Mitigation of resonance using digital models and direct measurement of harmonic impedances

A 12 kV 4800 kVAr power factor correction shunt capacitor bank for a 22 MW paper mill in the British Columbia Lower Mainland operated without problems for almost twenty years, until May of 1997. After that time, the 0.49 Ohm series reactor in the capacitor bank suffered repeated overloading and failures. Fifth harmonic current in the bank was measured by mill personnel and found to be, at times, 60 percent of the fundamental component, where only 5 percent was expected. It was suspected that the capacitor bank was involved in a resonance. Since no significant modifications or additions to the mill had taken place since 1984, the customer believed that changes to the utility subtransmission system must have been responsible for the sudden onset of problems. This paper describes the process of developing a digital model for the mill and its low voltage systems, including equivalents for a large number of induction motors, which was interfaced to a detailed pre-existing model of the utility supply. Furthermore, the results of a unique set of field tests to directly measure the harmonic driving point impedance of the mill and the utility network are presented. The measured impedances are used to fine tune the mill motor diversity factor assumed for the digital model of the induction motor loads. Using the models of the mill and external network thus established, the reason for the 1997 reactor failures is shown to be inappropriate rating of the original tuning reactors. A process is described for selecting a new reactor rating which will avoid further harmonic problems as well as meet customer operating requirements.