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.

Multi-terminal HVDC projects are increasingly developed in recent years to enhance the flexibility of energy transmission. Differing from the point-to-point HVDC systems, it is more challenging to design the multi-terminal HVDC system and ensure stable interoperability among converter systems, especially when the converters are manufactured from different vendors. Although impedance-based stability analysis allows for analyzing such systems based on black-box models, it is still difficult to utilize those black-box models to identify the root cause of potential instability. To tackle this challenge, this paper proposes a multi-level sensitivity analysis approach using frequency-domain sensitivity functions based on the impedance-based stability criterion. The proposed method differs from the classical sensitivity analysis based on state-space or transfer-function models, as it is purely based on black-box impedance models. Case studies on a four-terminal HVDC system are carried out for stability and sensitivity analysis based on the impedance measurement in PSCAD, through which the most sensitive HVDC station can be identified. The proposed theory and the analyzed results are finally validated by electromagnetic transient simulations.

The stability of multi-vendor, multi-terminal HVDC systems can be analyzed in frequency domain by black-box impedance models using the generalized Nyquist stability criterion. Based on the impedance stability analysis, a multi-level sensitivity analysis approach using frequency-domain sensitivity functions is proposed to identify the root cause of potential instability. Case studies on a four-terminal HVDC system are carried out for stability and sensitivity analysis based on the impedance measurement in PSCAD. The analysis results are finally validated by electromagnetic transient simulations.

The penetration of power electronic interfaced generation (PEIG) is expected to reach up to 65% in some parts of the European power system by 2030 (at least during some hours of the year). Under such grid conditions, system security challenges are observed with frequency stability, voltage stability and undamped converter control interactions being among the most important issues. This study presents a short-term voltage stability assessment of the Great Britain synchronous area under EMT modelling assumptions. The study provides a mapping of system stability and identifies the critical penetration level of PEIG that instabilities are observed. In addition, an application of a grid forming control scheme (namely the enhanced direct power control) is proposed as a mitigation option which is applied here on full-converter interfaced wind power plants (type-4). The simulation results reveal that the application of the grid forming control to a part of the total wind power generation fleet can mitigate the instabilities observed, while enabling the system operation with 100% PEIG.

This paper reviews ongoing planning for power-electronic-dominated power systems and for the technological implementation of multi-vendor multi-terminal HVDC transmission systems in hybrid AC/DC structures in Germany and Europe. Existing system needs and the possibilities to achieve embedded multi-terminal HVDC overlay grids are described. Additional research activities on technical topics in order to answer open research questions for the secure and reliable operation of power-electronic-dominated power systems are highlighted. The need for European cooperation including TSO, vendors and research institutes is described. As an example, simulation results are given and discussed with respect to multi-terminal HVDC planning options.

The frequency-coupling dynamics of the modular multilevel converter (MMC) complicate the stability analysis and controller design of MMC-based high-voltage direct current (HVDC) transmission systems. In this paper, a multi-frequency state-space model of MMC with the closed-loop control system is established, which allows to identify the critical oscillation modes and their sensitivities to the control parameters, and thus provides a design-oriented stability analysis. The mathematical relationships between the multi-frequency state-space models in different frames are established, which bridges the gap between the frequency-coupling dynamics of the MMC power stage and that of asymmetrical control loops. Design-oriented stability analysis is then performed based on a generic MMC-HVDC model, which confirms the effectiveness of the modeling techniques.

This paper presents an overview of the main technical challenges that are derived from the high penetration of power electronic interfaced generation (PEIG) units in the power system by 2030. In order to cope with all these challenges, innovation, research and development in the electrical energy business is crucial. The paper provides a roadmap towards the successful integration of large scale PEIG in the power system focusing on system observability, controllability, flexibility and coordination as boundary conditions. It is concluded that international and multilateral developments are essential to achieve a secure and sound socio-economic system operation. Coordinated decisions lead to enhanced network security and cost efficiency. The common understanding of roles, interoperable or common analysis tools and procedures are key factors for successful realisation of future networks. KeywordsHVDC, PEIG, High penetration of inverter based generation, System Operation Challenges, Power System Stability, Roadmaps for R&D.

This paper presents a comprehensive methodology for the investigation of SSTI in AC/DC transmission networks. The main advantage of the proposed methodology is the modular modeling approach, where various complex components can be modeled in a comprehensive way while at the same time preserving the level of detail needed to capture the relevant dynamics. This is done by enhancing the CCM with an algorithm for the formulation of network state equations. All power system component models necessary for SSTI analysis are developed separately using the CCM principles and are connected according to their topological relations. A small-signal model of a point-to-point MMC-based VSC HVDC is developed to assess its influence on SSTI. The proposed methodology together with the developed models are implemented in a software framework called SNAP. The proposed methodology and the developed models are validated against time-domain simulations in PSS (R) NETOIVIAC.

This paper reviews ongoing research activities towards the technological implementation of meshed HVDC transmission system structures in Europe. The paper touches on the existing needs of the system and addresses how existing HVDC links could be integrated in order to achieve meshed HVDC grids. It concludes that additional research activities on technical topics are still needed in order to answer still open research questions for the secure and reliable implementation of meshed HVDC grids. Moreover, it highlights the need for European cooperation including TSO, Vendors and research institutes in order to address those topics.

This paper presents a methodology for the state space matrix formulation of a multi-machine power system using the component connection method (CCM). The motivation is the development of a computational framework for the analysis of subsynchronous resonances (SSR) in multi-machine meshed networks possibly including VSC-HVDC systems. An alternative modeling approach of the turbine, as well as the generator unit modeling, is presented. The electrical transmission network dynamics is included, and a modified algorithm for deriving the network state equations is described. All modeled components are connected using the interconnection matrices defined by CCM. The methodology is validated against known benchmark models for computer simulation of subsynchronous resonances. The proposed methodology provides a modular and computationally-efficient approach for the calculation of the system state matrix, which is necessary for systematic analysis of SSR.

As a result of developments within the last 30 years, wind energy has become a fundamental pillar for the electrical energy supply in Germany. As of today and beside other renewable energy sources, wind power generation is at the center of the ongoing transition process of the German energy system. The chapter describes the state of play as well as future challenges with respect to large-scale wind energy integration into the German and European power system. Based on existing facts and figures and a long-term outlook, the developments in the onshore and offshore wind power technology together with the necessary grid development plans and relevant network codes are provided. Recent experiences with respect to wind integration in Germany call for complex interoperability analyses between transmission and distribution including the sector coupling of generation and storage plants. Furthermore, ongoing research activities concerning new intersystem phenomena are addressed.

This paper presents an approach for incorporating VSC - HVDC embedded in AC networks into the secondary voltage control strategy of the network, in addition to the dynamic voltage support requirements following grid short circuits. The models of the hierarchical two-stage slow and fast-acting controllers have been described in detail and the coordination of the steady state (set-point tracking) part and the component responsible for dynamic voltage support explained. Simulation examples demonstrate that the disturbance as well as the reference tracking components operate without interfering with the operation of one another, and thus confirm the possibility of incorporating VSC HVDC into the overall network var dispatch strategy.

The Network Code on HVDC Connections (NC HVDC) is the ninth European network code on electricity presently being developed by ENTSO-E. It covers a technically detailed, but increasingly important subject in a domain where present standards and national HVDC grid codes are relatively young, quite diverse, or possibly not even fully developed yet. The NC HVDC will complement the earlier ENTSO-E connection codes for generation and demand, and is linked to other codes on system operation and market rules. This paper presents the rationale behind the requirements of the present draft code, with special focus on the requirements for DC connected Power Park Modules.

This paper discusses a novel model for stability studies of a Multiterminal High Voltage Direct Current system (MT HVDC) as part of an overlay grid in a hybrid AC system. The dynamic model is set up for Multilevel Voltage Sourced Converter (VSC) technology. The new model provides the opportunity to analyse the impact of full- and half-bridge modules during AC and DC faults. In the present paper a radial DC system with four converter stations was built up and simulated in PSS®NETOMAC. In principle the model can be expanded to a multiterminal HVDC system with a higher number of converter stations. Generally the structure of the DC grid does not subject to any restrictions. For steady state control of a MT HVDC system the Voltage Margin Method (VMM) was implemented. The main focus of the presented model is placed on dynamic stability studies in case of DC faults and their effects on the AC grid. But due to the possibility of VSC converters to provide general system services, e.g. to supply reactive power, the effects and advantages of VSC converters during AC faults can also be analysed. In principle Insulated Gate Bipolar Transistor (IGBT) technology offers the possibility of clearing DC faults on the DC side. Depending on the type of modules (full- or half-bridge modules) used in a Multilevel VSC converter the fault clearing strategy and therefore the effects to the AC grid differ enormously. It is essential for the transient stability of a highly stressed AC grid to ensure a very low fault clearance time to keep the system stable. The proposed control design was designed as a two partition macro in PSS®NETOMAC and can be used for planning a Multiterminal DC system in any AC grid. It was applied to a small test grid in order to prove its performance. For more realistic results the model was implemented and applied in a dynamic model of the Continental Europe high voltage power transmission grid.

Large capacities of wind power have already been accommodated in Germany with around 26 GW (May 2010). Yet further increases can be expected in order to achieve Europe's 2020 targets for renewable energy. This poses big challenges for wind generation developers in terms of obtaining suitable sites and financing the associated investments. Such developments also impact the networks and especially in Germany with around 40% of the Europe's current installed wind power capacity. Flexibility measures are used to provide a temporary relief but distances between generation and consumption will more and more increase. This gives rise to new challenges for the entire system in particular for transmission system operators as far as a safe and reliable electricity supply is concerned. By the year 2030 a total wind power capacity of more than 60 GW is expected for Germany, which is much higher than 50% of the German peak load. The immediate transmission related challenges for Germany embedded in a broader European context were already published by the German TSOs as a result of national and European wide investigations. The increase in onshore wind power combined with foreseeable offshore developments will require further action to guarantee the appropriate quality of supply.