Evaluation of the IEC 61000-3-11 procedures for the connection of multiple items of equipment

One way to manage voltage fluctuations in power systems is by limiting flicker emissions from individual items of equipment. The limits of voltage fluctuations produced by equipment having a rated input current less than 75 A intended to be connected to a public low-voltage (LV) distribution system are specified in IEC Standards 61000-3-3 and 61000-3-11. Evaluation procedures are included in 61000-3-11 which shall be applied if the equipment cannot meet the technical requirements and is therefore subject to conditional connection. In this case, the equipment has to be connected to a supply having an impedance lower than the reference impedance so that equipment emissions comply with the limits of the standards and manufacturers can declare the equipment to be compliant. Multiple formulas are provided in Clause 6 of IEC Standard 61000-3-11 for the determination of the required system impedance as a function of the reference impedance. One of the formulas supposedly addresses the connection of multiple items of equipment connected at the point of evaluation. The combined flicker effects of multiple items of equipment using the described methods in the standard are presented in this paper. Also, the cubic summation law, which is widely recognized by the industry and the standards, is used to evaluate flicker effects of multiple items. A comparison of these two methods for combining flicker effects is also presented. The results of using the formula provided by IEC Standard 61000-3-11 are shown to be over- or under-conservative depending on the specifics of the situation and the characteristics of the equipment involved.