SINGLE PHASE W TNREE PHASE CONVERSION W I T n o u T DC FILTER

M-R- UDAYAGIRI, Member -; V - S - S - Sarma

C e n t r a l ELectrclzlics ErrgineerinGJ R e s e a r c h Institute

V S tvoSts\

PILANl -

Voltage 'Fundamental Components waveform

Magnitude Phase shift ( v o l t s ) (Degrees)

v 419.58 16.52

ABSTRACT

Single phase to three phase converters consisting of controlled rectifiers for converting unregulated single phase AC to regulated DC voltage,DC filter for reducing the ripple voltage and three phase inverter for converting DC t o balanced three phase voltages are the art of the day. The filter circuit consistihg of electrolytic capacitors and DC chokes claim 25% of material cost, 50% of weight and size of the total system. The proposed paper deals with a novel single phase to three phase converter without filter components.

The ' system basically comprises of a full wave bridge rectifier and a three phase conventional inverter. First the operation of this system with six step logic is discussed which results in unbalanced three phase output voltages. Further necessary modifications are suggested to achieve:

i . Balanced three Dhase output voltages ii. Inverter output voltage control and

iii. Harmonic Elimination

A specific solution to obtain balanced three phase output voltages with no third harmonic is presented. The theoretical results are supplimented with practical results obtained from a 3 KVA Single Phase to Three Phase converter using this novel method.

I. INTRODUCTION:

Single phase to three phase converters are extensively used in electric locomotives and rural areas where only single phase power can be made available because of either technical or economical reasons. Single phase to three phase converters consisting of controlled rectifiers for converting unregulated single phase AC to regulated DC voltage, DC filter fqr reducing the ripple voltage and three phase inverter for converting DC to balanced three phase voltages are commertially available (1,2). The DC filters with electrolytic capacitors and DC chokes not only increases the cost, weight and size but also decreases the reliability.

Novel. approaches to convert single phase to three phase without smoothing components were already published in li-terature ( 3 , 4 ) . This paper suggests a new apprwch for the same problem using conventional three phases bridge inverter.

11. THREE PHASE INVERTER OPERATION WITH FULL HAVE RECTIFIED AC INPUT

Fig.1 shows the system which consists of a full wave rectifier with single phase AC input feeding a three phase inTr?rter. Fig.2 shows the conventional six step logic gate pulses alongwith

533 031: INDIA

V

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ThG amplitudes of the fundamental components alongwith phase shift cab be calculated using standard Fourier analysis iiiethod. Table 1 gives the caculated results. It can be noted that the three phase outputs are unbalanced.

111. GENERATION OF BALANCED FUNDAMENTAL VOLTAGES

V S

The primary requirement for-balanced three phase voltage system is 120 degrees phase shift among the three output voltages with equal amplitudes. In order to achieve the above conditions VvO of Fig.2 with 90 degrees phase shift can be taken as reference voltage. It can be noted from Fig.2 that if a voltage chop is introduced in waveforms VRY and VDR between 60 to 120 degrees this will not effect the reference voltage waveforiii V y e . Fig. 3 shows the switching and output waveforms with this modification. d, and qLare the chopping angles. By properly choosing o(, and Mathe phase angles 8ayandeBRCan be iiiade +30 degrees so as to get 120 degree phase shift among the three outputs .

Waveform FUrIDAMENTAL HARMONIC AMPLITUDES

V 0 3rd 5th 7th

238.68 -30- 89.79 109.38 94.64 V

FIG. 3

Using V R ~ , Vv,gand Veawaveforms from Fig. 3 and Fourier equations the following expressions can be derived:

In order to get 120 degrees phase shift among the three wave shapes M, and o(% are t o be selected such thatdo, andean are equal to -30 and 210 degrees respectively. :'sing Newton's inethod

IV. OUTPUT VOLTAGE CONTROL

Froz FiG.3 it can 5; obscrved that any extra chop between 0 to 6C degree effects all the three output voltages. However o( should be varied accordingly to achieve balanced three phase fundamental output ;oltaGes. Fig.4 shows three phase logic and output waveforms with this modification. Table 4 shcri the output voltagc variation with Ml and o(.

T

FIG. &

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a 10 20 3 0 40 50 I EC

V. ELIMINATION OF LOWER ORDER HARMONICS

The main drawback of above method is its higher amplitude of lower order harmonics. These harmonics can be filtered by employing appropriate tuned line filters. However in this paper well known harmonic elimination method, in which the harmonics are eliminated by chopping the switching waveforms at appropriate chopping angles, is used for eliminating lower order harmonics. Since the wave patterns with rectified single phase AC input without. filter to inverter contains trignometric functions the fundamental and harmonic component equations are derived and solved using numerical techniques. The variables are scanned over the allowed range and th2 best possible solution for the required conditions is chosen. The present paper deals with 3rd harmonic elimination. However the same technique can be extended for elimination of more number of harmonics.

a

V

VI. THIRD ~ARMONIC ELIHINATION

I

29.08 26 .76 23.5: 21 1 9 18.2

9.6 37 .25 79 .5 1 3 1 . 5 186.5 235 .7

Fig.4 which is used for output voltage control shows the waveforms of Vey , V VB , and V B R with one chop between 0 to 6 0 degrees. The fundamental and 3rd harmonic components of these waveforms are as follows:

A~(\/RY) = h 0 5 - COS 1 4 3 7 COS 2qXl 3 IO B!L(VRV)

ASCVav)= 5 E C O S ~ M , -r Cas 4 % ~ LCOSPY, -9 COS w.] [L+ z~,-w - s l n W , + S i n - 2 s sn zq- 5 053 a I \

* \I 4 x

4T 63CvQY) =L [Sin4o(l- 5 n 4 4 ? + 2 5 ~ n ~ , - Z Sm142-C56m I3

Eqns. 10 to 21 are solved simulatenously ts obtain ths values o(\ , M~ and 4 to satisfy the following conditions.

First the set of% vlaues which satisfies the phase angle requirement are solved and same are substituted in the above expressions to scan O<L and ctzvalues from 0 to 60 degrees. It is observed that the third harmonic is not eliminating for the entire range. The best possible solution found is o( = 19.5 ,HI = 45 andC$= 60 . Table 5 shows the theoretical results when the input RMS voltage is 500 volts.

Table - 5

Harmonic amplitudes Fundamental Waveform

Amplitude I Phase angle1 3rd I 5th I 7th

1 162.3 I -30° I 31.77 I 38 I 47.6 1 I 160.0 1 90' 1 3.9~-51 66 c 1 33.33 I I 1E2.3 I 210' I 31.77 I 18.551 i4.63 I

From Table 5 it can be concluded that it is not possible to eliminate third harmonic in all the three outputs by a single chop between 0 to 60. As the third harmonic in Vy-is eliminated for the output waveforms shown in Fig.4 with 41 , O ( L and o( as 45, 60 and 19.5 degrees, a second chop for

eliminating third harmonic in VRY and VgQ between 60 to 90 degrees is introduced which should not effect V r ~ a s , during this period Vyg is already zero. Fig.5 shows corresponding waveforms with these modifications. Using similar techniques mentioned already d , W I , O(L , d g and o(4 values can be computed which can eliminate third harmonics in all the three line-to- line output voltages, still providing balanced three phase fundamental components. A unique solution with o< = 5.6 , d , = 45 , O ( l = 60 ,q(3 = 60 , o($= 76 gives balanced three phase outputs with no third harmonic voltages. Table 6 shows the theoretical results with the above modifications.

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v 0

'RY

V Y B

BR

IWaveform Harmonic amplitudes as % of V

llst 3rd 5th 7th 9tk 11th l3tt S

I I

hn

FIG. 5

Table 6

I I I .

I I I I I 1 I t

I I D I I I I l l I I

1 Note: No even harmonics are prescrrt

VII. CONCLUSIONS

A 3 KVA single phase to three phase converter using full wave rectifier bridge, transistorised inverter without filter components was fabricated at CEERI to verify the theoretical results. Microprocessor based logic is used to generate different logic control pulse patterns to turn on and off the inverter transistors ( 5 ) . The system is operating satisfactorily. At present input step up transformer is used to take care of voltage under utilization problem of the system to operate 400 volts three phase loads. A novel method to overcome this problem is thought off and the development is under Drogress.

VIII.ACKN0WLEDGEUENTS

The authors wish to thank all the Power Electronics group members of Central Electronics Engineering Research Institute, Pilani (India) for their valuable discussions and help during fabrication. The authors also wish to thank Dr. US Khokle, Director for providing permission to publish the work.

IX. REFERENCES

UM Rao, SS Shekhawat, GN Acharya,"A Single phae-to-three phase static converter", Proceedings of IEE Conf. on Power Electronics - Power Semiconductors & their applications, 1977.

N Umesh Rao, "The General Motors GE6C Electric Locomotive", IEEE Transaction On Industry Application: Vol IA-22, No.3, May/June, 1986.

SI Kalra, PD Zioas, MH Rasid, "A Novel Single-to-three phase Static Converter", IEEE Transaction on Industry Applications; Vol 25; No.1, January/Feburary, 1989.

Katsuji Shinohara, Yoshitaka Minari, Toshiyuki Ir isa , Yutaka Imamura, "Analysis and Fundamental Characteristics of Induction Motor Driven by Voltage-Source Type Inverter without DC Link Components".

VSS Sarma, "Microprocessor Based Single phase to three phase converter without filter components", submitted to Kakatiya University, Warangal (India).