Mechanical DC Circuit Breaker Research Articles

Protection strategy for multi‐terminal high‐voltage direct current grids using SFCLs at converter station output

This study proposes a non-selective protection strategy for multi-terminal high voltage direct current grids based on resistive-type superconducting fault current limiters (SFCLs). Located at the output of AC/DC converters, the SFCL limits the current contribution from the AC grid in case of DC fault. With this approach, the fault clearing time constraint is relieved allowing the use of mechanical DC circuit breakers for fault current interruption. Furthermore, the breaking capability and energy dissipation requirements of the breaker are greatly reduced. To achieve a fast restoration of the DC grid, a redundant SFCL is introduced in parallel to those in operation, bypassing them when the fault is effectively suppressed. In addition, primary and back-up protection schemes are described and tested using a three-terminal bipolar high voltage direct current (HVDC) grid based on half-bridge modular multilevel converters and cable transmission. Simulations are implemented in EMTP-RV® to analyse and discuss performances of the proposed fault clearing strategy.

Development and distribution of surface charge on epoxy resin under dc electric field in different gases

Under DC voltage, charge accumulates on the break insulation of mechanical DC circuit breakers, distorting the local electric field, which increases the risk of break insulation failure. Glass-fiber vacuum-impregnated epoxy resin can be used as an insulation material for DC circuit breakers owing to its excellent mechanical and electrical properties. We have created a system to measure the surface potential of an epoxy resin in different gases. The accumulation and dissipation of the surface charge in air and SF6 are determined, and the effects of air and SF6 are compared. The results show that the polarity of the surface charge is the same as that of the applied DC voltage, and that the main source of the charge is from Schottky charge injection. In SF6, the charge density is higher than that in air, and its decay rate is slower. The decay law of the surface potential in SF6 can be characterized by two exponential superposition formulas. In the early stage, surface charge is attenuated mainly by being transferred along the surface of the insulator and neutralized by gas ions. In the late stage, surface charge is attenuated mainly by transferring through the volume of the insulator.

Characteristics of DC Circuit Breaker Applying Transformer-Type Superconducting Fault Current Limiter

Transformer-type superconducting fault current limiter (TSFCL) interruption system is a current breaking technology proposed by applying a transformer, superconductor, and mechanical dc circuit breaker. When the fault occurs, the fault current is limited due to the operation of the counter electromotive force by the transformer and the quenching of the superconductor. The fault current is then interrupted by the mechanical dc circuit breaker. In this paper, the transition and the interrupting behavior was analyzed according to the applied voltage to secure the stability and reliability of a TSFCL interruption system using the PSCAD EMTDC program. As a result, it was able to improve the current breaking time and the breaking capacity. Also, the burden imposed on a mechanical dc circuit breaker was reduced fivefold.

Transient Current Interruption Characteristics of a Novel Mechanical DC Circuit Breaker

With the rapid development of multiterminal direct current (MTDC) grids, high-voltage direct current circuit breakers (HV-DCCB) capable of interrupting currents within 5 ms are in urgent demand. The mechanical DCCB has always been one of the main proposals for HV-DCCBs. The current commutation is the prerequisite for mechanical DCCBs to interrupt currents successfully. Focused on the current commutation difficulty, a novel mechanical DCCB has been investigated in this paper. A simulation model of the novel mechanical DCCB in MTDC system for fault current interruption was established in PSCAD. By analyzing the simulation results, the detailed transient varying patterns of current, voltage, and energy in the novel mechanical DCCB during its interruption process are revealed. To verify the feasibility of the novel mechanical DCCB, a prototype was developed in the lab, and current interruption tests were carried out on the prototype. Experimental results have shown that a current of 5.3 kA can be interrupted by the prototype within 3 ms. In the end, failures that occurred in initial experiments are discussed and relative protection measures have been proposed. By adopting these measures, failures are successfully avoided and the normal operation of the novel mechanical DCCB are ensured.

A Novel DC Fault Ride-Through Scheme for MTDC Networks Connecting Large-Scale Wind Parks

This paper proposes a novel fault ride-through protection scheme for multiterminal dc (MTDC) networks connecting large-scale offshore wind parks. The proposed scheme introduces an MTDC network with a hybrid fault current limiter (FCL), which consists of a high-temperature superconducting FCL (HTS-FCL) and series braking resistor (SBR) integrated with low-loss mechanical dc circuit breaker (DCCB). The aim of HTS-FCL is to provide fast quenching, limit the dc fault current within the interrupting capacity of the DCCB, and reduce the dc fault current to certain levels such that converters can sustain prolonged periods of operation without blocking. Also, SBR allows automatic reclosing of DCCBs following a temporary fault, thereby enhancing the network security and reliability. Mechanical DCCBs with the proposed hybrid limiter are installed at the terminating points of each dc cable. The proposed protection scheme is based on differential current approaches with fault clearing time of few milliseconds, which gives enhanced network selectivity and reliability. A four-terminal meshed MTDC network with the proposed hybrid FCL is modeled in PSCAD/EMTDC. The obtained simulation results confirm successful dc fault isolation and network recovery for different dc fault scenarios.

Energy dissipating resistors for the ITER switching network units

In ITER, before each plasma pulse the poloidal magnet system will be pre-charged with maximum energy up to 8GJ. Part of this energy will be used for the gas mixture breakdown and early plasma current ramp up. This will be provided with help of the switching network units (SNU) comprising of special hybrid (mechanical and semiconductor) DC circuit breakers and resistors, which, after being inserted into the coil circuits by opening of the circuit breakers, dissipate the energy extracted from the coils.The paper describes the design of the SNU resistor and reports on the results of the tests carried out with the aim to optimize the design, demonstrate its suitability and to confirm its compliance with ITER requirements.

Interruption analysis of the SFCL-combined DC circuit breaker system using current-limiting technology

In this study, a SFCL-combined DC circuit breaker system was proposed by applying the current-limiting technology for DC circuit breaking. The SFCL-combined circuit breaker system consists of a mechanical DC circuit breaker combined with superconductors. To ensure the reliable structure and operation of the SFCL-combined circuit breaker system, a simulation grid was designed using the EMTDC/PSCAD program, and simulation was conducted. The results showed that the SFCL-combined DC circuit breaker system with superconductors limited the maximum fault current by 37%. In addition, the burden on the DC circuit breaker was decreased by 87%.

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.

Continuous Operation of Radial Multiterminal HVDC Systems Under DC Fault

For a large multiterminal HVDC system, it is important for a dc fault on a single branch to not cause significant disturbance to the operation of the healthy parts of the dc network. Some dc circuit breakers (DCCBs), for example, mechanical type, are low cost and have low power loss, but have been considered unsuitable for dc fault protection and isolation in a multiterminal HVDC system due to their long opening times. This paper proposes the use of additional dc passive components and novel converter control combined with mechanical DCCBs to ensure that the healthy dc network can continue to operate without disruption during a dc fault on one dc branch. Two circuit structures, using an additional dc reactor, and a reactor and capacitor combination, connected to the dc-link node in a radial HVDC system, are proposed to ensure that overcurrent risk at the converters connected to the healthy network is minimized before the isolation of the faulty branch by mechanical DCCBs. Active control of dc fault current by dynamically regulating the dc components of the converter arm voltages is proposed to further reduce the fault arm current. Simulation of a radial three-terminal HVDC system demonstrates the effectiveness of the proposed method.

DC Transmission Grid With Low-Speed Protection Using Mechanical DC Circuit Breakers

This paper introduces a dc transmission grid with fault-tolerant inductor-capacitor-inductor (LCL) voltage-source converters (VSCs) and using a slow protection system based on mechanical dc circuit breakers (CBs). LCL VSC inherently regulates dc fault current to levels that converters can sustain for prolonged periods which avoids insulated-gate bipolar transistor tripping and brings significant advantage to security and reliability aspects. Simple mechanical dc CBs are used at dc busbars and connecting points of each dc cable, in the same manner as it is normal practice used with ac transmission protection. The protection logic is based on differential methods which gives excellent selectivity and reliability. The fault clearing time is in the order of 30–60 ms which allows for reliable protection decision making. The simulation results obtained from a four-terminal dc grid modeled on the PSCAD platform confirm successful dc fault isolation and grid recovery for a range of severe dc fault scenarios.

Topology assessment for 3 + 3 terminal offshore DC grid considering DC fault management

This study assesses four DC grid topologies where each of the terminals can exchange power with any other terminal and a DC fault on any DC cable can be isolated. The DC grids are built using the following components: hybrid DC circuit breakers (CB), mechanical DC CBs, DC/DC converters and DC hubs. The aim is to compare DC fault performance, technical feasibility/readiness, power transfer security, expansion and costs. The base case for comparison are three separate 300 km offshore HVDCs connecting 3 1 GW offshore wind farms with three onshore VSC terminals. The rating of DC CBs is limited by the state of technology and costs which introduces limitation on the length of DC cables (because of communication delays) and on the DC bus fault level (limiting the number of connecting DC lines). DC/DC converters inherently block propagation of DC faults and their rating is not sensitive to DC fault level. DC hubs have considerable cost advantages over multiple DC/DC converters in cases where multiple DC systems are connecting at the same DC station. The cost assumptions for all major components are analysed, including the offshore platform costs. The study concludes that overall DC grid costs are similar (within 8%) for all four topologies. Although power security is similar for all technologies, the expansion options are best with DC/DC converters or DC hubs. DC hubs nevertheless suffer from highest on-state losses.

Development of a Prototype Hybrid DC Circuit Breaker for Superconducting Magnets Quench Protection

A quench protection system for superconducting magnets mainly involves a mechanical dc circuit breaker (CB) and a dump resistor where the entire energy of the inductive magnet is dumped during quench. During the current commutation to the dump resistor, these mechanical CB experiences arcing at their contacts, which causes degradation in the performance as well as life expectancy of the CB, affecting the reliability of the entire quench protection system and appears as threat to the expensive and critical superconductor magnets also. A combination of mechanical and static breakers, which is termed a hybrid dc CB, presents a novel technique for arcless current commutation into the dump resistor. Furthermore, the current commutation time can also be reduced by adopting real-time switching schemes. A series of experiments has been done at the Magnet Technology Development Division Laboratory, Institute for Plasma Research, Gandhinagar, India, for testing the hybrid CB operation and operation at various current levels up to 800 A using a mechanical contactor as the mechanical CB and an insulated gate bipolar transistor as the static switch. A significant improvement in the voltage profile across the dump resistor has been observed using the hybrid CB exhibiting arcless current commutation into the dump resistor. Furthermore, an overall reduction in the current interruption time between a mechanical CB operation and a hybrid CB has been observed. The time for current commutation into the dump resistor with the conventional mechanical breaker is 48 ms, whereas that with the hybrid CB is observed to be 18 ms.