Adaptive Virtual Inertia Emulation and Control Scheme of Cascaded Multilevel Converters to Maximize Its Frequency Support Ability

To fully explore the frequency support ability of the cascaded multilevel converters (CMCs), this article has proposed an adaptive virtual inertia emulation and control scheme. First, the basic inertia emulation mechanism is introduced, which is achieved by absorbing and releasing the capacitor energy in the CMC. Second, considering the influence of reactive power generation of the CMC, the available variation range of the dc components of submodule (SM) capacitor voltages is redefined and recalculated. As a result, the maximum power exchange in SM capacitors in the CMC, which is used for frequency support, can be precisely characterized. Afterwards, the highest/lowest frequency value is estimated under different source/load power disturbance, and the adaptive virtual inertia emulation and control method is designed according to the dynamic frequency extremum estimation and the maximum variation range of SM capacitor voltages. Finally, simulation and experimental results verify that by the proposed method, the CMC can enhance its inertia support ability and reduce frequency deviation of the system. Under 0.02 p.u. load step condition, the frequency deviation is reduced by about 32.8%.