-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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The alternate arm converter (AAC) is a multilevel converter of the same family as the modular multilevel converter (MMC). Contrary to the MMC, the AAC offers dc-fault tolerant capabilities but requires complex submodule (SM) capacitor voltage/energy regulation and circulating current control. Such control is also limited due to the small overlap period where both arms conduct and energy between the arms can be exchanged. This paper develops a double-band hysteresis-based circulating current control strategy by introducing an asymmetric overlap period control of the director switches (DSs). The proposed controller ensures zero-current switching operation of the DSs, and increases the flexibility of SM capacitor voltage regulation owing to the asymmetric overlap period. It improves the performance of current control and energy regulation for the AAC at both near sweet-spot and non sweet-spot operation without disturbing the output voltages and currents. The performance and effectiveness of the proposed controller are illustrated through simulations on MATLAB-Simulink and PLECS.

Alternate arm converters-based HVDC model compatible with the CIGRE B4 dc grid test system