The simultaneous increase of grid connected converters and decommissioning of conventional power generation integrated via synchronous generators will lead to a change of the transmission system behaviour under faults. This paper reviews the possible behaviour of Modular Multilevel Converters (MMCs) under AC faults considering different grid conditions and control objectives as well as the resulti...

Abstract. The integration of offshore wind energy into the existing power system is continuously growing. With the increasing distance of the offshore wind farms (OWF) to the onshore AC transmission systems, HVDC systems are emerging as a preferable solution for the connection of OWF due to their techno-economic advantages in comparison to AC subsea connections. Integrating HVDC systems into the existing AC systems poses various planning and technological challenges. To be able to overcome these challenges a variety of studies has to be conducted, e.g. the HVDC system behaviour under faults. Simulations using electromagnetic transient (EMT) tools represent a generally accepted method to conduct the relevant studies. To increase the trust in the developed concepts subsequent hardware demonstrations would be preferable. However, performing these investigations with full-scale components is often not an option due to unavailability and high costs. As an alternative way, Power-Hardware-in-the-Loop (PHiL) approaches are considered. In this context, a new and worldwide unique laboratory demonstrator - the MMC Test Bench - is set up at RWTH Aachen University as part of the Horizon2020 project PROMOTioN. Here, laboratory-scaled Modular Multilevel Converters (MMCs) are used, which are connected on the DC side by cascaded Pi-line segments. The adjacent AC grids (i.e. offshore wind farms, AC transmission networks) are represented by real-time simulators (RTS) and connected to the MMCs via high bandwidth linear power amplifiers (PA). In this work, the MMC Test Bench is initially described. Afterwards, the PHiL set-up to demonstrate the implemented controls for an OWF connected to shore via an HVDC link is explained. To allow the joint operation of the hardware set-up and the RTS in a stable manner, adequate PHIL interfaces algorithms have to be designed and the scaling between the RTS, the power amplifiers and the hardware is explained. The connection of the OWF represents a special challenge for PHiL demonstrations as the OWF represents a weak AC system with the MMC in grid forming mode. In a next step, the results of the successful demonstration of the interconnection of the OWF via an HVDC link with the MMC Test Bench are presented. The system behaviour in stationary and transient operation is analysed based on the wind farm start-up sequence as well as different cases of wind infeed fluctuations. The results are compared to a simulated full-scale model and deviations are discussed.

Within the next decades, an increasing number of HVDC systems based on Modular Multilevel Converters is planned to support existing AC grids. For both socio-economical and technical reasons, recent network development plans suggest re-using infrastructure not only of AC, but also of existing DC systems. Therefore, partially-cabled HVDC systems might be routed in parallel on the same right of way. On the systems’ overhead line sections, intersystem faults between conductors of two independent HVDC links can arise. However, the impact of these new fault scenarios has not yet been investigated. This paper provides an in-depth analysis of voltage stresses imposed by DC-DC intersystem faults, taking into account different transmission system topologies of two parallel HVDC links. The results indicate that cable sections based on cross-linked polyethylene are exposed to transient polarity reversals exceeding the amplitudes reached under pure DC faults. In particular, asymmetries between the HVDC transmission systems lead to overvoltages on short cable sections that are not covered by the existing cable testing recommendations. Therefore, DC-DC intersystem faults need to be considered, when developing future cable test specifications applicable to parallel, partially-cabled DC links.

Multi-terminal HVDC grids based on Modular Multilevel Converters (MMCs) are considered a promising solution for the large-scale integration of renewable energy sources into existing transmission systems. Depending on the AC network constraints, the DC grids' extension and available breaker technologies, different protection schemes can be suitable for DC line fault clearing. Due to the absence of ...

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.

Future multi-terminal DC networks are envisioned for the large-scale integration of renewable energy sources into today’s power systems. To minimise the downtime of HVDC networks and thereby reduce the impact of DC contingencies on surrounding AC networks, faulted DC lines must be separated quickly and reliably from the remaining part of the network. Even though the fast separation of faults in HVDC networks is often associated with DC circuit breakers, protection systems based on fault blocking converters and DC high-speed switches have been proposed as a reasonable alternative for the protection of DC networks in the recent past. The focus of previous contributions regarding these protection systems has been on the separation of faulted lines. However, post-fault system restoration can have a significant influence on the power outage caused by a DC fault as well as the overall system recovery time. Particularly challenging during this restoration process is the post-fault DC voltage rebalancing in HVDC systems in monopole configuration, which still is the most frequently used configuration for MMC-based systems. This paper therefore investigates the applicability of different schemes to rebalance the pole voltages during the DC voltage restoration process (i.e. dynamic braking and AC grounding systems) and evaluates their impact on the performance of the protection strategy. Moreover, a rebalancing method, which does not rely on specific equipment, but only on the control of the fault blocking converters is proposed. This method appears to be a feasible alternative, since the overall protection system performance is competitive to the other methods involving specific equipment, even though no extra components need to be installed in the network.

Protecting high-voltage (HV) DC grids requires a different approach compared with that of ac power system protection and poses one of the major challenges that must be resolved before the realization of largescale HVdc grids that use equipment from multiple vendors. HVdc grid protection, which is essential for safe and reliable HVdc grid operation, entails the appropriate detection and fault clearing of dc-side short circuit faults (i.e., dc faults). In this context, fault clearing refers to interrupting the dc fault current and isolating the faulted component.

In the near future, HVAC and HVDC systems may be routed on the same towers in some applications. These so-called hybrid transmission systems raise several novel challenges, such as AC/DC intersystem faults. Previous investigations focus on the analysis of these faults in purely overhead line based HVDC systems. However, new HVDC corridors might be built as partially cabled systems to increase public acceptance. Intersystem faults in combination with mixed DC overhead and cable transmission systems are a novel challenge for both AC and DC protection systems and are analyzed within this paper. It is shown that even after DC fault current interruption the HVDC system is exposed to significant AC overvoltages and fault currents, which are caused by a resonant behavior of the system. These cannot be cleared by standard DC protection systems. To protect the DC system against intersystem faults the use of a fast earthing concept is proposed.

Within the framework of modernisation of the European electricity grid, multi-terminal high-voltage direct current (HVDC) offshore grids shall be integrated into future transmission systems. An essential aspect of multi-terminal HVDC systems is fast and selective DC-side fault handling and the separation of faulty lines. This study investigates the applicability of different control methods relying on full-bridge-based converters in combination with high-speed switches for a fast and selective separation of faulty line segments in a multi-terminal HVDC cable system in a symmetrical monopole configuration. It is shown that the proposed line current control method can significantly reduce the separation time of a faulty line compared with standard fault control methods. The analysis is based on simulations in PSCAD|EMTDC ™ with a converter model based on the CIGRE WG B4.57, which is enhanced for the use of full-bridge converters with fault current control schemes.

This paper presents an adaptive autoreclosing concept for HVDC transmission systems with modular multilevel converters in full-bridge topology for single pole-to-ground faults. Since HVDC transmission will be used in the German energy transmission system to support the heavily loaded ac grid for energy transport over far distances, the interruption time has to be kept as short as possible to preserve system stability. Within the presented autoreclosing concept, the necessary interruption time is determined adaptively. After fault detection, the converter control drives the fault current to zero and injects a low-level ac current. The nonlinear interdependence between the fault arc resistance and the current is used to detect the final arc extinction and to start the voltage recovery, enabling a fast resumption of the power transmission. PSCAD|EMTDC is used to model the electric arc behavior and the dielectric recovery inside an exemplary point-to-point HVDC grid model as an evaluation of the concept. The minimum required ac current is identified related to the used fault detection method. Additionally, the influences of fault position, line length, and fault resistance on fault detection are investigated. The results show that the required interruption time can be determined adaptively for transmission line length up to 400 km.

Future multi-terminal dc networks are envisioned for the large-scale integration of renewable energy sources into today's power systems. An essential aspect for the reliable operation of these systems is the fast and selective dc-side fault handling and separation of faulted lines. The objective of this paper is the development and evaluation of a multi-terminal dc protection strategy based on fault-blocking converters (e.g., full-bridge based modular multi-level converters) and high-speed switches. Both a suitable fault control and a new high-speed switch design for a fast separation of faulted dc lines are elaborated. The novel concept is intended to reduce the requirements on the high-speed switches compared to dc circuit breakers and to ensure a fast restoration of the active power transmission capability, as well as the dc voltage. The fault-clearing strategy is analyzed using electromagnetic transient (EMT) simulations under variation of the fault location, type, and resistance. The investigations are carried out in a cable-based multi-terminal HVdc system with full-bridge modular multi-level converters in symmetrical monopole configuration.

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.

In high voltage direct current (HVDC) transmission systems, based on voltage source converters (VSC) with overhead line (OHL) segments, DC line faults have a strong impact on the converter's power electronic devices. Fault tolerant multilevel converters are able to limit and control these fault currents after their detection. Especially for low impedance and AC/DC intersystem faults the detection can take several milliseconds, which can lead to high surge currents within the VSC. Thus, this paper presents a Dynamic Internal Overcurrent Control (DIOC) for Full Bridge Modular Multilevel Converters (FB-MMC). The control protects the power electronic devices of the converter against thermal overload without blocking of the converter. The DIOC does not depend on the fault detection and effectively limits the arm current in steady state and transient situations. A DC line protection concept based on the DIOC is proposed, which closes the gap between the occurrence and the detection of a fault. It is exemplarily shown, that this protection method is suited for pure DC and AC/DC intersystem faults. Thereby, the developed control algorithm enhances the reliability and the fault ride through capability of FB-MMCs. The DIOC has been tested in the simulation environment EMTDC TM |PSCAD TM .

The parallel routing of high voltage AC and DC systems on the same transmission tower is one new option for a timely integration of HVDC technology into existing AC power grids, enhancing the latter's flexibility. However, the rare event of an intersystem fault is a new fault type which might occur in these configurations. Previous studies indicate that existing protection concepts are necessary but need not be enhanced for fault handling in hybrid AC/DC systems and have to be adapted especially to this case. This contribution offers a conceptual approach to combine identification and classification of intersystem faults, resulting in a protection concept for hybrid AC/DC systems with subsequent fault handling. It shows that the fault current detection and localisation can be separated and run independently. Using the advantages in control of modern VSC with full bridge technology, the DC component can be removed very quickly by limiting DC fault currents and driving them to zero. Without blocking the converter it is possible to continuously stabilise the AC networks and reduce the time required to restore the normal power transmission capability after the fault have been cleared. Subsequently, the intersystem fault can be detected and classified by the DC system reliably. Since the AC system cannot detect a fault any more, it is also task of the DC system to trip the AC protection. The control was tested simulatively using EMTDC™/PSCAD™.