Frequency Control During LVRT Operation for CHB Converter in a Low-Inertia System

This work presents a virtual synchronous inertia implementation method for cascaded U-bridge (CUB) based battery storage systems. The virtual inertia is provided in a medium voltage grid, by coupling the derivative term of change in frequency to the change in reference active power of CHB converter. A frequency-locked loop is used instead of a phase-locked loop to estimate the frequency variation derivative signal with no high-frequency noise caused by using a differential operator. Additionally, as the penetration of inverter-based resources is increasing in a medium voltage grid, the converter must be connected to the grid during grid voltage sags and supply reactive power according to the grid codes. This paper presents the operation of a CHB-based battery storage system providing virtual inertia along with Low Voltage Ride Through (LVRT) operation. The controller meets all the LVRT requirements for both symmetrical grid faults (three-phase voltage sag) and unsymmetrical faults. It also ensures that the grid current magnitude is within the safe operating limits of the CHB converter while providing virtual inertial support as well as LVRT operation. The whole functioning of frequency control and LVRT operation in a low-inertia system is validated through MATLAB/Simulink.