Automated Three-Phase Transformer Connections in HIFREQ
HIFREQ can model both real and ideal single-phase transformers. In addition to offering the user maximum flexibility to connect these transformers together manually, SESCAD provides automated options to create three-phase transformer banks rapidly. Thus, even complex configurations such as Scott transformers, three-phase autotransformers with delta tertiary or zigzag transformers can be directly inserted in SESCAD with only a few mouse clicks.
TYPES OF TRANSFORMERS MODELED IN HIFREQ
Many types of three-phase transformer connections can be made automatically by SESCAD for HIFREQ. These building blocks can be imported in SESCAD, by selecting Insert | Object from Database from an already opened HIFREQ input file, then clicking on the 3-Phase Transformers folder, which yields the following list of available transformer connections:
The letters Y, D, Z and A refer to wye, delta, zigzag and autotransformer configurations, respectively.
Fig. 1 shows a 3D view of a complete HIFREQ model of a three-phase autotransformer bank with delta tertiary. (S) and (T) indicate the secondary and the tertiary outputs, respectively, while the tertiary loop is the shaded area. For such transformer banks, each phase is a three-winding autotransformer composed of three single-phase transformers. The underlying single-phase transformers can be either ideal transformers or more general transformer models including impedances.
Fig. 1. Three-Phase Autotransformer Bank with Delta Tertiary in SESCAD
Fig. 2 shows a 3D view of a zigzag transformer used in HIFREQ to establish a ground return path for an otherwise ungrounded source.
Fig. 2. 3D View of Zigzag Transformer Model in SESCAD
As can be seen in Fig. 3, complex, large-scale systems, including any number of three-phase transformers, can be modeled in HIFREQ in order to gain insight into the response of such systems during both load and fault conditions.
Fig. 3. Complete Network with Transformer Models
The figure above shows that HIFREQ can accurately model another complex system component besides the transmission line, grounding grid and realistic transformers: the Gas Insulated Switchgear (GIS) system. In fact, a GIS is simply a set of conductors completely contained within the outer pipe type enclosure which is filled with a gas having a high dielectric strength. This fits the description of a cable, albeit a large one, of which HIFREQ has two implementations. The Concentric Cable type can be used to model single-phase GIS, while the more general Pipe-Type Cable can be used to model a multi-phase GIS typically used at lower voltage levels. Modeling such systems in this way ensures proper distribution of fault currents because HIFREQ takes into consideration inductive effects between the conductors and the enclosure. A GIL can be modeled similarly.