Carolin Hirsching

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.

High voltage direct current (HVDC) grids are envisioned for large-scale grid integration of renewable energy sources. Upon realization, components from multiple vendors have to be coordinated and interoperability problems can occur. To address these problems, a multi-vendor HVDC system can benefit from a partially open control and protection system. Unwanted interactions can be investigated and solved more easily in partially open software compared to when applying black-boxed and vendor-specific software. Although a partially open approach offers these advantages, practical aspects, such as the implementation in a real station architecture, have to be addressed carefully. This paper covers this important topic, first by reviewing the required control and protection functions and second by discussing the choice for certain open and closed software parts, their implementation in physical units as well as the required communication and interfaces. The result from this discussion is a first proposal of a station architecture for a multi-vendor HVDC system using partially open control and protection. This architecture will be a helpful starting point to industry and academia working with research and harmonization on this topic as ad-hoc solutions in terms of practical aspects can be avoided.

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.

For bulk power transmission high voltage direct current schemes based on modular multilevel converter technology offer a high flexibility and reliability. This is especially valid, if bipolar schemes with dedicated metallic return or rigid bipolar configurations are utilized. In case of the latter configuration a floating neutral bus between positive and negative converter exists at one terminal. Therefore, new challenges in control and system design arise. While bipolar configurations with dedicated metallic return are well studied, investigations on the rigid bipolar configuration have been rarely addressed in literature. This paper presents the main differences as well as characteristic control and balancing aspects of the rigid bipolar configuration.

Bipolar high voltage direct current (HVDC) transmission systems based on modular multilevel converters (MMC) are intended to be used in several upcoming projects. This contribution performs a profound analysis of occurring overvoltages in a single point grounded rigid bipolar transmission scheme, where the MMC topology equipped with half-bridge (HB) submodules is utilized. In order to derive voltage stresses affecting the dc cable system, a detailed parametric study is performed by means of timedomain simulations. Obtained results are discussed with regard to different station grounding configurations.

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.

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.