Three-Phase Three-Limb (Core-Type) Two-Winding Transformer
This example illustrates saturation in a Three-Limb Core-Type transformer and compares the Simscape™ Electrical™ Specialized Power Systems model with a Simscape-based physical model.
The top circuit uses Simscape Electrical Specialized Power Systems blocks to implement a 300 MVA, 315 kV/25 kV, Yg/Yg transformer connected to a 315 kV grid. During asymmetrical voltage conditions, the zero-sequence flux returns outside the core, through an air gap, structural steel and tank. Thus, the natural zero-sequence inductance L0 (without Delta winding) of such a core-type transformer is usually very low (typically 0.5 pu < L0 < 2 pu) compared to a three-phase transformer using three single-phase units (L0 > 100 pu) or with a five-limb shell-type transformer. This low L0 value will affect voltage, current and flux imbalances both during linear and saturated operation.
The bottom circuit implements the same circuit using the Simscape electrical and magnetic foundation libraries. Look under the mask of the Simscape transformer to see how the magnetic circuit is implemented. Blue blocks represent steel reluctances of the 3 limbs and 4 yokes with specified nonlinear B-H characteristic. Yellow blocks represent reluctances of air paths modeling the winding leakage inductances. The orange block models reluctance of the zero-sequence flux path through the air. Its average length and section are adjusted in order to get a 0.5 pu zero-sequence inductance (as specified in Specialized Power Systems model).
The Three-Phase Programmable Generator block is used to simulate a 1.7 pu overvoltage on phase A (from 0.2 sec to 0.3 sec), producing saturation of phase A. Transformers in both models start with zero initial fluxes. In order to decrease the time required to reach steady-state, Anti-saturation Control blocks are used to quickly force the average fluxes in the three phases to zero.
Observe that both models produce the same voltage, current, and flux waveforms.