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The VSC HVDC back-to-back arrangement is used when two asynchronous AC systems need to be interconnected for bulk power transmission or for AC system stabilization reasons. Besides controlling the through power flow, it can supply reactive power and provide independent dynamic control at its two terminals. This paper investigates the steady-state and transient performance of high-voltage DC (HVDC) back-to-back transmission systems based on three-level voltage source converters. The study involves analysis of active-reactive power capabilities (P-Q envelope) including active power reversal and provision of voltage support to AC networks. The transient performance is explored by examining the VSC_HVDC response to external AC faults. Finally, the models and results are presented and tested by simulations using Matlab Simulink and its toolbox SimPowerSystems.