IJCST Vol. 3, Issue 1, Jan. - March 2012
ISSN : 0976-8491 (Online) | ISSN : 2229-4333 (Print)
Three-Phase Diode Rectifier with Active Current
Modulation and High Efficiency
1
1,2
Jeji.K, 2CH. Sujatha
Gudlavalleru Engineering College, JNTU Kakinada, Gudlavalleru, AP, India
Abstract
An active current injection network for a three-phase rectifier is
proposed. The proposed circuit uses three bidirectional switches
operating at low frequency and a half-bridge inverter operating at
high frequency. It also uses an inductor in order to make the current
modulation. This topology uses two active networks: an injection
network composed by three bidirectional switches operating at
low frequency and a current shaping network consisting of two
high frequency switches and only one inductance.
Keywords
Current Injection, High Power Factor, Rectifiers, Three-Phase
I. Introduction
The increase of nonlinear loads to the electric power distribution
has motivated to research for finding different options in order to
alleviate the difficulties associated with the power quality. Diode
rectifiers are extensively used; this is because they are cheap and
simple. However, for the three phase rectifier, not only is the
power factor penalized, but also a current with high harmonic
content is required. Three phase current injection rectifiers are
recently among the most attractive AC to DC energy conversion
topologies required in medium and high power applications. A
three phase current injection rectifier is typically the association
of three circuits: a classical diode bridge that is responsible for
the energy rectifying operation, a modulation circuit that has the
major role in the current shaping process, and a distribution circuit
which has the property of injecting a zero sequence current into the
source phases, avoiding thus the current discontinuities at the input
of the diode bridge. The modulation and distribution circuits can
be either passive or active. The use of a passive modulation circuit
yields a high power factor at the expense of a low efficiency. Based
on current modulation technique, this topology uses two active
networks: an injection network composed by three bidirectional
switches operating at low frequency and a current shaping
network consisting of two high frequency switches and only one
inductance. The current injection network selects the part of the
inductor current to be injected into the respective phase, where as
the current shaping network generates the third harmonic current
needed to improve the input current waveform and injects it onto
the respective phases through the current injection network.
Fig. 1(b): Proposed Rectifier Circuit
The fig. 1(a) and (b), shows a different topology of three phase
rectifier with active current modulation proposed. This circuit
consists of an inductor that operates at high frequency. This paper
is organized as follows. The proposed three phase rectifier with
active current modulation is explained in Section II, the power
processing analysis of the topology is shown in Section III, the
simulation and experimental results are discussed in Section IV,
and finally, conclusions are given in Section V.
II. Proposed Three Phase Rectifier with Active Current
Injection
The proposed circuit uses two active networks shown in fig. 1.
A. Current Shaping Network
It consists of two switches and one inductor that operate at high
frequency. This network main role is based on generating the
desired current il and on making the circuit adaptable to any load
variation. A reference current ilref is built depending of the input
ac mains and of the charge load. The two switches Sx and Sy
operate at high frequency in order to establish the closer image
to the current reference.
B. Current Injection Network
It is based on three bidirectional switches connected to the
three supply phases. The injection network selects the part of
the inductor current to be injected into the respective phase. The
related switches operate at low frequency equal to the double of
the source supply frequency.
Fig. 1(a): Proposed Rectifier Block Diagram
Fig. 2: Waveforms of the Proposed Converter
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IJCST Vol. 3, Issue 1, Jan. - March 2012
ISSN : 0976-8491 (Online) | ISSN : 2229-4333 (Print)
Fig. 2, shows the waveforms of proposed network for one phase.
Just a single device is turned on at a time; however a small overlap
between the control signals of the switches is needed in order
to avoid over voltages due to the inductor topology. Switches
sequence command in the injection network indicated in given
table 1 for intervals between 0o and 180o. Intervals between 180o
and 360o could be deduced similar to the first demicycle.
operations. Typical control signal is used for the respective switches
related to the main phases. Switches status varies 30 degrees
before and after each zero-voltage crossing of the respective phase
voltage. The given fig. 3, represents the control signals for the
switches in the injection network.
Table 1: Seuence of Command for Switches S1, S2 and S3
C. Operation of the Converter
1. During 0 − π/6 Radians
The current required by the injection network for phase “a” is
proportional to the respective phase input voltage (iai). This current
is the same with that of the current shaping network during this
time (il). The other two phase voltages have zero current demanded
by the injection network. The rectifier current during this time is
zero (iar). Then, the required current from the voltage source is
the same with the current demanded by the injection network.
2. During π/6 − π/2 Radians
The current demanded from the injection network by phase “a”
is zero during this time (iai). The current of the shaping network
il is now injected to phase “c.” The rectifier current during this
time is different to zero. The current demanded from the phase “a”
voltage source is the same with that required by the rectifier.
When the injection circuit is connected, the current is slightly
different; this is because the shaping circuit is connected to the
output voltage. The actual current for the proposed converter is
also shown in the fig. as iar.
3. During π/2 − 5π/6 Radians
The required current by the injection network for phase “a” is
zero during this time (iai). The current of the shaping network
il is now injected to phase “b.” The rectifier current during this
time is different to zero for phase “a.” Then, the required current
from the voltage source is the same with that demanded by the
rectifier (iar).
Fig. 3: Control Signals for Injection Network
The waveforms of the injected current is shown in fig. 4
Fig. 4:
And the fig. 5, shows the control sequence of the injection
circuit.
4. During 5π/6 − π Radians
The required current by the injection network from phase “a”
is proportional to the respective-phase input voltage (iai). This
current is, again, the current shaping network during this time (il).
The rectifier current during this time is zero. Furthermore, again,
the required current to the voltage source is the same with that
demanded by the injection network.
5. During the Following π Radians
The converter operation during the negative semi cycle of phase
“a” is similar to the positive semi cycle.
D. Control Strategy
The system requires two control circuits: the first one for the
injection network and the second one for the shaping circuit.
The injection network can be controlled with a simple open
loop controller using only some comparators and mathematical
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Fig. 5: Control Sequence of Injection Circuit
The current shaping network control requires a closed loop control.
The control key is based on stabilizing the inductor current near a
specific waveform. Thus, the two switches should operate at high
frequency in order to establish the closer image to the current
reference. It is obtained with the combination of some comparators
and mathematical operations on the subject of the three-phase
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IJCST Vol. 3, Issue 1, Jan. - March 2012
ISSN : 0976-8491 (Online) | ISSN : 2229-4333 (Print)
input voltage. A hysteresis current controller is also introduced
in order to get the control signals.
The inductor current reference can be built using the
following:
(1)
For a unity power factor, the following equations can be
obtained:
(2)
The required current from phase a is then
(3)
Knowing that:
(4)
The shaping network is controlled by closed loop controller. Fig.
6, shows the control block diagram of the shaping circuit. The
input voltages va, vb, vc and load value are detected in order to
calculate the inductor current reference. A hysteresis controller
is applied to restrain the switching ripple.
The converter operates with an input voltage of 220 Vrms, and
the switching frequency is 100 kHz. The ripple was selected at
0.25 A. The inductance L = 6 mH
III. Power Processing of the Converter
The converter has the important feature that only part of the power
released to the load is processed by the injection and shaping
networks. As a result of this, a high efficiency is obtained. Fig.
7, shows the power flow representation for the converter. Part of
the energy is processed by the three-phase rectifier; type another
part (k) is processed by the networks.
The three-phase rectifier losses are negligible, Pin is obtained
as
By solving the above equation the efficiency is
(6)
(7)
The rate k is around 3.7%, and considering the network efficiency of
80%, the efficiency obtained for the complete system is 99%.
The processed power by the injection and shaping networks can
be estimated with
(8)
Where T is the period of the input voltage and iai is the current
demanded by the network. By solving the above equation.
Where Ip = 3Vp/2R.
The output power of a three-phase rectifier is
(9)
(10)
Using (9) and (10) obtains the rate of the processed energy
(11)
When the efficiency ηn takes a value of 80%, the percentage k
is 3.7%.
From equations (7) and (11), the efficiency of the system is obtained
as a function of injection and shaping network efficiency
Fig. 6: Control of the Modulation Circuit
(12)
If ηn is reduced, the system efficiency is even quite acceptable
because the rate of the processed energy is minimum. For example,
if we consider ηn equal to 60%, the efficiency of the system is
98%; this becomes a high efficiency for the whole converter. If
we draw the graph between converter efficiency and complete
system efficiency we can observe the high efficiency.
Fig. 7: Power Flow Representation of the Converter
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IJCST Vol. 3, Issue 1, Jan. - March 2012
ISSN : 0976-8491 (Online) | ISSN : 2229-4333 (Print)
(a). Current Demanded by the Rectifier with the Networks
Working
Fig. 8: Improvement of Complete System Efficiency
(b). Current Injected by the Injection Network
(c). Compensated Current
Fig. 11: Represents Currents of the proposed system
Fig. 9: Graph Between k and ηn
IV. Simulation and Experimental Results
The system was mathematically simulated on Simpower System
Matlab. The phase-to-neutral RMS-voltage is 127 V, the mains
frequency is 60 Hz, the resistive load has a value of 61 Ω, the
adopted switching frequency is 100 kHz, and the inductor’s value
is 6.22 mH. The injected and compensated currents are shown in
fig. 8, when the system is submitted to load variations. The Total
Harmonic Distortion (THD) of the compensated current is 11.6%
and the power factor is 0.99.
In order to further reduce the total harmonic distortion, an
adjustment factor k is introduced:
The given Fig. 8 shows the relation between the coefficient factor
k and THD percentage.
The system under load variations the currents of the three phases
are shown in fig. 12.
(a). Phase ‘a’ Current
(b). Phase ‘b’ current
(c). Phase ‘c’ Current
Fig. 12: Phase Currents Under Load Variations
Fig. 10: Current THD Versus Adjustment Factor k
It is noted that for 1 < k < 2, the power factor is always equal
to 0.99. Fig. 9 shows the relation between THD percentage and
load value for k = 1.45. It is also noted that the power factor is
equal to 0.99.
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V. Conclusion
This paper has offered a different method to alleviate the harmonic
content of a three-phase rectifier. A system with two active networks
was suggested, with the first one for generating the third-harmonic
current and the second one for injecting the current to the ac
mains. In this paper a new adjustment factor k is introduced, for
the adjustable value of k, the system overall efficiency is improved
and the total harmonic distortion is also reduced.
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International Journal of Computer Science And Technology ISSN : 0976-8491 (Online) | ISSN : 2229-4333 (Print)
Jeji Katuri pursuing M.Tech (PEED)
in Gudlavalleru Engineering College
affiliated to Kakinada JNTU University.
She has received B.Tech degree in
electrical and electronics engineering.
Smt. CH.Sujatha is working as Associate
Professor at Gudlavalleru Engineering
College. She has received B.Tech,
M.Tech degree in Electrical and
Electronics Engineering and her area
of specializations are Machines and
Industrial Devices.
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