Modular Parallel Three-Phase Inverter System

237

Modular Parallel Three-phase Inverter System Jiann-Fuh Chen

Ching-Lung Chu

Yow-Chyi Liou

Department of Electrical Engineering National Cheng Kung University, Tainan, Taiwan, R.O.C. Abstract -This paper develops three-phase invefter modules that have the following functions: (1) inverters for stand-alone operation; (2) inverters in parallel; (3) inverters in parallel with utility system. For obtaining parallel operation, the parallel technique for a voltage-controlled PWM inverter and (N-1) current-controlled PWM inverters is proposed in this paper. Through the current control, the current-controlled PWM inverter can be operated in synchronism with the voltage-controlled PWM inverter. By a load-sharing center, the load is shared to each inverter in parallel, and accurately limits the output current of each inverter to its maximum rating. In this paper, system operations are described in detail and experimental results are developed. From the experiment results, this proposed scheme is adequate.

I. INTRODUCTION Unintermptible power supplies (UPS) are used to provide the potver to critical loads such as computers. communication systems, instrumentation plants. and hospital equipment. In order to expand the total output capacity. it is possible to increase the output capacity of a single inverter [ l ] or to employ modular parallel inverters that share the load requirements. Large capacity with a single inverter raises problems such as heaf dissipation, failure protection. and economical solution [2]. Modular parallel is a preferred method to e q a n d the system capaciq, to increase the overall $stem efficiency, and to increase the system reliability. Modular parallel systems have many advantages. Ho\vever, parallel operation is difficult. because the problem of operating the parallel inverters in phase must be considered. A typical control alternative is the use of PLL (phase locked loop). PLL has a slow response, which coupled with the phase dlfference between the inverters, gives rise to circulation current flow between the inverters. The circulating current will destroy the inverter system. especially under light load [ 3 ] . In parallel operation careful consideration must be given to the load share among. each inverter [4.5].The power supplied by each inverter to the system cannot exceed the rated capacity of the inverter. Any excess will damage the inverter.

Recently, some techmques have been devised [6,7] for acheLing better parallel output characteristics, but they merely have an output filter, so each inverter cannot operate indilidually. Thus giiing a multi-inverter system which is especially suitable for a large capacity system; but not for a modular inverter. In typical voltage-parallel systems. the output filters need to be redesigned for different numbers of inverters in parallel [8]. Othenvise unexpected resonant frequency will appear in the parallel inverter system. This degrades the advantages of modular parallel systems. This paper presents a new parallel three-phase inverter structure. The scheme of combining a voltage-controlled PWM inverter and (N-1) current-controlled PWh4 inverters. for parallel operation. is proposed. The application of current control enables the inverters to operate in synchronism. consequently there is no need for using PLL. Current control has an excellent transient and steady response [9]. and thus circulating current between the inverters is avoided. The output power of parallel inverters can be precisely controlled by load-sharing center. Moreover. in such a structure the output filter does not need to be redesigned in spite of " w i n g numbers of inverters in parallel operation. This method makes it easy for the parallel inverter to expand the system capacity.

l-L

d-, Controller

S

w

i

t

c

Current T L

,

Controller

Fig 1. Schematic diagram of the proposed inverter

IEEE Catalog Number: 95TH8081

Authorized licensed use limited to: Southern Taiwan University of Technology. Downloaded on March 6, 2009 at 01:50 from IEEE Xplore. Restrictions apply.

I

238

lzli Sinwi

I

Recfifier

Fig2 Block diagram of the volkigs control system.

11..THE PROPOSED rNVERTER The schematic diagram of the proposed in\.erter is shotvn in Fig. 1. The most frequentl!. used three-phase inttrter circuits have three legs. for each phase. This inverter maid!. consists of three power GBTs. an input capacitor Ci. an output filter fomied b! Ls and Cp. a mode selection stvitch SW. a voltage-controlled mode and a current-controlled mode. The proposed in\,erter has ttvo lunds of operating modes. ( 1) Voltage-controlled

mode: If the snitch SW is set to the "1" position. voltagecontrolled mode is selected. In t h s mode. the inverter can be operated as a voltage source. maintaining a constant sinusoidal voltage at the output terminal.

(2) Current-controlled mode: If the switch SW is set at to the "2" position. currentcontrolled mode is selected. In t h s mode. the in\.erter is operated as a current source. Its output current follo\i.s the command current.

A. Voltage-controlled mode Fig. 2 shows a block diagram of the voltage control qstem. It consists of two control loops, DC controller and AC controller. The gain at 60 Hz in the AC controller loop is insufficient to give full compensation no load to full load. therefore an ordinan DC regulator has to be added. The DC voltage controller located in the outer loop is used to maintain a constant level. In the DC controller loop.

*

sinusoidal voltage Vac is achie1,ed bj. multiplying the reference sinusoidal signal and the output signal of the DC voltage controller. For the PWh4 method. the triangle carrier comparison method is adopted. Thc control has been designed with instantaneous feedback and al'erage feedback for thc voltage source inverter [IO]. This approach git*esestreniely good transient characteristics as well as voltage regulation.

B. Current-controlled mode Current control is commonly and widell- used in high performance AC seno systems. AC to DC with unir poncr factors. and applications in actitt filters. It has the characteristic of good transient and steady response Current-controlled PMW inverters car, be viewcd as a current source Ivhose output current IS follot5.s the reference current I.;. Fig. 3 is a block diagram of the current control qstem. I n order to obtain a good output current regulation. a current regulator with load \.oltage feed-fonvard is used. The current regulator provides an instantaneous current limit capabilih. and increases the controller's robustness for handling suitchcd non-linear loads.

*.

Va\, is a three-phase average voltage command. Vac is supplied by the three-phase full-bridge rectifier. this is con\.erted via a low pass filter into a smooth DC voltage. A notch filter. with a notch frequency at 360 Hz. is used to reduce the effects of the ripple voltage The AC voltage controller positioned in the inner loop is uscd to maintain a sinusoidal wa\.eform. The desired

Fig 3 H l o i l diagram o f h current control system


239

Lood-Shoring

I;

I3

IN

(2) Inverters in parallcl. The master unit is set to \.oltagccontrolled operating mode. while the slave units arc set to current-controlled operating mode. Thc inverters are used in parallel. (3) Inverters and utilit! system in parallel: The theov for this kind of parallel opcration is the same as that of invertcrs in parallel in (2). The utility system is taken as the master unit and all the invertsrs in parallel are taken as sla,.e units. In t h s application. all the inverters are set to currentcontrolled operating mode. This alloivs operation in parallel with the utili5 system. The proposed inverter can be operated as a master unit or a slave unit. It is a modular inverter. In parallel operation. if an inverter breakdoljns. then that inverter must be removed. If the damaged inverter is a master unit. a slave unit must be upgraded to replace the removed a master unit. resuming operation.

Fig 4. Proposed parallel imertzr syslcm

111. PARALLEL OPERATION Our newly developed parallel structure includes a voltage-controlled P W M inverter and (N-1) currentcontrolled PWM inverters as shown in Fig. 4. Use of The voltage-controlled PWh4 inverter and the current-controlled PWM inverter is common. however. in this paper use as a master unit and slave unit. respectively. is proposed. The slave units can be operated in qnchronism with the master unit by using current control; and does not need PLL. The load can be shared to each inverter in parallel by using a load-sharing center. Since current-controlledPWM inverters have a good dynamic response. the parallel system can be operated very successfully. In a N-inverter parallel system. one inverter is selected as the master unit and the rest as the slave units. The master unit is a. voltage-eontrolled PWM inverter to maintain constant voltage in the parallel s\-stem. The slave units are cwrent-controlled PWM inverters, to track the reference current, which is distributed by the load-sharing center.

IV. INVERTER FUNCTION

V. LOAD-SH-ARING CENTER DESTGX As the load-sharing center can set the output current command of the s h e units. and the system yoltage is a constant sinusoidal wa\.eform maintained by the master unit. therefore the output currents of the slaye units can be directly controlled. Since S = JT (1) Where the symbol * denotes the complex conjugate. The output power of the inverter is proportional to its output current. Thus. the output power of the slave units is under control. Since Smastcr = S l O d J - Z.SS2in.E (2) Thus, the output poiver of the master unit is indirectly controlled. When setting the output current of the s1a.e units by using the load-sharing center. the following condition must be satisfied: P2 -+ P3 i-1”;

+

+P.v 5 Pr.

(3)

where

P L load ponsr phr : output pover of Nth imerter

By the law of consen ation of encrg! . The inverter is developed with WO operating modes. voltage-controlled mode and current-controlled mode. The operating mode of the inverter is selected according to the responsibilities of the inverter in the particular application. The proposed inverter has three kinds of function as €ollo\~s: (1) Inverter for stand-alone operation: The operating mode is set to voltage-controlled operating mode. and the invencr is used as a powcr supply.

P , + P? -k P;4-

4-P.y = PL

(4)

Based on equations (3) and (1).PI can be solved as

PI 2 0

(5)

Otherwise. P Iis negatij e M hich implies that the master unit is absorbing power from the parallel qstem instead of supplying powr IO the parallel p stem


240

Usually, the load can be shared according to the capacities of the inverters. For a numerical example, suppose we want to operate three inverters in parallel. The capacities are 2 kVA. 1 kVA and 3 kVA, respectively. Suppose the first inverter (2 kVA) is set as a master unit and the remaining inverters (1 kVA, 3 kVA) are set as slave units. According to the capacity of each inverter. the load-sharing center sets the power of each inverter to the load.

2+1 +3

where for first inverter for second inverter s R 3 : Capacity rating for third inverter

VI.OUTPUT FILTER DESIGN Fig. 5 shovs the equivalent circuit of the parallel inlerter qstem. When an inverter is operated for stand-alone. the transfer function of the filter can be represented by Laplace‘s transformation as follows:

The resonant frequenq H’l is many times greater more the fundamental frequency, the exact value depends on the filter design

SRI : Capacity rating S R :~Capacity rating

By KCL, I I can be obtained as:

A Typical voltage-parallel SJ stem

When another m’erter is added in parallel operation (see Fig ja). the SJ stem frequenc! becomes I+, = &SI

The output powers of each inverter are

+LS3)(CPI

+ cp3)/(~sILs;Iccyl CCp2)

(14)

Then, an unexpected resonant frequency will appear in the parallel inverter system In order to avold the unexpected resonant frequenc! the output filters need to be redesigned for daerent numbers of in1 erters in parallel [8] ~

B. The proposed parallel system:

Therefore, although the inverters have different capacities. they can still supply power to the system through the settings of the load-sharing center, accordmg to their respective capacities. Thus, inverters of Mering capacities can successfully be operated in parallel.

When an additional in!.erter is added for parallel operation (see Fig. jb). the system resonant frequency is not afiected. instead the original resonant frequenq LfS=-==.1 is maintained. The filter parameters of the p s 1 CpI

inverter do not need to be redesigned for different numbers of inverters in parallel operation. Thus a module of inverter is realized ,and parallel connections can be increased easily to expand the system capacity.

VI1 EXPERIMENTAL RESULTS U1

A three-phase 3-kVA experimental module w’as constructed and tested. The parameters of the proposed inverter system are listed below:

Inverter capacity 3 kVA Output voltage three-phase 110 Vrmsf60 Hz Dc voltage 210 V Ls = 1.98 mH Cp = 20 uF Inverter switching frequency 15 kHz Fig 5. Equivalent circuits ofthr parallrl system. (a) typical voltagc-parallel system.(b) thz propose parallel system

A. Inverter for stand-alone operation:


24 1

automatically adjusts in proportion with thc valuc of thc The Operating mode is set Io vo*tage-controlledOperating load-sharing center settings. Thus. transient load changcs do mode. Fig. demonstrates the dynamic performances Of the not activate the load sharing betlvecn the master unit and thc inverter for sudden variation in load. slave unit. Fig. 7 illustrates the inverter behavior with a balanced Fig. 1 1 shows behavior with a threethree-phase recMier load. phase rectfier load. T h s illustrates that the parallel system Fig. shows the and load current produces good characteristics. even under rectifier load. waveforms in 100% unbalanced single-phase non-linear load. This illustrates that the proposed inverter produces ~111. CONCLUSIONS good characteristics. even under unbalanced load. In t h s paper. using the recombination of a voltagecontrolled PVi'M inverter (or utility system). and (N-1) current-controlled PWM inverters. all the inverters can be operated in synchronism and there is no need for using PLL VU 0 which has a slolv response. Current-controlled PLVM inverters have an excellent transient and steady' response, so -150 the load can be shared predictably to each inverter in 40 parallel. In this proposal scheme. the other advantages. such as overload protection is easy by use of the load-sharing center. and the redesign of the output filter is not necessa? for different numbers of parallel inverters. This parallel vstem is easy to implement. From the experiment results. -40 I...*.. I this proposed scheme is adequate.

*

(6)

:..a.

Fig 6 . Step load variation output line current Iu.

I50

-40

(Oom+

100%):(a) output phase voltagz

0.00

: (b)

I

1

I

\'U

$eo. 1%.

I

Fig. 7. Balanced three-phase rectifier load. (a) output phase voltage output line current Iu.

-40

!

c.00

J 0O.LC..

\-U;(b)

B. Inverter in parallel with utility:

-'O

40

Utility system is taken as the master unit and the inverter is taken as the slave unit. The inverter operating mode is set to currentcontrolled, and the value of load-sharing center settings is adjusted to a 1:l ratio. Fig. 9 illustrates transient variation characteristics when the invener adds suddenly to the share load. and releases suddenly load. bet'veen the Fig. lo shows the load master unit and the slave unit. The individual load current.

( d i 1.

0'

I

[ 0.05

I

-40 0.00

1~0.00..

Fig. 8. Cnhalanced singlz-phase non-linear load: (a) output phase voltage Vu: line currentI ~ . lu; (c) output line cumen, I,.; (d) @) output lint


242

1

150

>12.3

.

512.3..

1

1

-150

1

100

I 3s1.3

SCI.1..

I

I

'

y-----l

J

I

-100

.f : .t:.

t.cc

':r.:s..

C.tC

11c.::..

I

I

16?.1

III.1..

Fig 9 Slave unit's sudden addition to the share load. (a) output phase vohags Vu. @) load current Iu; (c) the master output line current Imu. (d) the slave line

I

Fig. 11. Three-phase rectifisr load: (a) output phase voltage Vu: (b) load current Iu: (c)the master ourput line current Imu: (d) the slave line current Isu

IX REFERENCES

-150

I

100

I

5.l.i

'I

I.:.:..

1

A ( b ) lu

0

-100

5.C.O

).:.a..

3.Y.I

l.C.0..

100 A ( c ) Imu

-100

100 A

(d) Isu

T. hlizutani. T. Shimizu. K. KuroAi and T. Hoshi. " Polver transistorized unintsrmptible power suppl). " IEEE Trans. Ind. Appl.. vol. 1.4-20. no.4. pp.961-966. July'.4ug.1984. D. Boldin. High powsr ITS system using transistorized inverters." ~ O C ISTELEC'91. . pp.431- 435. NOV.1991. J. F. Chen C. L. Chu and C. L. Hirang. "The parallel operation of hvo LPS by the coupled-inductor method. " IEEE IE C o d . Rec., 1992. pp.733-736. Y . Kawata. H. KoiLs. F. Tominaga, T. Nakamizo and Ii. Harada. "Development of 45OkV.4 triport LTPS system." Proc. INTELEC'88. 23-1 pp.531-535, 1988. S. Ogasawvar& J. TaLagaki and H. Akagi, "A novel control scheme of a parallel current-controlled P W l l inverter." IEEE Trans. Ind. Appl.. v01.1.A-28. pp.1023- 1030. Sept.'Oct. 1992. J. Holtz and K. H. Wemcr, "Multi-inverter U P S system with redundant load sharing control." IEEE Trans. Ind. Elect., ~01.37.no.6, pp.506-513, Dec. 1990 T. Kawahata and S . Higashino, "Parallel operation ofvoltage source inverters," IEEE Trans. Ind. Appl.. vol.IA-24, no.2, pp.281-287, h.Iar..'Apr. 1988. J. F. Chen C. L. Chu and C. L. Huang. "Combination voltage-controlled and current-controlled PU'M inverters for parallel operation of UPS." IECOS93, vol. 2 of 3. pp. 111 1-1 1 16. Nov. 1993. A Nabas, S. Ogasauara and H. ALagi. "A novel control scheme for current-controlled PN'U inverters." IEEE Trans. Ind. Appl., vo1.1.4-22, pp.697- 701. J u l y ' A ~ z1986. [IO] C. M. Lian. T. H. Chen. T. C. Wang. G. J. Cho, C. M. Lee and C . T. Wang. "Design and implementation of a single phase current-forced switching mode bilateral convertor." IEE Proc.B, vol. 138. pp. 129-136.

O

-100

Fig 10. Load changes ( 1 0 0 " ~-+33.3?0) ofrated power: (a) output phase voltage Vu: (%) load current Iu: (c) the mastsr output linz arrent Imu; (d) the slave line currznt lsu.

Ma) 1991.
