Abstract
In this study, we will cover the basic methods used to distinguish between inrush current and fault current in power transformers. First, the nature of inrush current is presented compared to the fault current. Then the nature of the magnetizing current due to energizing a power transformer at no-load is explained. The first generation of methods used to disable the protective relay system during inrush current, namely the Desensitizing and Tripping Suppressor, is introduced. The second generation, the harmonic restraint method and the waveform-based restraint method with their different versions, is explained. Then we will explore thoroughly the fictitious equivalent resistance method as an example of the third generation of model type restraining or blocking methods. Finally, a comparison between these methods and conclusion is carried out.
Highlights
Power transformers are one of the most important elements in power systems
As α increases the magnitude of the fault current is(t) decreases and the highest value of the fault current occurs when energizing the transformer at α = 0 ̊
5) A model for simulating the transients of both inrush and internal fault currents is applied to a case study based on a closed-form analytical representation of the transformer’s magnetization curve, its leakage inductance as well as the no-load losses
Summary
Power transformers are one of the most important elements in power systems. The protection of power transformers and the prevention of tripping power transformer unnecessarily due to inrush current are crucial for the continuity of the power supply. Inrush current is drawn by the transformer when it is energized during commissioning, testing, or when it is connected to the network after maintenance. Inrush current is very high in magnitude compared to the normal magnetizing current and to the rated current of the power transformer which might reach hundred times the normal magnetizing current and few times the rated current. The time duration of the inrush current before reaching its steady state value—called the magnetizing current—is very small. (2014) Alternative Approaches for Distinguishing between Faults and Inrush Current in Power Transformers.
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