Differential Current Transformer Research Articles

A Wavelet-Based Transformer Differential Protection With Differential Current Transformer Saturation and Cross-Country Fault Detection

The current transformer (CT) saturation phenomenon has been one of the main problems for the power transformer differential protection, leading to incorrect current measurements and relay misoperation. This paper proposes a fast and efficient transformer differential protection scheme with additional differential CT saturation and cross-country fault detection modules after the external fault detection, all of them based on the differential wavelet coefficient energy with border distortions in order to stabilize the relay during external faults and distinguish accurately CT saturation from cross-country internal faults. The proposed method was assessed by using representative simulations of internal faults, transformer energizations, and external faults with CT saturation followed by cross-country internal faults, and good results were achieved.

Development of leakage resistance monitoring scheme adapted to the changes of network isolation parametres

Research materials show the effect of isolation capacitance on the conditions of leakage protection operation on alternating control current. The isolation monitoring method, based on using a differential current transformer was applied. To eliminate the destabilizing effect of changes in network isolation capacitance on the leakage relay operation conditions, conditions of adaptation of measuring element to this parameter were found. The research studies have allowed to develop functional diagram of leakage relay, in which functional failures such as the effect of unrecorded leakage currents from the rectified voltage poles on the measuring elements that are configured only to interact with the control current, formed by a special control current source; uncontrolled excess of value of leakage relay, allowable according to the performance charateristics by the actual network isolation capacitance, leading to functional failure because of the failure to provide standard for short-term leakage current through the human body were eliminated. The proposed scheme ensures performing protection functions such as continuous monitoring of the isolation resistance and current leakage values.

Transformer bridge for the high-accuracy comparisons of standard resistors at low frequencies

A transformer bridge has been developed at IMM for comparing low value standard resistors (0.1 /spl Omega/-100 /spl Omega/) at frequencies from 0.5 to 20 Hz. Two current sources and two resistance standards are connected in series in one current circuit. The potential leads of standards are respectively connected through the differential current transformer. The null-detector consists of two identical current amplifiers with a low input resistance. Each amplifier has a Schottky noise as the dominant component. The differential current transformer produces the inverse current pulsations in the input winding of another current amplifier. At 1.2 Hz, the current noise of the null-detector is equivalent to 1 pA in 100-/spl Omega/ resistor (or 10 pA in 1-/spl Omega/ resistor), i.e., a relative sensitivity below 0.001 ppm at room temperature. The bridge has no resonant circuits and can operate using a square-wave current.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Aging tests of amorphous current transformers used in ground fault interrupters

It is pointed out that the magnetic material for cores of a differential current transformer (DCT) of a ground fault interrupter (GFI) should be characterized by high initial permeability, little dependence on temperature, and a low remanence. These parameters have been taken into consideration during selection of the most suitable annealing regime of Co-based (CoFeMnMo)/sub 77/(SiB)/sub 23/ material. It is assumed that a GFI is required to have a service life of 20-years continuous operation. Hence, the magnetic properties of the core of the DCT must be highly stable and exhibit virtually no deterioration over this period. During its operational lifetime, the core may be subjected to rapid saturation due to short circuits and earth faults, etc., as well as cyclic temperature changes. The effects of temperature aging and high current transients on the performance of the DCT were investigated. It is concluded that current shocks do not significantly alter the small-signal magnetic permeability of Co-based amorphous cores of selected annealing. The material examined exhibits degradation of magnetic properties due to aging greater than that reported for Fe-based materials.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ground-Fault Interrupters

In a distribution circuit for connecting a load to an a-c source, it is known that a ground-to-neutral short tends to reduce the sensitivity of ground fault detectors that use a differential current transformer to sense line-to-ground leakage. A resonated coil on a core links the neutral conductor forming part of a protective device for interrupting the load circuit in the event of reduced Q of the resonated coil caused by a neutralto-ground fault. The core must also link the line conductor, for otherwise a loop including the line, the load and the neutral would wrongly have a like effect. The same core and coil are used with a common amplifier for the neutral-to-ground fault detector and for the line-to-ground leakage detector, so that the apparatus can be economical and compact. Furthermore, so long as there is no neutral-to-ground fault, the amplifier delivers a signal of normal strength that monitors the integrity of both detectors. The detector also responds to ''''run-away'''' of the monitoring signal generator.