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

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

• Sonel PQM-702 Power Quality Analyzer (see the product successor Sonel PQM-710)
• C-7A clamps
• Sonel Analysis software

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

 

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 (Fig. 2).
4. The power quality at the measuring point is threatened by the effects of the flow of higher current harmonic.

 

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

 

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.).

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

 

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

 

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

 

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.

 

Author:
Krzysztof Lorek