Method for identifying transformer excitation surge current

Abstract

The invention provides an identification method for transformer inrush current which belongs to power system main equipment relay protection technology field. The method has characteristics in that the method identifies inrush current and fault current base on waveform correlation degree. By comparison with each sampling point algebraic sum in dynamic differential current short data window, non-saturation region of transformer under inrush current and fault current is searched, two form standard sine waves are constituted of max value and position of sampling data in non-saturation region, then differential current sampling waveform in non-saturation region and two form standard sine waves correlation degrees are calculated respectively, inrush current and fault current are differentiated according with correlation coefficient average. The method can open longitudinal differential protection rapidly when internal fault and no-load closing with fault occur in transformer operation; and can lock longitudinal differential protection reliably when on-load closing occurs inrush current.

Description

technical field [0001] The invention belongs to the technical field of relay protection for main equipment in a power system, and in particular relates to a method for identifying transformer excitation inrush current. Background technique [0002] Although there are certain defects in the principle of transformer longitudinal differential protection, and the correct action rate is low, it has been used for a long time and has a wide range. Currently, it is still in an irreplaceable position as the main protection of transformers. The most serious problem faced by transformer longitudinal differential protection is the excitation inrush current, because the iron core of the transformer is saturated when the voltage recovers after no-load closing or external fault removal, and a large excitation inrush current will appear in the transient process, and the excitation inrush current acts as a differential current , if there is no corresponding preventive measures, the longitudi...

AI Technical Summary

Problems solved by technology

However, if the second harmonic ratio is reduced, due to the high voltage level of large transformers and the long transmission lines often connected at the ends, the capacitive effect of transmission lines is very obvious, and the large-capacity capacitors for static var compensation are widely used. When a serious fault occurs inside the transformer, the resonance between the inductance and the capacitance can significantly increase the harmonic content in the short-circuit current, which may cause a delay in the action of the longitudinal differential protection

In addition, since the second harmonic braking method is not easy to adopt the phase-separated locking method, when closing with a slight fault, the differential protection action is very slow

Although another excitation inrush identification method based on the principle of discontinuous angle does not have the above problems, when the current transformer is saturated, the inrush current transmitted to the secondary side of the transformer generates a reverse current and the waveform is deformed, causing the discontinuity angle of the inrush to disappear. , it is not easy to recover the original discontinuous angle exactly

The principle of second harmonic braking and the principle of discontinuity angle only take advantage of a certain aspect of the difference between the excitation inrush current and the fault current, so there are limitations

[0004] In order to solve the problem of low accuracy and reliability of the transformer excitation inrush current identification method, the present invention identifies the unsaturated working area of ​​the transformer through the algebraic sum of the differential current sampling points in the short data window, and further constructs a standard sine wave of the same length, Analyze the degree of correlation between the actual sampling waveform in the unsaturated region and the constructed standard sine wave, and identify the transformer excitation inrush current

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