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Current-transformer (CT) saturation causes severe distortion in the measured current waveform which may lead to maloperation of the protective devices. This paper proposes a low-cost, power-electronic device to prevent the CT from saturation. The proposed compensator is inserted in series with the relay in the CT secondary circuit and acts as a controlled voltage source (CVS). The proposed CVS generates a time-varying voltage to cancel the voltage developed across the CT burden; therefore, the CT magnetic flux remains almost constant and undistorted during the power system transients. It will be shown that this device can precisely compensate fault current, inrush current, and other probable transient currents despite its simplicity. The proposed device can be employed to compensate the already in-service CTs connected to nondigital and digital relays. Comprehensive computer simulations are carried out to validate the effectiveness of the proposed compensator. The performance of a sample compensator of this type is validated through special high-current laboratory experiments (carried out over several hundred amperes up to 1.6 kA), and the obtained results illustrate the capability of the proposed compensator to prevent CT saturation. [ABSTRACT FROM PUBLISHER]

To avoid saturation of a protective current transformer (CT) operating under an autoreclosing scheme, the CT size should be chosen much higher than a similar CT operating under a single-step fault clearing scheme. The main reason for this significant difference is that the residual flux (caused by the first stage of the fault) cannot be noticeably reduced during the deadtime interval of the reclosing process and, therefore, the occurrence of a subsequent fault can extremely saturate the CT. In this paper, a low-cost, low-power electronic device is developed and introduced to demagnetize the CT under the reclosing deadtime interval. It will be shown that by using this device, the required size of the CT is reduced by about 40%. To develop this device, an existing flux-based demagnetization technique is improved to operate faster and more efficient. The performance of this technique is independent of the CT characteristic parameters and provides promising results despite its simplicity. The proposed device can be employed to de-rate the new CTs to be installed in the system and/or to compensate the existing inservice CTs. Comprehensive computer simulations and laboratory experiments are employed to validate the reliability and effectiveness of the proposed compensation method. [ABSTRACT FROM PUBLISHER]

Current-transformer (CT) saturation threatens accurate discrimination between internal fault current and magnetizing inrush current of a transformer and endangers the proper operation of a differential relay. Thus, compensation of CT saturation could substantially improve the performance and security of transformer differential relays. So far, almost all of the technical literature in this field has focused exclusively on fault current compensation. However, this paper proposes an efficient compensation algorithm to reconstruct the fault and inrush currents which are distorted by CT saturation. Some of the attractive features of the proposed method are its accuracy, fast response time, desired sample-by-sample output, no dependency on power system topology, immunity against noise and harmonics, and its simplicity. These features are extracted through the application of numerous test cases reflecting a wide range of variations in fault and inrush currents. The proposed algorithm proves to be fast, accurate, and reliable. [ABSTRACT FROM PUBLISHER]