Dc Bus Design Paper Recommended

8/8/2019 Dc Bus Design Paper Recommended

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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 4, JULY/AUGUST 2001 1037

Operations of the Dominant Harmonic Active Filter(DHAF) Under Realistic Utility Conditions

Po-Tai Cheng , Member, IEEE, Subhashish Bhattacharya, Student Member, IEEE, and Deepak Divan, Fellow, IEEE

AbstractThis paper presents laboratory test results of theDominant Harmonic Active Filter (DHAF) prototype. The DHAFsystem achieves harmonic isolation at the dominant harmonicsusing square-wave active filter inverters. The key advantages of theDHAF system are the low rating and low bandwidth requirementsof the active filter inverter. Such characteristics allow cost-ef-fective and viable applications of the DHAF system to mitigateharmonic problems for high-power nonlinear loads (10100 MWand above). Several practical situations, including source-sinkresonance, ambient harmonic interferences, and unbalancedgrid voltages are applied to the DHAF prototype to validate itsperformance. The operation principles of the DHAF system andthe synchronous-reference-frame-based controller are discussed

to explain how harmonic isolation at the dominant harmonicsis accomplished. A design example of the DHAF system for a20-MVA rectifier load at an industrial site is also given to illustrateits application.

Index TermsActive filter,dominant harmonic active filter,har-monic filter, harmonic isolation, HVdc, square-wave inverter, syn-chronous reference frame.

I. INTRODUCTION

AS INDUSTRIES embrace modern power electronics

technologies with an unprecedented fervor and capi-

talize on the improved efficiency and productivity provided

by the workhorses like adjustable-speed drives (ASDs) and

uninterruptible power supplies (UPSs), the utility grid thatpowers the industries is being disturbed by these equipments

because of their rectifier front ends. Due to their nonlinear

nature, use of rectifiers results in significant harmonic current

in the utility grid. With the increasing use of power factor

correction capacitors installed in the grid for var compensation

and the inductance of the lines and transformers, severe L-Cresonances may be triggered by the harmonic current generated

by nonlinear loads. The harmonic current also causes higher

losses in the lines and transformers of the utility grid. Harmonic

Paper IPCSD 01004, presented at the 2000 Industry Applications Society

Annual Meeting, Rome, Italy, October 812, and approved for publication inthe IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Industrial PowerConverter Committee of the IEEE Industry Applications Society. Manuscriptsubmittedfor reviewApril1, 2000 andreleased forpublicationMarch23, 2001.This work was supported by the Wisconsin Electric Machines and Power Elec-tronics Consortium (WEMPEC), University of Wisconsin, Madison.

P.-T. Cheng is with the Department of Electrical Engineering, Na-tional Tsing Hua University, Hsin-Chu 30013, Taiwan, R.O.C. (e-mail:[email protected]).

S. Bhattacharya is with the FACTS and Power Quality Division, SiemensPower Transmission and Distribution, Pittsburgh, PA 15235 USA (e-mail: [email protected]).

D. Divan is with Soft Switching Technologies, Middleton, WI 53706 USA(e-mail: [email protected]).

Publisher Item Identifier S 0093-9994(01)05911-4.

Fig. 1. Proposed DHAF system.

standards, such as the IEEE 519, are strongly recommended by

the utilities to alleviate such problems.

Passive L-C filters have been the traditionally preferred har-

monic filtering solution mainly for their high efficiency, low cost

andsimplicity.However,L-Cfiltersaresusceptibletosource-sink

resonances [1][6].L-Cfiltersalso attract harmoniccurrent fromambient harmonic-producing loads and background distortion

of grid voltages [2], [7][9]. Filter loading due to background

distortionisakeydesignissue[10]. Theirfilteringcharacteristics

are affected by component tolerances, and the varying utility

system impedances in case of system configuration changes and

contingencies. Further, a stiff utility grid poses great difficulties

for L-C filter design because sharp and precise tuning will be

required to sink a significant percentage of the load harmonic

current.Withalltheseproblems,L-CfiltersmaynotmeettheIEEE

519standard[11].

Several active filter systems have been proposed to mitigate

harmonic current of industrial loads [12][14]. Pure series and

shunt active filters are suitable for small-rating nonlinear loads[12], [15][17]. Hybrid series and hybrid shunt active filters,

which are characterized by a combination of passive L-Cfilters

and active filters, are cost effective and practical for large-rated

nonlinear loads. Implemented with high-bandwidth pulsewidth

modulation (PWM) inverters, these active filters demonstrate

superb filtering characteristics [3][5], [18], [6], [19], [20], [8],

[12][14], [21], [22].

On the other hand, due to their high bandwidth requirement,

their applications are limited to nonlinear loads below 10 MW

[12], [13]. For nonlinear loads beyond 10 MW, hybrid active

00939994/01$10.00 2001 IEEE

uthorized licensed use limited to: NATIONAL INSTITUTE OF TECHNOLOGY KURUKSHETRA. Downloaded on August 17,2010 at 14:19:35 UTC from IEEE Xplore. Restrictions
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1038 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 37, NO. 4, JULY/AUGUST 2001

filter systems implemented with PWM inverters are not cost ef-

fective due to the high bandwidth and high rating requirement.

Passive L-Cfilters are usually used for this level of loads. How-

ever, as stated previously, using L-C filters may not meet the

IEEE 519 standard.

The authors have proposed the Dominant Harmonic Active

Filter (DHAF) [23][27] intended for high-power nonlinear

loads beyond 10 MW. The proposed DHAF system (Fig. 1)achieves harmonic isolation at the dominant harmonic fre-

quencies, i.e., at the fifth and seventh harmonics (for six-pulse

rectifier front ends), using square-wave inverters. The DHAF

system adopts the hybrid-shunt topology for the advantages

of simple protection and retrofit possibility with the existing

passive filters. The DHAF system does not have any compen-

sation limit in terms of of the nonlinear load because

its operations focus only on the dominant fifth and seventh

harmonics. The fifth and seventh harmonics of the supply

current are extracted by the DHAF controller to achieve domi-

nant harmonic isolation, therefore the DHAF system operates

independent of the load current profile. The passive filters

reduce the disturbances of high to the DHAF system.The DHAF system can also be installed at the PCC for a group

of nonlinear loads, including double-pulse type of front ends

widely used at low voltage levels. Filtering performances of

shunt active filters with double-pulse nonlinear loads can

be maintained if sufficient impedances (such as step-down

transformers) are provided in between [28].

Experimental and simulation results have shown that the

DHAF system meets the IEEE 519 harmonic current limits in

the supply [23], [27], [26]. In this paper, the synchronous-ref-

erence-frame (SRF)-based controller of the DHAF system is

explained in detail, and laboratory test results of the DHAF

prototype are presented. The DHAF system prototype is tested

under several practical utility interface situations including

source-sink resonances, ambient harmonic interferences, and

unbalanced grid voltages to validate its performance. The

hardware implementation of major components of the DHAF

prototype will also be presented.

II. SRF-BASED DHAF CONTROLLER

Fig. 2 shows the block diagram of the SRF-based controller

implemented for the fifth harmonic active filter inverter of the

DHAF system. The SRF controller achieves fifth harmonic iso-

lation by using closed-loop control on the fifth harmonic com-

ponent of the supply current. Three-phase supply currents ,

, and are measured and transformed into the synchronousreference frame ( axes) rotating at the fifth harmonic.

The fifth harmonic component of the supply current is trans-

formed into dc quantities in the and axes, and extracted

by the subsequent low-pass filters. and are then compared

to the references and . Note that references and

are zero in order to achieve harmonic isolation at the fifth

harmonic. The errors are fed into the proportional plus integral

(PI) regulators to generate the required voltage command for the

active filter inverter. A -to- transformation is applied

to convert the inverter voltage command back to three-phase

quantities. The modified sine/triangle modulation used in the

controller generates square-wave switching commands at the

fifth harmonic with slight fundamental frequency modulation to

achieve harmonic isolation at the fifth harmonic frequency and

dc-bus power balancing of the active filter inverter [23], [26].

For the DHAF prototype, AD2S100 vector rotator of Analog

Devices is used to implement the SRF transformation. The op-

erating frequency of the transformation is phase-locked to the

utility grid by a simple phase-locked-loop circuitry.In the laboratory, only the fifth harmonic SRF controller is

implemented to verify the operation of the DHAF prototype. In

real applications, a similar SRF-based controller will be imple-

mented for the seventh harmonic active filter inverter to achieve

harmonic isolation at the seventh harmonic frequency [29].

III. DHAF PROTOTYPE TEST RESULTS

The following test conditions are set up in the laboratory to

emulate various practical utility interface situations. The key

parameters are given as follows.

Supply: 244 V (rms, line-to-line), 60 Hz, with 1.0% fifth har-

monic distortion. mH. SCR 90 on 4.5-kVA basis.The IEEE 519 harmonic standard requires the total demand dis-

tortion (TDD) to be below 12%.

Load: Six-pulse diode rectifier. Details are given in Table I.

Note that and are the dc-side inductor and capacitor of

the rectifier load.

Passive Filter: Component parameters for each test condi-

tion are given in Table II.

Active Filter: A conventional three-phase voltage-source

inverter implemented with Toshiba MG100Q2YS40 (1200

V, 100 A) insulated gate bipolar transistor (IGBT) modules,

dc-bus electrolytic capacitors F, and ac-side

reactor mH. Turns ratio of the coupling transformer

is 20 : 1 (inverter side :L-Cfilter side).The instrumentations for the DHAF prototype are shown

in Fig. 3. The control circuit is implemented with digital and

analog circuit components to achieve the control functions

described in Section II.

A. Source-Sink Resonance

Passive L-C filters are very susceptible to source-sink

resonance formed by the passive filter and the utility system

impedance. If the resonant frequency is near the dominant fifth

or seventh harmonic frequency, highly distorted line currents

and voltages are likely to occur and cause line trip-offs and

possible equipment damages.The DHAF system test setup is shown in Fig. 4. The res-

onant frequency of the system inductance ( ) and the filter

components ( and ) is very close to the fifth harmonic

( , where rad s). Therefore, the supply current

is severely distorted with 38.2% of fifth harmonic as shown

in Fig. 5(a), and so is the filter current as shown in

Fig. 6(a).

After the active filter is started, the fifth harmonic component

of is reduced to 5.3%. THD of improves from 39.2% (be-

fore the DHAF system is started) to 11.0% (after) as shown in

Fig. 5(b). The distortion of the filter current is also reduced as

shown in Fig. 6(b).

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CHENG et al.: OPERATIONS OF THE DHAF UNDER REALISTIC UTILITY CONDITIONS 1039

Fig. 2. Controller of the DHAF system for the fifth harmonic active filter inverter.

TABLE IHARMONIC PRODUCING LOAD USED IN THE TEST BENCH

TABLE IIPASSIVE FILTER COMPONENTS FOR EACH TEST CONDITION

Note that the DHAF system achieves harmonic isolation in

presence of the background supply voltage distortion (approx-

imately 1.0% of the fifth harmonic). This experimental result

indicates that the DHAF system allows the filter to be

tuned at the fifth harmonic to maximize its harmonic filtering

effectiveness without the risk of inducing the source-sink har-

monic resonance.

Fig. 7 shows the line-to-line output voltage of the fifth

harmonic active filter inverter. The active filter inverter switches

in the square-wave mode to achieve harmonic isolation at the

fifth harmonic frequency. As indicated by the spectrum, the fifth

harmonic component of is the major component, veri-

fying the square-wave switching of the DHAF inverter.

also contains a small fundamental component for dc-bus power



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Fig. 3. DHAF prototype test bench instrumentations.

Fig. 4. Hardware test bench for the DHAF system. L , L , and C formresonance near the fifth harmonic frequency. Diode rectifier load.

(a) (b)

Fig. 5. Supply currenti

; under system resonance. (a)i

before DHAF isstarted. (b) i after DHAF is started.

balancing of the active filter inverter. The 11th and 13th side-

band harmonic voltages present in are the result of the

modulation strategy of the DHAF system. Detailed derivation

and calculation of the sideband components are provided in [26]

and [29].

B. Ambient Harmonics Interferences

Passive L-C filters are susceptible to ambient harmonic-pro-

ducing loads because excessive harmonic current from ambient

loads can cause passive filter overloading. In this test, the

(a) (b)

Fig. 6. Filter current i ; under system resonance. (a) i before DHAF isstarted. (b) i after DHAF is started.

Fig. 7. Active filter inverter voltage v (line to line); under systemresonance.

Fig. 8. Hardware test bench for the DHAF system. L and C are tuned at thefifth harmonic frequency. Ambient diode rectifier load.

DHAF system demonstrates its capability of blocking ambient

harmonics by achieving harmonic isolation at dominant har-

monic frequencies.

Fig. 8 shows the arrangement of the DHAF system and the

ambient harmonic-producing load. The current of the ambient

rectifier is given in Fig. 9. Note that the main load is discon-

nected. Before the DHAF starts, supply current contains

22.6% of the fifth harmonic component as shown in Fig. 10(a).

This high current distortion primarily results from the ambient

rectifier and background distortion of the supply voltage. After

the DHAF is started, the fifth harmonic component of is

reduced to 2.6%, and the THD of is reduced to 4.2% as

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Fig. 9. Ambient nonlinear load current.

(a) (b)

Fig. 10. Supply current i ; ambient diode rectifier load. (a) i before DHAFis started. (b) i after DHAF is started.

Fig. 11. Dynamic response of the DHAF system to the starting transient of theambient rectifier load. Top: ambient load current i ; middle: inverter dc-busvoltage; bottom: inverter output voltage v .

shown in Fig. 10(b). Fig. 11 shows the response of the DHAF

system to the starting transient of the ambient rectifier. As the

ambient diode rectifier load is started, the feedback controllerand the dc-bus voltage controller of the DHAF system respond

by charging up the inverter dc-bus voltage from 42.8 to 70.3 V

in the presence of increased disturbance from the utility side.

The envelope of the inverter output voltage follows the dc-bus

voltage because of square-wave switching. In actual appli-

cations, the utility needs to maintain the voltage THD below

5.0% with no individual harmonic component exceeding 3.0%

(under 69 kV) according to the IEEE 519 standard, therefore

the ambient load disturbances is limited. The DHAF system

is able to prevent overloading of the passive filters, even with

background distortion of the supply voltage, and thus allows

the passive filters to be rated based only on the main load.

Fig. 12. Hardware test bench of the DHAF system; unbalanced utility supplyvoltage.

(a) (b)

Fig. 13. Supply voltage v , v , and v ; unbalanced supply voltage.(a) Time domain. (b) Frequency domain.

(a) (b)

Fig. 14. Load current i , i , and i ; unbalanced supply voltage.(a) Time domain. (b) Frequency domain.

C. Unbalanced Grid Voltages

In practice, the three-phase supply voltages can be unbal-

anced for various reasons. The DHAF system is tested under

the unbalanced grid voltages and the resulting unbalanced diode

rectifier load current. As shown in Fig. 12, approximately 10%

voltage drop in phase A is fabricated using an autotransformer.

Fig. 13 shows the unbalanced lineline voltages, and the re-

sulting unbalanced rectifier current is shown in Fig. 14. Figs. 15

and 16 show the unbalanced three-phase supply currents before

andafterthe DHAF is started. Thefundamentaland thefifth har-

monic components are both unbalanced as shown in Table III.



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TABLE IIIHARMONIC CONTENTS AND THD OF SUPPLY CURRENT i , i , AND i . (a) FUNDAMENTAL. (b) THIRD. (c) FIFTH. (d) THD

(a) (b) (c) (d)

(a) (b)

Fig. 15. Supply current i , i , and i ; before the DHAF is started;unbalanced supply voltage. (a) Time domain. (b) Frequency domain.

(a) (b)

Fig. 16. Supply current i , i , and i after the DHAF is started;unbalanced supply voltages. (a) Time domain. (b) Frequency domain.

The third harmonic component is also significant due to the un-

balance.

After the DHAF is started, the fifth harmonic currents are

reduced to 0.32, 0.28, and 0.31 A, respectively, as given in

Table III.

The DHAF system only suppresses the negative-sequence

component of the fifth harmonic current. The DHAF controlleruses SRF transformation rotating at the negative-sequence fifth

harmonic, thus, the positive-sequence fifth harmonic current is

converted into ac components and then filtered out by the sub-

sequent low-pass filters. Therefore, only the negative-sequence

fifth harmonic component of is driven toward zero by the

PI regulators. The output lineline voltages of the active filter

given in Fig. 17 show a set of balanced three-phase voltageswhich achieve harmonic isolation at the negative-sequence fifth

harmonic frequency. Table IV shows that the negative fifth har-

monic component of the supply current is suppressed by the

DHAF system, while the positive-sequence fifth harmonic com-

ponent still exists. Compensation of the positive-sequence com-

ponent of the fifth harmonic current can be added if desired.

IV. DESIGN EXAMPLE

A DHAF system is designed for an industrial customer of

20-MVA nonlinear load connected to the 11-kV feeder to meet

Fig. 17. Active filter inverter output voltage v , v , andv ; unbalanced supply voltage. (a) Time domain. (b) Frequency domain.

TABLE IVPOSITIVE-SEQUENCE AND NEGATIVE-SEQUENCE FIFTH HARMONIC

COMPONENTS OF i

Fig. 18. DHAF design example for a 20-MVA industrial site.

TABLE VHARMONIC CONTENTS OF THE 20-MVA INDUSTRIAL CUSTOMER

the IEEE 519 harmonic standard as shown in Fig. 18. Similar

industrial installations have been presented [30][32]. The har-

monic content of the load current is given in Table V. The fifth

and seventh passive filters given in Table VI provide a total of 10

Mvar for reactive power compensation and harmonic filtering.

Assume that the grid voltage contains 2% each of fifth and

seventh harmonic distortion, respectively. As the DHAF system

achieves harmonic isolation at the fifth harmonic frequency, the

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TABLE VIPASSIVE FILTERS OF THE 20-MVA INDUSTRIAL CUSTOMER

active filter inverter produces the active tuning and background

harmonic distortion tracking voltage components [23]

V (1)

Note that designates coupling transformer primary side

(L-C filter side) quantities. The current flowing into the fifth

harmonic filter is calculated based on the fundamental reactive

current, the fifth harmonic component of load current, the sev-

enth harmonic current due to the 2% tracking voltage generated

by the seventh active filter, and the L-C filtered 11th and 13th

harmonic current

A (2)

With a 1:4 (L-C filter side : inverter side) coupling trans-

former, the secondary side quantities can be derived

V

A (3)

With square-wave switching, the inverter dc-bus voltage

requirement is

V (4)

The inverter dc-bus capacitor is designed for 5% ripple (as-

suming dc-bus voltage 1300 V) under full load

F (5)

Based on similar calculations, the voltage and current ratings

of the seventh harmonic active filter are

V

A (6)

The voltampere ratings of the fifth and seventh harmonic

square-wave inverters are 540 kVA (2.7%) and 510 kVA

(2.04%), respectively, based on peak values. The commercially

available 2500-V IGBTs can be used in this case for imple-

mentation.

Fig. 19. Photographs of the DHAF prototype.

V. CONCLUSIONS

This paper has presented operational test results of theDHAF system under practical utility interface conditions, such

as source-sink resonance, ambient harmonics interference, and

unbalanced grid voltages. The experimental results validate the

capability of the DHAF system to achieve harmonic isolation

at the dominant harmonic frequencies under various utility

system conditions as well as background supply voltage distor-

tions. The operational principles of the DHAF system and the

SRF-based controller of the DHAF system were presented. The

DHAF system is a viable and cost-effective solution for har-

monic mitigation of high-power nonlinear loads (10100 MW

and above). The design example shows that the DHAF system

can be implemented with commercially available IGBTs for a

20-MVA nonlinear load. Photographs of the DHAF laboratoryprototype are shown in Fig. 19.

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Po-Tai Cheng (S96M99) received the B.S. degreefrom National Chiao-Tung University, Hsin-Chu,Taiwan , R.O.C., and the M.S.E.E. and Ph.D. degreesfrom the University of Wisconsin, Madison, in 1990,1994, and 1999, respectively.

He is currently an Assistant Professor in the De-

partment of Electrical Engineering, National TsingHua University, Hsin-Chu, Taiwan, R.O.C. His pri-mary research interest are active filters, utility appli-cations of power electronics, power quality issues,and high-power converters.

Subhashish Bhattacharya (S86) received theB.E. (Hons.) degree in electrical engineering fromthe University of Roorkee, Roorkee, India, andthe M.E. degree from Indian Institute of Science,Bangalore, India, in 1986 and 1988, respectively,both in electrical engineering. He is currentlyworking toward the Ph.D. degree at the Universityof Wisconsin, Madison.

Since December 1998, he has been with the

FACTS and Power Quality Division, Siemens PowerTransmission and Distribution, Pittsburgh, PA.

His primary areas of interest are active filters, utility applications of powerelectronics and FACTS, and drives and control techniques.

Deepak Divan (S78M78SM91F98) receivedthe B.Tech. degree from Indian Institute of Tech-nology, Kanpur, India, and the M.S. and Ph.D.degrees from the University of Calgary, Calgary,AB, Canada, in 1975, 1979, and 1983, respectively,all in electrical engineering.

He has been a Professor at the University of Wis-consin, Madison, since 1985 and is an Assistant Di-rector of the Wisconsin Electric Machines and Power

Electronics Consortium (WEMPEC). He is Presidentand CEO of Soft Switching Technologies Corpora-tion, Middleton, WI, a manufacturer of power conversion equipment. He is theholder of 20 issuedand pending patents andhas authored more than 90 technicalpublications, including several prize papers.

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