The radial topology of the Multi-terminal High Voltage Direct Current (MTDC) power system is a preferred connection for the gigawatt- renewable power due to its scalability and reliability. However, a radial topology with a metallic return bipolar converter configuration MTDC network possesses technical challenges regarding DC fault current interruption and grid expansion. Furthermore, such HVDC networks are energized in a specific manner, usually involving a separate energizing controller. This paper proposes a design of DC Hubs with direct current circuit breakers (DCCBs) along with a network energization sequence without requiring a separate controller. Additionally, a PI-based controller for post-DC fault circulating current in MTDC's metallic return is proposed. This control operates after DCCB recloses, removing any offset in the metallic cable by regulating the power setpoint in the converters. The proposed control is investigated under a pole-to-ground fault occurrence in the DC Hub. The proposed solution is validated by RSCAD/RTDS^@ simulation by applying detailed and average equivalent models of turbines, DCCBs and converters. The results of this simulation show a successful suppression of the DC circulating current, which results in a balanced operation of the MMCs in the post fault steady state conditions. ... Read more

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. ... Read more

In this paper three new control modules are introduced for offshore wind power plants with VSC-HVDC transmission. The goal is to enhance the Fault Ride Thought (FRT) capability of the HVDC system and the connected offshore wind power plant during balanced and unbalanced AC faults. Firstly, a positive-seq uence-voltage-dependent (PSVD) active current reduction control loop is introduced to the offshore wind turbines. The method enhances the performance of the offshore AC voltage drop FRT compliance strategy. Secondly, an adaptive current limiting control strategy which operates simultaneously on the positive and the negative sequence current is discussed. It enables negative sequence current injection, while at the same time respecting the maximum fault current capacity of the HVDC converter station. Finally, a state machine is proposed for the VSC-HVDC system and for the offshore wind turbines respectively. It coordinates the fault and the post-fault response during balanced as well as unbalanced faults, ensuring a smooth shift from the normal operating point towards the fault and the post-fault period. The test system consists of a two level VSC-HVDC link, rated at ±250 kV, connecting an offshore wind power plant with 700 MW generation capacity. Simulation results with a detailed EMT type model in PSCAD/ EMTDC environment are presented. ... Read more

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. ... Read more

This paper presents a novel iterative procedure augmented by electromagnetic transient type simulations and the state of the art mean variance mapping optimization algorithm. The aforementioned procedure enables the optimal tuning of coordinated fault ride through compliance strategies for offshore wind power plants with VSC-HVDC transmission. In particular, the formulated optimization task minimizes the electrical stresses experienced by the VSC-HVDC system and the offshore wind power plants during onshore faults. Moreover, it ensures that the onshore and offshore grid code profiles are not violated due to unwanted dynamics associated with the combined response of the VSC-HVDC system and the wind power plants. Two state of the art coordinated fault ride through strategies are optimized, namely the voltage drop and the frequency modulation technique. Simulation results demonstrate that the optimal tuning of coordinated fault ride through compliance strategies by the proposed iterative procedure enables improved dynamic response and reduced electrical stresses for the offshore wind power plant and the VSC-HVDC transmission. ... Read more

The development of large offshore wind power generation in the North Sea has been significantly accelerated in the last years. The large distance from shore in combination with the need for large transmission capacity has raised the interest for the voltage source converter high voltage direct current technology (VSC-HVDC). Transmission system operators in order to ensure high degree of the power system security of supply, impose strict grid connection requirements to offshore wind power plants and their HVDC transmission. Based on these boundary conditions, the overall research objectives that have been assessed in the context of this thesis include the following. Assessment of the state of the art coordinated fault-ride through strategies for offshore wind power plants with VSC-HVDC transmission. Analysis of unbalanced grid faults for wind power plants with VSC-HVDC transmission. Investigation of the effect of negative sequence current control for onshore and offshore AC faults. Analysis of the effect of typical grid codes on the power system voltage and rotor angle stability. The developed methodologies, models and control schemes proposed within the context of this thesis could facilitate the analysis and stable operation of transmission systems with VSC-HVDC connected offshore wind power plants. ... Read more

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-toline 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. ... Read more

This report is built within the content of the MS.c course EE4545 at TU Delft. It aims to provide an introduction to typical grid codes for DC connected wind plant. In addition, the report discusses the modeling approach used for wind plants and VSC-HVDC systems and provides insight on the grid code compliance strategies. ... Read more

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. ... Read more

This paper proposes a new knowledge-based control philosophy for the direct voltage and power control of a multi-terminal voltage source converter based offshore HVDC grid. The limitations of the classical direct voltage droop control
strategy are discussed and mainly the difficulty to reach powerreference set-points is stressed. In that context, a knowledge based intelligent controller (namely Fuzzy) is proposed. It is capable of addressing these weaknesses by combining the advantages of the droop controller such as robustness and exceptional ability to compensate for imbalance during contingencies, and the constant active power controller which has the ability to easily reach power set points. In this context, the power dispatch of the HVDC grid converters is achieved without the need to solve before-hand HVDC grid load flow equations where the droop constant is included in the algorithm. The advantages of the new Fuzzy controller is the reduced computational effort, the high degree of flexibility, and the zero percentage error. The efficacy and robustness of the control strategy is demonstrated by means of time domain simulations for a three terminal voltage source converter based offshore HVDC grid system used for the grid connection of large offshore wind power plants. ... Read more

This paper proposes a new methodology for the optimal compliance of type-4 wind power plants in VSC-HVDC grid connection with the typical Fault-Ride-Through (FRT) requirements. Unlike the traditional chopper based solution, an improved offshore AC voltage droop FRT strategy, which is communication free and ensures robust faulted dynamic response is proposed to achieve FRT compliance. The calculation of the best parameters that enable successful FRT compliance is formulated as an optimization problem. The objective function aims at minimizing the electrical stresses imposed at the HVDC system and at the offshore wind power plants during the FRT and the post-FRT period, while simultaneously ensuring FRT compliance for the HVDC system and the wind power plants. The optimization is tackled based on an iterative procedure that combines dynamic modelling of the HVDC system and the connected offshore wind power plants, with a genetic algorithm based search process. Numerical results for a point-to-point connection that is extended to the three terminal HVDC grid connection case are demonstrated. ... Read more