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Medium voltage direct current (MVDC) grids are proposed as a solution for future distribution systems providing advantages of lower transmission losses and high controllability. Protection of a DC grid is more challenging than AC systems due to fast fault propagation and high peak currents as well as lack of current zero crossing. Until now, proposed hybrid DC circuit breakers have been based on the power electronic components and considerably more expensive than AC circuit breakers. In case when DC circuit breakers are applied at each individual feeder, the cost of the MVDC grid increases significantly. In this paper, a multi-feeder protection system is proposed that utilizes the main power electronic part of the circuit breaker for several feeders' protection. The proposed solution allows the reduction of the protection system costs maintaining fast fault clearing capability provided by hybrid DC circuit breakers. Design evaluation of the protection system components is described in detail. Prop...

The unbundling of power market and continuous increase of renewable generation over the years has resulted in bulk power transmission over long distances and cross-border links. Moreover, Integration of renewable generation is set to increase every year to fulfil EU's vision and other country specific goals. These require an expansion of existing conventional AC transmission network to hybrid/meshed AC-DC. In this regard, several point to point projects all around Europe have been planned, some of which are already up and running. Next step towards this is the building of multi-terminal HVDC grids. The construction of multi-terminal DC (MTDC) leads to major changes in the power flow of the existing grids. This paper investigates such meshed AC-DC grids and attempts to predict the behaviour of the future grids via load flow studies. To facilitate the power flow and easier flow control, a new type of interface node for the converter nodes has been proposed which has been tested on a small test system. Finally, power flow analysis has been done on a simplified model of North-Western European grid connected to Great Britain grid and Nordic grid via DC interconnections for several scenarios in future (until 2030). It's noteworthy to mention that in the test models (analysed scenarios), all the planned HVDC links and North Sea Transnational Grid (NSTG) project have been included depending on their estimated date of completion. A MATLAB based tool has been developed to run power flows with varying wind, load and conventional generation inputs. The results are presented by times series plots of essential result variables such as voltages, branch loadings etc.

Environmental concerns, system security and energy efficiency requirements are posing special demands on power transmission options. Particularly, increased bulk power transmission, as well as large-scale integration of renewable energy sources act as main drivers for the development of HVDC technology. This paper deals with the development of a reliability/availability model for Voltage-Source Converter HVDC (VSC-HVDC) transmission systems for offshore and onshore applications, such as offshore wind farms connection or DC interconnectors. The goal of the present work is to develop vendor independent model for system reliability studies and sensitivity analysis of system availability. The proposed method overcomes the constraints that the currently dominating reliability and availability analysis methods introduce. A novel modelling tool - based on Bayesian Network theory - will be capable of serving as valuable asset towards design optimization and operational strategy of VSCHVDC systems. Two component-based modelling approaches are demonstrated within this study. The first is based on the Boolean logic of “operating” or “not operating” element, promising identical results with Fault Tree Analysis, but enabling identification of the weakest point in the system and sensitivity analysis as well. The second approach takes into consideration the distributive character of real components availability data, integrating in such a way uncertainty in the estimated output system availability.