cascaded multilevel voltage source inverter based active power filter for harmonics and reactive...

7/31/2019 Cascaded Multilevel Voltage Source Inverter based active power filter for Harmonics and Reactive power compens

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INTERNATIONAL JOURNAL OF APPLIED ENGINEERING RESEARCH, DINDIGUL

Volume 1, No 4, 2011

Copyright 2010 All rights reserved Integrated Publishing Association

RESEARCH ARTICLE ISSN - 0976-4259

661

Cascaded Multilevel Voltage Source Inverter based active power filter for

Harmonics and Reactive power compensationKaruppanan P

1, Rajasekar S

2, KamalaKanta Mahapatra

1

1- National Institute of Technology- Rourkela, India-769008

2- Motilal Nehru National Institute of Technology-Allahabad, [email protected]

ABSTRACT

This paper presents a cascaded multilevel Voltage Source Inverter (VSI) based shunt Active

Power Filter (APF) for current harmonics and reactive power compensation due to non-linear

loads. The proposed approach comprises Low Pass Filter (LPF) in conjunction with

Proportional Integral (PI) controller which is used to estimate the peak reference current and

maintain the DC-bus capacitor voltage of the cascaded inverter nearly constant. The cascaded

multilevel active filter switching signals are derived from Triangular-carrier Current

Controller (TCC), Periodical Sampling Current Controller (PSCC) and proposed Triangular

Periodical Current Controller (TPCC). It gives better dynamic performance under transient

and steady state operating conditions. The APF system is validated through extensive

simulation under steady state and transient condition with different non-linear loads.

Comparative assessments of different current controllers are carried out. These simulation

results reveal that the cascaded active filter effectively compensates the current harmonics

and reactive volt amperes to improve the power quality.

Index Terms-- Active Power filter (APF), Proportional (PI) controller, Triangular periodical

current controller, Harmonics, Power quality

1. Introduction

Power quality and custom power have become topics of research interest because ofwidespread use of non-linear loads such as diode/thyristor rectifiers, SMPS, UPS, induction

motor drives etc (Alexander, 1993). These non-linear loads effect in harmonic or distortioncurrent and reactive power problems (Joseph, 1990). The harmonics induce malfunctions in

sensitive equipment, overvoltage by resonance, increased heating in the conductor, harmonic

voltage drop across the network impedance and affect the other customer loads at the point of

common coupling (W.M.Grady, 1990). Traditionally passive filters have been used to

compensate the harmonic distortion and the reactive power but passive filters are large in

size, have aging and tuning problems and resonate with the supply impedance (F.Barrerro,

2000). Recently Active Power Line Conditioners (APLC) or Active Power Filters (APF)

overcome these problems and are used for compensating the current-harmonics and

suppressing the reactive power simultaneously due to fluctuating loads (Bhim Singh, 1999).

The controller is the most important part of the active power filter and lot of research is being

conducted in this area (H.Akagi et.al., 1982). Conventional PI and Proportional Integral

Derivative (PID) controllers have been used to estimate the peak reference currents and

control the dc-side capacitor voltage of the inverter (A.Chaouhi, 2007). Most of the active

filter systems use PI-controller for maintaining the dc-side capacitor voltage. When thesource supplies a non-linear or reactive load, it is expected to supply only the active


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fundamental component of the load current and the compensator supplies theharmonic/reactive component (Helder et.al.,,2008). The outer capacitor voltage loop will try

to maintain the capacitor voltage nearly constant which is also a mandatory condition for the

successful operation of the APF. The low pass filter design followed PI-controller is proposedfor controlling the capacitor voltage and estimate the required reference current (Helderet.al.,,2008). The effectiveness of active power filter depends on the design and

characteristics of current controller (D.M.Brod, 1985) . Most of the current controller

techniques used in active filter is based on Pulse Width Modulation (PWM) control strategies.

There is various PWM current control strategies proposed for active power filter (H.Akagi

et.al., 2007). Three-different methods such as triangular-carrier, periodical sampling and

proposed triangular periodical current controllers are discussed for active power filter

(J.Dixon et.al., 1996).

This paper presents a proportional integral controller based cascaded multilevel shunt active

power filter for the harmonics and reactive power mitigation of the non-linear loads. The

cascaded multilevel H-bridge active power filters have been widely used for power qualityapplications due to increase in the number of voltage levels, low switching losses, low

electromagnetic compatibility for hybrid-filters and higher order of harmonic elimination(K.Corzine, 2002). The cascade M-level inverter consists of (M-1)/2 H-bridges and each

bridge has separate dc source (S.J.Huang and J.C.Wu, 1999). The cascaded inverter gateswitching signals are generated using TCC, PSCC and proposed triangular-periodical current

controllers. It provides a dynamic performance under transient and steady state operatingconditions. The compensation process is based on sensing main currents and multiplies with

estimated peak reference current using proportional integral controller by controlling the DCside capacitor voltage of the cascaded inverter. The shunt APF system is validated through

extensive simulation and it is investigated in terms of harmonics and VDC settling time understeady state and transient condition with different non-linear loads. Comparative assessments

of three different current controllers are disclosed.

2. Design of Shunt APF system

Cascaded active filter for power line conditioning system is connected in the distribution

network at the point of common coupling through filter inductances and operates in a closed

loop. The three phase active power filter comprises of 24-power transistors with

freewheeling diodes, each phase consist of two-H-bridges in cascaded connection and every

H-bridges having a dc capacitor. The shunt APLC system contains a cascaded inverter, RL-

filters, a compensation controller (unit current vector with proportional integral controller)

and switching signal generator (TCC or PSCC or TPCC) as shown in the Figure 1. The

instantaneous source current is )()()( tititi cLs -= and the instantaneous source voltage

is tVtv ms wsin)( = .


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Figure 1: shunt active power line conditioners system

The nonlinear load current contains the fundamental and harmonic current components,

which can be represented as

)1()sin()sin(

)sin()(

2

11

1

F++F+=

F+=

=

=

n

nn

n

nnL

tnItI

tnIti

ww

w

The instantaneous load power can be computed from the source voltage and load current and

the calculation is given as

)2()()()(

)sin(*sin

sin*cos*sincos*sin

)(*)()(

2

1112

tptptp

tnItV

ttIVtV

tvtitp

hrf

n

nnm

mm

ssL

++=

F++

+=

=

=

ww

fwwfw

This load power contains fundamental (active power), reactive power and harmonic power.

From this equation (2), the real (fundamental) power drawn from the load is

)3()(*)(cos*sin)( 12

1 titvtIVtp ssmf == fw

If the active power filter provides the total reactive and harmonic power, )(tis will be in a

phase with the utility voltage and will be sinusoidal. At the time, the active filter must

provide the compensation current )()()( tititi sLc -= The Current harmonics is achieved by

injecting equal but opposite current distortion components at the point of common coupling,

24VDC,ref

isa*,isb*,isc*

isa, isb, isc

ica,icb,icc

Rs,Ls

Reference current generator

Current

Sensor

Vdc Sensor

CDC

iLa, iLb, iLc RL

LL

Non-sinusoidal Load3-phase supply

TCC or PSCC or TPCC

Proportional Integral

(PI) Controller

Cascaded VSI

Unit current vector

vsa, vsb, vsc


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there by canceling the original harmonic and make sinusoidal in the supply source thatimproving the power quality on the connected power distributed system.

2.1. Power Converter

A cascaded multilevel active power inverter is constructed by the conventional of H-bridges.

The three phase active filter comprises of 24-power transistors and each phase consists oftwo-H-bridges in cascaded method for 5-level output voltage, shown in Fig 2. Each H-bridge

is connected a separate dc-side capacitor and it serves as an energy storage elements to

supply a real power difference between load and source during the transient period. The

capacitor voltage is maintained constant using PI-controller. Each H-bridge can produce three

different voltage levels VdcVdc -+ ,0, by four-switching operations. The ac-output of the

each H-bridge is connected in series such that the synthesized output voltage waveform is thesum of all the individual H-bridges.

Figure 2: Design of cascaded multilevel active power filter

The 24-IGBT switching operations are performed using proposed triangular carrier current

modulator and harmonics it is achieved by injecting equal but opposite current harmoniccomponents at a point of common coupling.

3. Proposed Control Strategies

The block diagram of the proposed control system is shown in Figure 3 it consists of twosections. One is reference current control strategy using unit current vector with PI-controller.Another is triangular current controller, periodical current controller and proposed triangular

carrier current modulator for switching signals of cascaded multiple voltage source inverter.A comparative assessment of these three-different current controllers is carried out.

Cdc

Cdc

CBA


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current maxI and controls the dc-side capacitor voltage of cascaded multilevel inverter. It

transferred by the function which is represented as,

)5(/)( sKKsH IP+=

Where, [ PK =0.7] is the proportional constant that determines the dynamic response of the

DC-side voltage control and [ IK =23] is the integration constant that determines its settling

time. The proportional integral controller is eliminating the steady state error in the DC-side

voltage. The PI controller is estimated the magnitude of peak reference current maxI and

current maxI takes response of the active power demand of the non-linear load and losses in

the distribution system. The peak reference current multiply with output of unit current vectorand has determined the desired reference current.

3.2. Current Controller

The effectiveness of an active power filter basically depends on the design characteristics of

the current controller. Most of the current control techniques used in active power filters is

based on Pulse Width Modulation (PWM) strategies. In this section, periodical sampling

current controller, triangular-carrier current controller, and triangular periodical current

controller methods are analyzed and execute the features, simplicity and effectiveness of the

each controller for active power converter applications.

3.2.1. Triangular-carrier Current Controller (TCC)

The triangular carrier current controller is one of the familiar methods for active power filter

applications to generate gate control switching pulses of the voltage source inverter. To

determine the switching transitions by means the error current [desired reference current (ia*)compared with the actual source current (ia)] is multiplied with proportional gain (Kp). The

output signal of the proportional gain is compared with triangular carrier signal.

Figure 5(a): Triangular-carrier current controller

The four triangular signals are generated same frequency with different amplitude for

cascaded multilevel inverter, because each phase in one converter does not overlap other

phase shown in Figure 5 (a). Thus the switching frequency of the power transistor is equal to

the frequency of the triangular carrier signal.

Kp

a

a*

.5

0.5

0

0.5

G1

G2NOT

G1

G2NOT

G1

G2NOT

G1

G2NOT


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667

3.2.2. Periodical Sampling Current Controller (PSCC)

The periodical sampling current control method is used to generate gate control pulses of the

active power converter. In each transition, a comparison between actual current (ia) andreference current (ia*) is made. Then, multiplies with proportional gain (Kp) for better

dynamic performance under transient conditions.

Figure 5 (b): Periodical sampling current controller

The output signal of the proportional gain is sampled and held D-Latch at a regular interval

Ts synchronized with the clock of frequency. The 4-external clock applied to each converter

andTs is set as 30 ns, because each phase in one converter does not overlap other phase, as

shown in Figure (b). The gate control switching pulses of ON and OFF depends on the falling

and rising edge of the clock. This type of control is very simple to implement: only a

comparator and a D-flip flop are needed per phase. The main advantage of this method is that

the minimum time between switching transitions is limited to the period of the samplingclock.

3.2.3. Triangular Periodical Current Controller (TPCC)

The proposed triangular-periodical current controller for active power filter line currents canbe executed in order to generate the switching pattern of the cascaded voltage source inverter.

These inverters provide features like quick current controllability, the suppression of theharmonics induced due to switching operation. The average switching frequency of each

inverter is equal and unconditioned stability. The five-level inverter systems of the currentcontroller are utilized independently for each phase. Each current controller directly generates

the switching signal of the three A, B, C phases. The A-phase actual source current

represented as Isa and reference current represent as isa* as shown in Figure 5 (c), similarlyderived the B and C phase currents. To determine the switching frequency by means the error

current is multiplied proportional gain (Kp) and compared with triangular carrier signal. The

four triangular signals are generated same frequency with different amplitude for cascaded

inverter. Thus the switching frequency of the power transistor is equal to the frequency of the

triangular carrier signal. Then, the output signal of the comparator is sampled and held D-

Latch at a regular intervalTs synchronized with the clock of frequency equal to Ts/1 . Note

that 4-external clock applied to each converter andTs is set as 30 ns, because each phase in


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668

one converter does not overlap other phase. The active power filter suppresses the harmonicscaused by the switching operation of the cascaded multilevel inverter.

Figure 5(c): Triangular periodical current controller

4. Result and Analysis

The performance of the PI-controller based cascaded active power filter is evaluated through

extensive simulation using Matlab/Sim power tools in order to model and test the system.

The cascaded multilevel voltage source inverter is constructed by the 24-power transistors for5-level output voltage. The 24-transistor switching pulses are generated using triangular

carrier, periodical sampling and proposed triangular-periodical current modulator. These

three different current controllers are simulated and investigated. The system parameters

values are in Table 1.

Table 1: System parameters

Parameters Values

Line to line source voltage (Vm) 440 V

System frequency (f) 50 Hz

Source impedance: Source resistor (RS)

Source inductor (LS)

1

0.5 mHNon-Linear Load: Diode rectifier

Load resistor (RL)

Load inductor (LL)

6-diode

20

100 mH

Filter: Inductor (LF)

Resistor (RF)

1 mH

1

DC-side capacitance (CDC) 2100 F

Reference voltage (VDC, ref) 150 V

Power Converter 24-IGBTs/diodes


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Case 1: Steady state

The diode rectifier load connected in the ac-grid and cascaded active filter joint in which its

parallel at PCC for inject the current harmonics. The rectifier R-L load current or sourcecurrent before compensation is shown in Figure 6 (a). PI-controller is used estimate the

magnitude of peak reference current maxI by controlling dc-side capacitor voltage of the

cascaded inverter. The peak reference current is multiplied with a unit current vector output

and determined the reference current that is shown in figure 6 (b).

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0. 2-80

-60

-40

-20

0

20

40

60

80

Time in Second

Loa

d

C

urren

tin

Am

p

iLa

0.02 0.04 0.06 0.08 0. 1 0.12 0.14 0.16 0.18 0.2-80

-60

-40

-20

0

20

40

60

80

Time in Second

R

ef

erence

C

urren

tin

Am

p

isa,ref

Figure 6: Simulation results (a) source currents before compensation or load current (b)extracted reference current.

The effectiveness of an APF basically depends on the design characteristics of the current

controller. TCC, PSCC and TPCC methods are used and executed for active power converter.The harmonic currents are reduced as if the switching frequency was increased. The

switching frequency of the each controller is almost same. TPCC is giving better performance

than TCC and PSCC in terms of transient response and harmonic contents. Fig 7 is shown the

gate control pulses of TCC, PSCC and TPCC methods.

0 0. 01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1TCC-Gate Control Pulses

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0. 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1PSCC-Gate control Pulses

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1TPCC-Gate Control Pulses

Figure 7: (a) TCC-gate pulses (b) PSCC-gate pulses and (c) TPCC-gate pulses

These gate control pulses apply to active filter for compensation current or harmonic current

as shown in Fig 8(a). Consequently current harmonics is achieved by injecting equal butopposite current harmonic components at the PCC by canceling the original distortion. Thesimulation result of source current after compensation is presented in Fig 8(b) that indicates

the current is sinusoidal. We have additionally achieved power factor correction as shown inFig 8(c) that result indicate a-phase voltage is in- phase with a-phase current.

(a) (b) (c)

(a) Time in Second

(a) (b)


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670

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-40

-30

-20

-10

0

10

20

30

40

Time in Second

Com

pensa

tio

n

Cu

rren

tin

Am

p

ica

0.02 0.04 0.06 0.08 0.1 0 .12 0.14 0.16 0.18 0.2-80

-60

-40

-20

0

20

40

60

80

Time in Second

Source

C

urrentin

Am

p

isa

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-400

-300

-200

-100

0

100

200

300

400

Time in Second

Source

Vo

ltage

an

d

Curren

t

isa

Vsa

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-20

0

20

40

60

80

100

120

140

160

180

200

Time in Second

D

C-

sid

e

C

apac

itor

Vo

ltage

Cdc

Figure 8: (a) Compensation current, (b) Source current after active filter (c) unit power factorand (d)DC-side capacitor voltage

These figures are focused in A-phase only other phases are just phase shifted by 1200

The DCside capacitors voltage is controlled by proportional integral (PI) controller. The PI-controller

maintains the capacitors voltage with small ripple in steady and dynamic conditions, shown

in Figure 8 (d). It serves as an energy storage element to supply a real power to operate three-

phase cascaded inverter.

Case 2 Transient condition

The three-phase 6-pulse diode rectifier (non-linear) load current or source current before

compensation is shown in Figure 9(a) that indicate the source current is distorted or having

harmonic currents. The harmonic compensation is achieved by cascaded active power filter,

injecting equal but opposite current harmonic components at PCC. The three-phase source

current after compensation is shown in Figure 9(b) this indicates the current is sinusoidal.

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-400

-300

-200

-100

0

100

200

300

400

Times in seconds

SourceVoltageinvolt

Vsa

Vsb

Vsc

(a) (b)

(c) (d)

(a)


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0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0 .2

-100

-80

-60

-40

-20

0

20

40

60

80

100

Times in Seconds

Sourcecu

rrentinAmp

isa

isb

isc

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

-100

-80

-60

-40

-20

0

20

40

60

80

100

Times in Seconds

LoadCurrentinAmp

iLa

iLb

iLc

Figure 9: (a) Source Voltage (b) Source current after active filter and (c) Load currents

The Fast Fourier Transform (FFT) is used to measure the order of harmonics with thefundamental frequency 50 Hz at the source. These orders of the harmonics are plotted under

steady state conditions in the distribution supply current. The order of the harmonics plottedin Figure 10 (a), without active power filter it indicate 5th 7th 11th and 17th order of harmonics

which is presented and lower order harmonics are absent. The order of the harmonics plotted

with active power filter, is shown in Figure 10 (b) this evidence proof indicates cascaded

active power filter is compensation the harmonics uptown less than 5%.

0 2 4 6 8 10 12 14 16 180

5

10

15

20

25

Orderof Harmonic

M

agn

itu

de

base

d

on

"B

as

e

Pea

k"

-P

aram

ete

r

0 2 4 6 8 10 12 14 16 180

5

10

15

20

25

Order of Harmonic

M

agn

itu

de

base

d

on

"Base

Pea

k"-

Parame

ter

Figure 10: Order of harmonics (a) the source current without active filter (THD=25.17%),

(b) with active power filter(THD=2.76%)

The Total Harmonic Distortion (THD) measured from the source current on the distribution

system. The proportional integral controller based compensator filter made linear source

current to the supply. The total harmonic distortion measured and compared is shown in

Table 2.

Table 2: THD measured without APF and with APF

with APFTHD without APF

TCC PSCC TPCC

Steady state 25.17 % 3.51% 3.11 % 2.76 %

Transient 26.14 % 3.95 % 3.79 % 3.02 %

Power factor 0.8767 0.9731 0.9797 0.9883

(c)

(b)

Steady state Steady stateTransient state

Stead state Transient state Stead state

(a) (b)


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The simulation is done in various non-linear load conditions. The proportional integralcontrol based compensating cascaded active filter made balance responsibility even when the

system is non-linear load. FFT analysis of the active filter brings the THD of the source

current less than 5% into adopted with IEEE 519-1992 and IEC 61000-3 standards harmonicunder non-linear and/or unbalanced load conditions.

5. Conclusions

This paper demonstrates that the cascade inverter based active power filter is suitable for

power line conditioning of power distribution systems. The cascaded PWM-voltage source

inverter provides lower costs, higher performance and higher efficiency for power line

conditioning applications. A novel method comprising low pass filter in conjunction with

proportional integral control scheme has been adopted for controlling the DC-side capacitor

voltage of the cascaded inverter and estimating the required reference current. The APLC

system including the proposed control method is validated through extensive simulation

under steady state and transient with different non-linear loads. These simulation resultsreveal that the cascaded active power filter effectively filters the harmonics and compensates

reactive volt amperes. The measured total harmonic distortion of the source currents is

2.59% that is in compliance with IEEE 519-1992 and IEC 61000-3 standards for harmonics.

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