Decentralized/distributed optimal power flow (OPF) in alternating current grids is currently a subject undergoing intense study to cope with computational burdens in large-scale grids and to maintain self-control of a regional system operator. However, future power grids will most likely be hybrid grids consisting of the conventional AC transmission system combined with high voltage direct current technology. Thus, we reformulate the full AC-DC OPF problem such that it becomes separable and therefore accessible to decentralized algorithms. Then we show in detail two different approaches on the decomposition of a hybrid AC-DC grid. Finally, we implement an improved Alternating Direction of Multipliers Method (ADMM) as well as the most recently proposed Augmented Lagrangian based Alternating Direction Inexact Newton (ALADIN) method. Simulation results show that optimality gaps below 0.01% are reached with both decomposition approaches and algorithms in two different test systems (5-bus and 66-bus). Furthermore, convergence rates and wall clock times are reduced by around one order of magnitude from ADMM to ALADIN.

Bipolar high voltage direct current (HVdc) links present one reasonable solution to realize bulk power transmission schemes. Especially if full-bridge modular multilevel converter topologies (FB MMC) are utilized, a broad variety of operational advantages exists. To completely unveil intrinsic technological potentials, comprehensive control and protection concepts to discriminate and handle ac, dc as well as internal converter faults are essential. The proposed scheme consists of a fast and selective multi-zonal fault detection paired with decoupled ac and dc current controls. Hereby, actively controlled dc fault current interruption as well as ac fault reactive current injection are incorporated. It is demonstrated that internal converter quantities remain within feasible technical boundaries, interactions are limited or even fully mitigated and protection thresholds can be chosen to enable selective tripping. To validate and evaluate the proposed concept, simulations for characteristic faults are performed in electromagnetic transients software.

The symmetrical monopolar configuration is the prevailing scheme configuration for HVDC interconnectors utilizing the modular multilevel converter topology. However, the rigid bipolar configuration is gaining significance, as first projects are currently in planning stage. The purpose of this paper is an analysis of both HVDC scheme configurations with regard to cable voltage stresses originating from faults. The study focuses on HVDC cables with extruded insulation connected to MMCs equipped with half-bridge submodules. Basic design aspects in rigid bipolar configuration such as the station grounding concept are discussed. Moreover, various station internal as well as dc faults are examined by means of electromagnetic transient simulations. Occurring cable voltage stresses are analyzed with respect to voltage polarity, peak value and wave front-parameters and compared in symmetrical monopolar and rigid bipolar configuration. It is demonstrated that cable overvoltages with same polarity as dc operating voltage are significantly reduced in schemes operating in rigid bipolar configuration. Voltage reversals caused by the discharge process during a cable fault are nearly independent of the selected scheme configuration. Obtained results are relevant with regard to upcoming projects in rigid bipolar configuration and provide insights to further refine insulation co-ordination aspects related to dc cable systems.

Due to decreasing inertia within the German transmission grid, HVdc transmission links are recently planned as a grid-forming asset in order to contribute to the overall system stability. Here, virtual synchronous machine implementations may be favoured as they enable inertia supply and operation at different grid strengths. However, their fault-ride-through capability is linked to several challenges such as current limitation, the preservation of transient stability and dynamic reactive current injection, especially, while providing high inertia. Moreover, diverse effects occur at the point of connection, such as voltage drop and phase jump effects, which can be traced back to the fault impedance characteristic during a symmetrical fault event and affect the transient stability. Therefore, this paper investigates these occurring effects in detail. Based on this analysis, the performance of the virtual synchronous machine is evaluated with respect to different design aspects - namely the reactive power control, the inertia model and design of the phase locked loop - resulting in a recommendation for a design compromise. Finally, observed relations are classified according to their applicability within a typical design of an HVdc converter equipped with half-bridge submodules. Derived design recommendations are validated by simulations in electromagnetic transients software.

The increasing number of electric vehicles poses new challenges to the power grid. Their charging process stresses the power system, as additional energy has to be supplied, especially during peak load periods. This additional load can result in critical network situations depending on various parameters. These impacts may vary based on market penetration, the energy demand, the plug-in time, the charging rate, and the grid topology and the associated operational equipment. Hence, the impact of electric vehicles (EVs) on the power grid was analysed for twelve typical German low voltage grids by applying power flow calculations. One main result was that thermal and voltage-related network overloads were highly dependent on market penetration and grid topology.

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 electromagentic transient (EMT) studies are required to determine the absolute maximum lightning impulse voltage the cable might experience. Within this paper, the general lightning behaviour of OHL-cable mixed systems subsequent to the occurrence of a backflashover (BFO) as well as a shielding failure is investigated for a 525-kV HVDC transmission scheme. This paper presents a modelling 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.