Morphological Lifting Scheme for Current Transformer Saturation Detection and Compensation

Abstract

Current transformer (CT) saturation can cause protective relay mal-operation or even prevent tripping. The wave shape of the secondary current is severely distorted as the CT is forced into deep saturation when the residual flux in the core adds to the flux change caused by faults. In this paper, a morphological lifting scheme is proposed to extract features contained in the waveform of the signal. The detection of the CT saturation is accurately achieved and the points of the inflection, where the saturation begins and ends, are found with the scheme used. This paper also presents a compensation algorithm, based upon the detection results, to reconstruct healthy secondary currents. The proposed morphological lifting scheme and compensation algorithm are demonstrated on a sample power system. The simulation results clearly indicate that they can successfully detect and compensate the distorted secondary current of a saturated CT with residual flux.

Electromotive Force Characteristics of Current Transformer According to the Magnetic Properties of Ferromagnetic Core

“…The induced voltage of the 2nd winding is written from the differential equation of the flux linkage for the closed loop as (9). (9) where N is the number of turns, B is the linkage flux density, and s is the area of winding.…”

“…The primary objective of the current transformer design is to ensure that the primary and secondary circuits are efficiently coupled, so that the secondary current has an accurate relationship to the primary current. A current transformer also isolates the measuring instruments from a possibly very high voltage in the monitored circuit [6][7][8][9][10]. Current transformers are commonly used in metering and protective relays in the electrical power industry.…”

Phasor Estimation Algorithm Based on the Least Square Technique during CT Saturation

“…The other approach is based on the regression technique [5]- [8]. The regression method in [5] detects the saturation using discrete wavelet transform and compensates the distorted section of a secondary current with features extracted from the healthy section using a least square fitting method.…”

“…However, these methods require the data after the first saturation is finished; this drawback may result in the delayed operation of protection relays. In [8], a method employing the morphological lifting scheme based on morphological wavelets is used to detect CT saturation. A slope of the magnetizing curve, which is regarded as a straight line in the saturated section, is then calculated to estimate and compensate the magnetizing current of the CT.…”

A High Efficiency Controller IC for LLC Resonant Converter in 0.35 μm BCD

“…If the PFC output is saturated and the supply voltage of the controller IC is stable, the operation of the LLC resonant converter is started. Also, the current transformer (CT) can be used to compensate for the over current of the primary side [15], [16].…”