a new high power factor bidirectional hybrid three

8/8/2019 A New High Power Factor Bidirectional Hybrid Three

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A New High Power Factor Bidirectional

Hybrid Three-Phase Rectifier.

Abstract This paper presents a new

bidirectional hybrid three-phase rectifier

suitable for medium and high powerapplications. The rectifier employs a three-

phase diode bridge rectifier and a Boost-type

three-phase PWM rectifier. The proposed

rectifier is capable of providing sinusoidal

input currents with low harmonic distortionand DC output voltage regulation. The

rectifier topology generation, the principle

of operation, control scheme and simulation

results are described in

the paper.


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INTRODUCTION

Nowadays, governments and internationalorganizations

have introduced the IEEE 519 and IEC 61000-3-2/61000-3-4 standards for limit the harmonic contentof the currents drown from the AC power line.

To address the problem a great number of newsingle-phase and three-phase rectifier topologies

have been developed.In high power applications, where three-phase

systems are required to avoid the system load

unbalance, the search for three-phase rectifiertopologies with high efficiency, high reliability,

simple control scheme and high quality inputcurrents were increased in the last decade.Three-phase diode bridge rectifiers are commonly

used asfront-end converters due their simplicity, reliability

and low cost. However, they do not meet theharmonic content imposed by the IEEE 519 and IEC

61000-3-2/61000-3-4 international standards.


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The use of multi-pulse three-phase rectifiers achievelower harmonic content of the input current bycancellation the harmonic components generated by

each group of six-pulse diode bridge rectifier.Moreover, they keep the simplicity and reliability

features. However, they

Three-phase PWM rectifiers meet the internationalstandards for harmonic current limit, providing

sinusoidal input currents with low harmonicdistortion. PWM rectifiers are more expensive and

less reliable than diode bridge rectifiers.In the last years, the search for rectifiers thatcomprise the

better features of diode bridge rectifiers and theadvantages of PWM rectifiers introduces a new classof three-phase rectifiers with low effect on the

mains, the Hybrid Rectifiers [1, 2, 4-6].


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The basic idea of the Hybrid Rectifiers is that theyare generated by the connection of an uncontrolled

rectifier with a rectifier operates with low frequency

and it handles the higher output power rating.Therefore, the controlled rectifier is designed tooperate with small power ratings and with highswitching frequency.

This conception proposes that the semiconductorswhich are operating with low switching frequency

will commutate with high power rating. Unlike, thesemiconductors which are operating with highswitching frequency will commutate with lower

power rating. It suggests that the efficiency of thesystem will increase.

This paper proposes a new bidirectional hybrid

three-phase rectifier with high power factor and DCoutput voltage regulation. The rectifier employs athree-phase diode bridge rectifier and a Boost-typethree-phase PWM rectifier connected in parallel.


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THE PROPOSED HYBRID RECTIFIER

The proposed hybrid three-phase rectifier isoriginated by the parallel connection of a three-

phase diode bridge rectifier and a bidirectionalBoost-type three-phase PWM rectifier.

However, it is not possible to connect the bothrectifiers directly due to the step up characteristic ofthe Boost-type PWM rectifier.

Thereby, the hybrid configuration should employ acircuitry to connect the both rectifiers. This circuit

can be connected inthe AC-side or in the DC-side of the hybrid rectifier.The AC-side connection is performed by means of a

threephase transformer (or autotransformer.)In this case, the outputs are directly connected; the

AC-side connection works in low frequency and itdid not allow the output voltage control.

0-7803- Otherwise, the DC-side connection can beperformed by means of a DC-DC Boost converter.In this hybrid rectifier, the inputs are directly

connected; the DC-side connection works in highfrequency and it allows the output voltage control.


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PRINCIPLE OF OPERATION

The proposed bidirectional hybrid three-phaserectifier employing Boost converter is showed inFig. 4. The line currents i1,2,3 are obtained by the

sum of currents i1a,2a,3a and currents i1b,2b,3b.Similarly, at the output, the load current io is

the sum of currents ioa and iob.To achieve high power factor, the line currents i1,2,3

must be sinusoidal and without displacement factor.This way, the currents i1a,2a,3a and currentsi1b,2b,3b will be controlled to provide the line

currents with a sinusoidal shape.The shape of the currents i1a,2a,3a are imposed bythe diode bridge and, by the control of the Boost

inductor current, the peak current of these currentscan be controlled. The DC-DC Boost converter must

be operating in Continuous Conduction Mode andBoost inductor current must be following a constant

reference current. Therefore, the diode bridgerectifier keeps his characteristic of low frequency

commutation. The currents i1b,2b,3b are controlledto follow a desired reference, so that the sum ofthese currents with the currents i1a,2a,3a result in

sinusoidal input currents.


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The power level processed by each rectifier can berelated with the peak currents. Theoretically, any

combination of power can be performed by each

rectifier and in all cases, input currents will presentsinusoidal shape. Therefore, it is interesting that thediode bridge rectifier processes the greatest

part of the output power, according to the conception

of the Hybrid Rectifiers.Each combination between the powers processed for

each rectifier represents a specific mode ofoperation. Fig. 5 shows the ideal currents waveformsfor two different modes of operation. The first

operation mode, called operation mode 1,is related with the situation where the diode bridge

and the Boost converter supply the output rated

power. Thus, the PWM rectifier processes just theenergy necessary to achieve high power factor at theinput. For this situation, the current waveforms are

presented in Fig. 5 a), Fig. 5 b) and Fig. 5 c).

The second operation mode, called operation mode2, is related with the situation where each rectifier

supplies a half of the output power. In this case, thecurrent waveforms are presented in Fig. 5 d), Fig. 5e) and Fig. 5 f).


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Fig. 5. The ideal currents waveforms for different modes of operation: a) line current i1, b)

current i1a and c) current i1b for operation mode 1; d) line current i1, e)current i1a and f) current i1b for operation mode 2.


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The control scheme of the hybrid rectifier is showedin Fig. 8. It is composed by four current control

loops and a voltage control loop.

The voltage control loop provides DC output voltageregulation and the references for the current controlloops.The Boost converter current control loop provides

the peak current regulation for input currentsi1a,2a,3a. Thus, the active power processed by the

three-phase diode bridge and the DCDCBoost converter is set up.The currents i1b,2b,3b are controlled indirectly by

the sensing and control of input currents. In thiscase, it is an interesting approach because it is easier

provides sinusoidal references than the references

with the shape of currents i1b,2b,3b.The control strategy is based on the analogmultiplier approach. This is typical for PFCimplementation of power converters operating in

continuous conduction mode and fixedfrequency.

The control can be implemented by analogcommercial integrated circuits for power factorcorrection or by a digital signal processor.


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This paper presents two new topologies of

bidirectional

hybrid three-phase rectifier suitable for medium andhigh power applications. The rectifiers employ athree-phase diode bridge rectifier and a Boost-typethree-phase PWM rectifier. The mathematical

analysis shows the region of operation ofthe hybrid rectifier with improved performance. The

region among the interval 0.75 0.91

minimizes the active power and the apparent power processed by PWM rectifier and maximizes theactive power processed by diode bridge rectifier

and the DC-DC Boost converter.The advantage of this hybrid system is the capability

of delivering high power levels due the parallelassociation of the rectifiers. Moreover, the increaseof the efficiency is another expected advantage.

The increase of component count does not affectstrongly the volume of the rectifier because the

components are designed for lower current rates.

As may be seen from the simulation results, thehybrid rectifier presents high power factor and DC

voltage regulation.


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The disadvantage of the system appears in thecontrol scheme, by the use of an extra current sensor

and an additional current control loop.

a new high power factor bidirectional hybrid three

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