This paper offers a new effective clustering approach for determination of congestion zones formed by transmission constraints on wholesale electricity market for further retrospective economic analysis. The approach can be used for nodal auctions with AC optimal power flow models and calculations of locational marginal or nodal prices. Analysis is conducted on power transfer distribution factors obtained from price decomposition. The paper proposes static and weighted representative clustering algorithms and gives the method of determination of congestion zones margins estimates that are stable in long-term periods. Determined congestion zones form local markets and can be used for market power detection and mitigation and general network infrastructure development.