Open phase condition or loss of a phase refers to one phase of three phases being physically and unintentionally disconnected on the supply side of the transformer. This could occur for several reasons such as a loose cable, a broken conductor, a blown fuse, a circuit breaker with one defective contact, etc.
Transformers used between the inverter (DER) and the utility (POI) are fed on both sides alternatively. Grid tied inverters are fitted with anti-islanding protection but when they experience loss of one phase in a three-phase system, they shut down as expected. However, in a three-phase system the situation becomes more complex if a transformer is present between the inverter (DER) and utility (POI). In this situation, the transformer construction makes a difference in an open phase condition.
Upon loss of a single phase in a three-phase transformer the downstream voltage could be re-generated by the transformer internal electric or magnetic actions (Voltage Reconstruction, VC) that makes the ani-islanding detection difficult. In a three-leg core YG:yg transformer the sum of the three phase fluxes, or the “zero-sequence flux”, must always be zero.
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