Sonel PQM analyzers case study #6: Inverter with a DC reactor

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Analyzes and evaluations of example power quality recording performed with SONEL PQM series analyzers

INVERTER WITH A DC REACTOR

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Power quality in industrial facilities is threatened by the widespread presence of a large number of power drives. To ensure the resistance of drives to network parameters and easy regulation of technological processes, the best solution is to use inverters.

Description of the identified problem The low power inverter is typically equipped with a 6-pulse rectifier. Therefore, it cooperates in a characteristic way with the power grid, and its distorted current has a negative impact on the quality of energy. The impact of using a DC series reactor on the improvement of the inverter power supply parameters should be assessed.

Measuring equipment used ďƒ˜ PQM-702 Power Quality Analyzer with C-7 cclamps ďƒ˜ Sonel Analysis software

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Fig. 1. Picture of THD I and Kf parameters against the background of three-phase apparent power without a DC reactor

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Fig. 2. Voltage and current oscillogram for the operation of the inverter without a DC reactor

Initial conclusions (Fig. 1. and 2.)

1. A high level of current harmonics has been observed: a) At power ~8.3 kVA – THD I ~=140% (See 1.); b) At power ~8.9 kVA – THD I ~= 158% (See 2.).

2. Current harmonics with a significant share of higher components, high value of the Kf factor (See 3.).

3. The instantaneous waveform of the phase currents clearly indicates the impulse consumption of energy from the supply network (See 4.).

4. The power quality at the measuring point is threatened by the effects of the flow of higher current harmonic.

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Fig. 3. Oscillogram of voltages and currents for the operation of the inverter with a DC reactor

INVERTER WITH A DC REACTOR

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INVERTER WITH A DC REACTOR

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Corrective action: - use of a DC reactor in the rectifier circuit (Fig. 3. to 7.).

Effects: 1. The width of the current pulses has increased (See 5.).

2. Harmonic distortion has decreased (See 6.).

3. The value of Kf factor for current has decreased (See 7.).

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Fig. 4. The image of THD I coefficients against the background of three-phase apparent power with a DC reactor

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Fig. 5. Image of the current Kf coefficients against the background of three-phase apparent power with a DC reactor

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Fig. 6. Harmonic levels of the phase currents of the inverter without a DC reactor

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Fig. 7. Harmonic levels of phase currents of an inverter with a DC reactor

Conclusions 1. The use of a reactor improved the shape of the current flow (Fig. 2. and Fig. 3.).

2. Current THD level decreased from 140% to 70%.

3. The share of higher harmonics after using a reactor has decreased from approx. 160% to 29% - 35% (Fig. 1. and Fig. 4.).

4. Clear, much faster drop in the content of higher harmonics after using a DC reactor (See 8. and 9.).

5. As a result of using a simple DC reactor, the losses from higher harmonic currents will be significantly reduced.

Recommendations 1. Consider modifications to the remaining inverters.

INVERTER WITH A DC REACTOR

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