One of the diagnosis methods for voltage instability due to tap changer operation is to use conventional power flow results. This paper discusses that divergence of conventional power flow calculations can show tap changer instability only in radial networks (i.e. in bulk power delivery transformers), but for analysis of tap changer instability in link networks (i.e. in transmission transformers), a new type of bus must be defined for those controlled by tap changers and, consequently, new power flow calculations must be performed. A wide range of tap changing is used in this study. (c) 2007 Elsevier Ltd. All rights reserved.