Generation of Overvoltages by Chop Current on Ag-WC and Cu-W/WC Contacts in Vacuum

摘要

As the electrical current nears zero during breaking operations, the current will often prematurely drop to zero. This behavior is called chop current, and appears in a wide range of electrical contacts including contacts in vacuum, oil, and SF_6 gas. The chop current is not directly a concern; rather it is the perceived risk of overvoltages generated by the rapid change in the current. This work presents the results of overvoltage measurements caused by chop current on Ag-WC and Cu-W/WC contacts in vacuum interrupters using the three-phase motor substitute circuit from IEC standard 62271-110. The chop current produced an oscillation in the voltage on the load with a frequency of 3.6-6.3 kHz and a rapidly decaying magnitude. The average value of the overvoltages on the load from chop current for Ag-WC was 1.1 PU (per unit) over the test, and the maximum value was 1.3 PU. Cu-W/WC had an average value of 1.5 PU and a maximum value of 3.4 PU. These values are below the peak of the power-frequency withstand voltage of 4.8 PU at this rating; therefore the chop current is of little concern. Based on these results, further analysis of overvoltage behavior should focus on multiple re-ignitions and virtual chop current events instead of chop current.