Download AN OVERVIEW OF UNITY POWER FACTOR POWER SUPPLY

An Overview of Unity Power Factor Supply Performance
Supplied From Low Power Naval Synchronous Generators
MICHAEL S. VICATOS
Hellenic Navy Detachment-Kiel
Werftstrasse 112-114, Kiel 24143
GERMANY
Abstract: The pulsed current required to charge the rectifier smoothing capacitor in power supplies,
creates current harmonics and consequently electromagnetic noise. In addition, it causes distortion to
the supply voltage, particularly when the rectifier is supplied from a source having non-negligible
impedance compared with the load. The Unity Power Factor Power Supplies, (UPFPS) being equipped
with power factor correction circuits, exhibit an input current waveform proportional to the supply
voltage. This reduces the current harmonics and introduces low distortion to the supply voltage. In this
paper, an overview of UPFPS performance is presented, supplied by low power rating synchronous
generators under various supplying and loading conditions. The harmonic distortion produced to the
supplying voltage by the UPFPS, is compared with the distortion produced by the conventional
rectifier equipped with a smoothing capacitor. UPFPS are mainly used in airborne and ship borne
applications.
Key words: Rectifier, Current Harmonics, Conditioner, Power factor correction, Unity power factor,
Distortion, Boost type chopper.
power supply cables. If more than one rectifier
are connected to the same power source, all
smoothing capacitors are more or less charged
simultaneously. This situation usually appears
in limited size grids, like aircraft and ship
mains. In the ship mains the load comprises
mainly electronic equipments with a rectifier
and a smoothing capacitor in their power supply
input and represents a significant percentage of
the source nominal power. In such a case a
highly distorted current flows through the
supplying network, reducing the mains
utilisation and causing significant distortion to
the supplying source.
In ships mains, where the generator size and
weight are constrained, a high power factor is
required for every loading condition.
The Unity Power Factor Power Supply
(UPFPS) is an AC to DC converter, equipped
with a rectifier and a power factor correction
circuit, which in addition provides output
voltage regulation. The power factor correction
circuit, most commonly a boost type chopper, is
pumping current from the supplying source to
the storage capacitor during the entire period.
Fig 1. The storage capacitor is always charged
to a level higher than the sinusoidal supply
voltage peak value. Therefore a controlled
1 Introduction
The increasing needs of DC power require new
sophisticated approach to the traditional AC to
DC conversion techniques. The commonly used
AC to DC conversion methods employs a
rectifying bridge, with an input filter and an
output smoothing capacitor. During operation,
the output smoothing capacitor is charged with
a high value current, for a few milliseconds
only. The charging occurs around the absolute
maximum value of the sinusoidal voltage
waveform, twice during the supplying voltage
period. For the rest of the period, the supply
voltage becomes lower than the charged
capacitor voltage and the rectifier current
remains practically zero. During the capacitor
charging interval, the inrush current causes a
significant voltage drop to the supply source
terminal voltage according to (1).
Vt(t) = V0(t) – R0I(t) – L0dI(t)/dt
(1)
Where Vt(t) is the supply voltage at the rectifier
input terminals, V0(t) is the supplying source
EMF, R0 and L0 the supplying source internal
resistance and inductance respectively and I(t)
the input current.
Additionally, the supply current contains
harmonics, which may cause interference to
sensitive devices located in the vicinity of the
1
D5
D1
L
D6
D2
Switching
control
D3
400Hz, as well as from the 50Hz mains through
a 5kVA transformer. For the cases where a
variable supply voltage was required, a 5A
(550VA) variac was used. The load was
resistive and was varying in steps, from 0 to
approximately 300W.
SW
C
Load
D4
Fig. 1 The UPFPS.
2.1 Variable Voltage Operation
current flow is maintained continuously. By
controlling the input current, the output voltage
is regulated to the desired value. The result is an
input current proportional to the supplying
source instantaneous voltage waveform.
This gives:
- Resistive input characteristic to the AC
to DC converter.
- Reduced low frequency voltage and
current harmonic contents to the supply
source and
- Regulated
DC
output
voltage,
regardless of mains and load variations.
In addition, the UPFPS, due to the boost type
chopper, exhibit a wide operation range in the
supply voltage and the supply frequency. The
generated high frequency switching harmonics
may be easily suppressed by the input filter.
There is a lot of research and development on
this topic in the recent years. Various UPFPS
types of single phase [1]-[6], three phase [7], or
even for domestic use, like fluorescent lamp
supplies [8] and UPS [9], have been described.
UPFPS of various operating principles,
configurations and performances, like PWM
[1], Buck-Boost/Flyback [2], Multi-level
operation [3], Resonant Commutated [4], Zero
Voltage/ Current Transition [5] and Delay Time
Control [6], are given.
The UPFPS exhibits a wide input voltage
operation range, since it is equipped with an
output voltage stabilizer. Thus, for a constant
load, the UPFPS operates at an almost constant
input power P and the input current Iin, varies
reverse proportionally to the supply voltage Vin.
Iin = P/Vin
(2)
The test was performed with a load of 312W
while the supply voltage was varying from 90V
to 140V. The supply source was the 60Hz and
the 400Hz, 7kVA generators, as well as the
50Hz mains. The input current, the input power
and the efficiency were measured versus the
variable supply voltage. The test results are
given in Figs 2, 3 and 4 respectively. Since the
UPFPS operates with constant power, the input
current decreases according to (2) as the supply
voltage increases, Fig.2. Thus, during operation
at higher voltage, lower loss and higher
efficiency, are expected, due to the reduced I2R
loss, Figs 3 and 4.
The supply frequency does not affect
significantly the UPFPS performance, since
there is no transformer employed, to justify iron
losses. Nevertheless, it has to be noticed that,
because of UPFPS self supply and the
additional high frequency switching loss, the
UPFPS efficiency is approximately 5-7% lower
than the corresponding rectifier one.
2 Experimental Results
In this paper, the performance test results of a
400W DC power supply, operating either as a
UPFPS or as a rectifier, supplied from various
frequency AC sources, are presented. During
unity power factor operation, the PWM boost
type chopper, as in Fig 1, provides power factor
correction and output voltage regulation. During
rectifier operation, the PWM control is
deactivated and only the smoothing capacitor is
used.
In both, unity power factor and rectifier
operation modes, the UPFPS was supplied
alternatively from two 120V single-phase 7kVA
generators, having frequencies 60Hz and
UPFPS operation at 312 W
Input current at 50 Hz
Input current at 60 Hz
Input current at 400 Hz
Current (A)
4
3
2
90
100
110
120
130
140
150
Voltage (V)
Fig. 2 Input current versus variable supply
voltage, for 50 Hz, 60 Hz and 400 Hz operation.
2
370
3,0
UPFPS operation at 312 W
Input power at 50 Hz
Input power at 60 Hz
Input power at 400 Hz
2,5
2,0
Power (W)
Current (A)
360
1,5
UPFPS operation at 120 V
Input current at 50 Hz
Input Current at 60 Hz
Input current at 400 Hz
1,0
350
0,5
90
100
110
120
130
140
0,0
150
0
50
100
Voltage (V)
150
200
250
300
350
Load (W)
Fig. 3 Input power versus variable supply
voltage, for 50 Hz, 60 Hz and 400 Hz operation.
Fig. 5 UPFPS input current versus load.
2.3 Operation Under Nominal Conditions
2.2 Variable Load Operation
At nominal operating conditions, a comparison
between rectifier and UPFPS operation is
performed, as far as the voltage and current
waveforms are concerned and their harmonic
contents.
The UPFPS may operate from no load, up to
maximum load maintaining a constant output
voltage, due to the output voltage stabilizer.
The test was performed at a supply voltage of
120V, while the load was varying from 0 up to
312W. The supply source was the 60Hz and the
400Hz, 7kVA generators as well as the 50Hz
mains. The input current, the input power and
the efficiency were measured versus load. The
test results are given in Figs 5, 6 and 7
respectively. From Fig. 6, the input power may
be expressed as a function of load as in (3):
Pin = P0 + Pout
2.3.1 Rectifier Operation
Rectifier operation was tested at 50Hz, 60Hz
and 400Hz supply frequencies. During rectifier
operation, the current harmonics cause
distortion to the supply voltage waveform
according to (1). This can be seen in Fig. 8,
where the corresponding undistorted voltage
waveform, for comparison reasons, is
superimposed.
The effect is emphasized when the load
represents a significant percentage of the
supplying source nominal power. This is true
for the 7kVA generators for the 60Hz and
400Hz operation, due to the significant value of
the generator synchronous and subsynchronous
reactances Xs and Xs´ (Xs is approximately
j2Ω) but mainly because of cabling resistance
(approximately 2Ω).
(3)
Where P0 represents the no load power
required for UPFPS self supply. P0, for this
particular UPSPS, is approximately 40W. Due
to P0, UPFPS efficiency is reduced particularly
at low loads, as it can be seen in fig 7.
0,90
400
350
300
250
Power (w)
Efficiency (%)
0,88
UPFPS operation at 312 W
Efficiency at 50 Hz
Efficiency at 60 Hz
Efficiency at 400 Hz
0,86
200
150
UPFPS operation at 120 V
Input power at 50 Hz
Input power at 60 Hz
Input power at 400 Hz
100
50
0,84
90
100
110
120
130
140
0
150
0
Voltage (V)
50
100
150
200
250
300
Load (W)
Fig. 4 Efficiency versus variable supply voltage,
for 50 Hz, 60 Hz and 400 Hz operation.
Fig. 6 UPFPS input power versus load.
3
350
1,0
120
100
Input Voltage (V)
Efficiency (%)
0,8
0,6
UPFPS operation at 120 V
Efficiency at 50 Hz
Efficiency at 60 Hz
Efficiency at 400 Hz
0,4
Supply Voltage spectrum
400 Hz Rectifier Operation
(Total Harmonic Distortion 7.7%)
80
60
40
0,2
20
0,0
0
50
100
150
200
250
300
0
350
0
1000
Load (W)
2000
3000
4000
5000
Frequency (Hz)
Fig. 7 UPFPS efficiency versus variable load.
Fig. 9. The supply voltage spectrum for 400 Hz
rectifier operation.
At 50Hz operation, where the rectifier is
supplied through the 5kVA transformer, the
voltage waveform distortion is smaller due to
the transformer leakage reactance XL
(approximately j0.2Ω) and the shorter cables
(approximately 1Ω). Such a supply condition is
typical for aircraft and ship mains.
The supply voltage and input current spectra,
at full load, are given in Figs. 9 and 10
respectively. The voltage waveform harmonic
distortion is approximately 7.7%, Fig. 9, due to
the third and the fifth voltage harmonic
components. The corresponding voltage
waveform at no load exhibit an harmonic
distortion value of approximately 1%.
The full load current waveform harmonic
distortion is approximately 68%, Fig 10. The
current harmonic components reduce the
supplying source utilisation, giving an
equivalent power factor of approximately 77%.
Operation at 50Hz and 60Hz, exhibit similar
results.
2.3.2 UPFPS Operation
When the power supply operates as UPFPS,
controlled current is pumped to the smoothing
capacitor all over the period, proportionally to
the instantaneous voltage value. Therefore the
current follows the voltage waveform, Fig. 11,
and the AC to DC converter input characteristic
becomes resistive. The supply voltage distortion
is practically the same as the distortion of the
unloaded voltage waveform.
In Figs. 12 and 13, the corresponding voltage
and current spectra are given for UPFPS
operation. It can be seen, that the current has
approximately the same waveform like the
voltage one, as well as that the current and the
voltage spectra are practically similar. Thus the
7% UPFPS reduced efficiency, is compensated
by the supplying source nominal power
utilisation due to the unity power factor.
Operation at 50Hz and 60Hz, exhibit similar
results.
2,5
10
100
5
0
0
1,5
1,0
0,5
-5
-100
Input Current Spectrum
400 Hz Rectifier operation
(Total Harmonic Distortion 68%)
2,0
Current (A)
400 Hz Rectifier operation
Supply Voltage
Input Current
Undistorted Vin
Input Current (A)
Supply Voltage (V)
200
0,0
-200
0,0
0,5
1,0
1,5
2,0
0
-10
2,5
1000
2000
3000
4000
Frequency (Hz)
Time (msec)
Fig. 10. The supply current spectrum for 400 Hz
rectifier operation.
Fig.8. The supply voltage and input current
waveforms for 400 Hz rectifier operation.
4
180
6
400 Hz UPFPS operation
Supply Voltage
Input Current
4
0
0
-2
-120
-4
-180
Input Current (A)
2
-60
-6
0
1
2
3
4000
5000
4 Acknowledgements
The author would like to express his gratitude to
the Hellenic Navy Submarine Base personnel
for the support and the technical assistance.
References:
[1] Jun-Young Lee et al., Design of a PowerFactor Correction Converter Based on
Half-Bridge Topology”, IEEE transactions
on Industrial Electronics, vol. 46, No. 4,
August 1999, pp. 710-723.
[2] Tsai-Fu Wu, Yu-Kai Chen, “Modeling of
Single Stage Converters With High Power
Factor and Fast Regulation”, IEEE
transactions on Industrial Electronics, vol.
46, No. 3, June 1999, pp. 585-593.
[3] Bor-Ren lin, Hsin-Hung Lu, “A New
Control Scheme for Single-Phase PWM
Multilevel Rectifier with Power factor
correction”, IEEE transactions on
Industrial Electronics, vol. 46, No. 4,
August 1999, pp. 820-829.
[4] Stefan V. Mollov, Andrew J. Forsyth,
“Analysis, Design, and Resonant Current
Control for a 1-MHz High Power Factor
Rectifier”, IEEE transactions on Industrial
Electronics, vol. 46, No. 3, June 1999, pp.
620-627.
Supply Voltage Spectrum
400 Hz UPSPF Operation
(Total Harmonic Distortion 1.3%)
60
40
20
0
2000
3000
The UPFPS AC/DC conversion efficiency is
lower than the corresponding rectifier one.
However, in small size mains like in shipboard
power systems, where the various DC loads
represent a significant percentage of the source
power rating and the grid utilisation is required
to be high, the use of UPFPSs may be
preferable.
120
1000
2000
Fig.13 The input current spectrum at 400Hz
UPFPS operation.
In this paper, the performance test results of a
400W AC to DC converter was presented,
operating either as a rectifier, or as a UPFPS,
supplied from AC sources of various
frequencies. The UPFPS performance was
tested at 50Hz, 60Hz and 400Hz supplying
frequencies, with varying supply voltage and
variable load. The UPFPS may operate in a
wide supply voltage and frequency range with
high efficiency. Since the output voltage is
stabilized, the input current decreases, following
an inverse proportional function of the
increasing voltage. The efficiency is higher at
high voltages because of reduced I2R loss. The
supply voltage and the input current spectra
were compared for rectifier and UPFPS
operation, at approximately nominal load, for
the 50Hz, 60Hz and 400Hz supplying
frequencies. At UPFPS operation, the current
waveform is similar to the supply voltage
waveform,
introducing
practically
zero
distortion to the supply voltage.
0
1000
Frequency (Hz)
3 Conclusion
80
1
0
Fig. 11 The supply voltage and input current
waveforms for 400 Hz UPFPS operation.
100
2
0
4
Time (msec)
Voltage (V)
Input Current Spectrum
400 Hz UPFPS operation
(Total Harmonic Distortion 2.7)
3
60
Input Current (A)
Supply Voltage (V)
120
4
3000
4000
5000
Frequency (Hz)
Fig.12 The supply voltage spectrum at 400Hz
UPFPS operation.
5
[5] Jung-Goo Cho et al., “Reduced Conduction
Loss Zero-Voltage-Transition Power
Factor Correction Converter with Low
Cost” IEEE transactions on Industrial
Electronics, vol. 45, No. 3, June 1998, pp.
395-400.
[6] Gun-Woo Moon et al., “Magnetic-Coupled
High Power Factor Converter with Low
Line Current Harmonic Distortions for
Power Factor Correction and Fast Output
Response”, IEEE transactions on
Industrial Electronics, vol. 45, No. 4,
August 1998, pp. 552-558.
[7] Karel Jezernik, “VSS Control of Unity
Power Factor”, IEEE transactions on
Industrial Electronics, vol. 46, No. 2, April
1999, pp. 325-332.
[8] Ricardo Nederson do prado, Saul Azzolin
Bonaldo, “A High Power Factor Electronic
Ballast Using A Flyback Push-Pull
Integrated Converter”, IEEE transactions
on Industrial Electronics, vol. 46, No. 4,
August 1999, pp. 796-802.
[9] Elias Rodrigues et al., “A Novel SingleStage Single Phase Dc UPS with Power
factor Correction”, IEEE transactions on
Industrial Electronics, vol. 46, No. 4,
December1999, pp. 1137-1147.
6