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This paper investigates the steady-state and transient performance of high-voltage DC (HVDC) transmission systems based on modular multilevel 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 VSCHVDC response to external AC faults. The stability of cell capacitors during power reversal and AC faults is also examined.