James Yu

A Comparison of AC and HVDC Options for the Connection of Offshore Wind Generation in Great Britain

This paper presents a comparison of two forms of cable connection of a distant offshore wind farm to a transmission system: AC and HVDC. The requirements of relevant industry standards in Great Britain (GB) that drive a connection design and, hence, its costs are highlighted along with an analysis of the ways in which AC cable connections might be made to comply while facilitating the export of active power. Dynamic studies investigating responses to grid-side short-circuit faults show that, in the particular scenarios studied, an AC connection of a wind farm in place of a large synchronous generator is marginally detrimental while an HVDC connection is beneficial. A comparison of costs shows that the crossover distance at which HVDC is cheaper than AC for wind farms of different sizes occurs at longer distances than have hitherto commonly been assumed, and AC connections benefit from reactive compensation not only at the point of common coupling and wind farm end, but also at the connection midpoint.

A novel modular multilevel step-up DC/DC converter for offshore systems

This paper presents a novel modular multilevel DC/DC converter suitable for integrating the oif-shore wind farm with the HVDC transmission system. The proposed converter consists of a wind farm-side three-phase MMC inverter and a series-connected rectifier linked by a special decoupled medium frequency transformer. The transformer decouples the three-phase input voltage into three isolated single-phase voltages with the 120-degree displacement. As the system requires only a unidirectional power flow, this enables the reduction of switching losses by using simple diode bridge rectifiers modules. The modular structure of the proposed system reduces also the voltage and current stresses of the power devices. A simple control strategy based on the stationary reference frame without involving the complex Park transformation is proposed in this paper. The system is managed through controlling the primary side converter only without any feedback from the output side. A generalized control strategy to extend the system into larger scale is also presented. Simulation results using MATLAB/Simulink are reported to demonstrate the feasibility and the effectiveness of the proposed converter.

Impact of load dynamics on torsional interactions

This paper evaluates the impact of load dynamics on torsional interactions by considering a mix of static and dynamic loads aggregated at the bulk transmission level. This is essential to investigate the importance of detailed load modelling for subsynchronous resonance (SSR) studies to accurately assess damping contribution and capture system dynamics. SSR interaction with dynamic loads is investigated for both direct on line and drive connected motor loads. Damping contribution from dynamic loads is also assessed based on their location and size. The interaction of dynamic loads with classical SSR phenomenon is observed and introduced as the new concept of (Subsynchronous Resonance Load Interactions (SSR-LI)). SSR-LI assumes critical importance for scenarios where the load and generation centers are in close electrical proximity and impact of loads on torsional damping is significant. Finally, the scope for using existing converter interfaced motors for torsional mode damping has been discussed.

Design and Control of Unidirectional DC/DCModular Multilevel Converter for Offshore DC Collection Point: Theoretical Analysis & Experimental Validation

This paper presents the design and control of an advanced unidirectional dc–dc modular multilevel converter (MMC), which enables the integration of off-shore windfarms with the high-voltage direct current (HVdc) transmission system. The proposed converter consists of a single-phase MMC inverter, coupled with series-connected rectifier modules through a medium frequency transformer of multiple secondary windings. The modularity feature of the proposed converter enables scalability for different voltage levels. In addition to the galvanic isolation, the transformer also provides stepping gain to the output voltage. The proposed converter shows superior performance in terms of efficiency, losses, and devices utilization, when compared with the most competitive unidirectional cascaded dc–dc converters such as input series output series and input parallel output series. Furthermore, unlike the conventional d-q control method, which involves multiple transformations, this paper employs a simple proportional resonant control strategy that directly acts on the ac output of the MMC, under the stationary reference frame. The analytical design along with the simulation and experimentally validated results, confirmed the excellent performance of the proposed converter.

Dynamic control of MVDC link embedded in distribution network: — Case study on ANGLE-DC

Distribution network operators are encountering varied challenges through restricted corridors for expansion, fault-level constrained by capacity increase, increased pressure on operational efficiency, and proliferation of distributed energy resources (DERs). This will in effect impose limitations in power-flow and voltage. Moreover, due to the vast installation of converter based DERs like photovoltaic, electric vehicles, an efficient approach is to minimize the AC to DC conversion so as to reduce the losses by enabling DC technologies in the distribution networks. To this end, this paper assess the performance of a 33 kV distribution network with a proposed medium voltage DC (MVDC) link between the island of Anglesey and the mainland in North Wales, UK. This project will be the first of its kind in Europe and will paves the way for a more integrated DC network for the future. Dynamic operation and control of the proposed MVDC link is tested through PSCAD/EMTDC simulations. Operation of the DC link under steady state and during a single-line-to-ground (SLG) fault was tested. Moreover, a dual control loop has been implemented to improve the operation of the proposed link under SLG fault.

Addressing emerging network management needs with enhanced WAMS in the GB VISOR project

This paper describes a number of novel Wide-Area Monitoring System (WAMS) capabilities and analysis tools, including a new monitoring solution that covers the 4-46Hz sub-synchronous oscillation range, and the deployment of a new oscillation source location algorithm. These are being demonstrated under VISOR - an innovation project that has created the first integrated GB WAMS in order to demonstrate the capabilities and tangible business benefits of WAMS to the GB system. Other work from the project is discussed including hybrid state estimation, dynamic model validation, line parameter estimation and a new boundary constraint approach that uses angle difference in addition to power. Finally, selected learning from a review of existing GB WAMS performance is presented, along with details of next steps in the project.

Mitigating SSR: The threat of hidden critical lines and WAMS as a solution

Major incidents of dangerous Subsynchronous Resonance (SSR) have been caused by the loss of one or more lines causing the generator to come radial or partially radial with the series capacitor. Identification of such lines in a meshed multi-compensated power network is not possible through mere topological inspection. The paper presents a methodology for identifying Critical lines that may be hidden in meshed networks and can be used by the System Operator to raise EMS alarms against SSR threats in case of planned or unplanned outages. The paper also describes how Wide Area Monitoring Systems (WAMS) can be used to provide an economical alternative to existing SSR mitigation techniques by making best use of existing WAMS assets and minimizing further investment. To provide visibility of torsional oscillations (4–46 Hz), the paper describes a new Waveform Measurement Unit (WMU) that offers this additional functionality using a software update of a contemporary PMU, which is being trialed for the first time as part of the GB VISOR project. The potential opportunity and challenges of using WAMS for SSR mitigation is studied using the IEEE FBM. The methodology for identifying Critical lines and generators is demonstrated using the IEEE 68 bus model.