170 / 2024-04-15 15:23:36

Minimizing inrush current and voltage dip at energization of transformer

transformer,Inrush current,Voltage sag,vacuum circuit breakers

Energization of transformers leads to inrush currents. The magnitude of the inrush current is affected by the phase angle at which energization occurs, as well as the remanent flux in the transformer core. This has been well known for a long time. Even so, most of the transformer energization events in the distribution grid are still performed without any means to minimize the inrush current. Even though the inrush current may not be extremely severe to the transformer itself, large inrush currents may also cause significant voltage drop in the grid. With the introduction of more renewable energy resources in the grid, switching transformers in and out also becomes more frequent – hence the importance of minimizing transient stresses related to the switching increases. A new engineering recommendation was for example released by the Energy Networks Association in 2019 “Voltage fluctuations and the connection of disturbing equipment to transmission systems and distribution networks in the United Kingdom” (P28, Issue 2), and a technical review of this one, especially focusing on voltage fluctuations caused by the energization of transformers, was published by Enspec.



In order to investigate the topic in depth, studies have been performed in a 20 kV, three-phase grid, in a laboratory of ABB Corporate Research in Västerås, Sweden. The 20 kV part of the laboratory is supplied via a transformer and consists of several parallel 20 kV cables of 100s of meters of length. One cable ends in a test cell where a 900 kVA transformer (20kV/690V) is connected. At the 20 kV level, a vacuum circuit breaker is used to energize and de-energize the transformer. Measurements were firstly performed with the transformer operating in no load conditions. Using the voltage in phase L1 as reference, circuit breaker closing was performed at all possible phase angles from 0° to 360°, evaluating the peak inrush current and the maximum voltage drop at all phase angles. Thereafter, similar experiments were performed, energizing the transformer in no load conditions, but connecting a load to the LV side before opening the circuit breaker. This corresponds to deenergizing the transformer in load conditions. Tests were performed with loads of different power factor. Laboratory equipment including an FPGA based control and acquisition unit was used to control the circuit breaker closing angle. Tests were performed using both simultaneous operation of the three circuit breaker contact pairs, as well as individual operation, allowing to close two circuit breaker contact pairs first and delay the third one by one quarter of a period. Inrush currents ranging from a few A to close to 200 A were obtained. The nominal current of the transformer is 26 A rms. Hence, a huge benefit can be obtained by choosing the optimal closing angle. The measurement results revealed the optimal closing angle as well as the optimal type of closing (simultaneous or single pole). This paper presents the test set up and the test results obtained.