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The Modular Multilevel Converter (MMC) topology is discussed intensively as a crucial part of future DC grids, especially to integrate offshore wind power. While generalized voltage source converter models are usually utilized for multiterminal investigations, conclusions on transient DC grid behavior and on ACDC interactions during distorted AC grid conditions are hard to draw or limited in their significance. In this paper, a MMC based DC grid framework with a tripartite focus on internal converter, DC and AC grid control is presented. Besides an evaluation of the implemented MMC model that incorporates the available arm sum voltage, a decoupled AC and DC current system control concept suitable to handle unbalanced voltage conditions is derived for HVDC applications. Combined with wind farm arrays, this collocation allows a meaningful examination of hybrid ACDC structures and the implementedDC grid control methodologies. To provide proof of the universal applicability of this framework, simulations are carried out on a five terminal system. Index Terms-generalized MMC modeling, HVDC grid control, MMC energy balancing, power converter, unbalanced operation, wind power integration 0885-8977 (c)

In high-voltage direct current (HVDC) transmission systems comprising overhead line (OHL) sections as well as cable sections, the dc cable system is exposed indirectly to lightning strokes. In case of HVDC cable systems with extruded insulation the present test recommendations do not specify lightning impulse test levels related to rated system voltage. Instead, preliminary electromagnetic transient studies are required to determine the absolute maximum lightning impulse voltage the cable might experience. Within this paper, the general lightning behavior of OHL-cable mixed systems subsequent to the occurrence of a backflashover as well as a shielding failure is investigated for a 525 kV HVDC transmission scheme. This paper presents a modeling method to calculate occurring lightning overvoltages along a dc cable embedded between OHL sections. Based on a detailed parametric study using time-domain simulation, maximum lightning impulse voltages along the cable system are determined, and evaluated with regard to impulse level and polarity. The impact of project related parameter such as cable length, tower design, and tower grounding conditions on lightning overvoltage affecting the cable is demonstrated.

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