This paper examines two new bidirectional hybrid dc circuit breaker topologies for application in meshed dc grids. The goal is to retain performance of hybrid DC CB with bidirectional current interruption, while reducing semiconductor count, DC CB size and weight. The fault current is routed to the unidirectional internal valve using multiple additional ultrafast disconnectors. Operation of both topologies is studied using a 320 kV, 16 kA simulation model, as well as demonstrated on a 900 V, 500 A lab prototype. The control systems are presented and discussed in detail. The low-voltage hardware prototypes verify performance of several new technical and operating solutions in laboratory conditions. A comparison is made with the existing DC CB topologies and performance and reliability compromises of each topology are assessed. The conclusion is that it might be possible to halve the DC CB semiconductor count while retaining same 2 ms opening speed and bidirectional operation.
This paper proposes a dc grid protection strategy based on temporary MMC blocking in combination with mechanical DCCBs on dc lines. MMCs are blocked for only a short period of time while DCCBs operate and resume operation afterwards. A comparison is made with a protection strategy in which MMC blocking is avoided. The study analyses the impact of dc faults on dc power flow, ac system, DCCB dimensioning and MMC's antiparallel diodes. Operation is demonstrated on a point-to-point HVDC and a three-terminal dc grid, also using thermal model of MMC's IGBT module. Main benefits of the proposed strategy are simplicity and low protection system cost. On the downside, antiparallel diodes are exposed to greater current and thermal stress.
Ultra‐fast disconnector (UFD) is a key component of hybrid DC circuit breakers and it is also studied as the main switch in some DC grid topologies. A UFD model suitable for DC grid studies and considering both normal operation and failure mode is presented. The dynamic motion of contacts is analysed in detail and it is concluded that Thomson coil inductances including parasitic parameters play an important role and it is recommended to use finite element modelling. The arcing mode of UFD is repressed using a variable resistance in series with an ideal switch. The variable resistance is calculated analytically based on the instantaneous position of contacts and the circuit conditions. Two different arc models are recommended: for the air‐insulated UFD and SF6 UFD, and in each case, two operating regimes should be considered: high and low currents. The UFD model is verified for both normal operation and failure mode using measurements on a 5 kV laboratory UFD and the results show very good matching. The 320 kV SF6 UFD model is evaluated using limited reported results from manufacturers.
A major setback for large scale electric vehicle market expansion compared to their internal combustion competitors consists in their high price and low driving range. One way of reducing the cost, dimensions and mass of electric vehicles is to eliminate the dedicated AC/DC converter used for battery charging. Alternatively, charging could be done using the motor windings as grid side inductors and controlling the inverter to operate as an active boost rectifier. The challenge in this approach is the unequal phase inductances which depend on the rotor position. Another problem appears when the battery gets discharged below the peak of the grid voltage, typically if the vehicle has not been used for a longer period of time. To avoid high currents that could damage the battery, a voltage suppression mode is introduced for safe depleted battery charging. This paper proposes and analyzes an integrated charger control algorithm to charge the battery through a permanent magnet synchronous machine (PMSM) windings. Keywords -electric vehicle, battery charging, space vector modulation, asymmetric, unbalanced, active rectifier, voltage suppressionI.
HVDC grid protection with adequate speed and reliability is required to minimise the impact of DC faults. In particular, fast breaker failure backup protection algorithms are needed to meet the expected reliability requirements of HVDC grids. In this paper, existing breaker failure backup protection algorithms are shown inadequate to detect partial failures like a single module failure of breakers with a modular structure. This paper proposes a backup protection algorithm which rapidly detects a DC breaker failure based on estimating the countervoltage created by the energy absorption branch during an interruption. The performance of the proposed algorithm is evaluated using a four-terminal test network with both hybrid and mechanical DC breaker technologies. The simulation results show that the proposed algorithm is able to quickly detect both complete and partial failures of the two breaker technologies even considering measurement errors, noise and ageing of the energy absorption components.
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