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 Abstract In this paper, a novel active power filter is proposed and implemented by sing a vol tage-source powe r converter wit h a series connected inductor and capacitor set. The power converter is controlled to generate a compensating voltage that is conv ert ed int o a compensat ing current via the ser ies conn ect ed ind uct or and capacitor set. The compensating current flows into the power feeder in order to suppress the harmonic currents generated by nonlinear loads. The salient advantages of the proposed active Power filter are lower voltage rating of dc ca pa ci tor and po we r swi tch in g devices, smaller filter inductor, smaller di mension, light wei ght, bett er fi lter   pe rfo rma nce and low ele ctr omag net ic interference (EMI). A three-phase 100 k VA acti ve po wer filt er is developed to demonstr at e the per formance of the  proposed method. The results show that the proposed active power filter has the expected performance.

A Noval Active Power Filter for Harmonic Supression

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    Abstract

    In this paper, a novel active power filteris proposed and implemented by sing a

    voltage-source power converter with a

    series connected inductor and capacitor

    set. The power converter is controlled togenerate a compensating voltage that is

    converted into a compensating current

    via the series connected inductor andcapacitor set. The compensating current

    flows into the power feeder in order to

    suppress the harmonic currents

    generated by nonlinear loads. The salient

    advantages of the proposed active

    Power filter are lower voltage rating ofdc capacitor and power switchingdevices, smaller filter inductor, smaller

    dimension, light weight, better filter

    performance and low electromagneticinterference (EMI). A three-phase 100 k

    VA active power filter is developed to

    demonstrate the performance of theproposed method. The results show that

    the proposed active power filter has the

    expected performance.

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    I. INTRODUCTION

    The power electronic related

    facilities may generate a large amount ofharmonic current due to the nonlinear

    input characteristic. The harmonic

    current may pollute the power systemcausing problems such as transformer

    overheating, rotary machine vibration,

    voltage quality degradation, destructionof electric power components, and

    alfunctining of medical facilities .In

    order to solve the problem of harmonic

    pollution effectively, many harmoniclimitation standards.

    The harmonic current can be suppressedby using a passive or active power filter.

    Conventionally, the passive power filter

    is used to solve the problems ofharmonic pollution in the industrial

    power system due to its low cost.

    However, it has the

    Following disadvantages:

    1) Sensitive to the variation of power

    system impedance;2) Sensitive to frequency variation of

    the utility;

    3) The risk of series/parallel resonance;4) The filter frequency is fixed, and not

    easy to adjust.

    Among those listed above, theseries/parallel resonance is the most

    serious disadvantage. It may result in

    over-current/ over voltage on the

    inductor and capacitor causing damageto the passive power filter. Since the

    system impedance has a significant

    effect on the performance of passive

    power filter, it is very hard to obtain anexcellent filter performance in practical

    applications. Moreover, the harmoniccurrent produced by neighboring

    nonlinear loads may flow into the

    Passive power filter and result in the

    overload of the passive power filter.Recently, the harmonic

    suppression facilities based on power

    electronic technique have beendeveloped. These active harmonic

    suppression facilities known as active

    power filter can suppress the differentorder harmonic components of nonlinear

    loads simultaneously According to the

    power circuit configurations and

    connections, the active power filter can be divided into parallel active filters,

    series active filters and other filter

    combinations The parallel active powerfilter is connected in parallel to the load

    and the generated compensation current

    opposes to the load harmonic current tobeing injected into the power feeder. The

    parallel active power filter has many

    configurations .Among these

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    configurations, the standard inverter type

    is widely used and discussed

    Fig.1.system configuration of active power filter

    (a) standard inverter type parallel active filter,

    (b) series active power filter, (c) hybrid power

    filter.

    Fig. 1(a) shows the system configuration

    of the standard inverter type parallel

    active power filter (conventional parallel

    active power Filter). The conventionalparallel active power filter can perform

    the harmonic current suppression,

    reactive power compensation and balancing three-phase currents. This

    filter consists of a voltage-source power

    converter and a filter inductor connectedin series. The role of the filter inductor is

    used to suppress the high frequency

    ripple current generated while switchingthe power electronic devices of the

    power converter. The inductance of the

    filter inductor depends on switching

    frequency, dc voltage, and ripple current

    limitation. The dc bus voltage must behigher than the peak value of the utility

    voltage to force the output current of the

    active power filter under the commandof compensating current in the

    conventional parallel active power filter.

    The use of high dc bus voltage has manydisadvantages such as large filter

    inductance, and high voltage rating of dc

    capacitor and power electronic devices.

    A larger filter inductor will result insignificant power loss, more heat

    dissipation, bulk dimension and weight,

    and degrades the performance of

    frequency response. The requirement ofhigh voltage rating of dc

    Capacitor and power electronic deviceslimits high power application of active

    power filters due to the high power

    rating of the power converter and cost.

    Fig. 1(b) shows the system configurationof the series active Filter. The major

    advantages of the series active filter over

    the parallel active power filter are that itcan maintain the output parallel active

    power filter, (b) series active power

    filter, (c) hybrid power filter

    Voltage waveform to be sinusoidal and

    balance the three -phase voltages.However, the series filter is less popular

    in the industrial applications due to the

    inherent drawbacks of series circuits,

    namely it must handle high loadcurrents, which increases their current

    rating compared with the parallel active

    power filtersIn some applications, the combinations

    of several types of filters can achieve

    greater benefits. The major combinationsinclude parallel active filter and series

    active filter, series active filter and

    parallel passive filter, parallel active

    filter and parallel passive filter, and

    active filter in series with parallel

    passive power filter .Among theseconfigurations, the active filter in series

    with parallel passive filter, also known

    as the hybrid power filter, is more

    widely discussed in the literature ThisConfiguration is shown in Fig. 1(c)

    where the passive filter filters the

    dominant harmonic, and the powerconverter is used to enhance the filter

    performance and to protect the passive

    filter from power resonance. Hence, thecapacity of the power converter is

    smaller than that of the parallel active

    power filter for the same nonlinear load.

    Besides, the voltage stress applied to the

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    power electronic switches in the power

    converter is low. As a result, the hybrid

    filter is suitable to high-powerapplications. However, the hybrid power

    filter requires a bulk passive power filter

    set and a voltage-matching transformer.

    Based on the type of compensation, theactive power filter can be divided into

    reactive power compensation, harmonic

    compensation, balancing of three-phase

    systems and multiple compensations.The conventional parallel active power

    filter belongs to multiple compensations,

    and it can compensate for the harmonic

    current and reactive power simultaneously .The hybrid power filter

    belongs to the harmonic compensation,and it only compensates for the

    harmonic current.

    In this paper, a novel active power filter

    is proposed. The proposed active powerfilter is implemented by using a voltage-

    source power converter with a series

    connected inductorAnd capacitor set. The proposed active

    power filter can be regarded as a new

    family of the hybrid power filter,combing a parallel active filter and an ac

    power capacitor. The proposed active

    power filter has the advantages of lowervoltage rating of dc capacitor and power

    switching devices, smaller filter inductor

    , smaller dimension, light weight, better

    filter performance and low EMI. Finally,a three-phase 100kVA prototype is

    developed to demonstrate the

    performance of the proposed activepower filter.

    II. SYSTEM CONFIGURATION

    AND OPERATION PRINCIPLE

    The system configuration of the

    proposed active power filter is shown in

    Fig. 2.

    . Fig.2.system configuration of proposed active

    power filter

    It consists of a series connected inductor

    and capacitor set, a power converter anda high frequency ripple filter. The

    voltage-mode control is used to control

    the power converter. The powerconverter generates a compensating

    voltage that is converted into a

    compensating current flowing through

    the series connected inductor andcapacitor set, and the compensating

    current flows into the power feeder in

    order to filter harmonic currentsgenerated by nonlinear loads. The

    configuration of the proposed active

    power filter is similar to that of thehybrid power filter in the first view.

    However, the function and dimension of

    the passive elements (L-C) are not the

    same. In the proposed method, theinductor of the series connected inductor

    and capacitor set is very small, and it is

    used to filter the switching ripple of the

    power converter. The capacitor in theseries connected inductor and capacitor

    set is used to supply a fixed reactivepower. However, the passive elements

    (L-C) in the hybrid power filter are used

    to tune the dominant harmoniccomponent of the load current. The

    inductance in the hybrid power filter is

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    larger than that used in the proposed

    active power filter, then, the dimension

    and weight of the inductor used in thehybrid power filter are also larger than

    that used in the proposed active power

    filter. The high frequency ripple filter isconfigured by a set of capacitors and

    resistor, and it uses to further filter out

    the switching ripple of the powerconverter.

    . Fig.3.Equivalent circuit of proposed active

    power filter

    Fig. 3 shows the equivalent circuit of the

    proposed active power filter. It consistsof two voltage sources, one is the utility

    and the other is the power converter. The

    compensating voltage generated by thepower converter is a dependent voltage

    source whose voltage depends on the

    harmonic components of the load

    current. The equivalent circuit shown in

    Fig. 3 can be further divided into thefundamental frequency equivalent circuit

    and the harmonic frequency equivalentcircuit as shown in Fig. 4.

    Fig.4.Equivalent circuit of proposed active power filter, (a) fundamental equivalent circuit

    (b) harmonic equivalent circuit.

    Fig. 4(a) shows the equivalent circuit

    under the fundamental frequency .If the

    power loss is negligible; thecompensating voltage generated by the

    power converter only contains harmonic

    components. Hence, the voltage source

    of the power converter can be regardedas a short circuit under the fundamental

    frequency. Also, it is evident that the

    impedance of the series connectedInductor and capacitor set is capacitive

    under the fundamental frequency.

    Hence, the series connected inductor andcapacitor set performs the fixed reactive

    power compensation. Since the

    inductance of the series connected

    inductor and capacitor set isVery small in the proposed active

    power filter, the compensating reactive

    power can be approximated as

    (1) Is the capacitance of the series

    connected inductor and capacitor set,

    and is the RMS value of theutility line voltage is. Fig. 4(b) shows the

    equivalent circuit under harmonic

    frequency. If the frequency is lower thanthe tuned frequency, the

    Series connected inductor and capacitor

    set is capacitive. On the contrary, the

    series connected inductor and capacitorset is inductive if the frequency is higher

    than the tuned frequency.The switching frequency of the power

    converter is significantly higher than the

    tuned frequency of the series connected

    inductor and capacitor set. As a result,the series connected inductor and

    capacitor set acts as an inductor to filter

    the switching frequency of the powerconverter. For suppressing the load

    harmonic current, he desired

    compensating voltage can be derived as

    (2) Where is the harmonic

    component of the load current, is

    the impedance of the series connected

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    inductor and capacitor set. If the power

    converter can generate a voltage as

    shown in (2), then this voltage isconverted into a compensating current

    that is opposite to the load harmonic

    current. Hence, the load harmoniccurrent can be suppressed. As shown in

    Fig. 4(a), the fundamental component of

    utility voltage drops on the seriesconnected inductor and capacitor set,

    hence, the compensating voltage

    generated by the power converter

    consists only the harmonic components.In addition, (2) shows that the desired

    compensation voltage is dependent on

    the load harmonic current and the

    impedance of the series connectedinductor and

    Capacitor set. This value is smaller thanthe peak value of the utility voltage.

    From the operation theory of the bridge

    power converter, the dc bus voltage of a

    power converter must be higher than thepeak value of the compensating voltage.

    Because the peak value of compensating

    voltage is smaller than that of the utilityvoltage, the dc bus voltage in the

    proposed active power filter can be

    reduced significantly as compared withthe conventional parallel active power

    filter whose voltage must be higher than

    the peak value of the utility voltage.Consequently, the voltage rating of dc

    capacitor and power electronic devices

    can also be reduced. Besides, the ripple

    current of the power converter isdependent on the dc bus voltage and

    filter inductance. This implies that the

    lower the dc bus voltage, the smallerfilter inductance required for specified

    ripple current limitation. Therefore, the

    filter inductance used in the seriesconnected inductor and capacitor set is

    smaller due to the lower dc bus voltage.

    Besides, the high frequency response of

    the proposed active power filter is better

    than that of the conventional parallel

    Active power filters due to the smallerfilter inductance. Compared to the

    conventional parallel active power filter,

    it shows that the proposed active powerfilter uses three additional ac capacitors

    to reduce the inductance of filter

    inductor. In practice, the core of aninductor with large inductance is made

    from the iron alloy, which results in the

    bulky volume, heavy weight and large

    loss. The core of an inductor with smallinductance can be made from the ferrite

    materials, which have the characteristics

    of small volume, light weight and low

    eddy current loss .The EMI, generatedby the switching of power converter, is

    also dependent on the dc bus voltage.Therefore, the salient advantages of the

    proposed active power filter are low

    voltage rating of dc capacitor and power

    switching devices, smaller filterinductance, smaller dimension, light

    weight, good filter performance and low

    EMI. Besides, the smaller filterinductance can improve the high

    frequency response performance of this

    active power filter. Since, the capacity ofthe dc bus voltage is dependent on the

    amount of compensation current and not

    the utility voltage, the application of the proposed active power filter could be

    extended to a wider voltage range. In the

    limited variable voltage application,

    such as 220 V to 480 V, the change inthe main components is only the voltage

    rating of series connected inductor and

    capacitor set. In addition, the proposedactive power filter can be applied in

    50/60 HZ power systems only adjusting

    the parameters of the control circuit. Forthe conventional active power filter, the

    voltage rating of both active and passive

    components must be changed. The

    hardware cost of the proposed active

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    power filter is very competitive in the

    nonlinear loads whose input is a diode-

    rectifier or phase-controlled rectifierwith a low level voltage below 480 V.

    The conventional parallel active power

    filter can supply the reactive power asthe variation of the load, and performs

    the unity power factor compensation.

    Nevertheless, the hybrid active poweronly supplies a fixed reactive power.

    This results in the leading power factor,

    as the load condition is light. The

    reactive power compensation performance of the proposed active

    power filter is similar to the hybrid

    power filter supplying a fixed reactive

    power.

    III. CONTROL THEORY

    Conventionally, the active power filter

    was controlled by the current-mode.

    However, it is hard to be implementedunder low filter inductance due to the

    high switching ripple, and it may

    generate multiple crossing during acarrier period of pulse-width modulator.

    The phenomenon of multiple crossing

    will result in more than one switchingoperation during a carrier period. In the

    proposed active power filter, the voltage-

    mode control is used. The three-phase power converter controlled by the

    voltage-mode control acts as a voltage

    amplifier with the gain represented by

    (3)

    Is the dc bus voltage and is theamplitude of the carrier signal of the

    pulse-width modulator. Hence, thecontrol circuit of the voltage-mode

    controller is used to determine a

    reference voltage by dividing the desiredcompensating voltage by the gain shown

    in (3). From the above section, it can be

    found that the desired compensating

    voltage generated by the power

    converter is derivedFrom (2). Hence, the first control signal

    where can be further derived from (2),

    and it is represented as

    (4)

    Where L and C are the inductance andcapacitance of the series connected

    inductor and capacitor set respectively,

    and R is the stray loss of active power

    filter. If the power converter cangenerate a harmonic voltage equal to the

    first control signal and convert into a

    compensating current by the series

    connected inductor and capacitor set, theharmonic components of the load current

    can be compensated theoretically. In practice, the filter performance is

    degraded due to the parameters of the

    series connected inductor and capacitor

    set that may be varied due to age,variable frequency, production and

    temperature. For improving the

    compensating performance, the secondcontrol loop must be used to modify the

    error of compensating result. Theconcept of the second control loop is based on the theory of conventional

    hybrid power filter .The second control

    signal is obtained by detecting the

    harmonic components of the utilitycurrent and then amplifying with a gain ,

    and it can be represented as

    (5)Where is the harmonic component of the

    utility current? If the power converter

    can generate a voltage equal to thesecond control signal, the utility

    harmonic current can be derived from

    Fig. 4(b) and represented as

    (6)

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    is added to the denominator when the

    power converter generates a voltage as

    the second control signal From (6), it canbe found that a term . Hence, the second

    control loop is used to control the power

    converter to act as a virtual harmonicresistor. The virtual harmonic resistor is

    in series with the utility to block the

    uncompensated harmonic components ofload current flowing back to the utility.

    In the proposed active power filter, the

    first control loop acts as rough tuning,

    and the second control loop is used forfine-tuning. Due to the use of voltage-

    mode control in the proposed active

    power filter, the series connected

    inductor and capacitor set may result inhigh frequency oscillation between the

    utility and the active power filters.Hence, the third control loop is applied

    to avoid high frequency oscillation. The

    third control loop is used to generate a

    virtual harmonic resistor to be connectedin series with the series connected

    inductor and capacitor set. The virtual

    harmonic resistor acts as a harmonicdamper. It can be realized by using the

    power converter to generate a harmonic

    voltage that is proportional to theharmonic components of the active

    power filter current. Hence, the third

    control signal can be represented as

    (7)

    Where the current harmonic componentof the active power filter is. Hence, the

    power converter can act as a virtual

    harmonic resistor. A dc capacitor locatedat the dc bus of the voltage-source power

    converter is used to supply a dc voltage

    to the power converter and act as anenergy buffer. The dc bus voltage is

    expected to be a constant voltage.

    However, the virtual harmonic resistor inthe second and third control loops and

    the switching loss of power converter

    will consume the real power. Then, the

    voltage variation at the dc bus cannot be

    avoided. To maintain a constant dc bus

    Voltage, the fourth control loop is used.The voltage regulation of the dc bus

    voltage can be obtained by using the

    power converter to generate afundamental voltage in phase or out-of-

    phase with the fundamental component

    of the active power filter current. Thefourth control signal can be represented

    as

    (8)

    Where the fundamental component of

    the active power filter is current. Then,the power converter acts as a

    positive/negative fundamental resistor to

    absorb/regenerate the real power from/tothe utility, so as to maintain the dc bus

    voltage at a constant value. From the

    above, the reference voltage of thecontrol circuit is the summation of the

    first, second, third, and fourth control

    signals, and it can be represented as

    (9)In (9), the first control signal is the

    dominant component. Hence, the

    compensating voltage generated by the

    power converter is almost the product ofthe impedance of the series connected

    inductor and capacitor set and the

    harmonic components of load current.

    IV. CONTROL BLOCK DIAGRAM

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    It consists of four control loops. From

    Fig. 5, it can be found that four feedback

    signals, namely the load current, theutility current, the output current of

    power converter and the dc bus voltage

    are used in the control circuit of theproposed active power filter to calculate

    the reference voltage of the power

    converter. The first control loop is usedto implement the product of harmonic

    components of the load current and the

    impedance of the series connected

    inductor and capacitor set shown in (3).The load current is detected and sent to

    the band-rejection filter I to filter out its

    fundamental component. From (3), it can

    be found that the product of harmoniccomponents of the load current and the

    impedance of the series connectedinductor and capacitor set can be

    obtained by feeding the harmonic

    component of load current to a

    Proportional Integral Differential (PID)controller.

    TABLE I

    MAJOR PARAMETERS OF

    PROTOTYPE

    Fig.5. Control block diagram of proposed active

    power filter

    The proportional, integral anddifferential coefficients are the resistor

    R, capacitor C and inductor L shown in

    (3), respectively. Then, the output of the

    first control loop is obtained. For

    TABLE II

    COMPARISON RESULT

    Avoiding the effect of noise, a low-pass

    filter is used in the front of Differential

    controller and a high-pass filter is

    inserted at the end of Integral controllerto reject the dc component due to the

    initial condition. Since, the series

    connected inductor and capacitor set is

    located at the output of the powerconverter, the capacitor also can block

    the dc component due to the initialcondition. Hence, the effect of the initial

    condition caused by the switch-on of the

    proposed active power filter can besuppressed. For improving the

    compensating performance, the second

    control loop is used to modify the error

    of the compensating results of the firstcontrol loop. In the second control loop,

    the detected utility current is sent to the band-rejection filter II to filter out thefundamental component. Then, the

    uncompensated harmonic components of

    the utility current are obtained. Theoutput of the band-rejection filter II is

    fed to the amplifier I to

    Obtain the output of the second control

    loop.

    The third control loop is used to generate

    a virtual harmonic resistor to beconnected in series with the series

    connected inductor and capacitor set to

    act as a damper. The output current ofthe power converter is sent to a band-

    pass filter to obtain the fundamental

    component, and then, the detected output

    current of the power converter and its

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    fundamental component are fed to a sub

    tractor to obtain the harmonic

    components. The harmonic componentsare fed to amplifier II to obtain the

    output of the third control loop.

    The fourth control loop is used toregulate the dc bus voltage. The fourth

    control loop comprises a low-pass filter

    to filter out the dc bus voltage ripple anda subtract or to subtract a setting value

    from the output of low-pass filter, then,

    the sub tractor result is sent to a PI

    controller. The output of the band-passfilter is the fundamental component of

    active power filter current, and the

    output of the fourth control loop is the

    product of the output of the PI controllerand the output of the band-pass filter.

    Finally, the modulated signal can beobtained by summing the outputs of the

    first, the second, the third and the fourth

    control loops. Then, the modulated

    signal is sent to a pulse-width modulatorso as to drive the power switching

    devices of the power converter.

    V. EXPERIMENTAL RESULTS

    For demonstrating the performance of

    the proposed active power filter, a three-

    phase 100KVA prototype is developed.

    The major parameters of the prototype

    are shown in Table I.The utility power is supplied by a three-

    phase three-wire utility system with 380

    V and 60 Hz. A comparison of the proposed active power filter and the

    conventional parallel active power filter

    is shown in Table II. Since theinductance of series connected inductor

    and capacitor set is only 58 H, a ferrite

    core can be used to reduce the power

    Loss, weight and volume. Hence, the

    volume and weight of the proposed

    active power filter is evidently smallerthan that of the conventional parallel

    active power filter. In addition, thehardware cost is also reduced

    significantly. The tested load is a 300

    Kva UPS with a six-pulse rectifier

    charger. Fig. 6 shows the experimentalresult of the proposed active power filter

    in the steady state. The load current

    shown in Fig. 6(c) is rich in harmonics;its total harmonic distortion (THD) is

    51%. However, the THD of the utility

    Fig.6. Experimental result of proposed active

    power filter under steady state, (a) utility

    voltage,(b) utility current, (c)load current, and

    (d) output current of active power filter.

    Current after compensating by the proposed active power filter is only

    4.5%. The waveform of the utility

    current after compensating by the proposed active power filter is nearly

    sinusoidal. The test result shows that the

    Harmonic suppression performance of

    the proposed active power filter isexcellent. Fig 7 shows the experimental

    result of the proposed active power filter

    under switching-in the nonlinear load.As seen in Fig. 7(b).

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    Fig.7. Experimental result of proposed active power filter under steady state, (a) utility

    voltage,(b) utility current, (c)load current, and

    (d) output current of active power filter

    The utility current is still nearly

    sinusoidal under the transient duration. Itverifies that the transient performance of

    the proposed active power filter is

    excellent.

    In the industrial distribution power

    system, a turbine generator is often usedas a back-up power. Because the

    capacity of the turbine generator is not

    large enough, the power source of theturbine generator can be regarded as a

    weak power source.

    Fig.8. Experimental result under weak power

    source before applying active power filter.

    Fig.9. Experimental result under weak power

    source before applying active power filter.

    The experimental results shown in Figs.8 and 9 are tested under the condition of

    weak power source. The weak power

    source is supplied by an 800 kva turbinegenerator. The load used in the test is the power equipment using a three-phase

    rectifier in the input port. As seen in Fig.

    8, the voltage waveform of the turbinegenerator is distorted seriously due to the

    nonlinear load.

    The THD of the utility voltage and theutility current are 12% and 25%

    respectively. The distorted utility voltage

    may disturb the normal operation of

    power equipment itself or theneighboring load using the same power

    source. Fig. 9 shows that both

    waveforms of the voltage and current ofthe turbine generator are nearly

    sinusoidal after applying the proposed

    Active power The THD % of the turbinegenerators voltage and current are 4%

    and 4.5%, respectively. Hence, this can

    demonstrate that the proposed activepower filter not only can suppress the

    input current harmonics but also avoidthe waveform distortion of the voltage

    under nonlinear loads.

    VI. CONCLUSION

    Recently, many power electronic

    application technologies have been used

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    to replace the role of conventional

    passive elements in the distribution

    power system for solving the problemsof power quality. The active power filter

    is used to solve the harmonic problems

    in the industrial power system, and this becomes popular gradually. However,

    the wide use of the active power filter is

    still limited due to the high cost and the power rating of power electronics. In

    this paper, a novel active power filter is

    proposed. The proposed active power

    filter has the advantages of lower voltagerating for dc capacitor and power

    switching devices, smaller filter

    inductor, smaller dimension, light

    weight, better filter performance and lowelectromagnetic interference (EMI). A

    three-phase 100 kva active power filteris developed to demonstrate the

    performance of the proposed method.

    The experimental results show that the

    proposed active power filter hasexcellent performance in suppressing

    harmonic current. The hardware cost of

    the proposed active power filter is verycompetitive in nonlinear loads whose

    input is a diode-rectifier or phase-

    controlled rectifier with a low levelvoltage below 480 V.