Modeling Autotransformers

The PSCAD Master Library offers limited 1-Phase Auto Transformer and 3-Phase Star-Star Auto Transformer models.  If preferred, this section describes how to build an autotransformer directly by utilizing the existing transformer models with suitable tap changers.

Figure 6-16 illustrates an equivalent circuit for a single-phase autotransformer using a classical single-phase transformer component and a tap changer on the secondary winding (this is one possible method of modeling an autotransformer).  The tap can be set up to cover a wide operating range.

Figure 6-16 - Autotransformer Equivalent Circuit in PSCAD

Some important notes on the use of the configuration in Figure 6-16, as compared with an actual autotransformer model are given as follows:

• The above configuration exactly represents an autotransformer at 100% tap setting.

• The change in tap setting is modeled as a change in the turns-ratio of the transformer.  The per unit leakage reactance and magnetizing currents specified for 100% tap are used to calculate admittances for the new voltage rating, corresponding to the tap setting.  The magnetizing branch (for a non ideal transformer) is considered to be in the middle of the two winding reactances.

For example, if the magnetizing current is assumed to be negligible, the conductance matrix with tap changer on the secondary winding is calculated as:

Where,

A similar but more complex expression is used if magnetising current is considered.