Distance protection using a time-frequency filtering technique

Abstract

Summary The paper presents a new time–frequency formulation similar to the conventional discrete S‐transform but with a significantly increased speed of computation suitable for digital protection of power transmission lines. The new formulation uses frequency scaling, band pass filtering, and interpolation techniques to achieve fast and accurate estimation of fault location in an interconnected power system network. The new transform uses half cycle of the data prior to the initiation of the fault and half cycle data after the fault to yield impedance measurement at the relay location with significant accuracy even in the presence of decaying DC components, harmonics, and distortions due to noise. Several fault case studies have been performed to validate the performance of this new algorithm for different fault resistances and fault location along the protected line. For comparison, a recently proposed robust Gauss–Newton algorithm is used to produce impedance trajectories for the case studies attempted in this paper instead of the widely used discrete Fourier transform, which gives erroneous results in the presence of decaying DC and harmonic components. Copyright © 2014 John Wiley & Sons, Ltd.

Minimizing the undesirable effect of coupling capacitor voltage transformer on DFT-based phasor estimation method

“…It must be noted that, due to severe disturbances in power system, the system frequency may differ from its nominal value f 0 . The latter causes undesirable instability in estimating phasor, and as a result, it must be considered in phasor estimation of numerical relays.…”

“…As it was mentioned before, in practical application, voltage signal does not experience even harmonic component, especially during fault condition. Also, it was mentioned that DFT‐based phasor estimation algorithms must minimize the unwanted instability of frequency deviation in off‐nominal frequency condition. To overcome the abovementioned drawbacks, applying half‐cycle Fourier transform and , the real and imaginary parts of the fundamental phasor component can be written as below: wherein and , U Re and U Im are real and imaginary parts of fundamental component of , respectively.…”

“…Having U hfr , U hfi , U lfr , U lfi , and U dc , the unwanted effect the CCVT components can be calculated, and as a result, by applying , the unwanted effect the CCVT components are subtracted from the input signal. Step 3 (Updating frequency coefficient) After subtracting, utilizing frequency tracking, expressions , , , and are updated. Except for the first cycle, this action is done in one cycle Step 4 (Phasor estimation process) Applying and , fundamental phasor will be calculated while analytically all the undesirable issues have been considered.…”

“…Especially in protection application in which the decision‐making of contingencies almost always needs to be accomplished in real time. Failure in quick as well as precise response in protective relays leads to improper decision‐making and consequently causes losing power system security and reliability …”

“…Failure in quick as well as precise response in protective relays leads to improper decision-making and consequently causes losing power system security and reliability. [1][2][3] Recently, several methods for estimating phasor and frequency of the current and voltage signals have been suggested for different purposes. For example, some methods have been addressed in other studies [4][5][6] that aim to estimate fundamental phasor component of the fault current waveform while they are dealing with the instability caused by the decaying direct current (DC) components.…”