Until today, HVDC systems based on Modular Multilevel Voltage Source Converters (VSC-MMC) have exclusively been realised with cross-linked polyethylene (XLPE) cable transmission in monopolar configuration. The next generation of VSC-MMC systems may comprise bipolar configurations, mixed cable and overhead line transmission or hybrid AC/DC corridors. While diverse and more complex fault characteristics are expected in these systems, suitable XLPE cable test standards addressing these impacts are not yet available. Within this work, the dynamic fault behaviour of next generation VSC-MMC systems is analysed to evaluate voltage and current stresses imposed on future cable applications. For this purpose, several system topologies and MMC setups are modelled and extensive fault simulation studies are carried out in an electromagnetic transient program (EMTP). As the results indicate, future cable applications may be exposed to severe transient stresses exceeding existing test levels. In particular, travelling wave reflections at the transitions of cable and overhead line segments may cause high-frequency voltage oscillations with amplitudes of up to −2 pu. Moreover, intersystem faults in hybrid AC/DC corridors may lead to superimposed AC and DC cable stresses. The identified fault characteristics need to be taken into consideration to develop suitable cable test standards for future HVDC systems.

The integration of distributed renewable energy resources in combination with advancements in power electronics make the application of multi-terminal medium voltage DC grids a promising and flexible solution. Fast and selective protection concepts in combination with DC (disconnecting) switches for fault handling are required to ensure high reliability and safe grid operation of these systems. Medium voltage DC grids may be realised based on several converter technologies with different fault clearing capability. Depending on the converter technology, switches for isolating faulted branches or DC circuit breakers (CBs) for fault clearance become necessary. DC medium voltage CB concepts have already been proposed. However, the switching times of these CBs vary significantly depending on the presence or absence of mechanical components resulting in different requirements for fault detection. This contribution evaluates the relations between different converter and DC CB technologies, DC grid topologies and fault detection methods according to selective fault clearing. Transient simulation studies are carried out for the development of protection concepts. Based on the results, reasonable technological combinations of the above-mentioned technologies are identified. Special consideration is given to the dimensioning of series reactors as fault current rise limiting devices.

The high penetration of renewable energy requires flexible transmission of electrical energy over long distances. Onshore high-voltage DC (HVDC) interconnectors based on overhead transmission lines have already reached an advanced planning stage. These interconnectors could be extended to multi-terminal grids in the second step for higher transmission redundancy and flexibility. Fast and selective protection concepts for fault handling are required to ensure high reliability and continuous operation of these systems. Modular multi-level converters with submodules in half-bridge topology and solid-state HVDC-circuit breakers (SSCB) provide fault clearing within several microseconds to prevent converter blocking. Within this publication, a selective protection concept for SSCB and multi-terminal HVDC systems based on the overhead transmission is developed and analysed. It is based on a combination of local voltage and current signals and does not require communication between grid nodes. Additional series reactors for limiting rising fault currents are not required. Subsequent simulations for validating the protection concept and identifying its limits are carried out in power systems computer-aided design/electro-magnetic transient design and control for an exemplary HVDC system. The combination of overcurrent protection with additional excitation signals enables selective fault clearing for all types of line fault scenarios.

The integration of distributed renewable energy resources in combination with advancements in power electronics make the application of multi-terminal medium voltage DC grids a promising and flexible solution. Fast and selective protection concepts in combination with DC (disconnecting) switches for fault handling are required to ensure high reliability and safe grid operation of these systems. Medium voltage DC grids may be realised based on several converter technologies with different fault clearing capability. Depending on the converter technology, switches for isolating faulted branches or DC circuit breakers (CBs) for fault clearance become necessary. DC medium voltage CB concepts have already been proposed. However, the switching times of these CBs vary significantly depending on the presence or absence of mechanical components resulting in different requirements for fault detection. This contribution evaluates the relations between different converter and DC CB technologies, DC grid topologies and fault detection methods according to selective fault clearing. Transient simulation studies are carried out for the development of protection concepts. Based on the results, reasonable technological combinations of the above-mentioned technologies are identified. Special consideration is given to the dimensioning of series reactors as fault current rise limiting devices.

Bipolar HVDC transmission systems typically offer up to 50% transmission redundancy in case of single pole-to-ground line faults. Continuous operation during permanent multiconductor faults, however, is not permitted, emphasizing the need for accurate fault type discrimination in such applications. For bipolar schemes with a dedicated metallic return (DMR) conductor, discrimination of pole-ground faults and pole-DMR faults represents a challenging task due to their similar electrical characteristics. In this paper, several approaches to detect DMR fault involvement are identified and analyzed, comprising both noninvasive methods based on measurement data and invasive methods based on DC current control strategies of full-bridge VSC-MMC stations (Modular Multilevel Converter based on Voltage Source Converter technology). An exemplified point-to-point HVDC system is modeled in PSCAD/EMTDC and extensive simulation studies are carried out. Based on the obtained results, a comprehensive fault type discrimination concept for permanent line faults is developed, which can be incorporated into fault handling schemes. Thus, successful fault type discrimination can be performed for all relevant fault types, even under exceptional fault conditions.

Due to recent political developments in Germany new High Voltage Direct Current (HVDC) transmission right-of-ways will have to be realised preferably as cable systems, especially near densely populated areas. Hence, overhead line (OHL) systems in combination with underground cable sections near urban areas - so-called partial underground cabling systems - become a viable option. This however leads to new challenges regarding system protection, since different converter control actions are required for OHL faults compared to cable faults. Within this paper the behaviour of HVDC systems with partial underground cabling and fullbridge Modular Multi-Level Converters (MMC) during line faults is investigated in a PSCAD|EMTDCTM model to identify possibilities to differentiate OHL and cable faults and subsequently adjust the converter operation. As the results indicate, differentiating the fault type based on local current and voltage measurements does not seem feasible, except for simple cable-OHL topologies and, if enhanced waveform analysis algorithms are applied. However, if instead cable current differential protection is used for the system's cable sections, faults can be characterised correctly within short time and the converter stations are able to alter their operating point accordingly.

In case of permanent pole-to-ground line faults bipolar High-Voltage Direct Current (HVDC) transmission systems can be kept in operation as asymmetrical monopoles. However, power transmission is limited to half of the system's rated power during that time. To reestablish bipolar operation maintenance measures are needed, which can only be performed, if the fault location is known. Within this paper, a new distance protection concept for HVDC systems based on full-bridge modular multi-level converter (MMC) modulation strategy is introduced. An enhanced converter control is used to evoke an alternating current (AC) component on the faulted pole enabling the evaluation of a reactive line loop impedance component. By that, the distance to the fault location can be estimated. The developed concept is applied in a PSCAD model and different fault scenarios are investigated. As the results indicate, the developed concept can successfully be used to localize HVDC transmission system faults, before the faulted line segment is isolated from the rest of the system. Thereby, the proposed technique does not require a communication infrastructure or high sampling frequencies.