A new current relay for capacitors in medium voltage LC harmonic filters

Static LC filters are usually used in industrial installations in conjunction with generating harmonic loads, for instance arc furnaces, for voltage quality control. The filter is matched at some frequency between the third and the seventh harmonic, but currents wider spectrum of frequency will flow through the filter. Peak and rms voltages, each on individual capacitor will vary with harmonic, difference of phase, and amplitude. These variations will depend on load operation and may go over capacitor ratings, diminishing the equipment life. As potential transformers are generally not installed in each capacitor set, a new type of current relay that computes the voltage from the current through the filter was developed. Errors and typical waveforms are shown. Settings of peak and rms voltage adjustments are discussed. The relay can be successfully used in shunt capacitor banks also.