Methods of Current Limitation in Low Voltage (up to 1 kV) Electric Networks

The article discusses and systematizes the causes of high values of short-circuit currents in electrical networks up to 1 kV and indicates their negative consequences. A brief analysis of some current limiting methods (both with and without additional investment) is given, which makes it possible to reduce the negative effects when significant short-circuit currents occur. The variants of the implementation of these current limiting methods recommended for study at the design stage or when replacing equipment that has exhausted its resource, are considered. The advantages and disadvantages of the methods of limiting short-circuit currents described in the article are indicated. Options for connecting equipment with low power consumption to nodes of the electrical network up to 1 kV with high values of short-circuit currents are also considered. Special attention is paid to current-limiting protection devices, a detailed description of the design of some of them is given, the advantages of their use in comparison with other current-limiting methods are indicated, the corresponding graphs of the limitation of the periodic component of the short-circuit current, shock short-circuit current and thermal pulse are presented. Based on the analysis of graphic materials, the issue of ensuring the selectivity of protections is considered. The degree of prevalence of equipment with the property of current limitation is determined. A method for determining selective current-limiting protection devices is given, viz. the method of energy selectivity, the corresponding graphs for determining energy selectivity are presented. The physics allowing to determine the degree of efficiency of limiting the short-circuit current is described. Recommendations on the use of current-limiting equipment are proposed.

The Coordinated Fault Current Limiting Strategy for a Hybrid Multilevel Modular Converter (MMC) based VSC-HVDC Applications

Background: High-voltage direct current (HVDC) is suitable for high capacity and longdistance power transmission, thus becoming ideal for connecting renewable energies such as solar power and wind power to grids. Objective: Overhead lines in HVDC are vulnerable to short-circuit faults. Non-permanent DC short circuit faults are the most common in HVDC transmission, which can lead to pause in power transmission and interruption in large grids. Thereby, it is crucial to deploy techniques to suppress fault current. Method: To lower the fault current economically, a coordinated fault current limiting strategy based on a hybrid multilevel modular converter (MMC) is proposed in this paper. Results: Combining hybrid MMC and small-capacity DC circuit breaker reduces total IGBTs required and avoids MMC blocking during pole-to-ground short-circuit fault. This approach is verified using a two-terminal MMC-based system in PSCAD/EMTDC simulation environment. Conclusion: By implementing the introduced scheme, the peak fault current can be lowered by 33.0% using hybrid-MMC with 80% of FBSMs. Economic efficiency can be improved by adopting proposed scheme.