Fault ride through control of medium-voltage converters with LCL filter in distributed generation systems

The fault ride through capability of distributed generation systems is an important factor for the stability of the grid. Especially for medium-voltage converters with low switching frequency this requirement is difficult to fulfill. In case of sudden voltage dips, the dynamics of a common grid control is normally too slow to prevent unacceptable converter current transients which can cause a destruction of the power semiconductors. To avoid this, a predictive control strategy is presented in this paper. For the steady state and in case of reference value changes, a common dq-oriented control of the positive and negative sequence is used. Only in case of serious grid faults, the proposed predictive control strategy will take over the calculation of the switching states for a few milliseconds. With the help of this short intervention, the system remains stable in case of grid faults, and unacceptable converter overcurrents are avoided.