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The paper begins with a review of techniques for computer based protection of multi-winding three-phase power transformers. After reviewing the principles of harmonic-restraint and voltage-restraint current differential protection, the paper develops a technique which computes and uses flux-current relationship of the transformer to obtain the restraint function. It is shown that this protection technique requires fewer computations as compared to the harmonic current computation, and hence can be implemented on a microcomputer of modest capability. The principle has been tested in the laboratory on a model power transformer. The relay based on this flux-restraint principle performs well on all the system phenomena simulated in the laboratory. It is expected that a relay based on this principle will be tested in the field on a power transformer.