Assessment of an electrical resonance issue in a fast electronic on-load tap-changer transformer based test-bench during renewable energy source tests

The growth of renewable energy source (RES) installations in remote locations and less favourable grid sites increases the grid stability challenges in transmission and distribution systems. The use of electronic on-load tap-changer (eOLTC) on electromagnetic power transformers will allow RESs to have faster voltage ride-through (VRT) and reactive power injection responses, thereby improving the grid operability. The validation of these RESs equipped with novel eOLTC technology demands test-benches that allow fast voltage variations of the medium-voltage grid to which the power generation unit (PGU) is connected. To do so, eOLTC transformers can also be used. Thus, operating conditions of conventional field RES installations composed of a cascaded connection of a mains transformer and a PGU are better reproduced. However, when the voltage tap positions of the eOLTC transformers change, their impedance changes as well, resulting in a variation of the equivalent impedance at the PGU connection point. Under these conditions, depending on the cable impedance and transformer impedance relation, an electrical resonance issue could arise. This work addresses the assessment and solution of an electrical resonance phenomenon observed when testing an eOLTC transformer of 20/0.69 kV and 5.5MVA connected to an upstream 30/20 kV and 7.4