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Analyzes and evaluations of example power quality recording performed with SONEL PQM series analyzers
SINGLE-PHASE LOAD WITH APFC
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Modern single-phase devices with higher power, made at a higher technical level, are partially protected against drawing distorted current from the network, resulting from the operation of typical rectifier bridges. At the same time, these solutions allow for partial correction of reactive power, keeping it at a low level. These are the most reasonable solutions that integrate a simplified active filter with a rectifier power supply.
Description of the identified problem A professional high-power power supply unit is equipped with an APFC (Automatic Power Factor Corrector) system, i.e. automatic correction of the power factor as seen from the mains side. From the observation of the quality of energy consumed by the power supply, the heterogeneous nature of the power is visible (Figs. 2 and 3). Additionally, a large variability of the quality parameters of the current consumed by the power supply unit was observed, as well as noticeable voltage dips at the moment of switching on (Fig. 1.). Are these disturbances related to the properties and operation of the power supply unit?
Measuring equipment used PQM-702 Power Quality Analyzer Set of C-7 current clamps Sonel Analysis software
Fig. 1. The image of phase voltage and current during connections and operation of the power supply unit with APFC
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Fig. 2. Image of the PF power factor against the current during connections and operation of the power supply unit with APFC
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Fig. 3. Image of THD I and Kf levels against the active power during operation of the power supply unit with APFC
Initial conclusions 1. A large range of network load changes caused by the operation of the power supply unit was found. 2. For small consumed powers, the power factor PF was very small (See 1.). 3. Above 25% of the load taken from the network, there is a significant increase in the effectiveness of the power factor correction (See 2.) at the level of 0.972, which is a result close to the expectation and a reduction in distortion (See 3.).
SINGLE-PHASE LOAD WITH APFC
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SINGLE-PHASE LOAD WITH APFC
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Only with the active power consumed above 220 W of the power supply with a nominal power of 750 W, the power factor PF improved significantly to the level of 0.972 and the THD I and Kf decreased. It proves the positive influence of the APFC function on the nature of the current consumed by the power supply from the network (See 3.). Looking at the oscillograms in detail, we also have the answer how the power supply works at low loads. Occasionally, there are charging pulses that do not have much importance on the harmonic level due to the low values (Fig. 4). At high loads, on the other hand, quite effective correction of the shape and phase of the current occurs thanks to APFC (Fig. 5).
Fig. 4. APFC does not work at low load
Fig. 5. APFC operates under heavy load
Fig. 6. The image of the current harmonics during the effective operation of the APFC in the power supply unit
Fig. 7. The moment of switching on the power supply unit with APFC - oscillographic image and RMS 20ms(1/2)
Conclusions: 1. Correction of the current shape does not work at low power drawn from the network by the power supply unit with APFC. 2. Above 25% of the rated load taken from the network, there is a rapid increase in the effectiveness of APFC correction, a visible improvement in the parameters of the current consumed from the network at high loads (Fig. 6.). 3. At the moment of switching on, there is a 10-fold half-period over-current, causing dips in the voltage of approx. 5V RMS, but only in the case of initial load of the power supply unit output (Fig. 7.).
Recommendations:
1. When selecting the power supply unit with APFC, pay attention to: declared inrush currents of the device, planning loads at >60% of rated power.
SINGLE-PHASE LOAD WITH APFC
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