Solid-State Transformer
K.R.M. Nair · 2021
The power grid is becoming increasingly complex due to the continued additions of renewable generation, electric vehicle charging, increase of nonlinear loads, demand for DC, etc. The demands from the user include voltage sag compensation, uninterrupted supply, harmonic filtering and isolation, reactive power compensation, voltage balancing, reduced footprint, elimination of oil/fluids, etc. The transformer to meet the needs of the “smart grid” is solid state using power electronics which steps down the voltage at medium frequency. The incoming medium voltage from the grid at power frequency is first converted into medium-voltage direct current (DC) and then inverted to medium-frequency alternating current (AC). The voltage sag compensation, power factor correction and harmonic isolation are done at this stage. The AC is stepped down by a medium-frequency transformer. The frequency and the material type of the core are selected by optimizing the total owning cost (TOC). The core materials available now are ultrathin cold-rolled grain-oriented steel, amorphous metal, nanocrystalline metal, ferrite, etc. Output from the transformer is low-voltage (LV) AC at medium frequency which is inverted to LV DC and again inverted to power frequency LV for end use. The entire system is bidirectional and capable of automatic control and communication. Basic design of a 1000-kVA, 11-kV/425-V three-phase transformer is worked out. The voltages and currents at the medium voltage conversion and inversion stage, the transformation ratio of the transformer and further the LV DC and AC and voltages are calculated. The objective function for optimizing the TOC of the transformer is in the form: Q = A × no-load loss + B × load loss + C × core weight + D × conductor weight + E × insulation weight + F where A and B are costs of no-load loss and load loss, respectively. C, D, E and F are the unit costs of core material, conductor material, insulation and other materials.