Mario Ndreko

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 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.

On grid code compliance of offshore mtdc grids: modeling and analysis

This paper studies the grid code compliance of an offshore multi-terminal HVDC grid used for grid connection of far and large offshore wind power plants during AC system failures. The paper stresses out control modules and parameter sensitivities which influence the combined dynamic response of the MTDC grid and the connected wind power plants. Common low voltage ride through strategies are evaluated and a variety of DC grid operating points are tested. Finally, the influence of the short circuit power level at the onshore grid connection point is presented. The results of this paper can be used as reference for the design of future grid code requirements related to the control and operation of HVDC converters connected in MTDC grid configurations.

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.

Metaheuristic approach for online optimal reactive power management in near-shore wind power plants

Mean-variance mapping optimization (MVMO) is an emerging metaheuristic optimization algorithm, whose evolutionary mechanism performs within a normalized search space. The most remarkable aspect of this mechanism resides in the application of a special mapping function to generate new values of the optimization variables based on their statistical significance throughout the search process. This paper concerns with the feasibility of the MVMO to tackle the problem of online optimal reactive power management in near-shore wind power plants. The main challenges reside in the restricted computing budget and mix-integer nature of the problem. To this aim, MVMO is configured to evolve a single solution throughout the search process, and a new mapping function is proposed to improve the global search capability. Numerical tests on a benchmark system proposed by the IEEE Working Group on Modern Heuristic Optimization as well as a real world wind power plant demonstrate the effectiveness of MVMO.

Investigation on different negative sequence current control options for MMC-HVDC during single line to ground AC faults

This paper presents the effect of the negative sequence current control strategy of an MMC-HVDC system on the single-line-to-ground (SLG) fault response of the high AC transmission lines. Four different methods reported in the literature are compared. The paper shows that the negative sequence current suppression by the MMC-HVDC station demonstrates increased double-frequency power oscillations in the DC link and high over-voltages in the AC terminals during single line to ground faults. On the other side, the adequate control of the negative sequence current during unbalanced faults in the grid improves the observed DC link voltage and power ripples. Furthermore, the negative sequence current injection, increases the zero sequence current amplitude measured at the PCC of the MMC-HVDC station. The later, enhances the fault detection capability of protection schemes in the AC transmission during SLG to ground faults in the vicinity of the MMC substation.

Impact of offshore wind and conventional generation outages on the dynamic performance of AC-DC transmission systems

The impact of offshore wind power and conventional generation outages on the dynamic performance of multi terminal DC interfaced AC systems is ascertained in this paper. In addition, the paper addresses the share of primary frequency reserves as well as synthetic inertia response between MTDC connected AC systems focusing on the interactions between the direct voltage control loop and the dedicated primary and synthetic inertia response control loops. The classical single machine modeling approach is used for the AC systems extended by common dynamic models of MTDC grids in a so called simple AC-DC model adequate for frequency stability studies. The results of this paper can be used as reference for grid code design and optimization of MTDC grid control loops operating next to AC power systems.

Optimal management of reactive power sources in far-offshore wind power plants

This paper introduces a new approach for the optimal management of reactive power, with emphasis on offshore wind power plants. The approach follows a predictive optimization scheme (i.e. day-ahead, intraday application). Predictive optimization is based on the principle of minimizing the real power losses, as well the number of On-load Tap Changer (OLTC) operations for daily time horizon (discretized in 24 hours). The mixed-integer nature of the problem and the restricted computing budget is tackled by using an emerging metaheuristic algorithm called Mean-Variance Mapping Optimization (MVMO). The evolutionary mechanism of MVMO is enhanced by introducing a new mapping function, which improves its global search capability. The effectiveness of MVMO to find solutions that ensure minimum losses, minimum impact on OLTC lifetime, and well as optimal grid code compliance is demonstrated by investigating the case of a real world far-offshore wind power plant with HVDC connection.