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This paper proposes an approach to mitigate adverse effects of the fault management of closed/ring grids with high shares of distributed generation (DG). The approach refers to conventional distance protection relays in particular. The rising share of DG, especially in distribution systems, can lead to overloadings and intolerable voltages. Closed-ring structures increase the grid capacity for DG due to more balanced load and voltages profiles. However, malfunctions of protection relays may occur because fault locating and selective fault clearing can be disturbed. This paper presents a distance protection algorithm for feeder relays that complies with the requirements of closed/ring grids with inter/infeeds. The proposed current comparison method (CCM) bases on the comparison of negative/sequence currents. It needs no voltage measurement and remains unaffected from inter/infeeds and fault resistances. Additionally, for relays located at the ring coupler, where the ring can be opened, a coupling relay method (CRM) is proposed. It locates unsymmetrical faults in closed/ring grids with inter/infeeds and enables fast and selective opening of the ring. This helps maintaining reliable power supply for customers in closed rings. Finally, the paper presents a grading strategy reducing the average fault clearing time. Lab tests prove the capability of both proposed algorithms. [ABSTRACT FROM AUTHOR]

An advanced approach of the internal energy flow control of a multiport device based on a modular multilevel converter (MMC) was proposed. Such a multiport device offers various connecting points as ac terminals, the main dc terminal as well as the dc terminals of the MMC modules. State-of-the-art multiport concepts have restrictions concerning the flexibility of connecting the available ports. The proposed concept makes an unrestricted energy interchange between all ports possible without affecting the ac grid and the main dc terminal. It contains a microscopic and macroscopic view of the entire system and an analytic description in a double synchronous reference frame. This approach allows the identification of degrees of freedom for the internal energy flow control to overcome the mentioned restrictions of state-of-the-art multiport devices. Beyond numerous simulations calculations, the proposed concept has been implemented to a prototype of a multiport device which consists of an MMC with integrated batteries in power electronics laboratory. The approach has been successfully tested and verified with measurement results. [ABSTRACT FROM AUTHOR]

• new and more detailed (submode-based) converter control classification. • shows interrelations of control concepts comprehensively and at a glance. • reveals new control concepts. • converter-grid interaction modelled through equivalent circuits. • creates a proper basis for understanding converter-dominated power systems. As decarbonized power systems are mainly based on power electronic converters, the operational behavior of electrical power systems will be more and more determined by the control concepts of these converters. The wide variety of control concepts developed over the last years, however, often causes ambiguity and misunderstanding among power system experts regarding the performance of converters and converter-dominated power systems. Reviewing papers and classification schemes for converter control exist in great numbers but they do not cover the full range of control concepts or only deal with specific applications like microgrids. Therefore, this paper proposes an advanced classification that brings together different existing classification schemes, control concepts and control features and shows their interrelations in a new and comprehensive way. The proposed classification scheme combines four different criteria: the steady state equivalent circuit of the converter, the operating characteristic (grid-following, grid-forming, grid-supporting), different control modes (and submodes) regarding power, voltage and frequency and the ability/necessity to operate grid-connected. The control modes also include control features like virtual inertia or virtual impedances. Based on a double synchronous reference frame converter model, the different control concepts were presented by means of their control loops and equivalent circuits. The comparison with existing classification schemes and the application to different control concepts found in literature showed that the proposed classification scheme does not only provide a more detailed (submode-based) converter control classification but it also revealed new control concepts that were verified by simulation and experimental results. Thus, the control concepts for power electronic converters were reclassified, restructured and extended. In this way, the classification scheme proposed in this paper creates a proper basis for a comprehensive understanding of converters and converter-dominated power systems. [ABSTRACT FROM AUTHOR]

The new characteristics and phenomena of hybrid AC/DC transmission grids challenge the conventional protection concepts. In hybrid AC/DC overhead line systems, AC and DC systems are located on the same support structure. As a result, electromagnetic and galvanic interactions between the AC- and the DC-line influence the AC-protection and the DC-control in a novel and complex way. This paper presents key findings and solutions from a current 400-kV-hybrid AC/DC overhead line project in Germany. The focus is on AC/DC intersystem faults, i.e. galvanic contact between DC and AC systems. Fault detection and fault response of the AC-protection and the DC-control as well as a proper AC/DC protection coordination are discussed. The paper illustrates the challenges of a secure and dependable fault clearing and indicates solution methods. The findings of this paper are based on EMT simulations with detailed PSCAD-models of the hybrid AC/DC overhead line and the VSC-converters. Laboratory protection tests have been conducted and confirmed the results. [ABSTRACT FROM AUTHOR]

In this paper, a contribution to the analysis of electromagnetic ac to dc interactions in complex ac/dc hybrid overhead lines was made. For accurate calculation of such coupling, standard transformations like the symmetrical components cannot be used. Therefore, a method based on adjusted modal decomposition was presented, which allows analysis of untransposed and partly transposed configurations in arbitrary switching states. The method was applied to the case of dc fault clearing using single-pole auto reclosing (dc SPAR), where parallel ac circuits could induce a secondary arc in the dc line. The magnitude of the secondary arc current was examined, which allows evaluating the necessary dead time of the dc SPAR. The influence of parameters like line configuration, load flow, and fault location was discussed. After that, a realistic study case hybrid line was analyzed. In particular, the comparison to secondary arcs in ac lines and the behavior as a function of fault location in the context of changing ac systems and line configurations along the hybrid dc corridor provide new insights into a relevant phenomenon. [ABSTRACT FROM PUBLISHER]

The aim of this work is to include the distance-protection behavior in direct methods for transient stability assessments. In this way the direct methods assess an electric-power system’s behavior more realistically and hence more reliably. In order to consider the delayed tripping zones of distance-protection relays, the time component was added to the Lyapunov energy function. The main innovation is that the dwell time of the post-fault impedance trajectory inside various tripping zones is calculated based on the speed of the transformation between the kinetic and the potential energy parts of the Lyapunov function. It enables the identification of unwanted trips during power swings. The tripping-zone settings can be revised accordingly. The method was verified by a comparison between the direct method and the well-known, time-domain, numerical-simulation method on a single-machine, infinite-bus test system where the results have to be identical. The application of the proposed method on a multi-machine power system gives good results. [ABSTRACT FROM AUTHOR]

An approach for a real-time adaption of the dead time for autoreclosing on high-voltage lines was proposed. The objective was the optimal adaption of the dead time of a currently running single-phase autoreclosing cycle by calculating the secondary arc current based on measurements. The algorithms for optimization were performed for real-time applications. For this purpose, the fault location plus the load currents of the parallel nonfaulty conductors were measured. The measurement and the calculation of the dead time took place during the fault and the dead-time period. In this way, an optimized dead-time setting with regard to the actual fault location and loading conditions were achieved. This approach results in an online determination of the minimum necessary dead time of a successful reclosing under consideration of the fault location and prevailing loading conditions. Analytical calculations and numerical examples as well as high-voltage field tests verified the results. [ABSTRACT FROM PUBLISHER]

• Protection devices independent assessment of grid security. • Applicable to multivariable grid structures. • New evaluation algorithm based on realistic simulation data and flexible fuzzy sets. • Full adaptive and automatic protection security assessment. • Specifically designed for the use with an optimization algorithm to generate best settings. System Protection Security Assessment is an important task in modern energy grids to ensure system security at all. The assessment system is particularly challenged by multivariate grid structures caused by volatile renewable infeeds. This paper presents an innovative strategy to evaluate the protection relay coordination of system-wide power grids. A way to calculate the quality of all protection relays based on realistic simulation data and independent of the protection method was sought. The hybrid algorithm consists of two major steps. First, a systematic analysis with various fault simulations is performed and the measurement results of all relays of all simulations are used as database. Subsequently, the use of fuzzy sets allows to express the quality of each relay setting regardless of its type. Specifically developed for the use of an optimization algorithm, finally, a new protection coordination is determined for an adapted version of the IEEE 9 bus grid. The results are validated, discussed and the effectiveness of the methodology compared to conventional setting rules. [ABSTRACT FROM AUTHOR]