A novel flux-based protection scheme for power transformers

Power transformers as an important and expensive apparatus in the power system must be protected against the faults and should be monitored permanently to increase power system reliability and to decrease maintenance costs. Internal Turn-Turn faults (TTF) are the most common failures in power transformers, which could seriously reduce the useful long life of these equipment. Although the common protection schemes such as differential relays are able to detect some of internal faults, some other ones (such as TTFs and short circuit near the neutral point) cannot be detected by them. Also, these relays may trip erroneously due to energizing inrush currents, transformer over excitation, and occurring CT saturation at one side. In this paper a novel Linkage Flux Based (LFB) scheme is proposed to detect TTF in power transforms, which uses some Search Coils (SC) located on the transformer legs to sense the related linkage flux. Any difference in induced voltage in the corresponding SCs (located in any leg) suggests passing unsymmetrical linkage fluxes through them (against the normal conditions), which stands for occurrence a fault inside the transformer. The proposed technique not only can be used to protect power transformers, but also can be employed to find the fault location during repair activities, as well.