Energy and Power Engineering, 2013, 5, 1425-1428

doi:10.4236/epe.2013.54B270 Published Online July 2013 (http://www.scirp.org/journal/epe)

The Discrimination of Inrush Current from Internal

Fault of Power Transformer based on EMD

Fanyuan Zeng, Qianjin Liu, Chao Shi

School of Electric Power, South China University of Technology,Guangzhou, China

Email: zeng_fanyuan@163.com

Received March, 2013

ABSTRACT

The method is based on that the waveform of the inrush distorts seriously, while the fault current nearly keeps sinusoid.

The complicated signal can be decomposed into a finite intrinsic mode functions (IMF) by the EMD, then define and

compute the projection area on X-axis of each IMF—, the specific gravity of SIMF—

ci

Sci

, and the maximum of

ci

—max

. We can get a new scheme of transformer-protection based on comparing the difference between inrush and

fault current. Theoretical analysis show that the method can precisely discriminate inrush and fault current, fault clear-

ance time is about 20ms. Moreover, it is convenient to achieve and hardly be affect by not-periodic component.

Keywords: Inrush Current; Transformer Protection; HHT; EMD; IMF

1. Introduction

At present, the domestic transformer primary protection

in power system configuration mainly uses second har-

monic restraint principle and longitudinal differential

protection based on current discontinuous corner braking

principle. The long-term operating experience shows that

the differential protection can not accurately distinguish

the difference between the transformer internal faults and

external faults, so the main contradiction is still focused

on the identification of magnetizing inrush and internal

fault.

2. Empirical Mode Decomposition

Empirical Mode Decomposition (EMD) can effectively

identify magnetizing inrush and internal fault. EMD is

suitable for the analysis of non-linear, non-stationary

signal sequence with a high signal-to-noise ratio. The

center of this technology is empirical mode decomposi-

tion, which can decompose complex signals into a finite

number of intrinsic mode functions (IMF).The decom-

posed IMF component contains the local features signal

of the different time scales of the original signal. The

empirical mode decomposition method can make the

non- stationary data become smooth and get the Hilbert

transform spectrogram and the frequency of physical

significance. Compared with the short-time Fourier

transform and wavelet decomposition methods, this

method is intuitive, direct, posterior and adaptive.

Basic principles of empirical mode decomposition:

In order to obtain th e intrinsic mode fun ction s , the d ata

signal must be decomposed with EMD. So it is necessary

to introduce the definition of the basic concepts of EMD

decomposition process: IMF. This is the basis of the

master of EMD method.

The intrinsic mode function must satisfy the following

two conditions:

1) In the entire time range, the number of lo cal extreme

points and zero-crossing points of the function must be

equal to, or up to a diffe rence of one;

2) At any time, the average of the local maximum of

the envelope (upper envelope) and the local minimum of

the envelope (the envelope line) must be zero.

The first condition obviously has the similar require-

ments with traditional narrowband stationary Gaussian

signal. For the second condition, it is a new concept

which is the classic global requirements modifies local-

ized requirements, so that the instantaneous frequency is

no longer subject to the asymmetric waveform with un-

necessary fluctuations. In fact, this condition should be

“the local mean of the data is zero”. However, for non-

stationary data calculating the local mean involves the

concept of the local time scale, which is difficult to de-

fine. Therefore, in the second condition, the envelope

flocal maxima and local minimum values of the average

envelope are instead of zero, which make the wave form

of a signal locally symmetric. Huang et al study shows

that, under normal circumstances, to use this instead of

the physical significance of the instantaneous frequency

is in line with the system studied. Intrinsic mode func-

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