Search

Overhead line energization can be performed under normal or faulty conditions. The latter indicates an occurrence of a fault along the line that exists at the moment of energization. This can lead to significant over-voltages that could endanger proper line operation. The issue gets more complex to protective devices when it comes to high-resistance faults, which impair the ability of relays to react promptly and accurately. Consequently, installation of additional detection devices becomes necessary. This paper proposes a novel method that properly detects and classifies line operating conditions at the moment of energization. The method is designed to be useful for utilities and it can be considered as a low-cost, fast, and accurate detection and classification approach, suitable for dealing with both low-resistance and high-resistance faults. Through comprehensive mathematical modeling it was found that both normal and faulty conditions during line energization are accompanied by the zero-sequence voltages of specific characteristics. The differences between zero voltage sequences are reflected in harmonic content, magnitudes of dominant frequencies, and their phase angles in regard to supply voltage. These findings are taken as the method's detection and classification criteria. The validity of the proposed model is verified by simulations and by field measurements. [ABSTRACT FROM AUTHOR]

An on-line monitoring system based on a two-stage voltage divider is presented for monitoring the switching and lightning over-voltages in 10–35-kV medium-voltage grids. A low-damped capacitive divider (voltage ratio of 1000:1) is designed as an over-voltage monitoring sensor for primary voltage division, and an R-C damped divider is designed for the secondary voltage division (voltage ratio of 10:1 for the 10-kV grid and 20:1 for the 35-kV grid). The divider's designing methods and response characteristics are analyzed. The result of the measurement indicates that the voltage divider has good frequency response, which can meet the requirements for monitoring switching and lightning over-voltages. Moreover, with the data double-buffering and high-speed sampling techniques, this device can collect longer data (64 Mbytes) and shorten the interval (less than 1 sec) at a high sampling rate (up to 20 MHz). Finally, the device has been installed in a 110-kV substation in parallel with a 10-kV bus for two years. Many over-voltage waveforms (including switching, induced lightning, and back-flashover lightning over-voltages) are captured on site. The actual measured results indicate that the device can monitor fast transient over-voltages precisely. [ABSTRACT FROM AUTHOR]