Loss of a Phase Condition in a Three-Phase PV System
Phase loss in a three-phase transformer can have significant and damaging effects on the transformer and the electrical system. Three-phase electrical systems are designed to operate with balanced load. When one phase is lost, it disrupts this balance.
In commercial photovoltaic (PV) systems, it is common practice to install a three-phase YG:yg step-down transformer between the PV inverter (Distributed Energy Resource, DER) and the utility at the Point of Interconnection (POI). In light commercial and residential apartment applications, a 480/277 V to 208/120 V transformer is often used. These transformers must be adequately protected against open-phase conditions.
An open-phase condition, or loss of a phase, refers to the unintentional disconnection of one phase on the supply side of the transformer. This condition can be caused by various issues such as a loose connection, a broken conductor, a blown fuse, or a malfunctioning circuit breaker contact.
Transformers installed between the inverter (DER) and the utility (POI) may be energized from either side depending on operating conditions. Grid-tied inverters typically include anti-islanding protection and are designed to shut down upon detecting the loss of a single phase. However, the presence of a transformer between the DER and the POI complicates this scenario, especially in open-phase conditions.
In such configurations, the transformer construction becomes critical. When a single-phase loss occurs in a transformer with a three-leg core and YG:yg configuration, the transformer may reconstruct the missing voltage on the open phase through internal electrical and magnetic effects. This phenomenon, often referred to as voltage reconstruction (VC), can hinder effective anti-islanding detection.
In a three-leg core transformer, the magnetic flux in all three legs must sum to zero due to the core’s magnetic path constraints—this is known as zero-sequence flux. If one phase (e.g., Phase C) becomes disconnected on the primary side, there is no direct excitation voltage for that phase. However, to maintain the zero-sum condition of flux, the magnetic flux from phases A and B will return through the third leg, often inducing nearly full voltage on the open phase under no-load conditions.
This voltage reconstruction poses several challenges:
– Detection Difficulty: Traditional protective devices may not recognize the open-phase condition because voltage appears normal at the secondary side.
– Magnetic Saturation and Heating: The high magnetic reluctance associated with zero-sequence flux in a three-leg core requires significant magnetizing current, which can lead to core saturation and overheating.
Therefore, the challenge is not just about deploying the right protective devices but also implementing reliable detection methods. Currently, the most effective technique appears to be zero-sequence current relaying, potentially supplemented by negative-sequence voltage relaying for enhanced reliability.
