Computationally Efficient Electromagnetic Transient Power System Studies using Bayesian Optimization

The power system of the future will be governed by complex interactions and non-linear phenomena, that should be studied more and more through computationally expensive software simulations. To solve the abovementioned problems, power system engineers face problems with following characteristics: (i) a computationally expensive simulator, (ii) non-linear functions to optimize and (iii) lack of abundance of data. Existing optimization settings involving EMT-type simulations have been developed, but mainly use a deterministic model and optimizer, which may be computationally inefficient and do not guarantee finding a global optimum. In this paper, an automation framework based on Bayesian Optimization is introduced, and applied to two case studies. It is found that the framework has the potential to reduce computational effort, outperform deterministic optimizers and is applicable to a multitude of problems. Nevertheless, it was found that the output of the Bayesian Optimization depends on the number of evaluations used for initialization, and in addition, careful selection of surrogate models, which should be subject to future investigation

Modeling of MMCs With Controlled DC-Side Fault Blocking Capability for DC Protection Studies

The fault current characteristics in dc systems depend largely on the response, and hence also the topology, of the ac-dc converters. The presently used ac-dc converter topologies may be categorized into those with controlled or uncontrolled fault blocking capability and those lacking such capability. For the topologies of the former category, generic models of the dc-side fault response have not yet been developed and a characterization of the influence of control and sensor delays is a notable omission. Therefore, to support accurate and comprehensive dc system protection studies, this paper presents three reduced converter models and analyzes the impact of key parameters on the dc-side fault response. The models retain accurate representation of the dc-side current control, but differ in representation of the ac-side and internal current control dynamics, and arm voltage limits. The models were verified against a detailed (full-switched) simulation model for the cases of a full-bridge and a hybrid modular multilevel converter, and validated against experimental data from a lab-scale prototype. The models behave similarly in the absence of arm voltage limits, but only the most detailed of the three retains a high degree of accuracy when these limits are reached

Power-System Level Classification of Voltage-Source HVDC Converter Stations Based Upon DC Fault Handling Capabilities

status: publishe

Communication-less Protection Algorithms for Meshed VSC HVDC Cable Grids

For the large-scale integration of renewable energy sources into the power system, transmission corridors with power ratings and lengths greatly exceeding those in the existing power system will be needed. To realize these corridors, Voltage Source Converter High Voltage Direct Current (VSC HVDC) offers several advantages over the currently widely used ac technology. The use of VSC HVDC in a large-scale meshed grid can provide the major reinforcements to the power system needed for the integration of massive amounts of renewable energy sources. Selective protection against dc side faults is essential to safely and reliably operate meshed HVDC grids. Since required operating times for HVDC grid protection are ten to hundred times faster than existing ac protection, HVDC grid protection algorithms are fundamentally different from those used in ac systems. Furthermore, the limited number of HVDC grid protection algorithms reported in the recent literature were only tested in specific small-scale test systems. For a generally applicable and reliable HVDC grid protection, a more fundamental approach towards the development of protection algorithms is needed. This work provides the necessary concepts to develop communication-less protection algorithms for meshed HVDC grids. A detailed overview of dc fault phenomena is provided and fault clearing strategies proposed in the literature are discussed and classified. The fault current contribution of the half-bridge modular multilevel converter is characterized and a reduced converter model for dc fault studies, is proposed. Guidelines for the design of fault detection methods, based on fundamental traveling wave theory, are provided. Furthermore, signal processing requirements for protection algorithms, in particular required sampling frequency and digital filtering, are investigated. Finally, fast and selective HVDC grid protection algorithms for primary and backup protection are developed. These algorithms are tailored for selective fault clearing in VSC HVDC cable grids with inductive cable termination.1. Introduction 2. HVDC Grid Protection 3. Cable and Converter Models for Dc Fault Studies 4. Grounding and Configuration of HVDC Grids 5. Fault Detection in HVDC Grids: TravelingWave Theory and Signal Processing 6. Non-Unit Protection of HVDC Grids with Inductive Cable Termination 7. Backup Protection Algorithms for HVDC Grids 8. Conclusionnrpages: 224status: publishe

Y-parameters Model of Cross-Bonded Cables Including Bonding Wire Impedances

status: Published onlin

Reduced Modular Multilevel Converter Model to Evaluate Fault Transients in DC Grids

This paper proposes a reduced model for a modular multilevel converter (MMC) that can be used to evaluate the first transient after a short circuit fault in a DC grid. Detailed modelling of an MMC involves a large number of electrical nodes, hence requiring high computational effort. Reduced converter models have been proposed in the literature. However, calculation times can still be high for large grids. The reduced model proposed in this paper is based on an RLC-circuit that models the capacitive discharge phase of the MMC during DC faults. Therefore, it can be used to efficiently evaluate fault detection criteria that must act within the first transient. By performing transient simulations for a pole-to-pole fault at the converter terminals, the suitability of the model to represent the MMC during faults is demonstrated. Furthermore, it is shown by transient simulations that the model can adequately represent the reflection of travelling waves due to faults in a multiterminal system.status: publishe

Cable Protection in HVDC Grids Employing Distributed Sensors and Pro-Active HVDC Breakers

Protection of HVDC grids against short circuits must act in the order of milliseconds to avoid damage to power electronic components and to maintain a stable dc voltage. Therefore, ultra-fast protection algorithms are needed which detect the fault and identify its location. Recently developed communication-less HVDC grid protection algorithms provide a high operation speed but are inherently limited in reach to provide selectivity. In this paper, a new method for protecting HVDC grids with long cables is proposed. The method combines protection algorithms, sensors integrated in the cable joints and pro-active hybrid breakers to achieve an increased performance compared with the state-of-the-art without adding to the overall fault clearing time. This paper describes the proposed method, provides the theoretical basis for the algorithms and presents the results of an in-depth analysis using EMT-type simulations. The simulations were performed in a detailed test system implemented in EMT-type software. Simulation results show that long cables can be protected along their entire length through a combination of the proposed algorithms, although the reach of the individual algorithms is limited.status: publishe

Classification of Fault Clearing Strategies for HVDC Grids

This paper defines and classifies fault clearing strategies for HVDC grid protection, based on the method of fault current interruption. HVDC grid protection must act on a much shorter timescale than AC grid protection and faces the challenge to interrupt fast rising fault currents without natural zero crossings. Conversely, the HVDC grid protection equipment, such as HVDC breakers or converters with fault blocking capability, offers several degrees of freedom for fault clearing strategies for HVDC grid protection. However, each of these strategies fulfils different objectives and imposes different requirements on HVDC grid components. Therefore, a classification of fault clearing strategies is needed. In this paper, the objectives and requirements for HVDC grid protection are described. Furthermore, constraints on HVDC grid protection are given. Available methods for fault current interruption and protective relaying algorithms are briefly reviewed. Different classes of fault clearing strategies are defined and the options for fault current interruption and protective relaying algorithm for each strategy are given.status: publishe