Control of VSC-HVDC with electromechanical characteristics and unified primary strategy

High voltage dc (HVDC) systems act as the prevailed solution for transmitting offshore wind energy to onshore main grids. Control of the voltage source converters (VSC) in HVDC systems is decisive for the performance. This paper proposes the control of VSC-HVDC with electromechanical characteristics and unified primary strategy, as a reaction to the updated requirements of the ac grid transmission system operators. As two important aspects of VSC-HVDC control, converter control and primary control are both designed in detail. Electromechanical characteristics make the VSC capable of providing inertia to the ac networks as well as simplicity in island operation. Besides, unified primary control is given as a universal primary strategy for VSC stations, and especially takes into account frequency support and control mode transition. The proposed converter control is validated in scaled-down 10 kW laboratory setups, while the proposed primary control is endorsed by the simulation tests on a CIGRE multi-terminal HVDC model.Peer ReviewedPostprint (author's final draft

Unified reference controller for flexible primary control and inertia sharing in multi-terminal voltage source converter-HVDC grids

Multi-terminal dc (MTDC) grids are expected to be built and experience rapid expansion in the near future as they have emerged as a competitive solution for transmitting offshore wind generation and overlaying their ac counterpart. The concept of inertia sharing for the control and operation of MTDC grids, which can be achieved by the proposed unified reference controller. The control objectives of the MTDC grids voltage source converter (VSC) stations are no longer limited to the stabilisation of MTDC grid, instead, the requirements of ac side are also met. The interaction dynamics between the ac and dc grid is analysed to illustrate the proposed concept. In addition, the voltage source converter stations can work in different operation modes based on the proposed unified control structure, and can switch among the operation modes smoothly following the secondary control commands. Simulation results exhibit the merits and satisfactory performance of the proposed control strategy for stable MTDC grid operation.Peer ReviewedPostprint (author's final draft

Control of multi-terminal HVDC networks towards wind power integration: A review

© 2015 Elsevier Ltd. More interconnections among countries and synchronous areas are foreseen in order to fulfil the EU 2050 target on the renewable generation share. One proposal to accomplish this challenging objective is the development of the so-called European SuperGrid. Multi-terminal HVDC networks are emerging as the most promising technologies to develop such a concept. Moreover, multi-terminal HVDC grids are based on highly controllable devices, which may allow not only transmitting power, but also supporting the AC grids to ensure a secure and stable operation. This paper aims to present an overview of different control schemes for multi-terminal HVDC grids, including the control of the power converters and the controls for power sharing and the provision of ancillary services. This paper also analyses the proposed modifications of the existing control schemes to manage high participation shares of wind power generation in multi-terminal grids.Postprint (author's final draft

Multi-terminal HVDC grids with inertia mimicry capability

The high-voltage multi-terminal dc (MTDC) systems are foreseen to experience an important development in the next years. Currently, they have appeared to be a prevailing technical and economical solution for harvesting offshore wind energy. In this study, inertia mimicry capability is added to a voltage-source converter-HVDC grid-side station in an MTDC grid connected to a weak ac grid, which can have low inertia or even operate as an islanded grid. The presented inertia mimicry control is integrated in the generalised voltage droop strategy implemented at the primary level of a two-layer hierarchical control structure of the MTDC grid to provide higher flexibility, and thus controllability to the network. Besides, complete control framework from the operational point of view is developed to integrate the low-level control of the converter stations in the supervisory control centre of the MTDC grid. A scaled laboratory test results considering the international council on large electric systems (CIGRE) B4 MTDC grid demonstrate the good performance of the converter station when it is connected to a weak islanded ac grid.Peer ReviewedPostprint (author's final draft

Ancillary Services in Hybrid AC/DC Low Voltage Distribution Networks

In the last decade, distribution systems are experiencing a drastic transformation with the advent of new technologies. In fact, distribution networks are no longer passive systems, considering the current integration rates of new agents such as distributed generation, electrical vehicles and energy storage, which are greatly influencing the way these systems are operated. In addition, the intrinsic DC nature of these components, interfaced to the AC system through power electronics converters, is unlocking the possibility for new distribution topologies based on AC/DC networks. This paper analyzes the evolution of AC distribution systems, the advantages of AC/DC hybrid arrangements and the active role that the new distributed agents may play in the upcoming decarbonized paradigm by providing different ancillary services.Ministerio de Economía y Competitividad ENE2017-84813-RUnión Europea (Programa Horizonte 2020) 76409

Management and Protection of High-Voltage Direct Current Systems Based on Modular Multilevel Converters

The electrical grid is undergoing large changes due to the massive integration of renewable energy systems and the electrification of transport and heating sectors. These new resources are typically non-dispatchable and dependent on external factors (e.g., weather, user patterns). These two aspects make the generation and demand less predictable, facilitating a larger power variability. As a consequence, rejecting disturbances and respecting power quality constraints gets more challenging, as small power imbalances can create large frequency deviations with faster transients. In order to deal with these challenges, the energy system needs an upgraded infrastructure and improved control system. In this regard, high-voltage direct current (HVdc) systems can increase the controllability of the power system, facilitating the integration of large renewable energy systems. This thesis contributes to the advancement of the state of the art in HVdc systems, addressing the modeling, control and protection of HVdc systems, adopting modular multilevel converter (MMC) technology, with focus in providing services to ac systems. HVdc system control and protection studies need for an accurate HVdc terminal modeling in largely different time frames. Thus, as a first step, this thesis presents a guideline for the necessary level of deepness of the power electronics modeling with respect to the power system problem under study. Starting from a proper modeling for power system studies, this thesis proposes an HVdc frequency regulation approach, which adapts the power consumption of voltage-dependent loads by means of controlled reactive power injections, that control the voltage in the grid. This solution enables a fast and accurate load power control, able to minimize the frequency swing in asynchronous or embedded HVdc applications. One key challenge of HVdc systems is a proper protection system and particularly dc circuit breaker (CB) design, which necessitates fault current analysis for a large number of grid scenarios and parameters. This thesis applies the knowledge developed in the modeling and control of HVdc systems, to develop a fast and accurate fault current estimation method for MMC-based HVdc system. This method, including the HVdc control, achieved to accurately estimate the fault current peak value and slope with very small computational effort compared to the conventional approach using EMT-simulations. This work is concluded introducing a new protection methodology, that involves the fault blocking capability of MMCs with mixed submodule (SM) structure, without the need for an additional CB. The main focus is the adaption of the MMC topology with reduced number of bipolar SM to achieve similar fault clearing performance as with dc CB and tolerable SM over-voltage

Ancillary services analysis of an offshore wind farm cluster-technical integration steps of a simulation tool

In this publication, the authors present methodology and example results for the analysis of ancillary services of an offshore wind farm cluster and its electrical power system. Thereby the operation tool Wind Cluster Management System (WCMS) is used as simulation tool to evaluate certain planning scenarios. Emphasis is made on two topics: 1) the integration of high voltage direct current (HVDC) technology to the WCMS, 2) the ancillary service analysis. As examples, voltage source converter based HVDC (VSC-HVDC) and the provision of reserve respectively balancing power are discussed in detail. The analyzed study case considers the Kriegers Flak area while the associated power system connects wind farms to Sweden, Denmark and Germany.EC/FP7/ENERGY-2011-1/ 28279

Fast frequency support control in the GB power system using VSC-HVDC technology

A fast frequency support control scheme for voltage source converter based high-voltage direct-current (HVDC) links has been designed, simulated and experimentally implemented and validated. The effectiveness of the proposed scheme has been tested on simplified GB power system models with both averaged and switched converter models. System performance has been initially assessed using different software simulation platforms (PSCAD and MATLAB/Simulink). System validation has been carried out using an experimental test-rig. It is shown that simulation and experimental results agree on well when the fast frequency support provision is enabled. For completeness, the effectiveness of the control scheme has been tested for two contingency scenarios: (i) when a high-voltage alternating-current interlink in parallel with the HVDC link is disconnected, and (ii) for a substantial increase in system load

Primary Frequency Regulation Using HVDC Terminals Controlling Voltage Dependent Loads

HVDC can provide frequency regulation during disturbances (e.g., faults) by controlling the power flow between two remote AC areas. While this action reduces the power deviation in the area affected by the disturbance, it causes a power imbalance in the other healthy AC area, leading to a frequency variation and endangering the system stability. In this work, a HVDC primary frequency regulation controlling voltage-dependent loads (PFRVDL) is proposed, where the HVDC terminal in the healthy area influences the grid voltage amplitude to shape (decreasing or increasing) the load consumption in order to cope with the power variation required by the fault-affected area. The PFR-VDL extracts the needed energy for the frequency support, not from the generators (with following frequency deviation) but from the voltage-dependent loads in the healthy area. This work analyzes the PFR-VDL performance, generalizing it with two possible HVDC connection cases: Asynchronous connection with single HVDC line, and embedded HVDC forming a parallel, hybrid connection with HVAC. The PFR-VDL application benefits and limitations are evaluated analytically and verified by means of PSCAD EMTDC simulations, and finally validated with a large interconnected IEEE 39 bus system

Impact of Grid Forming Power Converters on the Provision of Grid Services through VSC-HVdc Systems

High Voltage dc (HVdc) transmission systems have gained increased popularity as a flexible and efficient power transmission option with higher grid controllability. Widespread adoption of HVdc systems for interconnecting power systems and integrating large renewable energy generation facilities such as wind farms, has forced the power system to undergo a transition from a predominantly ac system into a hybrid ac-dc system, especially in the high voltage transmission grid. This paper attempts to provide an overview on the role of Voltage Source Converter based HVdc(VSC-HVdc) systems within the evolving power system as a grid services provider. Special attention is paid to discuss the impact of Grid Forming converter control approach on the provision of such services through VSC-HVdc systems