An On-line Monitoring System for Over-voltages Based on a Two-stage Voltage Divider and Field Measurement Results in Medium-voltage Grids

Abstract: 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.