Transformers and Continuity of Phases Between Primary and Secondary

The great variety of transformer connection schemes makes it difficult to maintain the notion of named electrical phases as continuous across the primary/secondary boundary at a transformer. Nonetheless it may be important in some contexts to know the primary phases upon which a given secondary circuit or a given service point may depend for its power (e.g. outage analysis in a system where sectionalizing equipment can have independently operating phases). ArcFM Electric Tracing, like Feeder Manager, recognizes a continuity of named phases between primary and secondary conductors, mediated by the transformer and using its phase designation to determine which phases are continuous across the boundary.

However, the standard ArcFM electric data model uses a single junction feature to represent a distribution transformer, with both primary and secondary conductors directly attached to the junction. It is implicitly understood in this model that power bypasses the transformer when flowing from one primary conductor into another at the transformer junction. However, power can flow between primary and secondary conductors only by passing through the transformer windings—which may only be connected to a subset of the available primary phase conductors, as indicated by the transformer’s phase designation.

Because transformers are modeled to treat primaries and secondaries differently, ArcFM Electric Tracing must distinguish primary from secondary conductors while tracing. It does this by examining the OperatingVoltage attribute of conductors (as encoded in MMElectricTraceWeight) whenever the tracing encounters a transformer junction. Any phases that reach the transformer junction via a conductor of a given operating voltage are allowed to pass unimpeded into any other attached conductor of the same operating voltage. This enforces the “primary conductors bypass the transformer” rule. When tracing into a conductor of a different operating voltage than the arrival conductor, however, any of the phases not included in the transformer’s phase designation will be barred from continuing into the different-voltage conductor. This enforces the “must go through transformer windings when changing voltage levels” rule.

Phase continuity through a transformer junction feature is determined by the operating voltage of attached conductors.