MVDC for utility scale power distribution and control

This poster outlines the case for converting selected network assets to medium voltage dc to increase network performance via a series of load flow studies. Technology readiness level and research requirements for the technology are also outlined

Control design of Modular Multilevel Converters in normal and AC fault conditions for HVDC grids

This paper describes a control design strategy of Modular Multilevel Converters (MMC) for High Voltage Direct Current (HVDC) applications to operate during normal and AC fault conditions. First, a steady state analysis of the converter is performed to identify the uses of the current components within the control strategy. Based on the initial stationary study, a complete converter control structure is proposed, which enables full control of the MMC internal energy during normal and AC fault conditions. A detailed design procedure is included for the current and energy regulators, in order to be able to ensure a dynamic response under any grid condition. Finally, theoretical developments are validated through simulation results by means of a detailed model in normal operation and during an AC voltage sag

The impact of MVDC upon conventional distance protection schemes in hybrid ac-dc distribution networks

This paper looks at the protection implications of introducing fully controllable, embedded, medium voltage dc (MVDC) power electronic links into power distribution networks. Studies have indicated, protection notwithstanding, that embedded MVDC can be used to provide economically attractive, enhanced capacity and control of power flows. Through a series of simulation studies, the impact on distance protection schemes (in terms of reach and response time) resulting from the introduction of a controlled MVDC link have been examined for symmetrical faults. The paper also considers under what conditions a dc side fault can be observed from the ac grid. A series of recommendations for system integrators are made

Design of a linear time-varying Model Predictive Control energy regulator for grid-tied VSCs

This paper presents an energy regulator based on a Model Predictive Control (MPC) algorithm for a Voltage Source Converter (VSC). The MPC is formulated to optimise the converter performance according to the weights defined in an objective function that trades off additional features, such as current harmonic distortion, reactive power tracking and DC bus voltage oscillation. Differently from most approaches found in the research literature, the MPC proposed here considers the coupling dynamics between the AC and DC sides of the VSC. This study is focused on the example case of a single-phase VSC, which presents a nonlinear relationship between its AC and DC sides and a sustained double-line frequency power disturbance in its DC bus. To reduce the burden of the MPC, the controller is formulated to benefit from the slow energy dynamics of the system. Thus, the cascaded structure typically used in the control of VSCs is kept and the MPC is set as an energy regulator at a reduced sampling frequency while the current control relies on a fast inner controller. The computational burden of the algorithm is further reduced by using a linear time-varying approximation. The controller is presented in detail and experimental validation showing the performance of the algorithm is provided

Transfverter: Imbuing Transformer-Like Properties in an Interlink Converter for Robust Control of a Hybrid AC–DC Microgrid

In a hybrid ac-dc microgrid, stiff voltage sources may appear in either the dc or ac subgrids which gives rise to multiple operation modes as power dispatch changes. This creates a challenge for designing the interlink converter between the ac and dc subgrids since the different modes require different interlink controls. To solve this problem, this paper proposes the concept of a transfverter inspired by how transformers link ac grids. Like a transformer, a transfverter can react to the presence of stiff voltage sources on either the dc or ac side and reflect the “stiffness” and voltage stabilizing capability to the other side. A back-to-back converter with droop control is used as the underlying technology to implement this concept. A novel optimization method called model bank synthesis is proposed to find control parameters for the interlink converter that offer the best controller performance across the different microgrid modes without requiring mode-changing of the controller. The effectiveness of the proposed solution is validated through both simulation and experiments

Blending HVDC-link energy storage and offshore wind turbine inertia for fast frequency response

This paper explores the benefits of combining the dc-link energy storage of a voltage source converter-based high-voltage dc (VSC-HVDC) link and the kinetic energy storage from wind turbines to facilitate in fast primary frequency control and system inertia to an ac network. Alongside physical and analytical justifications, a method is proposed which blends the energy stored in the HVDC link with the power control capabilities of the wind turbines to provide frequency response that is fast while not requiring excessive volume of capacitance nor demanding performance requirements on the wind turbines