Protection Of Power Transformers

Power transformers are essential and important elements of power systems, and the reliability of power transformers directly determines whether the power system works safely or not. At present, the main protection of internal faults of the power transformer is the differential protection. The backup protection is usually made up of compound voltage block direction over-current protection, zero-sequence direction over-current protection and etc. The transformers are damaged or burnt down because the existing backup protection is not sensitive enough and its pull-out time is long. So, it is essential to improve the transformers backup protection.At present, the main protection of internal faults of the power transformer is the differential protection. Its capability determines the transformer protection. This paper introduces the existing differential protection applying transformer protection. How to discriminate between internal fault and magnetizing inrush current are the important details of transformer protection. In this paper, second harmonic brake, dead angle, waveform symmetry, waveform correlation algorithm and virtual third harmonic brake which are based on different current, voltage brake, voltage equations which are based on different voltage or based on different voltage and current are introduced and analyzed. This paper also introduced some new application in transformers based on rising digital signal process method.This paper design the scheme of the power transformer protection in configuration, measurement input, hardware composition, software flow, protection algorithm, protection criterion.The disadvantages of transformers backup protection are analyzed detailedly, and a new scheme for transformer backup protection based on the directional unit applying fault components is presented in this paper. On the basis of existing backup protection, when fault occurs, the scheme can determine fault location and isolate fault quickly according to outputs of measurement elements in all sides of transformer. If there is an inner fault, the breakers in all sides of transformer can be tripped instantaneously. If there is an external fault, the protection scheme can determine fault location quickly, and can realize quick backup protection with selectivity and simple adaptation of pull out time. The protection scheme proposed in this paper can solvethe problem effectively that the transformer backup protection is not sensitive enough when faults occur in the medium or low voltage side. Directional relay based on fault components is selected as the measurement element in order to meet different operating mode requirements of all kinds of transformers. It deflates the influencing area of faults and reduces operation time delay of backup protection.In the end it is proved that the new scheme is correct by a mass of simulation using PSCAD/EMTDC, and the scheme is feasible and practicable.