Conclusion and Learning
The success of this case attributes to the joint effort of the plant and the power utility. Both parties should positively face the problem, conduct testing and determine the best possible and feasible solution.
Reduce Impact Caused by Harmonic Amplification on plastic plant
Background
A plastic plant has operated several years in southern Taiwan. There are three 30.4 MVA 6-pulse rectifier units. They produce 6k±1 th, k=1, 2, 3, ...... order harmonic currents. For this reason, the 18-pulse configuration with a phase shift in the primary side of the rectifier transformers is used. Four single-tuned filters (5th, 7th, 11th, and 13th order single-tuned LC filters) were installed to mitigate the harmonic distortion from three 6-pulse rectifiers and also to correct the power factor. After around four years running, there were many failures about the capacitors of filters. There have been many faults about harmonic amplification problems caused by single-tuned filters and the equivalent impedance of a power system when the frequencies of parallel resonance locate at or near harmonic orders. It was identified that those failure was caused by harmonic current. Production loss of this plant was at least 50 million NTD.
Investigation and Finding
The plastic plant and the research team met and aimed at finding the best solution to reduce the impact. Field measurements and computer simulations are used to verify the disturbance problems.
From the measurement data, for the past four years the filter capacitances have been reduced by 1.8 ~ 4.2 %. Then from the results of computer simulations, the series resonant frequencies are shifted from 300Hz, 420Hz, 660Hz, and 780Hz to 300Hz, 428Hz, 665Hz, and 789Hz, respectively. Meanwhile, the parallel resonant frequencies (orders) are shifted from 222Hz (3.77), 336Hz (5.6), 473Hz (7.9), and 715Hz (11.9) to 234Hz (3.9), 344Hz (5.7), 480.5Hz (8.0), and 726Hz (12.1), respectively.
