A modular multilevel converter for series compensation of an EVH transmission line with battery energy storage

This paper presents the design and analysis of a converter for integrating energy storage into the transmission grid. The system consists of a Battery Energy Storage System (BESS) interfaced to a Modular Multi-level Impedance Sourced Converter (MMZSC) acting as a bidirectional controllable source for a three phase isolating transformer arranged in a Series Compensation (SC) configuration that is dual-purposed for both power flow control and transient stability of distributed generation that is weakly coupled to the surrounding power system. The design example is detailed with respect to considerations of interfacing to 345 kV lines. First, a qualitative design review is given with respect to limitations of current technology and valuable improvements made possible by this paper's combination of system configuration, topology, and device considerations. A description is given of how the converter is operated via a modified space vector modulation scheme in conjunction with a sliding-mode direct power control method. Computer simulations are included that demonstrate operating modes of this system; including bidirectional power flow, superior efficiency, reduced DC impedance requirements, and lower distortion than equivalent voltage source converters (VSC). This paper provides a new contribution by combining the elements of bidirectional power flow with a multi-level Z-source converter. In addition the justification for this system as the best candidate for providing the grid regulation functions is considered.