Impact on Reliability of Transformers on Account of Phase Unbalances in EHV Network

Abstract

Indian power system is growing rapidly and to maintain the pace with the demand, transmission sector is also expanding in variety and size. The interconnection of the grids is an amalgamation of HVDC links (500 kV & 800 kV) and EHVAC (400kV & 765Kv) lines for both inter regional/ national and intra-regional power transfer. Synchrophasor at transmission level were first introduced in India in the year 2012 when a total of 14 PMUs were commissioned, however the usage for real-time visualization led to higher role of synchrophasor technology across control centers. By the year 2018, the number of PMUs had ramped up to 77 and with the better visualization tools, the usage of Synchrophasor for real-time operation by the system operator has increased. The enhanced visibility in grid operation with the use of synchrophasor has helped operator in identifying various aspects of integrated operation and take actions in real time. With the advent of extra high voltage transmission lines and high power corridors, the transformers are playing a crucial role in demand supply management. With transformers augmented rated from few VARs to hundreds of MVARs, the need to design transformer protection in coordination with 765 as well as 400kV line protection settings. In the event of fault in the system it becomes very important to isolate the transformers in time so as to minimize the electrodynamic as well as heating stress. The distribution transformers can be safely protected using back up overcurrent however such protections are time delayed.