Power Factor Correction with PWM Rectifier
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POWER FACTOR CORRECTION WITH PWM RECTIFIER
V.M. Mishra1, A.N. Tiwari2, N.K. Sharma3 and S.P. Srivastava4
1
G.B. Pant Engineering College, Pauri-Garhwal (Uttrakhand)
2,3
M.M.M. Engineering College, Gorakhpur (U.P)
4
I.I.T, Roorkee (Uttrakhand)
Abstract: The occurrence of power supply anomalies (e.g. voltage
sags, surges and swells, sustained under and over voltages, etc.)
originating on the secondary side of customers and high utilization
level can often damage and/or disrupt customers computerized
process, electric equipment and interrupt loads, a costly issue for
customers and utility society. For last two decades researchers
developed new algorithms and model for unity power factor
incorporating PWM rectifiers, so that cheap and good quality of power
can be made available to the customers without violating system
disturbances. Still research is in progress to meet the present day
congestion management problem with the help of PWM rectifiers,
efficiently. Therefore, the authors presented a technique high power
factor incorporating PWM rectifiers up to date.
Index Terms: Near unity power factor, PWM rectifiers
1. INTRODUCTION
The modern drive specifications are required to meet the new IEEE519 standard, as to avoid current and voltage harmonics distortions
of the utility. The possibility of the operating the power system at
minimal cost while satisfying specific transmission constraints and
securing drives capacity by the use of PWM rectifiers. For medium
to large size drives, twelve and eighteen pulse converters are readily
available. The twenty four and thirty pulse converters are under
development. The choice of pulse number is a matter of economic
verses harmonic control.
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The objective of power factor correction and PWM rectifiers is
to obtain smooth ramp- up and ramp down acceleration transients,
optimized mechanical load characteristics, equipment reliability,
reduced mechanical related maintenance, increased product quality,
reduced audible noise level and reduced physical space requirements. The motor drives optimize speed and torque of the motor,
thus saving on energy costs. Hence there is an urgent need of near
unity power factor and sophisticated power flow control while
incorporating PWM rectifiers. A PWM rectifier is capable to drawing
nearly sinusoidal input current with the phase angle between
0-360°. It also provides regulated dc link voltage and regenerative
capability hence, most suitable for four quadrant operation.
Conversion of ac line voltages from utilities has been done by
combination of rectification and a large capacitor. Such a conversion
approach leads to pulsed current being drawn from the ac distribution networks. These current pulse cause following problems:
•
Poor use of the ac source and distribution wiring volt ampere
capacity because of the high harmonic contents of the line
currents;
•
Distortion of line voltage waveform caused by harmonic
currents and the non-zero source impedance of the
distribution network(constructive combination of harmonic
current in neutral return lines can, in particular, lead to such
distortion);
•
Injection of noise in to equipment which operates from the
line voltage.
2. CONCEPT OF PWM CONVERTERS
The active rectifier front end of a pulse width modulated (PWM)
inverter drive has been attracting increased attention due to
incessantly growing power quality concern [1], [2]. The boost type
PWM rectifier has been increasingly employed in recent years since
it offers the possibility of a low distortion line current with unity
power factor for any load condition [3],[4],[5]. Another advantage
over traditional phase controlled thyristor rectifiers is its capability
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Power Factor Correction with PWM Rectifier
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for nearly instantaneous reversal of flow. [6] It has been shown that
unbalanced input voltages or impedance causes an abnormal second
harmonics at the dc bus, which reflected back to the input causing
a(non-zero sequence) third order harmonic current to flow. Next,
the third harmonic current causes a fourth order harmonic voltage
on the dc bus, and so on. This result in the appearance of even
harmonics at the dc output and odd harmonics in the input currents.
These additional components cause added losses in the dc link filter
capacitor [7].
The single-phase representation of the PWM rectifier circuit
presented in Fig. 1.1 a is shown in Fig. 2.1 b. L and R represent the
line inductance and resistance respectively, eL is the line voltage and
eS is the bridge converter voltage controllable from the DC-side.
Magnitude of eS depends on the modulation index and DC voltage
level.
Fig. 1: Simplified Representation of Three-Phase PWM Rectifier. (a) Main
Circuit (b) Single-phase Representation
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Fig. 2: Phasor Diagram for the PWM Rectifier (a) General Phasor Diagram (b)
Rectification at Unity Power Factor (c) Inversion at Unity Power Factor
Figure 2 presents general phasor diagram and both rectification
and regenerating phasor diagrams when unity power factor is
required [19].
The figure shows that the voltage vector eS is higher during
regeneration (up to 3%) then rectifier mode. It means that these two
modes are not symmetrical. Main circuit of bridge converter
(Fig. 1a) consists of three legs with IGBT transistor or, in case of
high power, GTO thyristor. The bridge converter voltage can be
represented with eight possible switching states (Fig. 3, six-active
and two-zero) described by equation:
Fig. 3: Switching States of the PWM Bridge Converter
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Power Factor Correction with PWM Rectifier
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ek + 1 =
2
udc e jkπ /3
3
(1)
for k = 0....5
=0
for k = 6, 7
3. PF CORRECTION WITH PWM CONVERTERS
The half wave and full wave (H-bridges) converters are used for
low performance and high performance applications, respectively.
The half wave topology can deliver power only for motoring in both
the directions without regenerative capability. In the full bridge
converters with two controlled switches having anti-parallel diode
per phase, bi-directional power flow is achieved. This topology is
very common for three phase ac drives [8], [9].
Hamid A. Toliyat et al [10], developed a low cost thyristor based
load commutated inverter(LCI) for brushless paramagnet motor
drive system. The developed drives is low cost, with high reliability,
adequate performance for high volume production and application
to commercial application.
Present assumption is that voltage distortion is the responsibility
of the user[11]. In, [12] a general methodology for computing
voltage and current distortion in accordance with IEEE-519-1992 is
presented.
The choice of pulse number is a matter of economic verses
harmonic control. Large ac drives (100 hp and above), typically use
thyristor converters for bus voltage and line current control for speed
regulation. The bus voltage is controlled by switching of thyristors
in the converters. At low speed, the 5th harmonic current distortion
is observed as high as 50% at thyristor converter fed ac drive
installation [13, 14]
B.T. Ooi et al, [15] described, an integrated ac drive system using
a controlled current PWM converter link is reported. Two identical
three phase, bipolar transistor, controlled current, PWM power
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modular are integral so that one function as a rectifier and other as
inverter in an ac drive system. The input currents maintain near
sinusoidal at utility frequency with unity power factor and also it
can work with leading power factor [16].
N.R. Zargari and Geza Joos, [17] presented, performance
investigation of a current controlled voltage -regulated PWM rectifier
in rotating and stationery frames. The current controlled PWM
rectifier provides near sinusoidal input currents with unity power
factor and low output voltage ripple. It provides a well defined input
current spectrum; exhibit fast transient response to load variations
and it is also capable of regenerative operation.
4. SCOPE FOR FURTHER USE
The boost type PWM rectifier has been increasingly employed in
recent years since it offers the possibility of a low distortion line
current with unity power factor for any load condition [5], [18].
Another advantage over traditional phase controlled thyristors is
its capability for nearly instantaneous reversal of power flow.
Fro the PWM view point, further prospects are mostly
dependable on a number of practical applications of the PWM
controllers. Expecting increased number of their installations, raised
concern appears within their coordination in overall planning and
operational procedure. System with several PWM rectifiers is to be
analyzed. Possible overlapping or interaction between controls
systems are to be investigated. Value added increased of drives
capacity by using PWM devices is compared with other solutions.
5. CONCLUSION
This paper has presented the technique used with PWM rectifiers
to achieve high power factor with regenerative capability. However
it is concluded that PWM rectifiers can essentially improve power
factor, hence improve the performance of modern power system
and drives.
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