Abstract
Multi-terminal high voltage DC transmission currently represents a leading technology in long-distance power transmission systems. Among the main technical challenges facing such technology, DC fault isolation, permitting different grounding schemes, providing interoperability, and high DC voltage stepping between different HVDC networks, and allowing high-speed power reversal without power interruption especially when connecting the pre-existing voltage source converters (VSC) and line commutated converters (LCC)-based HVDC networks. This paper introduces a new modular multilevel converter (MMC) based front-to-front DC-DC converter to interconnect two different types (LCC/VSC) of HVDC networks. The proposed topology comprises a voltage source MMC (VS-MMC) and a current source MMC (CS-MMC), while both are coupled via an AC link including the isolating transformer. The proposed topology can successfully provide an uninterruptible bi-directional power flow, high DC voltage stepping with a DC fault blocking capability, and low number of semiconductors due to the usage of only half-bridge SMs. The system design is provided with a detailed mathematical analysis. Furthermore, two active power control methodologies are proposed and compared. The first control technique is simpler and entails lower passive elements, while the second technique ensures a zero reactive power over the full range of active power flow. Furthermore, Losses analysis and comparison are provided between the two proposed control techniques. Finally, Control-Hardware-in-the-Loop (CHiL) test validation is employed to confirm the validity of the proposed system under healthy as well as different fault scenarios.
Highlights
RECENT literature has witnessed an extensive investigation to voltage source converters (VSCs) based high voltage direct current (HVDC) transmission networks as a promising technology in long-distance power transmission systems [1]–[3]
By applying the duality principle in CS-modular multilevel converter (MMC), all current source sub-modules (CS-SM) per arm are replaced by an equivalent variable inductor (L1i), as described in Fig. 6, whose inductance depends on the number of inserted CS-SMs as in (11); L1i where L1i, n1i are defined as the equivalent arm inductor and the number of inserted CS-SMs in the ith arm, and LSM is the inductance of each CS-SM
The DC link at current source MMC (CS-MMC) exhibits reduction in the same can be observed at the DC link voltage of voltage source MMC (VS-MMC). voltage due to control action of blocking the VS-MMC and Based on obtained results, the proposed DC-DC converter topology is fault tolerant during the fault at both converter sides
Summary
RECENT literature has witnessed an extensive investigation to voltage source converters (VSCs) based high voltage direct current (HVDC) transmission networks as a promising technology in long-distance power transmission systems [1]–[3]. Most of the existed VSC-based converters are MMC with half-bridge SMs. different power reversal strategies are proposed in literature regarding the hybrid LCC/MMC HVDC systems [9]. The DC modular choppers, which utilize lower number of SMs as introduced in [37], employ capacitive or inductive energy storing elements but these converters exhibit AC circulating current due to the hard switching of SMs, which increase conduction losses [38] Another autotransformer based DC-DC converter has been proposed in [39], which leads to partial conversion of total DC interchange between both DC sides [39], [40]. A new modular multilevel based DC-DC converter is proposed to interconnect different types of HVDC system networks (LCC and VSC). The AC current is divided among the arms depending on the equivalent capacitance of each arm as in (9) and (10)
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