Arjen A. van der Meer

Modeling and control of HVDC grids: A key challenge for the future power system

HVDC technology is developing fast and HVDC grids are increasingly seen as a possible and feasible solution to manage the future power system with large amounts of renewables in a secure and cost-effective manner. However, systems with significant amounts of DC transmission behave in a fundamentally different manner when compared to the traditional AC power system. The integration of HVDC systems introduces new fast dynamics on different time frames and adds controllability to the combined system. As a result, the modeling and control of the entire interconnected system needs to be re-evaluated in order to accurately compute the system behavior, both from the AC and DC system. This survey paper gives an overview of the current research in the field of HVDC grids focusing on the interaction of the AC and DC system. The converters and their behavior are discussed in greater detail. A second component which is discussed is the DC breaker. Both devices operate fundamentally different than their AC counterparts. The fast interaction between AC and DC systems requires changes in the manner in which the modeling and computation of the system is done, both at the DC and the AC side. Although these considerations are needed within all relevant time frames, two relevant cases are specifically addressed in this paper: the connection of offshore wind power through a HVDC system and the optimal operation of the power system with a strong presence of HVDC.

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.

Cosimulation of Intelligent Power Systems: Fundamentals, Software Architecture, Numerics, and Coupling

Smart grids link various types of energy technologies-such as power electronics, machines, grids, and markets-via communication technology, which leads to a transdisciplinary, multidomain system. Simulation packages for assessing system integration of components typically cover only one subdomain, while simplifying the others. Cosimulation overcomes this by coupling subdomain models that are described and solved within their native environments, using specialized solvers and validated libraries. This article discusses the state of the art and conceptually describes the main challenges for simulating intelligent power systems. This article, part 1 of 2 on this subject, covers fundamental concepts. Part 2 will appear in a future issue of IEEE Electrification Magazine and cover applications.

Combined stability and electro-magnetic transients simulation of offshore wind power connected through multi-terminal VSC-HVDC

Offshore wind energy has a high potential especially in Northern-Europe. Future wind power plants may be situated further from the shore and therefore a high-voltage direct current connection based on voltage sourced converters is most suitable for grid integration. Connection utilization can be improved by interconnecting several wind power plants leading to multiterminal schemes. This paper describes a modeling approach that facilitates the incorporation of such (offshore) dc-systems into transient stability simulations. It enables the possibility to use a different simulation approach for each side of the converters, i.e. to represent the acside by complex phasors and the dc-side by electro-magnetic transients. Coupling between the ac and dc-sides is obtained by the active power balance. To study the interaction between the multi-terminal scheme and the onshore network an illustrative test-network has been taken. A chopper-controlled braking resistor that protects the dc-circuit against overvoltages has been included and is expressed as a variable resistance. Methods to distribute the wind power among the onshore converters are explored and operation without a supervisory dispatch controller has been studied.

Fault ride-through requirements for onshore wind power plants in Europe: The needs of the power system

Wind power plants show different behavior than conventional (synchronous) generators. As the traditional power systems mainly consisted of centralized generation by synchronous machines feeding passive loads, it was well-understood how the system reacted in normal operation as well as during disturbances. As wind power plants are foreseen to increase in size and the amount of installed wind power will grow, the relative contribution of equipment not exhibiting this common behavior increases. At the same time power electronics offer opportunities for additional features to stabilize the power system. Transmission system operators impose requirements on the (dynamic) capabilities of connected new generation resources (including wind power plants) which are specified in grid codes. In this paper, the importance of such requirements is explained by looking at the needs of the power system and by showing simulation results for a test network. The paper facilitates a detailed understanding of the underlying phenomena related to grid code requirements with a focus on low-voltage ride-through and voltage support by reactive current boosting.

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 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.

Stability assessment of VSC-HVDC connected large-scale offshore wind power: A North-Sea region case study

Large amounts of offshore wind power plants (WPP) connected through voltage sourced converter high-voltage DC (VSC-HVDC) technology can have significant consequences for the dynamic behavior of the related onshore AC systems. This paper explores the rotor angle stability effects of the integration of 35 GW of wind power in the North Sea, connected to the European mainland, Nordic, and UK power systems. It investigates the influence of the VSC-HVDC topology (e.g. radial versus meshed) and the DC power flow control strategy on the dynamic response of the connected power systems after common disturbances such as the outage of an offshore wind power plant and short-circuits in the AC grid. The results show a significant influence of the power flow control method applied. Radial VSC-HVDC connections for offshore WPPs can be designed such that the stability effects can be minimized, whereas meshed links may propagate disturbances from one synchronous area to another.