An Overview of Unity Power Factor Supply Performance
Supplied From Low Power Naval Synchronous Generators
MICHAEL S. VICATOS
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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.
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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 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
Fig. 1 The UPFPS.
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.
2 Experimental Results
In this paper, the performance test results of a 400W DC power supply, operating either as a UPFPSor 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 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.
2.1 Variable Voltage Operation
The UPFPS exhibits a wide input voltage operation range, since it is equipped with an output voltage stabilizer. Thus, for a constant load, the UPFPSoperates 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.
Fig. 2 Input current versus variable supply voltage, for 50 Hz, 60 Hz and 400 Hz operation.
Fig. 3 Input power versus variable supply voltage, for 50 Hz, 60 Hz and 400 Hz operation.
2.2 Variable Load Operation
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 (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.
Fig. 4 Efficiency 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 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.
2.3.1Rectifier 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).
Fig. 6 UPFPS input power versus load.
Fig. 7 UPFPS efficiency versus variable load.
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.
Fig.8. The supply voltage and input current waveforms for 400 Hz rectifier operation.
Fig. 9. The supply voltage spectrum for 400 Hz rectifier operation.
2.3.2UPFPS 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.
Fig. 10. The supply current spectrum for 400 Hz rectifier operation.
Fig. 11 The supply voltage and input current waveforms for 400 Hz UPFPS operation.
3 Conclusion
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.
Fig.12 The supply voltage spectrum at 400Hz UPFPS operation.
Fig.13 The input current spectrum at 400Hz UPFPS operation.
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.
4 Acknowledgements
The author would like to express his gratitude to the Hellenic Navy Submarine Base personnel for the support and the technical assistance.
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