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In certain low voltage distributed generation (DG) networks, inverters do not detect unintentional islanding caused by voltage imbalances. This condition violates IEEE-1547 requirements that the networks detect and isolate an island within two seconds of its formation. This paper provides practical application of phase loss detection to inverter protection schemes using transformer networks to address voltage drop considerations. In these applications, phase loss detection can be shielded by the addition of a transformer impedance network, preventing inverter isolation during low generation conditions. Remote emergency stop initiation via current imbalance detection at the Point of Common Coupling (PCC) can be implemented utilizing a dedicated radio network configured fail safe, providing anti-islanding supplementation to ensure detection. This addition establishes islanding detection under all generation conditions, improving line worker safety by ensuring power injection removal. Transformer network current analysis is considered for various common transformer configurations, with an existing application example.
Most EPSs in service were not originally designed to accommodate low voltage distributed generation at the distribution level. As DG becomes less expensive and more readily available, grid penetration by DERs1 has created new problems for energy providers. One such problem is the occurrence of islanding. An island is created when a distributed generator continues to power a section of line and its corresponding load despite the absence of a grid. Some power systems, such as microgrids, are designed to be able to connect and disconnect from the grid at planned intervals with a dedicated switch. However, in DG systems that are designed to be gridconnected, unintentional islanding is a grid stability and safety concern.
Per IEEE 1547-2018, “for an unintentional island in which the energizes a portion of the Area EPS through the PCC the DER shall detect the island, cease to energize the Area EPS, and trip within 2 s of the formation of an island.” However, the protection on the DER must not be so sensitive that it creates false positives because that would be a failure to meet the voltage ride-through requirements of the standard.
Single-phasing, or the loss of one phase of a threephase system, occurs more frequently in rural, medium voltage feeders than in urban areas. Fusing and service laterals can increase the likelihood of singlephasing. DG sites are frequently located in rural areas with long distribution lines where utility protective schemes rely on fuses. Subsequently, these sites are more likely to see the loss of a single phase, which may become an undetected and unintended islanding event.
|Supplementing Anti Islanding Protection Applications in Distributed Generation Interconnections|