Modeling of Transformer

Modeling of Transformer

For transformer itself the pu (or percentage impedance) value is same either referred from HV side or LV side (see next coffee break). Thus use of pu values eliminates need of representing the transformer; and entire power system can be represented by a simple network consisting of basic elements such as resistance, inductance and capacitance.

To convert a one line diagram of power system to its’ equivalent pu network first any appropriate base MVA is selected for entire power system (usually 100 MVA) and selection of base kV in one part of the power system determines base kV in all other parts of the power system as per transformation ratio. That means in simple words rated voltage (Line-Line) of any element is base kV for any element is base kV for that element. Then using above formulae we can represent power system by its equivalent network. This will be clearer after going through an example explained here after.

Generally generator and transformer impedances are given in pu values (percentage value/100). When these values are given in pu, base for these values is rating of respective generator or transformer in MVA and base kV is its rated voltage (Line-Line). Hence to convert these pu values to new base MVA selected for system modeling (generally 100 MVA) following formula is used.

Thus 10% (0.1 pu) impedance of 25 MVA power transformer when converted to base of 100 MVA would become 10*(100/25) = 40% (0.4 pu). Whereas 14% impedance of 315 MVA ICT would become 14*(100/315) = 4.44%.

Readers who are interested in knowing how pu values are useful in case of electrical circuits with transformers may go through coffee break section. For those readers who are already comfortable with using pu values may directly go to solved example.

Modeling the power system for unsymmetrical faults will be discussed afterward.