Modular multilevel converter with modified half-bridge submodule and arm filter for dc transmission systems with DC fault blocking capability

Although a modular multilevel converter (MMC) is universally accepted as a suitable converter topology for the high voltage dc transmission systems, its dc fault ride performance requires substantial improvement in order to be used in critical infrastructures such as transnational multi-terminal dc (MTDC) networks. Therefore, this paper proposes a modified submodule circuit for modular multilevel converter that offers an improved dc fault ride through performance with reduced semiconductor losses and enhanced control flexibility compared to that achievable with full-bridge submodules. The use of the proposed submodules allows MMC to retain its modularity; with semiconductor loss similar to that of the mixed submodules MMC, but higher than that of the half-bridge submodules. Besides dc fault blocking, the proposed submodule offers the possibility of controlling ac current in-feed during pole-to-pole dc short circuit fault, and this makes such submodule increasingly attractive and useful for continued operation of MTDC networks during dc faults. The aforesaid attributes are validated using simulations performed in MATLAB/SIMULINK, and substantiated experimentally using the proposed submodule topology on a 4-level small-scale MMC prototype

DC fault protection structures at a DC-link node in a radial multi-terminal high-voltage direct current system

In a multi-terminal HVDC system, DC circuit breakers (DCCBs) are conventionally connected in a star-configuration to enable isolation of a DC fault from the healthy system parts. However, a star-connection of DCCBs has disadvantages in terms of loss, capacity, reliability, etc. By rearranging the star-connection DCCBs, a novel delta-configuration of DCCBs is proposed in this paper. As each terminal is connected to each of the other terminals through only one DCCB, the current flows through only one DCCB when transferring power between any two terminals compared with two DCCBs in the current path for the conventional star-arrangement. The total loss of the proposed delta-configuration is only 33.3% of that of star-configuration, yielding a high efficiency. Also, any DC fault current is shared between two DCCBs instead of one DCCB in the faulty branch suffering the fault current. As a result, DCCB capacities in the proposed delta-configuration are only half those in a star-arrangement. Additionally, in the case of one or two DCCBs out of order, the power can still be transferred among three or two terminals, thereby affording high supply security of all HVDC links. Based on the DCCB delta-configuration, two novel DC fault protection structures with external and internal DC inductances are proposed. Their characteristics are discussed and it is shown a DC fault can be isolated using slow DCCBs without exposing any converter to significant over-current. The results demonstrate DC fault tolerant operation is achieved by using the proposed DC fault protection structures with delta-configuration

DC Fault Current Analyzing, Limiting, and Clearing in DC Microgrid Clusters

A new DC fault current limiter (FCL)-based circuit breaker (CB) for DC microgrid (MG) clusters is proposed in this paper. The analytical expressions of the DC fault current of a bidirectional interlink DC/DC converter in the interconnection line of two nearby DC MGs are analyzed in detail. Meanwhile, a DC fault clearing solution (based on using a DC FCL in series with a DC circuit breaker) is proposed. This structure offers low complexity, cost, and power losses. To assess the performance of the proposed method, time-domain simulation studies are carried out on a test DC MG cluster in a MATLAB/Simulink environment. The results of the proposed analytical expressions are compared with simulation results. The obtained results verify the analytical expression of the fault current and prove the effectiveness of the proposed DC fault current limiting and clearing strategy

DC fault isolation study of bidirectional dual active bridge DC/DC converter for DC transmission grid application

Fast isolation and detection of DC faults is currently a limiting factor in high power DC transmission grid development. Recent research has shown that the role of DC/DC converters is becoming increasingly important in solving various DC grid challenges such as voltage stepping, galvanic isolation and power regulation. This paper focuses on an additional important feature of bidirectional dual active bridge (DAB) DC-DC converters which make it attractive for future DC grids; it's inherent fault isolation capability which does not need control intervention to limit fault current in case of the most severe DC faults. Detailed analytical, simulation and experimental study are performed by subjecting the converter to DC short circuit faults at its DC voltage terminals. The results obtained have shown significant advantage of DAB where fault current is less than rated current during the fault duration. Thus no control action is necessary from the non-faulted bridge to limit fault current and no external DC circuit breakers are required. This advantage makes DAB converter feasible for DC grid integration

Protection of large partitioned MTDC networks using DC-DC converters and circuit breakers

This paper proposes a DC fault protection strategy for large multi-terminal HVDC (MTDC) network where MMC based DC-DC converter is configured at strategic locations to allow the large MTDC network to be operated interconnected but partitioned into islanded DC network zones following faults. Each DC network zone is protected using either AC circuit breakers coordinated with DC switches or slow mechanical type DC circuit breakers to minimize the capital cost. In case of a DC fault event, DC-DC converters which have inherent DC fault isolation capability provide ‘firewall’ between the faulty and healthy zones such that the faulty DC network zone can be quickly isolated from the remaining of the MTDC network to allow the healthy DC network zones to remain operational. The validity of the proposed protection arrangement is confirmed using MATLAB/SIMULINK simulations

DC fault protection of diode rectifier unit based HVDC system connecting offshore wind farms

DC fault ride-through operations of the offshore wind farm connecting with diode rectifier unit (DRU) based HVDC link are presented in this paper. A voltage-error-dependent fault current injection is proposed to regulate the WT current during DC faults and to provide fault current. This contributes the control of the offshore AC voltage, which does not drop to zero but is remained relatively high to facilitate fast system recovery after clearance of a temporary DC fault. The WT converters operate on current limiting mode during DC faults and automatically restore normal operation after fault clearance. The full-bridge based modular multilevel converter (MMC) is adopted as the onshore station and its DC fault current control ability is explored to effectively suppress the fault current from the onshore station around zero, which reduces semiconductor losses and potential overcurrent risk of the MMC station. Simulation results confirm the robustness of the system to DC faults

DC current flow controller with fault current limiting and interrupting capabilities

Conventionally, the current flow control and DC fault protection issues of HVDC grids are supposed to be solved by the DC current flow controller (CFC) and DC circuit breaker (DCCB) separately, which may result in a high capital cost. This paper proposes a CFC topology with DC fault current limiting and interrupting capabilities. The topology and operating principle of the CFC are presented with theoretical analysis. The control strategies under normal and fault conditions are described. In order to reduce the use of IGBTs, an H-bridge inter-line CFC with fault current limiting capability is further proposed based on the first proposed CFC. The proposed CFCs are tested in PSCAD/EMTDC. Simulation results show that the proposed two CFCs can effectively control the current flow of two lines during normal operation and limit and interrupt DC fault currents

DC fault detection and location in meshed multi-terminal HVDC systems based on DC reactor voltage change rate

The change rate of the DC reactor voltage with predefined protection voltage thresholds is proposed to provide fast and accurate DC fault detection in a meshed multi-terminal HVDC system. This is equivalent to the measurement of the second derivative of the DC current but has better robustness in terms of EMI noise immunization. In addition to fast DC fault detection, the proposed scheme can also accurately discriminate the faulty branch from the healthy ones in a meshed DC network by considering the voltage polarities and amplitudes of the two DC reactors connected to the same converter DC terminal. Fast fault detection leads to lower fault current stresses on DC circuit breakers and converter equipment. The proposed method requires no telecommunication, is independent of power flow direction, and is robust to fault resistance variation. Simulation of a meshed three-terminal HVDC system demonstrates the effectiveness of the proposed DC fault detection scheme

A hybrid modular multilevel converter with novel three-level cells for DC fault blocking capability

A novel hybrid, modular multilevel converter is presented that utilizes a combination of half-bridge and novel three-level cells where the three-level cells utilize a clamp circuit which, under dc side faults, is capable of blocking fault current thereby avoiding overcurrents in the freewheel diodes. This dc fault blocking capability is demonstrated through simulation and is shown to be as good as the modular multilevel converter which utilizes full-bridge cells but with the added benefits of: lower conduction losses; fewer diode and semiconductor switching devices, and; fewer shoot-through modes. The semiconductor count and conduction loss of the proposed converter are reduced to around 66.5% and 72% of that of modular multilevel converter based on the full-bridge cells respectively, yielding lower semiconductor cost and improved efficiency. Dc fault ride-through operation is realized without exposing the semiconductors to significant fault currents and overvoltages due to the full dc fault blocking capability of the converter