Investigations of Switching Tests during Transformer Inrush with a 145-kV Vacuum Circuit Breaker

Switching in power systems needs to be analysed carefully since switching actions can lead to serious overvoltages that can in certain cases damage devices and equipment. While highvoltage vacuum circuit-breakers are already commercially available, field experience with this kind of equipment is limited.

To address this gap, the Austria’s transmission grid operator executed switching tests on a 123kV vacuum circuit-breaker in a 110-kV substation supported by the stakeholder of Austria’s grid operators. Several operational switching tests, including the switch-on (energise) and switch-off (de-energise) of a 220 / 110 kV 300-MVA no-load transformer from the secondary side, were performed. The tests also include the switch-off during an ongoing inrush with high magnetisation current, to also verify particular switching operation. This specific test was executed twice and is the main focus of this paper.

For the test setup special measurement equipment was installed, including CR-dividers for high-bandwidth voltage measurements. The transformer is energized via the 110-kV secondary circuit, which leads to inrush currents in the range of 1 to 1.5 kA.

The current measurements are performed with protection current transformers with additionally installed measurements operating at a sampling rate of 10 kHz. The voltage across the circuitbreaker is calculated by the difference of the voltage measurements of the CR-divider at both sides of the poles.

This paper analyses two test operations involving the switch-off of three-phase inrush currents.

While the first measurement did not show any abnormalities, the second measurement reveals a transient overvoltage with an amplitude of about 240 kV, which is 2.18 times the rated operating voltage. The reason of the occurring overvoltage for only one of the two tests is explained with an unfavorable timing of contact separation of the circuit-breaker for the second test. Phase L3 was the first pole-to-clear and the magnetic energy stored in the currents of phases

L1 and L2 charged stray capacitances and cable capacitances in phase L3. Because these capacitances are very low this leads to a high overvoltage peak in L3.

This analysis identifies that the timing of the arc clearing of the circuit-breaker is a main influence regarding overvoltages. In this context, the arc clearing of a circuit-breaker is significantly affected by the timing of the opening command and the breaker’s opening time. It is also shown that the remanence of the transformer strongly affects the frequency of the transient overvoltage while the cable capacitance is affecting the amplitude of the overvoltage.