Generic Electrical Models for HVDC-connected Offshore Wind Power Plants
Ömer Göksu, Oscar Saborío-Romano, Nicolaos Cutululis, Poul Sĝrensen
DTU Wind Energy, Roskilde, Denmark
As large Offshore Wind Power Plants (OWPPs) are being constructed far from shore, HVDC is becoming the solution for connection to the onshore grid. In order to guarantee stable offshore grid and provision of system services to the onshore grid, requirements for the HVDC-connected OWPPs have been introduced, such as the recently published ENTSO-E HVDC network code.
In order to design and analyze the offshore grid operation, electrical simulation models for the HVDC-connected Wind Turbines (WTs) and OWPPs are being provided by the WT manufacturers to be used mainly by OWPP developers. However, these models are usually manufacturer specific confidential blackbox models, which cannot be publicly shared or cannot be investigated or modified by the users. Hence, there is a need for generic and non-confidential model structures, which can be configured and used by the WT manufacturers, OWPP developers, TSOs, HVDC manufacturers, and also research institutes; both for the project specific simulation studies and problem-solving oriented investigations.
In this study, generic HVDC-connected WT and OWPP electrical simulation models are developed, based on the IEC 61400-27 wind turbine modeling standard, whereas the WT and OWPP control models have been extended in order to cover the requirements; such as active power setpoint control, frequency sensitive operation, synthetic inertia, reactive power management, and fault current injection. This study is being considered as a first step towards standard electrical models for HVDC-connected OWPPs, which will help the industry to perform effective analysis and design studies for offshore grids.
The performance of the developed models has been illustrated to perform the capabilities required by the grid codes, via simulation results. As the study does not focus on a specific stability problem, stability analysis has not been conducted. The developed WT and OWPP models are implemented using MATLAB Simulink software, while the SimScapePowerSystems toolbox is used for electrical components, both for root-mean-square (RMS) and electromagnetic transient (EMT) types. The developed RMS models have been compared against the detailed EMT models, which includes an average-value converter model with grid synchronization (PLL) and current control blocks. Generic values from the available literature are used for equivalent wind time-series, electrical component and control parameters.
A generic OWPP based on the developed WT models and OWPP controller is simulated using a test network with HVDC offshore converter, while the DC link and onshore converter are being represented as a constant DC voltage source. The following cases are simulated in order to show the compatibility of the models with the required capabilities in the grid codes; active power setpoint control (curtailment), active and reactive power operational range (PQ chart verification), reactive power management (voltage / power factor / reactive power control), frequency sensitive operation (frequency support using reserve), fault ride-through and fault current injection.
The simulation results show that the developed generic models can be utilized to represent the OWPPs performing the grid code required capabilities, while connected to HVDC converters. The open structure, which is based on the IEC 61400-27 standard, gives flexibility to investigate the offshore grid operation, such as during offshore faults and provision of system services to the onshore grid. Additionally, comparison of the RMS models against the EMT model gives insight about the validity range of the RMS models.
This paper provides an understanding of the basic structure of the IEC 61400-27 standard WT model and its extended use with HVDC. It also describes the difference between RMS and EMT models. More importantly, this study contributes to the literature with a generic open source modeling structure for HVDC-connected offshore WTs and OWPPs. It enables the academia and industry to work towards standard models for HVDC-connected WTs and OWPPs. Additionally, it represents performance of the generic models and the overview of recent grid code requirements from HVDC-connected OWPPs, via providing simulation results.