The use of VSC-HVDC grids for offshore wind farm integration will require the use of dc breaker systems and they, at present, will require dc reactors to limit the rate of rise of fault current. The introduction of large dc reactors throughout a VSC-HVDC system can have significant impact on its stable operation and will require additional control. This paper analyses this problem and proposes a PSS-like control (DCPSS) to aid dc grid stability and cope with this effect. A generalized analytical model for studies on dc voltage control is presented. Key stability and transient performance issues caused by the use of the dc reactors in a multi-terminal system, are investigated by analyzing poles, zeros and frequency responses of both open-loop and closed-loop models. Design and location identification methods for the DCPSS are provided. Excellent damping enhancement is achieved by this controller. The analytical studies and time-domain simulations in this paper are performed based on two VSC-HVDC models. T Disconnector Auxiliary DC Breaker Hybrid DC Breaker Main DC Breaker Current Limiting Reactor Residual DC Current Breaker Fig. 1. ABB Proactive VSC-HVDC Circuit Breaker [1].0885-8977 (c)

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-High voltage direct current grids will require the development of dc protections that provide fast fault isolation and minimize the disturbance caused to the existing ac power networks. This paper investigates how the dc fault recovery performance of a half-bridge modular multilevel converter (HB-MMC) is impacted by different dc protection design choices. A HB-MMC point-to-point HVDC system that is protected with dc circuit breakers (CBs) is simulated on a real time digital simulator (RTDS) using detailed switch models of the converters and switch gear. A dc CB controller has been developed and implemented in a software-in-the-loop fashion, and has been made available free for download. A novel blocking scheme for the HB-MMC is proposed, which limits the prospective dc-side fault current, benefiting dc switch gear. A comparison of circulating current controllers shows that the standard dq controller is likely to be unsuitable for fault studies. Finally, benchmarking shows that a 48% reduction in power flow recovery time and a 90% reduction in the energy dissipated in the circuit breaker can be achieved, along with other benefits, depending on the protection design.Index Terms -HVDC, RTDS, protection, dc circuit breaker voltage-source converter, modular multilevel, ac grid impact.

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