Differential Protection for Arbitrary Three-Phase Power Transformers

Abstract: This thesis describes how to provide standardized, current based, differential protection for any three-phase power transformer, including phase-shifting transformers with variable phase angle shift and transformers of all construction types and internal on-load tap-changer configurations. The use of standard transformer differential protection for such applications is considered impossible in the protective relaying standards and practices currently applied. The first part of the thesis provides the background for different types of power transformers and the differential protection schemes currently applied. After that a complete mathematical proof for the new, universal transformer differential protection principle, based on theory of symmetrical components, is derived. It is demonstrated that it is possible to make numerical differential protection relays which can properly calculate differential currents for any power transformer, regardless of whether it is of fixed or variable phase angle shift construction and whether current magnitude variations are caused by on-load tapchanger(s). It is shown how to correctly calculate differential currents by simultaneously providing on-line compensation for current magnitude variations, on-line compensation for arbitrary phase angle shift variations and settable zero-sequence current reduction on any power transformer side. By using this method differential protection for arbitrary power transformers will be ideally balanced for all symmetrical and nonsymmetrical through-load conditions and external faults. The method is independent of individual transformer winding connection details (i.e. star, delta or zigzag), but dependent on having the correct information about actual on-load tap-changer(s) position if they are built-in within the protected power transformer. The implementation and practical use of this new universal principle is quite simple, as all necessary transformer data is commonly available on the protected power transformer rating plate. Practical application of the universal method for the differential protection of standard transformers, special transformers and phase shifting transformer is presented. Detailed testing of this new universal differential protection method is given and it is based on actual field recordings captured by numerical relays in existing phase-shifting transformer installations and on simulations from the Real Time Digital Simulator for a practical dual-core, symmetrical phaseshifting transformer. The implementation of the universal transformer differential method for analogue and numerical transformer differential relays is also described. Problems for the differential protection caused by transformer inrush currents are discussed. The mathematical relationship between differential protection and directional protection is derived. Then it is shown that through the addition of supplementary directional criteria security and speed of the operation of the transformer differential protection can be improved. Finally, the use of additional directional criteria to significantly improve the sensitivity of the differential protection for transformer winding turn-to-turn faults is suggested. Captured disturbance files from numerical differential relays in actual power transformer installations, during internal and external faults, have been used to demonstrate the performance of additional directional criteria.