Existing problems：
The harmonic currents were 3rd, 5th, 7th and 11th currents
Superposed harmonics resulted in the system PF value as low as 0.75
Zero sequence harmonics superposed on the neutral line caused too large neutral current and led to air switch tripping.

Solution：
ZAPF-50/380-B parallel active power filtering device of SSE was applied.

Control effect：
The waveforms of voltage and current were improved obviously. All harmonic waves were filtered effectively, among which 5th harmonic current distortion rate Ah5 was decreased from 70.7% to 2%, and the improvement rate reached 97%;
At the same time, neutral current decreased from about 38A to about 10A, to avoid the air switch tripping hazards.