Using High-Resolution Oscillography to Improve the Performance of Controlled Switching for Transformers
[Presented at 2019 EEA Conference in New Zealand and Texas A&M 2020]
Energization of power transformers at a random point on wave (uncontrolled energization) can create inrush currents far exceeding the full load rating of the transformer. High inrush current can result in power quality issues and induce unnecessary stress on the connected equipment. In contrast, controlled switching, in which the breaker is closed at the point on wave chosen to minimize the inrush current, greatly reduces the inrush current and its negative effects.
This paper describes the design and commissioning of a controlled switching scheme to energize and de-energize a 485 MVA, 220/18 kV transformer. Energization of this transformer frequently resulted in a large reactive current draw from the power grid. The poor power quality was adversely affecting equipment in the adjacent power station and transmission yard. It was not unusual for energization events to cause flicker and brownouts. Excessive noise could sometimes be heard for several minutes from this transformer and sympathetically from other transformers already in operation at the nearby power station, signaling long-lasting negative effects from the initial inrush.
The paper explores several controlled switching schemes and describes the one chosen for this application. It discusses the design and commissioning process and highlights the use of high-resolution oscillography to supplement breaker timing tests to obtain a critical breaker advance time setting for the scheme. The results show that by fine-tuning the breaker advance time setting, inrush currents exceeding 3 pu have been reduced to less than 0.2 pu. The paper describes future improvements to the method as well as benefits to other areas of the power system, including inverter-based resources.