Evaluation of current levels in the presence of harmonics for proper coordination using transmittances

This paper presents two of the problems which can occur when both RMS and fundamental responsive protection devices are used in a radial distribution system which has both source voltage distortion and nonlinear loads. The transmittance equations used to calculate the current contribution from the source voltage distortion and the nonlinear loads for each harmonic at any location in the system are presented. A new method for the proper coordination of RMS protective devices with fundamental protective devices when current distortion is present is suggested. Two algorithms which can be used to calculate the current transmittances in steady state circuit analysis programs based on the frequency scan option in the EMTP are presented