Search

The missing capability of DC fault current limitation is an inherent disadvantage in the typical half-bridge modular multilevel converter (MMC), which restricts its application in long-distance high voltage direct current (HVDC) systems. The full-bridge submodule (FBSM) and clamped double submodule (CDSM) topologies allow for fault current self-clearing, but also suffer from additional hardware cost and power losses. In this paper, an improved submodule topology utilizing the new reverse-blocking insulated gate bipolar transistor (RB-IGBT) is proposed to improve system economy without compromising its performance. The operating principles and DC fault characteristics of the proposed topology are analyzed in details, including comparison with various topologies in terms of cost, losses, fault current clearing speed and thermal. Compared with the traditional CDSM topology, the proposed RBSM can clear DC faults faster with reduced cost of switching devices. A DC fault protection strategy for MMC-HVDC system is also presented. Simulation results under normal operating, permanent and transient fault conditions are carried out to verify the effectiveness of the proposed topology. [ABSTRACT FROM AUTHOR]

DC transformer is an essential equipment for formation of multi-voltage-level DC grid. This paper proposes a novel non-isolated Boost-type alternate arm DC transformer (Boost-AADCT) for medium- and high-voltage level DC grid application. The Boost-AADCT is comprised of three submodule-cascaded arms, two thyristor-cascaded director switches, and two buffer capacitors. Bidirectional power transmission is achieved by alternatively charging and discharging of the intermediate arm following the Boost principle, while the pulsating power component caused by intermittent transmission is actively smoothened by grid-side buffer subcircuits formed by the buffer capacitors and grid-side arms. The active power buffering method increases the tolerance range of voltage ripple of the buffer capacitor, and the required energy storage capacity is resultantly reduced. Circuit configuration, operation principle, and mathematical model are first introduced. Then, the control strategy is proposed, the main circuit parameters are designed, and bidirectional fault-blocking capability is analyzed. Comparison with other DC-DC converters shows that the Boost-AADCT significantly reduces the requirement of semiconductors and has relatively lower synthetic cost. At last, performance of the Boost-AADCT and the proposed control strategies are evaluated by simulation tests conducted in MATLAB/Simulink environment. [ABSTRACT FROM AUTHOR]

Pole-to-pole dc short circuit fault blocking capability is a major concern when modular multilevel converters (MMCs) are applied in high voltage direct current (HVDC) transmission systems. Traditional MMCs based on half-bridge submodules (HBSMs) cannot block the fault current, thus needing expensive direct current circuit breakers (DCCBs). There are MMCs based on improved submodule topologies that can block dc faults, but usually with significantly increased device cost. This paper proposes an improved HBSM-based MMC (HB-MMC) which needs fewer extra semiconductor devices. The improved HB-MMC is more competitive than other schemes when it comes to investment cost and loss. An H-bridge circuit composed of two director switches and two diode valves is installed around one arm of the HB-MMC, which will not change the direction of current during the normal operation, whereas it can change the direction of the fault current when a pole-to-pole dc short circuits fault is detected. After two director switches have been turned off, a reverse-biased voltage provided by one arm can block the fault current. The topology can be further improved to provide reactive power during faults. Simulation results are presented to validate the features of the proposed improved HB-MMC. [ABSTRACT FROM AUTHOR]

Two key problems in meshed high-voltage direct current (HVDC) transmission grids are managing line power flows and protection against dc faults. Current flow controllers (CFCs) will be required to balance cable currents in meshed dc grids, in order to prevent individual line power capacity limits restricting overall power flow in the grid. Direct current circuit breakers (DCCBs) will be also required to protect HVDC grids from dc faults. This paper demonstrates that the current flow controller functionality can be added into a hybrid CB's design. This paper proposes integrating an interline CFC into the load commutation switch (LCS) of a hybrid DCCB. The integrated design LCS/CFC is analyzed and a state-space model is derived. The control of the CFC is designed and the performance of the LCS/CFC during normal operation is verified by means of MATLAB Simulink and PSCAD simulations. A comparison of the integrated LCS/CFC and the separate design is given. The case studies show a reduction in total power losses, and improved protection operation times can be achieved. [ABSTRACT FROM PUBLISHER]

This paper presents a breaker arrangement concept, the multiline breaker (MLB), for the protection of multiterminal high voltage dc (MTdc) networks. Based on the design of a hybrid breaker, the MLB is an economically attractive solution for the protection of multiple dc lines in nodal connection using a single main breaker path. By using commutation units, the MLB directs the fault current through the main breaker in a unidirectional way, irrespective of the fault location. Hence, this study presents the design requirements for the MLB, regarding both hardware and control, and evaluates its operation within a grid. For this reason, a four-terminal half-bridge multilevel modular converters-based MTdc grid in radial configuration was used and pole-to-ground dc fault conditions were investigated. The dc fault response of the grid with one MLB at the central node is compared to the respective response of the grid when one hybrid breaker is employed at each dc line. The simulations show that the MLB is feasible and that the overall MTdc grid fault response for the two protection systems is very similar. As a result, the design advantages of the MLB make it a promising solution for the dc fault isolation in MTdc grids. [ABSTRACT FROM AUTHOR]

A challenge raised in today's power system is assessing the system protection security and dependability after the topology has been changed due to relay operation upon the occurrence of cascading faults or intentional operator switching action. This paper proposes an automated setting calculation module which could be used to review the adequacy of the distance relay settings following network topology changes. The calculation procedure is broken down into blocks which could be processed in parallel in order to improve the computation speed. A new concept called distance of impact is proposed to reduce the computational burden by conducting the calculations on a limited portion of the network affected by the topology change. The module performance is tested on a synthetic IEEE 118-bus and real-life Alberta transmission operator systems. A sensitivity analysis in the form of N-2 contingency impact on the network relay settings coordination is conducted on the test systems. [ABSTRACT FROM PUBLISHER]

The high voltage direct current transmission system based on modular multilevel converter (MMC-HVDC) technology is now studied and used widely. But traditional half bridge topology of sub module (HBSM) of MMC can't block dc link fault current because of the freewheeling effect of diodes. In order to solve this problem, firstly, this paper improves HBSM topology, a series connected double sub module (SDSM) is designed and dc link fault current blocking capability is realized without tripping ac circuit breaker, so MMC-HVDC with this improved topology can immediately rebuild dc link voltage and resume power transmission. Secondly, the self starting and fault disposing procedure are also designed. At last, simulations in the software PSCAD/EMTDC are carried out to verify its effectiveness and feasibility. [ABSTRACT FROM AUTHOR]

This paper compares how a dc fault affects a multiterminal dc (MTdc) network depending on the HVDC transmission system topology. To this end, a six-step methodology is proposed for the selection of the necessary dc fault protection measures. The network consists of four voltage-source converters converters radially connected. The converters natural fault response to a dc fault for the different topologies is studied using dynamic simulation models. For clearing of the dc faults, four different dc breaker technologies are compared based on their fault interruption time, together with a current direction fault detection method. If necessary, the converters are reinforced with limiting reactors to decrease the peak value and rate of rise of the fault currents providing sufficient time for the breakers to isolate the fault without interrupting the MTdc network operation. The study shows that the symmetric monopolar topology is least affected by dc contingencies. Considering bipolar topologies, the bipolar with metallic return exhibits better fault response compared to the one with ground return. Topologies with ground or metallic return require full semiconductor or hybrid breakers with reactors to successfully isolate a dc fault. [ABSTRACT FROM AUTHOR]

This paper proposes an LCL dc-dc converter concept which is capable of achieving very high stepping ratios with megawatt-level power transfers. The converter can find potential application in connecting high-power dc sources to high-voltage dc transmission including future dc transmission networks. This converter is based on two ac/dc insulated-gate bipolar transistor-based converters and an internal passive LCL circuit without internal ac transformers. The LCL circuit is designed to enable voltage stepping without any reactive power circulation and with potentially soft switching operation which minimizes the switching losses. The designed converter has the ability to achieve current regulation even under extreme external dc faults and, therefore, the converter can operate through dc faults. The switch utilization is better than similar topologies and losses are reasonably low. A dual-active-bridge transformer-based converter design is presented to compare with the proposed LCL converter. A detailed PSCAD model confirms conclusions using a 100-MW 20-kV/300 kV test system. An LCL 200-W 20-V/100-V dc/dc prototype converter is built to validate the proposed topology. [ABSTRACT FROM AUTHOR]