Modelling and Testing Transformer Differential Protection in Inverter-Based Generation Networks

With the increasing penetration of inverter-based power generation, the evaluation of network protection performance and coordination of protection systems deserves closer examination. The electrical protection of power transformers with fault contribution from electronic interfaced power sources imposes new application challenges that have to be dealt with by protection engineers. Even though transformer differential protection has been successfully employed as the main protection of power transformers for many years due to its simple principle of ampere-turn balance and first Kirchhoff’s law, one of the major challenges to its successful application is the discrimination of fault currents from inrush currents due to transformer energization and overexcitation. In the case of an electronic-based power interface, the current waveform and harmonics content (2
^nd
, 3
^rd
, 5
^th
) in the fault currents may significantly impact the inrush stabilizing algorithm used in conventional transformer differential protection. The advanced transformer protection may use the current waveform analysis (CWA) algorithm to discriminate internal faults from inrush. The CWA algorithm is able to detect internal faults just after the first cycle after fault inception and block the standard second harmonic inrush stabilization. The investigated power system is a large-speed drive with an active front end in a four-quadrant rectifier that can be operated in motor and generator mode. The active front end is a modular multilevel converter with LVRT capability. For a detailed analysis of the transients, the modular multilevel converter is modelled in MATLAB/Simulink with an adequate control strategy. The transformer is modelled with nonlinear characteristic. The MATLAB/Simulink environment tests included external faults, internal faults at the grid and converter side and turn-to-turn faults. Selected cases are shown and discussed in detail.