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According to the N-1 criterion, system security in the event of a failure of a network element (N-1 situation) must still be guaranteed. In transmission systems, this is traditionally realized with different preventive (i. e. pre-fault) actions, e. g. the provision of unexploited transmission capacities and redispatch of power plants. In contrast to this, the curative approach ensures a N-1 security reactively (i. e. post-fault). This approach is based on comprehensive information and communication technologies which allow a highly automated process for identifying critical system states and the determination of subsequent corrective actions, e. g. load shedding or generator rejection. These actions are commonly designated as Special Protection Schemes (SPS). However, with increasing applications of SPS, the reliability of SPS needs to be determined and the impact of SPS on the security of supply needs to be assessed. In this contribution analytical methods are applied to determine the probability of different states (in service, limited operation and outage) for five different SPS. Furthermore, the probability and maximum level of overloads, quantifying risk in situations where curative and preventive actions fail, are compared in certain network areas in the transmission system. Results show that a highly redundant design of SPS (especially communication networks, battery storages) is crucial to reach a similar level of reliability compared to conventional network elements. The risk analysis emphasizes that the probability and level of overloads can be reduced or is on a similar level compared to preventive actions.

This paper presents a hybrid network reduction method for static network models. The methodology accounts for the later dynamisation needed to obtain a dynamic network security assessment model. As a basis for the model forecast, datasets provided by the European Network of Transmission System Operators for Electricity (ENTSO-E) working group System Protection and Dynamics are used. The method is applied to the synchronously interconnected network model of Continental Europe, and a comparison between the reduced and original network is performed. The comparison results reveal very small deviations between both networks thus validating the approach.

In this paper the short-circuit current of overhead transmission lines is analyzed at a short circuit in an HVDC transmission network. The analysis focuses solely on the current contribution of the transmission line at a line-to-earth or a line-to-line short circuit. In this context it is shown which impedance is effective at the corresponding fault case. Special attention is given to the frequency dependence of the line parameters and how they affect the short-circuit current. The frequency of the current is derived with a simplified method. Finally, it is presented how the resulting time course of the short-circuit current can be determined based on travelling wave theory. The analysis covers different HVDC grid topologies.

At a short circuit in an HVDC system modular multi-level converters with half-bridge submodules contribute to the short-circuit current even after blocking of the submodules. DC circuit breakers are therefore necessary for the interruption of the short-circuit current especially for the reliable operation of multi-terminal HVDC systems. In this paper a calculation method of important parameters of the short-circuit current is presented. The DC short-circuit current is determined by the characteristics of the AC and DC system. The substantial influence factors are evaluated and based on the ratings of existing VSC-HVDC projects typical AC and DC impedances are derived. For different HVDC scenarios resulting short-circuit currents are calculated. The paper concludes with recommendations for the calculation of short-circuit currents in radial and meshed HVDC grids.

In this paper the contribution of HVDC cables to the short-circuit current during a fault in an HVDC system is analyzed. The cable is implemented as a frequency-dependent model in a monopolar and bipolar configuration. The first part of the paper covers the calculation of the line parameters in dependence of the frequency. Special attention is given to the grounding of the sheath. In the second part line-to-line and line-to-earth short circuit currents are simulated and calculated for different HVDC cable types in several line topologies.

The developed method of risk-based asset replacement consists of the three major parts: availability & reliability calculations, asset risk determination with a Value at Risk (VaR) method and the multiple choice knapsack optimized replacement strategy developer. The transmission network availability & reliability calculations are carried out with multiple load flow scenarios which cover all typical load states during a year of the investigated transmission system. The developed VaR-method includes a Monte Carlo simulation to generate and concentrate an amount of input values to one key figure per asset. The individual outage rates and costs which depend on the time of failure occurrence are taken into account by the VaR determination. The multiple choice knapsack optimized risk-based replacement strategy developer determines the decision which activity is chosen for the individual asset and the reliability & availability of the overall transmission network and minimum investment costs as target figures. This two-dimensional optimum is found with the branch-and-bound algorithm of Sinha and Zolters. The three different opportunities are immediately replacement, refurbishment or inspection with a change to a corrective replacement. The transition from a standard to a risk-based replacement strategy allows a significant budget reduction while keeping transmission system reliability & availability. The assets providing an increased risk in the network due to their age-related behavior and location in the transmission network are determined and get a higher priority for exchange. Thus, the major assets are replaced more quickly and due to the corrective replacement strategy the costs are reduced for the less important assets.

This paper deals with the short circuit current contribution of different HVDC converter types at a fault on the DC side. The analysis covers both current source converters (CSC) and voltage source converters (VSC). Bipolar and monopolar configurations are considered. The influence of the AC and DC system parameters (SCR, AC system equipment, DC transmission line length etc.) on the short circuit current is illustrated. The simulation results are verified by short circuit current calculations based on the system configuration.

This paper deals with the analysis of short circuit currents in radial HVDC networks. After the occurrence of a short circuit in a HVDC system different sources contribute to the resulting short circuit current. In this paper the various short circuit current sources with their corresponding fault currents are analyzed. The determination of short circuit currents is done by simulation in PSCAD®. The analysis covers both line-to-line and line-to-earth faults at different fault locations. The dependency of the fault location on the amplitude is shown. The results are verified by an analytical approach to calculate the single short circuit currents of the different sources. A short circuit in a DC system causes transients with high frequencies. Therefore special attention is given to the frequency dependence of transmission lines and their effect on the short circuit current. The paper concludes with general comments on the adaption of the results to meshed DC grids and a perspective to transmission grids in the future.