Mart A. M. M. van der Meijden

A Market-Based Transmission Planning for HVDC Grid—Case Study of the North Sea

Summary form only given. There is significant interest in building HVDC transmission to carry out transnational power exchange and deliver cheaper electricity from renewable energy sources which are located far from the load centers. This paper presents a market based approach to solve a long-term TEP for meshed VSC-HVDC grids that connect regional markets. This is in general a nonlinear, non-convex large-scale optimization problem with high computational burden, partly due to the many combinations of wind and load that become possible. We developed a two-step iterative algorithm that first selects a subset of operating hours using a clustering technique, and then seeks to maximize the social welfare of all regions and minimize the investment capital of transmission infrastructure subject to technical and economic constraints. The outcome of the optimization is an optimal grid design with a topology and transmission capacities that results in congestion revenue paying off investment by the end the projects economic lifetime. Approximations are made to allow an analytical solution to the problem and demonstrate that an HVDC pricing mechanism can be consistent with an AC counterpart. The model is used to investigate development of the offshore grid in the North Sea. Simulation results are interpreted in economic terms and show the effectiveness of our proposed two-step approach.

Advanced Hybrid Transient Stability and EMT Simulation for VSC-HVDC Systems

This paper deals with advanced hybrid transient stability and electromagnetic-transient (EMT) simulation of combined ac/dc power systems containing large amounts of renewable energy sources interfaced through voltage-source converter-high-voltage direct current (VSC-HVDC). The concerning transient stability studies require the dynamic phenomena of interest to be included with adequate detail and reasonable simulation speed. Hybrid simulation offers this functionality, and this contribution focuses on its application to (multiterminal) VSC-HVDC systems. Existing numerical interfacing methods have been evaluated and improved for averaged VSC modeling. These innovations include: 1) ac system equivalent impedance refactorization after faults; 2) amended interaction protocols for improved Thévenin equivalent source updating inside the EMT-type simulation; and 3) a special new interaction protocol for improved phasor determination during faults. The improvements introduced in this contribution lead to more accurate ac/VSC-HVDC transient stability assessment compared to conventional interfacing techniques.

The Effect of FRT Behavior of VSC-HVDC-Connected Offshore Wind Power Plants on AC/DC System Dynamics

Future power systems will contain more converter-based generation, among which are the voltage-source converter-high-voltage direct-current (VSC-HVDC)-connected offshore wind power plants (WPP). Their interaction with the onshore system influences power system dynamics in the transient stability timeframe. The respective protection and control methods which cause this interaction must be taken into account in grid-integration studies performed today. This paper gives insight into the effect of typically required fault ride through (FRT) and post-FRT measures of VSC-HVDC-connected offshore WPPs on the combined ac and HVDC system dynamics. Several important sensitivities are addressed, among which are: 1) FRT implementation, 2) the postfault active power-recovery rates, 3) the ac network dynamic characteristics, and 4) the HVDC topology. The analysis is first performed as a proof of concept on a small benchmark system, and subsequently generalized to a realistic dynamic model of the future Northwestern European power system. The results of this paper can be used as a reference for understanding the effects of large-scale VSC-HVDC-connected offshore WPPs on the stability of the onshore interconnected power systems.

Transient stability analysis of an onshore power system with multi-terminal offshore VSC-HVDC transmission: A case study for the Netherlands

This paper quantifies the dynamic interaction between an onshore power system and multi-terminal voltage source converter (VSC)-based HVDC transmission systems which are used for the integration of far and large amounts of offshore wind power. Both point-to-point and multi-terminal direct current (MTDC) connection of offshore wind power plants have been investigated. A 2025-2030 scenario for the power system of the Netherlands and its neighbors has been created as a case study. Special attention is given to the transient stability of critical generators in the Dutch power system for situations with large amounts of onshore and offshore wind power production. Time-domain simulations are performed using the commercial software PSS®E, augmented with user-defined models, for a fourteen-terminal MTDC network which extends from Germany via Netherlands to Belgium. The studies highlight fault propagation in a wider neighborhood when MTDC technology is used for the connection of wind power in the North Sea.

The impact of energy storage on long term transmission planning in the North Sea region

This paper presents a planning framework to investigate the impact of different levels of integration of large-scale energy storage on the development plan of a meshed HVDC grid in a power system with large-scale offshore wind. In our problem formulation, the charge/discharge schedules of energy storage are modeled in such a way that market conditions in the succeeding hours are taken into account in the power dispatch at present. Both unlimited and limited energy storage capacity scenarios are considered, and compared to a no-storage reference case. The optimal plan includes grid topology, transmission capacities, energy storage capacities and optimal energy storage schedules. The optimization model sets the transmission capacities in such a way that transmission congestion revenue collected throughout the lifetime of the infrastructure pays off the investment cost of building the grid. The proposed model is applied to study the future development of an offshore grid in the North Sea. Simulation results are assessed according to various economic indicators. Investing in energy storage is shown to be economically effective for windy offshore regions.

Reliability evaluation of substations subject to protection system failures

In power systems, substations play a key role in connecting generation and load centers to the grid. Substations are also a critical building block in connecting large-scale offshore wind and submarine interconnectors to the onshore grid. Therefore, reliability evaluation of substations is of great significance. Currently, most of the studies related to substation reliability only consider the substation connectivity and ignore the effects of protection system failures, which is not realistic. In this paper, the reliability of high-voltage substations subject to protection system failures is analyzed using event tree analysis. For this analysis, actual failure statistics from the Dutch power system are used. The influence of different substation configurations on the overall substation reliability is studied. This provides valuable insight about the reliability of different substation configurations, which can be useful for designing substations in the future power system.

The effect of the offshore VSC-HVDC connected wind power plants on the unbalanced faulted behavior of AC transmission systems

This paper studies the effect of the negative sequence current control scheme of a VSC-HVDC system on the positive, the negative and the zero sequence voltage and current components of a 380kV onshore AC transmission line during sustained unbalanced AC faults. It is assumed for this paper that the protection schemes in the AC transmission network fails. Hence, the unbalanced fault is sustained for a longer time period. In this frame the response of the AC transmission system is observed for two different applied negative sequence current control strategies at the onshore converter station. It is shown that the suppression of the negative sequence current, as it is mainly performed by vendors today or required by TSOs, might lead to difficulties in the detection and the isolation of the line-to-line AC faults. On the other hand, the case of negative sequence current injection proportionally to the negative sequence voltage, improves the ability to detect line-to-line faults close to the converter terminals. This paper uses detailed PSCAD/EMTDC time-domain simulations supported by a linear circuit analysis in the positive, the negative and the zero sequence circuits.

Impact of DC voltage control parameters on AC/DC system dynamics under faulted conditions

This paper discusses the influence of dynamics within a multi-terminal offshore DC grid (MTDC) on the onshore AC power system stability under faulted conditions. An AC-fault occurring at the HVDC converter station terminals may propagate via the offshore MTDC grid to the undisturbed asynchronously connected AC power system. This disturbance manifests itself as additional active power overshoot at the remotely connected converters. It is shown that the severity of this disturbance propagation is sensitive to the parameters of the DC voltage droop control employed for voltage regulation in the MTDC scheme. Furthermore, the effect of the MTDC grid topology on these dynamic interactions is illustrated by comparing a meshed with a radial topology. The analysis has been performed with phasor-mode time domain simulations, augmented with a user-defined state-space model for the MTDC grid. The AC system dynamics are based on available models of benchmark power systems in the stability simulation software PSS®E.

Impact of wind energy support schemes on the development of an offshore grid in the North Sea

There is significant interest in developing offshore wind energy in Europe. Offshore wind projects require large transmission and installation investments. Therefore supporting policies are essential to make these project more economically attractive. This paper presents a optimization planning framework to investigate the impact of implementing a support instrument at different stages of planning on the final grid design and installed offshore wind capacities. For a given level of wind support, the optimal plan includes the grid and offshore wind capacities. Optimal capacities are set in such a way that congestion revenue collected throughout the life time of the project pays off the investment cost of building the grid. At the same time the remuneration of the offshore wind farm recovers the initial investment cost. The model is used to investigate the impact of implementing a wind support plan on the development of the offshore grid in the North Sea. Numerical simulation results are interpreted in economic terms and demonstrate the effectiveness of the proposed approach.

Reliability of Transmission Links Consisting of Overhead Lines and Underground Cables

Underground cables will be applied more often within new connections of the extra-high-voltage (EHV) transmission network. Since EHV underground cables are a relatively new technology, not much is known about their behavior in large transmission networks. Underground cable connections (consisting of cables, joints, and terminations) are, in general, less reliable than traditional overhead lines, mainly because of their much longer repair time. This can negatively influence the reliability of the transmission network as well. It is therefore of interest to study the reliability of underground cable connections in detail. This paper concentrates on the reliability of EHV underground cable and overhead line connections. Based on actual failure statistics of individual components, the reliability of a connection is analyzed, and which factors are of influence and what measures can be taken to improve the reliability are studied. An optimized cable repair process and a configuration with additional disconnectors are two solutions.