A Delta-Connected Modular Multilevel STATCOM With Partially-Rated Energy Storage for Provision of Ancillary Services

This paper proposes a delta-connected Modular Multilevel STATCOM with partially rated storage (PRS-STATCOM), capable of providing both reactive, and active power support. The purpose is to provide short-term energy storage enabled grid support services such as inertial, and frequency response, either alongside or temporarily instead of standard STATCOM voltage support. The topology proposed here contains two types of sub-modules (SM) in each phase-leg: standard sub-modules (STD-SMs), and energy storage element sub-modules (ESE-SMs) with a dc-dc interface converter between the SM capacitor, and the ESE. A control structure has been developed that allows energy transfer between the SM capacitor, and the ESE resulting in active power exchange between the converter, and the grid. Injecting \text{3}^{rd} harmonic current into the converter waveforms can be used to increase the amount of power that can be extracted from the ESE-SMs, and so reduce the required ESE-SMs fraction in each phase-leg. Simulation results demonstrate that for three selected active power ratings, 1 pu, \frac{2}{3} pu, and \frac{1}{3} pu, the fraction of SMs that need be converted to ESE-SMs are only 69%, 59% & 38%. Thus, the proposed topology is effective in adding real power capability to a STATCOM without a large increase in equipment cost.