Andreas Wasserrab

Evaluation of short circuit currents in multi-terminal HVDC systems

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.

Contribution of HVDC converters to the DC short circuit current

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.

Risk-based asset replacement in EHV transmission networks

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.

Frequency-dependent cables for the calculation of line short-circuit currents in HVDC networks

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.