A Noval Active Power Filter for Harmonic Supression

8/4/2019 A Noval Active Power Filter for Harmonic Supression

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Abstract

In this paper, a novel active power filter is 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 of dc 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 the proposed method. The results show that

the proposed active power filter has the

expected performance.



I. INTRODUCTION

The power electronic related

facilities may generate a large amount of harmonic 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 suppressed by using a passive or active power filter.

Conventionally, the passive power filter

is used to solve the problems of harmonic 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 power filter is connected in parallel to the load

and the generated compensation current

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

parallel active power filter has many

configurations .Among these



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 conventional parallel 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 of high 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 power converter 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



power electronic switches in the power

converter is low. As a result, the hybrid

filter is suitable to high-power applications. 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 power filter is implemented by using a voltage-

source power converter with a series

connected inductor And 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 lower voltage 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 active power 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 power converter 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 reactive power. 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



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 power converter.

. 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 the power 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 capacitor set 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 power converter. 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



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 the peak 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 smaller filter 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 smaller filter inductance. Compared to the

conventional parallel active power filter,

it shows that the proposed active power filter 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, generated by 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 filter inductance, smaller dimension, light

weight, good filter performance and low

EMI. Besides, the smaller filter inductance can improve the high

frequency response performance of this

active power filter. Since, the capacity of the 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. For the conventional active power filter, the

voltage rating of both active and passive

components must be changed. The

hardware cost of the proposed active



power filter is very competitive in the

nonlinear loads whose input is a diode-

rectifier or phase-controlled rectifier with 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 power only 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)



is added to the denominator when the

power converter generates a voltage as

the second control signal From (6), it can be 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 of load 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 for fine-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 of the impedance of the series connected

inductor and capacitor set and the

harmonic components of load current.

IV. CONTROL BLOCK DIAGRAM



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 the proposed 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 controller to reject the dc component due to the

initial condition. Since, the series

connected inductor and capacitor set is

located at the output of the power converter, 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 of the 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



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 controller and 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 modulator so 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 smaller than 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).



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 of the turbine generator are nearly

sinusoidal after applying the proposed

Active power The THD % of the turbinegenerator’s voltage and current are 4%

and 4.5%, respectively. Hence, this can

demonstrate that the proposed active power 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



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 filter is 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.

Documents

A Noval Active Power Filter for Harmonic Supression