaviour of the set compensation equipment-load. Similar results areobtained to the band cphase.

Fig. 13. Voltage and current at the PCC, aphase. Voltage 48V/div and current 10 A/div. 5ms/div



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Table II summarizes the most important results. Harmonics, THD, powers and power factors areincluded. The THD voltage at the point of common coupling is improved with the active filter. Thisvoltage is distorted by the nonlinear load.The power factor is raised from 0.91 to 1. With the passive filter, the power factor is improved;however, the THD current and THD voltage are worse than without passive filter. It is due to thereactive power compensation with LC branches.

Conclusion

A control algorithm for a hybrid power filter constituted by a series active filter and a passive filterconnected in parallel with the load is proposed. The control strategy is based on the dual vectorialtheory of electric power. The new control approach achieves the following targets:

- The compensation characteristics of the hybrid compensator do not depend on the systemimpedance.

-

The hybrid filter and load set are resistive behavior. This fact eliminates the risk of overloaddue to the current harmonics of non-linear loads close to the compensated system.

- This compensator can be applied to loads with random power variation as it is not affected bychanges in the tuning frequency of the passive filter. Furthermore, the reactive power variationis compensated by the active filter.

- Series and/or parallel resonances with the rest of the system are avoided becausecompensation equipment and load are resistive behavior.

- The active filter improves the harmonic compensation features of the passive filter andcompensates the reactive power, achieving unit power factor.

- -The proposed control algorithm allows balancing asymmetrical loads.

Experimental results are presented. This allows the verification of the developed theoretical analysis.

Table II. Measured value in different situations

THD(%) RMS Fund. H3 H5 H7 H9 P(kW) Q(kvar) S(kVA) PF

Without

compensation

Phase aV 12.4 96 95.8 6.7 5.8 4.8 3.8

0.760.23(ind)

0.79 0.91I 28.5 8.7 8.3 2 1.1 0.6 0.4

Phase bV 9.1 97.1 96.7 4.4 3.9 3.6 3.1

0.540.17(ind)

0.57 0.91I 27.6 6.1 5.9 1.3 0.7 0.5 0.3

Phase cV 7.2 98.2 97.9 3.1 3.0 2.7 2.5

0.450.13(ind)

0.47 0.92I 25.9 5.0 4.8 1.0 0.6 0.4 0.3

Neutral I 5.4 3.3 4.1 0.5 0.4 0.9 - - - -

Withpassivefilter Phase a

V 11.3 97.8 97.1 9.4 3.3 1.2 2.30.79

0.05(ind)

0.79 0.93I 34.9 8.6 8.2 2.8 0.6 0.2 0.2

Phase bV 7.6 98.9 98.6 6.1 2 0.6 1.6

0.570.02(cap)

0.57 0.94I 32.4 6.1 5.8 1.8 0.4 0.1 0.2

Phase cV 5.6 100 99.8 4.2 1.3 0.5 1.3

0.470.07(cap)

0.78 0.94I 28.9 5.0 4.8 1.3 0.2 0.1 0.1

Neutral I 6.7 3.3 5.8 0.3 0.1 0.5 - - - -

Withactiveand

passivefilter

Phase aV 1.0 98.5 98.4 0.5 0.2 0.1 0.1

0.56 0.01 0.56 1I 2.6 5.7 5.7 0.1 0.0 0.0 0.0

Phase bV 1.0 98.5 98.4 0.5 0.1 0.2 0.1

0.54 0.01 0.54 1I 2.6 5.5 5.5 0.1 0.0 0.0 0.0

Phase c

V 1.2 98.8 98.8 0.5 0.1 0.1 0.2

0.56 0.01 0.56 1I 2.9 5.7 5.7 0.1 0.0 0.0 0.0Neutral I 0.5 0.3 0.4 0.0 0.0 0.0 - - - -



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Control Strategy for Hybrid Power Filter to Compensate Unbalanced and Non-linear, Three-phase Loads-2009