What occurs to a three-phase transformer bank with an ungrounded wye high side connection if one phase is lost?

In a three-phase transformer bank with an ungrounded wye high side connection, the system is designed to operate with all three phases present. If one phase is lost, the transformer bank enters a condition known as "single-phasing." This occurs because the loss of one phase disrupts the balance of the three-phase system, causing the remaining phases to experience conditions that can potentially lead to equipment damage.

When one phase is missing, the currents in the remaining two phases can rise significantly, potentially leading to overheating of motors and equipment connected to the transformer. Additionally, the voltage on the remaining phases may not be able to support the load as designed, further compounding the risk of failure. Therefore, the inability of the transformer bank to continue normal operation under these circumstances is why it is described as being rendered inoperable by "single-phasing."

In such cases, protective devices or relays are often needed to detect the loss of a phase and disconnect the transformer or associated equipment to prevent damage. This highlights the critical nature of balancing loads across all phases in a three-phase system.