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Abstract: This paper describes a near unity power factor

rectifier with low harmonics. Small size, low cost, high efficiency are the main features of this rectifier configuration. This configuration makes use of three active bidirectional switches to achieve power factor correction. The switches are rated at a small percentage of total power processed and is switched at a very low frequency. The analysis of this configuration is carried out in this paper with a design example and is simulated using MATLAB/SIMULINK and it demonstrates its high power factor and low harmonics. A simple SPWM inverter with a star connected R-L load is used as the load to the rectifier. Index Terms—Unity power factor Rectifier, AC-DC Converter

I. INTRODUCTION n the past years, several topologies of unity power factor rectifiers have been proposed. Unity power factor rectifiers

find application in several areas like ac drives wind energy conversion system etc. The dc-dc boost type configuration [1] draws very good quality input current. But in high power applications, the boost switch undergoes high voltage stresses and also the design of switch becomes complex. The configuration described in [2] makes use of bidirectional switches, switched at supply frequency. This circuit is of low cost and high efficiency; however it is required to be connected to the neutral of the ac supply which results in pulsed currents in the neutral. Also the charge stored in the inductors causes high switching stresses during turn-off

The disadvantages of the above topologies is eliminated to a large extend by the configuration described in this paper. The power circuit is as shown in Fig. 1. This configuration was first introduced in [3]. Unlike the configuration given in [2] a connection to the neutral is not required, shows lower harmonic content in the input current and no clamping circuit is required for the switches during turn off. This configuration has been used in literature for AC Drive applications [4] wherein the drive has been controlled by two different relationship based on behavioural patterns. The configuration has found its application also in wind energy conversion systems [5].

Sreekesh Kesava Pillai and Paulson Samuel are with the Department of

Electrical Engineering, Motilal Nehru National Institute of Technology, Allahabad, India. (e-mail: k.sreekesh@gmail.com, paul@mnnit.ac.in)

978-1-4799-4939-7/14/$31.00 ©2014 IEEE

II. DESCRIPTION OF CONFIGURATION The sources VA, VB, and VC represent the source voltage.

LA, LB and LC represent the source inductances. The capacitors CA and CB are provided to attain a balanced central node at the output. SA, SB and SC are three bidirectional switches, configuration of which is presented in Fig. 1.

In a conventional diode bridge rectifier, the input current gets highly distorted and is further worsened by the load side voltage filtering using capacitors. This results in peaky current being drawn from the supply. The diodes of the rectifier start conducting only 30° after the voltage and as a result of this only two diodes conduct at any given time. This results in high harmonic distortion and low power factor of source current.

The presence of high THD in line currents results in reduction of overall system efficiency, equipment malfunction and losses being high [6]. Low power factor will result in

Fig. 1. UPF Rectifier Configuration

Fig. 2. Bi-directional switch

Performance analysis for a Unity Power Factor Input Rectifier

Sreekesh Kesava Pillai, Student Member, IEEE, and Paulson Samuel, Member, IEEE

I

2

(a) 0°-30°

(c) 60°-90°

(e) 120°-150°

(b) 30°-60°

(d) 90°-120°

(f) 150°-180°

Fig. 3. The various stages during one half cycle of input voltage from (a) 00-300 to (f) 1500-1800

more current being drawn from supply i.e. more reactive power being drawn, which in turn reduces efficiency and increases losses.

The configuration hence aims at turning on the third diode and hence make the phase current start flowing before the diode is naturally forward biased. Three anti parallel switches are used to bypass the supply across the diode bridge rectifier to the central node of the two high value filter capacitors. It has been observed that if the anti-parallel switch conducts for about 1/12th of the supply voltage period, it will give significant improvement in the input power factor and THD.

The configuration of the bi-directional switch is shown in Fig. 2. This concept was first introduced in [5]. Each bidirectional switch is given a pulse of duration 30° after the zero crossing of the phase voltage of the corresponding phase. The output voltage of the converter is given by

( )137

36

πiV

outV =

Vout being the rated output voltage and Vi being the input line to line voltage. The various stages of the converter during one

3

half cycle of input voltage for every 300 are shown in Fig. 3. A control that actually considers the loading level was introduced in [7].

Assuming that input current is supplying the load during the period of 90° to 130°, the critical source inductance can be determined using

( ) ( )232

2322

736

°

−=fP

iVL

π

Where f is the rated frequency of the ac system and P0 being the rated power of the converter.

This converter is however only dealing with a constant load. As the switching is independent of the load, this method cannot be used for conditions where the load is varying to control the power factor and THD [5]. A different control strategy was proposed by [3]. This method provides an effective control for varying loads. But the control strategy makes use of two separate equations for loads below rated and above rated. Also the THD for low loads is high.

In this paper the author has given a switching strategy for a varying load. An equation was determined which will give the switch on period required to be given to the bidirectional switches. The equation was determined after going through the system behaviour thoroughly. The equation is as follows

( )394.5032.54

2

29.38 +−=⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

refI

I

refI

Iα

Where α gives the turn on period of the switches SA, SB, and SC. I here is the dc link current and Iref is the current corresponding to the rated load. Hence a normalized current value is used to control the switching.

III. DESIGN AND SIMULATION RESULTS The simulation was carried out for a system whose specifications were assumed as follows.

• AC line to line voltage- 220V • AC line frequency-50Hz • Rated output Power of converter-1.5kW

The critical line inductance is calculated using (2) as follows

( )°

−=fP

iVL

32

2322

736

π

°

−×=fPiV

2

22108489.3

mH85.24=

TABLE I SYSTEM PARAMETERS

AC input line voltage 220V AC input frequency 50Hz Input line inductance 32mH

Load Capacitors 1000uF Rated Load 1.5kW

The simulation was carried on a computer platform using

MATLAB software. Table I gives the various system parameters which were used for simulation. As described earlier, the conventional diode bridge rectifier distorts the input current and results in poor THD and poor harmonics. The results for this have been shown in Fig. 4.

It shows the input current and its harmonic content. The current THD is seen to be around 85% and power factor is around 0.75. Fig. 5 shows the input current in one of the phases along with the corresponding phase voltage for the rated load with the suggested control. The current has been amplified for better viewing. The harmonic spectrum has been shown for the rated load in Fig. 6. The harmonic spectrum for 50% of rated load is shown in Fig. 7. The simulation was also carried out for various loads above and below the rated speed.

The variation of performance parameters namely power factor and harmonics for varying loads has been tabulated in Table II. It is observed from the results that the converter gives good performance on loads near the rated load. For loads which are much lower than the rated load it can be seen that the performance of the converter is much affected. This is because the value of the input inductance, as can be seen from the equation 2, is inversely proportional to the power. Hence as the power becomes lower and lower the inductance required is higher and higher.

Fig. 4. Input Current for conventional diode bridge rectifier

Fig. 5. Input Phase Voltage and Phase Current for the rated load (current has been

amplified by 5 times for better viewing)

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This has been to some extend compensated by giving an input inductance larger than the critical inductance value obtained for the rated load. Hence 32mH inductors were used instead of 24.85mH which was obtained during design calculation.

Fig. 6. Harmonic spectrum of source current for the rated load

Fig. 7. Harmonic spectrum of source current for 50% of rated load

TABLE II PERFORMANCE PARAMETERS FOR VARIOUS LOADS

Load (P0) Power Factor THD (%) 50% 0.998 11.55 80% 0.99 6.53 100% 0.99 6.17 120% 0.99 5.32

Fig. 8. Input phase current for variation in load at t=1 from 60% to 100% of

rated load

Fig. 9. Injected current in phase A for the rated load

The Fig. 8 shows the variation of input current for a

dynamic change in load from 60% of rated load to 100% of

rated load. The change is made at time 1s while the system is in steady state. Fig. 9 shows the waveform of current injected through bidirectional switch to the load in the phase A.

IV. CONCLUSION The conventional three phase diode bridge rectifier injects high amount of harmonics and the THD of input current is 85% and the power factor is 0.75. This is very poor and affects the system efficiency dramatically. The converter discussed in this paper shows excellent performance for varying loads. It is simulated in MATLAB and it was seen that the THD varied from 11.58% to 5.32% for loads varying from 40% to 120%. The power factor was also consistently above 0.99 for all the loads.

V. REFERENCES [1] A. R. Prasad, P. D. Ziogas, and S. Manias, “An active power factor

correction technique for three-phase diode rectifiers,” in IEEE/PESC Conf. Rec.,1989, pp. 58–66.

[2] I. Barbi, J. C. Fagundes, and C. M. T. Cruz, “A low cost high power factor three-phase diode rectifier with capacitive load,” in IEEE APEC Conf. Proc.,1994, pp. 745–751.

[3] E. L. M. Mehl, and I. Barbi, “An improved high power factor and low-cost three-phase rectifier,” IEEE Trans. Ind. Appl., vol. 33, no. 2, pp. 485–492, Mar./Apr. 1997.

[4] A. I. Maswood, and L. Fangrui, "A novel unity power factor input stage for AC drive application," IEEE Transactions on Power Electronics, vol. 20, pp. 839-846, 2005.

[5] A.Venkataraman, A. I. Maswood, N. Sarangan, and O. H. P. Gabriel, “An Efficient UPF Rectifier for a Stand-Alone Wind Energy Conversion System,” in IEEE Transactions on Industry Applications, vol .50, no. 2, pp. 1421-1431 Mar./Apr. 2014

[6] S. Kim, P. Enjeti, D. Rendusara, and I. J. Pitel, “A new method to improve THD and reduce harmonics generated by a three phase diode rectifier type utility interface,” in Conf. Rec. IEEE IAS Annu. Meeting, 1994, pp. 1071–1077.

[7] F. Liu, and A. I. Maswood, “A novel near-unity power factor AC drive,” in Proc.of IEEE Int. Conf. Power Electronics and Drive Systems (PEDS), Singapore, 2003, pp. 1090–1094.